Stress release-induced interfacial twin boundary omega phase formation in it a beta type Ti-based single crystal displaying stress-induced alpha '' martensitic transformation

The omega phase transformation in numerous group IV transition metals plays a key role to change the phase stability and modify mechanical properties, although its formation mechanisms and effect to accommodate strains are still unclear. A <1 1 0>(beta) single crystal of beta-type Ti-24Nb-4Zr-8Sn alloy (wt. %) is used to investigate the aforementioned problems. This alloy displays beta to alpha '' stress-induced martensitic transformation, which is reversible and leads to superelasticity. Profiles obtained from in situ synchrotron X-ray diffraction during cyclic tensile tests reveal definitely the appearance of omega phase due to applied stress releasing after unloading while these identified omega peaks shall vanish totally on subsequent loading. TEM microstructural observations show its morphology as thin layer locating on the boundary of (1 1 2) <1 1 1>(beta) twins coinciding well with the feature of interfacial twin boundary (ITB) omega phase, which is proved to be formed passively during the reversion of (1 1 0)<1 1 0>(alpha '') twins. The martensitic twinning components of (1 1 01<1 1 0>(alpha '') are indeed confirmed from crystallographic reconstructions based on the orientation relationship between the beta and alpha '' phases. A geometrical model is hence schematically used to analyze in details the orientation relationships between both deformation (1 1 2)<1 1 1>(beta) twins and ITB-omega phase, as well as with the original (1 1 0)<1 1 0>(alpha '') twinning. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.