MICROSTRUCTURE OF LESS-THAN-110-GREATER-THAN SYMMETRICAL TILT-BOUNDARIES IN MOLYBDENUM

The fracture strength in molybdenum depends markedly on the grain bounday character. In order to clarify the relationship between the strength and the grain boundary structure, bicrystals with [110] symmetric tilt boundaries were prepared and observed by means of transmission electron microscopy. The observations were conducted using the bicrystals with near SIGMA-1 and SIGMA-3 boundaries, which are higher in fracture strength, and with boundaries of misorientation angle around 30-degrees, which are lower in fracture strength. The results obtained are as follows. (1) SIGMA-1 low angle and near SIGMA-3 coincidence boundaries have a good coherency. The grain-bounday dislocations are observed and they can be described by a boundary dislocation model. (2) The boundaries of misorientation angle around 30-degrees are poor in coherency comparing with SIGMA-1 and SIGMA-3 boundaries. However, the grain boundary structure can also be described by the grain-boundary dislocation model. (3) It is considered that the grain boundary structure in molybdenum with a high covalency in bonding is not greatly different from that in normal metals.