Direct Observations of Grain-Boundary Sliding in Bi-Crystals of Sodium Chloride and Magnesia

A study has been made of the grain-boundary behavior in bi-crystals of sodium chloride and magnesia held under load at elevated temperatures. Grain-boundary sliding was observed in both materials, of a form similar to that found in metals. The sliding occurred not smoothly, but in discreet jumps followed by periods of zero movement. In sodium chloride the over-all sliding rate was found to depend upon four variables examined: stress, temperature, atmosphere, and boundary misorientation. Two variables examined, stress and boundary misorientation, were found to influence the over-all boundary sliding rate in magnesia. Void formation as a result of sliding was observed in a sodium chloride sample which contained a well-defined jog in its boundary. Implications that the present results bring to bear upon general considerations of the high temperature deformation and fracture behavior of polycrystalline nonmetallic materials are discussed.