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MICROSTRUCTURE CHANGES INDUCED BY LOW-ENERGY HIGH-TEMPERATURE NITROGEN ION IMPLANTATION ON VANADIUM-TITANIUM ALLOYS.

M. Varela, J.A. García, R. Rodríguez and C. Ballesteros
Univ. Carlos III de Madrid, E.P.S, Dept. Physics, ES

Keywords: Ion implantation; transmision electron microsopy, Vanadium alloy; tribology

Abstract:
A detailed structural characterization of unimplanted and low-energy, nitrogen implanted V5at.%Ti alloy is presented. The alloys were produced by repeated arc melting in a high-purity He atmosphere. Samples were nitrogen-implanted at 1,2 kV and 1 mA/cm2, up to a dose of 1E19 ions/cm2, at a temperature of 673 K. Alloys were analyzed in cross-section by transmission electron microscopy (TEM) electron dispersive x-ray (EDX) and x-ray diffractometry (XRD). Vanadium-based alloys are promising candidates as high-performance structural materials for fusion power devices. However these alloys show a poor tribological performance. Low-energy nitrogen-implantations are an effective tool to improve their tribological properties. The ion beam treatment dramatically changes the microstructure of the material, as a function of deep from the surface. Three regions can be differentiated: The region close to the sample surface, up to "250nm deep, heavily contaminated with carbon and other impurities. Dipper in the sample, from "250nm to "800 nm, and free of contamination, the crystal structure is mainly amorphous, TEM images show a distribution of nanocrystals embedded in the amorphous matrix. The size of the nanocrystals increases as a function of deep from the surface. From "800nm to " 1000 nm, a full crystalline area is observed, where precipitates and dislocations are imaged. The nanocrystals and precipitates were analysed by EDX, microdiffraction and selected area diffraction. They were identified as a cubic f.c.c. structure with a lattice parameter of 0.42 nm, essentially the same that the corresponding to TiN. A clear correlation between the microstructure of the implanted layer and the reported improvement in the tribological prop erties is demonstrated

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