Microstructures and superplastic behavior of eutectic Fe-C and Ni-Cr white cast irons produced by rapid solidification
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I.
INTRODUCTION
U L T R A F I N E grain structure is a principal requirement in alloys which are to exhibit structural superplasticity. It is particularly important that the grain size remains stable during superplastic flow. This is often accomplished by having a duplex microstructure in which a certain amount of second phase is introduced to inhibit grain growth.l~2 White cast irons consist of a cementite network often with massive platelike cementite within a pearlitic or martensitic matrix. The massive cementite grains do not have enough slip systems to allow extensive plastic deformation. Thus, these irons are hard and brittle at all temperatures and exhibit white crystalline fracture s u r f a c e s . 3'4 They simply cannot be fabricated into useful structural components, and are mostly used as castings for their excellent wear resistance and abrasion properties. Significant changes in microstructures, thus in properties, in a white cast iron can be obtained by utilizing rapid solidification technology. As the solidification rate increases, the primary and the secondary dendritic arm spacings decrease. This structural change is associated with increasing undercooling of the liquid, either during nucleation or in front of the growing solid interface. Thus, powders of white cast irons with very fine microstructures can be obtained by rapid solidification technology processing. 5'6'7 Utilization of such processing to achieve superplastic materials was demonstrated by Bochvar, Davidov, and Druzhinin 7 and by Ruano, Eiselstein, and Sherby. 6 In the present study, a eutectic Fe-C white cast iron (henceforth designated as eutectic cast iron) and a type-I D.W. KUM, formerly with the Department of Materials Science and Engineering, Stanford University, is Research Materials Scientist, Korea Advanced Institute of Science and Technology, 150 Cheongryang, Seoul, Korea; G. FROMMEYER, formerly Visiting Scholar. Department of Materials Science and Engineering, Stanford University, is Professor, Max-Planck-lnstitut fffr Eisenforschung, GmbH, Max-Planck-Strasse 1, 4000 Diisseldorf, West Germany; N.J. GRANT is Professor of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA; O.D. SHERBY is Professor of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted June 16, 1986. METALLURGICALTRANSACTIONS A
Ni-Cr martensitic white cast iron (henceforth designated as Ni-Cr cast iron) were selected because of the large volume fraction of cementite present, which varies from 51 pct to 64 pct. The objectives of this investigation were to produce fine grained duplex microstructures within the "white cast irons" and to determine the superplastic behavior of these white cast irons at temperatures around the AI transition temperature. II. MATERIALS AND EXPERIMENTAL PROCEDURE Commercial grade castings of eutectic cast iron and Ni-Cr cast iron were remelted and melt-atomized into fine powders smaller than 200/~m in diameter. The chemical compositions of these sample
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