Effect of the thermal processing on the microstructural, functional and mechanical properties of cast polycrystalline NiMnTi alloys

The solid-state refrigeration and heat pumping field is gaining increasing attention as an alternative solution to the conventional vapour-based cooling devices. Among the shape memory alloys (SMAs), besides the reference NiTi system, outstanding elastocaloric performances are exhibited by the NiMnTi alloy. In this work, we produced two cast NiMnTi alloys which were characterized in terms of microstructural, structural, functional, and mechanical properties. Moreover, we assessed the effect of the heat treatments on the microstructure and on the physical properties of the produced alloys by calorimetric, structural and high-temperature microscope analyses. Furthermore, the heat treatments promoted partial mechanical cycling stability and a superelastic behavior without significant residual strains. Finally, the elastocaloric performance of the alloy was evaluated by direct adiabatic ΔT measurements upon elastocaloric cycles in four different loading conditions. The alloys exhibited promising elastocaloric performance for possible solid-state cooling applications. In particular, the best adiabatic ΔT value achieved in this work was 8.7 °C with a strain rate of 400 %/min and up to a strain of 5 % for the NiMnTi heat treated at 900°C.