The most commonly used base monomer in dental RBC is bis-GMA (bisphenylglycidyl dimethacrylate) which, due to its high viscosity, is mixed with other dimethacrylates such as TEGDMA (triethylene glycol dimethacrylate), UDMA (urethane dimethacrylate) or other monomers. [START_REF] Jeon | Dental restorative composites fabricated from a novel organic matrix without an additional diluent[END_REF][START_REF] Powers | Craig's restorative dental materials[END_REF] UDMA was first introduced as a dental resin by Foster and Walker in 1974 as an alternative monomer to bis-GMA. It was obtained by the reaction between an isocyanate and an alcohol and it has been shown that UDMA-based polymers would show similar or very slightly lower water absorption than bis-GMAbased polymers. [START_REF] O ¨rtengren | Water sorption and solubility of dental composites and identification of monomers released in an aqueous environment[END_REF][START_REF] Pearson | Water sorption and solubility of resin based materials following inadequate polymerization by a visible light curing system[END_REF] These resins modified with urethane dimethacrylate also have a further reduced water absorption when several radicals are added to the base molecule. [START_REF] Alshali | Long term sorption and solubility of bulk fill and conventional resin composites in water and artificial saliva[END_REF][START_REF] Sideridou | Sorption of water, ethanol or ethanol/water solutions by light cured dental dimethacrylate resins[END_REF] Absorption phenomena are correlated with the solubility of the material, and UDMA-based materials would appear to behave correctly. [START_REF] O ¨rtengren | Water sorption and solubility of dental composites and identification of monomers released in an aqueous environment[END_REF][START_REF] Pearson | Water sorption and solubility of resin based materials following inadequate polymerization by a visible light curing system[END_REF] New Ormocer-based matrices have been introduced. This is an acronym for 'Organically Modified Ceramics'. They constitute a range of dental RBC which have been available on the market for the past 10 years. [START_REF] Mortensen | Concise encyclopedia of composite materials[END_REF] They were developed after a collaboration between the Fraunhofer ® Institute and the Medical Polytechnic School of Hanover in Germany. While conventional RBC are made of a purely organic matrix, Ormocer-based materials contain polymers at the matrix level, resulting from the addition of small monomer units whose nucleus is essentially composed of silicon atoms combined with silanized inorganic fillers. [START_REF] Kalra | Ormocer: an aesthetic direct restorative material; An in vitro study comparing the marginal sealing ability of organically modified ceramics and a hybrid composite using an Ormocer based bonding agent and a conventional fifth generation bonding agent[END_REF] The Si-O-Si inorganic network is produced by a sol-gel process to produce an inorganic polymer by simple chemical reactions and at a temperature close to room temperature. [START_REF] Moszner | Sol gel materials 2. Light curing dental composites based on Ormocers of cross linking alkoxysilane methacrylates and further nano components[END_REF][START_REF] Xu | Synthesis and shape memory effects of Si O Si cross linked hybrid polyurethanes[END_REF][START_REF] Zhao | Preparation and characterization of polyurethanes with cross linked siloxane in the side chain by sol gel reactions[END_REF] The polysiloxane chains found in Ormocers, made up of a succession of silicon and oxygen molecules, combine with other chains. These chains react during the polymerization process and thus contribute to an organic functionalization of the inorganic network initially formed. [START_REF] Kalra | Ormocer: an aesthetic direct restorative material; An in vitro study comparing the marginal sealing ability of organically modified ceramics and a hybrid composite using an Ormocer based bonding agent and a conventional fifth generation bonding agent[END_REF] In dental RBC, for example, the additional polymer-organic structure is established in the cavity by light-induced radical-based polymerization. [START_REF] Sivakumar | A comparative evaluation of microleakage of restorations using silorane based dental composite and methacrylate based dental composites in Class II cavities: an in vitro study[END_REF] They therefore form cross-linked copolymers in a threedimensional network. The Ormocer matrix already contains a polymer even before the final polymerization. This skeleton formed by silicon dioxide (SiO 2 ) would give the material a low shrinkage compared to dimethacrylate matrices. [START_REF] Civelek | Polymerization shrinkage and microleakage in Class II cavities of various resin compo sites[END_REF] To explain this property, it should be noted that in ceramics, the bonds between the atoms adopt a character qualified as ionocovalent. [START_REF] Noort | Introduction to dental materials[END_REF] The ionic or covalent tendency of an interatomic bond is related to the electronegativity gradient, [START_REF] Dekock | Chemical structure and bonding[END_REF] given that the link will be more ionic if the difference in electronegativity is higher. [START_REF] Dekock | Chemical structure and bonding[END_REF] Silicon dioxide SiO 2 is considered ionic at about 51% so its links have the double ionic-covalent character. [START_REF] Narisawa | Silicone resin applications for ceramic precursors and composites[END_REF] In addition, the abundance of polymerization sites allows Ormocers to crosslink without leaving any residual monomer, thus providing better biocompatibility. [START_REF] Kalra | Ormocer: an aesthetic direct restorative material; An in vitro study comparing the marginal sealing ability of organically modified ceramics and a hybrid composite using an Ormocer based bonding agent and a conventional fifth generation bonding agent[END_REF] Both organic and inorganic fillers are incorporated into this three-dimensional network. In the end, the silicon dioxide skeleton of the Ormocers lends the product its name and explains their relationship to ceramics. Some dental RBC, including several Ormocers, are more translucent to have the ability to polymerize on the 4 mm layer instead of 2 mm. These composites are described as 'bulk-fill'. The photopolymerization light diffuses more deeply, but the range of shade is more restricted to preserve the translucency essential for correct photopolymerization.

The new class of materials, led by Ormocers, is expected to combine the properties of silicone, the strength of organic polymers and the hardness and thermal stability of ceramics. These alternatives are even more interesting as doubts are currently emerging as to the toxicity of bisphenol A, found in the composition of bis-GMA. [START_REF] Bakopoulou | Molecular toxicology of substances released from resin based dental restorative materials[END_REF] Nevertheless, further studies are needed to assess the chemical and physical properties of these materials.

Thanks to a heating-up process called thermogravimetry, which makes it possible to evaluate the degradation kinetics of composites as it would occur under the effect of ageing but at an accelerated rate, several questions can be resolved. Do these organo-mineral matrices have the same thermal stability after maintenance in a wet environment? Is their degradation kinetics the same as that of other organic matrix composites (Bis-GMA, UDMA)? As all these materials are photopolymerizable, does the curing time play an important role in the stability of the material? Having a degradation profile enables the practitioner to anticipate the phenomena of the ageing of materials and to highlight the importance of such clinical implementation.

The objective of this study is therefore to evaluate the thermal stability over time of four dental RBC with different matrices.

Materials and methods

Materials

Four materials were evaluated: a traditional bis-GMA composite resin (Grandioso ® , VOCO, Cuxhaven, Germany), an UDMA composite resin (G-aenial ® , GC, Leuven, Belgium), two Ormocers composites resins including a classic one polymerizable per 2 mm layers (Admira Fusion ® , VOCO, Cuxhaven, Germany) and the other polymerizable on 4 mm layers (Admira Fusion Xtra ® , VOCO, Cuxhaven, Germany).

The samples were constituted under the manufacturer's recommendations, as duplicates and by the same operator. Briefly, silicone cylinders 4 mm in diameter and 2 or 4 mm in height were filled with conventional composites (2 mm cylinders) and mass placement (4 mm cylinders), and then photopolymerized (1300 mW/cm 2 , CELALUX 3, VOCO ® , Cuxhaven, Germany) at 50 and 100% of the time recommended by the manufacturer (Table 1).

