Crystallographic analysis of ferrous martensite morphology

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Abstract
The crystallography of ferrous martensite, especially that of {111}f and {252}f habits, has been analysed in terms of the Bowles-Mackenzie theory. In order to specify the isotropic dilation parameter, the transformation is assumed to occur preferentially in a geometry such that the shape strain in the vicinity of the martensite lath or plate edges is easily accommodated in the parent austenite by the parallelism between the direction of shape strain and the austenite slip direction. The lattice-invariant shear plane is also assumed to vary between (101)f and (111)f about the [101]f lattice-invariant shear direction. In this analysis, it is concluded that low-carbon lath martensite consists of many small subunits within the (111)f slip band which accommodates the shape strain and that the coalescence of them leads to the [101]f (111)f lath-like habit. In medium-carbon steels, the habit plane close to (252)f is predicted by the above assumption. The (295)f habit of high-carbon steels is obtained by the lattice-invariant shear of (112)b twinning as in the previous analyses.