Unravelling precipitation behavior and mechanical properties of Al–Zn–Mg–Cu alloy

Abstract

We investigate the effect of aging temperature on precipitation behavior and mechanical properties of an Al–7.6Zn–2.7Mg–2.0Cu–0.1Zr–0.07Ti (wt.%) alloy by evaluating the matrix's microhardness, electrical resistivity, and tensile properties: additionally, employing X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and atom-probe tomography (APT) to characterize this alloy. The nanoprecipitates forming under peak-aging conditions vary with aging temperature, forming coherent GPI zones at 80 °C, GPII zones with minor η' at 120–150 °C, and η'/η with minor GP zones at 180–220 °C. GPI and GPII zones forming at 80–150 °C contain similar concentrations of solute atoms (11Zn–9Mg–(<1.0)Cu (at.%)), whereas the η'/η nanoprecipitates forming at 180 °C contain larger concentrations of solute atoms (28Zn–24Mg–3.4Cu (at.%)). The strength of the peak-aged alloy decreases with increasing aging temperature owing to the increasing size and decreasing number density of the nanoprecipitates. Under peak-aging conditions, precipitation strengthening originates mainly from dislocation shearing at 80–150 °C and from Orowan bypassing at temperatures above 180 °C. The shearable to non-shearable transition of the nanoprecipitates at 180 °C reduces the strain hardening rate, thereby decreasing the alloy's ductility.