Hydride Transformation Research Articles

Beneficial effects of O-phase on the hydrogen absorption of Ti–Al–Nb alloys

The beneficial effects of O-phase on hydrogen absorption/desorption were demonstrated in three Ti–Al–Nb alloys with compositions in the vicinity of Ti 2NbAl. The alloys were first quenched from high temperature and then aged for a certain period of time at a lower temperature to get O-phase precipitation in the matrix before pressure–composition ( P–C) isotherm measurements. All three alloys absorb hydrogen to β hydride at a very low equilibrium pressure. Further absorption to γ hydride is difficult for the quenched O-phase free specimens. β->γ hydride transformation occurs in all the aged specimens and the reversible absorption/desorption between β and γ hydrides is observed in some aged specimens with relatively high volume fraction of O-phase in the matrix. The hydrogen absorption/desorption abilities of these alloys become poor when the Nb content is decreased. The determined formation heat for β->γ hydride transformation is in the range of −30–45 kJ/mol H 2, which is larger than that of binary Ti 3Al. It is found that the value of formation heat becomes more negative with the increasing amount of O-phase and is relatively independent of the interface between O-phase and matrix. This indicates the beneficial effect is related to the O-phase structure itself.

Multiphase and microstructure effects on the hydrogen absorption/desorption behavior of a Ti–22Al–27Nb alloy

The hydrogen absorption/desorption behavior of hydride-forming alloys depends on their microstructure. We investigated multiphase effects on the hydrogen absorption/desorption behavior at 100°C of a ternary alloy [Ti–22Al–27Nb (at.%)] whose microstructure consists of β (bcc) [or its ordered phase β 0 (B2)] and/or O (orthorhombic, based on Ti 2AlNb) phases. On exposing β 0 single-phase specimens to hydrogen, β hydride (B2) is easily formed. However, further hydriding to γ hydride (bct) does not easily occur. When the O phase exists in the β matrix phase, the β→γ hydride transformation occurs and the reversible absorption/desorption of hydrogen between β and γ hydrides is observed. Roles of the O phase in the reversible hydrogen absorption/desorption behavior are discussed on the basis of the crystal structure correlation between the β phase, O phase and γ hydride phase and the ordering of hydrogen atoms in these phases.

Hydrogen absorption and desorption in a B2 single-phase Ti–22Al–27Nb alloy before and after deformation

The effects of deformation on the hydrogen absorption/desorption behavior of a B2 single-phase Ti–22Al–27Nb (at.%) alloy were investigated mainly by measuring the static pressure–composition ( P– C) isotherms from room temperature to 100°C for specimens quenched from 1200°C and deformed to 5–∼90% in compression or by cold-rolling. β hydride is formed at a very low hydrogen pressure similarly to bcc hydrogen absorbing alloys. In as-quenched specimens, further absorption to γ hydride is sluggish and there is no obviously reverse γ→β hydride transformation. Both the β→γ and reverse γ→β hydride transformations are promoted by deformation. The best hydrogen absorption/desorption properties are attained in specimens deformed to 5–10%, the deformation structure of which consists of well-aligned and uniformly distributed <111> screw dislocations. The beneficial effects of deformation on the hydrogen absorption/desorption behavior of the alloy are interpreted in terms of similarities between atomic displacements around straight screw dislocations and those taking place during the β–γ hydride transformation.

Precipitation and dissolution peaks of hydride in Zr–2.5Nb during quasistatic thermal cycles

Two internal friction peaks and corresponding discontinuity (‘knee’) points of elastic modulus have been observed in hydride-forming metals upon heating and cooling, respectively. In the present work, measurements of Young’s modulus as functions of temperature and hold time during a quasistatic thermal cycle were made in Zr–2.5Nb samples containing hydrogen using a composite oscillator technique. The increment of modulus during an isothermal hold is proportional to the decrease in hydrogen concentration in solid solution of the Zr alloy. As a result, elastic modulus measurements provide a means to determine the amount of transformed hydride during the transition. It is confirmed experimentally that the two peaks reflect the variation of the hydride transformation rate during heating or cooling. It is demonstrated by the present work that the maximum-slope point of each peak, on the high temperature side, is coincident with the knee point on the curve of modulus versus temperature and this point provides the most reliable and physically sensible indicator of the end of hydride dissolution during heating, or the beginning of hydride precipitation during cooling.

