The fatigue and fracture resistance of a Nb-Cr-Ti-Al alloy
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INTRODUCTION
RECENT studies have shown that fracture toughness on the order of 10 to 20 MPa!m can be achieved in Nbbased in situ composites, based on the Nb-Cr-Ti,[1,2,3] NbSi,[4–7] Nb-Al-Ti,[8,9] and Nb-Ti-Hf-Cr-Al-Si[10–13] systems. These materials are characterized by a two-phase microstructure comprised of intermetallic and metallic solidsolution phases. For composites of engineering interest, the intermetallic phase exists in the form of hard particles embedded in a solid-solution metallic matrix. In the NbCr-Ti system, the intermetallic particles are Cr2Nb, which is a Laves phase with the C15 structure at ambient temperature.[14,15] The Cr2Nb particles are brittle, with a fracture toughness of about 1 MPa!m.[1,2,3] Because of the low toughness of the particles, most, if not all, of the fracture toughness of the in situ composites, based on the Nb-CrTi system, originates from the matrix, which is a Nb solidsolution containing Cr and Ti. It has been demonstrated that a Ti addition in the range from 30 to 40 at. pct enhances the fracture resistance of NbCr alloys by reducing the Peirels–Nabarro (P–N) barrier energy and stress, thereby increasing the dislocation mobility.[16] This has the effect of promoting the emission of dislocations and suppressing the propagation of cleavage cracks from the crack tip. As a result, the plane-strain fracture toughness of Nb-Cr-Ti solid-solution alloys increases with increasing Ti contents. The presence of hard Cr2Nb particles increases the plastic constraint in the matrix and reduces the fracture resistance if the matrix phase does not have sufficient ductility. With a ductile Nb solid-solution matrix, a fracture toughness as high as 20 MPa!m has been achieved in Nb/Cr2Nb in situ composites containing 38 vol pct Cr2Nb particles. The oxidation resistance of Nb-Cr-Ti solid-solution D.L. DAVIDSON and K.S. CHAN, Institute Scientists, are with the Southwest Research Institute, San Antonio, TX 78238. Manuscript submitted November 3, 1998.
METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys and in situ composites, however, is inadequate for high-temperature applications. One possible means of increasing the oxidation resistance is to add Al to Nb-CrTi to form solid-solution alloys and in situ composites based on the Nb-Cr-Ti-Al system. An example of the former is Nb-40Ti-10Cr-10Al and, of the latter, is Nb-27Ti10Cr-24A1.[10] Other alloying strategies include the addition of Hf and Si, which have been used to produce a variety of in situ composites based on a solid-solution matrix with either silicide particles, (Nb,Ti,Cr)2Al, or (Nb,Ti,Hf)2Al particles.[10] The characteristics of these emerging alloys and in situ composites have been summarized in recent review articles.[9–13] The effect of an Al addition on the fatigue and fracture properties of Nb-Cr-Ti-Al alloys is not well understood. Limited previous work on Nb-40Ti-10Cr-10Al indicated that the addition of Al to Nb-Cr-Ti solid-solution alloys led to a reduction in the fracture toughness.[3,16] When correlated to the fracture tou
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