The microstructural evolution, crystallography, and thermal processing of ultrahigh carbon Fe-1.85 pct C melt-spun ribbo
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INTRODUCTION
FINE-grained ultrahigh carbon steels can have very good strength levels without being extremely brittle; in some cases, these steels can even exhibit superplasticity. For example, Sherby et a l . t~J describe various thermomechanical processing schedules w h i c h can produce ultrahigh carbon steels with microstructures in which both the ferrite and cementite phases have dimensions in the 0.5- to 2-/xm range. Steels with these structures can be deformed in the range o f 650 °C to 800 °C to large extensions at rates as high as 10 -2 per second. Attempts have been m a d e , employing rapid solidification processing techniques alone, to produce microstructures fine enough to permit superplastic consolidation o f white cast iron t2] and ultrahigh carbon steelL3] powders, but the deformation rates which could be produced with as-solidified powders were limited. These somewhat disappointing results were probably the result o f the microstructures being insufficiently fine. Studies have also been made o f the refinement o f microstructure in conventionally solidified ultrahigh carbon steels by repeated thermal cycling across the eutectoid temperature. [4] The microstructure became progressively finer with repeated G. S P A N O S , Metallurgist, J.D. A Y E R S , Metallurgist, and C.L. VOLD, Physicist, are with the Physical Metallurgy Branch, Naval Research Laboratory, Washington, DC 20385-5343. I.E. LOCCI, Research Scientist, is with the N A S A Lewis Research Center, Cleveland, OH 44135. Manuscript submitted July 10, 1992. METALLURGICAL TRANSACTIONS A
thermal cycling, and superplastic flow properties improved, but even after 14 cycles, the structures were coarser than those produced by thermomechanical processing. The current investigation was undertaken in o r d e r to determine if finer microstructures could b e achieved by employing rapid solidification to produce a fine-grained fully austenitic starting structure and then using thermal processing cycles to produce an even finer ferrite plus cementite structure. A thorough characterization was undertaken o f the evolution, mechanisms o f grain r e finement, and crystallography o f the resultant microstructures by employing transmission electron microscopy (TEM). A binary alloy was used f o r this initial study in o r d e r to eliminate potentially complex alloying element effects on kinetics and morphology, fS-SJ The present study has thus provided the processing and microstructural basis f o r follow-on studies currently underway in more highly alloyed steels.
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EXPERIMENTAL PROCEDURE
Specimens o f Fe-1.85 pct C ribbon were prepared by conventional melt spinning with a copper w h e e l rotating with a surface speed o f 20 ms -1. Specimens were cooled in liquid nitrogen to generate a large volume fraction o f martensite and subsequently heat-treated in deoxidized molten lead baths followed by quenching in room temperature brine in order to halt the isothermal reaction after
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VOLUME 24A, APRIL 19
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