Impact of atomic couples and pairs on quenched-in vacancies in Al-Mg-Si-Cu alloys

After solutionizing and quenching of Al-Mg-Si-Cu alloys at cooling rates differing by a factor of about 200, the quenched-in vacancy concentration difference as measured by positron annihilation lifetime spectrometry is only about 30 times. This contradicts the expected ∼200 times difference predicted by the recently developed FSAK model for vacancy generation and annihilation at dislocation jogs and grain boundaries. To address this discrepancy, we investigate the influence of atomic couples and pairs (C&Ps) on the quenched-in vacancy concentration. A combined theoretical framework incorporating C&Ps formation kinetics and vacancy trapping is developed and applied to an Al-Mg-Si-Cu alloy. The results indicate that Si-Si pairs in Al-Mg-Si-Cu alloys act as primary vacancy traps during quenching, capturing significantly more vacancies than isolated solutes. The simulations satisfactorily explain the experimentally measured ∼30 times difference in quenched-in vacancy concentration compared to the much larger difference predicted in the absence of vacancy trapping on C&Ps.