J. Zhang, X. Ren, National Institute for Materials Science, Tuskuba, Japan; G. Fan, National Institute for Materials Science, Tsukuba, Japan; K. Otsuka, National institute for Materials Science, Japan, Tsukuba, Japan
Recently we proved that there exists no order-disorder transition in near equiatomic Ti-Ni shape memory alloys. In that investigation we found that the oxidation affects greatly the composition and structure of the alloys, unless the atmosphere of the heat-treatments are proper. Since the heat-treatment is essentially important in the production of Ti-Ni shape memory alloys with good shape memory characteristics, we discuss the effect of oxidation at high temperatures in the present paper. We chose two atmospheres for the heat-treatment; (1) Ar-1%H2 mixture gas, (2) Ar-1%H2 mixture gas+Ti getter. Then we examined the effect of oxidation by using TG-DTA and SEM assisted by EPMA for chemical analysis. The oxygen partial pressure was measured by ZrO2 cell. In the above two atmospheres even the case (1) looks to give a good atmosphere for anti-oxidation. However, actually the selective oxidation of Ti develops with appearance of TiO2 and Ti4Ni2O, and consequently the composition of the matrix changes greatly to Ni-rich side, which thus may lead to erroneous interpretations. On the other hand, in case (2), which can generate much lower oxygen partial pressure, the oxidation is mostly avoided, and the reliable results are obtained. At the meeting we will show more experimental details including the oxidation mechanism, and how important it is to suppress the oxidation in the heat-treatments of Ti-Ni alloys.
Summary: With careful investigation on Ti-Ni in different atmospheres with oxygen partial pressure controlled, our results show how important it is to suppress the oxidation in the heat-treatments of Ti-Ni alloys.
