Gaseous thermochemical synthesis and characterization of mixed interstitial phases in the Ti-C-O system

Abstract

Titanium grade 2 thin foils were carbo-oxidized at 800 °C in a thermogravimetric analyzer in CO gas for 20 - 100 hours. The synthesized thin foils were characterized by infrared carbon determination for composition analysis, transmission X-ray diffraction (XRD), light optical microscopy (LOM) and nanoindentation. Thermodynamic equilibrium predictions were performed using ThermoCalc. Oxygen and carbon expanded h.c.p. α and NaCl type δ carbo-oxide were identified from the XRD patterns. Rietveld refinements of the XRD patterns revealed an increase in expansion of the h.c.p. α lattice from interstitially dissolved C and O: the c/a ratio increases from 1.588 for interstitially free h.c.p. α to 1.607 after 100 hours of exposure. The overall carbon and oxygen content after 100 hours was 4.2 wt% and 5.5 wt%, respectively; these overall compositions correspond to the two-phase α + δ region in the calculated ternary phase diagram. The microstructure from LOM revealed large grains of expanded α-phase and a surface zone consisting of porous δ carbo-oxide. The hardness, as determined by indentation, of the expanded α increased linearly with the c/a ratio, resulting in an increase to 11748 MPA after 100 hours exposure from 2193 MPa for the untreated Ti grade 2.