GRAIN-BOUNDARY STRUCTURE, ENERGY AND STRENGTH IN MOLYBDENUM

Molybdenum has many attractive properties for high-temperature structural applications. However, its usefulness as a structural material is impaired by brittleness at and below ambient temperature, resulting from its intrinsically weak grain boundary strength. According to our previous study, the fracture strength in molybdenum depends markedly on the grain boundary character and, in particular, on the orientation relationship between the two adjacent crystals and the orientation of the grain boundary plane. However, our present knowledge is still far from a complete understanding. In this study, the relationship between the strength, grain boundary structure and grain boundary energy has been investigated for purified bicrystals of molybdenum with various {110} symmetric tilt boundaries, by means of transmission electron microscopy and optical interferometry. The main results obtained are as follows. (1) There is a relatively good correlation between the fracture strength and the grain boundary energy. The energy cusps, for instance, are observed for (112) and (111) Sigma 3 coincidence boundaries, which are high in fracture strength, while the energy is higher for near (114) and (122) Sigma 9 boundaries, which are low in strength. (2) Sigma 1 small angle and near (112) Sigma 3 coincidence boundaries have a good coherence, which agrees well with the result of the boundary energy measurement. Grain boundary dislocations are observed and they can be described by the boundary dislocation model. (3) On the(ll4) Sigma 9 coincidence boundary with low fracture strength, the structure is poor in coherence compared with Sigma 1 and Sigma 3 boundaries. However, the grain boundary structure can also be described by the grain boundary dislocation (displacement shift complete (DSC) dislocation) model. (4) 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.