A check of the I.P.S. theory with the aid of an accurate determination of habit planes and orientation relationships in bainitic steels

With the aid of a new method of habit plane determination, which has been described earlier [1,2], it is possible to determine the orientation relationship in specimens of a commercial 35NiCr18 steel within a deviation of 1°, despite the fact that the orientations of the bainite plates are measured by means of electron diffraction. This new method is based on the orientation determination of the austenite prior to the bainite transformation at 365°C using annealing twin traces on a surface of the crystal. The observed habit planes relative to the f.c.c. and b.c.c. structure are irrational but the mean values are close to {569}γ and {2 7 10}λ respectively, while the orientation relationship is of the type Kurdjumov-Sachs. The I.P.S. theory can not account for the observed habit planes and orientation relationships because the choice of the shear elements of the lattice invariant strain is not unrestricted, and twinned bainite has never been observed in the present investigation as required by the double shear theory.