Experimental and Numerical Investigation on the Phase Separation Affected by Cooling Rates and Marangoni Convection in C
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CU-CR alloys are commonly used as contact materials for energy distribution.[1,2] During switching operations, e.g., the interruption of high currents, a vacuum arc is formed. As a consequence, the surface of the contacts is subjected to a nonuniform temperature field and the melt goes through a relatively high cooling rate after extinguishing of the arc. The gap between the contacts has to recover its dielectric properties. This recovery process determines the success of the electrical circuit interruption. The Cu-Cr phase diagram[3,4] is characterized by a complete miscibility in the liquid phase, a small solubility of Cr in Cu and a negligible solubility of Cu in Cr, e.g., the maximum solubilities of Cr in Cu and Cu in Cr are 0.0077 and 0.0008, respectively, in mole fraction at eutectic temperature.[3] A metastable liquid miscibility gap lies immediately below the flat portion of the equilibrium liquidus line and is probably stabilized by impurities. The expected high cooling rates after arc extinction have an effect of stabilizing the miscibility gap. Since Cu and Cr are mutually nearly insoluble, Cu-Cr alloys are in essence two-phase composites, usually
FEI WANG and RAJDIP MUKHERJEE, Research Assistants, and BRITTA NESTLER, Professor, are with the Institute of Materials and Processes, Karlsruhe University of Applied Sciences, Moltkestrasse 30, 76133 Karlsruhe, Germany, and also with the Institute of Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Haid-und-Neu Strasse 7, 76131 Karlsruhe, Germany. Contact e-mail: [email protected] KATHARINA VON KLINSKI-WETZEL Research Assistant and MARTIN HEILMAIER, Professor, are with the Institute of Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76131 Karlsruhe, Germany. Manuscript submitted January 23, 2014. Article published online 22 January 2015 1756—VOLUME 46A, APRIL 2015
consisting Cr-rich particles embedded in a Cu-matrix (Figure 1) considering the Cu-rich side of the phase diagram. During the cooling process, the melt of Cu-Cr separates into Cu-rich and Cr-rich liquids when the melt enters the spinodal region. As a consequence, a dispersion of Cr-rich droplets as the minority phase in a matrix of the majority Cu-phase evolves.[5–7] The properties of the surface melt layer (e.g., roughness and/or loose microparticles), which is formed due to the vacuum arc caused by the switching operations, are directly affected by the size of these droplets. To obtain a prediction of the microstructure, the local cooling rate, which determines the size of the droplets,[8– 11] is of significant importance. Moreover, the observation of a size gradient of Cr-particles, which seems to be caused by the temperature gradient, drives the motivation to shed light on understanding the influence of the Marangoni convection on the phase separation structures by a systematic simulation study. In this paper, we numerically calculate the mean radius of Cr-particles from phase separation in Cu-Cr alloy undergoing different cooling rate
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