Medium but highly stable superelasticity and elastocaloric effect in an aging-treated Ni51.7Ti47.3Ta1 shape memory alloy

This work demonstrates the effect of aging temperature on transformation path is primarily dependent on the size of Ni4Ti3 precipitates in a Ni-rich Ni51.7Ti47.3Ta1 (at%) alloy. B2↔R↔B19' and B2↔B19' transformations are found in the 773 and 873 K-aged samples, separately, that contain large Ni4Ti3 precipitates (average length exceeding 150 nm). The sample aged at 673 K undergoes single-stage R-phase transformation due to the suppression of coherent Ni4Ti3 nanoprecipitates on B19' martensite. In this sample, with the application of external stress, B2 → B19' transformation takes place at room temperature resulting in medium superelasticity (recovery strain is 4 %) and elastocaloric effect (adiabatic temperature change is −9.4 K). Ultrahigh cyclic stability for these functional properties is probably due to the strengthening effect of Ni4Ti3 nanoprecipitates and small transformation strain.