Tailoring Grain Size and Precipitation via Aging for Improved Elastocaloric Stability in a Cold-Rolled (Ni,Cu)-Rich Ti-Ni-Cu Alloy

Critical challenges remain in applying shape memory alloys to solid-state refrigeration. In particular, addressing the degradation of elastocaloric response during repetitive loading is an outstanding issue. Refined grains or dispersed precipitates provide high resistance to dislocation activity during transformation and therefore would be expected to improve elastocaloric stability in shape memory alloys. To this end, we age a cold-rolled (Ni,Cu)-rich Ti-Ni-Cu alloy at different temperatures to tailor the microstructure with refined grains and dispersed Ti(Ni,Cu)2 precipitates. We find that with increasing aging temperature, precipitates coarsen with a reduction in density. Simultaneously, the microstructure evolves from being in a nanocrystalline state to an equiaxed coarse grained state. The synergy between nanocrystalline structure and dispersed coherent Ti(Ni,Cu)2 nanoprecipitates in the alloy aged at 400 degrees C gives rise to stable elastocaloric response with a respectable adiabatic temperature change (delta T-ad) of 13.9 K and a recoverable strain of 3%. Further increase in the aging temperature increases the transformation strain and delta T-ad, however, this is at the expense of cyclic stability due to prominent plastic activity in parallel with phase transformation. These findings offer microstructural design insights into the development of high-performance elastocaloric materials and superelastic alloys.