To simulate ageing in the buccal environment, half of the samples were placed in a dark place at 37 C for 45 days in sealed compartments containing 2 ml of water.

Thermogravimetric analysis (TGA)

All the RBC produced were subjected to thermogravimetric analysis to measure the loss of mass as a function of temperature. The quantitative determination of the organic and inorganic parts of the RBC was performed with a TGAQ50 (TA Instruments, New Castle, DE) with 25 mg of each sample and the analysis was performed at 30 to 1000 C at 10 C/ min under synthetic air atmosphere. There was no systematization of a blank measurement before measuring a sample, but a temperature calibration based on nickel's Curie Temperature was performed every month on the device.

Results

The TGA and derivative thermogravimetric curves are shown in Figure 1. The slope of the derivative objectifies the speed of mass variation.

Two regions can be clearly identified for the samples of the Bis-GMA-based material, also for those of the Ormocer material, even with or without the ageing process, indicating a two-step degradation process (Table 2). It does not differ if the polymerization time is changed for the Bis-GMA-based material. The aged sample polymerized as recommended by the manufacturer has an offset of the order of þ20 C on the abscissa axis compared to the unaged samples. The weight loss after each step represents the organic part (15%), whereas the inert residues (85%) corresponds to the inorganic part (Table 3). For the UDMA-based material, samples polymerized at 100% of the time indicated by the manufacturer have a common general profile before and after ageing (Table 3). Only the peaks' amplitudes differ, with the peak at 340 C being less pronounced and the peak at 430 C being more pronounced (Figure 1). The differences are even more important when the material is poorly polymerized with a shift in the first peak from 151 C to 178 C and the amplitude inversion between the peaks at 340 C and 430 C.

In the case of the Ormocer material, after ageing, the thermogram shows a peak at 112 C, a double peak close to 400 C, and a shift of about þ10 C on the abscissa axis. With a polymerization time halved, the degradation kinetics of unaged samples show similar areas of stability. After ageing, a peak at 113 C is again found for samples polymerized at 50% of the recommended time.

Finally, all the samples of bulk-fill Ormocer composite resin, polymerized at 100% or 50% of the recommended time, have similar degradation kinetics before or after ageing with peaks at 415 C and 623 C (Figure 1, Table 3). However, there is additional degradation after ageing, meaning that some species degrade before the main degradation for 14% of the material.

Discussion

Our results show good wet stability of polymerized dental RBC according to the manufacturer's instructions, although the UDMA-based materials show more variation. Thermogravimetric analysis is used to measure the change in sample mass as the temperature rises. The use of TGA enables the researcher to monitor the evolution of the processes over time which occurred during the temperature rise and to evaluate whether monomers remain in the polymer. Additionally, it also defines which types of bonds have been established in the sample. [START_REF] Engelhardt | Comparison of flowable bulk fill and flowable resin based composites: an in vitro analysis[END_REF] Thermal degradation concerns the organic part of the RBC so the technique also allows one to determine the proportions of the organic and inorganic parts inside the structure.

In the case of dental RBC, mass loss peaks occur during decomposition, i.e. the depolymerization of chains found in the structure. It is therefore a method for characterizing the degradation and thermal stability of these materials. [START_REF] Sideridou | Physical properties of current dental nanohybrid and nanofill light cured resin composites[END_REF][START_REF] Engelhardt | Comparison of flowable bulk fill and flowable resin based composites: an in vitro analysis[END_REF][START_REF] Bonon | Physicochemical characterization of three fiber reinforced epoxide based composites for dental applications[END_REF] Although it does not allow one to qualify the reactions in detail that occur during the temperature rise, it does allow one to define the degradation kinetics of the evaluated sample. [START_REF] Engelhardt | Comparison of flowable bulk fill and flowable resin based composites: an in vitro analysis[END_REF] A number of biases, such as the temperature rise of the balance itself or the convection and turbulence effects caused by gas flow, are inherent in the physical processes involved in thermogravimetric analysis and must be taken into account when interpreting the results. The importance of these biases was limited by the use of a double-beam balance in our study; the use of a blank measurement before sample measurement was not performed in parallel because an instrument calibration is already carried out each month. In addition, the analysis software used (Universal V4, 1D TA Instruments, New Castle, DE) applies a correction to the recorded values to take into account the effects mentioned above.

Indeed, the application of thermogravimetric analysis to dental RBC, that are more or less polymerized and have undergone artificial ageing or not, enables the researcher to compare the differences in their structural stability and to relate them to their internal arrangement. In 2011, Sideridou et al. established a link between the high aromatic ring content of a bis-GMA-based material and its one-peak degradation curve, compared to a two-step degradation of a composite containing UDMA and fewer cycles. [START_REF] Sideridou | Physical properties of current dental nanohybrid and nanofill light cured resin composites[END_REF] Recently, Nascimento et al. have also described the organic and inorganic parts of several dental RBC. [START_REF] Nascimento | Physicomechanical and thermal analysis of bulk fill and conventional composites[END_REF] Our results are concordant with theirs for the same Ormocer bulk-fill composite as they have obtained 87% of residues after the thermogravimetric analysis. Concerning the other types of matrices, they have shown a range of residues comprised between 60.1% (for a UDMA-based biomaterial) and 84.2% (for a Bis-GMAbased biomaterial). [START_REF] Nascimento | Physicomechanical and thermal analysis of bulk fill and conventional composites[END_REF] Behaviour in a wet environment Bis-GMA-based composite resins and Ormocer materials have similar curves before and after soaking in a humid atmosphere. On the other hand, the curves of the UDMA composite resin are different before and after water ageing, meaning that water imbibition has modified the structure of the RBC and its degradation. It has already been shown that there are variations in absorption between the different polymers by differential penetration of water into the polymer network through porosities and intermolecular spaces, [START_REF] Ferracane | In vitro aging of dental composites in water effect of degree of conversion, filler volume, and filler/matrix coupling[END_REF][START_REF] Venz | NIR spectroscopic investigation of water sorption characteristics of dental resins and composites[END_REF] this phenomenon being classically measured higher for bis-GMA-based resins than for urethane-based resins. [START_REF] Sideridou | Sorption of water, ethanol or ethanol/water solutions by light cured dental dimethacrylate resins[END_REF] However, our results show an opposite effect with less degradation of bis-GMA materials in the presence of water.

The analysis of the results obtained for Ormocer composite resins before ageing, for which the polymerization time was respected, gives an overview of the expected profile of the thermograms for the two Ormocers materials. For the one incremented on 2 mm layers as was the case for bulk, a main peak at 430 C is observed, corresponding (according to some researchers) to the breaking of siloxane links. [START_REF] Tiwari | Deciphering the inert atmosphere degradation patterns in hybrid silicones[END_REF] Once maintained at 37 C for 45 days, the same behaviour is observed in both types of composite, i.e. degradation of the material at a higher temperature, indicating a gain in stability. There is also a peak between 150 and 200 . This peak can correspond either to entities that have not polymerized or to soft silicone bonds formed over time. [START_REF] Tiwari | Deciphering the inert atmosphere degradation patterns in hybrid silicones[END_REF] In addition, if after optimal polymerization the aged Ormocer bulk-fill material keeps a single degradation peak at 430 C, the other deteriorates over the same temperature range into two peaks. This would indicate a change in the quality of the inorganic bond or a structural arrangement of silicones during ageing process which, being less stable, become degraded at this peak.

