Abstract
This paper describes the results of a quantitative transmission electron microscopy (TEM) investigation of the coarsening process of the metastable δ′(Al 3Li) phase in a series of binary Al-Li alloys containing between 2.4 and 4.5 wt% Li. Isothermal aging times were varied from 12 h to 10 days at 200 and 225°C. The coarsening of δ′ was found to obey Ostwald ripening kinetics. Increasing the amount of lithium accelerated the coarsening rate of δ′. Furthermore, increasing the volume fraction of δ′ increases the number of particles whose aspect ratio significantly differed from 1.0. However, increasing the aging time results in particles whose aspect ratio tends toward 1; (i.e. the δ′ particles tend to become more spherical with increased artificial aging). A comparison of the observed results with those predicted by the existing coarsening theories indicates that for an accurate modeling of the coarsening process, especially at high volume fractions of δ′, the coalescence mechanism needs to be taken into account. None of the existing theories can predict the observed coarsening rate and particle size distributions observed in this investigation.
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