Stable and large superelasticity and elastocaloric effect in nanocrystalline Ti-44Ni-5Cu-1Al (at%) alloy

Superelastic behavior and elastocaloric effect were investigated in a Ti-44Ni-5Cu-1Al (at%) alloy subjected to various thermomechanical treatments. The specimen heat-treated at 673 K for 5 min after hot rolling and subsequent cold rolling exhibited excellent superelastic strain of 4.9% with a small stress hysteresis of 90 MPa when the maximum tensile stress was 500 MPa. This specimen also exhibited a large elastocaloric effect with a temperature decrease of 17 K when the stress of 500 MPa was removed adiabatically. No remarkable deterioration was observed for the superelastic strain and elastocaloric effect up to 5000 mechanical cycles. The maximum superelastic strain obtained was 6.8% under a tensile stress of 750 MPa. Transmission electron microscope observation and in-situ X-ray diffraction analysis under tensile stress revealed that the average grain size of the specimen is about 40 nm, and the specimen exhibits a successive B2-B19-B19' transformation.