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

Abstract

Shape Memory Alloys (SMAs) are popular due to their ability to regain their original shape after plastic deformation. Binary TiPt alloys have low shape memory effects and as such, alloying elements are added to improve these properties. The effect of alloying and increasing the Hf in binary Ti50Pt50 to form ternary Ti50Pt50−xHfx (x=6.25,12.5 and 18,25 at.%) on the microstructure, transformation temperatures and hardness in the as cast and solution heat treatment conditions were investigated. In the as-cast condition SEM EDS analysis showed that alloying with Hf resulted in the formation of dark grey interdendritic regions comprised of Ti-rich phases and the increase in Hf content (at. %) led to an increase in the size of these interdendrites. Solution heat treatment (SHT) of the alloy led to a decrease in the size of the dark grey interdendritic regions and the formation and increase of dark black regions with increasing Hf content (at. %). Differential scanning calorimetry showed that partial substitution of Pt with 6.25 at. % Hf decreased the transformation temperature of the alloy below that of Ti50Pt50 in both conditions. Increase in to Hf 12.5 and 18.25 at. % increased the transformation temperatures above those of Ti50Pt50 binary alloy in both conditions. SHT of 12.5 at. % Hf significantly reduced the AS and MS temperatures. Alloying and increase in Hf content (at. %) increased the hardness of the alloys above that of Ti50Pt50. SHT increased the hardness of the alloys above that of Ti50Pt50 and the as-cast alloys.