Pore nucleation in solidifying high-purity copper
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I. INTRODUCTION
INVESTIGATIONS [1,2,8] of homogeneous nucleation of pores in liquid metals have shown that the initiation pressure that is required for nucleation is extremely high, and therefore, homogeneous nucleation is unlikely to occur in practice. In contrast, it is generally agreed[1–5] that the difficulty of nucleation is reduced if nonwetted solid surfaces exist in the liquid. Furthermore, the work of formation of an embryonic bubble is much reduced on a concave substrate.[1,4,5] Pores in castings are more the norm than the exception. The question is whether these pores are nucleated heterogeneously or homogeneously. If pores are nucleated heterogeneously, how can solid inclusions be formed when highpurity metal is melted and solidified in an atmosphere of low oxygen potential? If pores are nucleated homogeneously, how is the high initiation pressure produced? Liquid copper dissolves oxygen,[6] and conversely nitrogen[7] and carbon[8] are insoluble in liquid copper. Therefore, oxides can be formed in liquid copper and nitrides and carbides can not. Furthermore, oxides are nonwetted in liquid copper.[9] The nucleation of a pore in liquid copper, therefore, is considered to occur against the surface of an oxide inclusion. In this study, high-purity copper that contained very small amounts of oxide-forming elements was melted and solidified in the atmosphere of a H2 1 Ar gas mixture. This condition is extremely unfavorable for the formation of oxide inclusions in the melt. The results obtained in this study could offer useful information on the mechanism of pore nucleation in metals. Nucleation mechanisms that are outside of the normal classical approach have been proposed by Campbell.[3] Among these mechanisms, the homogeneous nucleation by a particles was investigated in this study.
EIICHI KATO, Professor Emeritus, is with the Department of Materials Science and Engineering, Waseda University, Tokyo 169-0072, Japan. Manuscript submitted November 30, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. EXPERIMENTAL DETAIL High-purity 6 N (from Mitsubishi Material Corp., Tokyo, Japan) and 7 N (from Dowa Corp., Tokyo, Japan) copper were used for the experiments. The impurities in these metals are given in Table I. The concentrations of Na, Mg, K, Cr, Fe, Ni, Sb, and Pb were less than 0.01 ppm. The equipment for the unidirectional solidification experiments is shown in Figure 1. Gas mixtures of H2 and Ar were used for the melting atmosphere and the hydrogen content of the melt was controlled by changing the H2 partial pressure in the atmosphere. The flow rate of the gas mixture was 8.3 3 1026 m3/s. The purities of the gases used for these experiments were 99.999 pct. High-purity graphite crucibles (24-mm external diameter, 18-mm internal diameter, and 110-mm length) were used, and the crucible containing a specimen was placed on a water-cooled copper plug. The specimen was solidified unidirectionally by withdrawing the water-cooled copper plug downward. The solidification velocity was controlled by changing
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