In-situ small-angle neutron scattering study of the growth and dissolution of precipitates in Al-Li alloys

The growth of the delta' precipitates in Al-Li alloys was studied by in situ small-angle neutron scattering experiments in the temperature range from 406 to 487 K. The size, number density and volume fraction of the precipitates were determined as a function of ageing temperature and time. The growth kinetics at high temperatures and for long ageing times at lower temperatures follow the coarsening theory. Two sets of kinetic constants for the different ageing temperatures were determined; one set was calculated from the size increase of the precipitates and the other from the reduction in their number density. The energies for diffusion obtained from both sets are the same, having a value of 1.18 eV. Also good agreement is found for the surface energies calculated from the two sets of kinetic constants. For the shorter ageing times at lower temperatures the number density of the precipitates can be described from the long-time behaviour if a second-order correction and the effect of the incubation period are included. In addition, the size of the precipitates grows proportionally to the square root of time. Finally, the dissolution kinetics are adequately described by using the kinetic constants derived from the growth experiments.