High-Performance dispersion-strengthened Cu-8 Cr-4 Nb alloy
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Weight Percent Chromium 20 30 40 50 60 70 80 90
INTRODUCTION
IN the past 5 years, Cu-Cr-Nb alloys with the Cr:Nb ratio of 2:1 have received special attention as the next generation of elevated-temperature high-strength, high-conductivity dispersion-strengthened alloysy.2~ The strengthening is provided by the precipitation of the hard Cr2Nb intermetallic compound, which is stable up to its congruent melting point at 2006 K and is not soluble in solid copper. The Cr:Nb ratio of 2:1 was selected such that all Cr and Nb combine to form the Cr2Nb compound to preserve the high thermal conductivity of the pure copper matrix. Fully reacting the Nb with the Cr also avoids the formation of Nb precipitates that are susceptible to hydrogen embrittlement. For this purpose, a slight chromium excess is sometimes preferred. According to the Cr-Nb phase diagram, the Cr2Nb compound is formed in a composition range from 62.78 to 68.08 at. pct Cr (Figure 1 from Reference 3), thus allowing for some compositional deviations from the Cr: Nb ratio of 2:1. By using a Cr-rich CrzNb precipitate, the activity of the Nb can be significantly lowered and the pressure necessary for forming niobium hydrides greatly increased.t 4] Comparison of Cu-Cr-Nb alloys with different Cr and Nb content at the same 2:1 Cr:Nb ratio indicated the best overall combination of properties for an 8 at. pct Cr and 4 at. pct Nb (Cu-8 Cr-4 Nb) composition.P] Mechanical testing of commercially argon gas-atomized Cu-8 Cr-4 Nb in KEN R. ANDERSON, Graduate Research Assistant, and JOANNA R. GROZA, Associate Professor, are with the Chemical Engineering and Materials Science Department, University of California at Davis, Davis, CA 95616-5274. ROBERT L. DRESHFIELD, Senior Materials Engineer, is with NASA Lewis Research Center, Cleveland, OH 44135. DAVID ELLIS, Senior Resident Research Associate, is with Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted August 9, 1994.
METALLURGICALAND MATERIALSTRANSACTIONS A
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30 40 50 60 70 80 Atomic Percent Chromium
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Fig. 1--The most recent assessment of the Nb-Cr phase diagram drawn from Ref. 3.
as-extruded and aged conditions revealed promising roomand elevated-temperature properties, as well as good thermal stability.{ 1,5,6~For instance, the tensile strength of Cu-8 Cr-4 Nb alloy is 426 MPa at room temperature and 100 MPa at 975 K, substantially higher than the same properties of NARloy-Z (Cu-3 wt pct Ag-0.5 wt pct Zr), the alloy currently used in aerospace applications. While a precipitation-hardening effect was observed in Cu-8 Cr-4 Nb after short aging times, there was no significant decrease in mechanical strength after prolonged high-temperature exposure. For example, the tensile strength after 100 hours of exposure at 973 K decreased less than 9.5 pct when compared to the 10 hour aged specimen at the same temperature. Low-cycle fatigue
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