Density functional study of copper segregation in aluminum

a b s t r a c t (111) Al surface (100) Al surface Cu segregation Density functional calculations Surface alloys Surface energy The 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. Aluminum has the capacity to form a very stable oxide. Thus, it leads to high temperature resistant coatings with good resistance to oxidation and corrosion in aggressive environments. It is often alloyed to modify some of its intrinsic properties and various treatments such as precipitation hardening are needed to improve its mechanical properties. The properties of these alloys are not due simply to their chemical composition but are particularly influenced by the involved phases and the alloy microstructure. Copper-aluminum alloys that have good mechanical properties are the most used alloys in the aeronautical field. In microelectronics, Cu/Al joints are widely used in high-direct-currentsystemsto transmitthe electric current, andcould be used as alternative to Au/Al joint in high-power interconnections and fine-pitch bonding applications due to the very good mechanical, electrical and thermal properties of Cu (1,2). The oxidation of such alloys canhave crucialconsequences on the phaseproperties.We thus want to investigate the first stages of oxidation of copper-aluminum alloys. We need first to study the clean material and understand the Cu-Al interactions. We present here the results of our computations on copper segregation in aluminum. The copper bulk segregation and copper surface segregation are both studied. During the last two decades several studies on aluminum and its alloys were carried out using first principle calculations. Various bulk phases(perfectphasesorinpresence of bulk defects)aswell ascleanAl surfaces were fully investigated. Hoshino et al. (3) showed that the stability of an aluminum based binary alloy Al-M, with a transition metal M, is related to the middle range interactions between the transition atoms, by a strong sp-d hybridization (Al-M). The energy of interaction between two impurities depends strongly on the distance separatingthem.UsingthefullpotentialsGreenfunctionsKKR(4,5)fora