Ti-H System Research Articles

A thermodynamic analysis of the ti-h system

A partial phase diagram of the Ti-H system has been calculated by combining available thermodynamical and equilibrium solubility data of the system. This was accomplished by applying a two sublattice model to the interstitial solution phases having different lattice ratios. Excluding the position of the eutectoid point and the αsolvus the agreement between the calculated and experimental results is satisfactory. In particular, the assessment of the phase diagram produced optimized parameter values for the titanium-hydrogen and hydrogen-hydrogen interactions as well as an estimate for the lattice stability of the fcc titanium, which together with the sublattice model used are expected to be useful also in the study of multicomponent titanium-hydrogen systems.

Electronic structure and total energy calculations for transition metal hydrides

The self-consistent electronic energy band structure of palladium and nickel hydrides and titanium dihydride have been calculated. For comparison the band structure and total energy of palladium dihydride was also calculated. The first two exhibit a minium in the total energy curve as a function of lattice constant and in fact show an exothermic heat of formation. Palladium dihydride is repulsive and unbound. For titanium dihydride a minimum in the total energy vs. lattice constant is also observed. This minimum and its value were found to be a sensitive function of the core approximation utilized. A significant improvement over previous results for the heat of formation for the Ti-H system was obtained.

Elastic and plastic accommodation effects on metal-hydride solubility

A model is presented that is capable of accounting for the hysteresis observed in the measured terminal solid solubilities (TSS) of metal hydride systems. Two distinct TSS are identified: 1. (i) a TSS determined on cool-down governed by hydride nucleation and the elastic accommodation energy arising from the hydride-matrix misfit; 2. (ii) a TSS determined on heat-up and dominated by plastic accommodation effects. The energy associated with the plastic accommodation effects is expected to be much smaller than that due to elastic accommodation. The heat-up TSS is therefore expected to be a good approximation to the “equilibrium”, or stress-free, TSS. It follows that the hysteresis between the activation energies of the heat-up and cool-down TSS is approximately equal to the elastic accommodation energy. The predictions of the model are tested with reference to TSS data derived from measurements on the V, Nb, Zr and Ti-H systems and are found to be in reasonable agreement with the data. Possible effects that the hysteresis of the TSS may have on the hydrogen-induced delayed cracking mechanism are discussed.

Thermodynamics of the Ti-H system

Hydrogen gas has been equilibrated with the hydrogen dissolved in titanium. The equilibrium pressure (<10 MPa) of the gas and the hydrogen concentration (40 ∼ 70 mol pct) in titanium have been measured at 576 to 980 K by Sieverts' method. A Gibbs-Duhem integration has been made to obtain the thermodynamic activity of titanium and the formation properties of the “TiH2” phase. Interaction energies between neighboring atoms in the hydride have been evaluated by applying a quasi-chemical model of statistical mechanics.

The formation of metastable hydrides Ti 0.75Al 0.25H x with x < 1.5

Ternary hydrides Ti 0.75Al 0.25H x were formed at temperatures below 200 °C by absorption from hydrogen gas. The hydriding was studied by means of volumetric analysis and X-ray diffraction. Phases were identified as α(h.c.p.) for x < 0.2, β(b.c.c.) for 0.4 < x < 0.5 and γ(f.c.c.) for x < 1.5; this corresponds to the sequence of equilibrium phases formed in the Ti-H system at temperatures exceeding 300 °C. The low temperature phases were metastable and on heating to above 200 °C were succeeded by TiH 2 formation. The pressure of the β + γ plateau is given as JRTlnP H 2 = ΔH − TΔS with ΔH = −11.3 kcal and AS = −30.4 cal K −1. The β + γ plateau pressure was 1 atm at 100 °C which approaches a useful range for hydrogen storage applications. The alloy has an attractive hydrogen-to-metal ratio of about 1.5, but the γ phase formed irreversibly. The attainment of reversibility in the γ phase formation is the dominant problem in developing useful Ti-Al-H alloys for hydrogen storage.

Nuclear-magnetic-resonance study of the electronic structure of the Ti-H system

The proton spin-lattice relaxation time ${T}_{1}$ of hydrogen in titanium hydride was measured as a function of temperature and hydrogen concentration in the temperature range where the overriding relaxation mechanism is the interaction with the conduction electrons. The hydrogen-concentration dependence and the temperature dependence of ${({T}_{1e}T)}^{\ensuremath{-}\frac{1}{2}}$ is presented. A break in the temperature dependence curve of ${({T}_{1e}T)}^{\ensuremath{-}\frac{1}{2}}$ was found for hydrogen concentrations having atomic ratios greater than 1.8. This is associated with a tetragonal deformation which is known to occur for the hydride. A phenomenological model of the electronic structure based on a semirigid band is proposed to explain the data. The concentration and temperature dependence of the susceptibility, hyperfine interactions, and transport properties are discussed in terms of the proposed model as well as the cause of the tetragonal deformation and the low-hydrogen-concentration limit of the hydride.