Influences of Mn content on the microstructures and mechanical properties of cast Al-3Li-2Cu-0.2Zr alloy

In this work, the influences of Mn content on the microstructures and mechanical properties of cast Al-3Li-2Cu-0.2Zr alloy were investigated. The results showed that with the increase of Mn addition, the as-cast grain size was gradually reduced, and the primary Al20Cu2Mn3 phase was formed in as-cast higher Mn alloys (0.8Mn and 1.2Mn alloy). The formation of Al20Cu2Mn3 dispersoids can restrict the grain growth during solution treatment, but decrease the number density of Cu-rich precipitates during ageing treatment (mainly T1-Al2CuLi) because they consume the solute Cu available for precipitation. The tensile property results showed that Mn addition had little effect on the yield strength (YS) but a detrimental effect on the ductility of the as-quenched alloys due to the presence of Al20Cu2Mn3 dispersoids and/or primary Al20Cu2Mn3 phase. With increasing the Mn content, the YS of ageing-treated alloys was continuously decreased, and the highest elongation of 3.5% was obtained in 0.3Mn-bearing alloy after ageing for 32 h at 175 °C. From the comprehensive consideration of ductility and strength, the optimal Mn addition in cast Al-3Li-2Cu-0.2Zr alloy was 0.3 wt.%.