High-speed nanoindentation mapping (HSNM), electron backscatter diffraction (EBSD), electron microprobe analyses (EPMA), and high resolution-microscale laser induced breakdown spectroscopy (HR-mu LIBS), were used to characterize the evolution of the elastic anisotropy of a commercially pure titanium (CP-Ti) having a gradient of oxygen concentration. CP-Ti samples were pre-oxidized in air at 655 degrees C for 120 h to create a 35 mu m-deep gradient of oxygen within Ti, the oxygen-rich layer (ORL). Wedge-cut samples were prepared to spread the ORL over hundreds of micrometers instead of tens of micrometers for cross-sections. EPMA and HR-mu LIBS were used to quantify the oxygen distribution within the ORL in a relative and absolute manner, respectively. The Vlassak-Nix theory was used for inverse identification of the stiffness matrix terms as a function of the oxygen content. The stiffness matrix as a function of the oxygen concentration was used to simulate the stress-strain distribution at the sub-grain level in the ORL under tensile macroscopic loading. Configurations with and without external oxide were numerically tested to investigate the role of the oxide layer on the stress distribution within the ORL as well as the crystallographic texture.

High-temperature oxidation of titanium leads to the formation of an external oxide scale and oxygen ingress into the metallic titanium material. Oxygen ingress can be significant due to the high solubility of O within Ti. An oxygen-rich layer (ORL) thus forms beneath the external oxide scale, exhibiting a brittle behavior. Microtensile specimens were used in order to exacerbate surface effects, i.e., surface reactivity in the case of the oxidation of titanium. Playing with the specimen thickness and pre-oxidation durations, it was possible to evaluate the evolution of tensile strength as well as the reduction in ductility for deep extensions of ORL relative to the specimen thickness (high fraction of ORL). In addition, ultrathin specimen extraction at different locations within the ORL depth aimed at better identifying the gradient of properties within the ORL. This micromechanical approach was applied to a commercially pure titanium (CP-Ti grade 2) and to a structural titanium alloy (Ti6242s). Both strengthening and loss of mechanical properties (yield strength and ductility) were observed depending on the material and oxygen ingress. While CP-Ti demonstrated an increase in mechanical strength up to ORL representing 80 pct of the gage section, Ti6242s experienced a loss of mechanical resistance even for the shortest exposure times (the ORL representing 10 pct of the gage section).

Titanium and its alloys combine an important mechanical anisotropy and a high capacity to dissolve oxygen. The detailed evolution of the elastic compliance of titanium as a function of its oxygen content is only partially known, despite its importance in structural applications. Here, high speed nanoindentation mapping (HSNM) was conducted on a grade 2 commercially pure titanium (CP-Ti) to probe elastic and hardness anisotropy as well as property evolution as a function of the oxygen content within Ti using pre-oxidized specimens. The oxygen concentration investigated ranged from 600 ppm to 20% atomic. Pre-oxidation of the CP-Ti was performed at 700 degrees C for 100 h under air to create a gradient of oxygen content within the metal, denoted oxygen-rich layer (ORL). Local oxygen content was quantified using microprobe analyses (EPMA) and crystal orientation using electron backscattered diffraction (EBSD). Reduced modulus and hardness maps were obtained on the preoxidized sample within the ORL and far from the ORL using large but highly resolved nanoindentation technique in continuous stiffness measurement (CSM) mode. Data merging techniques were used on this multi-modal dataset to statistically link local mechanical properties to chemical and crystal orientation information. Oxygen insertion in the Ti lattice was found to significantly increase the hardness and elastic moduli of titanium and was correlated to orientation of the c-axis of the alpha-Ti as a function of the nanoindentation loading direction. Using the Vlassak and Nix theory, it was possible to identify the evolution of the Cij terms of the stiffness matrix as a function of the oxygen content up to 20% at. in O.

Micromechanical characterization of the oxygen-rich layer (ORL) of a Ti-6Al-4V alloy due to high-temperature oxidation was investigated at room temperature. The tensile strength of the pre-oxidized specimens linearly decreased as a function of the surface fraction of ORL in relation to the gage section, demonstrating a competition between oxygen strengthening and embrittlement. Electron-probe microanalyses and nanoindentation testing aimed at locally assessing the elastic and hardness response of the material as a function of the oxygen content. These properties were used in finite element simulations to quantify stress profiles within the oxygen-graded material for different ORL thickness/specimen thickness couples.

