Sensitivity of heterogeneous microstructure evolution and preferential variant selection to thermomechanical conditions in a near ? titanium alloy

Heterogeneous microstructures such as macrozones and Widmannstatten microstructures are intolerable defects in high-temperature aero-engine parts manufactured using titanium (Ti) alloys. Through thermo-mechanical operations, microstructure characterization, phase reconstruction, and crystal plasticity finite element method (CPFEM) modeling, a full-route analysis was conducted to uncover the microstructure evolution of the allotropic B and alpha phases and the variant selection of the B ->alpha phase transformation (PT) of a near-alpha Ti alloy. With an increase in temperature or decrease in strain rate, the continuous dynamic re-crystallization (cDRX) of the B phase gradually prevails over the discontinuous dynamic recrystallization (dDRX) of the alpha phase. The reconstructed high-temperature B microstructure combing the CPFEM simu-lation revealed that the prior B grains with the deformation-induced (100)//AD orientation, which are softer and grow preferentially via cDRX to form a simple cubic texture in the B phase. The coarse B phase transforms into the secondary alpha (alpha s) phase with a localized phase-transformed texture (the 1210 //AD and 1011 //AD components) during the subsequent cooling process. Through the reconstructed B phase and crystallographic analysis of the B ->alpha PT, we found that the low-angle grain boundary (LAGB) in the B phase weakens the anisotropic growth of alpha s grains and the preferential variant selection of [ 1210]/60 degrees alpha s/alpha s grain boundary (GB) caused by autocatalytic nucleation. A high strain-rate deformation can increase the in-tragranular defects (e.g., LAGB) of the B phase, which promote the formation of a basket-weave structure instead of a Widmannstatten structure and weakens the variant selection during B ->alpha PT.(c) 2023 Elsevier B.V. All rights reserved.