The microstructural evolution during the equal channel angular (ECA) pressing process of an α+θ′ two-phase Al-1.7 at%Cu alloy has been studied by transmission electron microscopy (TEM). The θ′ precipitates are severely deformed after the first pass of ECA pressing that gives rise to a strain of ε to approximately 1. After 8 passes of the ECA pressing, most of the θ′ precipitates are dissolved and the nearly single phase of α with a fine grain size of approximately 500 nm is obtained. When the single phase ultrafine grain microstructure which is obtained from a supersaturated Al-1.7 at%Cu is aged for 24 h at 100 °C, precipitation of the equilibrium θ phase is observed along the grain boundaries, whereas only GP zones are formed in the undeformed specimen. This suggests that the grain boundaries in the ultrafine grained microstructure provide heterogeneous nucleation sites for the θ precipitates. The supersaturation required for precipitation of the GP zones is lost quickly by diffusion of Cu to the grain boundaries when a larger strain is imposed by ECA pressing.
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