The effect of cold work, heat treatment, and composition on the austenite to R-phase transformation temperature of Ni-Ti shape memory alloys

The influence of cold work (CW) and heat treatment (HT) on the austenite to R-Phase (A-->R) transformation temperature (TT) of a near equiatomic and three other Ti rich NiTi SMA is examined. For the SMA having a near equiatomic composition, the A-->R TT increases with increasing CW at low HT temperatures. For the SMA having the maximum possible Ti content, the A-->R TT decreases with increasing CW at low HT temperatures. For all compositions, the A-->R TT is not sensitive to CW at high HT temperatures. At a Ti content slightly below the maximum possible, the A-->R TT is relatively insensitive to CW and HT temperature. For all of the SMA investigated, the A-->R TT increases with increasing Ti content for a specific CW and HT temperature, and this effect is greatest at low CW and at high HT temperatures.