Guinier-Preston (GP) zones in an Al-1.7 at. pct Cu alloy aged at 383 K for 8000 minutes were analyzed by atom-probe field ion microscopy (AP-FIM). Layer-by-layer concentration profiles in the 〈100〉 direction were obtained from several GP zones. Generally, the GP zones were found to consist of several copper-enriched layers, with the highest measured concentration of copper in any of these layers being 50 at. pct. Although some uncertainty exists as to the copper content of the GP zones, these results appear to be inconsistent with the existence of layers that approach 100 pct copper. Some GP zones showed two well-separated copper-enriched regions; others showed a broad single peak in copper concentration. Therefore, it was concluded that aging at 383 K for 8000 minutes causes a transition from GP 1 to GP 2. The present results together with the previous atom-probe data indicate that there is a distinct stage for GP 2 zones which have two separated copper-enriched layers of 200 planes.
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