Phase Formation in Ti–Ni Binary System during Solid-State Synthesis
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Much effort has been made to synthesize porous NiTi alloys using powder metallurgy techniques. However, the sintered products from elemental Ti and Ni powders often contain Ti2Ni and Ni3Ti in addition to the desired NiTi phase, and the thermal and mechanical properties of the sintered products are inferior comparing to cast or wrought NiTi alloys. This study investigated the solid-state diffusion reactions between elemental Ti and Ni powders during sintering to delineate the origin of the formation of these undesired intermetallic phases and proposed a physical model to explain the phase formation processes. The intermediate diffusion reaction products include Ti(Ni) and Ni(Ti) solid solutions, Ti2Ni, Ni3Ti and NiTi. The Ti2Ni and Ni3Ti intermetallic phases are the primary reaction products between Ti and Ni, and NiTi phase is formed only as a secondary reaction product from Ti2Ni and Ni3Ti. In addition, two NiTi phases of different Ni contents are formed from either Ti2Ni or Ni3Ti. These findings clarify some uncertainties and common misunderstanding on NiTi formation through elemental powder sintering and provide a guide for the design of powder metallurgy of Ti and Ni.
KeywordsNiTi Shape memory alloys Solid-state synthesis Sintering Phase formation Diffusion
This work was supported by the Australian Research Council in Grants DP160105066. We acknowledge the facilities, and the scientific and technical assistance of the Australian National Fabrication Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. Yinong Liu wishes to dedicate this work to Prof. Jan Van Humbeeck, from whom he has gained much support and scientific mentoring. Prof. Jan Van Humbeeck has worked extensively on selective laser sintering of NiTi in recent years. The study presented here may contribute to enrich our understanding on this topic.
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