Formation of core-type macroscopic morphologies in Cu-Fe base alloys with liquid miscibility gap
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3/12/04
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Formation of Core-Type Macroscopic Morphologies in Cu-Fe Base Alloys with Liquid Miscibility Gap C.P. WANG, X.J. LIU, Y. TAKAKU, I. OHNUMA, R. KAINUMA, and K. ISHIDA The effects of alloying elements on the macroscopic morphologies in Cu-Fe base alloys were experimentally investigated. It was found that macroscopic homogeneity can be achieved by the addition of Mn, Ni, Al, or Co in the Cu-Fe base alloys, while the core-type macroscopic morphologies with Cu-rich or Fe-rich cores, which are radially separated as two layers in the inner and outer parts of the ingot solidified in the cast-iron mold, were formed by the addition of C, Cr, Mo, Nb, Si, or V. It is shown that the formation of the core-type macroscopic morphology is strongly connected with the existence of a stable miscibility gap of the liquid phase in the Cu-Fe base alloy due to the addition of alloying elements. The liquid phase with less volume fraction always forms the center part. This result can be explained by a mechanism that the minor droplets as the second phase are forced to move into the thermal center due to Marangoni motion, which is caused by the temperature dependence of interfacial energy between two liquid phases.
I. INTRODUCTION
ONE of the main features of the phase diagram of the Cu-Fe binary system[1] is that the metastable miscibility gap of the liquid phase exists at higher temperatures,[2] as shown by the dotted line in Figure 1. Considerable attention has been paid to this feature, and the separated microstructure of the Fe-rich and Cu-rich phases using rapid solidification techniques and drop tube processing has been reported.[3–6] Some studies have indicated that the appearance of a stable miscibility gap of the liquid phase in Cu-Fe base alloy is due to the addition of carbon.[7–10] It is known that a small amount of residual Cu in steel scrap, which is difficult to remove by the conventional steelmaking process, affects the hot workability of the recycled steel. Yamaguchi and Takeda[9] and Marukawa et al.[10] recently proposed a recycling process for extracting Cu from the mixed iron-copper scraps using the feature of this phase separation between the Fe-rich and Cu-rich liquid phases in the Fe-Cu-C ternary system. The present authors recently reported the formation of powders with an egg-type core microstructure in the hypermonotectic Cu-Fe base alloys by conventional gas atomization.[11] It was also shown in that article that a core-type microstructure is formed by the Marangoni effect[12–15] if the temperature gradient from the surface to the center of the powder is sufficiently large even under conditions of normal gravity. This result suggests that a macroscopic core-type structure can be obtained in some ingots by conventional casting and that composite materials can be expected to be easily fabricated in hypermonotectic alloys. Composite materials composed of a Cu-rich phase with high electrical conductivity and an Fe-rich phase with high strength have an obvious advantage in terms of various
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