Structure-dependent triple junction hardening and intergranular fracture in molybdenum

The roles of grain boundaries and triple junctions in crack nucleation and propagation in polycrystalline molybdenum are discussed in connection with grain boundary and triple junction hardening. Hardness measurements were made by microindentation tests for molybdenum polycrystalline specimens. The character and the connectivity of grain boundaries at triple junctions were determined by OIM analysis prior to the indentation test. Grain boundary hardening at low-angle and low Sigma coincidence boundaries was found to be smaller than at random boundaries. However, the hardening observed at the low-angle and Sigma boundaries increased with increasing Brandon's ratio up to almost the same level as that at random boundaries. Triple junction hardening depended on the grain boundary connectivity. The degree of triple junction hardening was smaller at the junctions composed of low-angle and Sigma boundaries than at those composed of more random boundaries. The grain boundary connectivity and triple junction character favourable for preventing intergranular fracture is suggested on the basis of the results of structure-dependent grain boundary and triple junction hardening in brittle polycrystalline molybdenum.