First principles investigation of zinc-induced embrittlement in an aluminum grain boundary

Abstract

Aluminum–zinc alloys have an extensive range of structural applications, but their susceptibility to intergranular stress corrosion cracking limits their wider use. In the present work the influence of zinc on a ∑5(0 1 2)[1 0 0] aluminum grain boundary was investigated by means of first principles calculations with the full potential linearized augmented plane wave method using two approaches: (i) within the framework of the Rice–Wang thermodynamic model; (ii) by the ab initio tensile test method. We determined the most energetically favorable segregation site of Zn along the Al grain boundary, its segregation energy from the Al grain boundary, the possible fracture paths of the grain boundary with Zn and the corresponding fracture energies. We established that Zn has a large driving force (−0.19 eV atom −1) for segregation from the Al bulk to the asymmetrical grain boundary site, and its segregation reduces the grain boundary strength slightly. Unusually, segregation of larger atomic size Zn leads to grain boundary contraction. Through precise calculations it was confirmed that zinc is a weak embrittler with a potency of +0.05 eV atom −1. Analysis in terms of the relaxed atomic and electronic structures and bonding characters showed that aluminum–zinc bonds have a metallic character in both grain boundary and free surface environments. This work provides a fundamental quantitative understanding of Zn-induced grain boundary embrittlement in Al alloys on the electronic level and establishes that Zn segregation is one of the factors contributing to stress corrosion cracking in Al alloys.