Influence of Rolling and Annealing on the Structure and Properties of the Medium-Entropy Ti₄₀Hf₅Zr₅Ni₄₀Cu₅Co₅ Shape Memory Alloys

The influence of the hot rolling and heat treatment on the structure, martensitic transformations, and mechanical behaviour of the medium-entropy Ti₄₀Hf₅Zr₅Ni₄₀Cu₅Co₅ shape memory alloy has been investigated. The results obtained showed that, after hot rolling at 900°С, the alloy does not undergo martensitic transformations, which is caused by the high dislocation density. Heating to 500°С is accompanied by heat release at temperatures of 257–323°С. The martensitic transformations are restored; however, they are observed in a wide temperature range, which is typical of ultra-fine grain materials. Holding the sample at 500°С for 1 h leads to grain growth; however, the grain size remains less than 1 μm. This circumstance reduces the temperature ranges of martensitic transformations and increases the dislocation yield limit, as compared to the initial state (before hot rolling). Annealing at 700°С for 1 h is accompanied by grain growth to 3.5 μm, which leads to additional decrease in the temperature ranges of the transformations, increase in hysteresis, and decrease in the yield limit for dislocation slip. The strength and strain up to failure increase as compared to the values found in the sample after annealing at 500°С for 1 h.