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 AA6026 alloy and their evolution during homogenization treatments at 480-550°C were investigated using optical microscopy, SEM, and TEM techniques in combination with EDS analysis. In addition to the α-Al(Fe,Mn)Si phase with 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 represented thin sections of α-Al(Fe,Mn)Si microconstituent. The other set of plate-like microconstituents was identified as a tetragonal, silicon-rich δ-Al4(Fe,Mn)Si2 phase. The formation of the δ-Al4(Fe,Mn)Si2 phase was attributed to the chemical composition of the alloy. During homogenization, the metastable δ-Al4(Fe,Mn)Si2 transformed into the α-Al(Fe,Mn)Si phase and fragmented. The dendritic α-Al(Fe,Mn)Si microconstituents underwent fragmentation. However, while the α-Al(Fe,Mn)Si microconstituents preserved a b.c.c. crystal lattice throughout the process, the product of the transformation of the δ-Al4(Fe,Mn)Si2 phase exhibited primitive cubic lattice.

Ispitivanje prisustva faza koje sadrže Fe u mikrostrukturi livene legure AA6026, kao i njihova evolucija tokom postupaka homogenizacije na temperaturama od 480-550 °C, sprovedena su korišćenjem optičke mikroskopije, skenirajuće elektronske mikroskopije (SEM) i transmisione elektronske mikroskopije (TEM) u kombinaciji sa analizom EDS-a. Osim faze α-Al(Fe,Mn)Si sa dendritskom morfologijom, identifikovane su dve vrste pločastih mikrokonstituenata koji sadrže Fe u mikrostrukturi livene legure. EDS mikroanaliza i elektronska difrakcija pokazali su da jedan skup ploča predstavlja samo tanke sekcije mikrokonstituente α-Al(Fe,Mn)Si. Drugi skup pločastih mikrokonstituenata identifikovan je kao četvrtasta, silicijumom bogata faza δ-Al4 (Fe,Mn)Si2 . Formiranje faze δ-Al4 (Fe,Mn)Si2 povezano je sa hemijskim sastavom legure. Tokom homogenizacije, metastabilna faza δ-Al4 (Fe,Mn)Si2 transformisala se u fazu α-Al(Fe,Mn)Si i fragmentirala. Dendritski mikrokonstituenti α-Al(Fe,Mn)Si takođe su fragmentirali. Međutim, dok su mikrokonstituenti α-Al(Fe,Mn)Si sačuvali kubnu kristalnu rešetku tipa b.c.c. tokom procesa, proizvod transformacije faze δ-Al4(Fe,Mn)Si2 pokazao je prostu kubnu rešetku.