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Influence of strain rate and temperature on the deformation mechanisms of a fine-grained Ti-6Al-4V alloy

Abstract : Depending on their initial microstructure, titanium alloys, as the Ti-6Al-4V may have ac-tivation of different deformation mechanisms during hot forming processes. In this work,interrupted tensile tests and heat treatments are used to improve the understanding of themechanical and microstructural behaviour of a fine-grained Ti-6Al-4V alloy at two temper-atures (750◦C and 920◦C) and so for two differentβphase fractions. The microstructuralfeatures like,αgrain size and phase fraction, were determined by Scanning Electron Mi-croscope (SEM) and image analysis. Moreover, evolution of the preferred crystallographicorientation ofαgrains and local misorientations between and inside grains were obtainedby Electron Backscatter Diffraction (EBSD). The strain rate sensitivity parameter as wellas the activation energy were deduced from mechanical tests. It appears, from all these mi-crostructural and mechanical data, that several mechanisms are activated depending on thestrain level and on the temperature range. At 750◦C, for a high strain rate, the deforma-tion is mainly controlled by dislocations activity in theαphase (texture changes, dynamicrecrystallization) and, at very low strain rate, by probably GBS accommodated with dislo-cations activity intoα(recovery) andβ. On the contrary at 920◦C, a clear decrease of theoverall texture intensity associated with a highmvalue suggests that GBS is the dominantmode of deformation. Nevertheless, as theα/βvolume fraction is around 48 %/52 % at thistemperature, not only theαphase but also theβphase as well asα/αandβ/βboundariesmight contribute to the flow behaviour. During long deformation time (low strain rate andhigh temperature), dynamic coarsening behaviour (both intoαandβ), that is controlled bybulk diffusion, can occur and modify the type, the distribution and decrease the number ofα/β,α/αandβ/βboundaries. This can be partly related to the flow hardening observedat 920◦C and 10−4s−1.
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Laurie Despax, Vanessa Vidal, Denis Delagnes, Moukrane Dehmas, Hiroaki Matsumoto, et al.. Influence of strain rate and temperature on the deformation mechanisms of a fine-grained Ti-6Al-4V alloy. Materials Science and Engineering: A, 2020, 790, pp.1-14/139718. ⟨10.1016/j.msea.2020.139718⟩. ⟨hal-02863273⟩



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