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

Abstract

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.

Acknowledgements

This work was funded by the National Science Foundation through grants DMR-9628936 and DMR-9904034. Funding during manuscript preparation for R.E.H. was provided by the US Department of Energy (contract W-7405-ENG-36).

Notes

† Present address: Los Alamos National Laboratory, Materials Science and Technology Division, Mail Stop G770, Los Alamos, New Mexico 87545, USA.

† ORs are classified as of deep-cusp, intermediate-cusp and shallow-cusp natures to describe the relative depths of the most prominent interfacial energy minima (located in a cusp or well) on their Wulff plot (Aaronson and Lee 1999). ORs that allow deeper cusps give rise to lower interfacial energies as a result of better atomic matching across the heterophase interface at particular boundary plane orientations.

† This does not imply that the subunit has the same OR with respect to both grains, only that both ORs fall in the intermediate-cusp category.

† Grain-boundary M₂₃C₆ was observed by TEM after 3 and 6 hours at 560°C in this steel (Hackenberg 2001) and may have a similar effect on subsequent bainite nucleation.

Additional information

Notes on contributors

R. E. Hackenberg†

† Present address: Los Alamos National Laboratory, Materials Science and Technology Division, Mail Stop G770, Los Alamos, New Mexico 87545, USA.