IMPROVED OXIDATION RESISTANCE OF GROUP VB REFRACTORY-METALS BY AL-IMPLANTATION( ION)

Aluminum ion implantation of vanadium, niobium, and tantalum improved the metals' oxidation resistances at 500 degrees C and 735 degrees C. Implanted vanadium oxidized only to one-third the extent of unimplante d vanadium when exposed at 500 degrees C to air. The oxidative weight gains of implanted niobium and tantalum proved negligible when measure d at 500 degrees C and for times sufficient to fully convert the untre ated metals to their pentoxides. At 735 degrees C, implantation of van adium only slightly retarded its oxidation, while oxidative weight gai ns of niobium and tantalum were reduced by factors of 3 or more. Impla nted niobium exhibited weight gain in direct proportion to oxidation t ime squared at 735 degrees C. Microstructural examination of the metal s implanted with selected fluences of the 180 kV aluminum ions showed the following. The solubility limit of aluminum is extended by implant ation, the body centered cubic (bcc) phases being retained to similar to 60 at. pet Al in all three metals. The highest fluence investigated , 2.4 x 10(22) ions/m(2), produced an similar to 400-nm layer of VAl3 beneath the surface of vanadium, and similar to 300-nm layers of an am orphous phase containing similar to 70 at. pet Al beneath the niobium and tantalum surfaces, All three metals, implanted to this fluence and annealed at 600 degrees C, contained tri-aluminides, intermetallic co mpounds known for their oxidation resistances. Specimens implanted to this fluence were thus selected for the oxidation measurements.