Phase transformation and mechanical properties of HfPd ultra-high temperature shape memory alloys

The effects of chemical composition on the structural, functional and mechanical properties of hafnium palladium (HfPd) alloy system were investigated. Five selected compositions of Hf50-xPd50+x (x = -0.5, 0, 0.5, 2.5 and 5) alloys were studied to reveal their potentials as high-temperature shape memory alloys to be employed in the applications to operate above 400 degrees C. All the alloys showed reversible phase transformations at temperatures above 400 degrees C. The large hysteresis of more than 120 degrees C was observed for HfPd alloys which is higher than most of the other conventional high-temperature SMAs. Thermal hysteresis increases with Pd content, as it was 124 degrees C for Hf51.5Pd49.5 alloy and 158 degrees C for the Hf45Pd55 alloy. The alloys do not show superelasticity and have very low recoverable strain. It was also shown that hardness increased (to above 400 HV) with increasing Pd content. The alloys fail when they are compressed up to 1500 MPa.