Changes in polymerization time and wet atmosphere

After ageing, the Ormocer material photopolymerized on 2 mm layers with a polymerization time reduced by half presents a new peak of 113 C compared to the reference time, which indicates that when polymerized at a duration below optimal, it has a higher tendency toward degradation. This structural difference is also found in well polymerized but aged samples of this material: at Day 0 (D-0) a single peak is present at 400 C, while a second peak is noted for samples that have been stored in a humid atmosphere at 37 C for 45 days. When the difference between these two peaks is increased for aged samples, the first peak recorded between 78 C and 160 C occurs at a higher temperature (113 C compared to 160 C for polymerization times halved and complete respectively). This RBC thus acquires different conformations depending on the duration of polymerization and this considerably influences the structure it adopts during its ageing. It is therefore sensitive to complete respect of the optimal curing time.

When not polymerized to optimal duration, the Ormocer bulk-fill material exhibits an early peak at 74 before ageing. The latter part of its degradation is done in a similar way regardless of the polymerization time. The presence of early peaks during incomplete photopolymerizations indicates that the material still owns non-polymerized monomers in its structure, even if this composite resin presents a structure relatively similar whether it is well polymerized or not. After ageing, the similar appearance of the curves with those obtained on unsoaked samples indicates a similar intrinsic structure between these two samples and reflects a high tolerance of the material for an application under a polymerization time shorter than the optimal time. Indeed, if initially concrete differences are noted for a shorter photopolymerization on the day of the implementation (D-0), the soaking at 37 C during 45 days tends to give the material a conformation similar to that of the complete photopolymerization at D-0 time.

For fully polymerized aged samples, the occurrence of a new early peak at 202 C, which integrates into a curve similar to that observed without ageing, suggests the formation over time of soft silicone bonds, following a reorganization of the siliceous infrastructure. The hypothesis of this restructuring also enables one to explain the plot of the other two peaks. These occur at almost identical temperatures and amplitudes before and after ageing. It would therefore mean that Ormocer bulk-fill material is more permissive with little deviations in the polymerization time, leading to modifications at D-0 but ultimately leading to a comparable conformation after ageing.

The bis-GMA-based composite resin, completely photopolymerized, has a two-phase degradation kinetics (Figure 1), the second phase corresponding to the degradation of acrylate bonds. After ageing, the temperature shift on the curve indicates that the different chains initially noted are still present but that they take longer to degrade (Figure 1). The appearance of a new peak at 135 C reflects a change in the internal structure of the material. Since this peak does not occur when the photopolymerization time is halved, it would not correspond to non-polymerized monomers but rather to low energy bonds. If they were non-polymerized monomers, they would have been detected even at a reduced photopolymerization time. Thus, they are the result of reconfiguration of the links within the matrix, occurring in all samples of this composite during their ageing, as this peak appears at Day 45 between 135 C and 179 C whether the photopolymerization is complete or halved. The variation in the photopolymerization time has no influence on the samples at D-0 but modifies the degradation kinetics after ageing.

For the UDMA material, the degradation of unaged samples correctly photopolymerized has a four-step sequence. This brings it closer to the bis-GMA material, although the UDMA matrix material is distinguished by the occurrence of a peak at a very high temperature (868 C). It could indicate the presence of stronger bonds within its organic network. After ageing, the thermogram amplitude around 430 C is increased while the rest of its curve remains identical. A nonoptimal polymerization time results in a new peak at D-0 between 150 C and 175 C and after ageing in an increased peak at 429 C.

Thus, even if many DTG curves are similar within the same composite resin at different polymerization times or at different pre-and post-ageing times, it is rare to observe a common kinetics between two different composites. This highlights the differences between the matrices and therefore the different RBC. Their matrix networks are complex and so do the connections it exists inside. The great heterogeneity of the kinetics observed in the various samples of Ormocer materials confirms a multiplicity of bond types that the material establishes during the ageing process. It also depends on the strict respect of the photopolymerization time. The similar kinetics of the Ormocer bulk-fill material in all tested situations is proof of its stability, whether regarding the photopolymerization time or the ageing process. This may be related to the proportion taken by the polymerization initiators in its composition. Conventional RBC maintain a high degree of structural regularity despite the ageing they undergo or the polymerization variations to which they have been subjected.

Conclusion

Under optimal conditions, the two tested Ormocer materials degrade in two phases during the TGA. The one incremented on 2 mm layers is very sensitive to the respect of the polymerization time, which is not the case for the bulk-fill which also resists ageing very well. The UDMA-based composite tested has a poor initial tolerance to the variation in polymerization time, whereas the bis-GMA-based composite does not. These two show good behaviour after soaking for 45 days.

It therefore seems that Ormocer RBC with mass placement have ageing properties that can compete with those of conventional composites whereas those incremented on 2 mm layers are more sensitive to the time necessary for polymerization.

This study shows a very heterogeneous behaviour between the different materials and therefore the need to follow very precisely the operating procedures.

Table 1 .Figure 1 .

 11 Photopolymerization time related to manufacturer's instructions

Table 2 .

 2 Areas of degradation for the different materials. Peaks are noted in bold.

	Before ageing	After ageing

Table 3 .

 3 Relative proportions of organic and inorganic phases of materials.

	Materials

Acknowledgements

The authors really thank James Robert Bottriell and Dr. Francis Collins for providing language and content corrections.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Thibault Canceill https://orcid.org/0000 0002 0049 7622

Author contributions

TC, PP, JD and SJ contributed equally to this work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.


Antoine Casadebaigt
email: antoine.casadebaigt@hotmail.fr

Jonathan Hugues

Daniel Monceau
email: daniel.monceau@ensiacet.fr

Cirimat

High temperature oxidation and embrittlement at 500-600 °C of Ti-6Al-4V alloy fabricated by Laser and Electron Beam Melting

Keywords:

The oxidation kinetics of Ti-6Al-4V alloys fabricated by LBM and EBM, HIP-treated or not, are similar to that of a rolled annealed Ti-6Al-4V alloy, as regards oxide layer and oxygen diffusion layer thickness. Kinetics of oxygen ingress in the metal is independent of the alloy microstructure. At room temperature, the thickness of the brittle layer after a tensile test corresponds to the oxygen diffusion layer, with an oxygen concentration above a critical value that is clearly below 1 at %.

