Effect of off-stoichiometric compositions on microstructures and martensitic phase transformations in Ni-Cu-Pd-Ti-Zr-Hf high entropy shape memory alloys

High entropy shape memory alloys (HE-SMAs) represent a new class of materials which show reversible martensitic phase transformations at elevated temperatures. HE-SMAs were derived from binary NiTi, to which the elements Cu, Pd, Zr and Hf are added. HE-SMAs represent ordered complex solid solutions. Their high temperature phase is of B2 type, where the different elements occupy specific sites in the former Ni- and Ti-sub-lattices. In the present work, we study how deviations from the ideal equimolar composition affect microstructures and phase transformation behavior using advanced microstructural and thermal characterization methods. The ratios of Ni-equivalent (Ni, Cu, Pd) and Ti-equivalent (Ti, Zr, Hf) elements in HE-SMAs were varied to establish systems that correspond to stoichiometric, under- and over-stoichiometric binary alloys. The results show that basic features of the microstructures in HE-SMAs were inherited from nine different binary HE-SMA sub-systems. As a striking observation, the dependence of phase transformation temperatures on the level of off-stoichiometry in HE-SMAs differs from what is known for binary reference systems. This is due to specific effects of individual alloy components and to the higher numbers of degrees of freedom in chemically complex alloys. The findings are discussed in the light of previous work on the concentration dependence of transformation temperatures in shape memory alloys.