Phases and Microstructures of Carbon-Implanted Niobium
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PHASES AND MICROSTRUCTURES OF CARBON-IMPLANTED NIOBIUM J.S. HUANG, R.G. MUSKET, AND M.A. WALL Lawrence Livermore National Laboratory, Livermore, CA 94550
ABSTRACT Polycrystalline niobium was implanted with 200, 100, and 50 keV carbon ions to create a uniform distribution of carbon over a thickness of about 0.25 gim. Samples implanted with calculated carbon content of 0.6, 1.9, 5.8, and 16 atomic percent were prepared, and the uniformity of the carbon distribution with depth was confirmed by Auger electron spectroscopy analysis. Glancing-angle X-ray diffraction analysis and transmission electron microscopy were used to characterize the phases and microstructures formed. The results indicated that no detectable second phases were present except on the surfaces where an amorphous phase and many particles were formed from contamination. Despite the low equilibrium solubility limit of carbon in Nb, we have created metastable solid solutions of Nb and C with carbon contents as high as 16 at.%. INTRODUCTION High-dose ion implantation is a useful process to modify the mechanical properties of metals. It is known that the implantation of metalloid ions results in improvement of microhardness and increases wear resistance and fatigue life. Ion implantation is a non-equilibrium process and the laws of classical thermodynamics, for instance, the Gibbs phase rule can not always be applied. Although implantations of Fe alloys with C and N metalloids had been studied extensively, little has been done on Nb. The objective of this work was to analyze the phases formed by high-dose implantations of carbon into Nb. The Nb-C system is interesting since it has been shown [1] that the precipitation of niobium carbide, Nb2 C, involved large amounts of plastic deformation at low temperatures, which can suppress the kinetics of the precipitation. It is expected that the limit of solubility of carbon can be extended significantly beyond that predicted by the equilibrium phase diagram. EXPERIMENTAL DETAILS The Nb was a commercial grade material of 99.9 wt% purity with the carbon content less than 0.003 wt%. The material was annealed at 1473 K for one hour and mechanically polished to a finish of 0.1 pm before carbon implantation. The as-annealed material had an average grain size of 40 gim. The carbon ion implantatnins were conducted with, sequentially, 200, 100, and 50 keV C' ions near room temperature(
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