Mahdi Bennara
email: mahdi.bennara@liris.cnrs.fr

Michael Mrissa
email: michael.mrissa@liris.cnrs.fr

Youssef Amghar
email: youssef.amghar@liris.cnrs.fr

An Approach for Composing RESTful Linked Services on the Web

Keywords: RESTful Web services, linked services, semantic Web, composition Categories and Subject Descriptors H.3.5.2 [Web-based Services]: RESTful Web services, Service discovery and interfaces

In this paper, we present an approach to compose linked services on the Web based on the principles of linked data and REST. Our contribution is a unified method for discovering both the interaction possibilities a service offers and the available semantic links to other services. Our composition engine is implemented as a generic client that allows exploring a service API and interacting with other services to answer user's goal. We rely on a typical scenario in order to illustrate the benefits of our composition approach. We implemented a prototype to demonstrate the applicability of our proposal, experiment and discuss the results obtained.

INTRODUCTION

During the last decade, the emergence of Web services has been a major success to enable interoperability on the Web. For almost every simple task we would make on the Web, we can easily find one or several Web services that realize this task. Moreover, service composition, or mashups, enables valued-added processes that combine several services to answer complex user needs. The success of Web services is highlighted via Web sites such as ProgrammableWeb 1that referenced 105 APIs available on the Web in 2005 and more than 10000 APIs in 2014, not counting mashups.

However, while service-oriented computing becomes adopted, the supporting technologies change. During the last few years, the typical Web service protocol stack (SOAP, WSDL, UDDI) is slowly being abandoned for the profit of REST-based approaches. On the 14/01/2014, ProgrammableWeb counts 2125 SOAP-based APIs for 6833 REST-based ones. A RESTful service respects several constraints [START_REF] Fielding | Architectural styles and the design of network-based software architectures[END_REF]:

• It exposes the operations a service offers as a set of one or more Web resources (resource identification).

http://dx.doi.org/10.1145/2567948.2579222 .

• It links to others resources to allow for further interaction according to the HATEOAS principle (resource linking).

• It follows the semantics of HTTP verbs to interact with the exposed resource (stateless interaction), thus unifying the interaction possibilities (uniform interface).

• It exchanges self-describing messages.

The adoption of RESTful services changes the way services are designed and implemented. The uniform interface, that comes with the correct use of HTTP verbs and their semantics, replaces the typical API built around functions and input/output parameters. The management of the application state, which was usually handled server-side, is now at the charge of the client software (the browser).

In addition, recent advances in the semantic Web research area have been promoting linked data [START_REF] Bizer | Linked data -the story so far[END_REF] and a set of languages and tools such as JSON-LD [START_REF] Sporny | JSON-LD 1.0 -A JSON-based Serialization for Linked Data[END_REF], RDF [START_REF] Manola | RDF Primer. W3C Recommendation[END_REF], OWL [START_REF] Hitzler | Owl 2 web ontology language primer[END_REF], SPARQL [START_REF]The W3C SPARQL Working Group: SPARQL 1.1 Overview. W3C Recommendation[END_REF] and POWDER [START_REF] Archer | Protocol for Web Description Resources (POWDER): Description Resources. W3C Recommendation[END_REF], that allow to annotate Web data, resources and services with explicit, machine-readable semantics that can be utilized in conjunction with advanced reasoning mechanisms to connect resources to each other in a way that makes sense. Indeed, Web services also benefit from these advances, and can now be annotated with machine-readable, explicit semantics and exchange semantically annotated data. These services are referred to as linked services [START_REF] Pedrinaci | Toward the Next Wave of Services: Linked Services for the Web of Data[END_REF] 2 . Therefore, the typical approaches to discover, compose, orchestrate and utilize services on the Web need a complete rethinking to comply with -and get the benefits of-the REST principles and the semantic Web. Reaping the benefits from these advances requires adapting the way information systems and applications interact with the Web.

In this paper, we propose a solution to facilitate service composition based on the REST and linked data principles. In the remainder of this paper, services are semantic Web resources, they are identified via a URI and they are accessed through HTTP verbs. They also are semantically described and represented as JSON-LD documents to make semantics explicit when needed. The contribution of this paper is to propose a unified approach to discover the interaction patterns a resource offers and the orchestration possibilities with other resources, in order to enable resource composition, using the principles of REST and linked data.

Our paper is organized as follows. Section 2 introduces the scenario to illustrate our proposal and gives details on the motivation of our contribution. Section 3 details our contribution and shows its innovation. Section 4 shows how our prototype operates in the context of our scenario and demonstrates the applicability of our solution. Section 5 presents related work and highlights the advantages our solution offers. Section 6 discusses our results and gives some guidelines for future work.

SCENARIO AND MOTIVATION

In order to motivate our approach, we consider a typical scenario similar to the scenario presented in [START_REF] Pautasso | RESTful Web service composition with BPEL for REST[END_REF] involving a customer who buys a book from an online seller. At the beginning of the scenario, we assume the user only knows the URI of the book selling service. The user also knows his/her objective: ordering a book, paying online and getting delivered in less than 48h if possible. After choosing a book and creating an order, the client must choose a specific shipment method, give the address to which he/she wants to be delivered, and finally perform the payment operation.

In this scenario we identify a set of elementary services, which are, according to the REST principles, exposed as resources. The resources modeled within this scenario are the Book, Order, Shipment Method, Shipment and Payment resources.

Generic machine-client

Update the order

Submit a payment

Update the shipment Request/response client resource

Book selling service

Online payment service

Shipping service

Figure 1: Resources and data exchanges in our scenario Fig. 1 shows the resources and the different exchanges involved in the scenario, described as follows:

• Book selling service: it is composed of two sub-resources, a list of books and a list of orders. Book descriptions can be retrieved using GET and new orders can be created using POST and updated with the list of books to buy and payment confirmation with PUT (or PATCH)

• Payment service: a new payment can be created with POST and confirmed with PUT (or PATCH)

• Shipment service: a shipment method can be selected with GET and a new shipment can be created with POST and validated with PUT (or PATCH)

In a typical service-oriented architecture, the user's high level objective is described in a business process that is located serverside and orchestrated/executed by the composition engine. Service orchestration is typically either hard-coded under the form of a business process (or mashup) or dynamically generated on the server side using well-known reasoning techniques. Such a solution is not an option according to the REST principles as the handling of the composition needs to be performed client-side, relieving the server from handling the composition process and offering better scalability. Therefore, this scenario underlines the following scientific locks that motivate our work:

• There is a need for the user to interact with all these services using the same generic client.

• There is also a need for the client to be able to use several services together without manually connecting them, and to locally handle the state of the composite application.

• There is a need to provide semantically explicit links that allow the generic client to go from one service to another.

This scenario highlights our motivation: we want to show how the service discovery and composition tasks can be achieved using the Web, linked services and the REST paradigm. To do so, we provide a unified method for discovering resource interaction possibilities as well as external semantically linked resources that can be used in conjunction. This contribution is a first step towards automated discovery and composition of linked resources (or RESTful linked services) on the Web.

CONTRIBUTION

In this section, we explain how our unified method explores the features resources offer in terms of what HTTP operations are allowed, and how it allows discovering other resources that are semantically linked to the resource currently explored, according to the composition possibilities they offer. Then, it becomes very simple for a client-side software program to interact with resources and crawl from a resource to another to fulfill the user's objectives.

Resource Interaction Model

Our model involves specific interaction possibilities with resources on the Web. We extend a resource with a descriptor that contains meta-data about the resource together with information about related resources. Then, when the URI of a resource is available, it is possible to get its descriptor too, following a generic interaction pattern. To make this interaction possible, we use the HTTP LINK header and the POWDER [START_REF] Archer | Protocol for Web Description Resources (POWDER): Description Resources. W3C Recommendation[END_REF] describedby property, expressed in RDF 3 , in order to give a link with explicit semantics to the descriptor of a resource. The descriptor describes the resources pointing at it by giving information on what HTTP operation are available on the resource, together with information on available sub-resources and on other related external resources, as illustrated in Fig. 3.

We must underline the fact that all descriptors are resources also according to the REST principles. Then, as the descriptor is also modeled as a resource, the mechanism recursively applies. According to our proposition every resource must have a descriptor containing meta-data about it. One can get the descriptor of a descriptor via the Link field of the HTTP header received after a GET or HEAD operation. However, in such a case, the body of the answer contains meta-data about the descriptor, and the Link field in the HTTP header contains a link to the universal descriptor that describes all descriptors 4 . Figure 4 shows how resources, descriptors and the universal descriptor are linked to each other. At the moment, the meta data that describes a descriptor does not provide link to external resources, but such a possibility could be explored, for example for meta-data crawling.

Unified Resource Discovery

The way clients get access to internal and external resources is homogeneous. A client accesses related resources with the required information about each of them, helping to decide which path to follow.

The information available in the link to another resource must be helpful for the client to decide whether it should interact with the target resource or not. It must describe the semantics of the relation with the current resource and any other useful piece of information. However, the link information must not give too much details on the resource. Details about a resource can be found in its own descriptor. 4 Note that a GET or HEAD on the descriptor of all descriptors returns a link to itself. As the universal descriptor is at the same time a descriptor, it should imperatively describe itself.

In addition, giving too much information at this stage can generate data redundancy or bandwidth related issues. The best compromise is to only give the information that allows the client, at a given point, to decide whether to use the linked resource or not.

As well, describing the HTTP operations allowed on a given resource with information on how to correctly use the corresponding operation is useful. Our work relies on the Siren project [START_REF] Swiber | Siren: a hypermedia specification for representing entities[END_REF] in order to describe resources, links and HTTP operations. The information consists first of all in a description of the data-models expected and to be returned when the HTTP operation is called. The HTTP status codes that can be returned with additional details on the circumstances of each code gives details on the use of the HTTP operation. Other annotations on the operation itself, such as the CRUD basic operations (creates/reads/updates/deletes) and other advanced operations (initializes, cancels, confirms, etc.) can be useful for certain clients. Annotations on the data manipulated by the operation makes its semantics explicit in order to allow a better exploitation by the client. The HTTP operations that can be executed on the current resource can also be detailed in order to allow clients for unambiguous interaction. In particular, the semantics of the POST operation can be semantically linked to explicit descriptions to help the client drive the interaction.

