Martensitic transformation of high-entropy and medium-entropy shape memory alloys

As new generation of high-temperature shape memory alloys, high-entropy alloys (HEAs) have been attracted for strong solid-solution hardened alloys due to their severe lattice distortion and sluggish diffusion. TiPd is the one potential high-temperature shape memory alloys because of its high martensitic transformation temperature above 500 °C. As constituent elements, Zr expected solid-solution hardening, Pt expected increase of transformation temperature, Au expected keeping transformation temperature, and Co expected not to form harmful phase. By changing the alloy composition slightly, two HEAs and two medium entropy alloys (MEAs) were prepared. Only two MEAs, Ti₄₅Zr₅Pd₂₅Pt₂₀Au₅, and Ti₄₅Zr₅Pd₂₅Pt₂₀Co₅ had the martensitic transformation. The perfect recovery was obtained in Ti₄₅Zr₅Pd₂₅Pt₂₀Co₅ during the repeated thermal cyclic test, training, under 200 MPa. On the other hand, the small irrecoverable strain was remained in Ti₄₅Zr₅Pd₂₅Pt₂₀Au₅ during the training under 150 MPa because of the small solid-solution hardening effect. It indicates that Ti₄₅Zr₅Pd₂₅Pt₂₀Co₅ is the one possible HT-SMA working between 342 and 450 °C.