Delineating brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites
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I. INTRODUCTION
THE salient characteristic of Nb-based in-situ composites is a microstructure of hard intermetallic particles embedded in a Nb solid-solution (Nbss) matrix, or vice versa—a microstructure of ductile particles dispersed in a continuous intermetallic matrix.[1] The hard intermetallics, which include NbCr2,[1–8] Nb3Al,[9,10,11] and Nb5Si3,[12–22] are intended to impart high-temperature strength, oxidation, and creep resistance,[17–20,22], while the Nb solid-solution phase is intended to impart ambient-temperature fracture toughness and tensile ductility through a number of ductile-phase toughening mechanisms such as crack blunting, branching, deflection, and bridging.[23,24] The presence of a ductile Nb solid-solution phase, however, does not always produce a high fracture resistance in the in-situ composites. The reasons are that (1) the Nb solid solution might not be ductile because of reduced dislocation mobility resulting from solidsolution strengthening, and (2) the brittle intermetallics are nondeformable and induce high plastic constraints in the solid-solution phase. The use of alloying additions to improve the tensile ductility and fracture resistance of Nb has been addressed recently on the basis of a computational approach for designing ductile Nb-Ti-Cr-Al solid-solution alloys.[25] It has been demonstrated that the computational approach can be used to design alloy additions, such as a Ti addition, to reduce the Peierls– Nabarro (P–N) barrier energy[26,27] and increase the dislocation mobility. Enhanced dislocation mobility, in turn, improves the tensile ductility and fracture toughness of the solid-solution alloys. Conversely, the computational methodology can be used to identify alloy additions, such as Cr and Al additions, that lead to an increase in the P–N barrier energy and a corresponding decrease in dislocation mobility. These alloying effects of Al and Cr are undesirable and KWAI S. CHAN, Institute Scientist, is with the Southwest Research Institute, San Antonio, TX 78238. DAVID L. DAVIDSON, Consultant, Southwest Research Institute, is retired. Manuscript submitted January 25, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
should be avoided, because reduced dislocation mobility inevitably leads to reductions of tensile ductility and fracture toughness.[28] Enhancement of the fracture toughness of Nbbased in-situ composites by Ti additions has been reported by Davidson et al.,[3] Subramanian et al.,[21] and Bewlay et al.[22] Besides intrinsic effects due to alloying, a ductile phase can impart fracture resistance in intermetallic- and ceramicmatrix composites through extrinsic mechanisms such as crack branching, deflection, and bridging mechanisms.[23,24,29–33] Micromechanical modeling of the crack bridging by ductile-phase particles has established that the fracture resistance of these particulate composites increases with increasing lengths of the bridge zone in the crack wake.[23,24,29–32] In addition, the fracture resistance increases with the volume fraction and fracture to
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