Microstructural characteristics of Ni-Sb eutectic alloys under substantial undercooling and containerless solidification
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ears, rapid eutectic solidification has aroused extensive research interest in the field of materials science[1,2,3] in two respects. First, this phase-transformation process involves complicated microstructural evolution and phase selection under nonequilibrium conditions and, thus, offers an important research subject for fundamental study. Second, it is a practical processing to produce in-situ composite materials, which can find many applications in industry. Up to now, the traditional rapid quenching method and high-undercooling technique have been well developed to realize rapid solidification of alloy melts. Compared with the rapid quenching method, the high-undercooling technique is more advantageous in that it can realize bulk rapid solidification of liquid metals at a slow cooling rate and, thus, provides a possible way for intensive research on rapid-solidification kinetics.[3,4,5] Among various approaches to obtain high undercoolings, the glass-fluxing method and drop-tube technique have become two popular ones. By removing the heterogeneous particles from liquid metals through the physical adsorption of the molten glass and its chemical reaction with the heterogeneity, the glass-fluxing method makes alloy melts achieve large undercoolings. Meanwhile, during the undercooling experiment, the measurement of temperature and in-situ observation of rapid solidification is quite convenient. Drop-tube processing not only provides a technique X.J. HAN, Ph.D. Student, and B. WEI, Professor, Director, Laboratory of Materials Science in Space and Director, Department of Applied Physics, are with the Department of Applied Physics, Northwestern Polytechnical University, Xian 710072, Shaanxi, People’s Republic of China. Contact email: [email protected] Manuscript submitted June 19, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
for containerless solidification in a reduced-gravity environment, but also has an advantage of combining high undercooling with rapid cooling.[6] It is expected that the drop tube technique can obtain larger undercoolings than the glass-fluxing method. Therefore, these two methods have been frequently used to investigate the rapid eutectic solidification.[7–12] Due to the laborious investigations of many pioneering researchers, a relatively complete system of eutectic growth theories has been established. For over three decades, the classic Jackson–Hunt (JH) theory[13] has been proven to be the most successful eutectic model. Recently, Trivedi, Magnin, and Kurz (TMK) have extended the validity of the JH theory to the case of a high Peclet number and worked out a new physical model[14,15] for regular lamellar eutectic growth. In principle, the TMK model can describe the rapid growth of highly undercooled eutectic alloy melts, provided that lamellar eutectic growth morphology is insured. However, a kind of anomalous eutectic frequently forms from undercooled melts in various alloy systems,[1–3,16] which invalidate the applicability of the TMK model. Therefore, great attention has been paid to t
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