In situ synchrotron X-ray diffraction analysis of two-way shape memory effect in Nitinol

Abstract

Despite the fact that the Two-Way Shape Memory Effect (TWSME) has been demonstrated in most Shape Memory Alloys, the effective application of this unique functional behaviour is hindered by the lack of a proper training methodology and understanding of its mechanisms. In this study, a novel training routine has been established together with a home-designed device, enabling TWSME of customised spline curvature to be produced. An in situ high energy synchrotron X-ray diffraction experiment has been performed on Nitinol, followed by comprehensive analysis to reveal the micromechanics of TWSME. Multiple mainstream hypotheses have been examined. The important findings are: (1) The training process has negligible influence on the texture of parent phase; (2) The preferred variant of the B19’ phase exhibits tension/compression asymmetry in TWSME; (3) (100) compound twin is the preferred deformation mode for compression TWSME; (4) The mesoscale residual strain field is the dominant factor that induces TWSME; (5) Lattice defects (dislocations) are spatially rearranged after training; (6) Compression TWSME training retards the B2 to B19’ transformation, whilst tension has the opposite effect. The implications of these findings are further discussed.