Diffusional Transformations: A theory for the formation of superledges

H. K. D. H. Bhadeshia

Abstract

It is well known that the growth of allotriomorphic ferrite in steels often occurs by the superledge mechanism, in which the piecewise displacement of the interface is accomplished by the motion of steps, whose heights can amount to several hundreds of lattice spacings. The present work is an attempt to understand the factors controlling the height (h) of experimentally observed superledges. A theory based on step nucleation is developed, which predicts a lower limit for h, for a specified set of transformation conditions and steel composition. Published experimental data seem to be in good agreement with the theory, particularly with respect to the variation of h with transformation temperature. It is also shown tha the critical height below which the nucleation of a ledge is not possible, is related directly to the free energy of the singular interface on which ledge motion is supposed to occur.

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Physica Status Solidi A, Vol. 69, 1982, 745-750.

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This research paper by H. K. D. H. Bhadeshia investigates the mechanical formation of superledges, which are large structural steps that facilitate the growth of proeutectoid ferrite in steel. The author proposes a mathematical model based on nucleation theory to determine the minimum possible height of these steps under specific thermal conditions and chemical compositions. By examining the interfacial free energy and the difficulty of forming smaller steps, the study provides a theoretical basis for why these features reach such substantial dimensions. Experimental data from various low-alloy steels are compared against the model, showing a strong correlation between transformation temperature and ledge height. Ultimately, the paper concludes that the size of these superledges is fundamentally linked to the thermodynamic stability of the moving boundary between austenite and ferrite.