A single surface analysis of deformation twins in crystalline mercury

Abstract

Deformation twins are reported to occur in crystalline mercury on the {110} planes of the face-centred rhombohedral cell, but a recent study of slip in this material suggests that this result may be incorrect. In the present paper we first give an analysis of the geometry of the possible twinning modes in mercury, including modes of a new type with four irrational elements, and show that the mode which is most likely to arise in practice does indeed have a {110} habit. A detailed single surface analysis, not involving the use of x-ray techniques, of one grain of a mercury specimen exhibiting four sets of twins is then described. None of the twins belong to the {110} mode. However, the traces and surface features of all four twins are consistent with the Type II mode with K₁ K₂ η₁ η₂ elements given by: This mode has been confirmed using the same technique on further grains and also conventional single crystal methods. Neither the {110} nor the mode involves atomic shuffling but the mode has a much larger twinning shear. The occurrence of this mode thus violates the basic hypothesis of recent theories of deformation twinning. The significance of this result and of the power of the single surface analysis technique are discussed.