Superposed hardening from precipitates and dislocations enhances strength-ductility balance in Al-Cu alloy

The age hardening response and tensile properties in an Al-Cu alloy pre-deformed by 20-80% and post-aged at 120-165 degrees C have been examined to study the effect of mixed dislocations/precipitates on the strength-ductility balance. As compared to the T8 temper, both yield strength and uniform elongation are significantly improved by a pre-deformation of 20-40%, followed by ageing at 120 degrees C. As compared to the T6 condition, the yield strength is higher (by about 50%), while the uniform strain is similar. The dislocation self-organization evolves from forests to tangles and cell boundaries with the pre-deformation level increasing to 60-80% and keeps preserved after post-ageing. The heterogeneous formation of theta' phases at dislocations and the homogeneous matrix precipitation of theta ''/GP zones constitute the hardening particles. The quantitative analysis suggests dislocations and theta' precipitates are the major strength contributors. Off-alignment between the deformed precipitate and the un-deformed one occurs for both theta' and theta '' during tensile straining. The dislocations tend to accumulate around large theta', eventually leading to the theta' rotation by 2-5 degrees and severe matrix distortion near the precipitate. The fine and relatively sparse theta' phases in the 20-40% pre-deformed alloy post-aged at 120 degrees C firstly block and then transmit dislocations, as is evidenced by the shearing at multiple locations, resembling the theta '' phases. The correlation between the precipitate-dislocation interaction and the obtained properties supports that localized dislocation pile-up at hard barriers, e.g. large theta' and cell boundaries, could initiate void and deteriorate the plasticity. Suppressing the dense formation of non-shearable precipitates delivers exceptional strength-ductility synergy in the pre-deformed and post-aged alloy.