Mechanisms of deformation in gum metal TNTZ-O and TNTZ titanium alloys: A comparative study on the oxygen influence

International audienceIn this work, Ti-23Nb-0.7Ta-2Zr (TNTZ) and gum metal Ti-23Nb-0.7Ta-2Zr-1.2O (TNTZ-O) alloys were synthesized by cold crucible levitation melting with the objective of investigating the influence of oxygen on the deformation mechanisms. By tensile tests, electron backscatter diffraction, atomic force microscopy and transmission electron microscopy analyses, we showed that the deformation in the TNTZ-O alloy is only accommodated by dislocation slip. Thus, the addition of oxygen suppresses the formation of α″ martensite and prevents the twinning deformation mechanism, which were both observed in the TNTZ alloy. In addition, in situ tensile tests in a transmission electron microscope showed that conventional a/2〈1 1 1〉 dislocation slip occurs widely in the TNTZ-O alloy. Screw dislocations have a lower mobility than non-screw dislocations. Cross-slip is shown to be easy and multiplication of dislocations by a double cross-slip mechanism occurs extensively, leading to the formation of large slip bands

Dislocation mobility in gum metal β-titanium alloy studied via in situ transmission electron microscopy

International audienceIn situ tensile tests in a transmission electron microscope were carried out on a "Gum Metal" β titanium alloy. Conventional dislocation slip was observed to be the only mechanism occurring during the plastic deformation. The low mobility of screw dislocations was shown to be due to their core structure configuration. Nanometer-sized obstacles were also present but have a weaker effect of the dislocation mobility. The density of these obstacles and the variation in energy due to the core structure of screw dislocations were measured and compared to theoretical data of literature

An alternative way to orient the parent phase in the cubic/orthorhombic martensitic transformation of titanium shape memory alloys

International audienceTitanium-based shape memory alloys undergo a cubic to orthorhombic martensitic transformation (β to α'' martensite). A new easy way operating as a reconstruction of the parent microstructure is used from electron backscattered diffraction. It is shown that the electron backscattered patterns of the martensitic microstructure can be directly indexed as the parent phase in order to obtain the correct orientation of the high temperature microstructure. This method is however specific to the cubic/orthorhombic martensitic transformation in titanium shape memory alloys

Investigation of the martensitic transformation and the damping behavior of a superelastic Ti-Ta-Nb alloy

International audienceIn this study the α″ stress-induced martensitic transformation and damping behaviour of the superelastic β-Ti-25Ta-25Nb alloy are investigated by tensile tests at room temperature and by dynamic mechanical analysis (DMA) in tensile mode for different applied stresses. Tensile tests show a fully non-linear elastic domain and, consequently, a specific method is proposed to determine the elastic modulus. Due to the wide range of temperature over which the martensitic transformation occurs in this class of alloys, the martensitic start temperature, Ms, is not a relevant parameter to characterize the transformation and the temperature Mmax corresponding to the temperature of maximum transformation is used. The important gap between these two temperatures explains the fully non-linear elastic behaviour of this alloy during conventional tensile tests. It is observed that two main damping sources occur in this alloy: friction at austenite/martensite interfaces during the martensitic transformation and friction at martensite/martensite interfaces at lower temperature. However, a third unexpected damping peak is also observed at high stress. Its origin is discussed with respect to the orientation of the applied stress and with regard to the most favourably oriented martensite variants determined by Schmid factor analysis

In situ TEM study of dislocation slip in a metastable β titanium alloy

International audienceDislocation slip is investigated in a metastable β titanium alloy with the Ti-23Nb-0.7Ta-2Zr-0.4Si composition (at. %) by in situ straining experiments in a transmission electron microscope (TEM). Moving dislocations have a/2 Burgers vectors and glide in {110}, {112} or {123} planes. The mobility of screw dislocations is lowered by punctual defects and the existence of a stable and sessile core configuration that has to recombine in order to allow dislocations to glide

Etude des Micromécanismes de Déformation d'un Alliage Ti-6Al-4V par Déformation MET in situ : Influence d'un Traitement de Surface\ud