While the definition of precise semantics is to be completed in future work, we envision also to semantically annotate the relation to external services with several properties that characterize how the other resource can be used with the original resource. We deem appropriate to use different interaction patterns such as precedes, follows, complements, replaces, and so on, that may help the user know what is the next link to follow and discover. These different patterns help clients build the different paths available to our generic client during a composition Additional information on the resource itself, such as provider, description, service area, language, location etc. can be integrated to the descriptor as well, but such details remains out of the scope of the current paper. Our resource discovery and composition model relies on the fact that resources can keep track of the interactions they have been involved in, and publish these traces, after processing, via their descriptor. Several possibilities exist to record resource interaction, such as storing the addresses of incoming requests, these addresses may lead to other related resources. Another solution is to extract well-known mashups from Web sites such as ProgrammableWeb. In this paper, we assume the list of connectible resources is maintained for each resource available on the Web. How to build and maintain such a list is subject to future work.

The applicability of our proposal and in particular of the unified discovery process is demonstrated with our scenario. We implement a generic client that takes advantage of the given descriptors in order to decide, at every moment and according to its user's needs, what resource links it should follow. While interacting with the different resources, the generic client handles the application state until it fulfills the user's goal. Figure 5 shows how our generic client discovers the different resources needed in our scenario.

IMPLEMENTATION

In the following, we first present our implementation setup, then we explain how the scenario runs with this setup. We give details about how each service is implemented and how it interacts with the client software and the other services using the elements of our contribution. Our prototype implementation is available on our development platform 5 .

Shipments

Setup

We implemented the services defined in the scenario using the Java TM Jersey framework for developing RESTful Web services based on the JAX-RS API. We used Eclipse as an IDE and the Glassfish application server to accommodate our services. We have developed 3 services, each in a single Java TM project, to keep the services independent from each other. Other service URIs are obtained by retrieving the book selling (the entry point) descriptor that leads also to its own resources, books and orders. The communication between the generic client and the services is ensured using the application/ld+json media type. We use the json/POJO serialization in order to convert the data transferred into a manipulable format in Java. We currently rely on the Siren project [START_REF] Swiber | Siren: a hypermedia specification for representing entities[END_REF] to describe Web resources.

Our client is currently a Java application but we plan to quickly implement it in Javascript as a browser plugin. The client takes the URI of the first resource to discover as input. According to our scenario and contribution, the client follows links found in the metadata retrieved from the entry point (book selling service resource) and calls HTTP operations on the right resources with the right data in order to answer the client needs. At the moment, this process is manually performed via user interaction through the client interface, but future work includes developing a reasoner to be able to reason about the user's objectives and match these objectives with the states of the different resources available to the client.

The structure of the descriptor is implemented via a Java class that encapsulates both allowed HTTP operations of the resource with its associated information and links to other external and internal resources also with their associated information, as well as the properties of the resource itself. Therefore, all the descriptors as well as the universal descriptor are instances of this class. They are considered as resources just like the three services of the scenario.

Revised Scenario

We present here the scenario and how it operates according to the elements of our contribution. Typically, a HEAD (or GET) HTTP operation on any resource should return the URI of the resource description (http://example.com/resource/resource. md, where md stands for meta-data) in the LINK field of the HTTP header, as described in Fig. 2. A GET operation on the URI of the link returns the descriptor, which is also a resource. The descriptor contains links and all the necessary information about the sub-resources of the resource it describes, and about external related resources. As we talk about a unified method for semi-automatic discovery, the descriptions of the internal sub-resources and external ones are homogeneous and processed the same way on the client side. The description allows the generic client to explore both internal and external interaction possibilities according to the information given on each resource. The client knows what is the best HTTP operation to apply to the resource according to its needs and the information available in the obtained description. A discussion on resource description is provided in section 5.

In the following, we present the resources modeled in our scenario and give their corresponding URIs. The particular resources are stored in their respective repositories (books, shipping methods, shipments, payments) and have the following URI patterns:

• Particular book: /bookselling/books/book123

• Particular orders: /bookselling/orders/order123

• Particular shipping methods: /shipping/methods/method123

• Particular shipments: /shipping/shipments/shipment123

• Particular payments: /payment/payments/payment123

In our scenario, the user first wants to browse book descriptions 6(GET on books repository resource) to select one or several book resources, whose detailed descriptions are then stored clientside. The problem is that the client at first does not know the URI of the books repository. It discovers this URI using the descriptor of the book selling resource.

The link to this descriptor is found in the HTTP response to a HEAD (or GET) operation on the entry point, we suppose known by the client, represented by the URI of the book selling service /bookselling/. More precisely, in the very beginning of the execution flow, the client executes a HEAD operation on the entry point URI. It gets the URI that corresponds to the describedby property attached to the Link Header field of the HTTP response, which is /bookselling/bookselling.md in our scenario. Then, it performs a GET operation on this link to get the descriptor of the book selling service. In this descriptor, the client finds multiple links to other related resources. It chooses the one pointing at the book repository using the semantics provided with the link and according to the user's needs.

Another question that our contribution answers to is: how does the client know what HTTP operation to execute in order to obtain book descriptions. The answer is: the client consults the meta-data on allowed HTTP operations and decides which one responds to the objective of getting a book description, in this case it is the GET operation.

Second, there is a need to create an order (POST) on the orders repository resource. The latter is referenced via the meta-data contained in "bookseller.md" retrieved from the book selling also exactly the same as the books repository resource. The HTTP operation to execute is retrieved exactly as explained above.

Once the order resource created, it remains in the status "Unpaid". It can be updated (PUT) to "Paid" once the payment is confirmed 7 . Then, the link to the shipment resource is extracted with the help of the "bookseller.md" resource that includes external resources.

Third, a shipment method is selected (GET) and a new shipment is created (POST) and set to the status "Unpaid", waiting for an update on the payment confirmation (PUT). The link to the "payment" resource is also retrieved by reading the "bookseller.md" resource.

Finally, the payment resource is created (POST) and confirmed as effective ( after getting a response from the order and shipment after the PUT operation).

RELATED WORK

Invocation of RESTful Services

The Hydra project [START_REF] Lanthaler | Hydra: A Vocabulary for Hypermedia-Driven Web APIs[END_REF] proposes a generic client to discover and invoke the different HTTP operations Web resources offer. The client relies on the Hydra core vocabulary and on the JSON-LD serialization format for data exchange.

One of the main advantages of Hydra is the possibility to build a generic client. Hence, there is no need to develop and compile a specific client to run through the Web API of each service. Also, Hydra proposes a mechanism that allows Web APIs developers to document their Web APIs into a format that can be either interpreted by machine or read by humans. The machine also can generate a human-readable format based on the documentation. The main drawback of Hydra is that it requires an effort in order to adapt existing solutions, and that the model developed is specific to Hydra.

The Siren project [START_REF] Swiber | Siren: a hypermedia specification for representing entities[END_REF] is another effort aiming for representing entities as Web APIs using the JSON format. An entity models a URI-addressable resource. The description of an entity includes a class field that describes the concept of the entity (for example book), a properties field that contains a set of key-value pairs describing the entity , an entities field that contains a list of related sub-entities, a links field that lists items containing navigational links including link to itself, an action field that lists a set of available actions on the current entity, described with the corresponding HTTP method, and a title field that textually describes the entity in question. Siren descriptions answers our needs for the description of Web resources and have been reused in our work.

The principle of distributed affordance [START_REF] Verborgh | Distributed affordance: an open-world assumption for hypermedia[END_REF] allows user interaction with resources in a distributed way. An affordance is defined as the possibility to perform an action over a resource. The authors define an architecture that enables enriching resource representations with distributed affordances. The affordance creation process happens based on information about a given resource, for example a book title, and knowledge acquired about user preferences and action providers, for example a local library service. The architecture is provided as a service, based on an API description catalog to gather action possibilities as well as user preferences. This solution is different from our approach where such information is directly attached to resources via their descriptors.

RESTdesc [START_REF] Verborgh | Description and Interaction of RESTful Services for Automatic Discovery and Execution[END_REF] is another effort for semantic description of Web APIs. The purpose of the description is to allow for an efficient way to discover the different features Web APIs offer. The RESTdesc approach takes into account the principles of REST. Since REST is based on the correct use of the HTTP protocol, RESTdesc format relies on the HTTP vocabulary in RDF [START_REF] Koch | HTTP Vocabulary in RDF 1.0. W3C Working Draft[END_REF] to describe the semantics of HTTP exchanges. The main advantage of RESTdesc is the loose coupling and reuse of existing models it offers.

The main benefit of our approach is to require only a few efforts in order to apply to existing Web services. The work to carry out is to add to the header fields of the HEAD or GET HTTP response of every resource a Link element that contains the URI of the resource descriptor. Building the descriptor contents also does not require a lot of work. In fact, the properties of a resource itself are already well known. For the HTTP operations allowed, as we run through the service source code we can easily show up the available HTTP operations as well as the properties that go with it. Concerning the list of links to other resources, there are a multitude of approaches to build such a list, as mentioned in Section 3.

Description of RESTful Services

ReLL [START_REF] Alarcón | Hypermedia-Driven RESTful Service Composition[END_REF] is a solution for the description of RESTful Web services that focuses on their Hypermedia property (HATEOAS). For ReLL, a RESTful service is a set of resources related to each other. Every resource in the set has its unique identifier in addition to its name, description in human readable language and other optional properties. A resource may have different representations depending on the media-type or syntax in use and every representation may contain links to other resource representations. The application domain semantics are explicitly supported in a resource representation. These semantics are made explicit using resource annotations.

LRDD [START_REF]LRDD Internet Draft[END_REF] is an effort to describe and obtain information on Web resources that are identified by URIs. The resource descriptor link is indicated by the resource itself using different methods such as with the Link field of the HTTP header. The link to the descriptor is dependent on the resource URI. The content of the descriptor is a machine readable information allowing better interoperability and enhancing the interactions with the resources.

HTTP vocabulary in RDF [START_REF] Koch | HTTP Vocabulary in RDF 1.0. W3C Working Draft[END_REF] is a W3C working draft aiming to represent the HTTP protocol in RDF format. It aims to link the REST paradigm with the concepts of the semantic Web. It gives a set of RDF classes and properties aiming to represent the HTTP specification as concepts.

The descriptions we use in our work are mainly inspired from the work cited above. We aim to reuse existing formats as much as possible in order to enhance the interoperability of our solution.

Discovery of RESTful Services

The RESTdesc [START_REF] Verborgh | Description and Interaction of RESTful Services for Automatic Discovery and Execution[END_REF] discovery process is based on the HTTP OP-TIONS verb. The invocation of this operation on a given resource returns back information in Notation3 Syntax. This information informs the machine-client of the current resource interaction possibilities. Since OPTIONS presents some drawbacks8 , an alternative is to link to descriptions using the LINK field of the HTTP header.

