Counting the coupled strengthening of α-Al(Fe,Mn)Si dispersoids and β″ precipitates in an Al-Mg-Si-Mn-Fe alloy: optimizing the solid-solution temperature

Abstract

For Al alloys, it is generally preferred to have a high solid-solution temperature T to enhance the strength, as this allows for effective dissolution of coarse constituents and facilitates the formation of sufficient precipitation (i.e., β″ in Al-Mg-Si alloys). However, this work presents an interestingly nonmonotonic relationship between strength and solid-solution temperature T in a commercial 6082 alloy (Al-Mg-Si-Mn-Fe-based), where the highest strength is achieved at a moderate T of ∼530°C. Detailed microstructural characterizations further reveal that this behavior can be attributed to the competitive precipitation of α-Al(Fe,Mn)Si dispersoids and β″ precipitates, which are governed by the following mechanisms: (i) A relatively low T of 500∼530°C fails to dissolve coarse constituents adequately, resulting in insufficient precipitation of α-Al(Fe,Mn)Si and β″ phases and consequently leading to low strength. (ii) Further evaluating solid-solution temperature T to 530∼570°C greatly encourages the precipitation of α-Al(Fe,Mn)Si dispersoids by costing constituents. However, this also depletes Si solutes primarily required for β″ precipitation, thereby impairing overall strength. (iii) The optimal compromise between α-Al(Fe,Mn)Si and β″ particles occurs at a moderate T (∼530°C), resulting in the greatest strength of the alloy. The corresponding strengthening model is also exploited to clarify this evolution, aiming to offer some clues on managing the competitive relationship among multiple strengthening contributors in aging-hardenable Al alloys.