MICROSTRUCTURAL DEVELOPMENT DURING NUCLEATION AND GROWTH

The microstructural development during nucleation and growth processes is studied numerically. Most of the studies are for the simple case o f constant nucleation and growth rates, but a brief discussion is made of the effect of time-dependent nucleation and growth. A 3-D code is used which accounts for not only the nucleation and growth of individu al new grains, but also the effects of grain impingement, and which al lows for the study of both homogeneous and heterogeneous nucleation. T he microstructures are characterized by the grain-size distribution (G SD) and the cluster-size distribution (CSD). In the case of homogeneou s nucleation, the development of GSD and CSD can be scaled using the A vrami time tau(Av) and Avrami length delta(Av), which are related to t he nucleation and growth rates. Both scaling constants have a simple p hysical meaning: the average grain size after the completion of the ph ase transformation is given by delta(Av), and the transformation half- time is approximately equal to tau(Av). The formation of a continuous chain of new-phase grains (percolation transition) is observed at simi lar to 30 per cent transformation degree, and the geometry of the larg est cluster near the percolation threshold has fractal characteristics with a fractal dimension of similar to 2.5. The presence of preferred sites of nucleation (heterogeneous nucleation), such as grain boundar ies, significantly modifies the microstructures when the spacing of nu cleation sites is much larger than the Avrami length, the main effects being a reduced percolation threshold and an elongate grain shape. So me applications to the olivine-spinel transformation in subducting sla bs and to the crystallization in a hypothetical magma ocean are discus sed.