COMPUTER-SIMULATIONS OF DIFFUSIONAL PHASE-TRANSFORMATIONS - MONTE-CARLO ALGORITHM AND APPLICATION TO PRECIPITATION OF ORDERED PHASES

A Monte Carlo (MC) simulation technique which is well suited for study ing diffusional phase transformations is presented. This technique acc ounts for atom transport through vacancy migration, and allows a physi cally meaningful definition of time. This MC technique is used for stu dying the precipitation of an L1(2) ordered phase from a supersaturate d, disordered f.c.c. matrix. Three alloys have been studied of composi tions x(B) = 0.125, 0.15 and 0.175, with equilibrium volume fraction o f the L1(2) phase of 0.23, 0.47 and 0.71, respectively. The results sh ow that, during early stages of the transformation, both phase separat ion (i.e. formation of regions of different compositions) and ordering (i.e. creation of atomic order in the solute-rich regions) proceed si multaneously in all the three alloys. In particular, there is no evide nce for homogeneous ordering prior to phase separation even in the mos t concentrated alloy with x(B) = 0.175. Thus, the results are in direc t contradiction to those obtained in recent simulations by Chen and Kh achaturyan, and underscore the limitations of the Bragg-Williams appro ximation used by them. The late stage behaviour of all the three alloy s obey classical laws of coarsening: (i) the microstructures are self similar, and (ii) the cube of the characteristic microstructural lengt h scale increases linearly with time. Thus, the range of validity of t he classical laws appears to extend to large precipitate volume fracti ons. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd . All rights reserved.