Effect of second phases on microstructure, martensitic transformation, and mechanical properties of Au–Cu–Al ternary alloys

The effects of second phases on the microstructure, martensitic transformation, and mechanical properties of Au–Cu–Al ternary alloys were investigated across selected compositions. Alloys near the β-phase region formed second phases, identified as the α (fcc) phase or Au 4 Al, depending on composition. The introduction of these phases generally resulted in grain refinement. This observation is consistent with a pinning effect of second-phase particles on grain boundary migration. The martensitic transformation temperature tended to increase in alloys containing second phases. This behavior is likely associated with compositional changes in the matrix caused by second-phase formation, particularly Cu depletion, with possible additional contributions from changes in Al content and microstructural constraints. Mechanical testing revealed that alloys containing the α phase exhibited improved ultimate tensile strength (UTS) and fracture strain compared with β single-phase alloys. The α phase was frequently observed along grain boundaries in alloys exhibiting improved ductility, suggesting a possible contribution to ductility enhancement. In contrast, alloys containing Au 4 Al showed only limited improvement in mechanical properties. This behavior may be related to the limited contribution of Au 4 Al to plastic deformation. These results indicate that the introduction of the α phase effectively improves mechanical properties. The observed improvement is likely associated with grain refinement and grain-boundary modification. However, compositions that promote α-phase formation tend to exhibit martensitic transformation temperatures above body temperature. Therefore, achieving a balance between enhanced ductility and transformation behavior near physiological temperature remains a key challenge in the Au–Cu–Al system.