Maud Rio

Tatiana Reyes

Lionel Roucoules

A FRAMEWORK FOR ECODESIGN: AN INTERFACE BETWEEN LCA AND DESIGN PROCESS

Keywords: product design, eco-design, LCA, CAX modelling

Integrating environmental aspects in the design process is getting more and more covered by the actual scientific research. Based on various fields such as "eco-design", "Design for sustainability", "ecoinnovation", several specific software and methods have been developed to tackle this issue. Our work focuses on product designer software and eco-designer software that already exist. This research analyses product design and eco-design-considering their respective complexity-in order to determine the main difficulties to systematically integrate environmental aspects in the design process. Our observations have led us to work on three elements: information, software, and collaboration between designers and LCA expert. The principal objective is to facilitate the interface between product design and eco-design by linking the requirements and performances of both domains. It seems therefore necessary to help the collaboration between various experts and thus to develop a digital interface enabling designers to exchange data and to use their appropriate related software.

The aim of this paper is to argue the need of a digital tool interfacing the actual tools of the design process and of eco-design (among the related software such as CAD, CAx, PLM, and LCA). This "digital interface" is presented at the end of the paper and will be created next year.

INTRODUCTION AND INDUSTRIAL CONTEXT OF ECO-DESIGN

Introduction of the research context

The concept of eco-design was born in the seventies in the industrial society [START_REF] Papanek | Design for the Real World: Human Ecology and Social Change[END_REF]. Today, as asserted by Johan Rockström et al. "largely because of a rapidly growing reliance on fossil fuels and industrialized forms of agriculture, human activities have reached a level that could damage the systems that keep Earth in the desirable Holocene state"-which was the stable state for the past 10,000 years-"the result could be irreversible and, in some cases, abrupt environmental change, leading to a state less conducive to human development" [START_REF] Rockström | A safe operating space for humanity[END_REF]. This assertion entails quite evidently a growing necessity to reduce the impacts generated by our society. Several authors have stated that in order to be sustainable, the product system has to be re-structured at every scale: governments and international institutions (political issues, international laws), the industrial system (industrial metabolism optimization), the services (transition towards a product-service system), the products (eco-design strategies), and the functions (focus on functionality rather than on marketability) [START_REF] Allenby | Industrial Ecology[END_REF][START_REF] Erkman | Vers une écologie industrielle: comment mettre en pratique le développement durable dans une société hyper-industrielle[END_REF][START_REF] Brullot | Mise en oeuvre de projets territoriaux d'écologie industrielle en France : vers un outil méthodologique d'aide à la décision[END_REF][START_REF] Karlsson | Ecodesign: what's happening? An overview of the subject area of ecodesign and of the papers in this special issue[END_REF]. Since the seventies, eco-design has thus been implemented progressively.

1.2

Overview of the paper breakthrough Considering this context, this article focuses on eco-design as a approach that might help reducing the damages of the industrial activities. Three consecutive levels of eco-design integration have been identified so far. Firstly, the approach of eco-design can be partially implemented (Design for: Recycling (DFR), Disassembly (DFD), Remanufacturing, Product Retirement, Low Energy consumption). Secondly, eco-design integration processes can be seen as classical (Design for Environment (DFE), Eco-Efficiency, Environmentally conscious Design & Manufacture), or, thirdly, as innovative (Design for life cycle, Design for Eco-effectiveness, Eco-Efficient Services, Design for Sustainability) [START_REF] Brezet | A promising approach to sustainable production and consumption[END_REF][START_REF] Millet | Intégration de l'environnement en conception, l'entreprise et le développement durable[END_REF]. The objective of our research work is to contribute to help reaching the third level, which goes together with modifying the whole industrial eco-system.

The following part of the paper presents the objectives of the research work. This part includes the definitions of the life cycle, the design process and how they relate to each other. Numerous software have been developed in order to assist engineers to reduce the environmental impacts of their products throughout their entire life cycle [START_REF] Karlsson | Ecodesign: what's happening? An overview of the subject area of ecodesign and of the papers in this special issue[END_REF]. However, the integration of environment into the design process is complex and often fuzzy for the product designer [START_REF] Reyes | The Trojan horse method as a vector of ecodesign integration: a case study at a French SME[END_REF][START_REF] Ny | Strategic Life-Cycle Modeling and Simulation for Sustainable Product Innovation[END_REF][START_REF] Brezet | A promising approach to sustainable production and consumption[END_REF]. Consequently, the third part of the paper based on literature results aims to create a non exhaustive map of available design and eco-design software and then to crosscheck (i.e. interface) the information that should be exchanged. This map supports the relevant observation of the eco-designer's communities that argue how difficult it can be to establish an effective and fruitful collaboration with the design core team. It shows that specific knowledge are needed by the product designers to exchange information about the same product or project in order to use the appropriate software of ecodesign. A wide range of possibilities are offered to the designer to manage, learn and integrate environment in the design process. Considering this, the fourth part suggests a framework to create a digital interface between eco-design tools and product design tools. This "interface" enables data exchange and refers the designer to the specific tools that will support his activity.

OBECTIVES OF THE RESEARCH WORK

2.1

Objectives of the research This research is a technical approach devoted to technically assist designers to integrate environment in the design process so as to contribute to innovative eco-design at a higher scale. To have a better understanding of the subject, the first step in the following section is to define "life cycle assessment" and "design process", and how they relate to each other.

2.2

What is life cycle assessment? Eco-design is about integrating environmental concerns into the design criteria of products or services to reduce significantly their negative impacts. Eco-design is characterized by the life cycle thinking. Indeed, in our research, we decide to help designers to perform Life Cycle Assessment (LCA) because it is one of the most suitable scientific way to analyse the entire life cycle of the product based on a multi-criterion calculation. Concretely the related ISO standard defines LCA as the "compilation and evaluation of the inputs, outputs and the potential environmental impacts of a product system throughout its life cycle" [START_REF]Environmental management-Life cycle assessment-Principles and framework[END_REF]. This approach has its own limits but LCA is the only systematic and global approach, the goal of which is to avoid pollution transfers from one life stage to the other [START_REF] Grisel | L'analyse du cycle de vie d'un produit ou d'un service: application et mise en pratique[END_REF]. So, this research aims at supporting LCA in the design process, which means that the definition of life cycle from the "LCA standard" must be shared by all designers. Zwolinski and Brissaud explain the two main ways to understand the product life cycle [START_REF] Brissaud | Innovation in Life Cycle Enginnering and Sustainable Development[END_REF]. The first one makes sense for marketers and is used for strategic approaches. It is decomposed in five phases, which are introduction, expansion, maturity, saturation and decline. The second one is more closely associated to the functionality of the product and is thus (or at least should be) shared by all designers working in the design process. But there are three levels of analysis associated with this second definition of life cycle: the firm level, the team level and the expert level. Considering the team level, Niemann et al. [START_REF] Niemann | Design of Sustainable Product Life Cycles[END_REF] define the life cycle as the succession of the following steps: design (conceptual, detailed)-production (procurement, manufacturing, assembly)-logistics (distribution)use-maintenance (service)-collection -remanufacturing (disassembly, refurbishment, reassembly)reuserecyclingdisposal. But the eco-design expert has a slightly different definition: his holistic approach takes into account the environmental aspects and the potential environmental impacts along the successive steps of the product's lifecycle, from raw material extraction, production, use, end of life treatment, to the final elimination [START_REF]Environmental management-Life cycle assessment-Requirements and guidelines[END_REF] (the reference standard ISO/TR 14062 gives the definition of the life cycle as "the successive and linked phases of a product system from the acquisition of the raw material to their final elimination" [START_REF] Andreasen | The contribution of design research to industryreflexions on 20 years of ICED Conferences[END_REF]).

2.3

What is the design process?

The design or product development process is a collaborative process that appeals to multiple competencies. It impacts all the activities of a company which aim at bringing a new product to the market. It normally involves design, marketing and manufacturing departments in the company [START_REF] Andreasen | The contribution of design research to industryreflexions on 20 years of ICED Conferences[END_REF]. Pahl and Beitz describe the design process as a succession of four main phases: clarification requirementsconceptual designembodiment designdetail design, to go from a functional space to a physical one [START_REF] Pahl | Engineering design -A systematic approach[END_REF][START_REF] Wallace | Observations on some german contributions to engineering design -In memory of Professor W[END_REF].

2.4

Life cycle assessment in the design process, what are the issues? As seen previously, the definition of life cycle varies from a level to another and design is seen as a collective process in which multiple competencies are involved. Designers have to integrate transverse constraints such as environment (it is the same for cost or quality), a task that can be quite complex in most cases. Succeed in this task is the objective of numerous research programs and embrace a wide range of field such as DFE, DFR or DFD [START_REF] Veerakamolmal | Design for disassembly, reuse and recycling[END_REF][START_REF] Kuo | Design for manufacture and design for 'X': concepts, applications, and perspectives[END_REF]. Management support tools are developed to organise and coordinate the process with teams, tools and data. Product Lifecycle Management (PLM) provides solutions such as giving the access to model various product BOM (as-is, as-manufactured, etc.) linked to document lifecycle (creation, modification, validation, etc.) and according to the project organisation modelling (tasks, access rights,...). Based on the assumption that all eco-design approaches are complementary, the general idea of the paper resides in the technical appropriation of those approaches by product designers. Indeed, the experts that have no understanding of the involvements of the eco-design concept have nowadays to take that information into account during the design process and the digital design chain. Since there are enough tools-individual and collective-(part 3), the data circulation between eco-design tools and design tools can be optimized. An interface could help this appropriation at a very technical level.

