In-situ study of phase transformations during homogenization of 6005 and 6082 Al alloys

• Published in: Journal of alloys and compounds 2017-11, Vol.725, p.504-509
• Main Authors: Bayat, N., Carlberg, T., Cieslar, M.
• Format: Article
• Language: English
• Subjects: 6xxx Al alloys; Aluminum base alloys; Beta rays; Billet casting; Boundary layer; Diffusion; Diffusion layers; Diffusion rate; Extrudability; Extrusion; Extrusion billets; Heat treatment; Homogenization; In-situ TEM; Intermetallic compounds; Magnesium compounds; Metal silicides; Phase transformation; Phase transitions; Platelets; Pressure casting; Surface diffusion; Workability
• ISSN: 0925-8388; 1873-4669; 1873-4669
• DOI: 10.1016/j.jallcom.2017.07.149

Intermetallic β-Al5FeSi phase and coarse Mg2Si particles have negative effects on extrudability and workability of 6xxx Al alloy billets. To achieve extruded products with a high surface quality as-cast billets are therefore heat-treated before extrusion. During heat treatment the undesired intermetallic particles, i.e., β-AlFeSi platelets, are transformed to a rounded α-Al(FeMn)Si intermetallic phase. This transformation was studied in-situ by TEM for 6005 and 6082 Al alloys. It was observed that the Mg2Si particles precipitate in the Al matrix at about 250 °C; this precipitation also occurred at the edge and faces of beta intermetallic particles, and the Mg2Si particles were the preferred sites for α-Al(FeMn)Si particle nucleation. The transformation proceeded faster and at lower temperatures, 350–450 °C, than what has been reported earlier for homogenization studies of bulk samples and industrial billets. This could be associated with the thin characteristic of used samples in TEM giving contribution from fast surface diffusion, but it was also concluded that the phase boundary layer diffusion was important for the understanding of how the transformations proceed. Two types of phase transformation of β-AlFeSi platelets to a rounded α-Al(FeMn)Si intermetallic phase during homogenization in 6005 and 6082 Al alloys. [Display omitted] •The in-situ phase transformations during homogenization were studied by TEM for Al alloys.•The Mg2Si precipitations were the preferred sites for α-Al(FeMn)Si particle nucleation.•After nucleation, the α-Al(FeMn)Si particles grow and coarsen via two different modes on the expense of the β−phase.•An explanation of the possible mechanisms taking place during transformation from β- to α-phase is presented.