Redox-mediated sustainable solid-state synthesis of NiTi: An in situ study of alloying pathways and phase evolution mechanisms

NiTi shape memory alloys serve in advanced biomedical and electro-mechanical devices. However, conventional liquid metallurgical production of NiTi from primary Ni and Ti is energy and emission intensive. Here, we present a sustainable one-step approach to produce NiTi from NiO and TiH 2 in the solid state. It involves the decomposition of TiH 2 and the reduction of NiO, with concurrent diffusion-based alloy formation. The reaction kinetics were examined by thermogravimetric analysis and in situ synchrotron X-ray diffraction. The results show that the decomposition, reduction, and alloying behavior of the system are influenced by the hydrogen partial pressure. Based on these findings, we have developed design guidelines to increase the NiTi-phase yield, resulting in a bulk sample containing up to 33.7 vol.% of NiTi which exhibits the desired thermoelastic martensitic transformation. The findings of this work also provide fundamental insights into sustainable fabrication of application-worthy NiTi alloys fully in the solid-state.