High resolution electron microscopy of phase decomposition microstructures in aluminium-based alloys

Abstract

High resolution electron microscopy (HREM) recently applied to phase decomposition and microstructure analyses was demonstrated for aluminium-based alloys such as the AlCu, AlMg and AlLi systems. The HREM has provided direct information on local structures on an atomic-scale resolution. In AlCu alloys, typical Guinier-Preston (GP) zones of type GP (1) and GP (2) with a single copper-rich layer and two copper-rich layers separated by three aluminium layers were clearly observed in the HREM images. The HREM images also revealed the coexistence of multiple-layer zones composed of several copper-rich layers. Lattice distortion around a single-layer GP (1) zone was not necessarily symmetric owing to the neighbouring-zone effect. A fine periodic structure similar to that of a GP (2) zone was found in rapidly solidified AlCu alloys. This structure was proposed to be closely connected with the multiple-layer GP zones. In AlMg alloys the supersaturated solid solution decomposed to form an initially modulated structure and subsequently spherical GP zones with the L1 2-type ordered structure. The periodic lattice distortion was introduced into the modulated structure associated with the magnesium atom fluctuation. Rapidly solidified AlMg alloys suggested the possibility that the homogeneous ordering process takes place. The uniformly ordered structure of the L1 2 type was more clearly detected in the AlLi alloys in the initial stage of decomposition. Both the interface and the morphology of this structure were quite different from those of the discrete δ′ phase. Thus the initial ordered structure was regarded as the precursor to the δ′ phase. The decomposition process was also discussed on the basis of experimentally obtained microstructures.