Fatigue and fracture toughness of a Nb-Ti-Cr-Al-X single-phase alloy at ambient temperature
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I. INTRODUCTION
THERE have been significant efforts in recent years to develop new Nb-based alloys with improved oxidation resistance for very high-temperature applications.[1–25] Much of these efforts have been focused on additions of Ti, Cr, Al, and other alloying elements to form Nb-Cr-Ti-Al[16,17,18] and Nb-Cr-Ti-Al-X alloys[19–24] with the intent of achieving a combination of good oxidation resistance and mechanical properties. The alloying element, X, represents an element from a list that includes Si, Hf, Mo, W, and V. Progress in this area, up to 1996, is summarized in several recent publications.[14,19–22] Current understanding of the role of these individual alloying elements on the mechanical and oxidation properties of Nb-based alloys and in situ composites can be found in these overview articles.[14,19–22] The alloying strategy in developing the Nb-Cr-Ti-Al alloys is to rely on the addition of relatively large amounts of Cr and Al to improve the oxidation.[19–22] Both Cr and Al additions in large doses, however, can lead to embrittlement and are detrimental to tensile ductility at ambient temperature.[16,18] The amounts of Cr and Al that can be added to Nb without causing embrittlement can be increased with the addition of Ti.[18] This aspect of compositional effects on tensile properties has been studied by Jackson and Jones.[18] One of the key results of their work was the identification of the maximum amounts of Cr and Al contents that can be tolerated in Nb-Cr-Ti-Al quaternary alloys without causing the transition from a ductile to a brittle fracture behavior at ambient temperature. Independent work at SwRI on Nb-Cr-Ti alloys and in situ composites revealed that Ti additions improve the fracture toughness of this class of materials.[12–15] For example, NbCr-Ti solid solution alloys with 30 to 40 at. pct Ti have been shown to have very good ductility and fracture toughness characteristics. The composition of the toughest alloy is (in D.L. DAVIDSON, formerly Institute Scientist, Southwest Research Institute, is retired. K.S. CHAN, Institute Scientist, is with Southwest Research Institute, San Antonio, TX 78238. R. LOLOEE, Graduate Student, and M.A. CRIMP, Associate Professor, are with the Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824. Manuscript submitted April 20, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
at. pct) Nb-13Cr-37Ti, which has a fracture toughness of 80 to 100 MPa!m[12,13] and a tensile ductility exceeding 6 pct[26] at ambient temperature. On the other hand, the addition of about 9 to 10 at. pct Al has led to reductions in fracture toughness to values as low as 12 to 15 MPa!m.[16,17] Recently, a Nb-based alloy was formulated and made by commercial practices[27] so that its properties could be more completely evaluated. The composition of this Nb-based alloy was (in at. pct) Nb-35Ti-6Al-5Cr-8V1W-0.3Hf-0.5Mo-0.05C, which will be referred to as Nb35Ti-6Al-5Cr-8V-1W. All alloy compositions are reported in atomic percent unless specif
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