SOFTWARE-SUPPORTED PRODUCT DESIGN AND ECO-DESIGN

3.1

Global approach: software-supported product design and eco-design Ilgin and Gupta have classified 540 published references from the last decade into four major categories: environmentally conscious product design, reverse and closed-loop supply chains, remanufacturing, and disassembly [START_REF] Ilgin | Environmentally conscious manufacturing and product recovery (ECMPRO): A review of the state of the art[END_REF]. The task consisting in adding the environmental requirement to the simultaneous consideration of economic, functional and manufacturable criteria is supported by several techniques and methodologies, such as DFX, LCA and material selection. An overview of the main tools used in DFE, DFR and DFD shows the large possibilities offered by the actual research. In addition, new software in LCA and material selection complete the previous list. As far as PLM systems are concerned, current solutions provide various concepts and technologies to manage the entire lifecycle information while managing heterogeneous files coming from external software applications (e.g. CAD and CAx solutions...). The aim of the figure 1 is to show an organisation of possible software tools along the design process. This list of tools is not exhaustive and does not pretend to be the best combination of software tools, but is one possibility. This figure 1 introduces a global approach that must link:

 Local design activities based on specific requirements and objectives (requirements analysis, structure breakdown definition, manufacturing processes selection...) and supported by specific software (also located on figure 1);  Global assessment of the design solution regarding the entire product life cycle. Information has then to be exchanged horizontally (concurrent and integrated design concept) and also vertically (LCA concepts). Eco TRIZ [START_REF] Houe | Knowledge modeling for eco-design[END_REF][START_REF] Mathieux | ReSICLED: a new recovery-conscious design method for complex products based on a multicriteria assessment of the recoverability[END_REF][START_REF] Ardente | Eco-sustainable energy and environmental strategies in design for recycling: the software ''ENDLESS[END_REF] CES (material choice) [START_REF] Veerakamolmal | Analysis of design efficiency for the disassembly of modular electronic products[END_REF] DFD CAD-based Approach [START_REF]The Ecofaire tool[END_REF][START_REF] Baumann | Mapping the green product development field: engineering, policy and business perspectives[END_REF] ReSICLED [START_REF] Villalba | Using the recyclability index of materials as a tool for design for disassembly[END_REF] ENDLESS [START_REF] Chu | Economical green product design based on simplified computer-aided product structure variation[END_REF] [START_REF]CES Selector[END_REF][START_REF] Hallstedt | A foundation for sustainable product development[END_REF] Information/Inspiration Eco-innovation design [START_REF] Grote | An approach to the EuP Directive and the application of the economic eco-design for complex products[END_REF] Ecofaire [START_REF] Dewulf | A pro-active approach to ecodesign: methods and tools, ecodesign in central America[END_REF] Product Design process

PLM System

Global environmental Analysis (Strategies) Eco-Design Expert LCA softwares (Gabi, Simapro, EIME, Umberto, etc.) Figure 1 -An example of product design and eco-design software tools

2 Are those methodologies really used?

As stated above, it is clear that methodologies and software are existing from both sides: design and eco-design. The eco-design tools can be classified depending on their characteristics and their functionalities: impact assessment (environment and cost), performance improvement, creativity, hybrid (integration of the environmental criteria in the classical design tools), strategy, etc. However, from a general point of view, product designers have a partial understanding of the integration of environment in the design process. This may be due firstly because some eco-design tools present lacks of practical value. They require a lot of information or time to use and bring out uncertainties. Secondly, the environmental field introduces to new concepts linked with biology or chemistry for instance (eutrophication, acidification) and requires a global vision concerning the whole product life cycle (material flows, energy flows and pollutions from raw material to the end of life). Consequently, most of the eco-design tools are not used in industry [START_REF] Baumann | Mapping the green product development field: engineering, policy and business perspectives[END_REF][START_REF] Janin | Démarche d'éco-conception en entreprise, un enjeu: construire la cohérence entre outils et processus[END_REF] and their integration and appropriation by designers are usually mostly inefficient [START_REF] Reyes | The Trojan horse method as a vector of ecodesign integration: a case study at a French SME[END_REF][START_REF] Dewulf | A pro-active approach to ecodesign: methods and tools, ecodesign in central America[END_REF][START_REF] Jeswiet | Ecodesign and future environmental impacts[END_REF][START_REF] Cote | Analyse comparative de deux méthodes d'analyse de cycle de vie simplifiées utilisables pour la conception des produits, Mémoire de Maîtrise[END_REF][START_REF] Boks | The soft side of ecodesign[END_REF][START_REF] Pochat | Integrating ecodesign by conducting changes in SMEs[END_REF]. Becoming familiar to a guide or to new software for instance requires time and motivation, in other words, financial resources and acceptation by the rest of the team. Their use does not necessary give quick and visible improvements, but rather brings changes. For the designer there is a lack of criteria to choose the tools and methodologies within the design process and the coordination between them [START_REF] Pochat | Integrating ecodesign by conducting changes in SMEs[END_REF][START_REF] Lindahl | Engineering Designers' Requirements on Design for Environment Methods and Tools[END_REF][START_REF] Lofthouse | Ecodesign tools for designers: defining the requirements[END_REF]. For instance, it is rare to find suitable indicators to evaluate the tools in a given context: organization of the company, expertises, type of products [START_REF] Baumann | Mapping the green product development field: engineering, policy and business perspectives[END_REF][START_REF] Karlsson | Ecodesign: what's happening? An overview of the subject area of ecodesign and of the papers in this special issue[END_REF][START_REF] Lindahl | Engineering Designers' Requirements on Design for Environment Methods and Tools[END_REF]. To use the suitable tool, product designers have to be educated to environment. As stated by Ilgin and Gupta, "firms must educate their employees in environmental aspects of manufacturing to increase their competitive edge. Moreover, ECM (Environmentally Conscious Manufacturing) principles should be incorporated into engineering curriculums at universities" [START_REF] Ilgin | Environmentally conscious manufacturing and product recovery (ECMPRO): A review of the state of the art[END_REF]. This statement raises the question of knowledge management, which is obviously linked to the project of transferring data from an expert to another to perform a LCA.

3.3

From global to specific environmental knowledge The product designer (e.g. working in the preliminary design phase) has his own bundle of knowledge and know-how (material properties etc.). In addition, he also has a basic understanding of other specific domains, such as environment (life cycle thinking, LCA, etc.). In order to use specific software, which will help him to integrate the impacts of the environment in his activity; he will however need additional knowledge. Indeed, a general lack of environmental skills is noted at each stage of the industrial design process [START_REF] Millet | Intégration de l'environnement en conception, l'entreprise et le développement durable[END_REF], especially concerning the eco-design vocabulary [START_REF] Pochat | Integrating ecodesign by conducting changes in SMEs[END_REF][START_REF] Hallstedt | A foundation for sustainable product development[END_REF][START_REF] Jacqueson | Integration de l'environnement en entreprise: Proposition d'un outil de pilotage du processus de création de connaissances environnementales[END_REF]. Consequently, it is difficult to use the appropriate software and to share a global understanding about the way the environment should be integrated in the design process. The lack of coherence between the environmental stakes as understood by the firm departments and by its providers [START_REF] Sundin | Product and Process design for successful remanufacturing[END_REF], raises the question of the environmental management strategy, which is most of the time linear and positioned on a short term vision [START_REF] Baumann | Mapping the green product development field: engineering, policy and business perspectives[END_REF][START_REF] Reyes | The Trojan horse method as a vector of ecodesign integration: a case study at a French SME[END_REF]. In reaction to this, researchers are working on methodologies to choose the suitable software. The Design Society for instance covers all issues of the environmental integration in the design process (http://www/ecodesignsociety.org/). Some other design and eco-design communities, such as MCE (Méthodologies de Conception et d'Eco-conception) aim at developing, test and help the emergence of eco-design tools and methodologies in the design process. To illustrate some "macro tools", Ny has developed a Framework for Strategic Sustainable Development (FSSD) the goal of which is to unify methods and tools and to identify how they can be used strategically (proposition of a "toolbox" for Sustainable Product Innovation (SPI)) [START_REF] Ny | Strategic Life-Cycle Modeling and Simulation for Sustainable Product Innovation[END_REF]. Some guides have also been created to manage the design team along the design process, bringing environmental knowledge and referring to numerous tools [START_REF] Mcaloone | Environmental improvement through product development, a guide[END_REF].

PROPOSITION OF A RESEARCH PROGRAM: AN INTERFACE BETWEEN DESIGN AND SUSTAINABLE DESIGN

4.1

Proposition of a research program Resulting from the literature results the figure 2 shows the global frame of a design process that would integrate the suitable already-existing software tools. The data interface that will be created will be compatible with the majority of tools that are developed by the actual scientific community. In this framework various experts are working at each stage of the design process. As shown by the literature, these experts are using specific software systems that have access to local environmental tool, depending on their particular domain of expertise. Resulting from this, they would exchange the related local environmental information (material choice, origin of production, transformation process, etc.) with the "global eco-design expert" via an appropriate digital interface and specific environmental indicators. This support (i.e. interface) is a "channel" for data transfer. But, this interface is also a "linking support" to provide a common understanding, (i.e. a minimum of environmental knowledge from the expert and a minimum of expert knowledge from the eco-design expert). It is a "junction" in which the expert has access to the data he is related to and to his suitable support (tool, guide, database, etc.) that will help him change his data. The eco-design expert receives all pieces of information from each stage of the design process (at it is shown on figure 2), which confer a global understanding avoiding pollution transfer to his environmental assessment. In other words, this global framework guarantees the transversality of the environmental assessment during the design process, without obliging all designers to have a global understanding of the project. To support the local-global environmental data exchange, the authors will obviously base their work on existing models and information technology. As shown on figure 1, Expert software (TDC, CAx solutions), PLM solutions and LCA software will be then customized to manage and dynamically provide the right local information to the global eco-design expert at the right time in the design process. Local and global information models will be enhanced to tackle this issue (e.g. ID card in PLM systems). Data exchange would be based on XML or STEP standards. This idealistic framework is rather a support to help the authors focus on the specific links between practices along the design process. To illustrate the researches, the authors are thus considering more in detail the embodiment phase (surrounded by a circle and presented in figure 3) in order to analyse its specific arrangement of information flows. The data flows are presented by dark harrows on figure 3. One of the solutions given by the PLM support is to edit an ID Card of each component of the product X (for instance). This card can be parameterized to specify the list of requirements needed to perform an LCA (type of material for instance). Ones the card is filled up, the digital interface will transfer the data to the LCA software. The eco-designer will run the LCA software and obtain the detailed results. He will then communicate his analysis to the right designers (indication about material impacts for the material engineer for instance). The interface will transfer the remarks from the eco-design expert to the referring designer and suggest to the latter tools and methodologies to make another choice to his local expertise. The designer changes the parameter (for instance the material choice) and, again, the data is transferred to the LCA software to analyse the new combination that has been made (figure 3).

