Effects of Hf and Co contents on the functional properties of TiZrHfNiCu(Co) high-entropy shape memory alloys

This study systematically investigated the effects of Hf and Co contents on the martensitic transformation temperatures, microstructures, and functional properties (shape memory effect and superelasticity) of TiZrHfNiCu(Co) high-entropy shape memory alloys. Experimental results indicated that the martensitic transformation temperatures were significantly elevated by increasing the Hf content, whereas they were suppressed by the addition of Co in this alloy system. The volume fraction of the brittle (Zr, Hf) 7 Cu 10 phase increased with the elevation of Hf content. This precipitate hindered the martensitic transformation behavior, thereby limiting the reversible strain and increasing the irreversible strain. In contrast, the formation of the (Zr, Hf) 7 Cu 10 phase was suppressed by the addition of Co, and the brittleness induced by high Hf content was also alleviated, thereby enhancing the functional properties of this alloy system. These features illuminate how alloying elements influence the design of TiZrHfNiCu(Co) high-entropy shape memory alloys across a range of application temperatures, suggesting a pathway for enhancing and tailoring their functional properties.