Papers by Author: F. Sommer

Abstract: The formation of ferrite (α) from austenite (γ) and vice versa, upon thermo-mechanical processing of steels, are phase transformations of great technological importance. Often these transformations occur in the presence of externally or internally imposed stress. This paper provides an overview of recent research on the quantitative analysis of the transformation kinetics of the γ®a and a®g transformations subjected to uniaxial compressive stress below the yield stress of g and a, based on the application of the high-resolution differential dilatometry and the modular model of transformation kinetics. The application of uniaxially compressive stresses leads to antagonistic effects on the transformation kinetics: the stress applied upon the γ®a transformation prompts the transformation, while it obstructs the a®g transformation. These results can be quantitatively discussed in terms of chemical driving forces and transformation-induced deformation energies.

Abstract: A kinetic Monte Carlo method has been developed for the simulation of interface controlled solid-state transformations to overcome timescale limitations associated with other atomistic simulation methods. In the simulation method the atoms can take place on sites from (at least) two intertwining crystal lattices. To enable the atoms to also take positions between the ideal lattice sites, a collection of randomly placed sites can be included. These ‘random sites’ have a realistic chance to be occupied at the location of the transformation interface and thus allow for irregularities in the atomic structure of the transformation interface. The atoms move by independent, thermally activated jumps. The activation energy for the atomic jumps can be determined for every jump separately based on the arrangement of the neighbouring atoms. The simulation method has been used to study the interface mobility in the austenite to ferrite transformation in iron for different interface orientations. The results obtained indicate that the excess volume associated with the interface plays a key role for the activation enthalpy for the interface mobility. The rate controlling process is the rearrangement of free space at the interface by series of (unfavourable) jumps by different atoms to create a path from the parent to the product phase.