This sample has been aged after quenching to enable the growth of precipitates from a supersaturated solid solution. The strength of the alloy has been greatly improved by precipitation hardening. A precipitation sequence is observed eventually resulting in the formation of the stable θ phase through a series of metastable intermediates. The θ phase is an intermetallic compound with a composition close to CuAl2. The θ precipitates preferentially form on Al grain boundaries, as these are excellent heterogeneous nucleation sites. The depletion of Cu near the boundaries to these precipitates is one reason for the formation of adjacent precipitation free zones (PFZ).
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