Some kinds of dense strain contrast in beta -Si3N4 have been considered as
dislocation tangles formed during mechanical milling. However, our present
electron microscopic analyses indicate that the strain contrast with radial
contours is not in the least attributed to the dislocations but rather to
the subgrain boundaries associated with the initial growth of the crystal,
i.e. the nucleation. A number of growth variants, independently nucleated o
n the surface of a single Si-N cluster, coarsen and form closure mistakes w
hen they approach, giving rise to the formation of subgrain boundaries exte
nded from the core. The subgrain boundaries are found to locate only on {10
0}, {110} and {101} planes of the beta -Si3N4 crystal structure.