2003 Volume 44 Issue 9 Pages 1774-1782
In order to clarify the mechanism of the bainitic transformation, a new technique utilizing interference fringes in transmission electron micrographs was applied for observations of bainitic precipitates in copper alloys. Based on the theoretical consideration of image formation, the appearance of these fringes along the interface of a precipitate was taken to be evidence of the existence of a shear-type transformation strain. From the analysis on the spacing of interference fringes, the direction and magnitude of the shear strain were obtained. It was found that the shear strain associated with bainites has almost the same crystallographic direction and magnitude as the one associated with martensites in the same alloy. On the basis of these findings, a new model on the transformation mechanism is presented, in which the dislocation motion bringing about the lattice transformation is controlled by the atomic diffusion through a local concentration change. This model consistently explains both of existence of the transformation strain and the diffusion-control nature of the transformation.