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Displacive characteristics of β to α phase transformation and its impact on hot deformation behavior in Ti-7333 metastable β titanium alloy

Abstract : A thorough investigation has been conducted on the Ti-7333 alloy in terms of the β to α phase transformation. It was revealed that the structure transformation is realized by a contraction in the <1"1" ̅"2" ̅>β, an elongation in the <"1" ̅1"1" ̅>β and <110>β, and a shear on the {1"1" ̅"2" ̅}β<"1" ̅1"1" ̅>β slip system. Each α precipitate is composed of two kinds of nano-sized α domains. One is situated on the broad face of the major α precipitate (named interface α), and the other goes through the major α (termed penetrating α). The nucleation of such α is induced by the large shear strain generated by the formation of the major α. The penetrating α does not obey the BOR with the β matrix. The nucleation of such α is induced by the largest normal strain generated by the formation of the main α plate. The selection of the α variants during hot deformation at 700°C happens with dependence on the local crystal perfection of the β grains. In the slightly deformed β grains, 2 BOR variants are formed ‘cross-shaped’ clusters. The selected variants make maximum contribution to the macroscopic deformation and receive the maximum resolved shear stress from the external load resolved on their {1"1" ̅"2" ̅}β<"1" ̅1"1" ̅>β systems for transformation. In the heavily deformed β grains occupied mainly by dislocation slip, 2 to 4 BOR variants are selected in smaller numbers. The selection criterion is still obeyed but with restriction from the local deformation. The deformation behavior during 700°C compression presents three characteristic stress-strain stages: a linear stage (Stage I), a discontinuous yielding (Stage II) and a steady-state (Stage III). At the end of Stage I, more than 90% of the β boundaries are occupied by grain boundaries α (αGB). The discontinuous yielding of Stage II originated from the fragmentation of the αGB and the intensive formation of mobile dislocations near the β grain boundary areas. The steady-state of Stage III arises from two orientation dependent deformation modes of the β grains. For those in favorable activation orientation for their {110}β<1"1" ̅1>β systems with respect to the external compressive load, the deformation is realized by the dislocation slip and by the formation of slip bands; whereas for those with their {1"1" ̅"2" ̅}β<"1" ̅1"1" ̅>β systems - the system for β to α transformation - in favorable orientations, the deformation is achieved by intensive formation of intragranular α precipitates.
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Submitted on : Thursday, April 2, 2020 - 2:46:35 PM
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  • HAL Id : tel-02529689, version 1

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Ke Hua. Displacive characteristics of β to α phase transformation and its impact on hot deformation behavior in Ti-7333 metastable β titanium alloy. Materials. Université de Lorraine; Northwestern Polytechnical University (Chine), 2019. English. ⟨NNT : 2019LORR0213⟩. ⟨tel-02529689⟩

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