Satoshi Semboshi

Degradation of hydrogen absorbing capacity in cyclically hydrogenated TiMn2

Abstract Degradation of the hydrogen absorbing capacity in cyclically hydrogenated TiMn2 (Ti–60 at.% Mn) with Laves structure was studied by measurement of pressure–composition–isotherm (PCT) curves, X-ray diffraction and microstructure observation. Characteristic changes with cyclic hydrogenation are observed as follows. The hydrogen absorbing capacity remarkably decreases, the lattice constant increases, the amount of hydrogen retained in TiMn2 increases and it approaches almost zero by dehydrogenation at 673 K in vacuum. Nano-sized regions with Moire patterns are produced in TiMn2 with hydrogenation and Debye rings corresponding to titanium hydride δ-TiH appear in the diffraction pattern. Based on these observations it is concluded that the degradation of hydrogen absorbing capacity after cyclic hydrogenation is attributable to the introduction of retained hydrogen, heterogeneous strain and/or nano-sized regions.

Effect of aging in hydrogen atmosphere on electrical conductivity of Cu–3at.%Ti alloy

The electrical conductivities of Cu–3at.%Ti alloys aged at 773 K in a hydrogen atmosphere were investigated as a function of aging time. The electrical conductivity of the quenched alloy, 5.2% International Annealed Copper Standard (IACS), improved with aging time to 66% IACS after 48 h. This was mainly caused by the dilution of the Cu–Ti solid solution in the alloy, which is supported by the fact that the lattice parameter of the face-centered cubic (fcc) phase approaches that of pure Cu by aging in a hydrogen atmosphere.

Mechanical properties and microstructures of β Ti–25Nb–11Sn ternary alloy for biomedical applications

The mechanical properties and microstructures of β Ti-25%Nb-11%Sn ternary alloy rods were investigated for biomedical applications as a function of heat treatment temperature after swaging by an 86% reduction in cross-section area. An as-swaged rod consisting of a β (bcc) single phase shows a low Youngs modulus of 53 GPa, which is interpreted in terms of both the metastable composition of the β alloy undergoing neither an athermal ω transformation nor a deformation-induced ω transformation and <110>texture development during swaging. Heat treatment at 673 K (400 °C) for 2h leads to a high strength of approximately 1330 MPa and a high spring-back ratio of yield stress to Youngs modulus over 15×10(-3), with acceptable elongation. This high strength is attributable to needle-like α precipitates, which are identified by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and high-resolution electron microscopy (HREM).

Microstructural evolution of Cu-1 at% Ti alloy aged in a hydrogen atmosphere and its relation with the electrical conductivity

Copper alloys with titanium additions between 1 and 6at% Ti emerge currently as attractive conductive materials for electrical and electronic commercial products, since they exhibit superior mechanical and electrical properties. However, their electrical conductivity is reduced owing to the residual amount of Ti solutes in the Cu solid solution (Cu(ss)) phase. Since Cu shows only poor reactivity with hydrogen (H), while Ti exhibits high affinity to it, we were inspired by the idea that hydrogenation of Cu-Ti alloys would influence their microstructure, resulting in a significant change of their properties. In this contribution, the influence of aging under a deuterium (D(2)) atmosphere of Cu-1at% Ti alloys on their microstructure is investigated to explore the effects on the electrical conductivity. The specimens were investigated by means of transmission electron microscopy (TEM), field ion microscopy (FIM), computer-aided field ion image tomography (cFIIT), and atom probe tomography (APT). At an early aging stage at 623K in a D(2) atmosphere of 0.08MPa, ellipsoidal alpha-Cu(4)Ti precipitates are formed in the alloy, and during subsequent aging, delta-TiD(2) is competitively nucleated instead of growth of alpha-Cu(4)Ti particles. The co-precipitation of alpha-Cu(4)Ti and delta-TiD(2) efficiently reduces the Ti concentration of Cu(ss) matrix, particularly in the later aging stages in comparison to the aging in vacuum conditions. The electrical conductivity of the alloy aged in the D(2) atmosphere increases steeply up to 48% International Annealed Copper Standard (IACS) after 1030h, while it saturates to approximately 20% IACS in the alloy aged in vacuum. The outstanding increase of electrical conductivity during aging in D(2) atmosphere can be basically explained by the reduction of Ti solute concentration in Cu(ss) matrix.

