Nanohardness of molybdenum in the vicinity of grain boundaries and triple junctions

We performed a nanoindentation study of the inherent mechanical properties of grain boundaries and their triple junctions in molybdenum tricrystals of controlled geometry grown from the melt. A region with increased hardness is revealed in the vicinity of all grain boundaries studied. The width of this region is about 2 μm and the maximum relative increase of hardness at the grain boundary is about 30%. The local nanohardness at the triple junctions was found to be higher than that at the grain boundaries. This effect is analyzed in terms of the indentation size effect model of Nix and Gao, assuming that the grain boundaries represent ideal barriers for the first nucleated dislocation loops. We calibrated the parameters of the developed model using the data for nanohardness of grain boundaries. The model predicts values of nanohardness at the triple junctions which are in good agreement with the experimental results.