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Etude d'un alliage de titane β "transformable par déformation" : lien entre propriétés mécaniques, microstructures de déformation et mécanismes de rupture

Abstract : In order to tackle the lack of ductility and work-hardening of titanium alloys, a new family is being developed, namely, strain-transformable β titanium alloys. The deformation mechanisms involve transformation induced plasticity (TRIP effect) and twinning induced plasticity (TWIP effect). They have been widely studied over the last ten years. On the otherhand, the understanding of fracture mechanisms of TRIP/TWIP titanium alloys still remains poorly documented although this is a key to improve the development of these alloys. Therefore, this project is an in-depth study of the mechanical behavior of a Ti-8.5Cr-1.5Sn β TWIP alloy, under various loading conditions. The coarse microstructure of the alloy was used to investigate deformation and fracture on a grain-by-grain basis. First, a methodology for the systematic study of this family of alloys has been developed. We selected the most relevant tests for a complete study of the mechanical behavior of a TRIP/TWIP alloy by investigating heat treatment conditions (quenching thickness) and mechanical testing conditions (specimen geometry, loading mode). Then,this methodology was validated on the Ti-8.5Cr-1.5Sn alloy first at room temperature, then at subzero temperatures, in order to get closer to the Ms point. The mechanical behavior was evaluated under a wide range of conditions giving access to tensile strength, impact toughness, and fracture toughness. The respective contributions of crack initiation and crack propagation were determined. A multi-scale correlation (optical, SEM, TEM) highlighted fracture by localized plastic flow, without damage development, strongly correlated with the plastic deformation mechanisms and independent of variations in strain rate and stress triaxiality. At subzero temperatures, the absence of ductile-to-brittle transition was demonstrated on the failure mechanisms, as was thepreservation of the phenomenology observed at room temperature (no triggered TRIP effect). Low energy fracture encountered at lower temperatures still involved a ductile dimple mechanism. It remained governed by localized plastic flow, which is triggered all the earlier the lower the test temperature. This new and comprehensive study has improved the understanding of fracture mechanisms of a TRIP/TWIP alloy and provides a solid database for further studies of this family of alloys.
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Submitted on : Sunday, November 14, 2021 - 1:01:36 AM
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  • HAL Id : tel-03427468, version 1

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Chloé Varenne. Etude d'un alliage de titane β "transformable par déformation" : lien entre propriétés mécaniques, microstructures de déformation et mécanismes de rupture. Mécanique des matériaux [physics.class-ph]. Université Paris sciences et lettres, 2020. Français. ⟨NNT : 2020UPSLM030⟩. ⟨tel-03427468⟩

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