Abstract
The main purposes of this study were; (i) to apply the theory of martensite crystallography to martensitic transformations in low-symmetry materials, (ii) to investigate, using the computer simulation method, the microscopic structure of complex interphase boundaries which are not yet fully understood in terms of the martensitic mechanism. Although the unique symmetry of a twin boundary makes it a rather exceptional kind of interface, it is clearly a particularly appropriate starting point especially due to the role which twinning plays in martensitic transformations. The accuracy of twinning modes is vital for their use as lattice-invariant shears in theories of martensite crystallography. Potential twinning modes for zirconia were determined using the analysis due to Bilby and Crocker (1965) and the associated atomic shuffling was also considered. Twinning orientation relationships involving a screw axis and a glide plane have been established. The theory of martensite crystallography (Acton et al. 1970) was then applied to the tetragonal to monoclinic martensitic transformation in zirconia. The predictions for the habit plane, shape strain and the direction of the shape deformation were obtained and compared with available experimental observations. The application of the theory was also extended to the face-centred cubic to monocline martensitic transformation in plutonium alloys. The predictions of the crystallographic features for this transformation are reported. The computer simulation method was applied to investigate the relaxed atomic structure and energies of the complex interphase boundaries. The (100)b//(100)f and the (011)b /(111)f interphase boundaries were investigated using interatomic potential. Special consideration was given to the misfit dislocations at the interface which can accomplish the lattice-invariant shear of the phenomenological theories of martensite crystallography. A new equilibrium interatomic potential for iron was developed to study the relaxed structure of the (225)f b. c. c. /f. c. c. interphase boundary. These results are also compared with experimental information. Finally the general results of the thesis are discussed and main conclusions summarized.