Phase Formation in Ti–Ni Binary System during Solid-State Synthesis

摘要

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 Ti~(2)Ni and Ni~(3)Ti 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, Ti~(2)Ni, Ni~(3)Ti and NiTi. The Ti~(2)Ni and Ni~(3)Ti intermetallic phases are the primary reaction products between Ti and Ni, and NiTi phase is formed only as a secondary reaction product from Ti~(2)Ni and Ni~(3)Ti. In addition, two NiTi phases of different Ni contents are formed from either Ti~(2)Ni or Ni~(3)Ti. 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.