Two-Way Memory Effect in NiTi Shape Memory Alloys

Abstract

In general, the development mechanisms of TWME have long been understood as the nucleation and growth of preferentially oriented martensite guided by the internal stress. This work extends the study by investigating the effects of martensite deformation, constrained stress and retained martensite via partial reverse transformation through thermal arrest during heating on the stress-assisted two-way memory effect (SATWME) and TWME. It was observed that the generation of maximum SATWME was caused by the development of optimum internal stress. The increase of internal stress was accompanied by the increase of martensitic strain resulting from constrained cooling. When the martensitic strain exceeded the initial pre-strain, it directly influenced on the magnitudes of SATWME and TWME. The accommodation process of stress-assisted and detwinned martensite variants as a result of partial reverse transformation caused the formation of internal forward and back stresses. TWME was promoted by the dominant internal forward stress formation, while the dominance of internal back stress decreased the TWME by decreasing the martensitic strain.