Kinetics and morphology of the reverse β→α hydride transformation in thermodynamically open Pd–H system

Abstract The kinetics of degassing and reverse β→α hydride transformation initiated in the unstrengthened β-hydride of Pd in the thermodynamically open Pd–H system is studied. An isothermal kinetic diagram was constructed and its shape was similar to those of transformations in steel during heating. With optical microscopy in situ and video-registration methods the morphology of new α-phase precipitations is studied. It is shown that the process of α-phase generation in subsurface layers depends on the orientation of the grain crystal lattice to the surface. α-Phase nuclei grow in borders of the grain in which they were generated. It is shown that hydrogen macro-stresses may be generated throughout the whole volume of the β-hydride sample and they relax by means of grain borders appearance (mutual grains displacement). During the process of β→α transformation local internal hydrogen stresses may slow down both degassing and β→α transformation.

Kinetic and morphological peculiarities of β → α phase hydride transformations in the palladium-hydrogen system

The reverse β → α transformation in a well annealed low-defect-concentration β phase hydride of Pd has been investigated by an in situ method of optical microscopy in a special hydrogen-vacuum device. It was determined that the transformation proceeds by a classic mechanism of generation and growth and that initial α phase precipitations were extensive. After transformation folded deformed surfaces were irreversibly produced in the course of β → α phase hydride transformations in the Pd-H system.

Form changing of palladium plate under hydrogen treatment

Palladium plate behaviour under hydrogen diffusion was studied at temperatures above the critical point of the hydride transformation on set. It was determined that hydrogen influence on an elastically stressed palladium plate leads to its form changing, which has both reversible and irreversible constituents. It was found that as a number of barocycles increases, the plate deformation behaviour changes. Theoretical analyses of the elastic fields of stresses of plates, as well as the fields conditioned by hydrogen diffusion were carried out. A comparison of theoretical and experimental results was made.

Hydride embrittlement in ZIRCALOY-4 plate: Part II. interaction between the tensile stress and the hydride morphology

The effect of an applied tensile stress on the hydrides morphology in ZIRCALOY-4 was studied. To this end, the residual stresses around the hydride caused by the hydride precipitation was first evaluated. Considering the disability to predict hydride transformation stresses by ordinary macroscopical mechanical calculation in previous studies, X-ray diffraction (XRD) profile analysis and transmission electron microscopy (TEM) observations were carried out to quantify the microstructural evolution in hydrided ZIRCALOY-4. The residual microstrains and microstresses in the matrix and around the hydride were thus estimated. The big discrepancy between our results and the existing studies were explained by the major self-accomodation of phase transformation deformation remaining inside the hydrides and the local plastic accommodation of ZIRCALOY-4. In order to study the stress effect on hydride orientation and to estimate the hydride orientation threshold stresses, hydrogen was introduced into the specimens under tensile stress. A quantitative technique was used to evaluate the susceptibility to perpendicular hydride formation under the influence of texture, residual stresses, and externally applied tensile stresses, following an improved approach that had been first developed by Sauthoff and then applied to Zr-H system by Puls. Both analytical and experimental results indicate that the threshold stress for producing perpendicular hydrides varies with the microstructural features, the yield strength, and the residual stresses.

Martensite and hydride transformations in the V-N-H and V-O-H systems

Optical metallography has been applied to vanadium containing traces of oxygen and nitrogen on electrolytic saturation with hydrogen; a martensite phase is formed near room temperature and above (formation controlled by the H/V concentration), and hydrides segregate, which suppress further martensite growth.

Modelling Delayed Hydride Cracking in Zirconium Alloys

The most recent theory of delayed hydride cracking in zirconium alloys, herein called herein Dutton-Puls theory, is reviewed. The theory is examined in relation to recent experimental results pertaining to the partial molar volume of hydrogen in zirconium, the size of the crack tip hydrides and the effect of temperature changes on the cracking rate. The straition markings on the fracture surface are considered. An alternate theory of striation formation originally proposed for titanium alloys and involving a semi-cohesive zone at the crack tip is recommended.Arguments are presented from the literature that the hydride transformation is martensitic in nature and that the hydride constraint energy W t inc should be modified. Suggestions for further work in the areas of the precipitation hysteresis and with regard to the effect of hydride precipitation on crack tip stresses are made.