Computer simulation of the kinetics of order disorder and phase separation during precipitation of δ′ (Al₃Li) in Al-Li alloys

A computer simulation study has been performed of the reaction paths for the precipitation of δ′ (Al₃Li) ordered particles from a disordered matrix (α) in Al-Li alloys, using microscopic Langévin diffusion equations. It is found that the precipitation of δ′ occurs either by a congruent ordering process followed by decomposition, or by a non-classical nucleation mechanism which requires critical fluctuations of both composition and order parameter, except in a narrow range of compositions near the equilibrium phase boundary of the disordered phase, where classical nucleation theory seems to be applicable. Composition and order parameter profiles across a critical nucleus were obtained for different compositions in the nucleation and growth regime, and compared with those obtained from the continuum non-classical nucleation theory. Possible origins for the discrepancies in the precipitation mechanism, obtained from different theoretical and experimental studies, are suggested.