To create this interface a methodology in collaboration with industrials has been created. This methodology is based on observation, prospection and analysis, aiming at answering to the following questions:

 Which data have to be exchanged from local to global analysis as soon as possible during the design process?  What are the data formalism for those exchanges between designers and eco-designers during the design process?  Which tools are used and what are their possible compatibilities with eco-design tools? 5. CONCLUSION Despite the number of software at each stage of the product development process, the industrial society is not integrating environment efficiently in its design process yet. Through literature review and research observations, it appears that numerical interfaces between usual design and eco-design software are needed to help the various experts involved in the design process to interact with each other. The paper aimed at presenting a framework that might help resolving some difficulties that are preventing the product designer from systematically integrating environmental considerations in his activities. This framework links expertises more efficiently and enables pro-activity. In our future research work, a comprehensive model of the interactions and information flows between the designer and the eco-designer will be created. Once the several constraints are identified, a methodology will be developed in order to create the most suitable interface on a given context. The model will be tested through an application case.

Figure 2 -

 2 Figure 2 -Proposition of a general interface framework along the design process

Figure 3 :

 3 Figure 3: proposition of a digital interface between product designers and eco-designers during the embodiment design stage


Keywords: Semi-solid, steel, thixoforging, X-ray microtomography, SEM-EDS

The distribution of liquid at the semi solid state is one of the most important parameters for steel thixoforging. It has a great influence on the viscosity of the material, on the flows and finally on the final shape and mechanical properties of the thixoforged parts. Both ex situ and in situ 3D X-ray microtomography characterizations have been carried out to determine the quantity and localization of liquid at high temperature of M2 steel slugs. Microtomography was first performed ex situ at room temperature on samples heated and quenched from semi-solid state. The specimens were also scanned in situ directly at high temperature. The obtained results have been compared to 2D observations using EDS technique in SEM on heated and quenched specimens. They showed a good correlation making both approaches very efficient for the study of the liquid zones at the semi-solid state.

Introduction

Semi-solid forming is an innovative technology offering the opportunity to manufacture complex components with high mechanical properties in one step. Thixoforging which involves heating the feedstock into the semi-solid state with a low liquid fraction is of interest to scientists and industries. Therefore, fundamental knowledge on the microstructure evolution in the heating step is of great importance, including the spheroidization of solid grains, formation, location and volume fraction of liquid phase. These parameters are critical for effective process control, since they affect the microstructure development [START_REF] Amin-Ahmadi | Semisolid structure for M2 high speed steel prepared by cooling slope[END_REF], rheological behavior [START_REF] Bigot | Characterisation of semi-solid material mechanical behavior by indentation test[END_REF] and therefore the material flow [START_REF] Becker | Impact of experimental conditions on material response during forming of steel in semi-solid state[END_REF] during processing.

Classically, the characterization and quantification of these parameters are normally performed on a 2D polished and etched surface of solid material quenched from the semi-solid state. However, a random 2D section may lead to some error in shape factor, grain size distribution and liquid quantification, in particular-because the solid grains are not spherical but have a complex shape due to the agglomeration process in the semi-solid state. These parameters can only be precisely determined by 3D investigation after quenching, when the microstructure is preserved from the semi-solid state. The serial sectioning technique can be used for observing the 3D microstructures [START_REF] Rowenhorst | 3D crystallographic and morphological analysis of coarse martensite: Combining EBSD and serial sectioning[END_REF], but also non-destructive X-ray microtomography [START_REF] Salvo | X-ray micro-tomography an attractive characterization technique in material science[END_REF][START_REF] Gu | Quantification and localization of the liquid zone of partially remelted M2 tool steel using X-ray microtomography and scanning electron microscopy[END_REF]. Moreover, the microstructure evolution during heating can also be investigated insitu at high temperature with this technique [START_REF] Limodin | In situ investigation by X-ray tomography of the overall and local microstructural changes occurring during partial remelting of AL-15.8wt.% Cu alloy[END_REF]. However, such experiments are mainly performed on aluminum alloys.

In the present work, two different states of M2 steel are investigated: as-received and semi-solid state. The instrumental device and the experimental procedure are described. The microstructure evolution during heating to the semi-solid state is analyzed and discussed. By comparing the results obtained by various techniques, the former liquid zones are finally well identified and quantified.

Experimental Methods

The M2 high-speed tool steel (solidification range: 1200°C ~1420°C) was chosen because of its large freezing range and low solidus temperature which are suitable for thixoforming process. Especially, its high content of alloying elements permits a good absorption contrast between solid and liquid phases for X-ray microtomography. The chemical composition of M2 steel is given in Table 1. In order to study the microstructure evolution, the M2 slug was partially induction heated to semi-solid state by a direct remelting route. Due to the skin effect, the temperature field was not uniform in the slug, meaning some zones were in the semi-solid state at high temperature, while others remained totally solid, similar to the as-received state as shown in Fig. 1. During heating, the temperature was measured by a pyrometer. After heating for 120s, the temperature at the point C reached to 1250°C and the slug was directly quenched in the water in order to preserve the former structure at the semi-solid state. The cylindrical samples with a diameter of 1.2mm were electrodischarge-machined from the quenched slug in different directions for the X-ray microtomography experiments. Here, we mainly focus on one sample as shown in Fig. 1. X-ray microtomography experiments were realized at room temperature at the ESRF, ID15A beamline, in Grenoble. Before testing, the sample was mounted on a translation-rotation stage which is used for aligning the sample with the X-ray beam. In the test, X-rays go through the sample which is continuously rotating 360° with a high constant energy (60keV). A fast DALSTAR Pantera 1M60 CCD is used to record the transmitted beam. The exposure time of this camera is 25ms. For one scan, 3600 projections with the effective pixel size of 1.2µm were obtained. After reconstruction, the 3D representation of the microstructure was processed and analyzed with ImageJ.

In order to identify the microstructure represented in 3D, 2D microstructure observations were also performed. The distribution of various chemical compositions was studied by Energy Dispersive Spectrometry (EDS). A Jeol 7001FLV Scanning Electron Microscope (SEM) was used to characterize the microstructure, such as the phases, grain size and shape. Here, the observed samples in 2D were etched by 2% Nital (2 ml HNO 3 + 98ml ethanol).

Results and discussion

High energy X-ray microtomography experiments were first performed on the as-received M2 steel grade. Fig. 2 presents a 2D tomographic slice on transversal section and a 3D reconstructed stack of a sample at the as-received state. The black zones represent porosities. In Fig. 2(a), typical segregation bands are observed for this commercially rolled M2 grade. In order to characterize the white and grey zones, SEM and EDS experiments were performed. Comparing the results obtained by these two techniques, the white zones in tomographic slices are confirmed to be carbides particles. However, due to the resolution (3µm) of X-ray microtomography, some small carbide particles (<1µm) are not distinguished or some isolated carbides particles with a distance less than 3µm are probably shown as one big large particle.

3D tomographic images and SEM micrographs of heated and quenched M2 steel at different heights are shown in Fig. 4. The tomographic images show the spatial distribution of the carbides in the material: the white corresponds to carbide particles, black to pores and dark gray to the matrix. These images can be directly compared since the same amount of material is concerned with the same magnification in each case. As shown in Fig. 1, the temperature field is not uniform in sample due to the penetration depth and conduction: the temperature increases with increasing height. Thereby, the microstructures at different height are different in grain size and morphology. At the bottom (Fig. 4(c)), due to the heat exchange between the material and ceramic, the material remains at the solid state showing a similar microstructure to the one at the as-received state as shown in Fig. 2: separated fine carbide particles are distributed in the matrix and along grain boundaries. With increasing height, the temperature increases. The original carbide particles are dissolved and the alloying elements start diffusing to the liquid zones where the solubility is higher. As well known, liquid preferably develops from the grain boundaries due to their high content of alloying elements. During quenching, new carbides are created from the liquid zones as illustrated in Fig. 4(b) (H ≥15mm in Fig. 1). The diffusion process is enhanced with increasing height; the original carbides are further dissolved. Finally, the original carbides are completely dissolved with the diffusion of the alloying elements to the liquid zones forming interconnected new carbides after quenching. One can also observe the entrapped liquid which corresponds to the fine carbide particles in the matrix. From the images, it can be found that there is a significant grain growth from 10µm to 45µm at high temperature as compared with the one at the as-received state. As well known, the grain growth is thermally activated: the grains grow with increasing temperature and holding time. However, the grain size at high temperature is still relatively small which can be explained by pinning effect [START_REF] Roberts | Tool steels[END_REF]: a big amount of carbides (see Fig. 3) contained in the M2 steel is responsible for inhibiting the grain growth during heating. Omar et al. [START_REF] Omar | Solid-liquid structural break-up in M2 tool steel for semi-solid metal processing[END_REF] mentioned that the carbides restrict the grain growth during the ferrite-to austenite phase transformation of M2 which takes place around 800°C. When comparing SEM micrographs and X-ray microtomographic images, they are in quite good agreement. The real volume fraction of liquid can be measured by image analysis with an image processing software ImageJ. Fig. 5 shows the evolution of the volume fraction of carbide as a function of height. As can be seen from the curve, over the height interval from 0mm to 12mm, the carbide fraction of material stabilizes at 11±1%. Then, from the height of 12mm to 17mm, the carbide fraction grows rapidly from 11±1% to 15±1%. Subsequent to the rapid growth, it increases slowly until reaching the value of 21±1%. The material from the bottom up to the height of 12 is assumed to be at the solid state, because the microstructure (showing small separated carbide particles are distributed in the matrix and along grain boundaries) and the volume fraction of carbides (11±1%) are similar to those at the as-received state. When the carbides are dissolved, the material contains liquid phase and austenite grains. The new carbides are formed instead of liquid zones during subsequent quenching process. In this case, the volume fraction of carbides (F C ) is equal to the former liquid fraction (F L ). Using SEM, it is found that at the height of 15mm, the original carbide particles are completely dissolved and new carbides are formed. Based on this, the carbide fraction over 13±1% is considered to be equal to the volume fraction of liquid.

