High Entropy Shape Memory Alloys - Effects of Chemical Complexity on Martensitic Transformation

High entropy alloys have recently received significant scientific attention as they allow to study how chemical complexity affects elementary aspects such as phase stability, diffusion and deformation behavior. A few years ago, a new compositionally complex shape memory alloy (SMA) of type Ni-Cu-Co-Ti-Hf-Zr was introduced by G.S. Firstov and co-workers. This material, which is derived from binary Ni-Ti, shows a reversible martensitic transformation at elevated temperatures. In the present study we investigate how the chemical complexity affects the martensitic transformation in NiTi-based SMAs in terms of phase transformation temperatures, latent heats, cyclic stability and microstructures. We consider Firstov-type high entropy shape memory alloys (HESMAs) and SMAs with compositions from HESMA subsystems with lower configurational entropy. The different SMAs were prepared by arc melting and subsequent heat treatments. The phase transformation behavior was characterized by differential scanning calorimetry (DSC). HESMA microstructures were characterized by scanning electron microscopy (SEM) and electron probe micro analysis (EPMA). In the present work, we present first results on the formation of martensite in chemically complex SMAs during in-situ cooling in a scanning electron microscope.