Comparative analysis of the cellular and Johnson-Mehl microstructures through computer simulation

The geometric properties of polygranular microstructures of the Johnson-Mehl cellular types have been studied through computer simulation. These prototypic microstructures arise naturally from the classical model of a phase transformation in a one-component solid through growth from a random distribution of nucleation sites. The Johnson-Mehl microstructure results in the kinetic limit of a constant nucleation rate over an essentially constant density of available nucleation sites; the cellular microstructure is produced in the kinetic limit of simultaneous activation of the available nucleation sites. Members of each of these microstructure types are similar to one another in all aspects of their geometrical statistics; they differ only through a homogeneous expansion or contraction. Their geometric features have been characterized through a combination of analytic and computer simulation studies. Comparison with available experimental results shows that the Johnson-Mehl microstructure compares well with such metallurgically diverse experimental structures as the crystallization structure of silicon iron and the intermediate structures established during the ordering of lithium ferrite. These correspondences suggest that the idealized microstructures studied here may be physically relevant as well as being pedagogically useful.