Coupled Precipitation of Dual-Nanoprecipitates to Optimize Microstructural and Mechanical Properties of Cast Al–Cu–Mg–Mn Alloys via Modulating the Mn Contents

The effect of Mn content on the microstructure evolution and mechanical properties of Al-Cu-Mg-x Mn alloys at ambient temperature was investigated. The findings show that in the Mn-containing alloys at the as-cast state, the blocky primary T(Al₂₀Cu₂Mn₃) phase coexisting with the Al₂Cu phase appeared. With the increase in Mn content, the majority of the Al₂Cu phase dissolved, nd a minor amount of the T phase remained at the grain boundary after solution treatment. The rod-like T_Mn (Al₂₀Cu₂Mn₃) nanoprecipitate was simultaneously distributed at grain boundaries and the interiors, while a high density of needle-like θ″ (Al₃Cu) nanoprecipitate was also observed in the T6 state. Further increases in Mn content promoted the dispersion of the T_Mn phase and inhibited the growth and transformation of the θ″ phase. Tensile test results show that 0.7 wt.% Mn alloy had excellent mechanical properties at ambient temperature with ultimate tensile strength, yield strength, and fracture elongation of 498.7 MPa, 346.2 MPa, and 19.2%, respectively. The subsequent calculation of strengthening mechanisms elucidates that precipitation strengthening is the main reason for the increase in yield strength of Mn-containing alloys.