THE EFFECT OF NIOBIUM MORPHOLOGY ON THE FRACTURE-BEHAVIOR OF MOSI2 NB COMPOSITES

The morphology of the niobium reinforcement added to MoSi2 affected the fracture behavior (and hence toughness) of MoSi2/20 vol pet Nb composites. The addition of discontinuous random niobium in the form of particles or short fibers deflected cracks that propagated through the MoSi2 matrix. However, this did not result in any improvements in toughness (estimated from the area under flexural stress-displacement curves), as matrix cracks preferentially propagated through the Nb/MoSi2 interphase region. The addition of aligned niobium fibers, oriented perpendicular to the direction of matrix crack propagation, directly participated in the fracture of the composite. Depending on the diameter of Nb embedded in the MoSi2 matrix, these fibers either fractured in a brittle manner or ruptured in a ductile manner. Small (400-mu m) diameter continuously aligned Nb fibers fractured by brittle cleavage during testing. Therefore, the addition of these fibers was not as effective in improving the toughness of MoSi2 as the addition of larger (800-mu m) diameter continuously aligned Nb fibers, which ruptured in a ductile manner. It was observed that the larger diameter fibers had separated from the matrix through the propagation of cracks in the reaction zone adjacent to the fibers and that these cracks formed prior to yielding of these fibers. In contrast, the smaller diameter fibers remained well bonded to the matrix and, thus, were constrained by the MoSi2 matrix from yielding. This resulted in brittle fracture behavior of the Nb fiber. There appeared to be an effect of aspect ratio on the fracture of the ductile embedded fibers. Shorter length 400-mu m-diameter fibers separated from the matrix, behavior similar to the continuous 800-mu m-diameter fibers and not the continuous 400-mu m-diameter fibers.