Part IX - Papers - Hydrogen Solubility in Alpha Ti-O Alloys

Isothermal sections of the Ti-0-H phase diagvam at 600°, 700°, and 800°C have been constructed from equilibrium hydrogen pressure measurements over a range of Ti-0-H alloys containing up to 34 at. pct 0. Oxygen reduces the hydrogen solubility at fixed hydrogen pressure but increases the terminal solubility in the a solid solutions at the a/(a + P) phase boundary. Metal-lographic studies of alloys annealed below the eutectoid temperature show that oxygen also increases the ter-ninal solubility of hydrogen in the a solid solution in equilibrium with the hydride phase. The effect of oxygen on hydrogen solubility is discussed in terms of the electronic and geometric structure of interstitial solid solutions. THE isothermal solubility of hydrogen in a Ti-0 alloys at 800°C has been measured by Hepworth and ~chuhmann' who subsequently used ternary Gibbs-Duhem integrations to calculate oxygen activities in binary Ti-0 alloys. The present study aims to extend Hepworth and Schuhmann's measurements to alloys richer in oxygen and to a range of temperatures. This paper presents the results of hydrogen equilibrium pressure/concentration measurements in Ti-0 alloys as well as observations on the effect of oxygen on the terminal solubility of hydrogen in a titanium. The thermodynamic properties of binary Ti-0 alloys, derived from these measurements, will be published separately.' EXPERIMENTAL METHODS Specimen Preparation. Ti-0 alloys were prepared from iodide-refined titanium and titanium dioxide obtained by hydrolysis of redistilled titanium tetrachlo- ride. Specimens containing up to 16 at. pct 0 were prepared by electromagnetic levitation3 whereas a conventional tungsten arc furnace with a water-cooled copper hearth was used to melt oxygen-rich alloys. The specimen homogeneity was indicated by the constancy of lattice parameters of samples from various parts of the ingot as well as by the sharpness of the reflections on X-ray powder patterns. Oxygen-rich specimens in the composition range (20 to 34 at. pct), where ordering has been ~bserved,~ were annealed for 300 hr at 600°C prior to use in hydrogen solubility measurements. Apparatus. The hydrogen pressure/concentration measurements were made with a modified Sieverts apparatus shown diagrammatically in Fig. 1. Specimens were placed in a clear silica tube and impurity pickup by direct reaction with silica was prevented by lining the heated end of the silica tube with thin molybdenum sheet. A double silica tube was required to eliminate loss of hydrogen by diffusion through silica at high temperatures and high hydrogen pressures, the space between the tubes being maintained at a hydrogen pressure equal to that above the specimen. Hydrogen was purified by diffusion through a Ag-Pd tube.= Hydrogen pressures were measured with a mercury McLeod gage in the range 0.001 to 4 Torr