CuZr-based shape memory alloys are being considered for use in high-temperature applications. We have conducted a study on the shape memory properties of these alloys using thin-film samples. We report that the high-temperature austenitic phase of CuZr, CuZrNi and CuZrCo can be supercooled below the normal martensite finish temperature and that supercooled austenite samples can explosively transform to martensite across an entire sample within a few microseconds, causing a sudden spike in sample temperature. This unusual explosive behavior only occurs in samples that are annealed above the austenite finish temperature prior to cooling. We believe that the martensitic transformation is suppressed due to a lack of nucleation sites and a large energy barrier. If, by contrast, the sample is quenched right after the austenitic transformation, there is no supercooling and the austenite transforms gradually to martensite as the temperature is lowered below the martensite start temperature. We suggest that the transformation in this case is facilitated by the presence of defects from previous transformations that serve as nucleation sites for martensite. To determine the kinetics of defect annihilation, we have used nanocalorimetry to measure the fraction of sample that transforms explosively after a certain period of anneal time. The resulting apparent activation energy is approximately 300 kJ/mol, which is too high for a diffusion-limited process. Transmission electron microscopy of CuZrCo samples suggests that the defects may be related to residual martensite above the austenite finish temperature. 

Supercooling of austenite and explosive transformations in CuZr-based thin film shape memory alloys

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