Identification of Fe-bearing phases in the as-cast microstructure of AA6026 alloy and their evolution during homogenization treatment

Abstract

The Fe-bearing intermetallic phases present in the as-cast state of an AA6026 alloy and their evolution during homogenization treatments at 480-550?C were studied by optical microscopy, SEM, and TEM techniques combined with EDS analysis. Besides the ?- Al(Fe,Mn)Si phase of dendritic morphology, two types of plate-like Fe-bearing microconstituents were revealed in the microstructure of the as-cast alloy. The EDS microanalysis and electron diffraction showed that one set of platelets represents just thin sections of ?-Al(Fe,Mn)Si microconstituent. The other set of plate-like microconstituents was identified as a tetragonal, silicon-rich ?-Al4(Fe,Mn)Si2 phase. The formation of the ?- Al4(Fe,Mn)Si2 phase has been attributed to the chemical composition of the alloy. During homogenization, metastable ?-Al4(Fe,Mn)Si2 transformed into the ?-Al(Fe,Mn)Si phase and fragmented. The dendritic ?-Al(Fe,Mn)Si microconstituents underwent fragmentation as well. However, whereas ?-Al(Fe,Mn)Si microconstituents preserve b.c.c. crystal lattice throughout the process, the product of the transformation of the ?-Al4(Fe,Mn)Si2 phase exhibited primitive cubic lattice.