The discovery mechanisms in RESTdoc [START_REF] John | RESTDoc: Describe, Discover and Compose RESTful Semantic Web Services using Annotated Documentations[END_REF] format distinguishes two different aspects of the RESTful service discovery. Discovery as a client, or discovery as you browse, concerns client-side browsers. This kind of discovery relies on HTML Link elements on a Web site in order to point to other resource descriptions. Discovery as a service, also called automated discovery, is the ability for a service to access and link to other related resources in the same application domain. The main idea is to construct a graph by running through links and identifying resources. This graph can be subject to later extension to explore related resources. The RESTdoc solution relies on a microformat-like syntax to annotate resource representation directly in the HTML source and on an adapter to convert into RDF.

In our solution, the discovery mechanism helps find the path that clients may follow, through the progressive exploration of resource descriptors accessible from the HTTP Link Header embedded in HTTP responses. The resource meta-description is clearly separated from the resource representation and located in the descriptor. Resource discovery is performed locally from a given resource by consulting the list of related resources. The generic client guides the discovery process as it gets the list of related resources and decides whether or not to follow a given link according to the user's needs.

Composition of RESTful Services

Several works have been carried out in order to resolve the composition problem for RESTful Web services. Some of these works propose to reuse the BPEL language as mentioned in [START_REF] Pautasso | RESTful Web service composition with BPEL for REST[END_REF].

There are two different ways to use the BPEL principles to compose RESTful Web services. The first one suggests to use the WSDL 2.0 description language without changing the current BPEL. The solution relies on the new HTTP binding element introduced in WSDL 2.0. In fact, the RESTful Web resource API is wrapped behind a WSDL document that acts as an interface between the REST Web resource and the BPEL code, using the HTTP binding with the REST resource and operation invocation on the BPEL side. The second way to use BPEL with REST resources is more direct than the first one but it requires an extension to BPEL in order to be able to support the HTTP operations on the resource API. However the main drawback of BPEL comes with its centralized approach. In fact, it relies on a static composition engine, which does not fit with the REST principles that rely on the resource notion and HATEOAS. Additionally, a centralized execution process makes it less interesting in the large scale context of the Web, for which RESTful Web services have been designed.

Another approach is proposed in [START_REF] Zhao | Towards automated restful web service composition[END_REF] for composing RESTful Web services that takes full advantage of their characteristics. The authors propose a situation calculus-based state transition system that ensures the automatic composition for RESTful Web services. The basic idea is to take the composition problem and transform it into as state transition problem. The latter can be resolved using the situation calculus principles and components. However this proposal does not mention any implementation, hence it is difficult to evaluate the solution in terms of complexity, accuracy and response time, as well this kind of approach is centralized and presents the same drawbacks as the BPEL-based approach.

CONCLUSION

In this paper, we have proposed a novel approach to compose RESTful linked Web services. Our proposal relies on the description of Web resources in order to allow a unified discovery process for both internal sub-resources and external services. Our generic client is responsible for carrying the composition steps by getting resource descriptions, discovering new resources and sending data from one resource to another. The composition process is guided by the generic client according to the user's needs. Our scenario motivates our contribution and demonstrates the feasibility of our approach. Our proposal consists in attaching a descriptor resource to each Web resource. A link to the descriptor is obtained from the LINK header of a HTTP HEAD (or GET) response from the resource URI. A descriptor contains useful information about the resource and how it interacts with clients and other resources. It contains links and information about internal and external resources. By following these links and making use of the available information, the client handles the composition process in order to fulfill the user's objectives. As future work, we aim to focus on how to use reasoning mechanisms to process a high level client request into a set of composition steps. The user request should be processed by a reasoner, and the output of this analysis should be a set of partially ordered HTTP requests to perform on resources, in order to answer to the initial user request. Also, as future work, we plan to extend the resource description with additional information on related resources such as their availability, quality, average response time in order to facilitate resource selection.

Figure 2 :

 2 Figure 2: Discovering a resource

Figure 3 :

 3 Figure 3: Discovery of a resource descriptor

Figure 4 :

 4 Figure 4: Links between resources, descriptors and the universal descriptor

Figure 5 :

 5 Figure 5: Service Discovery in our Scenario

http://www.programmableweb.com

Linked services are Web services that exchange linked data and are described with linked data. We do not use the term linked data services to avoid confusion with data services that only provide data.

Alternative solutions are available at http://www.w3.org/TR/ powder-dr/#httplink

http://soc.univ-lyon1.fr

We refer here about the actual book description with its URI, author, title, price, and so on.

For the sake of brevity, we omit security concerns, but such an exchange may involve key sharing and secured protocols such as Oauth (http://oauth.net/).

As mentioned in http://www.mnot.net/blog/2012/10/29/ NO_OPTIONS


Marie-Pierre Bacos

Mikael Perrut

Denis Boivin

Nicolas Horezan

Etienne Rimpot

Catherine Rio

Claire Sanchez

Effect of Au diffusion in Au-coated (gamma + alpha2) titanium aluminides

Keywords: A, Intermetallics (aluminides) B, Oxidation C, Coatings 1

Introduction

TiAl-based alloys are continuing to attract attention from the industry as a high-temperature material, due to their exceptional combination of light weight and mechanical properties. Titanium aluminides have been found to exist in three different phases, Ti 3 Al ( 2 ), TiAl (), and TiAl 3 . However, most TiAl-based alloys developed for structural applications are essentially two-phase (γ + α 2 ) mixtures. According to the Ti-Al diagram, the aluminum content of the Ti 3 Al ( 2 ) phase is between 22% and 39% (at.%), while the TiAl () phase contains 48.5% to 66% (at.%) aluminum.

The attractive features of titanium aluminides are their lower density, higher stiffness, higher strength, improved creep resistance up to moderately high temperatures, lower coefficient of thermal expansion, higher thermal conductivity, and higher melting temperature compared to the conventionally used Ni-based super-alloys, which makes them suitable for potential hightemperature applications in the aerospace, automotive and power generation industries [START_REF] Kothari | Advances in gamma titanium aluminides and their manufacturing techniques[END_REF][START_REF] Wu | Review of alloy and process development of TiAl alloys[END_REF][START_REF] Thomas | Processing and Characterization of TiAl-based Alloys: Towards an Industrial Scale[END_REF][START_REF] Das | Sheet gammaTiAl: status and opportunities[END_REF][START_REF] Djanarthany | An overview of monolithic titanium aluminides based on Ti 3 Al and TiAl[END_REF].

However, the alloys are known to possess a number of limiting properties, such as poor corrosion resistance to chlorine agents and low oxidation resistance above 700°C, which have continued to restrict their wider application. Many studies have already been carried out to successfully improve the oxidation resistance of TiAl, as stated in the review by R. Pflumm et al [START_REF] Pflumm | Oxidation protection of -TiAl-based alloys[END_REF]. To a lesser extent, some corrosion studies are described in the literature [START_REF] Li | Cyclic oxidation and hot corrosion behaviour of Ti-48Al-2Cr with aluminide coatings[END_REF][START_REF] Tang | Hot-corrosion Behavior of TiAl-Base Intermetallics in Molten Salts[END_REF][START_REF] Nicholls | Hot salt corrosion of titanium aluminides[END_REF][START_REF] Yao | NaCl-induced hot corrosion of a titanium aluminide alloy[END_REF].

Therefore, to enhance the corrosion resistance of TiAl, we modified its surface composition by adding a noble and inert element. Given its high compatibility with titanium, we developed a specific coating based on a gold (Au) electrolytic deposit to protect a γ -48-2-2 alloy (nominal composition Ti-48Al-2Cr-2Nb in at. %) against corrosion by NaCl and oxidation [START_REF] Bacos | Influence of an oxidation protective coating upon hot corrosion and mechanical behaviour of Ti-48Al-2Cr-2Nb alloy[END_REF]. The coating obtained after diffusion of 20 μm of gold, for 1 hour at 900°C and at 10 -5 Pa, comprised two layers: an aluminoformer upper layer TiAlAu 2 and a lower layer TiAlAu.

After 1000 hours of cyclic oxidation at 800°C, the formation of a dense but non-homogeneous protective alumina layer was observed over the entire surface of the sample. Indeed, some more degraded areas appeared locally under the layer of alumina. Since the γ -48-2-2 alloy has a gamma-grain equiaxed microstructure (TiAl) with a few small  2 -phase precipitates (Ti 3 Al) at the grain boundaries, it was assumed that these degradations could be related to the presence of the  2 precipitates at the grain boundaries of the alloy.

In order to separate the contributions of each phase, so as to be able to validate the latter hypothesis, the present work investigates the microstructures of Au-diffused coatings formed on single-phased samples: a stoichiometric TiAl alloy (called -TiAl), i.e., an alloy whose microstructure is a -grain microstructure with no  2 phase precipitates, and a stoichiometric  2 -Ti 3 Al alloy without any  phase.

To check for destabilization of the upper TiAlAu 2 layer formed over the -TiAl alloy after long Au diffusion treatments, the stability of the compositions of the two layers, TiAlAu 2 and TiAlAu, has been studied. For this purpose, the experimental thicknesses of these layers have been compared to those calculated by a typical growth model for layers with a very narrow homogeneity range [START_REF] Prasad | Growth and consumption rates of the phase layers during interdiffusion in a diffusion couple with finite end member[END_REF].

In order to characterize the phases formed on the 2 -Ti 3 Al alloy after Au diffusion, different analysis techniques were combined. Finally, isothermal oxidation tests at 800°C were conducted to determine the ability of the Au-diffused 2 -Ti 3 Al alloy to form dense and protective alumina.

Experimental procedure

-TiAl and  2 -Ti 3 Al alloys prepared by double vacuum arc melting, re-melted seven times and annealed, were used as substrates for the coating experiments. Their bulk composition (at.%) was measured to be Ti50Al50 and Ti75Al25 for the  and  2 alloys, respectively (induction coupled plasma technique). The alloy microstructures were checked by means of scanning electron microscopy.

Specimens were cut into 10*15*2.5 mm parallelepiped blocks by electro-discharge machining. After being ground down to an 800-grid finish using SiC paper, the specimens were sandblasted and then ultrasonically cleaned using alcohol. The Au deposition experiments are described in Reference [START_REF] Bacos | Influence of an oxidation protective coating upon hot corrosion and mechanical behaviour of Ti-48Al-2Cr-2Nb alloy[END_REF].

After Au deposition, the specimens were heat-treated in a vacuum furnace (10 -5 Pa, heating rate of 10°C/minute up to 900°C with a threshold from 1 to 113 hours). The cooling rate was 15°C/minute. Specimens coated according to these steps (Au deposit and vacuum heat treatment) are called Au-coated specimens throughout the remainder of this article.

