Effects of precipitate microstructures near grain boundaries on strength and ductility in Al-Zn-Mg(-Ag) alloys

Abstract

The relationship between strength, ductility and precipitate microstructures in the vicinity of grain boundaries was evaluated for Al-4.9%Zn-1.8%Mg(-0.3%Ag) (in mass%) alloys using transmission electron microscopy (TEM), three-dimensional atom probe (3DAP) and tensile test. In the ternary alloy aged at 433 K, larger widths of precipitate free zones (PFZ) were observed by TEM and elongation was smaller, regardless of the size of grain boundary precipitates. On the other hand, in the ternary and Ag-added alloys aged at 373 and 393 K, elongation was larger due to both of the much smaller widths of PFZ and the much smaller size of grain boundary precipitates. These suggest that the presence of PFZ is harmful to the fracture of the investigated alloys. The measured proof stress and elongation were correlated to the width of PFZ and size of grain boundary precipitates, enabling a quantitative prediction of tensile properties from the corresponding microstructural parameters. Furthermore, on the basis of a 3DAP analysis result of the grain interior region where Ag atoms are condensed within nanoclusters of Zn and Mg, it was also proposed that Ag atoms around grain boundaries efficiently trap Zn and Mg atoms, resulting in the prevention of both solute depletion and widening of PFZ.