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

Abstract

Heterogeneous microstructures such as macrozones and Widmannstätten microstructures are intolerable defects in high-temperature aero-engine parts manufactured using titanium (Ti) alloys. Through thermomechanical 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 β and α phases and the variant selection of the β→α phase transformation (PT) of a near-α Ti alloy. With an increase in temperature or decrease in strain rate, the continuous dynamic recrystallization (cDRX) of the β phase gradually prevails over the discontinuous dynamic recrystallization (dDRX) of the α phase. The reconstructed high-temperature β microstructure combing the CPFEM simulation revealed that the prior β grains with the deformation-induced 〈100〉//AD orientation, which are softer and grow preferentially via cDRX to form a simple cubic texture in the β phase. The coarse β phase transforms into the secondary α (αs) phase with a localized phase-transformed texture (the 1̅21̅0//AD and 101̅1//AD components) during the subsequent cooling process. Through the reconstructed β phase and crystallographic analysis of the β→α PT, we found that the low-angle grain boundary (LAGB) in the β phase weakens the anisotropic growth of αs grains and the preferential variant selection of 1̅21̅0/60°αs/αs grain boundary (GB) caused by autocatalytic nucleation. A high strain-rate deformation can increase the intragranular defects (e.g., LAGB) of the β phase, which promote the formation of a basket-weave structure instead of a Widmannstätten structure and weakens the variant selection during β→α PT.