: Due to high solubility of oxygen and nitrogen in titanium alloys, the influence of the diffusion zone on the macroscopic tensile properties of pre-oxidized annealed Ti-6Al-4V tensile specimens was examined at room temperature. Thin microtensile specimens were prepared with different thicknesses ranging from 100 µm to 500 µm and then exposed at 750°C for durations between 5 and 200h. A dedicated gripping technique was developed in the present study to investigate the brittleness of such pre-oxidized and ultrathin specimens at room temperature. Tensile testing was paired with digital image correlation techniques to assess both macroscopic deformation and full-field strain maps. High temperature pre-oxidation treatments significantly decreased the ductility of the specimen and the tensile strength of the materials (yield strength and ultimate tensile strength). Fractographic examinations revealed typical brittle fracture features in the oxygen/nitrogen-affected diffusion zone in the periphery of the cross-section while the fracture remained ductile in the core of the specimen for most of the specimens. Some specimens fully failed in a brittle manner for “(pre-ox. duration) 1/2 /thickness” configurations with ratio equal or higher than 0.45 h 1/2 .µm -1 .

The paper demonstrates how defects inherited from the deposition processes can severely impair the lifetime of MCrAlY coatings in service. The oxidation behavior of two NiCoCrAlY coatings was investigated at 1150 degrees C up to 500 h. The coatings had the same nominal composition but were processed by two different projection techniques: air plasma spray (APS) and high velocity oxy fuel (HVOF). Freestanding coating specimens were extracted from the coated system and thinned down to different thicknesses ranging from 520 to 15 mu m in order to investigate size effects inherent to the oxidation response. The oxidation rate of the APS coating was found to be insensitive to the specimen thickness, while that of the HVOF coating increased with the specimen thickness, due to greater intersplat oxidation. APS specimens thinner than 60 mu m experienced intrinsic chemical failure (InCF) due to Al consumption to form the Al2O3 scale. In comparison, HVOF specimens with a thickness of 367 mu m were subject to InCF after 250-350 h oxidation. This first stage of InCF resulted in the formation of a Cr2O3 layer at the Al2O3/metal interface once Al activity in the MCrAlY coating was low enough to thermodynamically allow Cr2O3 to form. In addition, thick HVOF specimens developed mechanically induced chemical failure (MICF) resulting in the formation of (Ni,Co)(Cr,A1)(2)O-4 spinels on top of the Al2O3 scale and within oxide intrusions. The occurrence of MICF was associated with the concomitant effects of Al consumption due to intrusive oxidation and the spallation of the external Al2O3 scale.

The purpose of this study is to develop novel speckle pattern techniques for digital image correlation (DIC) kinematic measurements of mechanical tests at high temperatures, typically from 400 to 700°C. In this context, the speckle pattern should not only meet morphological criteria (size, density, distance) in order to improve spatial resolution, but it should also present a high contrast and resist high temperature and strain levels. To find a speckle pattern matching these specifications, a comparison was performed on six types of speckle made using different techniques. First, a computer‐generated speckle pattern that meets DIC criteria was numerically designed to produce six types of speckle pattern. Next, the speckle patterns produced using these six techniques were compared in terms of speckle morphology, image quality and adherence to titanium alloy TA6V material at high temperatures. From 25 to 600°C, the speckle pattern made by the technique combining anodisation and laser engraving named M5 technique gave the best contrast (highest value of mean intensity gradient [MIG] and Shannon entropy value) and the adherence of 200% of strain measurements to the TA6V material. At 700°C, speckle image quality is considerably reduced due to oxidation of the titanium alloy, and this may not be suitable for DIC measurements. Only the speckles produced by painting in which the paint plays a protective role provide with a better speckle contrast compared with other techniques. However, these speckle patterns enable only a strain measurement of 22% by the DIC method. This article concludes with guidelines for producing a speckle pattern suitable for high‐temperature mechanical tests.

Due to high solubility of oxygen and nitrogen in titanium alloys, the influence of the diffusion zone on the macroscopic tensile properties of pre-oxidized annealed Ti-6Al-4V tensile specimens was examined at room temperature. Thin microtensile specimens were prepared with different thicknesses ranging from 100 µm to 500 µm and then exposed at 750°C for durations between 5 and 200h. A dedicated gripping technique was developed in the present study to investigate the brittleness of such pre-oxidized and ultrathin specimens at room temperature. Tensile testing was paired with digital image correlation techniques to assess both macroscopic deformation and full-field strain maps. High temperature pre-oxidation treatments significantly decreased the ductility of the specimen and the tensile strength of the materials (yield strength and ultimate tensile strength). Fractographic examinations revealed typical brittle fracture features in the oxygen/nitrogen-affected diffusion zone in the periphery of the cross-section while the fracture remained ductile in the core of the specimen for most of the specimens. Some specimens fully failed in a brittle manner for “(pre-ox. duration)1/2/thickness” configurations with ratio equal or higher than 0.45 h1/2.µm-1.

