A computational approach to designing ductile Nb-Ti-Cr-Al solid-solution alloys
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THERE has been considerable interest in recent years in developing Nb-based in-situ composites for structural applications at high temperatures.[1–22] The salient characteristic of this class of emerging high-temperature materials is a multiphase microstructure containing one or more intermetallic phases embedded in a Nb solid-solution phase or vice versa, depending on the composition. The design of the composite microstructure is influenced by considerations of oxidation, strength, and creep properties at elevated temperatures, as well as ductility and damage tolerance at ambient temperatures.[1,4,17–20] The intermetallic phases of the insitu composites are expected to provide high-temperature strength, creep resistance, and oxidation resistance, while the Nb solid solution is intended to impart ambient-temperature ductility and fracture toughness through one or more ductilephase toughening mechanisms, including crack blunting, deflection, and bridging.[23,24] The presence of a Nb solid-solution (Nbss) phase in an in-situ composite does not automatically guarantee ambienttemperature ductility and fracture resistance, because these properties depend critically on the composition of the solidsolution phase, its microstructure, defect structure, and interstitial contents. While the latter three factors are important, the present article is focused on the alloying effects of the solid-solution phase only because of the lack of space. Extensive work by Begley[25] on binary Nb solid-solution alloys has established that alloying additions such as Cr, Al, W, Mo, V, Zr, and Re cause embrittlement in Nb and raise the brittle-to-ductile transition temperature (BDTT). From Begley’s study,[25] Ti and Hf are the only elements that are known not to increase the BDTT of Nb. More recent results KWAI S. CHAN, Institute Scientist, is with the Southwest Research Institute, San Antonio, TX 78238. Manuscript submitted October 23, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
have indicated that Ti enhances both the tensile ductility and fracture toughness of Nb solid-solution alloys,[26] while Cr and Al exert the opposite effects.[27,28] While they are undesirable from considerations of tensile ductility and fracture toughness, Cr and Al alloying additions in Nb are often necessary because they enhance oxidation resistance, which is the life-limiting property of current Nb-based in-situ composites and solid-solution alloys intended for high-temperature service.[17–20] Since alloying elements such as Cr and Al, which improve oxidation resistance, embrittle Nb, alloying additions to Nb solid-solution alloys and in-situ composites must, therefore, be chosen judiciously in order to attain a balance of properties such as oxidation and fracture resistance. The traditional approach to alloy design relies heavily on experimentation, usually by systematic variations of alloy composition followed by characterization of material properties. Regression analyses are often employed to correlate properties as a function of alloy addition in ord
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