A macroscopically inhomogeneous stress-induced R-phase transformation in Ti50Ni48.5Fe1.5 with enhanced elastocaloric effect

The stress-induced transformation from austenite phase to R phase in NiTi and TiNi-based shape memory alloys has a small stress hysteresis and a high stability to cycling across the transformation due to a small lattice distortion. However, the elastocaloric effect is limited due to the intrinsic small transformation strain and this significantly hinders its application in solid-state refrigeration. Here, we report a macroscopically inhomogeneous stress-induced R-phase transformation in Ti50Ni48.5Fe1.5 wire with a stress plateau in the stress-strain curve, showing difference from the conventional macroscopically homogeneous transformation with an associated continuous increase of the stress. This novel transformation behavior permits reaching a temperature decrease of 5.8 K with a transformation strain of only 0.4 %, which brings a near 100 % increase in the elastocaloric effect of R-phase transformation compared to Ni50.8Ti49.2 and an excellent cycling stability. Infrared tensile tests and in-situ X-ray diffraction measurements prove the existence of transformation bands, confirming the localized deformation of this transformation behavior. Transmission electron microscopy results show that the microstructure condition for inducing such a macroscopically inhomogeneous R-phase transformation in Ti50Ni48.5Fe1.5 is the limited hindrance effect from the local stress induced by doped Fe atoms. The stress-induced R-phase transformation with an enhanced cooling capacity and an excellent fatigue property makes Ti50Ni50-xFex alloys a new alternative material for solid-state refrigeration. [Display omitted]