Synthesis, Microstructure, and Mechanical Properties of FeCo-VC Composites
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0980-II05-14
Synthesis, Microstructure, and Mechanical Properties of FeCo-VC Composites H. Bei1,2 and E. P. George1,2 1 Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN, 37996-2200 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6115 Abstract Fe-Co-V-C quaternary alloys were drop cast and directionally solidified to obtain an in situ composite. It is found that the fully eutectic structure occurs at a composition of Fe - 40.5Co -10.4V- 8.6C (at. %) in a drop-cast alloy. Directional solidification of this composition in a high-temperature optical floating zone furnace produces a well-aligned microstructure, consisting of sub-micron VC fibers (~19% by volume) embedded in a FeCo-5V solid solution matrix containing ~ 1% C. The temperature dependencies of mechanical properties of this composite were examined by tensile tests and the composite was found to have higher yield strength and lower ductility than the matrix. Introduction B2-structured FeCo with 25~75 at.% Co forms the basis of an important class of soft magnetic alloys with some of the highest saturation magnetizations [1-3]. Upon heating it disorders from B2 to body centered cubic (bcc) with the highest order/disorder temperature (730 ºC) at the stoichiometric composition. It is well known that two major disadvantages of FeCo are its brittleness at low temperatures and relatively poor strength at elevated temperatures [3-4]. One way to improve strength is to make a composite, which combines the properties of an FeCo matrix with those of a suitable reinforcement. Directional solidification of eutectic alloys containing intermetallic phases has been used to produce in situ composites in which a good balance of high temperature properties and low temperature ductility can be achieved [5-13]. We show here that well-aligned VC fiber reinforced FeCo based composites can be produced by directional solidification under carefully controlled conditions in a high-temperature optical floating zone furnace. The structural morphologies, phase equilibria, and mechanical properties of Fe-Co-V-C quaternary alloys were examined in this study. Experimental Procedures Alloys having the nominal composition Fe - 40.5 Co-10.4 V-8.6 C (at. %) were arc melted, drop cast, and directionally solidified in a high-temperature optical floating zone furnace in flowing argon gas at growth rates of 10 - 40 mm/h and a fixed rotation rate of 60 rpm. This furnace has been used previously to produce well-aligned eutectic microstructures of NiAlMo, Cr-Cr3Si, Fe(Al)-Fe2Ta and V-V3Si alloys over a wide range of growth conditions [14-20]. Additional details of the processing conditions are given elsewhere [15, 19]. Total weight losses
after melting and casting were less than 0.05%, so nominal (starting) compositions (in at. %) are used throughout this paper. Representative samples were cut from the directionally solidified (DS) rods along the transverse and longitudinal directions, polished using standard me
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