Diffraction based identification of an elusive FCC phase in carbo-oxidized titanium

A face-centered cubic (FCC) phase formed during the carbo-oxidation of hexagonal close-packed (HCP) α-Titanium at 600 to 700 °C. The FCC crystal structure was confirmed using transmission electron diffraction, X-ray diffraction and high angle annular dark field scanning transmission electron microscopy. The a lattice parameter for the FCC resulting from the three methods was consistent with FCC sublattice of titanium atoms in δ-TiH_2-x. The FCC phase is hypothesized to form partly due to hydrogen pick-up, the stress state in the foil caused by the mismatch between body centered cubic iron-stabilized β-Titanium and HCP α-Titanium and the tensile stress present in front of the oxygen concentration profile. A transformation of α-Titanium into δ-TiH_2-x is accomplished by introducing Shockley partial dislocations on every 2nd closest paced plane, resulting in an orientation relation that has previously been observed for thermally induced FCC γ-Ti. The TiH_2-x phase was shown to decompose upon vacuum annealing at 500 °C.