Activation and Variant Selection of α″ Martensite in Ti-5Al-2Sn-2Zr-4Mo-4Cr Alloy during Compression Deformation

This article aims to understand the grain orientation-dependent stress-induced martensite (SIM) transformation in Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) alloy. The variant selection of β → α″ transformation is particularly focused on as the activation of different variants largely determines the microstructure evolution and mechanical properties. Profuse SIM transformation is observed in Ti17 alloy after compression deformation at room temperature. The habit planes of the activated martensites are analyzed in a statistical manner, and then Schmid factors (SFs) for the martensitic transformation are calculated. It is revealed that the activation of α″ martensites in Ti17 alloy is highly dependent on the initial orientation of β grains. Over 75% of the martensites select the variant with the first or second-largest SF, and about 63% prefer the invariant planes with higher SF as their habit planes. Geometric compatibility factor (m′) is for the first time, used to evaluate the strain compatibility of martensite (or slip)-induced martensites at grain boundaries (GBs). It is revealed that the selection of the habit planes seems to be largely affected by the strain compatibility required at GBs, which sheds light into understanding the activation and variant selection of SIM in meta-stable titanium alloys.