An aluminium-4% copper alloy has been heat treated to give the supersaturated solid solution, G.P. zones, and θ′ precipitates. Specimens have then been deformed at various strains up to a true strain of 5 by cold rolling. The tensile properties have been determined and then related to the microstructural changes and fracture modes observed with the transmission and scanning electron microscopes. At low strains solute atoms and G.P. zones strongly interact with gliding dislocations giving a high and uniform density of dislocations. Slip and eventually shear banding become the major modes of strain accommodation. At high strains G.P. zones are completely disrupted by dislocations passing through them and the mechanical properties are then very similar to those found in the supersaturated condition. In the alloy containing θ′ precipitates work softening occurs in the true strain range 0.3-1.0. The θ′ precipitates are then disintegrated to give small spherical particles which subsequently interact with dislocations to give high uniform dislocation densities and consequent work hardening. It has been found that the deformation modes are not homogeneous, and when high and uniform dislocation densities are achieved, further strain is accommodated by slip and shear banding.
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