Fracture behavior and toughness of NbSi2-based single crystals and MoSi2(C11b)/NbSi2(C40) duplex crystals with a single set of lamellae

The fracture behavior and toughness of (Mo0.85Nb0.15)Si2 silicide crystals were investigated by three-point bending tests. Crystals with two different microstructures, namely the C40 single-phase single-crystalline form and MoSi2(C11b)/NbSi2(C40) duplex crystals with an oriented single set of lamellae, were prepared, and their fracture behaviors were compared. The duplex microstructure improved the toughness, exhibiting higher values than the C40 single-phase single crystals at almost all loading orientations. An increase of more than 20% in the fracture toughness was obtained in a duplex specimen, A-2, in which the lamella boundaries are parallel to the side surface and have a notch face parallel to (12¯10)C40. A complicated morphology appeared in the fracture surface of specimen A-2 as a result of significant deflection of crack propagation. Crack deflection occurred predominantly along the plane parallel to {101¯0} cleavage planes in the constituent C40 phase. The C11b lamella effectively prevented the initial rapid propagation of cracks along the notched plane, because the ease of gliding of its dislocations and its deformability effectively accommodated stress concentration. This induced crack deflection, resulting in improved fracture toughness.