COARSENING OF DELTA' (AL3LI) AND COMPOSITE PRECIPITATES IN AN AL-2.5-PERCENT LI-0.15-PERCENT ZR ALLOY

The early stage coarsening behaviour of delta' (Al3Li) precipitates an d composite precipitates, consisting of beta' (Al3Zr) core enveloped b y a delta' shell, in an Al-2.5% Li-0.1 5% Zr alloy has been studied at 443, 473 and 503 K by conventional transmission electron microscopy u sing superlattice dark-field images. Coarsening kinetics of delta' pre cipitates was found to exhibit considerable deviation from the t1/3 la w corresponding to diffusion control at 473 and 503 K. This deviation may be explained either by the operation of a surface-reaction control led coarsening mechanism represented by t1/2 law, or by the presence o f a slower precoarsening stage preceding diffusion-controlled coarseni ng. The particle-size distributions of delta' precipitates were not in accordance with either the earlier theoretical predictions or the nor mal distribution. They were symmetrical or slightly skewed to the left , and particle sizes twice the average were observed. The coarsening k inetics of composite precipitates, was found to be rather different fr om that of delta'; a diffusion model which yields t1/n law with n chan ging from 1/4 to 1/6 as the coarsening progresses, was predicted. The composite precipitates have exhibited quite a narrow and rather symmet rical size distribution.