LAUSR

Abstract CuZr-based alloys are being considered as potential shape memory alloys for use in high-temperature applications. We have conducted a study on the effects of several alloying elements on the shape memory properties of these alloys using polycrystalline thin-film samples. Here we report on the explosive formation of martensite in supercooled CuZr, CuZrNi and CuZrCo samples. This explosive transformation behavior is characterized by the following observations: 1) The high-temperature austenitic phase can be supercooled below the martensite finish temperature Mf. At a critical temperature below Mf, austenite transforms to martensite across the entire sample in less than a microsecond. 2) The critical temperature has a narrow distribution and decreases slightly with higher cooling rate. 3) Observation of supercooling and explosive transformation behavior depends on the temperature history above the austenite finish temperature Af. If a sample is quenched immediately after heating above Af, martensite forms gradually on cooling below Ms; if a sample is allowed to dwell a few seconds above Af, the martensite forms explosively. We suggest that the gradual transformation proceeds by martensite growth on defects that accumulate during successive transformation cycles. If the sample is allowed to dwell at a temperature above Af, however, these defects are annihilated and the transformation is nucleation-limited. Nucleation of martensite then requires significant supercooling. The defect annihilation process is highly sensitive to temperature and has an apparent activation energy of 326 kJ/mol, which is too large for a simple diffusion-limited process. Transmission electron microscopy of CuZrCo samples suggests that the defects may be related to the presence of residual martensite.