The influence of morphology on grain-boundary and twin-boundary bainite growth kinetics at the bay in Fe–C–Mo

The growth kinetics morphology and crystallography of grain-boundary bainite (GBB) and twin-boundary bainite (TBB) formed in Fe–0.24?wt%?C–4?wt%?Mo were reassessed. Optical microscopy and transmission electron microscopy observations of these coalesced allotriomorphic slabs showed that the bainite–austenite growth fronts were rough, even spiky in place, characteristic of Widmanstätten growth. Furthermore, a significant substructure of regions having different ferrite orientations (subunits) was revealed, the GBB subunit arrangement was quite complex, suggestive of competitive growth between subunit, whereas TBB had a simpler subunit arrangement that yielded less subunit competition. These crystallographic and interfacial structure effects are instrumental for explaining the more rapid thickening of GBB relative to TBB. The differing subunit arrangements in GBB versus TBB originate in the differing ferrite–austenite orientation relationships established during nucleation at austenite grain boundaries versus twin boundaries. The complexities inherent in a multicrystalline allotriomorphic slab migrating with a rough growth front prevents direct interpretation of thickening kinetics data in terms of a fundamental growth process.