Delineating brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites

The fracture toughness of Nb-based in-situ composites typically decreases with increasing volume fractions of hard intermetallic phases, despite the presence of a ductile niobium solid-solution phase in the microstructure. For composites with a continuous intermetallic matrix, the fracture toughness can be more than double that of the monolithic intermetallics, but is still low in absolute terms, indicating that the solid-solution phase is not very effective in inducing ductile-phase toughening. The lack of enhancement of the fracture resistance appears to arise from an embrittlement effect instigated by the brittle phases in the microstructure, whose nondeformability results in a high plastic constraint acting on the ductile phase. In this article, an analytical model is developed for treating both brittle-phase embrittlement and ductile-phase toughening in terms of constituent properties and microstructural variables. The model is then used to (1) delineate brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites, and (2) design fracture-resistant in-situ composites based on Nb-Ti-Cr, Nb-Ti-Al, and Nb-Ti-Si systems.