Abstract
The normal structure of mercury is unique in that it is the only metal having rhombehedral symmetry to possess a single lattice structure. This fact, together with the nearness of the structure to f.c.c., make its deformation behaviour particularly interesting. The present investigation describes the orientation dependence of the deformation characteristics of mercury at two different temperatures. The large standard stereographic triangle associated with the low symmetry structure of mercury occupies one sixth of the full stereogram. Theoretical considerations have shown that, within this one triangle, slip on three different variants of the observed mode is possible, the boundaries between the corresponding regions having two distinct characteristics. These predictions have been tested by experiments on single crystals at 77°K and the role of twinning and kinking in the deformation process assessed. Single crystals tested in tension at 4.2°K have been found to undergo a stress-induced phase transformation. Evidence for this reaction is presented in the form of superconductivity and electrical resistance measurements and the martensitic nature of the transition established. Both the properties of the new phase and the nature of its production served to distinguish this transformation from the previously reported low temperature alpha - beta transition. It has thus been labelled the gamma phase. Detailed metallographic observations determined the shear elements associated with this transformation as {113} , g = 0.47, the indices referring to the f.c.r. cell of alpha-mercury. The morphology of the transformed crystals has been interpreted in terms of the accommodation of the martensitic plates in the parent; matrix and the crystallography of the habit plane and associated shear direction. A marked dependence of the occurrence of the transformation on the orientation of the crystal has been interpreted by regarding the shear process associated with the transformation as a conventional deformation mode. Further explanation of the results using various approaches such as lattice geometry and anisotropic elasticity theory has been attempted. The application of the current theories of martensite crystallography to this transformation based on a product crystal structure predicted using the pseudopotential theory of metals is also reported. Finally, an account of an experimental investigation, using an X-ray diffraction technique, to determine the actual product structure is presented.