Structural Characterization of Binary Al-Cu Alloy Processed by Equal Channel Angular Pressing at Half of the Melting Point

Microstructure evolution in Al-3 wt pct Cu alloy during equal-channel angular pressing (ECAP) at 423 K (150 degrees C) (similar to 0.5 T-m) was studied. The processing temperature roughly corresponded to the transition point from cold to hot deformation associated with preferred formation of either cellular/deformation band structures, or more equilibrium subgrain structure, respectively. Similar to cold deformation, the main feature of warm ECAP at e = 1-2 was development of dislocation structures consisting of cell/microshear bands and larger-scale deformation bands, leading to grain fragmentation. With further straining, the cell bands gradually transformed into subgrain structure, and the microshear bands became the main sources of grain refinement. New ultrafine grains were formed due to mutual crossing of microshear bands, followed by increase in their number and misorientation. Simultaneously, coarsening of Theta(Al2Cu)-phase precipitates, the average size of which increased from 15 to 35 nm, facilitated dislocation rearrangement. As a result, an inhomogeneous structure with a fraction of high-angle boundaries of about 0.5, consisting of arrays of new grains about 1.2 mu m in size and fragments of original grains, was formed at e = 8. It was concluded that grain refinement occurred mainly due to continuous dynamic recrystallization.