A study of TiNiCr ternary shape memory alloys

Abstract

Effects of Cr on transformation sequence and wear characteristic in Ti 50− X Ni 50Cr X , Ti 50Ni 50− X Cr X and Ti 51Ni 49− X Cr X alloys with X = 0.5–1.5 at.% are investigated. The lattice parameters of Ti 51Ni 48.5Cr 0.5 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 Ti 50.8Ni 49.2 alloy. Experimental results indicate that the Cr atoms replace Ni atoms, instead of Ti ones in these alloys. In addition, in Ti 50Ni 49.5Cr 0.5 and Ti 51Ni 48.5Cr 0.5 alloys show a two-stage B2 → R → B19′ transformation upon cooling and a one-stage B19′ → B2 transformation on heating. Both Ti 50Ni 48.5Cr 1.5 and Ti 51Ni 47.5Cr 1.5 alloys exhibit a two-stage B2 ↔ R ↔ B19′ transformation; Ti 49.5Ni 50Cr 0.5 alloy has a one-stage B2 ↔ B19′ transformation and Ti 48.5Ni 50Cr 1.5 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 Ti 48.5Ni 50Cr 1.5, Ti 51Ni 47.5Cr 1.5, Ti 50Ni 48.5Cr 1.5 and Ti 49.5Ni 50Cr 0.5 alloys have a better wear resistance than the Ti 50Ni 49.5Cr 0.5 and Ti 51Ni 48.5Cr 0.5 alloys due to their higher hardness, high cyclic hardening and/or pseudoelastic behaviors of B2 structure.