Difficulties with Ti-Ni Powder Metallurgy Process and Multi-Stage Sintering Technique for TiNi SMA

Abstract

A study of the exothermic behavior of reactions that occur within Ni and Ti elemental powder mixtures during heating in medium vacuum (4 x 10 -3 Pa) has been conducted. In comparison with several other ordered intermetallic compound mixes, a much more violently released exothermic heat was observed in the Ti-50Ni mixture. It indicates that the difficulties associated with a common powder metallurgy process for TiNi shape memory alloy (SMA) include the possible oxygen interaction in a low vacuum condition and the large and rapidly generated exothermic heat in a medium or high vacuum condition. Therefore, a multi-stage sintering process is proposed to fabricate the TiNi SMA powder metallurgy (P/M) specimens and their composites. In order to prevent the violently released exothermic heat, the sintering process was divided into several repeated cycles with intermediate cold deformation, and a relatively low sintering temperature, 1030°C (or 1080°C), was employed. Further study of the SMA composites, Ti 50 Ni 50 + 5vol%TiC and Ti 50 Ni 50 + 10vol%TiC, with this technique was presented in this article. The obtained sintering density is above 85%. In the meantime, 94.5% (50%) recovery of Ti 50 Ni 50 + 5vol%TiC (Ti 50 Ni 50 + 10vol%TiC) after 1.0% tensile strain then heated to 80°C were observed. The results of phase transformation and microstructure examination will also be discussed.