Enhanced homogeneous dispersoid precipitation by adding Si and Cr in an Al-Mg-Mn alloy

For deformation-strengthened Al-Mg-Mn alloys, regulating the precipitation of Mn-rich dispersoids and ensuring their uniform distribution is conducive to improving recrystallization resistance and mechanical properties. This work investigates the effects of Si, Cr and heat treatment on the formation and distribution of Mn-rich dispersoids inAl - 4.0 wt% Mg - 0.8 wt% Mn alloy, as well as the relationship between microstructure and mechanical properties. The results indicate that the optimal double-stage heat treatment (DH) parameters for the precipitation of Mn-rich dispersoids are 280 °C / 12 h + 375 °C / 48 h. After the triple-stage heat treatment (TH) of 280 °C / 12 h + 375 °C / 48 h + 450 °C / 12 h, the area fraction of dispersoids free zone (DFZ) in 0.1 wt% Si alloy was significantly reduced compared to the base alloy. However, further increasing the Si content does not significantly reduce the DFZ, which is due to the consumption of a part of Si by the Mg2Si primary phase and the micro-segregation of Si in the as-cast alloy (the equilibrium distribution coefficient of Si is much less than 1). In the alloy containing 0.2 wt% Si and 0.2 wt% Cr, the Mn-rich dispersoids were uniformly distributed in the whole dendrite due to the opposite equilibrium distribution coefficients of Mn and Cr. Therefore, after hot-rolled and annealing heat treatment, the alloy with 0.2 wt% Si and 0.2 wt% Cr exhibits excellent mechanical properties, which are manifested as ultimate tensile strength of 327.0 MPa, yield strength of 199.0 MPa, and elongation of 14.0 %. Compared with the sparsely distributed Mn-rich dispersoids in the base alloy, the uniform distribution Mn-rich dispersoids and the smaller average spacing in the 0.2 wt% Si and 0.2 wt% Cr alloys enhance the hindrance to the movement of dislocations during the tensile process and improve the Portevin-Le Chatelier (PLC) effect of the alloy.