In order to reduce production costs and environmental impact, the cycle time reductions associated with a decrease in temperature levels are relevant. This study focuses on the behaviour modelling of Ti-6Al-4V alloy at temperatures between 400 degrees C and 500 degrees C in order to obtain greater formability than at room temperature, whilst remaining below SPF conditions to reduce tool, workshop and energy costs. Mechanical tests are conducted to identify elasto-viscoplastic model parameters. They use displacement field measurements obtained by Digital Image Correlation (DIC) and based on innovative surface preparation patterns adapted to high temperature exposures. Different material parameters are identified to define a model that is able to predict the mechanical behaviour of Ti-6Al-4V alloy under hot/warm stamping conditions. Then, an omega shape forming test is developed to validate the behaviour model. Finally, the experimental results are compared with numerical simulations which require the implementation of the behaviour model formulation into an FE code.

The aim of the global study is to develop a technique to improve kinematic fields measurements during mechanical tests performed on Ti-6Al-4V alloy under hot forming conditions. In order to contribute to the optimization of Digital Image Correlation (DIC) procedures for this specific context, the generation of a suitable speckle is studied. Firstly, speckle made of different fabrication techniques is compared in terms of speckle quality. To proceed objectively, the same synthetic pattern is systematically used. Secondly, images of the speckled surfaces are then captured at different temperatures and compared in terms of contrast, grey level distribution, etc. At room temperature, the first results show that speckle made by direct anodization method lead to a better contrast. However, at high temperature, because the oxidation layer develops, this observation no longer holds true. Thirdly, the adhesion of speckle produced by different methods on Ti-6Al-4V surfaces is evaluated at high temperatures. Large strain tensile tests are performed. Speckle made by laser methods withstands large strains until necking. Meanwhile, speckle made by painting methods flakes off when the plasticity develops. In conclusion, a guideline for making a speckle suitable to perform mechanical tests under hot forming conditions is suggested. Mots clefs : Digital Image Correlation, speckle, tensile test, high

Dans l'industrie aéronautique, les alliages de titane sont utilisés pour leur excellent comportement mécanique associé à une faible masse volumique. Ils sont largement employés sous forme de tôles dont la mise en forme peut se faire par le biais de trois procédés : à température ambiante par opération d'emboutissage, à très hautes températures (T≈900°C) par formage superplastique (SPF) et à des températures intermédiaires (T=730°C, 880°C) par formage à chaud (HF). Le projet repose sur le développement du procédé d'emboutissage à chaud d'une tôle d'alliage de titane Ti-6Al-4V en conditions isothermes à des températures inférieures à 700°C. Par conséquent, la détermination des paramètres procédés et matériaux constitue une étape importante pour la mise en œuvre de simulations numériques et contribue à la réussite des opérations d'emboutissage de pièces industrielles. Ces paramètres procédés sont liés à la vitesse du poinçon, aux efforts de serre-flan et au frottement induit entre le flan et l'outillage. Leur analyse a permis de déterminer deux niveaux de températures (400°C et 500°C) offrant une chute drastique du coût énergétique, en comparaison des procédés HF ou SPF, tout en conservant des niveaux d'allongement suffisants. Les paramètres matériaux influençant le comportement de l'alliage sont analysés et quantifiés. Ils peuvent être influencés par plusieurs mécanismes : élasticité, viscosité, anisotropie (Hill48, Barlat91) et nature de l’écrouissage (isotrope, cinématique). Dans cette étude, un modèle de comportement élasto-viscoplastique anisotrope, capable de considérer les trajets de chargement subis par la tôle lors de sa mise en forme, a été formulé pour les deux niveaux de température. L’implantation du modèle de comportement a été réalisée dans le code de calcul éléments finis Abaqus/Standard 6.14® interfacé avec le logiciel ZMAT®. Elle a permis d’une part des simulations d’emboutissage de profil Omega pour lesquelles des comparaisons avec les expériences ont été réalisées et d’autre part, des calculs sur une pièce de forme complexe.

The Ti-6Al-4V titanium alloy is widely used for the manufacture of aeronautical and automotive parts (solid parts). In aeronautics, this alloy is employed for its excellent mechanical behavior associated with low density, outstanding corrosion resistance and good mechanical properties up to 600 degrees C. It is especially used for the manufacture of fuselage frames, on the pylon for carrying out the primary structure (machining forged blocks) and the secondary structure in sheet form. In this last case, the sheet metal forming can be done through various methods: at room temperature by drawing operation, at very high temperature (similar or equal to 900 degrees C) by superplastic forming (SPF) and at intermediate temperature (>= 750 degrees C) by hot forming (HF). In order to reduce production costs and environmental troubles, the cycle times reduction associated with a decrease of temperature levels are relevant. This study focuses on the behavior modelling of Ti-6Al-4V alloy at temperatures above room temperature to obtained greater formability and below SPF condition to reduce tools workshop and energy costs. The displacement field measurement obtained by Digital Image Correlation (DIC) is based on innovative surface preparation pattern adapted to high temperature exposures. Different material parameters are identified to define a model able to predict the mechanical behavior of Ti-6Al-4V alloy under hot stamping conditions. The hardening plastic model identified is introduced in FEM to simulate an omega shape forming operation.