The Effect of Heat Treatment on the Microstructural and Superelastic Behavior of NiTi Alloy with 58.5 wt. % Ni

Abstract

The effect of aging process on hardness as well as the superelastic behavior of NiTi shape memory alloy with 58.5 wt. % Ni has been investigated in this paper. The aging process has been performed at different temperatures ranging from 400^oC to 800^oC and for various exposure times from 30 min to 8 hours. The results showed an increase in hardness in the temperature range from 400^oC to 500 ^oC and then a decrease occurs. At constant temperature with prolonged aging time a gradual increase of hardness occurs. SEM observations showed that there is some precipitation in specimens aged from 600^oC to 800 ^oC. The decrease of hardness can be due to an increase in precipitate size. The superelastisity investigation at 25 and 60 ^oC showed that the maximum superelastisit results are related to specimens aged at 400 ^oC. The superelastisity properties of NiTi were discussed in terms of hardness and microstructure.