Rationalisation of Shear Transformations in Steels

H. K. D. H. Bhadeshia


Experimental evidence is provided in support of the theory that Widmanstatten ferrite formation involves the co-operative growth of mutually-accommodating plates. The adjacent plates generally turn out to be similarly oriented, presumably because the simultaneous nucleation of such variants is relatively easy. The mechanism of Widmanstatten ferrite growth is considered to be compatible with an atomic correspondence being maintained for the iron atoms.

It is demonstrated that the concept of ferrite growing with a partial supersaturation with respect to carbon is not tenable, and that the expected growth rates must correspond either to carbon diffusion control or to interface friction control (when the ferrite forms with a full supersaturation). Assuming that this natural distinction between the possible growth rates corresponds to the difference between Widmanstatten ferrite and bainite, thermodynamic conditions are established and tested to enable quantitative distinction between the two products. These conditions are then demonstrated to be consistent with the conventionally accepted morphologies of Widmanstatten ferrite and bainite, and it is shown that the hypothesis that the former involves the equilibrium partitioning of carbon during growth while the latter does not is reasonable in terms of the maximum amounts of transformation permitted for a given set of conditions. An attempt is also made to incorporate nucleation into the overall thermodynamic treatment, and the outcome of all this is that it seems possible to rationalise the shear transformations that occur in steels

Acta Metallurgica, Vol. 29, 1981, pp. 1117-1130.

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