Theory and computer simulation of the precipitation of ordered intermetallic compounds in binary and ternary alloys

The precipitation of L1{dollar}sb2{dollar} ordered Al{dollar}sb3{dollar}Li precipitates in Al-Li alloys, and of DO{dollar}sb{lcub}22{rcub}{dollar} ordered Ni{dollar}sb3{dollar}V and L1{dollar}sb2{dollar} ordered Ni{dollar}sb3{dollar}Al precipitates in Ni-Al-V alloys has been studied. In addition, the coarsening kinetics of the Al{dollar}sb3{dollar}Li precipitates have also been studied.; It is found that a critical nucleus consists of fluctuations of both composition and long-range order parameter profiles. It is shown that only when the composition of the initial disordered matrix is near the phase boundary of the disordered phase, are the composition and order parameter values inside the critical nucleus close to those of the equilibrium ordered phase, and that the critical profiles become increasingly diffuse as the composition of the disordered matrix approaches the ordering instability line. Based on the non-classical nucleation theory, the size of critical fluctuations, the critical free energy change and the nucleation rate are estimated for the specific case of the precipitation of metastable {dollar}deltaspprime{dollar} ordered phase from a disordered matrix in the Al-Li alloy.; The morphological evolution and coarsening kinetics of L1{dollar}sb2{dollar} ordered {dollar}deltaspprime{dollar} (Al{dollar}sb3{dollar}Li) precipitates in a disordered Al matrix ({dollar}alpha){dollar} has been investigated using computer simulations based on both stochastic and deterministic microscopic diffusion equations. A projected two-dimensional model of a three-dimensional fcc lattice has been employed. The coalescence or encounters among precipitates as well as the accompanying morphological evolution are automatically taken into account. The coalescence of two in-phase precipitates forms one large particle while encounters between two out-of phase precipitates results in an antiphase domain boundary (APB) wetted by a layer of the disordered phase. Volume fractions ranging from 20 to 65% have been studied. It is found that the cube of the average particle radius varies linearly with time in the scaling regime for all the volume fractions studied. It is shown that the particle-size distribution (PSD)s become increasingly broad and their skewness changes sign from negative to positive with increasing precipitate volume fraction. During coarsening, the volume fraction is not constant, approaching the equilibrium value only asymptotically with time.; The kinetics of diffusional phase transformations in the pseudobinary Ni{dollar}sb3{dollar}Al-Ni{dollar}sb3{dollar}V system have been studied using a computer simulation technique based on microscopic diffusion equations. The focus is on the atomic ordering and compositional clustering process during the phase transformation of an initially homogeneous disordered Al ternary alloy {dollar}rm(Nisb{lcub}75{rcub}Alsb{lcub}25-x{rcub}Vsb{lcub}x{rcub}){dollar} into a two-phase mixture of L1{dollar}sb2{dollar} (Ni{dollar}sb3{dollar}Al) and DO{dollar}sb{lcub}22{rcub}{dollar} (Ni{dollar}sb3{dollar}V) ordered phases. The computer simulations demonstrate that at small vanadium content, the L1{dollar}sb2{dollar} ordered domains appear first, followed by the nucleation of DO{dollar}sb{lcub}22{rcub}{dollar} ordered domains at the antiphase domain boundaries of L1{dollar}sb2{dollar} phase, whereas at large vanadium content, precipitation of DO{dollar}sb{lcub}22{rcub}{dollar} domains precedes L1{dollar}sb2{dollar} domain formation. The simulation results are discussed employing the thermodynamic stability analysis. The ordering and clustering kinetics predicted from our computer simulation seem to be consistent with recent experimental observations on the alloys. (Abstract shortened by UMI.)...