Thermodynamics and kinetics of mixed interstitial phases in the titanium systems

This Ph.D. thesis addresses interstitial alloying of titanium with oxygen, nitrogen, carbon and hydrogen and the impact on mechanical properties. Moreover the crystallography, thermodynamics and microstructural evolution of Ti containing interstitials are investigated. To this end thin foils of Ti with different loads of interstitials were synthesized applying different thermochemical routes.

Four main topics are investigated in the thesis: I) Mechanical properties and crystallography of Ti(N) and Ti(O) interstitial solid solutions, and the effect of pile-up and sink-in on the area determination using nanoindentation. II) The surprising synergistic effect between O and H resulting in an extreme crystallographic expansion and extreme solubility of H in Ti. III) The reversibility of hydrogenation/dehydrogenation and the effect on microstructure in Ti and Ti6Al4V. IV) Carbooxidation using CO to attain a low O partial pressure over a wide temperature range and formation of compounds based on C and O.

I) The effect of alloying Ti with N and O on the hardness and indentation modulus was studied over a wide range - from very low interstitial loads to the solubility limit of hcp α-Ti, attaining 80 times as much interstitial in solid solution compared to the preceding literature results. The lattice expansion exerted by O and N agreed well with previous studies. Atomic force microscopy and indent fitting successfully validated the applied finite element modelling based nanoindentation method’s ability to account for both sink-in and pile-up.

II) Titanium with 25 at. % O and 19 at. % N in solid solution, resulted in an increase in hydrogen solubility from zero to 50 at. % H, causing a spectacular 12.5 % anisotropic expansion of the hexagonal closed packed (h.c.p) lattice.

III) The hydrogenation and subsequent high vacuum annealing at 350 – 550 °C of Ti and Ti6Al4V, resulted in exceedingly complex microstructures and phase compositions, explained by the effect of cooling, heating and different hydrogen contents.

IV) The carbo-oxidizing of titanium foils enabled the study of the kinetics of C and O uptake, where the formation of an outer layer of face centered cubic (f.c.c.) TiC_xO_y significantly inhibited the ingress of O above 800 °C. At 600 – 700 °C an elusive f.c.c. phase in the center of the foils, was analyzed with advanced characterization methods and found to be a hydride. The development of this hydride was a consequence of mismatching body center cubic (b.c.c.) particles, tensile
stresses present at the advancing oxygen concentration profile and H present in the foil.

The experimental techniques utilized in the study were transmission X-ray diffraction (XRD), thermogravimetry (TGA), differential thermal analysis (DTA), high vacuum annealing (HVA) nanoindentation, hardness measurement, light optical microscopy (LOM) atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area diffraction (SAED), electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA). Along with thermodynamic and diffusion simulations in ThermoCalc and DICTRA to supplement the experimental data. The use of Rietveld refinements (full diffractogram fitting) on XRD diffractograms was also an integral part of the study.