A study of TiNiCr ternary shape memory alloys

Effects of Cr on transformation sequence and wear characteristic in Ti50−XNi50CrX, Ti50Ni50−XCrX and Ti51Ni49−XCrX alloys with X = 0.5–1.5 at.% are investigated. The lattice parameters of Ti51Ni48.5Cr0.5 martensite are calculated from the XRD spectra as a = 0.2894 nm, b = 0.4109 nm, c = 0.4639 nm and β = 97.49° and the parameters a, c, β are slightly smaller, but b nearly the same, than those in binary Ti50.8Ni49.2 alloy. Experimental results indicate that the Cr atoms replace Ni atoms, instead of Ti ones in these alloys. In addition, in Ti50Ni49.5Cr0.5 and Ti51Ni48.5Cr0.5 alloys show a two-stage B2 → R → B19′ transformation upon cooling and a one-stage B19′ → B2 transformation on heating. Both Ti50Ni48.5Cr1.5 and Ti51Ni47.5Cr1.5 alloys exhibit a two-stage B2 ↔ R ↔ B19′ transformation; Ti49.5Ni50Cr0.5 alloy has a one-stage B2 ↔ B19′ transformation and Ti48.5Ni50Cr1.5 alloy shows a B2 ↔ R transformation. The shape recovery of these alloys can be improved by Cr solid-solution hardening. With higher matrix hardness in these alloys, there is better shape recovery. The wear mechanisms of TiNiCr ternary alloys are found to be similar to those of binary TiNi alloys. The Ti48.5Ni50Cr1.5, Ti51Ni47.5Cr1.5, Ti50Ni48.5Cr1.5 and Ti49.5Ni50Cr0.5 alloys have a better wear resistance than the Ti50Ni49.5Cr0.5 and Ti51Ni48.5Cr0.5 alloys due to their higher hardness, high cyclic hardening and/or pseudoelastic behaviors of B2 structure.