Achieving superior strength-ductility balance by tailoring dislocation density and shearable GP zone of extruded Al-Cu-Li alloy

Abstract

Pre-stretching is commonly employed to accelerate ageing precipitation kinetics in wrought Al-Cu-Li alloys, but uneven precipitation resulting from dislocation pile-ups often degrades ductility. Herein, the strength and ductility of extruded Al-Cu-Li alloy are significantly improved through a novel thermomechanical treatment, involving pre-ageing and pre-stretching, followed by low-temperature interrupted ageing. A superior balance between high yield strength (∼ 657 MPa) and good ductility (elongation to fracture of ∼ 13.5 %) is obtained, with elongation increased by 105 % compared to the conventional T8 temper, while maintaining a respectable yield strength. Microstructure analysis reveals that dense Guinier–Preston (GP) zones induced by pre-ageing effectively dissipate energy from dislocation sliding, resulting in a uniform dislocation configuration even at 8 % pre-stretching. However, the GP zone density is greatly reduced due to their dissolution following pre-stretching. Upon interrupted ageing, the reprecipitation of GP zones forms a homogeneous mixture of δ′, GP zones, and T₁ phases. This combination alleviates local stress concentrations and lengthens the dislocation mean free path during tensile testing by shearing the GP zones at multiple sites, thereby improving ductility. Simultaneously, T₁ precipitates strengthen the alloy by pinning dislocations and promoting dislocation cross-slip, improving work hardening capacity. The dissolution of GP zones also redistributes the Cu atoms within the matrix, further enhancing the intrinsic ductility of the Al matrix. These findings offer valuable insights for developing high-performance wrought Al-Cu-Li alloys.