Conclusion

The microstructure evolution of the M2 steel during heating to the semi-solid state was examined with both 2D and 3D techniques. Different types of carbides (MC-type and M 6 C-type) at both as-received state and quenched from semi-solid state were identified by EDS. 3D X-ray microtomography illustrates the distribution of carbides: the as-received material showed microsegregation bands in longitudinal section and small carbides in transverse section, while after being induction-heated and water-quenched, new large interconnected carbides were formed, corresponding to the liquid zones at semi-solid state. The grains were growing, and the solid phase was surrounded with the liquid zones at high temperature. The real volume fraction of carbide was calculated on X-ray tomographic stacks. Furthermore, the good agreement between 2D SEM-EDS microstructures and 3D X-ray microtomography results proves that both kinds of techniques are just as efficient in characterizing high-alloyed steels in the semi-solid state. The high energy X-ray microtomography was proven to be a powerful tool for this kind of study. In addition, owing to their high resolution and easy access, SEM and EDS techniques should also be applied for characterization.

Fig. 1 .

 1 Fig. 1. Schematic view of temperature distribution during induction heating.

Fig. 2 .

 2 Fig. 2. X-ray microtomographic results of as-received M2 steel in (a) 3D and (b) 2D.

Fig. 3

 3 Fig. 3 presents the SEM micrographs of as-received M2 steel on transversal and longitudinal directions. Two different types of carbides (MC and M 6 C) presented in the as-received material were confirmed by EDS, Fig.3: the whiter particles are M 6 C carbides which are rich in W and Mo, while the darker ones are MC carbides riching in V, W and Mo. These carbide particles are distributed in the martensite matrix and along grain boundaries. The microsegregation bands are observed in longitudinal surface (shown in Fig. 2(a) and Fig. 3(b)). The average equiaxed grain size is 10µm measured adopting the mean lineal intercept method as outlined in ASTM E112-96.

Fig. 4 .

 4 Fig. 4. Tomographic stacks (above) and SEM images (below) of quenched M2 at various heights.

Fig. 5 .

 5 Fig. 5. Volume fraction of liquid as a function of the height for Sample 1.

Table .

 .

				1 Chemical composition of M2 steel
	Weight %	C	Mn	Mo	Si	Cr	V	W
	M2	0.85	0.25	5.0	0.035	4.1	1.9	6.4

Solid State Phenomena Vols. 192-193

Acknowledgement

The authors warmly thank Luc Morhain and Marc Wary (Arts et Métiers ParisTech CER Metz) for their technical support. The authors are also grateful to ESRF for providing beam time on the ID15 beam line and to all stuff members of the line for their help and discussion during the experiments.

Determination of quantity and localization of liquid in the semi-solid state using both 3D X-ray microtomography and 2D techniques for steel thixoforming

Guochao GU 1, a , Raphaël PESCI 2,b , Eric BECKER 1,c Laurent LANGLOIS 1,d , and Régis BIGOT 1,e


Muhammad Sadiq
email: msadiq3@gatech.edu

Raphae ¨l Pesci

Mohammed Cherkaoui

Impact of Thermal Aging on the Microstructure Evolution and Mechanical Properties of Lanthanum-Doped Tin-Silver-Copper Lead-Free Solders

Keywords: Lanthanum, SAC alloys, intermetallic compounds, aging temperature, aging time

An extensive study is made to analyze the impact of pure lanthanum on the microstructure and mechanical properties of Sn-Ag-Cu (SAC) alloys at high temperatures. Different compositions are tested; the temperature applied for the isothermal aging is 150°C, and aging times of 10 h, 25 h, 50 h, 100 h, and 200 h are studied. Optical microscopy with cross-polarized light is used to follow the grain size, which is refined from 8 mm to 1 mm for as-cast samples and is maintained during thermal aging. Intermetallic compounds (IMCs) present inside the bulk Sn matrix affect the mechanical properties of the SAC alloys. Due to high-temperature exposure, these IMCs grow and hence their impact on mechanical properties becomes more significant. This growth is followed by scanning electron microscopy, and energy-dispersive spectroscopy is used for elemental mapping of each phase. A significant refinement in the average size of IMCs of up to 40% is identified for the as-cast samples, and the coarsening rate of these IMCs is slowed by up to 70% with no change in the interparticle spacing. Yield stress and tensile strength are determined through tensile testing at 20°C for as-cast samples and after thermal aging at 150°C for 100 h and 200 h. Both yield stress and tensile strength are increased by up to 20% by minute lanthanum doping.

INTRODUCTION

Sn-Pb solder has long been used in the electronics industry, but due to its toxic nature and environmental effects, certain restrictions are made on its use by the European RoHS directive, and therefore, many researchers are looking to replace it. This is because Pb and Pb-containing compounds, as cited by the US Environmental Protection Agency (EPA), is one of the top 17 chemicals posing the greatest threat to human beings and the environment. 1 The new lead-free solders are mostly Sn-containing binary and ternary alloys. 2 Due to their better thermomechanical properties, SAC alloys are considered as the best choice. 2,3 However, because of their coarse microstructure, iron (Fe), cobalt (Co), and nickel (Ni) have been used as refining elements, and their effects on the microstructure have been investigated. 4 Many combinations of, e.g., indium (In), antimony (Sb), bismuth (Bi), copper (Cu), and silver (Ag) are also used as alloying elements. 2 Extensive knowledge and understanding of the mechanical behavior of the emerging generation of lead-free solders is required to satisfy the demands of structural reliability.

Rare-earth (RE) elements are used in different studies and are considered as the vitamins of metals because of their special surface-active properties which help in overall refinement of the microstructure and hence improve the mechanical properties. [5][START_REF] Pei | Proceedings of the 57th Electronic Components and Technology Conference[END_REF][START_REF] Anon | [END_REF][8][9][10] They are widely used in Sn-Zn, Sn-Ag, and Ag-Cu alloys. [START_REF] Pei | Proceedings of the 57th Electronic Components and Technology Conference[END_REF][START_REF] Anon | [END_REF][8] Lanthanum (La) is considered as a good additive due to its lower cost, wide availability, and low melting point 4 as compared with other RE elements.

REs are used to control the growth of IMCs at high temperatures and hence play a major role in improving the overall mechanical properties of SAC alloys, but overdoping can deteriorate the solder mechanical properties. They have been used in different studies; e.g., cerium (Ce) and lanthanum (La) are used in Sn-9Zn alloys. [START_REF] Anon | [END_REF] Lutetium (Lu) is also used in many studies to improve Sn-Au and Sn-Ag solder alloys. [START_REF] Anon | [END_REF] In Ref. 8, addition of small traces of RE elements reduced the size of Ag 3 Sn and Cu 6 Sn 5 IMCs, while in Refs. 6,11 the impact of lanthanum on the microstructure evolution due to isothermal aging was studied for eutectic Sn-3.5Ag alloy. In Ref. 12, the impact of long-term isothermal aging is studied for the growth of intermetallic interfacial layers on Sn-Ag and Sn-Pb eutectic solders on Cu substrates. Similarly, in Ref. 13, the impact of isothermal aging on RE-doped Sn-Cu alloys was studied. Hao et al. 14 systematically investigated the evolution of microstructure and IMCs for SAC solder modified with trace amounts of the rare-earth element Er (addition of 0.15 wt.% Er to SAC387) during isothermal aging. All these elements refine the grain size, leading to a fine microstructure which ultimately improves the mechanical properties of SAC lead-free solders, including the yield stress and tensile strength, thus increasing solder joint reliability. 15,16 In Ref. 17, the impact of isothermal aging on mechanical properties of SAC alloys was studied. Modern electronic devices are subjected to severe thermal conditions varying up to 175°C. Moreover, many assemblies are composed of different electronic materials with a wide range of thermal expansion coefficients, which causes extra stress generation. Therefore, different compositions of La are tested in this study to identify an optimum SAC-La alloy, and mechanical properties are determined in varying environmental conditions.

The high temperature selected for aging is 150°C, and aging times of 10 h, 25 h, 50 h, 100 h, and 200 h are tested. Specimens are exposed to these conditions in a furnace after polishing and etching. They are analyzed at the end of each aging time and are put back into the furnace after analysis. Elemental mapping and chemical composition of each phase are achieved using an SEM equipped with an EDS system, and grain size is calculated using an optical microscope with cross-polarized light. The gauge area is focused on during the entire period of study.

