Abstract
Mercury is the only element with a crystal structure based on a single rhombohedra lattice. Its deformation characteristics are therefore of particular interest. However, prior to the present investigation, there was considerable doubt concerning the interpretation of its mechanical behaviour. In particular, the reported deformation twinning mode had been questioned. Therefore, a two surface analysis of twin traces and surface tilts on a deformed square cross-section single crystals was carried out. The preliminary results of this analysis enabled earlier observations of twin traces on a single surface to be interpreted in terms of a new twin mode. Both methods established that the operative twins have shear elements K1 K2 eta1 eta2 given by '{1 3 5 }'{111}'' relative to the face centred rhombohedral lattice basis. This is the only twin mode with an irrational composition plane occurring in a single lattice structure. It is also unique because it is preferred to an alternative possible mode with a smaller shear. In order to investigate twinning in mercury further, a series of single crystals were deformed by bending at liquid air temperature. This enabled twin growth and the influence of slip on the twin boundary to be observed. A certain characteristic type of twin intersection similar to cross-twins in b.c.c. materials frequently occurred. This, together with straight incoherent twin boundaries which were also analysed, can be accommodated because of an almost exact crystallographic relationship between slip and twin systems. The influence of possible growth and nucleation mechanisms on the choice of the operative mode has been discussed. Finally, the atomic structure of mercury has been represented by models constructed from spheroids in a close packed configuration. In particular, models of the twin boundary have helped in elucidating the twinning process.