Mechanistic origin of grain size and oxygen interstitial effects on strain-induced martensitic transformation in Ti-12Mo alloy

Chong, Y.*; Tsuru, Tomohito   ; Mitsuhara, Masatoshi*; Guo, B.*; Gholizadeh, R.*; Inoue, Koji*; Godfrey, A.*; Tsuji, Nobuhiro*

Strain-induced martensitic phase transformation (SIMT) critically affects the mechanical properties of metastable titanium alloys. In this study, the effects of grain size and oxygen content on SIMT in a Ti-12wt.%Mo alloy were systematically investigated. It is found that SIMT is promoted by a decrease in grain size and in oxygen content. The mechanistic origins of the anomalous grain size dependency and the acute oxygen content dependency of SIMT are discussed based on multi-scale microstructural characterization and state-of-the-art simulations. Grain refinement does not raise the energy barrier for SIMT but rather provides more nucleation sites for strain-induced martensite, thereby promoting SIMT in fine-grained Ti-12wt.%Mo alloy. In contrast, for the Ti-12wt.%Mo-0.3 wt.%O alloy, oxygen atoms substantially increase the energy barrier for SIMT, due to a change in the local configuration of oxygen atoms during the phase transformation. In addition, atom probe tomography reveals for the first time that oxygen atoms segregate at phase boundaries, thereby further restricting the growth of martensite.