Surface and cross-sectional examinations were performed using a high-resolution field emission gun scanning electron microscope (FEG-SEM Zeiss Merlin and Gemini). Secondary (SE) and back-scattered electron (BSE) imaging modes were used. Energy dispersive X-ray spectroscopy (EDS-SDD with SAMx microanalysis software [13]) was used to determine the elemental composition and mapping of the regions of interest. In order to ensure state-of-theart quantitative composition measurements, long-time acquisition and metallic standards (Al, Ti, Au, and Ti-50Al) were used. X-ray diffraction (PANalytical Empyrean diffractometer, Cu-Kα radiation) was also used in the Bragg-Brentano reflection geometry. To identify the crystallized phases of the inner layers of the coating, mechanical grinding down to grid 4000 was performed, followed by surface SEM examinations.

Electron backscatter diffraction (EBSD) is used in this study to locate crystallized phases previously identified by X-ray diffraction. Indeed, EBSD has been proven to be a powerful tool for phase identification in multiphase materials, by analyzing Kikuchi patterns or electron backscattered diffraction patterns (EBSPs) [START_REF] Dingley | Progressive steps in the development of electron backscatter diffraction and orientation imaging microscopy[END_REF]. At each pixel, an EDS spectrum is acquired while the EBSD pattern is collected. The EDS peaks are identified to determine the chemical element present and databases are searched for all crystals that match the chemistry. These tables are compared sequentially with the interplanar angles and lattice parameters measured from the EBSD pattern. The phases having provided the best match and fit are selected and compared with those determined by surface XRD analysis. All of these experiments were performed on a FEG-SEM ZEISS Merlin with an EBSD system (TSL instrumentation software and NORDIFF detector) and EDS system (SAMx software and SDD detector).

Moreover, a dedicated EDS mapping experiment at 8kV provided not only spectral imaging to qualitatively differentiate intermetallic phases presenting a similar composition, but alsothrough a special quantification procedure available in the SAMx softwarechemical quantitative imaging, as reported in a previous article [START_REF] Perrut | Thermodynamic Modeling by the CALPHAD Method and its Applications to Innovative Materials[END_REF]. 800°C isothermal tests were carried out in an open, horizontal furnace.

All of the compositions are given in at.%.

Experimental results

Characterizations of Au-coated -TiAl and  2 -Ti 3 Al specimens

Influence of the heat treatment duration on Au-coated--TiAl specimens

The characteristics of the Au-coated -TiAl specimens analyzed in this section are given in As already published [START_REF] Bacos | Influence of an oxidation protective coating upon hot corrosion and mechanical behaviour of Ti-48Al-2Cr-2Nb alloy[END_REF], when vacuum heat treating (900°C, 1 hour) a 29 µm Au coating that was deposited on a -TiAl alloy, the initial Au layer was consumed completely and the formed coating was composed of TiAlAu2 and TiAlAu layers from the surface to the bulk alloy (Fig. 1, Spots 1 and 2, respectively). A zone enriched in Au is present (Fig. 1, Spot 3) at the interface between the lower TiAlAu layer and the -TiAl alloy. Semi-quantitative EDS measurements of the layers are given in Table 2. Previous work [START_REF] Bacos | Influence of an oxidation protective coating upon hot corrosion and mechanical behaviour of Ti-48Al-2Cr-2Nb alloy[END_REF] showed that the upper TiAlAu 2 layer, which contains around 25% at. of Al, transformed into an adequately dense and protective alumina layer during oxidation at 800°C, but the TiAlAu layer did not effectively provide good resistance to oxidation. Table 2: Semi-quantitative EDS analysis of a 29 µm Au-coated -TiAl alloy (Specimen A, spots in Fig. 1).

The thicknesses of the TiAlAu layer and of the diffusion zone enriched in Au increased according to the duration of the diffusion dwell period, while that of the TiAlAu 2 layer decreased (Fig. 2). The duration of the diffusion treatment leading to the total consumption of the TiAlAu 2 layer was estimated at 20 hours (Fig. 3). Even after long diffusion treatments, no additional phase was identified. It should be noted that although the layer thicknesses vary, their chemical composition remain constant over a long diffusion dwell time. As shown in Fig. 4 with the example of Specimen D, the EDS profiles from the substrate -TiAl to the surface indicated that the TiAlAu 2 and TiAlAu formed phases have a very narrow homogeneity range, the variation of composition in these phases being less than a few percent of the molar fraction. For the 1-hour Au-coated Specimen G, the mean composition of the outer phase is Ti24Al13Au63 (overall composition %at., Spot 1 in Fig. 6). In its lower part, there are some islands of a phase with a composition close to TiAlAu 2, which gathered into a continuous layer (Spot 2 in Fig. 6) resting on a very thin layer (~ 1 µm) that has a composition similar to that of TiAlAu (Spot 3 in Fig. 6). Just below, there is another layer composed of Ti50Al26Au24 (Spot 4 in Fig. 6). Finally, in contact with the substrate there is a two-phase layer composed of elongated and alternating phases (Spots 5 and 6 in Fig. 6). Specimen G, spots in Fig. 6).

The top surface X-ray diffraction pattern (Specimen G, 1h-Au-coated  2 alloy, Fig. 7)

indicates that the upper layer is mainly composed of two phases: a (TiAl)Au 2 (refined) tetragonal phase and a TiAlAu 2 cubic phase. For the (TiAl)Au 2 tetragonal phase, calculation using the Fullprof software, with the deduced parameters a = b = 3.40 Å and c = 8.55Å, gives a good fit to the positions of all of the intense lines (Table 4). The two parameters "a" and "c" are close to those of the TiAu 2 tetragonal phase (a = 3.429 Å-c = 8.538 Å), suggesting that the phase is significantly richer in Ti than in Al, as confirmed by the EDS analysis of Since longer diffusion dwell times permit layer growth, EDS/EBSD and XRD analyses were conducted on a 24h-Au-coated  2 alloy for a better identification of the different phases of the coating (Fig. 8 and Fig. 9). The 24h-Au-coated Specimen I presents a quite unchanged overall morphology of the layers, compared to the 1h-Au-coated Specimen G, but another "lamellar phase" located under the TiAlAu 2 phase was identified (Spot 3 in Fig. 8 and Table 5). Combined EBSD analysis and chemical EDS mapping performed on Specimen I (Fig. 8, Table 5) allows the XRD results (Fig. 9) to be refined, since it gives the topological distribution of the phases over the entire coating. Indeed, as shown in Fig. 10 with the example of Spot 7, the tetragonal phase with parameters as determined by XRD analysis (c/a = 1.045) provided the best match and fit for the two phases in Spots 5 and 7 of Fig. 8. By the way, from the outer layer to the substrate, the layer sequence can be described as The EBSD phase distribution map (Fig. 11) shows a crystallographic continuity between the single-phased layer (Phase 5 of Fig. 8), which is just beneath TiAlAu, and the elongated dark phase (Phase 7 of Fig. 8) of the two-phase layer that is in contact with the substrate. It suggests that these two phases, although they have different compositions according to the EDS analysis, have similar tetragonal crystallographic structures.

EDS quantitative imaging of the Au-coated α 2 -Ti 3 Al alloy (Specimen I) was used to differentiate these two phases appearing after interdiffusion of Au, Ti and Al at 900°C, allowing us to propose a partial diagram for the Al-Au-Ti corresponding isotherm section (Fig. 12, Table 7). This mapping was performed over a zone covering the interdiffusion layers from α 2 -Ti 3 Al (substrate) to TiAu, bringing out two unknown ternary intermetallics: Ti 2 AlAu (Phase 5 of Fig. 8) and Ti 3 Al 2 Au (Phase 7 of Fig. 8), compatible with these two intermetallics, which are undifferentiated in the EBSD map of Fig. 11. The tie-lines and tie-triangles were measured through a manual postprocessing procedure of the quantitative 2D-data. For each tie-line between two particular phases, 1D-composition profiles across the interface between these two phases are being plotted, so that the near-interface compositions of the coexisting phases can be measured. Every profile is chosen perpendicular to the interface. The tietriangles are deduced by three different tie-lines evaluated near a trijunction point. The TiAu compound seems to show a quite high solubility in Al in Fig. 12a, but it has been difficult to analyze this properly near the edge zone of the EDS map. It is worth noting that Au was never found in the  2 -Ti 3 Al substrate.

The analyses performed on the 24h-Au-diffused Specimen I enables the completion of Table 3 Table 7: Measurements associated with the proposed Al-Au-Ti diagram (Fig. 12).

Characterizations of oxidized coated  2 -Ti 3 Al specimens

A few coupons of 1-hour Au-coated  2 Ti 3 Al were oxidized at 800°C in isothermal tests that were carried out in an open, horizontal furnace. For comparison with a previous study [START_REF] Bacos | Influence of an oxidation protective coating upon hot corrosion and mechanical behaviour of Ti-48Al-2Cr-2Nb alloy[END_REF],

the same isothermal oxidation test was also conducted on an Au-coated γ -48-2-2 TiAl alloy, which contains  2 precipitates, as described in [START_REF] Bacos | Influence of an oxidation protective coating upon hot corrosion and mechanical behaviour of Ti-48Al-2Cr-2Nb alloy[END_REF].

As shown in Fig. 13, Au-coated γ-48-2-2 TiAl possesses a quite good oxidation resistance.

Formation of a thin alumina covers over much of the TiAlAu 2 upper layer surface, but some highly oxidized areas appear locally (around 2 µm in depth). In contrast, the 1-hour Au-coated  2 Ti 3 Al does not resist oxidation at 800°C. Pitting oxidation appears as of 100 hours of oxidation at 800°C, and more than a quarter of the thickness of the coating is attacked in the highly altered areas (Fig. 14).

Discussion

During the vacuum heat treatment of a 29 µm Au coating deposited on a -TiAl alloy, the Au diffused and formed, from the surface to the bulk, the TiAlAu 2 and TiAlAu phases in agreement with the work of Jorda [START_REF] Jorda | Phase relationships in the ternary system Ti-Au-Al at 775K[END_REF]. The EDS profiles measured through the Au-diffused coatings show that these phases have a very narrow homogeneity range and that their thicknesses vary according to the duration of the diffusion treatment. EDS analyses did not permit any additional phase to be identified during long diffusion dwell times, suggesting a good stability of the formed layers in terms of chemical composition and aluminum content.

To assess this stability, a comparison between the experimental thicknesses of the layers and those calculated from a growth model of layers with narrow homogeneity range (or line compounds) is discussed. In a binary distribution system A/B, A being a coating with a finite thickness and B being the semi-infinite substrate, Prasad [START_REF] Prasad | Growth and consumption rates of the phase layers during interdiffusion in a diffusion couple with finite end member[END_REF] developed a model, based on the integrated diffusion coefficient, as published by Wagner [START_REF] Wagner | The evaluation of data obtained with diffusion couples of binary single-phase and multiphases systems[END_REF], to calculate the consumption rate of the end-member A and the growth rates of the product phases A 2 B and AB. This model can be applied to our case, with A corresponding to Au, B to TiAl, A 2 B to TiAlAu Given that the Au finite layer is consumed completely after the 1h diffusion dwell period, the system can be reduced to the growth of a single-phased TiAlAu between the end-member TiAlAu 2 and the semi-finite substrate (TiAl).

x tc A 2 B and x tc AB are the thicknesses of the product phases A 2 B and AB respectively, after the 1h (t c ) diffusion dwell period (Specimen A).

