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

Abstract

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. • The different methods gave an a lattice parameter of ca. 0.44 nm for the FCC phase, which was consistent with δ-TiH 2-x . • The orientation relationship from TEM was determined as {1¯11¯} FCC ||{0002} HCP , indicating thermally promoted FCC phase. • High vacuum annealing removed the “FCC” phase (XRD) proving that it was indeed δ-TiH 2-x. • δ-TiH 2-x was made by SP dislocations promoted by hydrogen ingress during carbo-oxidation and the specimen stress state.