Introduction

Titanium alloys

Titanium alloys are used in aeronautics and in the aerospace industry because of their very high specific mechanical properties and their corrosion resistance at low temperature. Since the 1950s the quantity of titanium alloys in aircrafts has been continuously increasing. Moreover, the operating temperature of engines also increases constantly to improve efficiency and reduce fuel consumption. However, depending on the applications, titanium alloys are limited to temperatures below 600 °C, e.g. 300-400 °C for Ti-6Al-4V (wt.%) [START_REF] Boyer | Titanium for aerospace: rationale and applications[END_REF][START_REF] Boyer | An overview on the use of titanium in the aerospace industry[END_REF][START_REF] Boyer | Titanium airframe applications: brief history, present applications and future trends[END_REF][START_REF] Peters | Titanium alloys for aerospace applications[END_REF]. This limitation is mainly due to insufficient creep resistance at high temperature [START_REF] Badea | Creep behavior of Ti-6Al-4V from 450°C to 600[END_REF] and to oxygen embrittlement resulting from the high temperature oxidation. In high temperature environments, titanium alloys form an oxide layer (OL) [START_REF] Gemelli | Oxidation kinetics of commercially pure titanium[END_REF][START_REF] Champin | Oxydation des alliages de titane au voisinage des températures d'utilisation dans les turbomoteurs[END_REF][START_REF] Du | Air oxidation behavior of Ti-6al-4v alloy between 650-degrees-C and 850-degrees-C[END_REF]. For Ti, Zr or Hf alloys however, the high solubility of oxygen in the metal induces the formation of an oxygen diffusion zone (ODZ) [START_REF] Stringer | The oxidation of titanium in oxygen at high temperatures[END_REF][START_REF] Kofstad | Oxidation of titanium in the temperature range 800-1200[END_REF][START_REF] Kofstad | High Temperature Oxidation of Metals[END_REF][START_REF] Chaze | Influence of aluminium on the oxidation of titanium between 550 and 750°C[END_REF][START_REF] Frangini | Various aspects of the air oxidation behaviour of a Ti6Al4V alloy at temperatures in the range 600-700 °C[END_REF][START_REF] Güleryüz | Effect of thermal oxidation on corrosion and corrosion-wear behaviour of a Ti-6Al-4V alloy[END_REF][START_REF] Gaddam | Study of alpha-case depth in Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V[END_REF][START_REF] Gaddam | Influence of alpha-case layer on the low cycle fatigue properties of Ti-6Al-2Sn-4Zr-2Mo alloy[END_REF][START_REF] Brice | Oxidation behavior and microstructural decomposition of Ti-6Al-4V and Ti-6Al-4V-1B sheet[END_REF][START_REF] Li | Study of Ti-6Al-4V alloy spectral emissivity characteristics during thermal oxidation process[END_REF][START_REF] Maeda | Experimental and theoretical study of the effect of Si on the oxidative behavior of Ti-6Al-4V alloys[END_REF], which can be more detrimental than the external oxide scale. This ODZ affects mechanical properties: tensile strength increases while ductility decreases [START_REF] Finlay | Effects of three interstitial solutes (Nitrogen, Oxygen and Carbon) on the mechanical properties of high-purity alpha titanium[END_REF][START_REF] Kofstad | Investigation on the oxidation mechanism of titanium[END_REF][START_REF] Shamblen | Air Contamination and Embrittlement of Titanium Alloys, Science, Technology, and Application of Titanium[END_REF][START_REF] Bendersky | The effect of exposure on fracture of Ti-6AL-4V alloy[END_REF][START_REF] Shenoy | Oxidation and embrittlement of Ti-6Al-2Sn-4Zr-2Mo alloy[END_REF][START_REF] Fukai | The effects of the oxygen-enriched surface layer on mechanical properties of alpha+beta type titanium alloys[END_REF][START_REF] Kakehi | Influence of oxidation on mechanical properties of the heat-resistant titanium alloy DAT54[END_REF]. Ti-6Al-4V alloy is one of the oldest titanium alloys, it accounts for nearly 60 % of titanium alloys produced [START_REF] Boyer | An overview on the use of titanium in the aerospace industry[END_REF] and is frequently used in structural aircraft and jet-engine components [START_REF] Lütjering | Titanium[END_REF].

Additive manufacturing of Ti-6Al-4V

Additive manufacturing (AM) processes are new processes giving the opportunity of a technological leap in the manufacturing of metal parts. In the fields of aeronautics and aerospace, AM processes are a means of reducing part weight and drastically reducing wasted matter, thereby inducing cost savings. Powder bed fusion processes such as Laser Beam Melting (LBM) and Electron Beam Melting (EBM) are processes that have been studied since their development in the 1980s. Both processes use a heating source to selectively melt a powder bed and build the final part layer by layer. LBM uses a laser beam as heating source, whereas EBM uses an electron beam. One of the main differences between the two processes is the pre-heating step of EBM, the powder is maintained at approximately 700 °C to avoid sputtering. Another difference is the atmosphere in the chamber. LBM is done under argon, whereas a vacuum atmosphere is used in EBM [START_REF] Sing | Laser and electron-beam powder-bed additive manufacturing of metallic implants: a review on processes, materials and designs[END_REF].

These layer-by-layer processes make it possible to build parts with complex geometry which were impossible to build by using conventional processes such as forging or casting [START_REF] Beaman | Solid Freeform Fabrication: A New Direction in Manufacturing[END_REF]. However, even when parameters are optimized, AM processes may induce defects such as lack of fusion or gas pores. Hot isostatic pressing (HIP) treatment is a heat treatment performed under high pressure, about several hundreds of bars, that enables to reduce residual porosity and improve structural integrity [START_REF] Al-Bermani | The origin of microstructural diversity, texture, and mechanical properties in electron beam melted Ti-6Al-4V, Metall[END_REF].

For the Ti-6Al-4V alloy fabricated by LBM, the high cooling rate

Microstructure and surface characterisations

Five samples of 15 × 10 × 2 mm 3 were used to study the influence of processing and HIP treatment on microstructure. Two samples were fabricated by LBM, one of which was HIP-treated. Two samples were fabricated by EBM, one of which was HIP-treated, and the last sample was rolled β-annealed. The chemical composition of as-built LBM and as-built EBM Ti-6Al-4V alloys are presented in Table 1. The aluminium loss amounts to approximately 1 at.% and almost no change in C, H, O and N concentrations was detected for both LBM and EBM processes. The volatilisation of aluminium is more important for the EBM sample. Juechter et al. studied the evaporation phenomenon during EBM processing of Ti-6Al-4V alloys and showed that aluminium was the most prone to evaporate [START_REF] Juechter | Processing window and evaporation phenomena for Ti-6Al-4V produced by selective electron beam melting[END_REF]. Rolled β-annealed samples show a chemical composition that is similar to additively manufactured samples except for oxygen, whose concentration is twice as high.

Microstructure characterisation was performed with a Nikon Eclipse Optical Microscope (OM) after samples were ground with P2400 -SiC abrasive paper, polished with a SiO 2 solution and etched with Kroll reagent for 5 s. Finer observations were obtained using a Scanning Electron Microscopes (SEM) FEI Quanta 450 and LEO 435 VP at 15 kV. The chemical composition was measured with instrumental gas analysis (IGA) and inductively coupled plasma-optical emission spectrometry (ICP-OES) at EAG, France. Phase identification was realised by X-ray diffraction (XRD) at room temperature with a Bruker D8-2 using Cu-Kα radiation in the 2Θ range from 20° to 120° with a step size of 0.02°, a scan step time of 8 s and spinning on samples ground with P1200 -SiC abrasive paper.

In order to study the roughness induced by LBM and EBM processing, as-built samples were characterized and compared to an LBM sample ground with P600 abrasive paper. Roughness was characterized by confocal microscopy. A Zeiss Axio Imager Z2m microscope equipped with a Zeiss LSM800 module was used. For each sample, a square area of 1.6 mm sides was analysed. Results were analysed with ConfoMap ST software, which consisted in removing the abnormal points, filling the holes and adding a 19 × 19 median filter.

