Composition: Al-4%Cu.

Processing: Sand casting, Annealing (525 °C, 16 h) and Water quenching. 

Etching: 15-20 s immersion in mixture of acids (HCl+HNO₃+HF).

According to the Al-Cu phase diagram, annealing at 525°C brings the alloy to the monophasic κ field. As a result, dendritic segregation disappears (see X16). Quenching avoids the precipitation of the θ phase. Micrographs reveal equiaxial grains of supersaturated κ phase with a uniform distribution of copper. Grains boundaries meet at 120° due to equilibrium of surface energies. 

Al-Cu diagram

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