Effects of small pre-stretching on the microstructural evolution and strengthening of an Al–Cu–Mg–Ag alloy

Pre-stretching prior to aging is known to degrade the performance of Al–Cu–Mg–Ag alloys at elevated temperatures. In this work, small pre-stretching by 0.5% and 1.8% is selected to explore its potential influences on the microstructure and strengthening capacity of an Al–Cu–Mg–Ag alloy. The strength and microstructure of Al–Cu–Mg–Ag alloy are found to be sensitive to the pre-stretching. Even 0.5% pre-stretching can reduce the alloy strength by more than 10 MPa. Quantitative transmission electron microscopy (TEM) and atom probe tomography (APT) analysis indicate that this detrimental effect of pre-stretching on strength is directly associated with the suppressed Mg–Ag co-clustering, which greatly delays the Ω precipitation and induces a weaker strengthening effect. Quantitative calculation reveals that Mg–Ag co-clusters exhibit the greatest strengthening contribution to the yield strength of the underaged material because of its strongest cluster hardening, which is mainly originated from its order hardening capacity. As the pre-stretching increases from 0.5% to 1.8%, the θ coarsening is progressively intensified during exposing at 200 °C, accompanied by a gradual drop in the number density of Ω phase. Besides, the formation of a cubic σ phase is found to be considerably enhanced after exposing at 200 °C for 500h as the pre-stretching increases