Dendritic Growth of Undercooled Nickel-Tin: Part III
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INTRODUCTION
THISis the third in a series of papers on dendritic growth and structure of undercooled nickel-tin alloys. In Part I,l cinematographic measurements of dendritic growth were reported and compared with the analyses of Lipton, Kurz, and Trivedi (LKT), and of Boettinger and Coriell (BC). The dendrite growth velocities measured agreed well with the LKT-BC theory, but the observed tip radii were several orders of magnitude larger than expected. It was concluded that the large dendrites observed comprised an array of much finer, solute diffusion controlled dendrites. In Part II, z high-speed thermal measurements were reported; these provided additional information on processes occurring behind the dendrite tips during and after recalescence. It was discovered that recalescence occurs in two stages. In the initial, rapid recalescence stage, an array of fine dendrites sweeps across the alloy sample. In the subsequent, slow stage of recalescence, the sample temperature rises slowly, following a relationship strongly suggestive of coarsening of the dendritic structure. The maximum recalescence temperatures reached by hypoeutectic Ni-25 pct Sn samples with different initial undercoolings were in good agreement with the equilibrium states predicted using a thermal balance combined with the equilibrium lever rule. A similar model appeared to apply for solidification of the Ni-32.5 pct Sn eutectic alloy. In this paper, we describe the structures and microsegregation measured in the Y. WU, formerly a Graduate Research Assistant in the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, is a Senior Engineer with ALCOA, Alcoa Center, PA 15069. T.J. PICCONE, Graduate Research Assistant, and M. C. FLEMINGS, Toyota Professor of Materials Processing and Department Head, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Y. SHIOHARA is a Research Associate in the Materials Processing Center, Massachusetts Institute of Technology, Cambridge, MA 02139, and also a Special Research Fellow at the Castings Research Laboratory of Waseda University, Tokyo, Japan. Manuscript submitted April 27, 1987.
METALLURGICAL TRANSACTIONS A
samples produced, and relate these to the results of the previous two papers. A number of studies have been reported on structure of undercooled iron and nickel base alloys, including nickeltin. Walker 3 first reported the observation in pure nickel of a critical undercooling (about 170 K) at which grain size abruptly decreased. This critical undercooling was also observed in a number of iron and nickel base alloys by Kattamis and Flemings. 4'5'6 Their work and the later work by Kattamis and co-workers 7 showed that the dendritic structure gradually changes as this critical undercooling is approached, and at a critical undercooling is replaced by a spherical morphology, the size of which is on the order of the secondary dendrite arm spacing of the dendritic structure that has
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