Powder metallurgical processing of NiTi shape memory alloys with elevated transformation temperatures

The production of high quality powder metallurgical NiTi alloys with elevated phase transformation temperatures is challenging. During processing, an unavoidable pickup of impurity elements (especially oxygen and carbon) results in a decrease of phase transition temperatures and in the formation of brittle secondary phases. We introduce a processing route including melting, gas atomization and hot isostatic pressing for binary NiTi shape memory alloys which minimizes these problems. We demonstrate that the microstructure of the Ti-rich NiTi alloy contains precipitates of Ti2Ni type which can be exploited to dissolve oxygen picked up during later process stages. In this study, three powder fractions with different grain sizes and impurity contents were subjected to hot isostatic pressing. The evolution of microstructures and material properties was studied by chemical analysis, microscopy, differential scanning calorimetry, and mechanical testing. Exploiting the solubility of oxygen in Ti2Ni, the processing route presented in the present paper succeeds in producing powder metallurgical NiTi shape memory alloys with good structural and functional properties.