Influence of Mn Addition on the Evolution of Precipitates in Al-Cu Alloys

Al-Cu alloys are widely applied in the automobile and aircraft industries because of their low density, good wear resistance, low cost, and excellent mechanical properties. Microalloying, particularly using Mn, can further enhance the mechanical properties. Thus, a combination of high-angle annular dark-field scanning transmission electron microscopy, STEM-EDX elemental mapping, and tensile tests were used to investigate the effects of Mn content on the nucleation, growth, and structural evolution of θ’-Al₂Cu and Mn-rich precipitation phases of Al-4Cu-xMn alloys (x = 0.1  wt.%, 0.2  wt.%, 0.3  wt.%, and 0.5 wt.%). The results revealed that adding Mn to Al-Cu alloys can effectively promote the precipitation of θ’-Al₂Cu; particularly, increasing Mn content from 0 wt.% to 0.2 wt.% reduced precipitate size and size distribution. Moreover, the 0.3-wt.% Mn addition reduced θ′-Al₂Cu precipitation and coherent Mn-rich phase precipitation with butterfly contrast in the alloy. The addition of 0.5-wt.% Mn resulted in disappearance of the θ’-Al₂Cu phase and precipitation of a rod-like Mn-rich phase in the alloy, which was found to have coherent and semi-coherent interfaces with an Al matrix. Promotion of precipitation and refinement in the θ′-Al₂Cu phase is primarily attributed to the excellent miscibility and interfacial segregation of the Mn element, which effectively enhanced heterogeneous nucleation in the θ′-Al₂Cu phase. Additionally, two morphologies of the Mn-rich phase exhibited a relatively large coherence area with the matrix relative to θ′-Al₂Cu; thus, our study provides a new strengthening and toughening technique for Al alloys.