Effects of Ti Addition on the Liquid-Phase Separation of Cu 71 Cr 29 Alloy during Rapid Cooling
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INTRODUCTION
CU-CR alloys containing 29 to 55 pct Cr (mol pct) are important electrical contact materials and are widely used for high voltage applications due to their considerably high interrupting ability, high voltage withstanding stress, and high electric and thermal conductivities.[1,2] It was indicated that the smaller the Cr-rich phase in Cu-Cr alloys was, the better the electric properties could achieve.[2–4] Powder sintering is widely used to produce Cu-Cr contacts in industry, because finer Cr particles can be obtained in comparison with traditional melting and casting, where the microstructures consist of some coarse Cr-rich dendrites and Cu-rich matrix. The diameters of Cr particles in sintered products are in the range of 30 to 100 lm.[4,5] Therefore, many special preparation processes have been employed to improve the microstructures of Cu-Cr alloys during the past 20 years.[2–10] Melt spinning is an effective method to refine the microstructures of metallic materials. Cu-Cr ribbons with up to 7 pct Cr were prepared by melt spinning,[11,12] where nanoscaled Cr-rich particles can be obtained. Cu71Cr29 (mol pct) is a representative alloy used for electric switches. The ribbons were prepared by melt spinning in our prior experiments.[13] Cr-rich particles have been refined to smaller than 200 nm. In 2005, Zhou et al.[14] reported some similar results. Yang et al.[15] studied the arc behaviors of melt-spun Cu71Cr29 ribbons. The refinement of Cr-rich particles in the ribbons could clearly improve the electric properties. ZHANBO SUN, JUAN GUO, QIAN LI, and XIAOPING SONG, Professor, are with the School of Science, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China. Contact e-mail: [email protected]. edu.cn YAN LI and YAOMIN ZHU, Professors, are with the School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471003, People’s Republic of China. Manuscript submitted November 5, 2006. Article published online March 28, 2008 1054—VOLUME 39A, MAY 2008
Recently, our further experiments have proved that the Cu-Cr alloys experienced a liquid-phase separation during rapid cooling, because a large positive mixing heat existed in undercooled Cu-Cr melts.[16] The liquidphase separation has limited the microstructural refinement of melt-spun ribbons. Enhancing the cooling rate of the melt is a method to refine the liquid-phase-separated microstructures, while this method will be limited by equipment situations, especially in industry. According to our previous work,[17] alloying is another effective method to suppress the liquid-phase separation at large under coolings if the addition of a third component can decrease the large positive mixing heat of a binary system. Cu-Ti is a binary system with large negative mixing heat.[18] A smaller positive or negative mixing heat exists in the Cr-Ti system depending on Ti contents.[19] Slade[1] indicated that the addition of Ti could improve the properties of Cu-Cr contac
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