Resistance-curve toughening in ductile/brittle layered structures: behavior in Nb/Nb₃Al laminates

A study has been made of the fracture toughness and resistance-curve behavior of a laminate consisting of alternating layers of brittle Nb₃Al intermetallic and ductile Nb metal, using layer thicknesses of ∼500 and 125 μm, respectively. Effective resistance-curve toughening of Nb₃Al was achieved in such a coarse-scale layered structure with only 20 vol.% of the Nb reinforcement phase. Specifically, the toughness of Nb₃Al was increased from ∼1 MPa√m to well over 20 MPa√m (and as high as 70 MPa√m in certain samples) after several millimeters of stable crack growth. These values are significantly greater than other Nb/Nb₃Al composites containing Nb as ∼20 μm sized particulates or 1-2 μm thick Nb layers (in the form of a microlaminate), both containing at least 40 vol.% of the ductile phase. The source of such ductile-phase toughening was attributed to crack blunting at, and renucleation across, the ductile Nb layers, which in turn led to extensive bridging and plastic deformation within the Nb layers in the crack wake. Since the extent of crack trapping by the ductile layer and plastic deformation are limited by layer thickness, the present coarser-scale laminates tend to display better fracture resistance compared to composites with finer-scale ductile reinforcements.