The fracture toughness of niobium-based, in situ composites

The fracture resistance of Nb-Cr-Ti alloys or in situ composites of three different compositions, Cr2Nb, and a Nb-10Si in situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. The in situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in the in situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.