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

Abstract

Foils of ticanium grade 2 were carbo-oxidized in CO ac cemperarures ranging from 600 co 11 0D°C in sceps of SD°C. carbo-oxidation was performed for up co 130 hours in a thermogravin1etric serup, enabling the investigation of the kinetics of carbon and oxygen upcake from the gas mixrure. The carbo-oxidized specimens were characterized with transmission X-ray diffraction, lighc optical microscopy, scanning elecrron microscopy, elecrron probe microanalysis and nanoindentation co invescigace the microsrruccural evolmion along with the upcake 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. ticanium increased with cemperacure and firsc reached ics maxinmm solubilicy ac l00D°C and above. TiO developed within the core for high interstitial contents. The TiCxOy phase developed in-becween TiC. and the oxygen containing h.c.p. ticanium core for cen1perarures above soo•c. For cemperarures up co 8500C, the uptake of C and O in the cicanium foils (initially) obeys a parabolic time dependence. Arrhenius analysis of the thermogravimerry results indicates chat volume diffusion of oxygen in h.c.p. titanium is (initially) the race decermining scep. For cemperacures above 850°C, the activation energy is reduced by a factor 3, suggesting chat shore-circuit diffusion of species in TiCx conrrols mass increase. The hardness of h.c.p. titanium determined with nanoindentation scales with the incersticial content and the associaced c/a ratio of the h.c.p. unie cell The evolution of the microstructure with tin1e and cen1perature are discussed in relation co the observed kinetics of mass uptake.