Improved oxidation resistance of group VB refractory metals by Al + ion implantation
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INTRODUCTION
T H E group VB metals, vanadium, niobium and tantalum, and alloys based on these metals provide materials that retain high strengths at elevated temperatures, a result of their strong bonding and high melting temperatures. In their elemental form, the metals rapidly oxidize at temperatures above a few hundred degrees centigrade, greatly limiting elevated-temperature applications except at low pressures, where gas impingement governs oxidation rates. Therefore, protective coatings are applied, such as those based on the metal aluminides (VA13, NbA13, and TaAI3) suitably modified with chromium and yttrium to promote AlzO3 formationI~] and increase ductility.[2] Ion implantation offers an alternative to coatings, one which produces no change in the appearance of the surface to which it is applied nor any detectable dimensional changes to the part as a whole. Though clearly limited by its small depth of penetration and the tendency of implanted elements to diffuse into the part so treated, it is less likely to exhibit decohesion or delamination since the ion implanted layer resides beneath the original surface. Previous workt3] shows that even a few percent of implanted elements, particularly aluminum, affects the oxidation of niobium. The so-called paralinear oxidation kinetics are unchanged in form, in that a short, transient parabolic stage is followed by a pronounced acceleration in oxidative weight gain, the gain thereafter increasing in direct proportion to time. However, implantation greatly ex-
J.M. HAMPIKIAN, Assistant Professor, is with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245. M. SAQIB, Research Scientist, is with the Department of Mechanical and Materials Engineering, Wright State University, Dayton, OH 45435. D.I. POTTER, Professor, is with the Metallurgy Department and Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136. Manuscript submitted May 10, 1995. METALLURGICALAND MATERIALSTRANSACTIONSB
tends the slower, parabolic stage, thus retarding the onset of the rapid, linear oxidation. This result, demonstrated at temperatures up to 500 ~ unfortunately did not extend to higher temperatures. Implantation of higher concentrations of the beneficial elements may promote further improvements in oxidation resistance.t4j For example, a continuous layer of an amorphous form of TaA13, fabricated by implanting A1ยง ions into tantalum, extended to a depth of - 3 0 0 nm. This layer prevented oxidation of the underlying tantalum at temperatures up to at least 600 ~ The protection deteriorated somewhat by temperatures near 735 ~ but still reduced the oxidation rate constant by a factor of 5 compared to untreated tantalum. The purpose of the present work is to establish whether this higher concentration implantation treatment is effective in improving the oxidation resistance of vanadium and niobium and to determine the extent to which these improvements are retained as the oxidation temperature increases. II.
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