INFLUENCE OF A PRECIPITATE-FREE ZONE ON MECHANICAL PROPERTIES OF AN AL-MG-ZN ALLOY

The influence of precipitate-free zones along grain boundaries on the mechanical properties is studied in the alloy Al-6% Zn-2% Mg. The width of the precipitate-free zones has been varied by using different homogenization- and ageing temperatures. It is found that the σ0.2 and εf-values are practically constant, whereas the ductility decreases with decreasing width of the precipitate-free zones. These results are attributed to the particular mode of plastic deformation in these alloys. The influence of a double ageing treatment is also studied. This treatment comprises of first ageing to maximum hardness at one temperature and then continues the ageing at a reduced temperature. It is found that the ageing at the reduced temperature affected a progressive reduction in the ductility, whereas the σ0.2- and σf-values were less affected. This is attributed to a slight coarsening of the precipitate in the matrix and a more pronounced coarsening of the big, stable particles on the grain boundaries. © 1968.