High-Speed imaging and analysis of the solidification of undercooled nickel melts
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INTRODUCTION
NUMEROUSinvestigators have studied dendrite growth velocities in undercooled high-purity elements, particularly nickel, since the undercooling phenomenon was first observed in the Au system by Van RiemsdykIU in 1880. WalkerfZi made the first thorough investigation of undercooled nickel melts using photodiodes to measure the time required for an interface to propagate along a column of molten material. Colligan and Bayles[31used photodiodes in conjunction with high-speed cinematography, determining surface solidification morphology in addition to bulk solidification velocities. Later velocity studies have primarily involved refinements in the placement and precision of the photodiodes involved. Piccone et al.,t41 for example, employed a single photodiode focused on a portion of an undercooled, quartzencased nickel ingot in conjunction with a digital oscilloscope, using signal rise time to determine solidification velocity, Other researchers have employed electromagnetic levitation (containerless processing) to study undercooled Ni melts. One such example is the work of Willnecker et al.,Isl who used two hemispherical photodiodes and calculated solidification velocity using signal rise time and geometric considerations. Hofmeister et aL ~61used a single, tightly focused photodiode in order to minimize the effects of potential multiple nucleation events. Eckler and Herlach~v~ used a polar photodiode and a unique capacitance trigger in their work, defining more accurately the duration of the solidification event. Bassler et al.ISl were among the first to use a linear array of photodiodes--later moving to a twodimensional array--in an effort to improve the spatial resolution of their studies. With the exception of Hofmeister and Bassler, the results of the preceding work are qualitatively and quantitatively similar (Figure 1). At small undercoolings, the investigators show an approximate power-law relationship between unJOHN W. LUM and DOUGLAS M. MATSON, Graduate Researchers, and MERTON C. FLEMINGS, Toyota Professor of Materials Processing, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted January 11, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
dercooling and solidification velocity, V = k (AT)n, where /3 varies between 2 and 3. This behavior is relatively well predicted by the " L K T " theory of Lipton et al.,tgl particularly when a kinetic parameter is included in the analysis. At larger undercoolings, the results of prior investigations diverge more sharply, but many conclude that there exists some critical undercooling, AT*, in the range 170 to 190 K, above which a normal LKT analysis is no longer valid. In addition, several researchers have observed a corresponding microstructural transition at AT*,t~o,~,~21marking the shift from a columnar dendritic grain structure to a finer, equiaxed structure. The scatter of data at undercoolings above AT* is quite remarkable, bounded at the lower end by the resul
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