On the kinetics of carbide precipitation during reaction between graphite and Fe-Ti liquids under microgravity
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I.
INTRODUCTION
THE TiC/iron-based alloy composites are a unique group of materials, based on the superhard titanium carbide reinforcement and various matrix structures of steel and cast irons. Composites produced by mixing, compacting, and sintering TiC particulate and steel powders using conventional powder metallurgy techniques have been applied in the production of wear parts with a high resistance to thermal fatigue erosion-resistant materials and high-performance tooling and for other engineering purposes.[1,2,3] Powder metallurgy processes have several limitations, however; for example, the structural homogeneity depends on a thorough mixing and the initial size of the powder particles. The shape of the components must be simple to avoid density differences during compacting and sintering. The possibility of the in situ formation of this composite by solidification processes has been studied.[4–11] Titanium carbide particles were formed in iron solution by a reaction between carbon and titanium after casting a supersaturated Fe-Ti-C melt, causing precipitation of TiC particles,[4,5] or by a reaction between graphite and Fe-Ti alloy, between Ti and Fe-C alloy, or between Fe-Ti and Fe-C alloy.[6–11] In earlier experiments, it was found that convection has a large effect on the reaction process. Due to this, it is impossible to study the reaction kinetics on Earth. Therefore, experiments under microgravity have been performed. The growth rate of the precipitation layer and the nucleation behavior of titanium carbide during reaction between graphite and Fe-Ti melt and between Fe-Ti and Fe-C melts under microgravity were investigated. By using parabolic flights, a microgravity of 1022 g can be repeatedly achieved for a period of 20 seconds. It is most Z. LIU is with Volvo Lastvagnar AB, 541 87 Sko¨vde, Sweden. H. 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
convenient to study nucleation and growth of dispersed crystals in a solution in the early stage, by performing experiments using this method. Some of the experimental results have been presented previously.[10] However, the growth behavior of the carbide precipitation zone has not been thoroughly discussed. The present article will, consequently, mainly focus on the precipitation kinetics of the carbide formation, including the dissolution rate of graphite, the growth rate of the precipitation zone, nucleation of carbide crystals, and their dependence on alloy compositions. As a comparison, samples produced on earth at short process times are presented in a previous article.[9] II.
EXPERIMENTS
The experimental configuration shown in Figure 1 has been used to study the reaction between graphite and FeTi alloys. The compositions of the alloys are shown in Table 1, column 1. One piece of Fe-Ti alloy was encapsulated in the center of a high purity quartz tube, and pieces of graphite were plac
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