Heat transfer in shape memory alloy thin films

Abstract

The thermal response of periodic, shape memory alloy (SMA) island structures is addressed in this paper. Incorporated into the thermal modeling are several ingredients that are encountered at length scales typical to micron-sized thin films: (i) the microstructure may be heterogeneous, e.g., there may be a mix of an amorphous layer, a non-transforming crystalline layer and a crystalline layer that does undergo phase transformation; (ii) given the typical large surface-to-volume ratios and the attendant thermal losses to the environment, the strength of the heat source as well as its duration during a martensite to austenite transformation become very important. These issues, especially the latter, are investigated for a range of periodic structures with different spacings including the limiting structures of vanishing inter-island spacing on the one hand (infinitely extended thin film) and infinite inter-island spacing on the other (isolated islands). It is seen that a reduced inter-island spacing results in lower heat losses to the substrate, that a heterogeneous microstructure has a minimal effect on the thermal field, and that there appears to be a threshold in the strength of the heat source (represented by a current density, if the heat source is electrical in origin) above which the reduction in the heating time does not improve substantially. Interestingly, the evolution of martensite volume fraction with respect to time seems to be insensitive to the thickness of the film undergoing transformation to austenite.