Behaviour of valence electrons in the ageing process of Al-2·2at.% Cu crystals

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Abstract
The formation processes of Guinier-Preston (GP) zones, θ- and θ-phase structures in an Al-2.2 at.% Cu alloy have been investigated by a combined method of Auger valence electron spectroscopy and high-resolution transmission electron microscopy. An Auger valence electron spectrum of pure aluminium was subtracted from those of the aged specimens to analyse a small change in the L2,3 VV spectrum of aluminium atoms in the alloy. The spectrum consists of three peaks as follows. One peak is a positive peak and always appears in all ageing stages including the asquenched state. The intensity of this peak shows a good correspondence to the coherent strain around microstructures formed by ageing. The other two peaks are negative and appear in GP-1 formation. Subsequently, their intensities show the maximum values in θ-phase formation and decrease again in θ-phase formation. We conclude that these two peaks are closely related to the local chemical bonding between aluminium and copper atoms. On the basis of these results, the formation processes of GP zones, θ- and θ-phase structures are discussed.