Nanocalorimetry Study of Phase Transformations in Thin-Film Shape Memory Alloys

Abstract

The nanocalorimetry technique enables measurement of novel thin film materials at non-equilibrium conditions. This study utilizes the synergy between nanocalorimetry, transmission electron microscopy, density functional theory simulations, and theoretical models to explore the Cu-Zr and Ni-Ti shape memory alloy systems, revealing interesting phenomena and their underlying mechanisms. We have investigated the phase evolution of sputter-deposited equiatomic Cu-Zr thin films. The nanocalorimetry heat treatment to form the shape memory phases are determined. We found that in CuZr and some of its ternary alloys, the austenitic phase can be supercooled below the normal temperature range of martensitic transformation, resulting in an explosive-type transformation that converts the entire sample in microseconds. This phenomenon is due to a lack of nucleation sites and can be controlled by annealing of defects. We carried out a computational-experimental study on the ternary alloying effect of CuZr-based shape memory alloy. The computed energy difference between martensite and austenite suggests that both Co and Ni increases the transformation temperature. However, experiments show that the effect of Co goes in the opposite direction. We attribute this discrepancy to the microstructure-related twin boundary energy and strain energy terms. We found that the crystallization of NiTi and NiTiFe splits into two steps at ultrafast heating rates. The first step forms large grains of supersaturated austenite, and the second step forms secondary phases within the large grains. When a small percentage of ternary element Fe is added, the apparent activation energies of both steps drop significantly, and the resulting grain size is much smaller. Further increase of heating rate to above 10,000 K/s results in less time for nucleation and rapid grain growth of NiTiFe.