The integrated diffusion coefficient of the product phase TiAlAu can be written as: x' AB is the total thickness of the product layer AB formed during the diffusion dwell period.

Ď AB int = ((
x' AB = ((a AB +b AB ) 2 Ď AB int t) / ( a AB b AB )) (Eq.4)

By equating the number of atoms A consumed from the A 2 B (A 2/3 B 1/3 ) with the amount of atoms A present in x AB (A 1/2 B 1/2 ), and knowing the molar volumes V m of the phases, we can write:

x A 2 B /V m A 2 B ) = (1/2)(x AB /V m AB )  Eq x A 2 B /V m A 2 B ) = (3x AB /4V m AB )   withx' A 2 B = x tc A 2 B -x A 2 B  andx' AB = x tc AB + x AB
Then, the thickness of the product phase A 2 B at the end of the diffusion dwell period can be written as:

x' A 2 B = x tc A 2 B -(3V m A 2 B /4 V m AB )(x' AB -x tc AB ) (Eq. 6)
The relation between the thicknesses of the TiAlAu 2 and TiAlAu layers is then described by Eq.7.

x'

A 2 B = x tc A 2 B -(3V m A 2 B /4 V m AB ) (k t d + t c   -x tc AB ) (Eq.7)
A good general match between the experimental thicknesses of the TiAlAu 2 and TiAl layers and the theoretical ones is obtained (Fig. 3), with an estimated integrated diffusion coefficient of the product phase TiAlAu equal to 10 -14 m²/s and the numerical values of Table 8. For longer diffusion (24 h), the upper (TiAl)Au 2 tetragonal phase that was found to be depleted in Al transformed into a TiAu 2 tetragonal phase, and the TiAu orthorhombic phase clearly appeared between the TiAlAu 2 cubic phase, which was depleted in Al, and the TiAlAu hexagonal phases. Comparison of the 7 h and 24 h EDS profile points shows that the TiAlAu layer grows from the aluminum consumption of the TiAlAu 2 layer, leading to the formation of the TiAu layer between them. Since the TiAlAu phase has a very narrow homogeneity range and has a greater aluminum content (33% at. Al) than the semi-infinite substrate (Ti 3 Al, 25% at. Al), the system quickly freezes. The upper layer of the  2 Au-diffused coating consists of a non-protective TiAu 2 layer [START_REF] Declairieux | On the potential of Ti50Au50 compound as a high temperature shape memory alloy[END_REF] and a TiAlAu 2 layer that is too depleted in Al to be effective against oxidation. The first oxidation isothermal tests of a 25 µm Au-coated  2 -Ti 3 Al substrate (1-h Au diffusion treatment) confirmed this poor resistance to oxidation (Fig. 14)

Conclusion

In this study, the microstructures of Au-diffused coatings on  2 -Ti 3 Al alloys, compared to those on -TiAl alloys, were investigated in order to understand the role of  This may also explain why some pitting oxidation appears locally during the oxidation of the Au-coated γ -48-2-2-TiAl. After Au diffusion, if the surface of each TiAl gamma grain transformed into an upper aluminoformer TiAlAu 2 , the Ti 3 Al precipitates present at the boundary grains would have transformed into a non-protective alloy that is highly oxidized.

Fig. 1 :

 1 Fig. 1: Typical SEM micrograph of a 29 µm Au-coated -TiAl alloy -Specimen A.

Fig. 2 :

 2 Fig. 2: SEM micrographs (BSE) of 29 µm Au-coated -TiAl specimens according to the

Fig. 3 :

 3 Fig. 3: Experimental and theoretical curve thicknesses of the TiAlAu 2 and TiAlAu layers

Fig. 4 :

 4 Fig. 4: EDS profiles of the 29 µm Au-coated -TiAl Specimen D (diffused for 7hours). (Color

Fig. 5 :

 5 Fig. 5: SEM micrographs (BSE) and corresponding EDS profiles along the red lines of a 25

Fig. 6 :

 6 Fig.6: SEM micrographs of a 25 µm Au-coated   -Ti 3 Al specimen, diffused for 1 hour at

Fig. 7 :

 7 Fig. 7: XRD pattern of the top surface of Specimen G, 1h-Au-coated  2 alloy

Fig. 8 :

 8 Fig. 8: EDS and EBSD combined analysis of Specimen I, EBSD diffraction patterns and

Fig. 8 )

 8 -Analysis with standards.The top surface X-ray diffraction pattern of Specimen I (24h-Au-coated, Fig.9a- line)indicates that the (Ti,Al)Au 2 (refined) tetragonal peaks that were visible after 1 hour of diffusion have disappeared. The outer layer was only composed of TiAu 2 tetragonal and TiAlAu 2 cubic phases. Concerning the TiAlAu 2 cubic phase, it is worth noting that the peaks are slightly shifted with respect to those given in the 00-041-0937 sheet. Parameter "a" of the TiAlAu 2 cubic phase was found to be equal to 3.215 Å (instead of 3.198 Å). Given that a TiAu similar cubic phase (Pm-3m) exists (with a = 3.254 Å), it suggests, in agreement with the previous EDS analysis of Table5 (Spot 2), that the stoichiometry of the TiAlAu 2 cubic phase is depleted in Al. XRD analysis of Specimen I performed after grinding (removal of matter down to a depth of around 25 µm, Fig.9b-line) identifies the underlying phases as a TiAu orthorhombic (martensitic) phase with a = 4.61 Å, b = 3.00 Å, c = 4.81 Å, and a TiAlAu hexagonal phase. After another sequential grinding (Fig. 9c,  line), new peaks are visible. They can be assigned to a Ti 3 Au cubic phase and to a tetragonal phase, close to the -TiAl phase, but with the following parameters: a = b = 3.97 Å and c = 4.15 Å, c/a = 1.045,

Fig. 9 :

 9 Fig. 9: XRD patterns; ) top surface of Specimen I - line; b) and c) surfaces of Specimen I

Fig 10 :

 10 Fig 10: a) EBSD pattern (Spot 7) and b) overlay of the best solution (Tetragonal)[P4/mmm],

  TiAu 2 tetragonal/ TiAlAu 2 cubic /TiAu orthorhombic/TiAlAu hexagonal/tetragonal phase [P4/mmm] /Ti 3 Au cubic+ tetragonal phase [P4/mmm]. The image quality (IQ) map constructed from electron backscatter diffraction data shows a typical martensitic microstructure of the TiAu phase (herringbone morphology[START_REF] Inamura | Crystallography of Martensite in TiAu Shape Memory Alloy[END_REF]) (Fig.11).

Fig. 11 :

 11 Fig. 11: upper) EBSD phase distribution map of Specimen I and color coded map; lower)

  Fig. 12: a) Automatic retrieval of the compositions obtained by quantitative EDS imaging; b)

Fig 13 :Fig 14 :

 1314 Fig 13: SEM micrograph of a 29 µm Au-coated -48-2-2-TiAl specimen (1 hour diffusion)

  2 and AB to TiAlAu, with the following assumptions: (a) the species Ti and Al diffuse at the same speed; (b) the influence of one species on the diffusion of the other two is negligible; (c) the variations of composition in the TiAlAu and TiAlAu 2 phases are neglected; and (d) Au and TiAl do not diffuse in a solid solution in the TiAl and Au phases respectively.

  a AB b AB )(x' AB )²) / ((a AB +b AB ) 2t) (Eq.1) With a AB = N AB B -N - B b AB = N + B -N AB B N + B and N - B being the initial molar fraction of Species B, in B and A respectively; N AB B being the initial molar fraction of Species B in AB;

Table 8 :

 8 Numerical values used to solve Eq.7.After the heat treatment (900°C for 1 hour) of a 25µm Au coating deposited on an  2 -Ti 3 Al specimen, the following successive product phases were identified: (TiAl)Au 2 tetragonal/TiAlAu 2 cubic /TiAlAu hexagonal/Ti 2 AlAu tetragonal/Ti 3 Au cubic + Ti 3 Al 2 Au tetragonal phase. The two ternary compounds TiAlAu and TiAlAu 2 were clearly identified by Jorda et al.[START_REF] Jorda | Phase relationships in the ternary system Ti-Au-Al at 775K[END_REF] when investigating the phase equilibria of the ternary Ti-Au-Al at 500°C. Two new ternary compounds, Ti 2 AlAu and Ti 3 Al 2 Au, not reported in the Jorda 500°C ternary diagram, were found to exist after Au diffusion at 900°C. The Ti 2 AlAu participates in a three-phase equilibrium with Ti 3 Au and another ternary compound (Ti 3 Al 2 Au), and in a three-phase equilibrium with TiAlAu and TiAu. In the EBSD mapping the Ti 2 AlAu and Ti 3 Al 2 Au are associated with the same tetragonal structure as that determined by XRD analysis (P4/mmm, c/a = 1.045). However, the existence of duplicate peaks on the large diffraction angle at close inter-reticular distance suggests close but slightly different cell parameters for these two phases. Additional experiments and analyses would be necessary to properly assess their structures.

  2 precipitates in the resistance to oxidation of Au-coated -48-2-2-TiAl specimens. When vacuum heat treating (900°C, 1 hour) a 29 µm Au coating that was deposited on a -TiAl alloy, the Au diffused and formed, from the surface to the bulk, an upper aluminoformer TiAlAu 2 layer and a lower non-protective TiAlAu layer. The thicknesses of the TiAlAu 2 and TiAlAu layers could be estimated using the Prasad model and an estimated integrated diffusion coefficient of the product phase TiAlAu equal to 10 -14 m²/s. After the vacuum heat treatment (900°C, 1 hour) of a 25µm Au coating deposited on  2 -Ti 3 Al alloy, the following successive product phases were identified: (TiAl)Au 2 tetragonal/TiAlAu 2 cubic /TiAlAu hexagonal/Ti 2 AlAu tetragonal/Ti 3 Au cubic + Ti 3 Al 2 Au tetragonal. For longer diffusion (24 h) the upper (TiAl)Au 2 tetragonal phase transformed into a TiAu 2 tetragonal phase and the TiAu orthorhombic phase appeared between the TiAlAu 2 cubic, which was depleted in Al, and the TiAlAu hexagonal phases. Two new ternary compounds, Ti 2 AlAu and Ti 3 Al 2 Au, were found in this study. The Ti 2 AlAu participates in three-phase equilibria with Ti 3 Au and Ti 3 Al 2 Au, and also in three-phase equilibria with TiAlAu and TiAu. A partial diagram for the Al-Au-Ti 900°C isotherm is proposed. Absence or depletion of Al in the upper layers (TiAu 2 and TiAlAu 2 ) of an Au-coated  2 -Ti 3 Al substrate is probably the cause of its low resistance to oxidation.

