High-strength porous NiTi shape memory alloys with stable cyclic recovery properties fabricated using elemental powders

Based on previous research regarding the high-vacuum sintering of elemental powder (EP) NiTi alloys, this study investigates the preparation of high-strength, highly cyclically stable porous NiTi shape memory alloys. The microstructures, martensitic transformation behaviors, compressive and compressive recovery properties, and cyclic stabilities of the sintered and aged porous NiTi alloys were compared and analyzed. The porosity and average pore size increased with the addition of NaCl, with the oxygen content increasing slightly. The sintered alloy displayed an inhomogeneous distribution of the Ni4Ti3 precipitate, which exhibited a dispersed and homogeneous distribution after aging. The sintered porous NiTi alloy exhibited a multistage phase transformation. The martensitic transformation was suppressed after aging owing to changes in the size and distribution of the precipitates, and only R-phase transformation was observed. Porosity and aging precipitation influenced the dual yield points and recovery properties of porous NiTi. The pinning effect of aging precipitation increased the yield point and reduced the residual strain in the recovery. However, at a high porosity, the volume fraction of NiTi matrix decreased, resulting in a small strengthening contribution of precipitation. The disruption of porous structure was the dominant causes of irreversible strain. As the porosity increased, the porous structure became increasingly susceptible to damage during cyclic loading. Overall, the EP porous NiTi alloy exhibited a higher compressive strength, enhanced compression recovery properties and cyclic stability compared to those reported in previous studies. This is because of the optimization of high-temperature homogenization via sintering at 1250 °C and enhancement effects of aging precipitation. This study provides a novel approach for developing porous EP NiTi alloys.