Understanding the nucleation mechanism in solid-solid transformation in polycrystalline materials has been a long-standing problem due to the experimental difficulty in observing such processes. Molecular dynamics simulation, which has been proven to be a possible method of investigating phase transformations at the atomistic scale, was used to study the nucleation and growth of bcc phase in pure iron with low-angle fcc/fcc grain boundaries. The simulation was done at a constant temperature of 100 K under atmosphere pressure using an embedded-atomic method potential. The studied low-angle fcc/fcc grain boundary was constructed by rotating part of a single fcc crystal by about 5° around the [0 1 0]fcc direction leading to a grain boundary consisting of an array of edge dislocations.
Heterogeneous nucleation of the bcc phase is found to start at the dislocations of the fcc/fcc grain boundary, whereas homogeneous nucleation is not observed. The bcc nuclei forming at the low-angle grain boundary all show a special orientation relationship (OR) with at least one of the parent fcc grains, mainly the Nishiyama-Wassermann (NW) or the Kurdjumov-Sachs (KS) OR. When two neighbouring bcc nuclei have the same OR with the two fcc grains, a small misorientation exists between them. Such two neighbouring bcc nuclei may merge into a small bcc grain without any boundary movement after a long simulation time. When two neighbouring bcc nuclei have different orientations, one of the bcc nuclei is in an NW OR and the other is in a KS OR with one of the parent fcc grains. In such case, one bcc grain grows gradually into its neighbour bcc grain by movement of the grain boundary between them. Thermodynamic analysis of nuclei forming at the propagating facetted bcc/fcc interface is based on the classical nucleation theory, allowing the determination of activation energy, interface energy and number of atoms of a critical bcc nucleus, coinciding well with published data.
The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. .