Table 1 .

 1

		Vacuum	TiAlAu 2	TiAlAu
	-TiAl	diffusion	thickness * (µm)	thickness * (µm)
		duration (h)	± 2 µm	± 2 µm
	Specimen A	1	48	21
	Specimen B	2	43	32
	Specimen C	3	37	35
	Specimen D	7	24	40
	Specimen E	13	15	60
	Specimen F	113	0	74

Table 1 :

 1 Characteristics of the 29 µm Au-coated -TiAl specimens. Temperature of the vacuum diffusion treatment: 900°C.

* Thicknesses measured from SEM micrographs.

Table 3 :

 3

	Spot	Al (at.%) Ti(at.%) Au(at.%)	XRD corresponding phase of Fig. 7a
	1	13.1	24	62.9	(TiAl)Au 2 tetragonal phase
	2	22.2	28.8	49	TiAlAu 2 cubic
	3	28.7	38.0	33.3
	4	26.2	50	23.8
	5	38.2	47.6	14.1
	6	10.3	66.3	23.3

EDS analysis results of a 25 µm Au-coated- 2 -Ti 3 Al specimen (1 hour diffusion,

Table 3 (

 3 Spot 1). This (TiAl)Au 2 tetragonal phase presents a preferential orientation (110). The phases of the lower layers could not be identified due to the low intensity of the peaks.

	hkl	d(obs) Å	d(cal) Å
	002	4.2667	4.275
	101	3.1596	3.16
	110	2.4070	2.405
	103	2.1836	2.184
	112	2.0972	2.096
	200	1.6984	1.7005
	114	1.5976	1.5976
	202	1.5793	1.5801

Table 4 :

 4 X-Ray diffraction pattern of (TiAl)Au 2 (I4/m m; a=b= 3.401 Å , c=8.550 Å).

Table 5 :

 5 EDS analysis results for a 25 µm Au-coated- 2 -Ti 3 Al alloy (24h diffusion, Specimen I, spots in

	14

Table 6

 6

	, i.e., a more


A Julian-Jankowiak
email: aurelie.jankowiak@onera.fr

P Sallot

Microstructure and mechanical properties of Nb 4 AlC 3 MAX phase synthesized by reactive Hot Pressing

Keywords: MAX phases, Hot pressing (A), Grain size (B), Mechanical properties (C), Engine components (E)

The present study aims at synthesizing the Nb 4 AlC 3 MAX phase by reactive hot pressing using Nb:Al:NbC as starting materials. In order to identify the reaction path, interrupted tests at intermediate temperatures were performed as well as differential thermal analyses (DTA) of powders. Coupling between DTA and XRD data and SEM/EDS analyses of the samples allows a better understanding of the reaction mechanisms. Pure and fully dense Nb 4 AlC 3 samples were obtained and characterized for the first time by EBSD and SEM to assess, using an original method, grain size and microstructure. For instance, in the present study, an average grain length of 5-7 µm was obtained.

Standard mechanical characterizations showed interesting properties: K Ic ≈6 MPa.m -1/2 , E≈350 GPa and α≈7.10 -6 °C-1 . Oxidation performance of Nb 4 AlC 3 was evaluated at 1100°C under cyclic conditions. A breakaway regime was instantaneously established for this condition, thus demonstrating the impossibility of using such an unprotected material for structural applications at high temperature in air environment.

Introduction

MAX phases are ternary compounds with general formula M n+1 AX n (n=1 to 3) where M is a transition metal, A a group IIIA and/or group IVA element and X either C or N . They possess good thermal and electrical conductivities, high resistance to thermal shocks, good stiffness and damage tolerance as well as relatively low coefficients of thermal expansion. Some of these compounds have also a very good oxidation resistance due to the formation of a dense and adherent alumina layer. Among all the possible configurations leading to MAX phase compounds, only a few of them have been extensively studied, notably in the Ti-Al-C, Ti-Si-C, Cr-Al-C systems. To the authors' knowledge, only a few studies are related to the development and characterization of more refractory MAX phases, such as those corresponding to the Nb-Al-C system. Salama et al. took advantage of the complete solid solubility between Ti and Nb on the M site to produce Nb 2 AlC and (Ti,Nb) 2 AlC compounds to optimize microstructures and mechanical properties. Hu et al. managed to produce textured nanolayered Nb 4 AlC 3 ceramics having a shell-like microstructure configuration and to measure mechanical properties. Finally, Hu et al. proposed different synthesis procedures for hot pressing (HP) and spark plasma sintering (SPS) processes. Phase compositions were investigated as a function of temperature, dwell time and the pressure applied during sintering; the resulting thermophysical properties were also reported. Using hot pressing, the same authors have studied the reaction synthesis above 1500°C and have obtained a fully dense Nb 4 AlC 3 MAX phase with a low amount of secondary phases . They have reported its high temperature mechanical stability: the flexural strength and the Young's modulus of Nb 4 AlC 3 are maintained up to 1400°C and 1580°C, respectively, which confirms the promising application opportunities of this MAX phase at high temperature.

However, these results were obtained starting with Nb:Al:C mixtures and it was also demonstrated that the molar ratios of the starting powders could have a strong impact on the purity of the final material and notably on the amount of retained Nb 2 AlC and Al 3 Nb impurities.

In this paper, a new initial powder mixture is studied to obtain a dense Nb 4 AlC 3 material with a very low content of impurities. The precise reaction pathway is analyzed using a systematic coupling between differential thermal analyses (DTA), X-ray diffraction (XRD) analyses and microstructural results collected during interrupted sintering tests. New insights into microstructure of bulk Nb 4 AlC 3 are brought by fine electron backscattered diffraction (EBSD) and scanning electron microscope (SEM) analyses after sintering, adding new data to the relation between microstructure and final mechanical properties / oxidation resistance.

Experimental details

Commercial powders of niobium (Alfa Aesar, 99.8%, -325 mesh), aluminum (Alfa Aesar, 99.5%, d 50 : 9 µm) and niobium carbide (Alfa Aesar, 99%, < 10 µm) were used as starting materials.

The selected molar ratio of Nb:Al:NbC was set to 1.2 : 1.1: 2.8 for this investigation. Al was introduced in excess to prevent potential evaporation during sintering. The mixture is deficient in C as it is often the case for Al-containing MAX phases. The powders were mixed in alcohol using WC media for 2 hours. Alcohol was then evaporated and the mixture was finally dried for 10h at 90°C. Nb 4 AlC 3 was obtained by in situ reaction in a hot pressing device using BN-coated graphite dies. The sintering cycle comprised heating up to 1735°C (5°C/min) with a 2 hours dwell time before cooling down to room temperature. All the experiments were performed under a flowing argon atmosphere and a uniaxial pressure of 30 MPa. Following this procedure, 35x35x3mm 3 plates were successfully manufactured.

To evaluate the reaction pathway, several sintering cycles were interrupted at different temperatures to analyze the nature and the abundance of phases. Interruption levels were identified through DTA performed on a Nb:Al:NbC mixture in flowing Ar (20 ml/min) up to 1550°C using a Setsys evolution 1750 apparatus (SETARAM, France). Closed alumina crucibles were used to limit Al evaporation. Phase identification was accomplished on polished surfaces using an Empyrean Panalytical diffractometer in the Bragg Brentano configuration. The diffraction range was from 8° to 94° with a 0.01° scanning step. Rietveld refinement was achieved through the MAUD® software to identify the phases and their volume fractions. Two criteria,  (≈2) and R wp (<15), were used to assess the quality of the refinement. The XRD analysis being performed on the surface of monolithic samples, an arbitrary texture was taken into account in the Rietveld refinement.

Cross sections of the samples were embedded in resin before polishing down to 1 µm (mirror finish). As-polished samples were characterized using an SEM (DSM 962, Carl Zeiss) equipped with energy-dispersive spectroscopy (EDS). EBSD (Merlin, Zeiss) was performed on the same samples after a more cautious polishing with a colloidal silica suspension using a VibroMet polisher.

Fully dense pellets were prepared after in situ reaction at 1735°C for 1 or 2 hours and machined using electro discharge machining (EDM) to prepare mechanical and oxidation testing samples. Rectangular samples 35 x 5 x 1.5 mm 3 were polished on one side (1 µm) before dynamic Young's modulus measurement at room temperature (GrindoSonic Mk5i).

Flexural strength was evaluated on, at least, three samples using four-point bending tests conducted in the Z010 Zwick machine (1 kN) running at a cross-head speed of 0.3 mm/s. To avoid the presence of any surface defect which could act as crack initiation sites, the polished surfaces were oriented towards the side under tension. Finally, Vickers hardness was measured on the uncracked part of the samples. A 200 g load was used to indent the polished surface and measurements were repeated, at least, 5 times to determine an averaged value.

Toughness was evaluated using three-point bending tests performed in the Z010 Zwick machine through single edge notched beam tests (SENB). Samples were similarly rectangular in shape (35 x 3 x 1.5 mm 3 ) with an EDMed notch (a/w=0.5). Crack propagation was observed using a digital video camera; thus, the observed surface was polished to 1 µm as well.

Thermal expansion measurements were carried out on rectangular samples 16 mm in length and 5 x 2 mm in cross section. The Setaram Setsys Evolution 1750 device was operated under flowing argon (20 ml/min) with alumina rigs. The thermal cycle consisted in heating up to 1450°C (5°C/min) and cooling down to room temperature (5°C/min). Results were corrected using a sapphire blank test.

Finally, cyclic oxidation tests were performed using Safran facilities (vertical tubular furnace) on cylindrical samples (Ø25 mm, t= 2 mm) machined from the plates. Samples were positioned on ceramic holders directly on an automated elevator, exposing samples, alternately, to the high temperature of the furnace and then to the room air temperature. The thermal cycles, thus obtained, were carried out in air with holding at 1100 °C. The duration of each thermal cycle was approximately one hour; it included heating up to 1100°C (≈5-10min), a 45 min dwell time and, finally, cooling down to room temperature.

Results and discussion

Phase identification and reaction mechanisms

The linear shrinkage of the Nb:Al:NbC mixture during the reactive hot pressing is shown in figure 1. The corresponding differential thermal analysis is superimposed to allow direct comparisons.

