NEW CARBIDES OBSERVED BY HIGH-RESOLUTION ELECTRON-MICROSCOPY IN A MOLYBDENUM BICRYSTAL CONTAINING A SIGMA=5 SYMMETRICAL BOUNDARY

A molybdenum Sigma = 5 [000](310) symmetrical tilt boundary has been e xamined following carburization heat treatments. The resulting structu re was studied by 400 kV high-resolution electron microscopy and compa red with the structure of the pure boundary. It is shown that two diff erent morphologies and carbide structures exist. One is the standard M o2C hexagonal phase which precipitates at the boundary, while a tetrag onal structure can also be present within the grains as well as in the boundary itself. In this latter case, the carbide represents a thin ( about 1 nm) continuous layer located at the boundary plane.