The main focus of this work is to provide a quantitative analysis of the microstructure evolution under heat treatment of Sn-Ag-Cu lanthanum-doped alloys and its impact on mechanical properties. IMC particle size (average diameter), grain size, and interparticle spacing measurements have been carried out at different aging times at 150°C with different doping of lanthanum.

Since exposure of solder joints to high temperatures causes IMC particle coarsening, the mechanical properties may also be affected and early failure can be expected. As-cast samples and samples exposed to temperature of 150°C during 100 h and 200 h are then analyzed to determine the impact of thermal aging on mechanical properties through tensile testing at 20°C.

EXPERIMENTAL PROCEDURES

Solder alloys were prepared from pure metals, in the form of 200 g ingots with the following compositions (wt.%): Sn-3.0Ag-0.5Cu, Sn-3.0Ag-0.5Cu-0.01La, Sn-3.0Ag-0.5Cu-0.05La, Sn-3.0Ag-0.5Cu-0.25La, and Sn-3.0Ag-0.5Cu-0.5La. A three-part casting die, composed of two parts in aluminum and a central steel plate, was used to achieve ''cast by melt'' diecasting to make dog-bone-shaped tensile specimens. The front part of the die contains the path for the flow of molten alloy. The central plate contains five cavities for sample casting which are designed in a dog-bone shape. The back side of the die is used to tighten the plate to the front side and also provide fins to expedite the cooling rate during the solidification process. A temperature of 260°C is applied in the oven, and the die is heated for about 45 min before putting the molten metal into it. The 200 g ingots are put into a crucible and then placed in the oven at 260°C for about 25 min. Water at temperature of 15°C is used for quenching; the cooling rate of the specimens was measured with a K-type thermocouple. Only a small part of the die is dipped into the water to obtain a slow cooling rate of about 3°C/s, which is close to the actual reflow soldering process. The thickness of the resulting specimens is 2 mm. One of these cast specimens is shown in Fig. 1; all dimensions are in mm. Differential scanning calorimetry (DSC) tests were performed first to investigate the impact of lanthanum doping on the melting temperature of SAC alloys. The electrical resistivity of the SAC and SAC-La doped alloys was also measured using a four-point method for as-cast and thermally aged samples.

For quantitative microscopy, specimens of each composition were mechanically polished with silicon discs as well as 6-micron, 3-micron, and 1-micron diamond pastes. Chemical etching was performed for a few seconds using 5% hydrochloric acid/95% ethanol solution.

Quasistatic tensile tests at strain rate of 2 9 10 À4 s À1 were finally carried out with an MTS tensile machine equipped with a 5 kN load cell to determine the mechanical properties as a function of lanthanum doping for all compositions mentioned above except Sn-3.0Ag-0.5Cu-0.01La.

RESULTS AND DISCUSSION

The microstructure of the SAC alloys studied is composed of a soft Sn matrix and hard IMCs of Ag and Cu with Sn. 16 Figure 2 shows an SEM image of a SAC alloy: the black zone is the matrix that is mainly composed of Sn, and the white particles are the IMCs. These IMCs, rich in Ag and Cu, are brittle in nature in comparison with the soft Sn matrix and affect the mechanical properties of leadfree solders. 13 The size of these particles depends on many parameters such as alloy composition, cooling rate, and environmental conditions during service with temperature up to 175°C and aging time up to several hundred hours. In order to constrain the size of these IMCs, many researchers attempted a change in the cooling rate during the solidification process. [START_REF] Lewis | [END_REF]19 A fast cooling rate can generate a fine microstructure, but in many cases, this is not feasible because of the large thermal stresses in the substrates.

Lanthanum is well known for its surface-active properties. 11 During solidification, it moves to the free surface. In the SEM micrographs, small pores are observed only in doped alloys for the unpolished samples. EDS study also showed that lanthanum is uniformly distributed over the free surface. 20 The presence of lanthanum at the natural surface has many advantages. When this alloy is used as a solder joint, the elemental lanthanum in the bulk solder alloy at the interface will improve the wetting behavior of the solder alloy by creating chemical bonds with other materials. 11 Extensive wetting balance test study has been performed at 250°C and 260°C by the authors, 20 and it was observed that lanthanum doping increases wetting forces and reduces contact angles. Table I summarizes the wetting force and contact angle after performing 10 tests for each composition. Rosin mildly activated (RMA) flux was used. The results are quite consistent with Ref. 13.

DSC tests were conducted between the two peaks of 20°C and 300°C for both SAC and SAC-La doped alloys with a heating and cooling rate of 5°C/min. DSC curves are presented in Fig. 3. It is seen that there is no significant change in the melting temperature (starts at 217°C and ends at 230°C) between the undoped and maximum-doped SAC alloy, which is consistent with Ref. 11. Finally, the electrical resistivity measurements show that lanthanum doping has no influence on either as-cast or thermally treated samples. 20 The values for all samples with all test conditions are typically in the range of 10 lX cm to 15 lX cm.

Particle Size

An SEM FEG Jeol 7001F was used to collect images for each composition at different resolutions, in the initial state and after different aging times. The as-cast and thermally aged SEM images for Sn-3.0Ag-0.5Cu and Sn-3.0Ag-0.5Cu-0.5La alloys are shown in Fig. 2. With thermal aging, coarsening of both Ag-Sn and Cu-Sn IMCs takes place, at a rate depending on the temperature applied and aging time. It could be easily concluded from the micrographs that the coarsening rate for lanthanumdoped alloy is much slower than for the undoped alloy. Microstructure evolution is observed in terms of the IMC average particle size, its volume fraction, and the interparticle spacing. These measurements were done as a function of aging time, aging temperature, and lanthanum composition. ImageJ software was used to measure the average particle size at every aging interval. At least three SEM images of every composition were provided for measurements using three different magnifications: 8509, 25009, and 50009. The particle size was also measured manually in terms of average surface area using ASM handbook standards [START_REF]ASM Handbook vol. 9, Metallography and microstructures[END_REF] supported by ImageJ analysis. The results are plotted in Fig. 4a as a function of lanthanum doping and in Fig. 4b as a function of aging time at 150°C.

Stoichiometric calculations have shown that the volume fraction of IMCs in Sn-Ag alloy is around 0.07 in general. [START_REF] Dutta | [END_REF] Since the amount of La doping is quite small in the current work, the volume fraction of IMCs can be considered as close to 0.07. If the IMCs are assumed to be spherical and uniformly distributed in the eutectic region, as also considered in Ref. 11, then the interparticle spacing (sideto-side distance) can be calculated from the volume fraction of eutectic region. The results are presented in Table II. It is observed that this spacing does not change a lot with La doping. This validates the fact that La doping reduces the size of the particles, which inevitably increases the number of particles to keep the same interparticle spacing. Moreover, if the interparticle spacing is not changed, this justifies the fact that the eutectic region would be increased for higher La doping.

Figure 4a indicates a drastic decrease in the particle size due to lanthanum doping, whereas Fig. 4b shows that the coarsening rate of these IMCs is smaller for the lanthanum-doped alloys than for the undoped alloy.

As previously mentioned, the IMCs consist of Ag-Sn and Cu-Sn. An Oxford EDS system in the SEM enabled elemental mapping for each specimen. The spatial distribution of Ag, Cu, and Sn could be visualized in the IMCs, as shown in Fig. 5. To follow the growth of these IMCs, another SEM image and spatial distribution of these elements are provided in Fig. 6 for a 100 h thermally aged specimen. The Cu-based IMCs are much darker than Ag-based IMCs, as also reported in Ref. 23, and are therefore easily distinguished and confirmed by the EDS for two different zones as shown in Fig. 6. About 25 IMCs were analyzed, and an average measurement for the atomic % is provided in Table III. The atomic % may not correspond to the exact ratio for Ag 3 Sn and Cu 6 Sn 5 IMCs due to the limitation of our experimental setup, which considers more Sn to a depth of a few microns, which may not be the actual depth of these IMCs.

Thermal aging causes the evolution of both Ag 3 Sn and Cu 6 Sn 5 IMCs, and this process is governed by r 3 µ t, where r is the average radius of the IMC particles and t is the isothermal aging time. 23,24 Both types of particle coarsen at different rates due to the different rate-controlling mechanisms. The diffusion rate of Cu into Sn is higher than the diffusion rate of Ag into Sn. 23 Therefore, with time, the coarsening of Cu 6 Sn 5 is much greater than that of Ag 3 Sn 24 , due to which the Cu 6 Sn 5 IMCs can be easily differentiated from Ag 3 Sn IMCs. In Ref. 10, it was reported that the size of Cu 6 Sn 5 IMCs is much larger than that of Ag 3 Sn IMCs in SAC alloys. The EDS mapping for SAC-La alloy after thermal aging is presented in Fig. 6. Contrary to Ag 3 Sn particles, which show a normal increase with submicron size, the thermal evolution of Cu 6 Sn 5 IMCs shows a significant growth up to a few microns. This is consistent with Ref. 10. The rapid growth starts after 50 h of thermal aging at 150°C.

It is reported in Refs. 6,11 that La may be present in elemental form, but they could not identify it due to experimental limitations. In this work, however, La was confirmed to be present in the form of oxides in the bulk alloy. EDS elemental mappings were performed for La and other elements and are presented in Fig. 7 along with the SEM micrograph. This form is dispersed thoroughly in the bulk alloy. 5,8 Due to the small size of these oxides, they have not been determined exactly.

Grain Size

Grain size was measured for the as-cast samples and then continuously followed during thermal aging. Figure 8 shows grain size images for Sn-Ag-Cu and Sn-Ag-Cu-0.05La as-cast samples. Optical microscopy with cross-polarized light was used for this study, in which grains can be observed as different shades. A significant decrease in grain size, due to the addition of lanthanum, can be seen.

