Carbide formation in Nb and Ta implanted by carbon ions at room temperature

Niobium and tantalum single crystals with <111> orientation were implanted with 200 keV C+ ions at room temperature to fluences of 7.6x1017 and 1.5x1018 carbon ions per cm2, respectively, which gave a calculated maximum carbon concentration of ∝46 and 52 at.% in each material. The depth profiles of carbon were analyzed with Auger spectroscopy, and the phase formations were investigated with cross-sectional transmission electron microscopy. Face-centered-cubic nonstoichiometric NbC and TaC were formed in the implanted Nb and Ta, respectively; no Nb2C or Ta2C was formed. Based on the observed crystallographic relations between the fcc carbides and the bcc host lattices, and on the morphologies of the carbides, it is suggested that the fcc carbides are formed by a diffusionless martensitic transformation. (orig.)