Kinetics of interstitial uptake during gaseous carbo-oxidizing of titanium foils

Foils of titanium grade 2 were carbo-oxidized in CO at temperatures ranging from 600 to 1100°C in steps of 50°C. Carbo-oxidation was performed for up to 130 hours in a thermogravimetric setup, enabling the investigation of the kinetics of carbon and oxygen uptake from the gas mixture. The carbo-oxidized specimens were characterized with transmission X-ray diffraction, light optical microscopy, scanning electron microscopy, electron probe microanalysis and nanoindentation to investigate the microstructural evolution along with the uptake of oxygen and carbon. At the surfaces of the foils TiCx developed, while the core remained h.c.p. titanium. The interstitial content in h.c.p. titanium increased with temperature and first reached its maximum solubility at 1000°C and above. TiO developed within the core for high interstitial contents. The TiCxOy phase developed in-between TiCx and the oxygen containing h.c.p. titanium core for temperatures above 800°C. For temperatures up to 850°C, the uptake of C and O in the titanium foils (initially) obeys a parabolic time dependence. Arrhenius analysis of the thermogravimetry results indicates that volume diffusion of oxygen in h.c.p. titanium is (initially) the rate determining step. For temperatures above 850°C, the activation energy is reduced by a factor 3, suggesting that short-circuit diffusion of species in TiCx controls mass increase. The hardness of h.c.p. titanium determined with nanoindentation scales with the interstitial content and the associated c/a ratio of the h.c.p. unit cell. The evolution of the microstructure with time and temperature are discussed in relation to the observed kinetics of mass uptake.