QUANTITATIVE-ANALYSIS OF GP ZONES FORMED AT ROOM-TEMPERATURE IN A 7150-AL-BASED ALLOY

The first steps of the decomposition of A1-rich supersaturated solid solutions of A1-Zn-Mg-Cu alloys, generally lead to a distribution of extremely fine and complex particles of different phases. Although crystallography and morphology of these features have been extensively studied, little information on their chemistry is available. An atom probe study of these phenomena has been undertaken and the first results are reported in this paper. After long term ageing at room temperature, GP zones (GPZ) are formed with a high number density. Although a large scatter exists in the measurements, the mean GPZ composition has been determined. Zinc and magnesium are found up to a total solute level of about 30-35 at% and the Zn/Mg atomic ratio is close to 1.3. The copper level in GPZ appears to be similar to that of the solid solution. A preliminary tomographic atom probe study was undertaken. Three-dimensional reconstruction shows that the sphere model used to describe GP zones in this type of alloy and for such ageing conditions, is a rough simplification of a much more complex reality.