The nanocluster formation and vacancy behavior of step-quenched Al-Mg-Si alloy and its effect on transition to β" phase via advanced methods

Abstract The effect of step-quenching (SQ) on two/multi-step aging and the bake-hardening response in Al-0.9Mg-1.0Si (in wt.%) alloy was investigated in terms of nanocluster formation and vacancy behavior using advanced methods, including atom probe tomography (APT) and positron annihilation lifetime spectroscopy (PALS) analysis. The SQ treatment was found to be highly effective at suppressing the negative effects of natural aging during two/multi-step aging, compared with pre-aging (PA) treatment. The characteristic behaviors of nanoclusters formed by PA and SQ treatments were completely different. SQ is likely to preferentially form Si-V complexes, which are different than Cluster (1), and Si-rich clusters, as detected by the distribution of cluster compositions using APT analysis. Si-V complexes pre-formed during SQ treatment result in the formation of Mg-Si clusters with higher atomic density, inducing favorable transformation to the pre-β" phase because there are fewer vacancies inside the Mg-Si cluster. Also, the Si-V complexes reduce the fraction of Si-rich clusters commonly formed during natural aging (NA). The step-quenched Al-Mg-Si alloy with great bake-hardening response has promising potential for automotive outer panel applications.