cellular microstructure of chill block melt spun Ni-Mo alloys
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INTRODUCTION
R A P I D solidification of metallic alloys is known to result in refined microstructure, extended solute solubility, reduced microsegregation, and formation of metastable phases. Chill block melt spinning has been extensively used to produce rapidly solidified thin ribbons of many alloys. Microstructure of these melt spun alloys is studied because the microstructure controls mechanical properties. Chill block melt spinning is inherently a very complex process with many variables, including alloy composition, wheel material, speed and surface finish, atmosphere, melt temperature, ejection pressure, melt flow rate, angle of melt jet impingement, etc. These determine the physical and microstructural properties of the ribbon. Several simplified approaches have been taken to model the chill block melt spinning process. Momentum boundary layer models ~'2'~ have treated melt spinning as a material flow problem to determine the foil width and calculate the foil thickness from the known flow rate and wheel speed. These approaches, however, cannot shed any light on the microstructural variation with ribbon thickness, especially for crystalline ribbons. Process models based on solidification principles, 4'5 on the other hand, provide the possibility of understanding microstructural evolution, in addition to predicting ribbon dimensions. Correlation of ribbon thickness with solidification time (melt puddle residence time) has been recently reported for a vacuum cast nickel base superalloy. 6 This correlation suggested that the ribbon formation can be assumed to occur under ideal cooling conditions at the meltwheel interlace, with the only heat transfer resistance being through the thickness of the solidified metal. This approach takes no special account of the cellular or dendritic growth front of an alloy; it assumes alloy growth is similar to plane front growth of a pure element. A range of microstructures, plane front, cellular, and dendritic, is observed across melt spun crystalline ribbons due to varying local solidification conditions. Solidification during melt spinning has been treated both as growth of the solid in an undercooled melt puddle (negative temperature gradient ahead of the liquid-solid interface), 7'8'9 and as steady state growth of the solid-liquid interface in the melt
S.N. TEWARI is Associate Professor, Department of Chemical Engineering, Cleveland State University, Cleveland, OH 44115 T.K. GLASGOW is Manager, Microgravity Materials Science Laboratory, MS 105-1, NASA-Lewis Research Center, Cleveland, OH 44135. Manuscript submitted October 22, 1986. METALLURGICALTRANSACTIONS A
with a positive thermal gradient. 10.11.12The plane front solidified microstructures are believed to form either by planar growth with equilibrium partitioning (as explained by Mullins and Sekerka's Absolute Stability criterion") or by partitionless solidification (solute trappingS4). Prior studies of cellular/dendritic microstructures have concentrated on refinement of the secondary dendrite arm spacing in mel
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