Influence of homogenisation time on evolution of eutectic phases, dispersoid behaviour and crystallographic texture for Al–Zn–Mg–Cu–Ag alloy

Abstract

Microstructural changes for Al–Zn–Mg–Cu–Ag alloy at the time of homogenisation (0–48 h) at 465 °C were examined by optical microscope (OM), field emission scanning electron microscope (FESEM), energy dispersive X-ray Spectroscopy (EDS), transmission electron microscope (TEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) in detail. α (Al), η [Mg (Zn, Cu, Al)2], Al2Cu, and coarse Al3Fe emerged as predominant phases in the as-cast condition. The grain boundary areas were enriched with the main alloying elements i.e. Cu, Zn, Mg, and Ag. After 6 h of homogenisation, a phase change from η to S (Al2CuMg) was found to be initiated. By prolonging the homogenisation time, the dendritic network and the secondary eutectic phases were gradually dissolved in the matrix and eventually vanished after 48 h of homogenisation, which corroborated well with the proposed kinetic model. TEM micrographs demonstrated a high density of fine dispersoids after a homogenisation period for 6 h. In addition, higher homogenisation time accelerated the coarsening of dispersoids, following Ostwald ripening mechanism and lowering of Zener pinning pressure (ZPP). Crystallographic texture analysis demonstrated the presence of two completely different sets of texture components during casting and subsequent homogenisation treatments. Dominant Goss{011}<100>, Brass{011}<211>, P{011}<111>, CuT{552}<115> and S{123}<634> texture components appeared after casting, while Cube{100}<001>, rotated cube{110}<011>, E{111}<110> and F{111}<112> components became dominant after homogenisation.