The effect of geometrical assumptions in modeling solid-state transformation kinetics

In the quest for the ideal transformation model describing the austenite de composition in steel, emphasis shifts from empirical to physical models. Th is has resulted in the widely used description of the transformation by mea ns of the interface velocity between the parent phase and the newly formed phase, a description which yields reliable predictions of the transformatio n behavior only when combined with a realistic austenite geometry. This art icle deals with a single-grain austenite geometry model applied to transfor mations in which the interface velocity is constant throughout the transfor mation, e.g., certain types of massive transformations. The selected geomet ry is a regular tetrakaidecahedron, combining topological features of a ran dom Voronoi distribution with the advantages of single-grain calculations. The simulations show the influence of the ferrite-nucleus density, the rela tive positions of the ferrite nuclei inside the austenite grain, and the gr ain-size distribution. From simulations with a constant interface velocity, the transformation behavior for a tetrakaidecahedron is in agreement with transformation kinetics in terms of the Johnson-Mehl-Avrami (JMA) model. Us ing the tetrakaidecahedron geometry, one can simulate transformation curves that can be experimentally obtained by calorimetry or dilatometry, in orde r to study the quantities affecting the transformation behavior.