Effects of Al(MnFe)Si dispersoids with different sizes and number densities on microstructure and ambient/elevated-temperature mechanical properties of extruded Al–Mg–Si AA6082 alloys with varying Mn content

Abstract

The effects of Al(MnFe)Si dispersoids, with different sizes and number densities, on the evolution of microstructure and ambient/elevated-temperature mechanical properties of extruded AA6082 alloys, with varying Mn content, under T5 conditions, were investigated. Compared to the low density of coarse dispersoids formed during conventional homogenization, the high density of fine dispersoids formed during a new low-temperature homogenization was more effective in increasing the material’s resistance to plastic deformation during extrusion, resulting in the dissolution of more constituent Mg2Si particles into the α-Al matrix. A large amount of β", some β′ precipitates and fine dispersoids co-existed in the α-Al matrix of 0.5% Mn containing alloy, which afforded this alloy a substantial increase in ambient-temperature yield strength of 65–75 MPa under T5 conditions compared to the base alloy without dispersoids. A further increase in the Mn content decreased the number density of the β" precipitates, resulting in a decline in the mechanical properties. Upon thermal exposure at 300 °C for 100 h, β"/β′ fully transformed into an undesirable equilibrium β phase and lost their strengthening effect, while fine and dense dispersoids became the dominant strengthener, leading to a 55–70% increase in the elevated-temperature yield strength relative to the alloys either without dispersoids or with coarse dispersoids. Dispersoid strengthening was more pronounced at 0.7% Mn addition as further increasing the Mn content mainly contributed to the fraction of insoluble Mn-containing intermetallics.