The Atomic Order in Guinier-Preston Zones of Aluminum-Silver Alloys

The formation of the spherical Guinier—Preston zones in an aluminum-silver alloy is governed by a metastable miscibility gap, which consists of two different sections. The lower section occurs below 170°C (η state), the higher section up to 420°C (∊ state). The zones in the two sections differ in their silver concentration and in their atomic order. To prove the change in order, a combination of X-ray small-angle scattering and electric resistivity measurements was used. As the resistivity depends on the zone size and the atomic order, the change in order can be found when the zone size is known. This size was measured by the X-ray technique. To complete the results, X-rays ingle-crystal diffraction patterns with monochromatic radiation were taken at different stages. According to these patterns, three different states must be distinguished.The η′ state exists at room temperature after quenching from 550°C. The silver atoms prefer a layered arrangement in the zones, which is not very stable. It is destroyed after short annealings above 100°C. The η state is developed during annealing below 170°C. A three-dimensional atomic order is built up with increasing zone size, which results in a marked decrease in the resistivity. For the ∊ state (above 170°C), a nearly random atomic distribution exists. Step-quenching experiments prove that the ordered η state can also be developed at room temperature.