On the precipitation of TiC in liquid iron by reactions between different phases
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I.
INTRODUCTION
SUPERHARD titanium carbide bonded by steel is a unique metal matrix composite (MMC). Its special properties, which are maintained at elevated temperatures and machinability when in the annealed condition, were studied as early as the 1950s.[1] The advantages of this MMC over the conventional cemented tungsten carbide, as well as its applications to high performance tooling and other engineering structures, have been extensively discussed recently.[2] However, the fabrication process of this kind of MMC by conventional powder metallurgy has several limitations for the homogeneity of the material. In recent studies, solidification processing has emerged as one of the most economical and versatile techniques to produce MMCs. Aluminum particulate-reinforced composites manufactured by solidification processes have attracted much attention, whereas little work has been done on steel bound composites, as reviewed recently.[3] Production of in situ carbide-reinforced composites by solidification processing would be one of the simplest techniques to produce MMCs. The possibility of producing TiC-ferrous matrix composites through the liquid route has been studied either by cooling an Fe-Ti-C melt saturated with respect to TiC at low temperature from a temperature above the TiC liquidus with different cooling speeds to obtain carbide precipitation[4,5] or by reaction between titanium and carbon in an iron melt by additions to the melt.[6,7] The effect of cooling rate on the ripening kinetics of carbide particles has been studied.[4,5] This method places great demands on the melting facility since the melt temperature is over the liquidus of titanium carbide. For processing at steelmaking temperatures, some preliminary results obZHENSHENG LIU, Ph.D., is with Volvo Lastvagnar AB, 541 81 Skovde, Sweden. HASSE FREDRIKSSON, Professor, is with the Department of Materials Processing, Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted February 21, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
tained by adding graphite to Fe-Ti alloy and Ti to Fe-C alloy seemed very promising.[6] However, nothing is known about the reaction mechanisms, reaction kinetics, nucleation and growth of the carbide particles, and particle distributions in the liquid solution, which are necessary to understand and control the process. Different types of morphologies can be formed, i.e., a fine distribution of crystals, crystal agglomerates, dendrites, and thin films. A fine distribution of small crystals is always desired. In order to study the mechanisms and the precipitation kinetics during various reactions and to understand the solid/liquid type of reaction, a series of experiments was performed under microgravity during a number of parabolas in an aircraft. The present report will mainly concentrate on the general precipitation behavior and the reaction phenomena observed on Earth. The effect of natural convection on the precipitation of carbide under normal gravity is also discussed. The studies under mi
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