On the apparent rotation of transformation twins in Fe-Ni martensites

Fe-Ni alloys containing 28 to 33 wt pct Ni have been studied by transmission electron microscopy and by selected area diffraction to elucidate the phenomenon of the apparent deviation of the twin interface from the twinning plane 112m. Trace analyses of the twin interfaces are reported for 35 cases. Deviations of the twin trace, as projected onto the image plane, of up to 46 deg from those expected from 122m transformation twins were observed. Contrary to previous interpretations, the conclusion of this study is that the deviations are due to the uncertainty of the orientation of the foil surface with respect to the electron beam. Thus, it has not been shown explicitly that this phenomenon provides a basis either for invoking multiple lattice invariant shears or for explaining habit plane scatter.