Ductile Chromium in Heavily Cold-Drawn Cu 75 Cr 25 Alloy
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I.
INTRODUCTION
CHROMIUM is always considered as inherently brittle metal. Numerous studies have shown the effects of various impurity elements on the deformation properties of chromium.[1] It is reported that the deformability of chromium can increase to about 2 pct when the purity was improved.[2] Binary Cu-Cr alloys have been widely used in electrical contacts and high strength and high conductivity fields because of their high mechanical strength and electrical conductivity.[3–9] Because of limited solubility in the Cu matrix (close to zero at room temperature) and high stability of precipitation particles below 673 K (400 C), Cr is an effective reinforcing phase for Cu-based composites.[10] In order to achieve Cu-Cr alloys with higher strength, higher Cr content in Cu-Cr alloys is expected. However, the strength does not improve obviously with the increase of Cr content because of the weak strengthening effect of Cr dendrites in Cu matrix. On the other hand, Cu–Cr alloys with Cr content higher than 15 wt pct are difficult to prepare by conventional casting and plasticity deformation processes.[11,12] One reason is that the temperature of completely smelting Cu-Cr alloys with increased Cr content is higher (about 2273 K), which may induce YANLI CHANG, Doctoral Student, and ZIQIN GUO, Master Student, are with the MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi’an 710049, Shaanxi, P.R. China. ZHIMING ZHOU, Professor, is with the School of Material Science and Engineering, Chongqing University of Technology, Chongqing 400054, P.R. China. YAPING WANG, Professor, is with the MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, and also with the State Key Laboratory for Mechanical Behavior of Materials, School of Science, No. 28, Xianning West Road, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 14, 2015. Article published online November 16, 2015 504—VOLUME 47A, JANUARY 2016
significant reaction of liquid Cr with crucible materials and then the chromium compounds forming in the reaction could decrease the plasticity of Cu-Cr alloys. Another reason is that the increased Cr content may decrease the plasticity of Cu-Cr alloys. Up to now, the preparation and mechanical property of Cu-Cr alloys with Cr content as high as 15 wt pct are reported.[13–17] For Cu-Cr alloys with higher Cr content, the effect of microstructure on mechanical strength, plasticity, and deformation behavior has not been mentioned.[18,19] On the other hand, the deformation mechanism of Cr phase in the Cu-Cr alloys is still not clear.[20] In this work, Cu75Cr25 (with 25 wt pct Cr) ingots are prepared by vacuum induction melting (VIM) in calcia crucible. It is expected to reduce oxide pollution during the smelting process because of the lowest Gibbs energy of CaO in the metal oxides, and therefore, the reaction incli
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