Improved Oxidation Resistance of Group VB Refractory Metals by AI§ Ion Implantation J.M. HAMPIKIAN, M. SAQIB, and D.I. POTTER Aluminum ion implantation of vanadium, niobium, and tantalum improved the metals' oxidation resistances at 500 ~ and 735 ~ Implanted vanadium oxidized only to one-third the extent of unimplanted vanadium when exposed at 500 ~ to air. The oxidative weight gains of implanted niobium and tantalum proved negligible when measured at 500 ~ and for times sufficient to fully convert the untreated metals to their pentoxides. At 735 ~ implantation of vanadium only slightly retarded its oxidation, while oxidative weight gains of niobium and tantalum were reduced by factors of 3 or more. Implanted niobium exhibited weight gain in direct proportion to oxidation time squared at 735 ~ Microstructural examination of the metals implanted with selected fluences of the 180 kV aluminum ions showed the following. The solubility limit of aluminum is extended by implantation, the body centered cubic (bcc) phases being retained to -60 at. pct A1 in all three metals. The highest fluence investigated, 2.4 • 1022 ions/ms, produced an -400-nm layer of VA13 beneath the surface of vanadium, and -300-nm layers of an amorphous phase containing -70 at. pct A1 beneath the niobium and tantalum surfaces. All three metals, implanted to this fluence and annealed at 600 ~ contained tri-aluminides, intermetallic compounds known for their oxidation resistances. Specimens implanted to this fluence were thus selected for the oxidation measurements. I. INTRODUCTION THE group VB metals, vanadium, niobium and tanta- lum, 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 mod- ified with chromium and yttrium to promote AlzO3 forma- tionI~] 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 delam- ination since the ion implanted layer resides beneath the original surface. Previous workt3] shows that even a few percent of im- planted elements, particularly aluminum, affects the oxi- dation of niobium. The so-called paralinear oxidation ki- netics 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 di- rect 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. 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 amor- phous form of TaA13, fabricated by implanting A1 § ions into tantalum, extended to a depth of -300 nm. This layer pre- vented 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 tan- talum. 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 in- creases. II. EXPERIMENTAL PROCEDURES Strips of the metals were cut from cold-rolled stock. An electrical current passed through these 4-mm-wide strips, one at a time, heated them to the annealing temperature in a vacuum near 10-6 Pa. The annealing temperatures, mea- sured with an optical pyrometer, and times were 1150 ~ h for vanadium, 1440 ~ h for niobium, and 1550 ~ h for tantalum. Each treatment produced grain sizes near 0.1 mm. The oxidation specimens, measuring 4 • 10 mm, included a hole of 0.85-mm diameter for sample suspen- sion. They were 0.13-mm thick for vanadium and niobium and 0.17-mm thick for tantalum. Specimens for micro- structural and microchemical examination with analytical transmission electron microscopy (TEM) and Rutherford backscattering spectrometry were 3 mm in diameter and 0.25-mm thick in all cases. METALLURGICALAND MATERIALS TRANSACTIONSB VOLUME27B, JUNE 1996--491