Solute Sn-induced formation of composite beta '/beta '' precipitates in Al-Mg-Si alloy

The composite beta'/beta '' precipitates rather than well-defined beta' phase form in trace-Sn containing Al-Mg-Si alloy peak-aged at 250 degrees C, leading to considerable morphological refinement. The preferential occupation of 2 similar to 6 adjacent Si sites of the tiny beta' phase by Sn and typical beta '' substructure units covering the precipitates are frequently observed. The substructures grow upon further ageing into differently oriented beta '' with 2 similar to 3 complete structural unit cells. It is inferred that the beta' (Mg9Si5-xSnx) forms initially on the Sn/vacancy complexes and then acts as heterogeneous nucleus for beta ''. These findings demonstrate Sn could fundamentally alter the pre-/precipitation behaviors in Al-Mg-Si alloys. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.