Oxidation experiments

In order to study the influence of surface roughness and alloy microstructure on oxidation behaviour, 39 samples were fabricated by LBM, 39 samples by EBM and one sample was rolled and β-annealed. LBM and EBM samples were 28 × 28 × 2 mm 3 and the rolled β-annealed sample was 15 × 10 × 2 mm 3 . Six LBM (As-built LBM) and six EBM (As-built EBM) samples were kept as-built. 19 LBM (LBM P600), 19 EBM (EBM P600) and one rolled β-annealed (rolled P600) samples were ground with P600 -SiC abrasive paper. 14 LBM (LBM HIP P600) and 14 EBM (EBM HIP P600) samples were HIP-treated and ground with P600 -SiC abrasive paper. Grinding was carried out so as to assess the influence of microstructure on oxidation behaviour, independently of the effect of surface roughness of as-built samples. Each sample was cleaned in acetone and then in ethanol in an ultrasonic bath for 1 min prior to oxidation.

Samples were inserted in a pre-heated Carbolite furnace LHT 6/60 with forced convection. An air velocity of about 1 m•s -1 was measured induces a non-equilibrium microstructure. The microstructure of LBM Ti-6Al-4V alloy is entirely composed of acicular martensitic α' phase with prior-β grains elongated in the building direction [START_REF] Murr | Microstructure and mechanical behavior of Ti-6Al-4V produced by rapid-layer manufacturing, for biomedical applications[END_REF][START_REF] Simonelli | Further Understanding of Ti-6Al-4V Selective Laser Melting Using Texture Analysis[END_REF]. Due to the pre-heating step of the EBM process, the microstructure of EBM Ti-6Al-4V is composed of thin lamellar α and β phase with prior-β grains elongated in the building direction [START_REF] Koike | Evaluation of titanium alloy fabricated using electron beam melting system for dental applications[END_REF][START_REF] Murr | Fabrication of metal and alloy components by additive manufacturing: examples of 3D materials science[END_REF][START_REF] Scharowsky | Influence of the scanning strategy on the microstructure and mechanical properties in selective Electron beam melting of Ti-6Al-4V[END_REF]. The HIP treatment of Ti-6Al-4V is usually performed at 920 °C during 2 h at 1000 bars. For EBM Ti-6Al-4V alloy, HIP induces a coarsening of the microstructure, whereas for LBM, it induces the decomposition of the α' martensite into α and β phase [START_REF] Al-Bermani | The origin of microstructural diversity, texture, and mechanical properties in electron beam melted Ti-6Al-4V, Metall[END_REF][START_REF] Qiu | Microstructure and tensile properties of selectively laser-melted and of HIPed laser-melted Ti-6Al-4V[END_REF].

High temperature oxidation of additively manufactured Ti-alloys

As seen above, most literature papers on Ti-6Al-4V alloy fabricated by AM focused on the influence of process parameters on microstructure, defects or mechanical properties. Up to now, only very few studies have focused on the oxidation of titanium alloys fabricated by additive manufacturing. Among them, Karlsson et [START_REF] Terner | Initial oxidation behavior in air of TiAl-2Nb and TiAl-8Nb alloys produced by electron beam melting[END_REF][START_REF] Som | Thermally oxidized electron beam melted γ-TiAl: in vitro wear, corrosion, and biocompatibility properties[END_REF][START_REF] Wang | Characterization of wire arc additively manufactured titanium aluminide functionally graded material: microstructure, mechanical properties and oxidation behaviour[END_REF]. Zhou et al. published the first study on the oxidation of a titanium alloy (Ti-5.5Al-3.4Sn-3.0Zr-0.7Mo-0.3Si-0.4Nb-0.35Ta wt.%) fabricated by LBM [START_REF] Zhou | A novel titanium alloy manufactured by selective laser melting: microstructure, high temperature oxidation resistance[END_REF]. More recently, Liang et al. published a study investigating the oxidation behaviour of Ti-6Al-4V alloy fabricated by Selective Laser Sintering (SLS) [START_REF] Liang | High-temperature oxidation behavior of the Ti-6Al-4V alloy manufactured by selective laser sintering[END_REF].

LBM and EBM processes induce a rough surface state presenting partially-melted powder particles. Following the approach developed by Sanviemvongsak et al. [START_REF] Sanviemvongsak | High temperature oxidation of IN 718 manufactured by laser beam melting and electron beam melting: effect of surface topography[END_REF] for alloy 718, Casadebaigt et al. studied the influence of LBM-and EBM-induced roughness on the oxidation behaviour of Ti-6Al-4V alloy [START_REF] Casadebaigt | Influence of microstructure and surface roughness on oxidation kinetics at 500-600 °C of Ti-6Al-4V alloy fabricated by additive manufacturing[END_REF]. In this last paper, the effect of sample roughness, but also the effect of the presence of unmelted powder particles at the surface of the samples, were taken into account to quantify oxidation kinetics. This paper showed that oxidation kinetics of additively manufactured samples, compared to manufactured samples, are similar in terms of oxide layer and oxygen diffusion zone thicknesses. It also showed that the higher net mass change per unit area found for additively manufactured samples is due to the combined action of the samples' higher surface area and of fast oxidation kinetics due to the cracking of partially melted powder grains attached to the sample's surface [START_REF] Casadebaigt | Influence of microstructure and surface roughness on oxidation kinetics at 500-600 °C of Ti-6Al-4V alloy fabricated by additive manufacturing[END_REF].

The present paper follows on from these previous studies and focuses on the influence of the specific surface roughness and microstructure of Ti-6Al-4V titanium alloy, fabricated by LBM and EBM, on its oxidation behaviour. These results are compared to those obtained on a conventionally rolled Ti-6Al-4V alloy. In addition, the influence of HIP treatment on microstructure and oxidation behaviour was studied. The consequences of oxidation on the tensile properties of LBM and EBM Ti-6Al-4V alloys were also assessed.

Materials and experimental procedures

Materials

Ti-6Al-4V alloy samples were fabricated by Laser Beam Melting and Electron Beam Melting. LBM samples were fabricated using an EOS M280 machine equipped with an Yb-fibre laser, at FUSIA, France. Fabrication was made using standard EOS parameters for Ti-6Al-4V alloys to obtain fully dense parts. To limit thermal oxidation, LBM processing was carried out under Ar atmosphere with concentrations below 10 ppm of N 2 , 3.0 ppm of O 2 and 1 ppm of H 2 O. Plasma atomised Ti-6Al-4V grade 23 powder supplied by AP&C, with a particle size of 15-45 μm, was used. The chemical composition of the powder is presented in Table 1. EBM samples were fabricated using an Arcam Q20+ machine equipped with a LaB 6 cathode, at the IRT Saint Exupéry in the cold furnace, prior to heating, with a RS-90 Mini Anemometer. For hanging purposes, all samples were drilled with 1.5 mm diameter hole in order to have the highest contact-free area. An opening was present in the top wall of the furnace for atmosphere renewal. Isothermal oxidation treatments were carried out at 500, 538 and 600 °C, for different durations ranging from 25 to 5 000 h. Sample mass were measured before and after oxidation using an electronic balance Sartorius LA 75 3200D with an accuracy of ± 20 μg. Net mass change per unit area (mass gain) values were averaged on three to five measurements for each oxidation test.

Oxide layer characterisation

The oxide Layer (OL) thickness of samples oxidized at 600 °C was measured using ImageJ. Cross sections, ground with P2400 -SiC abrasive paper and polished with a SiO 2 solution were prepared, and optical microscopies were carried out with a Nikon Eclipse OM. For each sample, 5-10 images were taken and analysed. The image magnification depended on the thickness of the OL. OL thickness was determined by averaging 30-150 measures. For the thinnest OL, cross sections were obtained using a Focused Ion Beam (FIB) at the R. Castaing Microanalysis Centre in Toulouse, France with a SEM/FIB HELIOS 600i. OL thicknesses were determined from SEM images by averaging 10-15 measures.

