Rapid solidification characteristics in melt spinning a Ni-base superalloy
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I.
INTRODUCTION
DENDRITIC, cellular, and planar solidification, in that order, produce microstructures of greater compositional homogeneity. The solidification morphology is the manifestation of solid-liquid interface stability (with the planar interface being the most stable mode), and is generally controllable by adjusting processing parameters according to stability guidelines. ~-4 At ordinary solidification rates, where the constitutional undercooling principle ~'2 applies, the interface increases its stability with an increase in the ratio between the liquid thermal gradient and solidification velocity, G~/V. At high solidification rates, where the absolute stability theoryz'3 prevails, the interracial stability increases with the rate of solidification, V. Note that the solidification velocity has reverse effects on the interface stability in the two solidification regimes. Also, in the rapid solidification regime, the solidification vetocity becomes the sole process parameter controlling the interface stability and morphology (at a fixed alloy composition). An adequate knowledge of the rapid solidification kinetics is therefore critical for an understanding of the rapid solidification microstructure. In this paper the measurement of solidification front velocity is reported for the process of melt spinning a Ni-base superalloy. The measurement technique involves the correlation of ribbon thickness with the length of the melt puddle residing on the surface of the melt spinning wheel, s Since the melt puddle length defines the solidification time in which a ribbon with a certain thickness is formed, the above correlation allows a direct derivation of the propagation velocity o f the solid-liquid interface. This result from the ribbon solidification kinetics is then used with the absolute stability theory2'3 to interpret the cellular-to-dendritic transition in superalloy ribbons reported previously. 6-9 The kinetic result is also used to explain the ribbon texture characteristics as affected by the melt puddle convection. Furthermore, the solidification correlation is analyzed using heat transfer considerations to yield information about the S.C. HUANG and A.M. RITTER, Staff Metallurgists, and R.P. LAFORCE, Associate Materials Scientist, are with General Electric Corporate Research and Development Center, P.O. Box 8, Schenectady, NY 12301. R.P. GOEHNER, formerly with General Electric Research and Development Center, is now with Siemens-Allis Inco., 1 Computer Drive, Cherry Hill, NJ 08034. Manuscript submitted October 11, 1984. METALLURGICALTRANSACTIONS A
ribbon-wheel interface heat transfer coefficient and the ribbon cooling rate. These thermal results are compared to those deduced from other measurement techniques) 't~
II.
EXPERIMENTAL
The superalloy studied had a composition of 53Ni, 14Co, 17Cr, 8W, 1.75A1, 3.5Ti, 0.65Nb, 2Re, 0.02C, and 0.01B (in weight percent). The gamma prime volume fraction was about 30 pct and the concentrations of the strongly segregating elements (carbon and boron) were typical
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