Concerning densification of samples, two stages are visible in figure 1. A first increase in densification rate is observed around 670°C (① in figure 1) before it reduces drastically, leading to a shrinkage plateau. Then, a second densification peak is identified at 1385°C (② in figure 1) leading to complete densification.

In order to explain such a sequence, it was chosen to interrupt tests after the first and the second densification peaks, i.e. at 740°C and 1420°C, respectively. An interruption at 1000°C allows analyzing the evolution occurring during the shrinkage plateau while stops at 1630 and 1735°C enable the analysis of the microstructure at the final stages of densification. SEM examinations of the related microstructures are presented in figure 2. XRD analyses of the corresponding specimens are presented in figure 3.

In addition, for each interrupted test, the open porosity was measured and reported in Table 1. The data reported in Table 1 clearly demonstrate that densification is only effective after the second densification peak, i.e. 1385°C.

Table 1: Thermal treatment conditions, open porosity and densification levels of NAC materials.

Reference

Regarding all the different characterizations, the reaction sequence can be interpreted as follows.

Melting of aluminum/ Formation of Nb-Al intermetallics

The first endothermic peak at 653°C can be attributed to the melting of aluminum present in the initial mixture. It matches with the beginning of the first shrinkage step observed between 650°C and 700°C. The sample stopped at 740°C reveals that the material is still very porous (≈30% in volume) and the microstructure looks like the initial powder mixture (figure 2a). To summarize and conclude on this analysis of results, the following reaction path can be proposed in the present study: Not surprisingly, the reaction path is strongly dependent on the starting materials but also on the grain size distribution of the starting materials and on the processing parameters such as the heating rate. This could explain why, in the study of Hu et al. [START_REF] Hu | In situ reaction synthesis, electrical and thermal and mechanical properties of Nb4AlC3[END_REF] where sintering was performed with a heating rate of 15°C/min, Al (30 µm) was detected up to 1200°C, whereas, in our study, it was completely reacted at 1000°C with a 5°C/min heating rate and finer Al particles (9 µm).

653°C Al melting Al (s)  Al (l) (1)
667°C NbAl 3 formation Nb (s) + 3 Al (l)  NbAl 3(s) (2)
Apart from the initial choice of the starting materials, the formation of NbAl sample core and at the edges, respectively. This contamination could be attributed to the reaction at high temperature between Al and either the intrinsic/atmospheric oxygen or the carbon of the graphite dies, to form both phases. However, as the sample is mainly constituted of Nb 4 AlC 3 , reaction ( 7) is assumed to be predominant, compared to reaction (8).

Microstructural characterization

EBSD coupled with EDS analyses

Fine characterization of such materials, constituted by elongated (large aspect ratio) grains exhibiting a strong anisotropy, is difficult and barely documented, to the authors' knowledge, in the literature. Such measurements are however essential when considering mechanical properties or oxidation resistance . The high spatial resolution of EBSD can help analyzing the tangled microstructure observed with SEM and to better understand the relations between grains.

EBSD maps of Nb 4 AlC 3 microstructures, sizes of the analyzed domains and number of analyzed grains as a function of the dwell time at 1735°C (i.e. 1h or 2h) are presented in figure 4. It is clearly evidenced that grains are larger after a 2h material sintering, compared to those of the material subjected to only 1h sintering.

However, to further process these results, the OIM Analysis TM software was used to run grain size distribution characterization and several filters were applied on data to take into account the shape of the particles. First, grains located at the edge of the picture are excluded, then, only grains having the c-axis parallel to the surface (15° tolerance angle) and with an aspect ratio lower than 0.6 are considered.

For both materials, more than 5000 grains are still considered after data processing for the microstructural analyses. Thus, each grain was modeled by an ellipse, useful approximation for the determination of shape factors, grain sizes, areal fractions, … The particle size distribution is presented in figure 5.

For both materials, a lot of grains are very small and the median length (in grain number) is around 3.6 µm and 4 µm for NAC-1735-1 and NAC-1735-2, respectively (figure 5, table 2).

However, they represent less than 7% of the total area for NAC-1735-1 and less than 4.5% for NAC-1735-2. From table 2 and figure 5, it can be noted that the grain size distribution is large.

Comparing NAC-1735-1 to NAC-1735-2, the mean and the median lengths of the grains are similar. However, the areal fraction of the larger grains (d>d 90 ) is higher in the case of the NAC-413-2 sample. Indeed, only 10% of the grains represent around 65% of the total area in NAC-1735-1 and 74% in NAC-1735-2. It indicates that above d 90 the fraction of the longest grains is higher in the NAC413-2 sample than in the NAC413-1 sample. Compared to literature data, the grain size analyzed here is very small. As reported in Table 3, the particles described by Hu et al. are at least 3 times longer and 4 times wider although a similar process was used. Two explanations can be proposed. First, it can be noted that even using three filters to eliminate EBSD artifacts, a lot of small particles are still considered in the treatment of the data. Secondly, the grain size achieved in this work can be limited by the small size of the starting materials and the Nb-Al intermetallics nucleated during the reaction process. 128°C and 282°C, some authors calculated a CTE of 6.2x10 -6 °C-1 which is smaller but still coherent with this measurement.

Mechanical and oxidation properties

Coefficient of thermal expansion

According to these results, the coefficient of thermal expansion is dependent on temperature and the temperature derivative gives the following linear equation:

(T) = 1
.3678*10 -9 T + 6.586*10 -6 Equation 1Some authors have also shown an increase in the CTE as a function of the temperature for the Nb 4 AlC 3 material.

Mechanical properties

The Young's modulus evaluated using dynamic measurement is equal to 350 GPa. This result is in good agreement with the literature which reports values in the 306-365 GPa range .

Regarding hardness, measured values are around 4-5 GPa at 2 N for both materials sintered at 1735°C. This is lower than the values reported by Hu et al. for sintered and highly textured Nb 4 AlC 3 : 6.2 GPa at 3 N and 7.5-8 GPa at 2 N, respectively.

Four-point bending tests at room temperature were performed on four NAC-1735-1 samples and two NAC-1735-2 samples. In both cases, mechanical behavior is brittle by nature and the discrepancy between tests is restricted (Figure 7). The mean flexural strength is around 380 MPa for both materials even if grain sizes are not exactly identical. This result is slightly higher than the values obtained on hot pressed Nb 4 AlC 3 but lower than that measured on SPSed samples . However, the authors have shown that the mechanical properties strongly decrease at 1000°C on SPSed samples as compared to those measured on HPed samples, which remain approximately constant up to 1400°C. These values are coherent with the grain size of the Nb 4 AlC 3 samples tested in this study.

Finally, toughness was evaluated using three-point bending tests on notched specimens at room temperature and the results appear to be nearly similar for both materials. The analysis of a fractured sample is presented in figure 8. The crack, propagating from the notch to the bulk material, is deflected by grain boundaries (pull-out), but can propagate through cleavage of conveniently oriented grains of the laminated structure. All the mechanical properties at room temperature for NAC-1735-1 and NAC-1735-2 materials are summarized in Table 3 and compared to the available literature data, taking the grain size into consideration.

It is clear from Table 3 that texture has a strong impact on mechanical properties, especially on toughness and flexural strength. It can be noted that fracture toughness in the direction parallel to the basal planes is largely lower than that in the direction parallel to the c-axis direction. As observed in Si 3 N 4 ceramics, toughness is correlated to the presence of rod-shaped grains and more particularly to their aspect ratio (W/L) . The lower the aspect ratio, the higher is the fracture toughness. As the aspect ratio for MAX phase particles is quite low, it could explain the high toughness values measured on textured samples. Moreover, comparing the toughness values of the hot pressed samples of this study to the literature data, it should be noted that similar values are obtained for similar aspect ratios. Regarding the flexural strength of textured samples, the authors suggested that grain pull-out and bridging are the main damage mechanisms. Thus, at a small scale, the mechanism for crack propagation depends on the orientation of each grain and pull-out can be favored when the crack propagates parallel to the c-axis whereas cleavage is the main mechanism when it propagates perpendicularly to the c-axis.

Comparing flexural strength of NAC-1735-1 and NAC-1735-2 samples with the literature data, materials with 2 or 3 times larger grain sizes exhibit higher flexural strength whereas materials with approximately 7 times larger grain size have a lower flexural strength. This indicates that an optimal microstructure can be found without influencing notably the toughness.

Oxidation results

Cyclic oxidation performance of Nb 4 AlC 3 at 1100°C has proved to be disastrous, despite the large amount of aluminum available in the structure to form an oxide layer. From the first cycle, the samples entered directly into an oxidation breakaway regime, with the generation of a non-protective layered structure (figure 9).

This very brittle oxide structure is generated from all the surfaces exposed to the air. The experiment was stopped after 18 cycles and a pile-up of several oxide layers is observed.

Counting the number of layers on each side of the samples gives an average value of 15, which is close to the number of cycles the samples were subjected to. Thus, one potential explanation of this behavior is the formation at 1100°C of a fast growing and dense oxide layer, which is delaminated from the substrate during cooling steps.

Conclusions

In the present study, fully dense Nb 4 AlC 3 MAX phase material was synthesized using in situ hot pressing reaction. The synthesis sequence was analyzed by interrupted tests and the maximal purity of the phase was obtained for temperatures as high as 1735°C. The materials thus obtained were subsequently characterized by EBSD and SEM, leading to an average grain length of 7-10 µm.

Standard mechanical characterizations were performed and are in good agreement with already published results, i.e. K Ic ≈6 MPa.m -1/2 , E≈350 GPa and α≈7.10 -6 °C-1 . Oxidation performance of Nb 4 AlC 3 was evaluated at 1100°C under cyclic conditions. A breakaway regime was instantaneously established for this condition, thus demonstrating the impossibility of using this material for structural applications at high temperature in air environment.

