Complex multi-step martensitic twinning process during plastic deformation of the superelastic Ti-20Zr-3Mo-3Sn alloy

The Ti-20Zr-3Mo-3Sn (at.%) alloy is investigated before and after deformation by conventional and cyclic tensile tests, electron back-scattered diffraction (EBSD), in situ synchrotron X-ray diffraction (SXRD) and transmission electron microscopy (TEM). On one hand, the maximum recovery strain of 3.3% is obtained, which is attributed to the favorable {111} < 101 >(beta) texture of beta phase after short solution treatment. On the other hand, plastic deformation mechanisms are studied from specimens strained to 5% and 8%. The reversible stress-induced martensitic (SIM) alpha " transformation is preliminary detected by SXRD during loading and after unloading in both specimens. Then, the deformation microstructures are further inves-tigated in details by TEM. At 5% of strain, primary alpha " deformation bands are observed with an abnormal orientation relationship (OR) with beta phase. From crystallographic reconstruction, this OR is shown to be due to {130}< 310 >(alpha) " twinning followed by {111}alpha?? type I reorientation twinning during loading. At 8% of strain, a complex hierarchical microstructure composed of primary and secondary alpha " bands is observed, which corresponds to residual primary {130}< 310 >(alpha) " twins with secondary {130}< 310 >(alpha) " twins occur-ring after {111}alpha?? reorientation twinning of primary {130}< 310 >(alpha) " twins. The sequence of plastic defor-mation is then composed of primary {130}< 310 >(alpha) " twinning, followed by {111}(alpha)?? reorientation twinning and finally secondary {130}< 310 >(alpha) " twinning. A thin layer of omega phase is also observed at the beta/alpha " inter -phase boundary. Occurrence of this unprecedented complex multi-step martensitic twinning process is rationalized by means of Schmid factor analysis and transformation strain calculations. (C)2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.