OL nature was characterized by XRD at room temperature with a Bruker RX D8 GIXRD using Cu-Kα radiation in the 2Θ range from 20° to 80° with a step size of 0.02° and a scan step time of 2 s. In addition, an EDS analysis was performed with a FEI Quanta 450 SEM on the cross section of oxidized samples ground with P2400 -SiC abrasive paper and polished with SiO 2 solution.

Oxygen diffusion zone characterisation by EPMA

The Oxygen Diffusion Zone (ODZ) was characterized by Electron Probe Microanalyzer (EPMA) at R. Castaing Microanalysis Centre in Toulouse, France, with a CAMECA SX Five FE operating at 15 kV and 20 nA. Oxygen profiles were performed with a machine error of 3 000 wt. ppm on oxygen concentration measurement. For each sample characterized, 3-6 profiles were performed. Due to the presence of a native OL on the surface of the freshly polished cross section, the oxygen signal was biased and had to be corrected. For each profile, three count values were measured in the bulk to determine the count value corresponding to the native OL. The mean count value of the bulk was brought back to zero because the machine error of measured points is six times higher than the oxygen concentration in the bulk. Then, oxygen concentrations in at.% and wt.% were calculated.

Oxygen diffusion zone characterisation by SIMS

The following samples, all oxidised at 500 °C were analysed by Secondary Ion Mass Spectroscopy (SIMS) : EBM samples oxidised for 100 h and 500 h, and an LBM sample oxidised 500 h. All these samples were ground with P600 SiC paper before oxidation. The depth profiles were obtained through the external oxide scale and in the oxygen diffusion zone below the scale, down through where the oxygen intensity remains constant.

Intensity-depth profiles were measured by SIMS using a Cameca IMS 7f instrument. A Cs + source with a primary current of 40 nA was used in order to reduce the matrix effect. The raster area was 125 × 125 μm², and the analysis area 33 × 33 μm². The intensities recorded corresponded to the following mass: 133 Cs, 27 Al 133 Cs, 48 16 O 133 Cs 2 . Oxygen concentration profiles were assumed to be given by the following normalisation: the intensity of the signal 16 O 133 Cs 2 was divided by the average intensity of the same signal found in the core of the sample and multiplied by the oxygen concentration in the core of the sample. The oxygen concentration in the core of the sample was measured by IGA and is given in Table 1. This normalisation procedure led to an oxygen concentration below the oxide scales lying between 20 at.% and 30 at. %. The position of the metal/oxide interface was chosen close to the inflexion point of the oxygen concentration profile, where the concentration was close to 25 at.% which was consistent with EPMA measurements.

All sputtering craters were analysed by contact profilometry with a Bruker Dektak 8 instrument. This measurement was used to calculate the depth of the SIMS analysis from the duration of the sputtering. Average crater depth was about 10 μm with a roughness of about +/-0.5 μm at the bottom of the crater. The average oxide thickness was about 70 nm. Depth measurement uncertainty leads to a 15 % error on the determination of D.

Tensile tests

Tensile tests were performed on a MTS2 Criterion at MIDIVAL using a high temperature Epsilon extensometer of 20 mm working distance to measure displacement. Tensile specimens had a gauge area of 30 × 5 mm 2 , a thickness of 1.3 mm and were ground with P600 -SiC abrasive paper. In order to investigate the influence of oxidation on tensile properties, samples were pre-oxidized at 600 °C during 500 h in the Carbolite furnace LHT6/60 before running tensile tests. Rupture surfaces were observed using LEO 435 VP SEM. ODZ thickness was determined by measuring the distance between two points in order to draw a line perpendicularly to the metal/oxide interface. For both oxidized samples, the average value of ODZ thickness was obtained from 40 to 80 measures.

Results

Microstructures

Fig. 1 shows the microstructure of Ti-6Al-4V titanium alloys in five metallurgical states observed using OM and SEM. The rolled β-annealed sample presents large prior-β grains without any preferential grain orientation, whereas as-built LBM and as-built EBM samples both have prior-β grains elongated along the building direction. HIP treatment partly "erases" prior-β grains in HIP-treated LBM samples whereas in HIP-treated EBM samples, the solidification structure is still clearly visible. SEM observations reveal that rolled β-annealed microstructure is composed of α and β phase, similarly to as-built EBM microstructure but with α-laths being about 1 μm thick, as shown in Table 2. As-built LBM microstructure is composed of fully acicular martensitic α'-laths, with a maximum thickness of 0.9 μm, as reported in Table 2. HIP treatment on as-built LBM samples induced the transformation of α'laths into α and β phase. In addition, HIP treatment induced a large coarsening of α-laths in both as-built LBM and as-built EBM samples, as presented in Table 2. After the HIP treatment, the microstructures of LBM and EBM samples are similar. Fig. 2 shows the X-ray diffractograms obtained for the different Ti-6Al-4V alloys, i.e. rolled β-annealed, as-built LBM, as-built EBM, HIPtreated LBM and HIP-treated EBM. XRD of as-built LBM indicates the presence of an hexagonal phase only, corresponding to α' martensite. XRD analyses of as-built EBM and HIP-treated LBM/EBM samples show peaks corresponding to both α and β phases. In the XRD diffractogram of rolled β-annealed the α peaks appear clearly but the main peak corresponding to the (1 1 0) planes of the β phase of Ti-6Al-4V at 39.5° does not appear. However, a small peak corresponding to the (2 0 0) planes of the β phase appears at 57°. The intensity of β peaks is rather low because the volume fraction of β phase is small. In addition, in the diffractogram of the as-built EBM sample, the (1 1 0) β peak around 39.5° is not well separated from the (1 0 1 1) α peak around 40.5°. Table 3 presents the lattice parameters of each sample estimated from XRD. All five samples have similar α-phase lattice parameters. The β-

Table 2

Thicknesses of α-laths of rolled, LBM and EBM Ti-6Al-4V alloys.

Rolled β-annealed

As-built LBM (α' laths) As-built EBM LBM HIP treated EBM HIP treated α-laths thickness (μm) 1.0 ± 0.3 < 0.9 0.7 ± 0.2 2.1 ± 0.7 2.5 ± 0.4 LBM P600 oxidized at 600 °C was due to its metastable microstructure. LBM HIP P600 and EBM HIP samples have similar microstructures. Despite differences in chemical composition they present similar oxidation kinetics. The influence of slight variations in chemical composition on oxidation kinetics is therefore excluded. EBM P600 and EBM HIP P600 results summarized in Table 6 show that despite the fact that the HIP-treated sample has a microstructure twice as coarse and a lower density of α-lath interfaces, oxidation kinetics as well as OL and ODZ thicknesses are similar. These results confirmed that the diffusion of oxygen in Ti-6Al-4V is not controlled by the diffusion of oxygen at the α-lath interfaces, contrary to what Sugahara et al. explained in a previous study [START_REF] Sugahara | The effect of Widmanstatten and equiaxed microstructures of Ti-6Al-4V on the oxidation rate and creep behavior[END_REF]. This finding is also in contradiction with Leyens et al. and Sai Srinadh et al. who found that the oxidation kinetics of IMI 834 and TIMETAL 1100 changed depending on microstructure [START_REF] Leyens | Influence of microstructure on oxidation behaviour of near-alpha titanium alloys[END_REF][START_REF] Sai Srinadh | Oxidation behaviour of the near alpha-titanium alloy IMI 834[END_REF].

