Mk. Aydinol et As. Bor, COARSENING OF DELTA' (AL3LI) AND COMPOSITE PRECIPITATES IN AN AL-2.5-PERCENT LI-0.15-PERCENT ZR ALLOY, Journal of Materials Science, 29(1), 1994, pp. 15-25
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.