Effect of Cyclic Loading on Apparent Youngs Modulus and Critical Stress in Nano-Subgrained Superelastic NiTi Shape Memory Alloys

A series of uni-axial tensile cycling tests were conducted at room temperature in superelastic NiTi strip specimens with nano-grain size. The NiTi superelastic strip specimenʹs Apparent Youngʹs Modulus (AYM) and the critical stress decrease when the specimen is subjected to an external uni-axial stress and the strain being higher than 1.5%. Both of the AYM and the critical stress become steady after 10-time cycling. The number of the (111)[143] oriented grains increases with extending the strain value. The sub-grain size grows with increasing mechanical cycling number due to the annihilation of the small angle boundaries. The AYMsoftening is related to the grain re-orientation (texture evolution) and the formation of irreversible-stabilized B19ʹ martensitic variants. The softness of the critical stress is principally attributed to the aspect that the grains re-orient to align along the two textural components (111)[110] and (111)[143] when the external stress being applied. The rotation of grains towards the observed orientation gives higher Schmid factor for the transformation and is one of the reasons for the decrease in AYM and critical stress. The orientation relationships between B2 parent phase and the strain-induced B19ʹ martensite are observed to be: [111]B2|| [101]M, (110)B2|| (010)M and [111]B2|| [110]M and (110)B2|| (001)M.