Transition from metastability to instability in dynamics for the precipitation of δ'(Al₃Li)

A computer simulation study on dynamics for the precipitation of δ'(Al₃Li) ordered particles from a disordered matrix (α) in binary Al-Li alloys is performed using the microscopic Langevin equation. A unique precipitation mechanism is found near the ordering spinodal line. Different from the classical nucleation mechanism in the metastable region and the congruent ordering followed by spinodal decomposition in the instable region, a nonstoichiometric single ordered phase with composition fluctuations is formed by non-classical nucleation, and this ordered phase decomposes spinodally. It can be concluded that the precipitation dynamics of δ′ phase from metastability to instability is gradual, and no sharp transition occurs near the mean-field spinodal line as the mean-field theory predicts.