The crystallography of the precipitation of β vanadium hydride

Abstract

The phenomenological theory of martensitic transformations is applied to the precipitation of β vanadium hydride from b.c.c. solutions of hydrogen in vanadium. The predictions of the theory are examined for a body centred tetragonal hydride structure and for a range of monoclinic structures compatible with the reported mode of hydrogen ordering. This reaction, which involves substantial diffusion of hydrogen, exhibits martensitic relief effects and the habit plane near {221} is distinctly different from the {h0k} habit planes exhibited by a number of transformations involving similar changes in structure. It is shown that the habit plane and the reported orientation relationship are predicted, within experimental error, when the complementary shear is assumed to occur on the (112) B[1̄1̄1] B system. These predictions are obtained using the initial and final lattice parameters; correct predictions are not obtained if it is assumed that transformation occurs in regions of high hydrogen concentration. Since the complementary shear plane is not a plane of symmetry, (112) twins cannot form with equivalent correspondences and the hydride plates are not expected to be twinned. The interface should therefore contain dislocations and the small magnitude calculated for the complementary shear implies that these should be resolvable in the electron microscope.