The effect of ternary alloying on the shape memory properties of titanium-platinum alloys

Abstract

Among many shape memory alloys, Ti-Pt alloys have potential as a high temperature shape memory alloy. The alloy undergoes a reversible martensitic transformation at temperatures above 1000 oC. However, the alloy’s negligible shape memory effect could limit its application. Solid solution strengthening has been suggested as an alternative to improve the shape memory properties of Ti-Pt. In this study, the effect of partially substituting either platinum (Pt) or titanium (Ti) with vanadium (V) on the shape memory properties of Ti-Pt was investigated in the as-cast and solution heat treated conditions. The study focused on investigating the effect of V on the structure, phase transformation, mechanical properties and shape memory behaviour of Ti-Pt alloy using scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), X-ray diffraction analysis (XRD), differential scanning calorimeter (DSC), compression tests and a macro Vickers hardness tester. Ti-Pt50-x-Vx and Ti50-x-Vx-Pt (x = 0, 6.25, and 12.5 at.%) alloys were produced by an arc melting method using elemental powders of Ti, Pt and V. Solution heat treatment (SHT) was carried out at a temperature 1250 oC for 72 hours, followed by ice water quenching. The SEM-EDX revealed that the addition of V to Ti-Pt resulted in multiple phase microstructures in both the as-cast and solution heat treated conditions. The partial substitution of Pt with V did not occur, since the alloy composition was within in a three-phase region of the Ti-Pt-V ternary phase diagram with increasing V content. Solution heat treatment increased the amounts and size of the other phases with increasing V content. The partial substitution of Ti with V resulted in alloy compositions that were in a single TiPt phase region. The addition of 6.25 and 12.5 at.% V did not change the structure of B19 martensite phase and this phase was the major matrix phase in all compositions. The partial substitution of Pt with V decreased the transformation temperatures of the Ti(Pt,V) phase with increasing V content, while the partial substitution of Ti with V increased the transformation temperatures of (Ti,V)Pt phase with increasing V content. Both substitutions widened the thermal hysteresis compared to TiPt, with Ti(Pt,V) exhibiting the highest. For mechanical properties, the addition of V to Ti-Pt exhibited a single yielding behaviour and, the yield strength and hardness of Ti-Pt were significantly increased with the increasing V content. Ternary alloying with V degraded the ductility and plastic strain of Ti-Pt-V with increasing V content. The partial substitution of Pt with V did not improve the shape memory effect and shape recovery of Ti-Pt-V in the as-cast condition. However, the partial substitution of Ti with 6.25 at.% V improved the shape memory effect and shape recovery of Ti-V-Pt in the as-cast condition. The SHT improved the shape recovery of Ti-Pt-V and Ti-V-Pt alloys with increasing V content, but were still lower than in the as-cast condition.