  SEM/EDS examinations reveal the presence of NbC and Nb particles as well as Al particles with a melted aspect surrounded by NbAl 3 phase. The formation of the NbAl 3 phase in conjunction with intermetallic compounds such as Nb 2 Al and Nb 3 Al is confirmed by XRD (figure3). The Nb 2 Al phase was also detected by several authors but at a higher temperature, around 1000°C. Nevertheless, the very low amounts of Nb 2 Al and Nb 3 Al intermetallic phases compared to that of the NbAl 3 phase indicate that the exothermic peak at 670°C can be attributed to the formation of NbAl 3 .Formation of Nb 2 AlCBetween 740°C and 1000°C, two exothermic peaks are detected through DTA. A first peak at 824°C and a second one, poorly defined, around 920°C. The sample stopped at 1000°C is still very porous (≈30% in volume) but the microstructure has evolved (figure2b). Indeed, free Al is no more detected through SEM examinations and Nb particles are surrounded by NbAl 3 phases. Moreover, very thin particles are detected with a Nb/Al ratio between 2 and 3 according to EDS analyses. XRD analyses confirm that Al is almost no more present in the material and that the volume fraction of NbAl 3 has increased. Thus, for the following interpretation, the authors consider that Al disappears at this temperature. The proportion of NbC remains unchanged, which seems to indicate that, at this temperature, Al reacts preferentially with Nb particles to form NbAl 3 . Two other phases, identified as Nb 2 AlC and Nb 2 Al, were also detected. Thus, it can be assumed that the exothermic peak at 824°C corresponds to the formation of Nb 2 Al which immediately reacts with Al or NbC to form NbAl 3 and Nb 2 AlC phases (reactions 4 and 5). Formation of Nb 4 AlC 3 Around the second peak of densification rate (≈1385°C), the interpretation of DTA results is difficult. The third interrupted test at 1420°C exhibits a decreasing open porosity (21% in volume) and the reaction (figure 2c) of NbC particles to form Nb 2 AlC, Nb 4 AlC 3 and intermediate phases in the Nb-Al-C system such as Nb 3 Al 2 C, in a small proportion. NbC and NbAl 3 are still present whereas free Nb has disappeared at this temperature. Moreover, small amounts of Nb 3 Al and Nb 2 Al are still detected in the diffractograms.Final transformation Above 1500°C, linear shrinkage of samples becomes very large and open porosity decreases rapidly. The sample stopped at 1620°C contains a few pores (2% in volume) but its microstructure is still not single-phased. Reaction is not complete since starting materials (NbC) and intermediate phases (NbAl 3 , Nb 3 Al 2 C or Nb 2 AlC) are still present (figure 2d). Nevertheless, volume fractions of Nb 2 AlC and Nb 4 AlC 3 MAX phases increase while NbC, NbAl 3 and Nb 2 AlC proportions decrease according to XRD results (figure 3). Finally, complete densification was reached at temperatures up to 1735°C for 1 and 2 hours. In both cases, open porosity is lower than 1 % in volume. SEM characterization (figure 2e) reveals a homogeneous microstructure without any starting material or intermediate phase. Grains are elongated in particular directions and decorated with some isolated Al 2 O 3 particles randomly distributed. Their proportion seems to increase with the dwell time but is, however, too low to be detected through XRD analyses. It confirms that the major constituent is the Nb 4 AlC 3 MAX phase with remaining NbC particles. From Rietveld refinements (figure 3), the calculated cell parameters are a=3.1314(6) Å and c = 24.1494(3) Å for NAC-1735-1 (considering the hexagonal P6 3 /mmc crystal structure) which are close to the results published in the literature .

  (s) + 2 NbC (s)  Nb 4 AlC 3(s) NbAl 3(s) + 3 NbC (s)  Nb 4 AlC 3(s) + 2 Al (l) s) + 3 NbC (s)  Nb 4 AlC 3(s) + 2 Al (l) Nb 2 AlC (s) + 2 NbC (s)  Nb 4 AlC 3(s) Al:C and Nb:Al:NbC as starting materials, Hu et al. and Zhang et al., respectively, have also studied the formation of Nb 4 AlC 3 and Nb 2 AlC. Zhang et al. have shown that Al reacts with Nb to form NbAl 3 and was completely consumed at 700°C. Then, they propose that Nb 2 Al is formed, due to a partial reaction between Nb 3 Al and Nb. Finally, Nb 2 AlC appears around 1100°C and its volume fraction increases up to 1650°C, thanks to the reaction between NbAl 3 , Nb 2 Al and NbC. Hu et al. have used different starting materials, but they have also observed the formation of Nb 2 Al around 1000°C. Moreover, they suggest that Nb 2 C forms at this same temperature and report that Al was still detected at 1200°C. They propose that Nb and Al first react to form NbAl 3 , which can then be transformed into Nb 2 Al (reaction with Nb). This latter phase can form Nb 2 AlC thanks to its reaction with free C. Then, Nb 2 Al and NbC or Nb 2 C and C react to form the desired Nb 4 AlC 3 phase.

3 and Nb 2

 2 Al phases in the reaction sequence seems obvious. Nevertheless, the formation temperature and the proportion of the Nb 2 Al phase can be discussed. In our study, this phase is detected in an extremely low quantity and it was not observed in SEM pictures. Thus, it is assumed that it only consists in an intermediate phase which reacts rapidly to form Nb 2 AlC or Nb-Al-C phases, which are detected through SEM examinations.The transient condition in temperature imposes also an evolution in the phase composition, making several reactions possible for the Nb 2 AlC formation (e.g. equations (5) and (6)). Once formed, Hu et al. suggested that at very high temperatures the Nb 2 AlC phase can decompose and produce NbAl 3 . In this study, only Nb 4 AlC 3 was detected at 1735°C. Thus, NbC observed in NAC-1735-1 is assumed to be a starting material. Furthermore, SEM examinations reveal the formation of Al 2 O 3 and Al 4 C 3 inclusions in the

Figure 6

 6 Figure 6 presents the thermal expansion obtained under heating and cooling stages on the

  The oxygen pathway created by delamination gives rise to further oxidation and the growth of a new layer when samples are re-heated. The thickness of the oxide layer is around 70 µm±10 µm and the safe material is around 1.131mm±0.02mm. In other words, after 18 cycles at 1100°C only, half of the thickness of the material was converted into oxides.The nature of the oxide scale was determined through XRD and EDS measurements (figures 9, 10 and 11). XRD analysis on crushed oxide layers reveals that it is composed mainly of Nb 2 O 5 , with some NbAlO 4 . SEM examinations, performed on oxidized material after elimination of the layered structure, confirm that an almost monolithic layer is formed, which could be attributed to Nb 2 O 5 . The presence of alumina nodules in the oxide layer confirms an inward oxidation of the substrate from the surface, the stable nodules being directly incorporated from the bulk into the oxide scale, due to the oxidation front propagation. Some areas with different aluminum and niobium contents are detected from figures 9 and 11.It appears that the aluminum activity in the Nb 4 AlC 3 MAX phase is not high enough to promote alumina formation at 1100°C. In the literature, it has been demonstrated that alumina scales can be formed on NbAl 3 intermetallics , provided that the aluminum content is greater than 50 at% in the bulk. It has to be compared to the[START_REF] Barsoum | MAX Phases : Properties of Machinable Ternary Carbides and Nitrides[END_REF] at% aluminum content of the Nb 4 AlC 3 MAX phase. Results regarding oxide nature and proportions obtained in this study are consistent with analyses of oxide scales previously published by Zheng et al. , in which Nb 2 O 5 and NbAlO 4 oxides were detected in the [500-900°C] temperature range. Compared to NbAl 3 oxidation, which is accompanied by the formation of a Nb 2 Al interlayer, neither transformation nor interdiffusion has been observed at the Nb 4 AlC 3 /oxide interface. This can be explained by the fast diffusion of oxygen through the non-protective oxide scale , leading to the fast oxidation of Nb and Al. The strong spallation of oxide scales could be explained by the large volume of Nb 2 O 5 oxide, 1363 Å 3 compared to approximately 200 Å 3 for Nb 4 AlC 3 . Furthermore, the CTE mismatch between the oxide layer and the substrate, i.e. 5-6 .10 -6 °C-1 for Nb 2 O 5 versus 7-8 .10 -6 °C-1 for Nb 4 AlC 3 , does not act in favor of an extended lifetime of oxide scales on the Nb 4 AlC 3 MAX phase.

Figure 1 :Figure 2 :Figure 3 :Figure 4 :Figure 5 : 2 Figure 6 :

 1234526 Figure 1 : Heat flow and shrinkage during reactive hot pressing of Nb:Al:NbC mixture. Temperatures indicate exo or endothermic peaks. ① and ② indicate the first and second densification peaks at 670°C and 1385°C, respectively. The temperatures achieved for each stop are indicated below the graph.

Figure 7 :

 7 Figure 7: Strain-stress curves of Nb 4 AlC 3 materials manufactured at 1735°C-1h (a) and 1735°C-2h (b).

Figure 8 :Figure 9 :

 89 Figure 8: SEM examination of fracture propagation of in the NAC-1735-2 sample after an SENB test.

Figure 10 :

 10 Figure 10: XRD diffractogram of crushed oxide layer.

Figure 11 :

 11 Figure 11: EDS maps of elements at the oxide/substrate interface.

Table 2 :

 2 Areal fraction as a function of the grain size (d x in grain number)

		NAC-1735-1	NAC-1735-2
		Length	Areal	Length	Areal
		(µm)	fraction	(µm)	fraction
			(%)		(%)
	d 10	< 2	<1	< 2	< 1
	d 50	3.6	7	4	4.5
	d 90	12	35.7	14	26
	Shape factor	0.4		0.4

Table 3 :

 3 Properties at room temperature of NAC-1735-1 and NAC-1735-2 materials and comparisonwith literature data.

	Reference	NAC-1735-1 NAC-1735-2	Nb 4 AlC 3 (HP)	Nb 4 AlC 3 (HP)	Nb 4 AlC 3 (SPS)	Nb 4 AlC 3 (textured)
	Density (g/cm 3 )	6.90 ± 0.03	6.93 ± 0.01	6.97	6.89± 0.06	6.71	-
	/ th (%)	98.0 ± 0.4	98.5 ± 0.2	99	97.6	95	-
	Mean grain size (µm)	6(L) 2(W)	7(L) 2(W)	50(L) 17(W) 14(L) 4(W)	21(L) 9(W)	-
	Aspect ratio (W/L)	0.33	0.3	0.34	0.3	0.43
	E (GPa)	350 ± 2	350 ± 2	306	340 ± 4	-	353 c-axis 365 c-axis
	 f 4pts (MPa)	372 ± 8	379 ± 6	346 ± 38	573 ± 39	455± 2 (3pts)	881 c-axis 789 c-axis
	K IC (MPa.m 1/2 )	6.8 ± 0.5	6.3 ± 0.3	7.1 ± 0.3	6.6 ± 0.1	-	14.1 c-axis 9.3 c-axis
	H v0.2 (GPa)	4.0± 0.4	5.0± 0.9	2.6± 0.2 (3 N)		3.7± 0.5 (10 N)	7.5-8 (2 N)
	(x10 -6 . °C-1 )	7.7 (100-1450°C)	-	7.2 (200-1100°C)		6.7 (282°C)	-

Acknowledgments

The authors would like to thank D. Boivin and N. Horezan (ONERA) for EBSD-EDS analyses, Dr. C. Sanchez (ONERA) for XRD analyses and A. Mavel (ONERA) for mechanical tests.