A positron lifetime study of cellular precipitation in Al-22 at.% Zn alloy

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Abstract
The cellular precipitation of β-Zn which occurs on ageing quenched Al-22 at.% Zn alloy at temperatures within the two-phase region has been studied by positron lifetime experiments in conjunction with transmission electron microscopy and X-ray diffraction (XRD). The as-quenched alloy shows a single lifetime component of τ1 = 155 ps. On ageing the as-quenched alloy within the two-phase region, the positron lifetime spectrum is seen to develop a second component, τ2 = 230 ps, whose intensity I 2 increases with increasing ageing time and saturates in about 100 min. The occurrence of the second component in the aged samples is understood in terms of positron trapping at the misfit dislocations produced at the α-Al/β-Zn interface. The increase in I 2 with increasing ageing time correlates with the increase in the volume fraction of β-Zn precipitates as obtained from XRD measurements. The variation in positron annihilation parameters with ageing time, which reflects the kinetics of cellular precipitation, has been analysed in terms of the time-temperature-transformation diagram and the Avrami equation.