About the Al–Cu–Si isothermal section at 500 °C and the stability of the ɛ-Cu 15Si 4 phase

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The 500 °C isothermal section of the Al–Cu–Si system has been determined in the whole composition range, with special attention to the Cu-rich corner. A number of Cu–Si binary alloys equilibrated at 500–780 °C have also been investigated. A ternary point compound at 1.5 at% Al and 21 at% Si has been identified. It has the same crystal structure of the binary phase previously reported as ɛ-Cu 15Si 4. This phase however is not stable in the binary system, at least between 500 and 780 °C, and it is stabilised by small amounts of impurities. The γ 1-(Al,Cu) phase dissolves about 11.5 at% Si and ternary solubility extends in the direction of a constant valence electron concentration. The binary κ-(Cu,Si) phase is stable in a ternary solubility range close to the (Cu) terminal solid solution.