Observation of Transformation Strain Arrest During Partial Thermomechanical Cycling of an NiTi Shape Memory Alloy

Abstract

In the present study, an arrest in transformation strain was observed during the partial thermomechanical cycling (thermal cycling under constant stress) of an equiatomic NiTi shape memory alloy. Sheet specimens of the alloy were thermomechanically cycled between the transformation temperatures (from below M_f to below A_f) under constant stress, ranging from 50 to 150 MPa. The upper temperature for cycling was sequentially arrested at different temperatures in the subsequent cycles till T>A_f. After every partial cycling, the arrest in transformation strain was observed in the immediate subsequent cycle. Unlike the temperature memory effect (due to thermal arrest during partial thermal cycling), the strain arrest effect remembers the magnitude of the previous cycle strain. It requires slight overheating than the previous cycle arrest temperature in order to continue to undergo further transformation strain. As a result, there is a deviation in the local strain path from the global strain path. These results suggest that the shape memory alloy can remember a temporary intermediate shape, between the transformation temperatures, other than the global shape, which is commonly called the one-way shape memory effect. The effect of applied stress levels on the strain arrest phenomenon was also studied. A methodology to determine the strain arrest characteristics is developed and presented in detail in this paper.