Computer simulation of morphological evolution and coarsening kinetics of δ′ (Al3Li) precipitates in Al-Li alloys

The morphological evolution and coarsening kinetics of Ll(2) ordered (Al3Li) precipitates (delta') in a f.c.c. disordered matrix (alpha) were investigated using computer simulations based on microscopic diffusion equations. The effective interatomic interactions were fitted to the phase diagram using a two-neighbor mean-held model whereas the kinetic parameter in the microscopic diffusion equation was fitted to the chemical diffusion coefficient in the equilibrium disordered phase. The coalescence or encounter among precipitates which belong to any one of the four different antiphase domains of the Ll(2) ordered phase is automatically taken into account. Volume fractions ranging from 20 to 65% were studied. Structure, scaling and particle-size distribution (PSD) functions were calculated. It is shown that the PSDs become increasingly broad and their skewness changes sign from negative to positive with increasing precipitate Volume fraction. It is found that the cube of the average particle radius varies approximately linearly with time in the scaling regime for all the volume fractions studied, with the rate constant increasing with volume fraction. During coarsening, the volume fraction is not constant, approaching the equilibrium value asymptotically with time. The results are compared with existing analytical theories and experimental measurements. (C) 1998 Acta Metallurgica Inc.