Enhancing shape recoverable ratio in (Ni _50.1 Ti _34.9 Hf ₁₅ ) ₈₅ Nb ₁₅ high-temperature shape memory alloy by thermal-mechanical treatments

The low shape recoverable ratio in NiTiHf high-temperature shape memory alloys was mainly attributed to dislocation generation due to low matrix strength. This work proposes that dislocation generation is also a result of the high critical stress of martensite variant reorientation due to a large atomic mismatch at the twin boundary. Thus, the shape recoverable ratio can be improved by avoiding the reorientation of martensite variants by introducing parallel martensite variants along the external stress direction. Two thermal-mechanical treatments, cyclic deformation-recovery training and pre-deformation training, are applied with a shape recoverable ratio improved from 77% to 88.5% and 97.5%, respectively.