Influence of Mn on nanoscale precipitate formation in Al-Cu-Mn alloys*

Aluminum alloys having high strength and low density are promising for use in transportation industry. Recent studies have shown that the addition of Mn to Al-Cu alloys stabilizes metastable precipitates by segregating to alloy-precipitate interfaces, and allows the alloys to retain strength at higher temperatures. In this work, we fabricated Al-Cu-Mn alloys with varying Mn concentration (0.3-1% wt.) and investigated the formation of nanoscale precipitates viz. θ' and T-phases. While at high Mn content (1% wt.), we observed an overwhelming majority of T-phase precipitates and overall higher hardness under isothermal aging. With decreasing Mn content, the precipitation of T-phase is suppressed and a high density of θ' precipitates are observed. First-principles density-functional-theory calculations show that the interface energy of θ' with Al matrix is lower compared to that of the T-phase, which is one of the key factors that govern nanostructure evolution with varying Mn addition. We will also investigate other factors including Mn segregation and elastic-strain energy on alloy-precipitate interfaces.