Transformations de phases et mécanismes de déformation dans les alliages de titane bêta

Abstract : The aim of this Phd was to design a Ti-bêta alloy that would have a good Y.S/ductility balance. The mechanical properties of these alloys depend on their deformation mechanisms (glide system, twinning, stress induced phase transformation) and the parameters to be controled are the chemical composition and the grain size metastable alloys that deform essentially by twinning show low yield strength and large elongation. On the contrary, stable alloys deform by gliding and are characterised by higher Y.S. and smaller elongation. The ideal alloy should therefore start deformation by glide and later in the deformation, activate twinning. For two specific chemical compositions, Ti-20V and Ti-20V-5Al, we have carried out thermomechanical heat-treatments to obtain different grain sizes. The influence of the microstructure on the deformation mechanisms has been studied. During plastic deformation, the Ti-20V alloy deforms by 332 <113> twinning. In addition, stress induced [omega] s transformation and 110 <111> glide are also involved in the deformation. Twinning is always activated first, except when the grain size lower than 40[mu]m. In this case, its contribution to the deformation is limited. Twinning occurs more easily for sufficiently large grain sizes and high deformations rates. Modeling of the texture evolution during rolling allowed us to verify these observations through the analysis of the main texture componenets. The presence of these components can be associated to 110 <111> glide. For the Ti-20V-5Al loaded in a static deformation mode, only glide was activated. During the first percentage of strain, only one glide system is actived. Using a plasticity model, we have shown that the main texture components were associated with 112 <111> glide. Comparison of the various alloys has allowed to select the optimum alloys as well as its optimum microstructure for improving mechanical properties. The reasons of this choice have been discussed in the light of our experimental results
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Angela-Maria Nicotra-Barbe. Transformations de phases et mécanismes de déformation dans les alliages de titane bêta. Autre. Université Paul Verlaine - Metz, 1996. Français. ⟨NNT : 1996METZ047S⟩. ⟨tel-01777160⟩

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