Microstructural size effects in high-strength high-conductivity Cu-Cr-Nb alloys
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INTRODUCTION
TERNARY Cu-Cr-Nb alloys, particularly with a composition of 8 at. pct Cr and 4 at. pct Nb, have demonstrated high strength and high conductivity coupled with good thermal stability.[1,2] This behavior is due to the insoluble and strong Cr2Nb intermetallic phase (with a 2:1 Cr/Nb ratio) in a copper matrix. The Cr2Nb phase is distributed as large primary precipitates, formed congruently from the melt, and fine secondary Cr2Nb particles, precipitated from solid solution.[3] Previous studies of extruded Cu-8Cr-4Nb exposed to temperatures of up to 1323 K for up to 100 h indicated a drop in strength of only 25 to 30 pct. Such behavior was mainly a manifestation of Cu grains stabilized by Cr2Nb dispersoids characterized by a sluggish coarsening behavior.[4,5,6] It was found that the primary particles, usually situated at grain boundaries and triple points, provide a direct grain boundary pinning effect and, hence, an indirect grain boundary strengthening (Hall– Petch) effect. The secondary Cr2Nb particles provide Orowan strengthening. For extruded material, it was established that grain boundary strengthening accounts for about two-thirds of the overall strength of material, with Orowan effects essentially contributing the remainder. This thermal stability, along with strengthening effects and, more importantly, strength retention, was the driving force to improve these alloys. They are strong candidates for uses ranging from welding electrode and highvoltage switch applications to more advanced active-cooling applications such as first wall, divertor interactive components and magnetic confinement in fusion reactors, combustion chamber liners for next-generation reusable launch vehicle engines, and rocket nozzle liners. For all of these applications, in addition to strength and KEN R. ANDERSON, formerly Research Assistant with the Chemical Engineering/Materials Science Department, University of California, Davis, is Senior Engineer, Bettis Atomic Power Laboratory, West Mifflin, PA 15722-0079. JOANNA R. GROZA, Professor, is with the Chemical Engineering/Materials Science Department, University of California– Davis, Davis, CA 95616. Manuscript submitted August 10, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
stability, thermal conductivity of the material is an important factor. For higher conductivity, a smaller amount of stable and strong Cr2Nb particles is favorable, thus making an alloy with 4 at. pct Nb and 2 at. pct Cr (Cu-4Nb-2Cr) a better choice. However, the lower Cr2Nb content has an inevitable adverse effect on strengthening. To maximize the strengthening of this alloy, i.e., to take advantage of the substantial Hall–Petch effect derived from the pinning capability of Cr2Nb particles, mechanical milling (MM) was selected as a convenient processing approach. In addition to general size refinement, it was also expected that MM can bring primary and secondary particle sizes closer together to further enhance thermal stability. Thus, the impetus of the present study is to apply MM in order to refin
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