Influence of the Hf/Zr Ratio on the Structure and Functional Properties of Medium-Entropy Ti–Hf–Zr–Ni–Cu–Co Shape Memory Alloys

Abstract The influence of the Hf/Zr ratio on the structure, martensitic transformations, and functional properties of cast senary medium-entropy Ti–Hf–Zr–Ni–Cu–Co shape memory alloys with a total Hf+Zr concentration of 10 at % was studied. It was found that the alloy structure did not depend on the Hf/Zr ratio. All alloys underwent the B2 ↔ B19′ martensitic transformation regardless of the Hf/Zr ratio. An increase in the Zr concentration from 2.5 to 7.5 at % decreased the martensitic transformation temperatures by 10–15°C. Mechanical tests carried out in the austenitic and martensitic states showed that on an increase in Zr concentration, the martensite reorientation stress decreased, whereas the yield limit for dislocation slip in the austenitic phase increased. A study of the recoverable strain variation on cooling and heating under stress showed that its maximum value was equal to 6% and did not depend on the Hf/Zr ratio. The recovery coefficient increased with a rise in the Zr concentration due to a suppression of the dislocation slip on cooling under stress.