Abstract
A model for the nucleation of ferrite on austenite grain boundaries proposed by Sharma and Purdy is reexamined using the Cahn-Hoffman construction. The construction, which offers a rigorous method of predicting the shape of heterogeneous nuclei, indicates that the original proposal of a disk shaped nucleus should be modified to that of a spherical segment. Based on currently available thermodynamic and physical data, it is shown that the spherical segment model is very sensitive to the austenite grain boundary interfacial tension. Even small changes in interfacial tension of several mN/m, which could result from the equilibrium absorption of boron into austenite boundaries, is sufficient to delay the austenite transformation by a significant time.
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