Au_52.5-xCd_47.5Ag_x has a single β phase in the composition range of x ≤ 41 and a wide temperature range extending to room temperature or below. Coexistence of the β and α phases has been observed for 41 < x < 47, and the coexistence of the β, α, and ζ phases has been observed for x ≥ 47, as thermal equilibrium states. We found that the α phase appears and coexists with the β phase for x ≤ 41 in the aged alloy at room temperature. The mole fraction and the stability of the α phase depend on the atomic order of the β phase. Moreover, it was found that the ζ phase is stress induced in the composition range of x ≤ 41. The ζ phase disappears with heat treatment to release strain, and so this ζ phase is not stable.