Transforming detrimental intermetallics by accumulative thermal and strain energies in the Al–Fe–Si alloy

It is well known that the plate-like Al₁₃Fe₄ intermetallics are detrimental to the mechanical properties of recycled Al alloys which have higher Fe levels than primary Al. In wrought Al alloys, it is difficult to avoid the formation of primary plate-like Al₁₃Fe₄ intermetallics due to its equilibrium nature during solidification and its adaptions of many impurities such as Si, Mn, and Cr to form complex substitutional (Al, Si)₁₃(Fe, Mn)₄ crystals during subsequent homogenization and hot deformation. In this study, the influences of hot processing with different Mn and Cr additions and subsequent homogenization and hot transformation have been investigated. When 0.3Mn is added to Al–0.9Fe–0.1Si alloy, the percentage of Al₁₃(Fe, Mn)₄ decreases from 0.75 to 0.45% after homogenization of 10 h. When both 0.3Mn and 0.3Cr are added, it will decrease from 1.57 to 0.65%. After 30% hot deformation, it further decreases to 0.40% and 0.35% in 0.3Mn and 0.3Mn + Cr alloy, respectively, resulting in an improvement in elongation by 11.15% and 49.35%. Combined with cold deformation by 20%, Al₁₃(Fe, Mn)₄ eventually decreases to 0.30% and 0.24% in 0.3Mn and 0.3Mn + Cr alloys, achieving complete conversion from Al₁₃(Fe, Mn)₄ to α-Al₁₅(Fe, Mn)₃Si₂, and the tensile strength is increased by 23.52% and 19.90%, respectively. To reveal the transformation mechanisms of Fe-rich intermetallics, first-principle calculations are adopted to compare the interfacial energy of Al₁₃Fe₄ and α-Al₁₅(Fe, Mn)₃Si₂ with the Al matrix. It is found that the Al₁₃Fe₄ shows a much more faceted nature than the α-Al₁₅(Fe, Mn)₃Si₂ due to its higher Jackson's α-factor and greater interfacial energy but can be transformed into α-Al₁₅(Fe, Mn)₃Si₂ at certain thermodynamic conditions.