Eutectic intermetallic formation during solidification of a Mg-Sn-Al-Zn-Mn alloy

Multiple primary and eutectic intermetallic compounds (IMCs) often form during the solidification of multicomponent magnesium alloys. Here, we comprehensively explore eutectic intermetallic formation in Mg-4Sn-3Al-1.5Zn-0.6Mn (wt%) as-cast alloy, in comparison with Mg-3Al-1Zn-0.3Mn, to better understand Mg2Sn formation during complex multiphase solidification and how it affects the formation of other IMCs. Various IMCs formed at different stages of solidification: τ-AlMn and Al8Mn5 early during solidification and Mg2Sn, β-Mg17Al12, and τ-MgAlZn as late forming eutectic phases. Most Mg2Sn phase shared a faceted interface with β-Mg17Al12 and τ-MgAlZn grains, forming a fully divorced eutectic particle containing multiple phases. EBSD-based trace analysis in combination with XCT and TEM results revealed that β-Mg17Al12 and τ-MgAlZn phase grew adjacent to the {100} growth facets of Mg2Sn. Within multi-phase particles, seven types of reproducible orientation relationships (ORs) were measured between each intermetallic. The ORs between Mg2Sn and τ-MgAlZn were found to have the best lattice match among the observed phase pairs. For the defined ORs, Mg2Sn can be correlated with the icosahedral coordination in τ-MgAlZn.