Considerations of solute-drag in relation to transformations in steels

H. K. D. H. Bhadeshia

Abstract

In recent years, solute-drag theory has been widely quoted to explain apparent discrepancies in kinetic measurements of transformations in steels. Since conventional impurity drag theory is concerned solely with drag at grain boundaries, many new, or "special" drag models have been developed to apply at interphase interfaces. An assessment is made of these special drag theories, and of the experimental evidence claimed in their support. It is found that our understanding of solute-drag at interphase interfaces is weak, and that the experimental evidence for the existence of solute drag at transformation interfaces in steels is very doubtful.

This research analyses solute drag dynamics, specifically examining how impurity atoms interact with moving grain boundaries and interphase interfaces to slow down structural changes. The text critiques conventional theories that rely on solute segregation and diffusion across interfaces, noting that these models often fail to account for the anisotropic nature of atomic movement. A significant portion of the work evaluates special drag effects in steels, questioning previous claims that carbide-forming elements like molybdenum significantly hinder transformation through unique binding forces. By re-evaluating experimental data, the author suggests that observed kinetic discrepancies may actually stem from carbide precipitation, interface faceting, or complex attachment processes rather than a simple drag mechanism. Ultimately, the source concludes that current evidence for these specialised effects remains scientifically unproven and requires more advanced analytical validation.

Journal of Materials Science, Vol. 18, 1983, pp. 1473-1481.

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