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

¹ Advance Materials Manufacturing, Manufacturing Cluster, Council for Scientific and Industrial Research, P.O. Box 395. Pretoria, 0001, South Africa
² Department of Metallurgy, School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, Doornfontein Campus, Johannesburg, 2028, South Africa

^* Corresponding author: emoletsane@csir.co.za
^* Corresponding author: tmotsai@csir.co.za
^* Corresponding author: emakhatha@uj.ac.za

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 Ti₅₀Pt_50−xHf_x (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 A_S and M_S 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.