Insights into the chemical composition of the cubic αc phase with 3d-transition metal elements V, Cr, Mn, Fe, Co, Ni, Cu, Zn as occurring in Al-Si alloys

The so-called cubic α c phase is frequently present in recycled transition-metal containing Al−Si alloys, in intermetallic layers evolving during reactive diffusion occurring when bonding an Al alloy to a steel or cast iron, and even in meteorites. Thereby, the α c phase is known for its broad homogeneity ranges and various morphologies. The present study provides own systematic results on the chemical composition of the α c phase evolving in Fe-containing Al−Si alloys with the 3d-transition metal elements V, Cr, Mn, Co, Ni, Cu, Zn and combines these results with an extensive literature review. It is concluded that the α c phase can be considered as a solid solution which spans a broad homogeneity range via thermodynamically connected multicomponent systems. That range originates from the ternary Al-Si-Mn system, reaches towards the ternary Al-Si-Fe system and exists without Mn also in the Al−Si−Fe−Cr, Al−Si−Fe−V and Al−Si−Fe−Cu systems. In order to dissolve Co and Ni, also presence of Mn or Cr is required. Zn was not detected in the α c phase. Thereby, the α c phase accepts a system-dependent Si content from 9-12 up to 6-17 at% while the total transition metal content scatters around the crystallographically ideal value of 17.4 at% except for higher contents with Cu. A phenomenological electron counting based analysis (Hume-Rothery like approach) of the structurally contributing number of electrons per atom shows that Fe, Cr and V contribute a similar number of electrons as Mn while Co and Ni contribute approx. 1 electron per atom fewer.