Electronic structure, microstructure, and crystal structure of the precipitation-hardened alloy Cu₉₈Be_1.8Co_0.2

Abstract

Precipitation hardening of copper alloys results in improved elastic properties but is accompanied by reduction in electrical conductivity. We study the electronic structure, microstructure, and crystal structure of precipitation-hardened Cu:Be:Co (98:1.8:0.2 weight %) alloy to look for coupled changes accompanying the precipitation hardening. X-ray diffraction is used to study the strain in the Cu matrix upon Guinier-Preston zone formation and the subsequent precipitation. Using x-ray photoemission spectroscopy (XPS) and scanning electron microscopy (SEM), we compare the Cu matrix and Co beryllides of well-characterized as-obtained and precipitation hardened alloys. SEM confirms the evolution of the microstructure typical of Guinier-Preston zone formation and precipitation. The binding energies and line shapes of Cu 2p, Co 2p, and Be 1s core levels are investigated using XPS. In spite of the Co beryllides migrating to the grain boundaries as an entity, XPS indicates that the Be atoms get oxidized upon migration, while the Co atoms remain metallic. The Cu 2p core levels shift 0.3 eV to higher binding energy in the as-obtained and partially hardened alloys. In addition, a line shape change observed only in the partially hardened alloy is attributed to strain in the Cu matrix upon Guinier-Preston zone formation. In contrast, for the fully hardened alloy, the binding energy and line shape reverts back to that of pure copper. But for the chemical potential shift, the valence band spectral features exhibit negligible changes in spectral shape upon hardening. The results are consistent with a change in the chemical potential due to metastable Co beryllides and increasing strain in the initial stages of hardening due to Guinier-Preston zones. In the fully hardened alloy, the observed reduction of the chemical potential shift is related to precipitation and a corresponding readjustment of the Fermi energy.