Microstructure and properties of spark plasma sintered Al-Zn-Mg-Cu alloy

Abstract

The microstructure of an aluminum alloy containing 53 wt% Zn, 2.1 wt% Mg and 1.3 wt% Cu as main alloying elements has been studied with the focus on the precipitation behavior during the spark plasma sintering process. The starting material was an atomized Al-Zn-Mg-Cu powder with the particle size below 50 μm. The particles showed a solidification microstructure from cellular to columnar or equiaxed dendritic morphology with a large fraction of the alloying elements segregated in form of intermetallic phases, mainly (Zn, Al, Cu)49Mg32 and Mg2(Zn, Al, Cu)11, at the cell and dendrite boundaries. The microstructure of the sintered specimens followed the microstructure of the initial powder. However, Mg(Zn, Al, Cu)2 precipitates evolve at the expense of the initial precipitate phases. The precipitates which were initially continuously distributed along the intercellular and interdendritic boundaries form discrete chain-like structures in the sintered samples. Additionally, fine precipitates created during the sintering process evolve at the new low-angle boundaries. The large fraction of precipitates at the grain boundaries and especially at the former particle boundaries could not be solved into the matrix applying a usual solid solution heat treatment. A bending test reveals low ductility and strength. The mechanical properties suffer from the precipitates at former particle boundaries leading to fracture after an outer fiber tensile strain of 3.8%.