Combinant une faible densité à d'excellentes propriétés mécaniques, le Ti-6Al-4V est l'alliage de titane le plus utilisé en aéronautique. Malgré cela, les mécanismes élémentaires de déformation du Ti-6Al-4V, responsables de ses propriétés mécaniques, ne sont pas encore totalement connus. L'objectif principal de ce travail est donc d'apporter une meilleure compréhension de ces micromécanismes. Cet alliage présentant d'assez mauvaises propriétés de surface, notamment au niveau de l'usure mécanique, nous nous sommes aussi intéressés aux modifications apportées par un traitement de surface protecteur (nitruration).\ud Pour mener cette étude à bien, nous avons réalisé un grand nombre d'essais de traction in situ dans un microscope électronique en transmission (MET). Nous avons ainsi déterminé que la résistance de cet alliage est essentiellement due à la structure de cœur des dislocations vis de vecteur de Burgers de type a et que l'ordre à courte distance (OCD) a une influence notable dans la déformation des nodules. Le rôle des différents types d'interface dans la déformation a aussi été clairement défini.\ud Concernant l'alliage traité en surface, les observations post mortem d'échantillons pré-déformés montrent que la couche nano-cristallisée en surface est une barrière à la propagation des dislocations et que, dans la couche de diffusion, l'OCD évolue vers la formation de nano-précipités ordonnés à longue distance conduisant à un mouvement plus collectif des dislocations. Enfin, nous avons réalisé pour la première fois un essai de traction MET in situ sur un échantillon préparé en section transverse grâce à la mise au point d'une méthode originale utilisant un FIB._____________________________________________________________Ti-6Al-4V is the most used titanium alloy in aeronautics because of its low density combined with excellent mechanical properties. However, the elementary mechanisms of deformation responsible for mechanical properties of Ti-6Al-4V alloys are not yet well understood. To provide a better knowledge of these micromechanisms is the main purpose of this work. As this alloy has relatively bad surface properties, particularly regarding mechanical wear, we are also interested in modifications induced by a protective surface treatment (nitridation).\ud Numerous in situ transmission electron microscope (TEM) tensile experiments have been performed during this study. They have allowed us to determine that the strength is mainly due to the core structure of screw dislocations with a-type Burgers vectors and that short range order (SRO) has a non negligible effect in the deformation of nodules. The role of the different interfaces has been also clearly enlightened.\ud Concerning the surface treated alloy, post mortem observations of pre-deformed samples show that the nano-crytallised layer at the surface is a barrier for the propagation of dislocations. In the diffusion layer SRO evolves to the formation of long range ordered nano-precipitates leading to a more localised deformation. Finally, an in situ TEM tensile experiment has been performed for the first time on a cross-section sample prepared with an original way using a FIB\ud \u

Twinning system selection in a metastable β-titanium alloy by Schmid factor analysis

International audienceElectron backscattering diffraction and Schmid factor analysis were used to study the twinning variant selection in a Ti-25Ta-24Nb (mass%) metastable β-titanium alloy. The two twinning systems {1 1 2}〈1 1 1〉 and {3 3 2}〈1 1 3〉 were observed. For each system the Schmid factor was shown to be a relevant parameter to determine the activated variant. Moreover, selection between the two twinning systems depends on the crystallographic orientation of the grain with respect to the tensile direction

Deformation twinning in the full-α″ martensitic Ti–25Ta–20Nb shape memory alloy

International audienceDeformation twinning of the Ti–25Ta–20Nb (mass%) shape memory alloy is characterized using EBSD and TEM. The selfaccommodating α″ microstructure is shown to be first deformed using \111\α″ type I and <211>α″ type II reorientation twinning. Plastic deformation occurs further by plastic twinning with a new \130\<310>α″ twinning system. Maximum lattice deformation calculation is a relevant parameter to predict the variant of martensite that is favored during the reorientation process. Conversely, Schmid factor analysis can be used to predict the selection of \130\<310>α″ twinning variants during deformation. Analogy between \130\<310>α″ and \332\<113>β twinning systems is highlighte

Texture investigation of the superelastic Ti-24Nb-4Zr-8Sn alloy

International audienceIn this work, the influence of crystallographic texture on mechanical properties was investigated by X-ray diffraction in the superelastic Ti-24Nb-4Zr-8Sn alloy. Different textures were obtained by changing the cold rolling reduction rate and the following thermal treatment (solution treatment or flash thermal treatment). The tensile tests performed show that Young's modulus, elongation at rupture and ultimate tensile strength are not influenced by texture. However, the superelastic property of the Ti-24Nb-4Zr-8Sn alloy after solution treatment clearly increases with the textural change into the γ-fiber texture with a (1 1 1)View the MathML source main component due to the increase of cold rolling rate. Contrarily, the texture change induced by the increase of cold rolling rate has no influence on superelasticity after flash thermal treatment. Flash thermal treatment gives also higher recovery strain than solution treatment due to a smaller grain size