Effects of manganese on microstructure and mechanical properties of A206 alloys containing iron

The effects of Mn and Fe contents on the mechanical properties of aluminum-based A206 alloys were investigated quantitatively. Results showed that the addition of Fe caused a loss in both ductility and yield strength. Further addition of Mn could recover the ductility, but it caused a further loss in yield strength. In low-Mn alloys (0.29 wt% Mn) the primary constituent was the needle shape of Cu₂FeAl₇. Upon further addition of Mn, the Chinese script configuration of Mn-bearing particles formed instead. The Cu₂Mn₃Al₂₀ particles formed in high-Mn alloys during solution treatment and resulted in grain-growth inhibition. The needle, Mn-bearing, and Cu₂Mn₃Al₂₀ particles caused the solid solution level of copper in the matrix to decrease; meanwhile, increasing the Mn solution level retarded the precipitation of the strengthening phase. Differential scanning calorimetry analyses showed the kinetics and amount of decrease in θ′ phase precipitation when the contents of Fe and/or Mn were increased. The smaller grain size induced by the Cu₂Mn₃Al₂₀ particles and the θ′ phase were the factors that determined the hardness of A206 alloys under as-quenched and T7-treated conditions, respectively.