Type of presentation: Oral

MS-4-O-1787 Re-examining the role of silver in aluminium alloys: interfacial segregation, growth kinetics and chemical order.

Rosalie J. M.¹, Dwyer C.^2,3, Bourgeois L.²

¹National Institute for Materials Science, ²Monash University, ³Ernst Ruska-Centre and Peter Gruenberg Institute, Forschungszentrum Juelich

rosalie.julianmark@nims.go.jp

Aluminium alloys are an ideal example of materials where an understanding the microstructure at a nanometer scale is critical to effective materials design. A broad range of aluminium-based alloys develop high strength from the controlled precipitation of one of more intermetallic phases, with sizes in the nanometer regime. These alloys have formed the backbone of the aviation industry since its inception and continue to be widely used in transport applications ranging from military and civilian aircraft to automobiles and bicycles. Electron microscopy is a key technique used to study the nucleation and growth, morphology and orientation relationships and interactions with defects of these precipitates in order to understand these systems and thus optimise alloy design.

Although aluminium alloys are considered mature materials with a long history in industrial service, recent advances in microscopy have revealed a wealth of new information about alloys that were considered thoroughly-understood, including simple binary and ternary alloys used as models for the more complex industrial systems.

Recent studies have shown that interfacial Ag segregation is not limited to the well-known Ω-phase in Al-Cu-Mg-Ag alloys, but occurs in different, well-defined ways in other alloys.  In Al-Cu-Ag alloys this segregation takes the form of an Ag bilayer around θ´ (AlCu₂) precipitates [1] and affects the growth behaviour [2] but not the precipitate structure or orientation relationship to the matrix.  In contrast, Ag segregates as a monolayer to γ´precipitates (AlAg₂) in Al-Ag alloys.  The presence of excess Ag solute around the precipitates suggests that growth of the γ´ phase is initially controlled by the rate of migration of the interface, rather than the supply of solute [3].

 These studies have also revealed surprising results about the γ´ phase itself.  This hexagonal, close-packed phase was thought to be chemically ordered, with alternate Ag-rich and -poor layers and a high density of stacking faults.  Recent investigations using aberration-corrected HAADF-STEM and CBED have not only found no evidence of long-range chemical order, but also determined that the phase is essentially free of stacking-faults [3].

This study adds to a growing body of evidence calling for careful re-examination of these well-studied systems to better understand their precipitation behaviour and facilitate further improvements in the performance of industrial aluminium alloys.

[1] Rosalie and Bourgeois, Acta Mater., 60:6033-6041, 2012.

[2] Rosalie and Bourgeois, Light Metals, 365-371,  2013.

[3] Rosalie, Dwyer and Bourgeois, Acta Mater., 69:224-235, 2014.

The authors acknowledge the support of the Australian Research Council via the Centre of Excellence for Design in Light Metals. The authors are also grateful for the use of the facilities at the Monash Centre for Electron Microscopy and engineering support by Russell King.

Fig. 1: Ag segregation to a θ´ (AlCu₂) precipitate.  The micrograph shows Ag bilayers on the θ´-matrix interfaces in an Al-Cu-Ag alloy.  The curve shows the HAADF-STEM intensity profile.	Fig. 2: Chemical order in γ´ precipitates. The HAADF-STEM micrograph (Exp.) is compared with simulations for ordered and  disordered structures. The experimental image shows no evidence of chemical order.
Fig. 3: Ag segregation to a γ´ (AlAg₂) precipitate in an Al-Ag alloy.  HAADF-STEM micrograph (Exp.) and simulations (for foil thickness=42.9 nm) with monolayer and bilayer Ag segregation. The boundary between the hcp γ´ phase and the fcc matrix is indicated. Monolayer segregation best reproduces the experimental image.