Density functional study of copper segregation in aluminum

Abstract

International audienceThe structural and electronic properties of Cu segregation in aluminum are studied in the framework of the density functional theory, within the projector augmented plane-wave method and both its local density approximation (LDA) and generalized gradient approximation (GGA). We first studied Al-Cu interactions in bulk phase at low copper concentration (≤3.12%: at). We conclude to a tendency to the formation of a solid solution at T=0 K. We moreover investigated surface alloy properties for varying compositions of a Cu doped Al layer in the (111) Al surface then buried in an (111) Al slab. Calculated segregation energies show unstable systems when Cu atoms are in the surface position (position 1). In the absence of ordering effects for Cu atoms in a layer (xCu=1/9 and xCu=1/3), the system is more stable when the doped layer is buried one layer under the surface (position 2), whereas for xCu=1/2 to xCu=1 (full monolayer), the doped layer is more accommodated when buried in the sub-sub-surface (position 3). First stage formation of GP1- and GP2-zones was finally modeled by doping (100) Al layers with Cu clusters in a (111) Al slab, in the surface then buried one and two layers under the surface. These multilayer clusters are more stable when buried one layer beneath the surface. Systems modeling GP1-zones are more stable than systems modeling GP2-zones. However the segregation of a full copper (100) monolayer in an (100) Al matrix shows a copper segregation deep in the bulk with a segregation barrier. Our results fit clearly into a picture of energetics and geometrical properties dominated by preferential tendency to Cu clustering close to the (111) Al surface

DFT Modelling of Cu Segregation in Al-Cu Alloys Covered by an Ultrathin Oxide Film and Possible Links with Passivity

“…A converged K-points mesh of (6 × 6 × 6) was used for each bulk Al, Cu and (4 × 1 × 4) for Al 2 O 3 . Within this approach, the bulk lattice constants are a = 4.05 Å for Al, a = 3.63 Å for Cu, and a = 9.78 Å, b = 8.32 Å and c = 13.56 Å ((111) γ-Al 2 O 3 ) in good agreement with experimental values and previous studies [6,15]. The unit cell of the supported oxide film model for segregation study has dimensions of (8.58 × 9.90 × 58 Å 3 ) and a surface of 0.85 nm 2 .…”

“…In preliminary tests, we increased the Al(111) slab thickness up to nineteen layers, as recommended in reference [15]. We concluded that a six layer slab is enough to reproduce the main electronic features of the metal.…”

“…As explained in the reference [15], the segregation energy is a way to measure the copper tendency to form aggregates in a given layer. At the lowest concentration of copper, we substituted one atom by layer and buried this atom to the layer l = 4.…”

“…Hoshino et al [14] showed that because of the strong hybridization of Aluminium sp orbitals and metal d orbitals of transition atom in Al-M type alloys (where M is the transition metal), the stability of the bulk alloy depends on the medium range interactions of M atoms. Benali et al [15] also investigate theoretically the Al-Cu interactions in bulk phase at low Cu concentrations and found weak negative values of the mixing enthalpies, indicating a low tendency to form a solid solution at T = 0 K. Models of copper segregation within a given layer of (111) aluminium surface were proposed. The layer formation energy has a strong dependence on its composition and its position with respect to the surface.…”

“…Other studies have analysed the atomic structure and the formation enthalpy of mixed Al-Cu layer [26][27][28]. Benali et al [15] found that GP1 (one monolayer of Cu atoms on a {100} lattice plane) and GP2 (multilayers of Al and Cu atoms) zones segregation one and two layers under the surface is favoured, GP1 being more stable than GP2.…”