Phase transformations in equiatomic CuZr shape memory thin films analyzed by differential nanocalorimetry

Abstract We have investigated the phase transformations in sputtered CuZr shape memory thin films using a differential nanocalorimetry technique that is capable of making calorimetric measurement on thin-film samples with a sensitivity as small as 12 pJ/K. We first present a general procedure to accurately measure the heat capacity and enthalpies of transformation of a sample, even if there is a significant difference in the heat capacities of sample and reference. We then demonstrate the technique by analyzing the phase evolution of equiatomic CuZr thin films and explore the conditions for the formation of the martensitic phase responsible for the shape memory properties of this alloy. We show that fast, low-temperature cycling through the martensitic transformation increases the hysteresis, which we attribute to the accumulation of defects during the martensitic transformation. If the austenitic phase is given sufficient time at elevated temperature to annihilate these defects, the transformation is stable under thermal cycling conditions.