Grain size, as a function of lanthanum composition, is plotted in Fig. 9a. A huge refinement is observed. Figure 9b describes the grain size as a function of aging time at high temperature. Since the number of grains is not large, error bars are provided in both plots. It is important to note that the grain size for the undoped alloy is too large and only one grain boundary is observed in the gauge area. Hence, the grain size is assumed to be 8 mm as given in Fig. 9a. It is also observed that the alloy composition with the smallest grain size is SAC-0.05La. This is consistent with the work performed by Pei and Qu [START_REF] Pei | Proceedings of the 57th Electronic Components and Technology Conference[END_REF]11 for Sn-3.5Ag alloy with similar La doping. Previous studies 20 found that, due to the pinning effect of IMCs, thermal aging does not affect the grain size in the undoped alloys. Figure 9b shows that almost no change in grain size occurred due to thermal aging. It can be concluded that lanthanum doping not only refines the grain size but also restricts its growth at high temperatures. This is consistent with Ref. 11. This refinement is due to the particular effect of lanthanum adsorption at different planes in the SAC alloys. 8,13,25 Mechanical Properties

Figure 10a shows the yield stress and tensile strength for the as-cast samples at room temperature. A significant increase in both yield stress and tensile strength is observed. This is consistent with Ref. 16, in which an increase in tensile strength is investigated for Sn-0.7Cu, Sn-3.5Ag, and Sn-9Zn alloys due to RE elements (mainly Ce and La). On the other hand, an increase in La doping also leads to a decrease in elongation to failure. This may be due to an increase in the quantity of hard, RE-bearing particles. 8,13 In Ref. 26, it is shown that lanthanum results in the formation of LaSn 3 compounds. The average microhardness of this phase, in a slowly cooled alloy, has a high value of 48 HV as compared with the average microhardness of the matrix of 13.5 HV. 9 The evolution of yield stress and tensile strength for 100 h and 200 h aging at 150°C are given in Fig. 10b andc, respectively; all values are summarized in Table IV. The increase in yield stress and tensile strength due to RE doping in tin-based alloys is quite consistent with Ref. 27.

Thermal coarsening greatly affected the microstructure of the SAC alloys. We can see a significant decrease in both yield stress and tensile strength from the as-cast alloys to thermally aged alloys. This is because of the IMC growth, which causes early failure and reduces solder joint reliability. In all three cases, the mechanical properties of the doped solders are significantly higher than those of the undoped alloys. Even an increase of 2 MPa could be appreciable, since this corresponds to a 10% increase in the yield stress of 20 MPa.

Since creep tests are conducted under constant stress, less than the yield stress, an increase in the yield stress would increase the gap between the applied stress and the yield stress of the material. Thus, it is expected that La doping will increase the creep life of SAC alloys, as also presented by many researchers. 9,13,[START_REF] Lin | [END_REF] CONCLUSIONS Doped SAC alloys have been studied to determine the influence of lanthanum on their microstructure evolution and mechanical properties at high temperature. Several conclusions can be made. The addition of lanthanum drastically reduces the IMC particle size. It also controls the growth of these IMC particles during thermal aging. It leads to a huge refinement of grains for the as-cast specimens, but no evolution is observed after thermal aging. There is no significant increase in the interparticle spacing. A significant increase of yield stress and tensile strength with lanthanum doping is observed for as-cast and thermally aged specimens. Thus, lanthanum refines the microstructure, reduces the drastic effects of IMCs, and hence improves the mechanical properties of SAC solders.

Fig. 1 .

 1 Fig. 1. Dog-bone-shaped specimen for tensile tests.

Fig. 2 .

 2 Fig. 2. SEM images of (a) SAC and (b) SAC-La before thermal aging, and (c) SAC and (d) SAC-La after thermal aging.

Fig. 3 .Fig. 4 .

 34 Fig. 3. DSC curves for SAC and SAC-0.5La alloys.

Fig. 5 .

 5 Fig. 5. (a) SEM image and EDS elemental mappings of (b) Sn, (c) Ag, and (d) Cu for the as-cast SAC sample.

Fig. 6 .

 6 Fig. 6. (a) SEM image and EDS elemental mappings of (b) Sn, (c) Ag, and (d) Cu for SAC-La after 100 h thermal aging.

Fig. 7 .

 7 Fig. 7. (a) SEM micrograph and EDS elemental mappings of (b) Sn and (c) La of SAC-La alloy.

Fig. 8 .

 8 Fig. 8. Optical microscopy images of SAC (left) and SAC-La (right).

Fig. 9 .

 9 Fig. 9. Average grain size versus La composition for as-cast (a) and thermally aged (b) samples.

Fig. 10 .

 10 Fig. 10. Stress versus La composition for (a) as-cast, (b) 100 h thermally aged, and (c) 200 h thermally aged samples.

Table I .

 I Wetting forces and contact angles

Alloy

Wetting Force (mN) Contact Angle (°) SAC 5.8 ± 0.15 47 ± 1.1 SAC-0.05La 6.1 ± 0.14 45.5 ± 0.9 SAC-0.25La 6.7 ± 0.09 43 ± 1.2 SAC-0.5La 6.4 ± 0.10 44 ± 1.1

Table II .

 II Interparticle spacing

	La (wt.%)	0	0.01	0.05	0.25	0.5
	Spacing (lm)	0.058	0.063	0.062	0.068	0.063

Table III .

 III Atomic composition of IMCs

			Atomic %
	IMC	Sn	Ag	Cu
	Ag 3 Sn	74 ± 0.7	26 ± 0.7	9
	Cu 6 Sn 5	52 ± 0.4	9	48 ± 0.4

Table IV .

 IV Yield stress and tensile strength of SAC-La alloys

	Alloy	Condition	Yield Stress (MPa)	Tensile Strength (MPa)
	SAC	As cast	23	37
		100 h aged at 150°C	18.5	21.5
		200 h aged at 150°C	19.5	24
	SAC-0.05La	As cast	35	40
		100 h aged at 150°C	21	24.5
		200 h aged at 150°C	22.6	26
	SAC-0.25La	As cast	36	42
		100 h aged at 150°C	29	30.5
		200 h aged at 150°C	2 6	2 7
	SAC-0.5La	As cast	37	43
		100 h aged at 150°C	3 0	3 2
		200 h aged at 150°C	2 9	3 2

ACKNOWLEDGEMENTS

The authors would like to thank Ste ´phanie Blanc (Electrical Engineer at Schlumberger) for her useful contribution to the project, Claude Guyomard and Olivier Naegelen (Arts et Me ´tiers ParisTech) for the die design and sample casting, respectively, and Jean-Marc Raulot for his enriching discussions.

CONFLICT OF INTEREST

The authors declare that they have no conflict of interest.


Éric Leclercq
email: eric.leclercq@u-bourgogne.fr

Marinette Savonnet
email: marinette.savonnet@u-bourgogne.fr

Adding Semantic Extension to Wikis for Enhancing Cultural Heritage Applications

Keywords: Semantic Wiki, Ontology Engineering, Cultural Heritage application

Wikis are appropriate systems for community-authored content. In the past few years, they show that are particularly suitable for collaborative works in cultural heritage. In this paper, we highlight how wikis can be relevant solutions for building cooperative applications in domains characterized by a rapid evolution of knowledge. We will point out the capabilities of semantic extension to provide better quality of content, to improve searching, to support complex queries and finally to carry out different type of users. We describe the CARE project and explain the conceptual modeling approach. We detail the architecture of WikiBridge, a semantic wiki which allows simple, n-ary and recursive annotations as well as consistency checking. A specific section is dedicated to the ontology design which is the compulsory foundational knowledge for the application.

Introduction

Collaborative platforms that manage scientific knowledge are essential tools for scientists to help them to formalize their ideas, to develop theories collaboratively, to publish results (research articles, technical reports, data sets, etc.) and to produce knowledge for different kind of users. Moreover, a collaborative platform should be able to integrate other services such as visualization tools, or spatial analysis tools.

Wiki solutions meet the requirements of a web platform with collaborative capabilities. Easy setup and rich editing support are primary reasons for the widespread adoption of wikis. Users can enter text and others types of data (pictures, video) and connect content through hyperlinks. Most of wikis also provides a versioning system to track content changes and a full-text search engine for querying wiki pages.

The narrative structure is one advantage of wiki documents centric approach, compared to a database centric approach. In a database centric approach, the database schema is built upon entities identified in the first step of analysis, and thus based on an instant knowledge. In domains characterized by a rapid evolution of knowledge, such as biology or archaeology, a static database schema is not suitable and can be proscribed by the cost of evolution. Nevertheless, a mere document management system is not sufficient to catch interdependent structures of knowledge. For example, domain specialists often need to comment on primary data. Adding semantic annotation capabilities to documents allows different levels of interpretation and can sustain: 1) knowledge evolution by keeping track of the successive annotations; 2) better quality in the query evaluation process; and 3) amenable result displayed according to user skills. Annotations can be defined at a coarse grained level (whole document) or at a fine grained level (i.e. attached to a piece of text). An ontology must be associated to the annotation system to provide a semantics for annotation terms according to domain knowledge.

Semantic wiki solutions meet the requirements of annotation system and knowledge description. Adding semantics yields two dimensions of enhancements to a wiki: 1) adding a more formal structure to the wiki; 2) exporting, integrating and reusing information by the adoption of standard semantic technologies. Compared to a traditional database, a semantic wiki allows: 1) to expand the structure of documents content; 2) to enable a data model emergence from the usage; and 3) to support collaborative, distributed workflows and processes. Semantic wiki thus seems to combine the best from two worlds: structure from databases as well as expandability and collaboration capabilities from wiki systems.

The rest of the paper is organized as follow: section 2 gives an overview of the CARE project, section 3 describes the requirements and WikiBridge architecture, section 4 describes semantic tools for archaeology, and section 5 discusses related works. Finally, section 6 concludes the paper.

