A Direct Study of Grain Boundary Allotriomorphic Ferrite Crystallography

S. S. Babu and H. K. D. H. Bhadeshia


The crystallography of grain boundary allotriomorphic ferrite has been studied with the aim of estimating the fraction of ferrite-austenite interfaces capable of developing into Widmanstatten ferrite. The direct measurement of austenite orientation was made possible by using steels containing relatively large concentrations of silicon, heat treated in two stabes to enable the retention of a substantial volume fraction of austenite to ambient temperature. The fraction of allotriomorphs exhibiting a reproducible orientation relationship with the austenite is found to be rather small, as a consequence of the higher mobility of more incoherent ferrite-austenite boundaries. Allotriomorphs showing an orientation relationship with both the austenite grains are found to be very rare. The tendency for the allotriomorph to adopt an orientation which optimizes the fit with respect to both the austenite grains with which it is in contact is also found to be small. Microanalysis data an their theoretical interpretation confirm that the transition from equilibrium to paraequilibrium growth cannot be predicted by the usual method based on an estimate of the solute diffusion distance in the austenite.

Materials Science and Engineering A, Vol. A142, 1991, 209-220.

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Allotriomorphic and Idiomorphic ferrite

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