Oxygen diffusion zone and O diffusion coefficient

New data for low temperature oxidation kinetics and oxygen diffusion

Fig. 6 shows that at 600 °C, the oxidation kinetics of additively manufactured LBM and EBM samples is similar to that of the rolled β annealed Ti-6Al-4V alloy used in this study, and also to that of the different conventionally processed Ti-6Al-4V alloys from the literature. Very few literature data exist regarding temperatures below 600 °C. At 500 °C, we found that oxidation kinetics are slightly lower than the extrapolation from high temperature literature data. This can be explained by the formation of a less-protective OL at oxidation temperatures above 600 °C. Based on literature results, it is a well-known fact that the OL cracks and loses its cohesion with the metal at the metal/ oxide interface when it reaches a critical thickness [START_REF] Du | Air oxidation behavior of Ti-6al-4v alloy between 650-degrees-C and 850-degrees-C[END_REF][START_REF] Stringer | The oxidation of titanium in oxygen at high temperatures[END_REF][START_REF] Sarrazin | Oxidation of titanium-alloy Ta6v4 by dry oxygen between 700 degrees centigrade and 1000 degrees centigrade. 1. Experimental results[END_REF][START_REF] Motte | Comparative-study of oxidation with water-vapor of pure titanium and of Ti-6al-4v[END_REF][START_REF] Coddet | Essai d'interprétation du mécanisme d'oxydation de l'alliage de titane TA6V4 par l'oxygène sec entre 700 °C et 1000 °C[END_REF]. It seems this critical thickness was not reached for the oxidations performed at 500 and 538 °C studied here. According to literature, the delamination of the OL is thought to be induced by the presence of internal stresses due to the large molar volume difference between the metal and the oxide [START_REF] Stringer | Stress generation and relief in growing oxide films[END_REF][START_REF] Philibert | L'oxydation à chaud des métaux[END_REF]. Stringer explained that nanometric pores form inside the OL during its growth. These pores become larger as the OL layer thickens and internal stresses induce the breakdown of the OL [START_REF] Stringer | The oxidation of titanium in oxygen at high temperatures[END_REF]. This phenomena is clearly depicted in Fig. 17 with the delamination appearing where pores weaken the integrity of the OL.

Table 6 presents the ratio of ODZ thickness to OL thickness for samples oxidized 500 h at 600 °C and 2000 h at 500 °C. For both oxidation conditions, the ratio is similar for LBM P600 and EBM P600 samples, but it strongly depends on temperature. Indeed, after an oxidation of 500 h at 600 °C, ODZ thickness is about 3 times higher than OL thickness, whereas after 2000 h of oxidation at 500 °C, ODZ thickness is about 50 times higher than OL thickness. For all samples, Table 6 also shows which proportion of the mass gain is due to the oxide growth and which is due to the oxygen dissolution in the metal. These proportions were calculated using two methods. The first one is based on the total mass gain and on OL thickness measures with the hypothesis of a fully dense oxide. The second method consists in integrating EPMA concentration profiles with the hypothesis that the effect of volume expansion on alloy density, due to the oxygen ingress in the metal, is negligible. After 500 h of oxidation at 600 °C, the OL is thick and accounts for most of the mass gain (≈ 85 %). Whereas after 2000 h of oxidation at 500 °C, the OL is very thin and the mass gain is mainly due to the ODZ. At 500 °C, the oxide growth accounts for only 15 % of the mass gain, and most of the oxygen uptake is due to its dissolution into the metal.

Most of the oxidation studies on Ti-6Al-4V alloy reported in the literature were performed at temperatures above 600 °C, as seen in Fig. 6. The Ti-6Al-2Sn-4Zr-2Mo (Ti6242) alloy is a titanium near-α alloy used at temperatures up to 600 °C. Gaddam et al. studied both Ti-6Al-4V and Ti6242 alloys and observed that, at 593 °C, the latter alloy had much lower oxidation kinetics -in terms of mass gain -than the former [START_REF] Gaddam | Study of alpha-case depth in Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V[END_REF]. For 200 h at 593 °C, Ti-6Al-4V had a mass gain of about 0.8 mg•cm -2 whereas Ti6242 had a mass gain of about 0.2 mg•cm -2 .

Table 6

Net mass change per unit area proportion due to oxide layer (OL) growth and due to dissolved oxygen in the metal (ODZ). (* calculated from OL thickness measurements; ** calculated from EPMA measurements). However, in their study Gaddam et al. also showed that for 500 h at 593 °C Ti6242 and Ti-6Al-4V alloys have the same "alpha-case" of 30 μm. It seems that, at 593 °C, the oxidation kinetics of Ti-6Al-4V is higher than the oxidation kinetics of Ti6242 only because the oxide scale grows faster. As mentioned previously, the higher OL thickness on Ti-6Al-4V at 593 °C is probably due to the presence of numerous defects in the oxide scale and to its delamination, which increases its formation rate. For oxidation tests carried out for 2000 h at 500 °C, the critical thickness was not reached and the formation rate of the OL is indeed relatively low, as shown by the thin OL thicknesses presented in Table 6. Moreover, by comparing EPMA-measured ODZ thickness of samples oxidized 2000 h at 500 °C in this study with the ODZ thickness, measured by optical microscopy and EPMA, of the Ti6242 alloy, with different oxygen concentration in the bulk, oxidized 500 h at 500 °C from Gaddam et al. [START_REF] Gaddam | Influence of alpha-case layer on the low cycle fatigue properties of Ti-6Al-2Sn-4Zr-2Mo alloy[END_REF], it can be seen that Ti-6Al-4V and Ti6242 alloys both have quite similar ODZ thicknesses.

ODZ and embrittlement

Fig. 13 shows that the rupture mode of Ti-6Al-4V changed from ductile to brittle due to the presence of oxygen. As seen in Fig. 11, for LBM P600 and EBM P600 oxidized 500 h at 600 °C, the oxygen concentration at 25 μm from the metal/oxide interface is much lower than 1 at.%. It can therefore be assumed that a small concentration of oxygen (< 1 at.% of oxygen) is enough to change the rupture mode. The quantification of the oxygen concentration responsible for the change in rupture mode cannot be determined precisely with EPMA. As explained previously, the machine error of EPMA measurement is about 0.3 wt.%, i.e. 0.9 at.%. Nevertheless, the modelled diffusion profile allows to estimate the oxygen concentration at a depth of 25 μm. This value is about 0.4 +/-0.9 at.% taking into account the initial bulk oxygen concentration of 0.3 at.%. This value of "less than 1 at.%" is consistent with literature results [START_REF] Finlay | Effects of three interstitial solutes (Nitrogen, Oxygen and Carbon) on the mechanical properties of high-purity alpha titanium[END_REF][START_REF] Shenoy | Oxidation and embrittlement of Ti-6Al-2Sn-4Zr-2Mo alloy[END_REF]. In addition, Yan et al. used an atomic probe and found that an oxygen concentration of 0.33 wt.% (≈ 0.9 at. %) is enough to have a drastic drop in ductility and "reach total brittleness" in the Ti-6Al-4 alloy [START_REF] Yan | A transmission electron microscopy and three-dimensional atom probe study of the oxygen-induced fine microstructural features in as-sintered Ti-6Al-4V and their impacts on ductility[END_REF]. Similarly, Liu et al. confirmed that an oxygen concentration of about 0.65 wt.% induced a change in rupture mode from ductile to brittle for an α titanium alloy resulting in a rupture along α-lath boundaries and a cleavage through α-laths [START_REF] Liu | Literature survey on diffusivities of oxygen, aluminium and vanadium in Alpha Titanium, Beta titanium and in rutile[END_REF].