Project overview

The aims of the international project CARE (Corpus Architecturae Religiosae Europeae) is the setting up of a corpus describing Christian edifices in Europe (http://care.u-bourgogne.fr). Italy, Spain, Czech Republic, Poland, Slovakia, France and Croatia have been included in the project four years ago. Each edifice is described in a document that focuses on the description of states of evolutions from the 4th century to the 11th century. The French corpus focuses on the 7th and 8th centuries with very rich decades in terms of number of monuments. The French part of the project is supported by a French ANR funding (ANR-07-CORP-011).

Representing and managing knowledge in cultural heritage require a deep understanding of specific concepts. Building collaborative platform brings out some challenging characteristics: 1) complexity of data (heterogeneous, incom-plete, uncertain, inconsistent, spatial, temporal); 2) domain knowledge barrier; 3) evolving knowledge; and 4) skills of actors.

The CARE community

From an organizational perspective, the CARE project takes the form of an expert network collecting and providing information on edifices, analyzing historical sources, filling documents and collaborating in the exploitation of the corpus through smaller research groups. Furthermore, the project involves undergraduate students that help in collecting information but lack expertise required to interpret data. In France, more than sixty researchers are collecting and analyzing data concerning approximately 2700 monuments. Two key characteristics describe the CARE community: multi-disciplinarity: The data collecting process involves archaeologists, historians, art historians, topographers, draftsmen. It is designed as a collaborative process which merges information from various disciplines; inter-disciplinarity: The interpretation of data brings together all the actors which also enrich their respective practices by the confrontation of methods or problems.

Conceptual modeling for the foundational knowledge

Linster in [START_REF] Linster | Viewing Knowledge Engineering as a Symbiosis of Modeling to Make Sense and Modeling to Implement Systems[END_REF] shows that the interaction among domain experts, knowledge engineers and tools creates the knowledge. He has shown that the process of elaborating a knowledge-based system is a constructive model-building process that includes: a discussion process between knowledge engineers and domain experts as well as the construction of a conceptual model (i.e. a general and abstract framework). Thus, the knowledge engineering activity encompasses the design of two kinds of models: model to make sense and model to implement systems.

We have applied Linster's guidelines to the CARE project in order to initiate a foundational knowledge from the corpus of documents. The first stage is the salient concepts identification. The key concept is the edifice to which it is essential to model changes. All constituent elements of a building need to be described. They can delimit space or define religious function (baptismal, funerary, etc.). All changes of space or religious function determine a new edifice state. Edifices and their evolutions are described in a set of documents. Salient concepts let us to build a conceptual model, three groups of elements have been identified (figure 1):

1. spatial concepts without temporal relationship (light grey): concept EGS refers to edifice, group of edifices or space inside edifice such as nave or apse. Composition relationships can be identified between EGS. 2. spatio-temporal concepts (grey) called SEGS, represent variations of spatial concepts in time. Spatio-temporal concepts are linked to a date or a period. Dating elements can be determined by documents, or described by methods such as C14, thermoluminescence or stratigraphy.

3. identification of vocabulary terms for the project domain (black). While concepts EGS and SEGS are used to structure the descriptions, terms are used to describe specific elements, properties of edifices, religious functions or manufacturing techniques.

Religious function

Edifice space (nave, transept, altar, burial, ...)

Fig. 1. Conceptual model of the CARE corpus

The description of the construction of the ontology is detailed in Section 4.

WikiBridge's architecture

In a survey authored by Uren et al. [START_REF] Uren | Semantic Annotation for Knowledge Management: Requirements and a Survey of the State of the Art[END_REF], authors study semantic annotation, identify a number of requirements, and review some semantic annotation systems. WikiBridge's design principles, in agreement with the CARE community, are following of the seven requirements given by Uren et al.: easy to use interface, user collaborative design, support of different user skills, support of heterogeneous format, compatibility with Semantic Web standards, annotation capabilities and storage, support for reasoning.

In the next subsection we develop the most important requirements with regards to the architectural design of WikiBridge.

Requirements

In a knowledge engineering process, it is common that non-technical domain experts work together with experienced knowledge engineers. To support different levels of users skill certain advanced functionalities should be hidden from novice users but made available to experienced users. Thus, we use an Access Control List (ACL) mechanism to describe privilege control depending on user identity and group affiliation. Advanced users can define forms to help users to enter descriptions of edifices, each part of a form generates automatically annotations. During the annotation process, a wizard suggest terms of the ontology according to highlighted section in the form.

To be able to exchange data with other applications (e.g. ontology editors, Web Services, other wikis), a compliance to Semantic Web standards is required. WikiBridge is purely based on existing Semantic Web standards such as the Web Ontology Language OWL for describing ontologies and W3C's RDF for annotations.

We consider reasoning as one of the most important functionalities as it allows: 1) to emerge knowledge that is not explicit in the data; 2) to check the meaning of annotations with regards to the context of the annotation; and 3) to enhance navigation and search.

Architecture

One of the most famous semantic wiki is Semantic MediaWiki (SMW) which is based on MediaWiki [START_REF] Völkel | Semantic wikipedia[END_REF][START_REF] Krötzsch | Semantic MediaWiki[END_REF]. In 2009, when we have started the project, complex annotation and consistency checking were identified as mandatory functionalities. In 2009, SMW doesn't provide complex annotation and doesn't have consistency checking in its roadmap. We have extended MediaWiki with the following semantic components: form based acquisition interface with automatic annotation, annotations wizard, annotations validation based on the context of a document, semantic rules and a semantic query engine. The semantic components are structured in three layers.

User interaction layer The user interaction layer is covered by MediaWiki and structured data control needed for unexperienced users is managed by Semantic Forms (an extension 1 for MediaWiki). A specific description language allows administrators and advanced users to define new forms (figure 3). Modules corresponding to the interaction layer are represented on the top of figure 2.

Structuration of wiki article

Creation of a form Semantic Layer To improve the quality of information during the input and annotation processes, we propose three semantic components in WikiBridge Core (white boxes in figure 2) developed on the top of third party tools (RAP -RDF API for PHP, Pellet and Jena). The syntax and the semantics of annotations, made by experts, are guaranteed by an application ontology. Values lists used in annotation wizard are filled by individuals retrieved from ontology concepts, through SPARQL queries that are used to select individuals. The annotation wizard helps users to construct simple or complex annotations by selecting ontology terms in lists and giving them properties and values (figure 4). Each document is identified by its URL in the wiki and annotations use this URL as a basement for identifying part of content. Simple annotation allows to annotate a subject by describing a property by a literal. Complex annotation allows to annotate a subject with two or more values and references to other elements (subjects). For example we can annotate an altar with its dimension, its building material, its location in the nave. The nave is detailed in another part of the document. Annotation construction is a context sensitive process, initial terms displayed to users are directly connected to fields of forms. Annotation consistency checking is then operated by a set of specific components interacting with RAP, Pellet and Jena. A subset of first order logic constraints are checked with a Java component that interacts with Pellet and Jena. So, WikiBridge extension connects to the Java constraint checker by the means of a web service. Moreover, rules can be added to query ontology and annotations in order to test new facts and thus to produce new knowledge that can be inserted in the set of semantic constraints.

Two kinds of constraints can be check by using the ontology knowledge: 1) domain values of properties using ABox capabilities; and 2) structural consistency of properties using TBox capabilities (for instance, a cathedral can have a nave but cannot have an atrium). Nevertheless, some domain dependent constraint cannot be embedded in the structure. For example "In France, there is no church with rammed earth wall for the studied period however, this technique is used in Ireland and in other countries" can be translated by the following constraint that must remains consistent: hasRammedWall(?x) ∧ edificeType(?x, ?t) ∧ edificeCountry(?x, ?c) ∧ c='France' ∧ t = 'church' Persistency layer The persistency layer includes four types of storage (bottom of figure 2): documents content, semantic annotations, ontology, and constraints.

The content of documents is stored by MediaWiki specific database. Annotations are stored as triple in RAP triple store and they can be retrieved by WikiBridge user layer to display annotations with icons and colors (different colors are used to distinguish centuries) in a document or by the SPARQL query engine. The ontology imported as an OWL file is stored in a specific schema managed by RAP. Ontology terms can be then queried using SPARQL and results can generate wiki pages. Constraints are stored in plain text using Jena rules syntax. A type attribute specifies if the rule can be applied to check the ontology structure or to check annotation consistency.

Users with different skills

Information access has been designed with taking into account some features about users. We have thus identified a usage typology in accordance to 1) kind of usage (reader, investigator, annotator); 2) knowledge degree of the domain (domain specialists like historian researchers and non specialists). To handle these different types of users, we offer three types of queries:

1. faceted browsing allows users to explore by filtering available information with the ontology structure; 2. form based searching provides semantic search by filling in parameters of parametric queries identified during the analysis of requirements; 3. aggregate view for each article, all annotations related to the edifice are displayed in a factbox.

Nevertheless all types of queries rely on the SPARQL query engine which also allows to process in line queries into wiki pages in order to summarize informations.

To operate spatial and temporal analysis on annotations a set of web services has been developed (figure 5). Some specific services allow to retrieve edifices and their coordinates according to a set of conjunctive properties. Moreover, a generic web service have been developed to handle SPARQL queries.

