The effect of vanadium and nickel on the microstructure and transformation temperatures of Ti₅₀Pt₅₀ alloy

Abstract

Significant research has been done to produce shape memory alloys that have good shape memory properties and high martensitic transformation temperatures. The Ti50Pt50 alloys have been found to have high transformation temperature of around 1050℃ however, they exhibit negligible shape memory properties. The solid solution strengthening, and improved shape memory properties could be enhanced by ternary alloying. Therefore, this work investigates the effect of varying V and Ni contents, in the range of 6.25 to 12.5at%, on the austenitic and martensitic transformation temperatures, and hardness of the equi-atomic Ti50Pt50 alloy. Arc melting followed by casting and solution heat treatment was carried out to produce the alloys. As-produced alloys were characterized by using scanning electron microscopy, differential scanning calorimetry and hardness testing. The microstructures showed high volume fraction of second phases formed in the TiPtV alloy compared with Ti50Pt50 and TiPtNi alloys. The multiple phases formed in the TiPtV alloys could be the cause of high hardness values observed in these alloys as compared withTi50Pt50 and TiPtNi alloys. Thermal transformation studies revealed that TiPtV alloys exhibit transformation temperature close to Ti50Pt50 alloy, in contrast with TiPtNi alloys. TiPtNi alloys thermal behaviour was improved by solution heat treatment.

The effect of increasing hafnium on the microstructure, phase transformation temperatures and hardness of TiPtHf potential shape memory alloys

“…% Hf. A similar observation was also made in prior studies [4] [16] where alloying with vanadium (V) and nickel (Ni) lowered the martensitic transformation temperature and widened the thermal hysteresis. The benefit of a widened thermal hysteresis is that SMAs may be used in coupling applications that require large temperature range [2][3] Solution heat treatment of Ti 50 Pt 43.75 Hf 6.25 increased the ∆T of the alloy above that of all alloys in the as-cast and solution heat treated conditions.…”

“…Substitution of Pt with either Hf, Co, Zr, V, Ni, Ru, or Cu to form ternary alloy lowers the transformation temperatures of these alloys below that of the binary Ti 50 Pt 50 [16][17]. Previous studies [16] show that the partial substitution of Pt in the Ti 50 Pt 50 alloy with either V or Ni at 6.25 at %, Ti 50 Pt 43.75 V 6.25 and Ti 50 Pt 43.75 Ni 6.25 suppressed the austenite to martensite temperature during the exothermic process. The martensite start (M S ) temperature decreased from 1015 to 791℃ and from 1015 to 903℃ with 6.25 at.…”

“…The increase in thermal hysteresis was also observed in the solution heat treated alloys. The latter was also observed with increased V content (12.5 at %) where the austenite to martensite transformation is suppressed leading to wide thermal hysteresis [4] [16]. The enthalpy change (or peak size) of the endothermic and exothermic process is reduced with the initial partial substitution of Pt with either V, Ni or Ir and the peak widens with an increase in the concentrations (at %) of these elements [4][10] [16].…”

“…It was reported that partial substitution of Pt with V resulted in a microstructural matrix of Ti-rich (Ti, V) 3 Pt and Ti 5 (Pt, V) 3 interdendritic and dendritic Ti(Pt,V) phases in the ascast condition [4] [16]. Increase in V (12.5 at %) decreased the volume fraction in dendritic Ti(Pt,V) and increased the eutectic regions composed of (Ti, V) 3 Pt, Ti 4 (Pt, V) 3 and Ti 3 (Pt, V) precipitates [4].…”

“…The increase in Hf content transformed the main matrix of alloy A to the solidification in the grain boundaries of alloy B [8] resulting in the Ti-rich microstructures of increased grain size [8]. Solution heat treatment of these alloys at low and increased V (6.25 and 12.5 at %) decreased the volume fraction of the transforming Ti(Pt,V) phase and increased the volume fraction of other phases with increasing V [4] [16]. It has been reported that heat treatment of alloys improves mechanical properties and transformation temperatures of SMAs [18 -19].…”