On the two-way shape memory behavior in NiTi alloy—An experimental analysis

The present research aims to understand the mechanism of the two-way memory effect (TWME) in NiTi alloy through experimentally investigating shape recovery in the course of various thermomechanical processes. Particular attention was paid to the effect of martensitic strain, either due to pre-deformation or constrained forward transformation, on the TWME. Attempts were made to clarify the correlation of the stress-assisted two-way memory effect (SATWME) and the TWME to martensitic strain resulting from pre-straining and/or constrained cooling. When the martensitic strain resulting from constrained forward transformation exceeds the initial pre-strain, it directly influences the SATWME and TWME. Partial reverse transformation through thermal arrest during heating leads to accommodation of stress-assisted and detwinned martensite variants that result in formation of internal forward and back stresses. TWME is promoted through dominant internal forward stress formation, while the dominance of internal back stress reduces the TWME by reducing the martensitic strain.