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

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• The 20-40% pre-deformation plus post-ageing at 120°C improves both strength and ductility compared to the conventional temper in Al-Cu alloy. • The similar uniform strain with the T6 condition is obtained in the pre-deformed alloy with a strength higher by about 50%. • The quantitative analysis suggests dislocations and θ' precipitates together determine the excellent strength-ductility balance. • Off-alignment between the deformed precipitate and the un-deformed one occurs for both θ' and θ" during tensile straining. • The fine θ' phase leads to the transmittance instead of accumulation of dislocations and sustained plasticity. The age hardening response and tensile properties in an Al-Cu alloy pre-deformed by 20-80% and post-aged at 120-165°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°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 θ' phases at dislocations and the homogeneous matrix precipitation of θ"/GP zones constitute the hardening particles. The quantitative analysis suggests dislocations and θ' precipitates are the major strength contributors. Off-alignment between the deformed precipitate and the un-deformed one occurs for both θ' and θ" during tensile straining. The dislocations tend to accumulate around large θ', eventually leading to the θ' rotation by 2-5° and severe matrix distortion near the precipitate. The fine and relatively sparse θ' phases in the 20-40% pre-deformed alloy post-aged at 120°C firstly block and then transmit dislocations, as is evidenced by the shearing at multiple locations, resembling the θ" phases. The correlation between the precipitate-dislocation interaction and the obtained properties supports that localized dislocation pile-up at hard barriers, e.g. large θ' 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.