Atomic-Scale Computer Simulation Of Al₃Li Precipitation

In this paper, the atomic-scale computer simulation programs of Al3Li ( ' phase) based on the microscopic diffusion equation and nonequilibrium free energy were firstly worked out, which can be applied to all precipitation process, including nucleation, growth and coarsening , and the compositional regions from metastable to instable. The precipitation mechanism of ' was systematically investigated by simulating the atomic pictures and calculating the order parameters, volume fraction of ordered phase, etc., and the variation in precipitation mechanism with composition, particularly the precipitation behavior across the mean-field spinodal lines were interpreted for the first time. The main simulation results were confirmed by comparing the experimental results and theories of others, meanwhile, the phenomenon and laws were also obtained, which are difficult or unable to discover in experiments at present. The main conclusions are summarized as follows:Classical nucleation and growth mechanism only occurs near the phase boundary of the disordered phase, by which 8' phase precipitates from supersaturated solid solution directly. The precipitation sequence is.- supersaturated solid solution-stoicheometric ' phase- growth. With the increase of the composition there is more characteristic of non-classical nucleation and growth, and the order parameter values of critical ordered nucleus decrease. The precipitation sequence is: supersaturated solid solution -nonstoicheometric ordered phase - stoicheometric ' phase - growth. Ordering and clustering occur simultaneously, and no GP zones are found.In instable region the precipitation mechanism of ' phase is congruent ordering followed by spinodal decomposition, and ordering occurs prior to clustering. By congruent ordering, a nonstoicheometric single ordered phase is produced with the order domains being separated by antiphase domain boundaries (APBs). Spinodaldecomposition occurs predominantly at APBs, which results in the replacement of the APBs by the two order/disordered interfaces; Spinodal decomposition at APBs promotes the decomposition in domains, however, the disordered phase in domains disappear as time follows. The precipitation sequence in instable region is as follows: Supersaturated solid solution congruent orderingspinodal decomposition stoicheometric phaseIt is found for the first time that with the increase of compositions, congruent ordering is enhanced, incubation period is shortened, spinodal decomposition is decreased, and volume fraction of ordered phase is increased.Computer simulation was firstly performed on the precipitation mechanism at mean-field spinodal lines in complicated system with ordering and clustering. No incontinuous transition has been found at mean-field spinodal lines and the transition of precipitation mechanism transform gradually across the mean-field spinodal lines. Precipitation mechanism at disordering instability line is non-classical nucleation and growth, similar to those in metastable region, and precipitation mechanism at ordering instability line is non-classical nucleation and growth, followed by spinodal decomposition, similar tothose in instable region. The sequence is:non-classical NGSupersaturated solid solutionspinodal decomposition The variation in precipitation mechanism of 8' phase in intermediate region is systematically investigated firstly, and it is found that with the increase of composition the precipitation characteristic transforms from that of metastable to instable region gradually. And ordering occurs earlier and quicker than clustering; the composition in critical nucleus is lower and the size is larger; spinodal decomposition occurs in ordered phase.There are different forms of coarsening in different regions, which overlaps (nucleation) growth in metastable region, spinodal decomposition in instable region, andthe mixture of growth and spinodal decomposition in transitional region. However, the cube of mean radius of the ordered phase...