High-resolution electron microscopic studies were made of small-angle and special boundaries in samples which had been prepared by means of cold isostatic pressing and sintering. The structures of 90 rotation twin boundaries, small-angle tilt boundaries, and a  = 17 boundary were examined. A 90 rotation domain with a (001) planar interface was found to have steps and was observed to result from a domain growth process. The use of geometrical arguments indicated that grain boundary dislocations with a Burgers vector of [001], in the small-angle [100] tilt boundaries, could dissociate into only 3 partial dislocations. In small-angle boundaries which were parallel to [100], the misorientation was accommodated by 1/3[001] shifts over some 3c distances perpendicular to the (001) planes; with spacings of about 12nm along the (001) planes. Orientational differences between the grains which formed small-angle tilt boundaries with both [100] and [010] tilts were accommodated by a reorientation of the (001) planes and misfit dislocations. A grain boundary which was close to the  = 17 orientation was shown to be perpendicular to the (001) planes which were contiguous across the boundary, and formed via an a-b in-plane rotation, by about 30, around the c-axis.

A.Singh, P.Kuppusami, E.Mohandas, S.Raju, V.S.Raghunathan: Physica C, 1996, 261[3-4], 273-83