Papers by Author: Alfred Cerezo

Abstract: Hardness measurements and 3-dimensional atom probe analysis have been used to characterise the precipitation behaviour in two 6xxx series aluminium alloys, one Cu-free alloy (Al-0.78at%Mg- 0.68at%Si) and one Cu-containing alloy (Al-0.78at%Mg-0.68at%Si-0.30at%Cu). The heat treatments consisted of either natural ageing or pre-ageing at 353K followed by a paint-bake treatment at 453K. Natural ageing was seen to increase the hardness, and hence reduce formability compared to pre-ageing. In addition, the strengthening effect of artificial ageing was less after natural ageing than after pre-ageing. In the Cu-free alloy, needle-like β″ was observed to form only after a pre-ageing treatment during the first 60 minutes of a paint-bake treatment. In the Cucontaining alloy, needle-like β″ formed during paint bake in both the naturally-aged and pre-aged material, although it is formed more rapidly after pre-ageing. This was accompanied by an increase in strength over the Cu-free alloy and indicates that Cu reduces the deleterious effect of natural ageing.

Abstract: The quantitative correlation between strength, ductility and precipitate microstructures in the vicinity of grain boundaries with precipitate free zones (PFZ) was evaluated for Al-Zn-Mg(-Ag, Cu) alloys using transmission electron microscopy (TEM), three-dimensional atom probe (3DAP) and tensile test. In the Al-Zn-Mg ternary and Cu-added alloys aged at 433K, larger widths of PFZ were observed by TEM and resulted in lower elongations to fracture, independent of the size of grain boundary precipitates. On the other hand, the elongation of the Ag-added alloy was higher, if compared at the same levels of proof stress, due to the much smaller width of PFZ. This strongly suggests that PFZ is harmful to fracture of the investigated alloys. From a 3DAP analysis, furthermore, it was revealed that Ag and Cu atoms are incorporated in the nanoclusters from the initial stage of aging. In this work, the elongation was well correlated to the width of PFZ, size of grain boundary precipitates and the level of proof stress, enabling to predict ductility of the alloys from known microstructural factors.