The crystallography of the bainite transformation—I

Abstract

The crystallography of the bainite transformation in an Fe-8% Cr-1.1 % C alloy was studied. Measurements of the habit plane by two-surface analysis and the specific variant of the orientation relationship have been made for the first time for a lower bainite transformation. The habit plane is near (254) in the austenite, and corresponds closely to (1̄22) in bainitic ferrite. The magnitude of the shape strain accompanying this transformation was also measured. The above bainite crystallographic parameters are different from those in martensite formed in the same alloy. Phenomenological analyses of the bainite crystallography on the basis of present theories show that the experimental habit plane and orientation relationship can be predicted if one assumes either that 1. (a) the habit plane is exactly an invariant plane and that the lattice invariant shear is irrational in both plane and direction, or 2. (b) the habit plane is unrotated but undergoes an isotropie contraction of about 1.2%. In the latter case the lattice invariant shear consists of a double shear on the (11̄1) and (101) austenite planes along the common [1̄01] direction. These planes correspond to the (101) and (112) slip planes in the bainitic ferrite along the common slip direction [1̄1̄1]. The bainite crystallography is not consistent with internal twinning as the inhomogeneous shear. This conclusion is further supported by thin foil electron microscope studies which show no internal twinning in this bainite. These results suggest that lower bainite and martensite in the same alloy are likely to have different transformation inhomogeneities, as well as other differences.