The Structure of the Broad Faces of Ferrite Plates

H. K. D. H. Bhadeshia

Abstract

Published experimental data on the variation of the highest temperature at which Widmanstatten ferrite can be seen to form at a detectable rate, as a function of steel composition, are analysed theoretically. It is found that the data can be predicted to a fair accuracy if it is assumed that the nucleation of Widmanstatten ferrite occurs by a mechanism which is similar to that of martensite, but with the diffusion of carbon during nucleation, and if it is additionally assumed that the growth of Widmanstatten ferrite can only be sustained when the chemical driving force exceeds a specific stored energy term.

It is argued that Widmanstätten-ferrite forms through a displacive transformation rather than a diffusional process. Central to this claim is the characterization of the austenite-ferrite interface as glissile, meaning its defects can move conservatively to facilitate growth. The author contends that the macroscopic shape change and specific strain fields observed in these steel structures are inconsistent with uncoordinated atomic movement. By analyzing dislocation structures and crystallographic orientations, the source disputes alternative models that favor sessile, low-energy interfaces. Ultimately, the text highlights how the invariant-plane strain and plate morphology distinguish this transformation from purely diffusional growth seen in other ferrite types.

Materials Science and Technology, Vol. 6, 1990, pp. 781-784.

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