Transformation sequence and second phases in ternary Ti–Ni–W shape memory alloys with less than 2 at.% W

Transformation sequence and second phases in Ti50−XNi50WX, Ti50Ni50−XWX and Ti51Ni49−XWX alloys with X = 1–2at.% are investigated. Two different second-phase particles located at grain boundaries are observed. The lattice parameters of Ti50Ni49W1 martensite are calculated from the selected area diffraction patterns (SADP) and all of them are larger than those of Ti50Ni50 alloy. Experimental results indicate that the W atoms replace Ni atoms, instead of Ti ones in these alloys. In addition, Ti50−XNi50WX and Ti51Ni49−XWX alloys have a two-stage B2 → R → B19′ transformation, but the Ti50Ni50−XWX alloy has a one-stage B2 → B19′ transformation because the former Ms temperatures are depressed significantly. The shape recovery of these alloys can be improved by the W solid-solution hardening. With higher matrix hardness in these alloys, there is better shape recovery.