Microstructure of ⟨110⟩ Symmetric 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 Σ1 and Σ3 boundaries, which are higher in fracture strength, and with boundaries of misorientation angle around 30°, which are lower in fracture strength. The results obtained are as follows.(1) Σ1 low angle and near Σ3 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° are poor in coherency comparing with Σ1 and Σ3 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.