Microstructural features and functional properties of NiCuTiZrAl high entropy shape memory alloys

In the present study, the Ni₂₅Cu₂₅Ti_35-XZr₁₅Al_X (X = 5, 7.5, 10, 12.5) high-entropy shape memory alloys (HESMAs) with the lower costs were designed and optimized. The results revealed that the pronounced segregation behavior existed in the as-casted NiCuTiZrAl high-entropy shape memory alloys, which resulted in the generation of dendrite-interdendrite structure. Among, the dendrite region was mainly comprised of B2 TiNi type matrix phase, while the NiAl-rich phase with BCC structure and a AlCu₂Zr phase with FCC structure can be observed in interdendrite region. With Al content increasing, the volume fraction of B2 phase increased firstly and then decreased. In proportion, the mechanical properties of the as-casted Ni₂₅Cu₂₅Ti₃₂Zr₁₅Al_X HESMAs firstly increased and then decreased with the increase of Al content. In contrast, the as-casted Ni₂₅Cu₂₅Ti₂₅Zr₁₅Al₁₀ HESMAs exhibited a superior combination of higher compressive strength and microhardness as well as the excellent superelasticity with the recoverable strain of 5 % and the excellent cyclic stability, which can be the solution strengthening of multiple principle elements and precipitation strengthening of higher density of nano-scale precipitates.