Grain size dependent martensitic twinning behavior in superelastic Ti-20Zr-12Nb-2Sn alloy: A comparative study

The superelastic Ti-20Zr-12Nb-2Sn (at. %) alloy is investigated before and after deformation by cyclic/conventional tensile tests, optical microscopy, in situ synchrotron X-ray diffraction (SXRD), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) after 700 °C and 900 °C solution treatments for 30 min. Both of the specimens show the same {111}<101> β recrystallization texture, but a different average grain size of respectively 8 μm and 105 μm. Occurrence of the reversible stress-induced martensitic (SIM) α" transformation is validated through in situ SXRD. After plastic deformation, this alloy displays a grain size dependent martensitic twinning behavior. In the fine grain material, only residual SIM α" phase is observed with no twinning up to rupture, which was never reported in plastically deformed Ti-based superelastic alloys. In the coarse grain material, twins are observed in residual SIM α" phase. For the 5 % strained specimen, primary/secondary martensitic α" band undergoes a novel three-step twinning process (primary {130}<310> α" twinning + <211> α" type II twinning + secondary {130}<310> α" twinning). We also showed that the final product of this three-step process can be misinterpreted as {021}<512> α" type II twinning. This primary/secondary α" twin band is then further partially twinned again by a third {130}<310> α" twinning or {110}<110> α" twinning at 8 % of strain, highlighting an unprecedented fourth step twinning process. {130}<310> α" twins are systematically reorientated by <211> α" type II twinning in order to maximize the transformation strain value with respect to their β phase counterpart. This work comprehensively sheds light on the mechanism of superelasticity optimization via grain size refinement strategy.