The formation of three phases containing Fe and Mn in 5182 aluminum alloy

The categories, crystal types, morphology, dimension, distribution and selectivity of precipitation positions of alloy phases containing Fe and Mn in the DC casting ingot of 5182 aluminum alloy under industrial production conditions after being treated with different homogenization processes were studied through TEM and EPMA with FIB sample preparation. The formation process of three phases containing Fe and Mn was clarified as well. The results showed that there was only one category of alloy phase containing Fe and Mn in the as-cast 5182 aluminum alloy, which was one category of skeletal nonequilibrium constituent Al19(Fe,Mn)4′ with orthorhombic structure formed by the eutectic reaction L → α(Al)’ + Al19(Fe,Mn)4′. After the ingot was homogenized at a high temperature above 460 °C, there were three categories of alloy phases containing Fe and Mn, which were skeletal equilibrium phase Al19(Fe,Mn)4 with orthorhombic structure, short rod-like intermetallic phase Al3(Fe,Mn) with monoclinic structure and acicular alloy phase Al6(Fe,Mn) with orthorhombic structure, respectively. The metastable constituent Al19(Fe,Mn)4′ transformed into the equilibrium phase Al19(Fe,Mn)4 with orthorhombic structure and the alloy phase Al3(Fe,Mn) with monoclinic structure by desolation transformation Al19(Fe,Mn)4’ → Al19(Fe,Mn)4 + Al3(Fe,Mn). In addition, acicular alloy phase particles Al6(Fe,Mn) with orthorhombic structure were precipitated from the alloy matrix with supersaturated solute Fe and Mn atoms through the desolation reaction α(Al)’ → α(Al) + Al6(Fe,Mn). With increasing homogenization temperature, the skeletal constituent Al19(Fe,Mn)4 particles with orthorhombic structure were almost the same, and the short rod-like alloy phase Al3(Fe,Mn) particles, with monoclinic structure changed unobviously in morphology and size, but its number increased significantly. At the same time, the number and the size of acicular Al6(Fe,Mn) alloy phase particles with orthorhombic structure precipitated from the alloy matrix increased.