Compositional mappings of room-temperature phases and mechanical properties of β-phase Au–Cu–Al shape memory alloys

Au–Cu–Al ternary alloys are promising candidates for biomedical shape memory and superelastic applications; however, their composition-dependent behavior in bulk form has not been fully elucidated. In this study, bulk Au–Cu–Al alloys were fabricated over a wide compositional range within the single β-phase region, and their martensitic transformation behavior, phase constitution, and mechanical properties were systematically investigated. Martensitic transformation was observed for all alloys. The martensitic transformation start temperature ( M s ) decreased primarily with increasing Al content and valence electron concentration ( e/a ). Even at identical e/a values, both M s and phase constitution exhibited a strong dependence on the Cu/Al ratio, with increasing Cu/Al ratio leading to a systematic transition from the L2 1 parent phase to orthorhombic and subsequently monoclinic martensite. Mechanical properties were also strongly influenced by the Cu/Al ratio, and alloys in which monoclinic martensite was stabilized exhibited both higher strength and improved ductility. Furthermore, depending on composition, both superelasticity and twinning pseudoelasticity were observed; however, a clear trade-off between ductility and superelasticity at room-temperature was identified. These results demonstrate that the Cu/Al ratio is a key compositional parameter governing martensitic transformation behavior and mechanical properties, and provide fundamental guidelines for designing Au-based shape memory and superelastic alloys.