Bainite: an atom-probe study of the incomplete reaction phenomenon

H. K. D. H. Bhadeshia and A. R. Waugh

Abstract

An imaging atom-probe combined with a high mass-resolution energy-compensated time-of-flight spectrometer has been used to study localised composition changes at the interfaces whose motion leads to the formation of bainite in steels. "Bulk" compositional analysis of retained austenite and bainitic-ferrite has also been carried out, and the results are interpreted in terms of a new thermodynamic analysis which allows a better distinction between diffusion-controlled growth involving the formation of bainitic-ferrite with a partial supersaturation of carbon, and that in which the ferrite grows with the precise composition of the parent austenite (i.e. in essence martensitic). The fine scale distribution of substitutional alloying elements (Si, Mn) has also been examined. It is believed that the results can be best understood in terms of the martensitic growth of bainite sub-units.

Acta Metallurgica, Vol. 30, 1982, 775-784.

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The provided documents detail a scientific investigation into the bainite transformation in steel using high-resolution atom-probe field-ion microscopy. Researchers H. K. D. H. Bhadeshia and A. R. Waugh analyzed the chemical composition of different phases and interfaces to understand how carbon and alloying elements redistribute during the reaction. Their findings reveal that carbon concentrations are unevenly distributed in the remaining austenite, supporting the theory that bainitic ferrite initially grows through a martensitic mechanism with a full excess of carbon. The study also utilizes thermodynamic modeling to explain why the reaction stops before reaching equilibrium, a state known as the incomplete reaction phenomenon. Ultimately, the evidence suggests that substitutional elements like silicon and manganese do not move across the transformation interface, nor does carbon, confirming the displacive nature of the growth process.