The coherent phase diagram of Aux(Ag0.24Cu0.76)1-x was studied by electron microscopy. Various regions were found in the present experimental coherent phase diagram consistent with the theoretical coherent phase diagram obtained by Yamauchi using the cluster variation method: (i) a single-phase region comprising a disordered solid solution of f.c.c. α0 phase, (ii) a two-phase decomposition region containing copper-rich f.c.c. α1 and silver-rich f.c.c. α2 phases, (iii) coexisting regions of a disordered phase and an ordered phase, α2 + Cu3Au(L12), α2 + AuCuII(L10-s) and α2 + AuCuI(L10), (iv) coexisting regions of two disordered phases and an ordered phase, α1 + α2 + Cu3Au and α1 + α2 + AuCuII, and (v) coexisting regions of a disordered phase and two ordered phases, α2 + Cu3Au + AuCuII and α2 + AuCuI + AuCuII. Microstructural features of these regions were described in association with phase identifications.
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