Shape recovery of NiTi shape memory alloy under various pre-strain and constraint conditions

The thermally induced shape recovery behavior of NiTi shape memory alloy (SMA) wire under constraint conditions and variable pre-strain conditions was investigated. Two kinds of deformation procedures have been performed: test (1), fixed pre-strain with variable constrained stress; and test (2), fixed constrained stress with variable pre-strain. As studied previously, when subjected to stress-free thermal cycling the maximum two-way memory effect (TWME) is generated at about 13% pre-strain, owing to the optimum amount of dislocations and preferentially oriented martensite produced. The present study showed that under test (1) conditions, the effect of constrained stress on the (generation of the stress-assisted two-way memory effect (SATWME) is found to be associated with the macroscopic increase in martensitic strain and internal stress. The test (2) condition with fixed constrained stress of 200 MPa showed no significant change in SATWME with increasing pre-strain, and its maximum value is obtained at 13%. On the basis of these observations, it is concluded that the development of maximum SATWME and TWME is independent of deformation procedures and is determined predominantly by the magnitude of martensitic strain generated during either pre-deformation or thermal cycling. The mechanism is further discussed in association with relevant thermodynamic properties.