High resolution electron microscopy on the layered structures of GP zones in an Al-1.7at%Cu alloy

Abstract

Summary Layered structures of GP zones were clearly observed in the high resolution electron microscope images under optimum imaging conditions. The majority of GP zones were typical GP(1) zones with a single-layer structure and typical GP(2) zones with a structure consisting of two Cu-rich layers separated by three Al layers. Atypical multi-layer structures with several variations of stacking of Cu and Al layers were also found to coexist in the variously aged specimens. Single-layer GP(1) zones predominated in the specimens aged at room temperature for 441Ms (14 years), at 373K for 86.4ks and at 423K for 1.8ks, 12.6ks, respectively; the frequency was determined to be 81.0–86.3%, whereas atypical multilayer structures were found locally up to about 12.9–15.9%. For the aging at 423K for 86.4ks, the predominant structure was the typical GP(2) zone, the frequency being 53.0%, and the atypical GP zones was also found to be mixed up to 17.8%. The sizes of the atypical GP zones were almost the same as these of the typical GP(1) and GP(2) zones. The frequency of the atypical GP zones increased slightly at the aging stage of θ′ phase due to the formation of the combined layered structures of GP(2) zone and θ′ phase.