Cu-Cr alloys have been found many applications in industries for they exhibit an excellent combination of high mechanical strength and high electrical conductivities. Especially the Cu-Cr alloys with high Cr contents are predominant contact materials for medium-voltage vacuum interrupters. However, there exist two types of Cu-Cr phase diagram in literature, one being a simple eutectic-type and the other a monotectic-type, which give quite different liquidus temperature for the high Cr content alloys. In addition, there has been a discrepancy regarding the existence of a liquid miscibility gap in the binary Cu-Cr phase diagram. In order to clarify the type of the Cu-Cr phase diagram and directly verify the metastable miscibility gap in the undercooled Cu-Cr alloys, researches have been carried out as following:
To accurate determination of the type and liquidus of the Cr-rich part of the binary Cu-Cr system, we used the electromagnetic levitation-based containerless solidification in combination with contactless pyrometry for temperature measurements as well as the use of a high purity protective atmosphere and high purity raw materials. In such a way, contamination of crucibles, thermocouples and protective atmosphere to the samples can be avoided. We found that the binary Cu-Cr phase diagram is monotecti-type, rather than eutectic-type.
The liquidus of Cu-Cr alloys with high Cr contents was optimized and calculated by the subregular solution model. The calculated liquid interaction can be given by
Ωl＝(86088-10674XCr)-T(34.39-17.05XCr). The stable liquid miscibility gap ranges from 53at.% to 84at.% Cr at an invariant monotectic temperature of 2023±20K. The computed critical point corresponging to the closure of miscibility gap occurs at 2118.5K and XCr=0.6964.
We have successfully prepared the rapidly solidified Cu-Cr alloys with large-scope of Cr content by splat-quenching. When the Cr content reaches 15at.%, the metastable liquid phase separation occurred in splat-quenched Cu-Cr melt droplets. In addition, the size of Cr-rich spheres from liquid phase separation increased as the Cr content increased. As the Cr increased to 65at.%, the microstructure of both the large Cr-rich spheres and the large Cr-rich bandings occurred. At the same time, it has been observed that lots of the Cu-rich phase existed in the splat-quenched Cu-Cr melt droplets with high Cr content, which can be attributed to the second phase separation.
Because of the strong reaction of Cu-Cr alloys with the crucible materials at the high temperature, both the inducing melted and the melt-spun Cu-Cr alloys were contaminated, which clarified the arguments beween Müller and Li. Liquid phase separation occurred in the inducing melted alloys with the Cr concentrations more than 40at.%, which might be attributed to the reactant particles promoting the liquid phase separation. Liquid phase separation also occurred in the melt-spun Cu71Cr29 and Cu45Cr55 alloys. With increasing the wheel speed, the size of the Cr-rich spheroids was reduced significantly. The microhardness increased obviously, which was attributed to the refined microstructure and the supersaturated solid solutions.
Liquid phase separation occurred in the arc melted Cu-Cr alloys with the Cr concentrations up to 45at.%, which was ascribed to the rapid solidification. Contrary to the non-homogeneous microstructure of the arc melted Cu-Cr alloys, the electromagnetic levitation melted Cu-Cr alloys showed the homogenous microstructure for the strong convection of the electromagnetic stirring.
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