Effect of undercooling on the microstructure of Ni-35 at. pct Mo (eutectic) and Ni-38 at. pct Mo (hypereutectic) alloys
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I. INTRODUCTION
SOLIDIFICATION
in a cooling alloy melt is usually initiated due to the heterogeneous nuclei present either in the melt itself or on the container walls. In the absence of such nuclei homogeneous nucleation is expected to occur at temperatures well below the equilibrium melting point. The probability of heterogeneous nucleation can be reduced by using high purity materials and contamination free alloy making process. The heterogeneous nuclei can also be made ineffective and eliminated by the thermal conditioning treatment of the melt in a suitable slag.' Such an alloy melt undercools significantly before the solidification begins. Solidification of undercooled alloys is known to result in many interesting microstructural changes, such as reduced dendrite spacing,2s3refined grain ~ i z e , ~reduced . ~ . ~ chemical segregati~n,~ formation of metastable phase^,^^'.^ particulate and anomalous e u t e c t i ~ s , ~ increased -'~ solute solubility,14~15.16etc. Even though similar microstructural advantages can also be obtained by the rapid quenching techniques now being extensively investigated, these are limited to producing very thin ribbons and powders. The solidification of undercooled alloys, on the other hand, offers a potential technique of producing rapidly quenched microstructures in bulk. It is therefore important to study the microstructural evolution in solidified undercooled alloys. Several eutectic alloys which generally show well aligned and regular two phase distribution in their microstructure yield an "anomalous eutectic" microstructure due to rapid solidification, e.g., Al-Cu," Pb-Sn,I7 and Cd-Zn.16 Solidification of undercooled eutectic alloy melts has also been observed to result in such "anomalous eutectic" microstructures, e.g., AI-Cu,18 Ni-Sn,19 Co-Sn,15 Pb-Sn,I4 and Ni-Sb.I2 This microstructure has been called "anomalous e u t e ~ t i c " "degenerate ,~~ eutecti~",~' ''uncoupled eutectic",1° "irregular eutectic",19 and "particulate eutectic"" in literature. Their microstructure appears to consist of a particulate distribution of one phase distributed in the matrix of another phase, and does nit contain the usual aligned two phase eutectic type of distribution. However, it has been observed S. N. TEWARI is with the Department of Chemical Engineering, Cleveland State University, Cleveland, OH 44115. Manuscript submitted May 6, 1985. METALLURGICAL TRANSACTIONS A
by successive polishing that these particulate features were not isolated, bit were interconnecteh.19Several mechanisms have been proposed for the formation of such anomalous eutectic microstructures. These are (i) dendritic growth of solute rich nickel phase in a partitionless manner followed by intradendritic melting and re~olidification,'~ (ii) a transition from regular coupled growth to a simultaneous uncoupled growth, lo (iii) independent simultaneous nucleation and growth of both phases of e ~ t e c t i cand , ~ ~(iv) growth of the solid in a nondendritic fashion without solute distribution followed by solid-state pr
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