Offering semantic tools for archaeology

Cultural heritage collections can be annotated with different thesauri. The web page http://tinyurl.com/5u8bjer maintains about twenty thesauri, classified by content and organization. The CIDOC Conceptual Reference Model (CRM) provides an extensible ontology for concepts and relationships in cultural heritage domain. Since 2006, it is an ISO standard (21127:2006) for exchange of cultural heritage information [START_REF] Crm | The CIDOC Conceptual Reference Model (ISO/CD 21127)[END_REF]. The CIDOC CRM emerged from the ICOM/CIDOC Documentation Standards Working Group (http://www.cidoc-crm.org). CIDOC-CRM aims at treating all types of material collected and/or displayed by museums: sites, monuments as well as collections of fine and applied arts. It is intended to encompass the detailed description both of individual materials as well as groups of materials as a whole. It covers also contextual information: historical, geographical context in which materials are placed and which gives them much of their significance and value. The event notion in CIDOC-CRM is represented by the concept of event. An event describes environment of the material over the time and what could happen to him. The central notion is complemented by: 1) the TimeSpan concept describes the moment it happened; 2) the concept of Place ; 3) who did it (Actor ); and 4) what is being described. CRM offers notions of Physical Objects that can be natural or have been manufactured by man, and Conceptual Objects.

Since CRM is a reference in the field, we use it as a starting point to establish CARE ontology. The view of the ontology CARE as a specialization of CRM allows us to relate to a standard and comply with it. The CARE ontology has two parts: 1) religious concepts, their spatial relationships and characteristics (class EGS in figure 1) and 2) timeline to track evolutions.

Religious concepts in the CARE project

Religious concepts in CARE are edifices, represented by the concept Building, with its decomposition into different constituent elements (nave, transept, apse, etc.) represented by the concept of Structure. To detail parts of an edifice, we introduced the concept of ElementArchitectural to describe masonry, floor, opening, etc. Liturgical installations (altar, ambo, ciborium, etc.) are represented by the concept of InstallationLiturgique and burials represented by the concept Tombe. These concepts have been placed under the concept E24 Physical Man-Made Thing CIDOC-CRM. Indeed, CIDOC-CRM defines this concept as "all persistent physical items that are purposely created by human activity". Figure 6 represents all these concepts (concepts with EXX are CIDOC-CRM concepts).

Modeling spatial relationships in archaeology

The geometry implementation in the textual descriptions that are analyzed is a complex geometry. Indeed, these descriptions do not refer to an absolute and orthonormal space: it is rather, a space perception or a cognitive space whose structure is largely based on the functional aspects and objects described, and the perspective of the archaeologist. From the analysis of textual descriptions of religious concepts (description of the position and shape) given by archaeologists we have found four types of spatial properties:

-orientation properties: forward, back, bottom, next to, on one side, under, below, at a lower level, at the same level, above, right, left, center, prior, and the cardinal directions; -boundary properties: outside, inside; -distance properties: near, far, next to, around; -topological properties: flank, join, open on the side, link to, stand against, surround, isolated. To represent the topological properties, we used the work of Hegenhofer and Herring [START_REF] Hegenhofer | Categorizing Binary Topological Relations Between Regions, Lines and Points in Geographic Databases[END_REF]. The authors have defined a minimum set of eight relations (disconnected, externally connected, partially overlap, equal, tangential proper part, non-tangential part, tangential proper part inverse, non-tangential part inverse) describing the relations between two regions.

Modeling temporal knowledge to track evolutions in CARE project

When writing of his excavation report, the archaeologist graphically summarizes the results obtained with a timeline which is often organized by anterior/posterior relationships: materials are considered in relation to each other. In the CARE project, time model is based on following criteria: some absolute benchmarks and a relative chronology based on intervals. We have established a convention for century division and boundaries. Centuries start at year 1 and end at year 100. We also have established subdivisions terms such as early (1 → 32), mid (33 → 66) and late (67 → 100). These century divisions are placed under E52 TimeSpan. CIDOC-CRM offers specific concepts related to time [START_REF] Binding | Implementing Archaeological Time Periods Using CIDOC CRM and SKOS[END_REF][START_REF] Doerr | Thesauri of Historical Periods -A Proposal for Standardization[END_REF]. The concept E2 Temporal Entity describes all phenomena which happen over a limited extent in time. It includes notions such as: E3 State Conservation comprises the states of objects characterized by a certain condition over a timespan; E4 Period includes sets of coherent phenomena or cultural manifestations bounded in time and space. Some Allen' relationships [START_REF] Allen | Maintaining Knowledge about Temporal Intervals[END_REF] are properties used.

The CARE project aims to follow the evolutions (creation, modification, deletion) of an edifice and its constituent elements. The concept of activity is important for CARE project because a state may be characterized by an activity in the CIDOC-CRM ontology. The concept E7 Activity is defined as follows: "The action or sequence of actions intentionally carried out by Actors that result in changes of state in the cultural, social, material systems which interest". This notion includes both complex and long-lasting actions such as building an edifice, as well as simple and short-lived actions. Following the concepts of the CIDOC-CRM ontology, we use seven concepts to model states of edifices:

-E6 Destruction includes events that destroy one or more instances of the concept E18 Physical thing; -E11 Modification includes all instances of E7 Activity that modify E24 Physical Man-made thing; -E81 Transformation corresponds to the destruction of one or more objects and the simultaneous production of others using parts or materials from the first. Transformation preserves recognizable substance from the first element, but has a different nature; -E63 Beginning of Existence includes events that bring into existence to any E77 Persistent Item; -E64 End of Existence includes events that end the existence of any E77 Persistent Item; -E79 Part Addition includes activities that result in the fact that an instance of E24 Physical Man-made Thing is enlarged or increased by the addition of a party; -E80 Part Removal includes activities that result in the fact that an instance of E18 Physical Thing is reduced by the removal of a party.

The CARE ontology has been designed using Protégé and it actually encompasses 124 classes and 715 individuals.

Related works

Several semantic wikis have been developed or used specifically for cultural heritage applications.

Witte et al. [START_REF] Witte | Converting a Historical Architecture Encyclopedia into a Semantic Knowledge Base[END_REF] present an approach to cultural heritage data management which integrates different technologies: a wiki user interface, text mining support using a Natural Language Processing (NLP) framework and ontologies based on OWL and RDF. Authors have implemented the ideas for the German Handbuch der Architektur, a comprehensive multi-volume encyclopedia of architecture. A volume (506 pages) of the encyclopedia is converted into wiki pages. Authors have to capture two sub-domains by ontologies: the domain of document management (i.e. sentence, noun, page number, etc.) and architectural domain (i.e. wall, building material, etc.). NLP allows to connect architectural concepts with document-specific one, e.g. sentences that mention construction elements of a certain material. A public version is available at http://durm.semanticsoftware.info/wiki.

The HermesWiki [START_REF] Reutelshoefer | Taking OWL to Athens: Semantic Web technology takes Ancient Greek history to students[END_REF] is a semantic wiki in the historical domain in German language. The main objective is to provide an overview on Ancient Greek History for teaching purposes of undergraduate students. The wiki consists of three parts: a collection about twenty essays giving a comprehensive domain walk-through, translations of the describing ancient sources and a glossary. The entries in the glossary are tagged. It has been implemented as a plugin for KnowWE [START_REF] Baumeister | KnowWE: community-based knowledge capture with knowledge wikis[END_REF], reusing as much of the core components as possible. A public version is available at http://hermeswiki.informatik.uni-wuerzburg.de.

NavEditOW is a framework for ontology driven web site. It has been exploited to support a semantic description of two projects: 1) a web portal and a set of advanced services supporting the sharing of knowledge about Prehistory and Protohistory in the Italian context [START_REF] Bonomi | A Framework for Ontological Description of Archaeological Scientific Publications[END_REF]. In particular, one of the services is represented by a digital library, in which entries (i.e. bibliographic description of publications) will be ontologically described. The system is currently on-line at URL http/www.archeoserver.it and; 2) SilkRoDE (Silk Roads in the Digital Area) project that aims to collect, structure and diffuse all knowledge about the Cultural Heritage of Central Asia from fields such as archeology, geography or history [START_REF] Bonomi | Integrating a Wiki in an Ontology Driven Web Site: Approach, Architecture and Application in the Archaeological Domain[END_REF]. The ontological approach provides the required expressiveness and flexibility to support rich forms of navigation among stored contents. The framework integrates a wiki engine for rendering documents stored in the ontological tier.

In the same view, MANTIC is a web application that realizes a portal for archaeological information about the city of Milan [START_REF] Mantegari | Rapid Prototyping a Semantic Web Application for Cultural Heritage: The Case of MANTIC[END_REF]. MANTIC integrates different data sources and the global schema is based on CIDOC-CRM.

Our approach of semantic wiki is directed towards scientific application domains which contribute to produce knowledge. These kind of applications rely on core ontologies that act as a consensus. Knowledge is enhanced by querying and analyzing data, new concepts can emerge and new constraints can be found out. In contrast with above projects, WikiBridge provides a rich annotation model [START_REF] Leclercq | Access and annotation of archaelogical corpus via a semantic wiki[END_REF] based on semantic values [START_REF] Sciore | Using semantic values to facilitate interoperability among heterogeneous information systems[END_REF] and supports semantic consistency checking upon the ontology and constraints.

Conclusion

In this article, we have presented WikiBridge which provides users with advanced functionalities such as rich annotation model, consistency checking. The first experiment of the use of WikiBridge shows many interesting possibilities for scientific community, mainly the possibility given to scientists share and collaboratively build annotated knowledge. A knowledge that can be re-organized at any time, to fit the needs of the scientists. We have demonstrated that flexibility and data quality required by scientific applications can be achieved by using wiki with Semantic Web technologies. The semantics of annotation is guaranteed by an ontology including constraints which allow to describe accurately domain knowledge. Our dual approach allows to cope with evolution of knowledge by dynamically modifying the ontology and annotations without modifying database schema.

Fig. 2 .

 2 Fig. 2. WikiBridge's Architecture

Fig. 3 .

 3 Fig. 3. User interaction layer

Fig. 4 .

 4 Fig. 4. Annotation wizard

Fig. 5 .

 5 Fig. 5. OpenLayers interface interacting with a web service of WikiBridge

Ontologie C A RE Expression des règles de l'ontologie

Chose