Conclusions

The main purpose of this study was to determine the influence of the specific surface state and microstructure of the Ti-6Al-4V alloy fabricated by Laser Beam Melting and Electron Beam Melting on high temperature oxidation, and to compare the behaviour of these materials with conventionally processed Ti-6Al-4V. The present study led to the following conclusions:

1) The surface of LBM and EBM Ti-6Al-4V alloys are rough. The presence of partially-melted powder grains increases the surface area by a factor of 2-5. It was shown that the higher net mass change per unit area obtained for as-built samples in this study were only due to their higher surface area. As far as OL and ODZ thicknesses are concerned, the oxidation kinetics of as-built LBM and EBM are similar to ground LBM and EBM samples respectively, and they do not explain the differences in mass gains. 2) Oxidation kinetics of Ti-6Al-4V titanium alloys fabricated by LBM and EBM are similar to that of conventionally processed Ti-6Al-4V alloys at 600 °C, and to that of other conventional Ti-6Al-4V alloys reported in the literature. Oxidation treatments performed on HIPtreated EBM and EBM samples show that α-lath thickness and the number of α interfaces inside the metal does not have an influence on oxidation kinetics nor on oxygen diffusion in the metal. The oxidation of LBM samples shows that the α' microstructure is not stable during oxidation treatment at 500 and 600 °C and transforms into a (α + β) microstructure.

3) The oxide layer is mainly composed of rutile (TiO 2 ) with presence of alumina (Al 2 O 3 ) when the oxidation is long enough. Results also show that there is a high density of pores inside the oxide layer and at the metal/oxide interface, and this porosity increases as oxidation progresses. These pores weaken the oxide layer which deteriorates due to the presence of internal stresses once a critical thickness is reached. At 600 °C, the oxide layer accounts for about 80 % of the oxygen intake, whereas at 500 °C less than 20 % of the oxygen is in the oxide scale. At 600 °C the critical oxide layer thickness is reached before a few dozen hours, whereas, at 500 °C, it still had not been reached after 5 000 h oxidation tests. 4) With the ingress of oxygen in the metal, the ductile rupture mode of the Ti-6Al-4V alloy fabricated by LBM and EBM switches to a brittle rupture mode. It was found that an oxygen concentration lower than 1 at.% was sufficient to embrittle the alloy.

Fig. 1 .

 1 Fig. 1. Optical (OM) and Scanning Electron microscopies (SEM) of rolled, LBM and EBM Ti-6Al-4V alloys.

Fig. 2 . 8 S

 28 Fig. 2. Comparison of X-ray diffractions of unoxidized Ti-6Al-4V alloys (intensities are normalised by their highest value for each diagram).

Fig. 4 .

 4 Fig. 4. Characterisation of surface roughness by confocal microscopy of Ti-6Al-4V alloys before and after grinding.

Fig. 10 shows

 10 Fig. 10 shows an example of experimental EPMA profile obtained from an LBM P600 sample oxidized 500 h at 600 •c. To determine the effective diffusion coefficient of oxygen (D) in the metal, experimental EPMA profiles were fitted using the solution of the second Fick Jaw, Eq. (1), with the oxygen concentration at the metal/oxide interface, Cs, and D being constant. To determine the ODZ, the criteria ODZ = 4°.f[f:i was chosen, based on the ratio ��:�; = 0. 005, presented in Eq. (2). This corresponds to the metal thickness value for which an oxygen concentration is about 0.5 at.o/o for most samples. This criteria has the advantage of being independent of the operator.

Fig. 8 .Fig. 9 .

 89 Fig.8. Optical microscopies of a) as-built LBM oxidized 500 h at 600 °C, b) as-built EBM oxidized 500 h at 600 °C. SEM observations in BSE mode of FIB cross sections of c) LBM P600 oxidized 500 h at 500 °C, d) EBM P600 oxidized 500 h at 500 °C, e) LBM P600 oxidized 500 h at 538 °C, f) EBM P600 oxidized 500 h at 538 °C, g) LBM P600 oxidized 500 h at 600 °C and h) EBM P600 oxidized 500 h at 600 °C.

Fig. 15 .

 15 Fig. 15. XRD measurements of the bulk microstructure of LBM P600 samples after oxidation treatment.

Fig. 16 .Fig. 17 .

 1617 Fig. 16. V and Al content in α and β phases of Ti-6Al-4V calculated with TCTI 1 database in Thermo-Calc.

  al., Caballero et al. and Bermingham et al. have studied the oxidation of Ti-6Al-4V alloy during EBM processing and Wire and Arc Additive Manufacturing (WAAM) [37-39]. Three other studies from Terner et al., Som et al. and Wang et al., focused on the oxidation behaviour of titanium aluminides

  Ti 133 Cs, 51 V 133 Cs, 133 Cs 2 , 1 H 133 Cs 2 , 12 C 133 Cs 2 , 14 N 133 Cs 2 ,

Table 1

 1 Chemical composition of Ti-6Al-4V alloys used in this study (measured by Instrumental Gas Analysis (IGA) for O, N, H and C and Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) for Al, V, Fe, Cr and Ti).

		Al (%)	V (%)	Fe (%)	Cr (%)	O (ppm)	N (ppm)	H (ppm)	C (ppm)	Others (%)	Ti
	LBM Powder (wt)	6.36	4.03	0.21	0.01	900	100	20	100	< 0.4	Bal
	(at)	10.7	3.60	0.17	0.01	2600	300	900	400		Bal
	EBM Powder (wt)	6.35	3.68	0.19		800	200	10	100	< 0.4	Bal
	(at)	10.7	3.29	0.15		2300	600	500	400		Bal
	Rolled β-annealed (wt)	6.41	3.93	0.16		1800	100	40	40	< 0.4	Bal
	(at)	10.8	3.51	0.13		5100	300	1800	200		Bal
	As-built LBM (wt)	5.99 ± 0.02	4.00 ± 0.02	0.2 ± 0.002	< 0.05	1000	100	30	100		Bal
	(at)	10.13	3.58	0.16	< 0.04	2900	300	1400	400		Bal
	As-built EBM (wt)	5.59 ± 0.02	4.01 ± 0.01	0.19 ± 0.003	< 0.05	800	140	10	100		Bal
	(at)	9.49	3.6	0.16	< 0.04	2300	500	500	400		Bal

Acknowledgments

This work performed at CIRIMAT laboratory falls within the framework of the Andduro project hosted by the French Institute of Technology IRT Saint Exupery, supported by the Occitanie Region and industrial partners. The authors gratefully acknowledge Claudie Josse, Sophie Gouy (UMS Castaing, Toulouse, France) for their technical assistance on FIB cross-section observations and EPMA measurements respectively, Julien Milanese and Jean-Baptiste Osio (MIDIVAL, France) for their technical assistance on tensile tests and François Jomard (GeMac, UVSQ, Versailles, France) for SIMS measurements.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.