First-principles studies of complex hydride YMn2H6 and its synthesis from metal hydride YMn2H4.5

First-principles calculations were performed for a complex hydride YMn2H6 to investigate its electronic structure and thermodynamic stability. The results indicated that an Y atom and one of two Mn atoms were ionized as Y3+ and Mn2+, respectively, and another Mn atom bound covalently to H atoms to form a [MnH6]5− complex anion. Based on the enthalpy change of −65 kJ/mol estimated from the calculation, we experimentally verified a possible low-pressure synthesis of YMn2H6 from a metal hydride YMn2H4.5. X-ray diffractometry confirmed the formation of YMn2H6 after hydrogenation below 5 MPa, much lower than the previously reported value of 170 MPa.

Effect of composition on hydrogen absorbing properties in binary TiMn2 based alloys

Abstract The hydrogen absorbing properties of binary TiMn 2 based alloys have been investigated as a function of composition in the range between Ti–56.4 and –66.8 at% Mn. The volume fraction of TiMn 2 phase increases with increasing Mn content. The Mn content of TiMn 2 phase is almost constant at compositions between Ti–56.4 and –59.4 at% Mn, while it increases with increasing Mn content at compositions between Ti–59.4 and –66.8 at% Mn. Ti–59.4 at% Mn containing TiMn 2 phase with 60 at% Mn exhibits the superior hydrogen absorption capacity and the least degradation of capacity during repeated hydrogen absorption and desorption. It is concluded that both hydrogen absorbing capacity and cyclic property in binary TiMn 2 based alloy are improved by increasing the volume fraction of TiMn 2 phase with keeping Mn content as low as possible.

Effect of microstructure on hydrogen pulverization of Nb3AlNb two phase alloys

Hydrogen pulverization is expected to be used as a method for producing powder with high quality and low cost. In this study, hydrogen pulverization is investigated using a Nb-16 mol%Al alloy, which is composed of Nb solid solution (Nbss) and Nb3Al, in terms of volume fraction of Nbss and Nb3Al and fracture toughness. The volume fractions were controlled through heat treatments of 1473, 1773 and 1873 K for 86.4 ks in vacuum. Before hydrogenation, the micro-structures were observed by a scanning electron microscope (SEM). The volume fractions were evaluated by analyzing the SEM images. An equation is proposed to evaluate Nbss solvus using volume fraction ratio of NbssNb3Al by taking atomic volume change into account. In addition, electron probe micro analysis (EPMA) was carried out to determine phase equilibrium compositions. Thus, we propose phase boundaries of Nbss and Nb3Al in the NbAl phase diagram. Vickers hardness tests were conducted and fracture toughness was estimated from length of a crack introduced by indenter. With rising heat treatment temperature, hardness decreases and fracture toughness increases, partially because volume fraction of Nbss increases with rising temperature. After surface treatment, hydrogenation was carried out and pressure-composition-isotherm (PCT) curves were measured under hydrogen pressure from 0 to 3.4 MPa at 313 K. By analyzing the distribution of powder sizes after hydrogen pulverization, it is found that the powder size becomes large with rising heat treatment temperature. It is concluded that powder size can be easily controlled by changing volume fraction of constituent phases, microstructure and fracture toughness through heat treatments.

Hydrogenation-induced fragmentation in Ta–Ni alloy

Abstract Hydrogen-induced fragmentation behavior of Ta–Ni binary alloys is investigated to understand the mechanism of hydrogen pulverization of refractory metals and alloys and to develop the powder fabrication process by hydrogenation. Single-phase alloy of Ta solid solution (Ta ss ) exhibits no change in sample shape by hydrogenation at room temperature, while two-phase alloys of Ta ss and Ta 2 Ni Laves phase are broken into fragments. Crack propagation occurs preferentially in the brittle Ta 2 Ni phase rather than in the ductile Ta ss phase. When the volume fraction of brittle Ta 2 Ni increases with increasing Ni content, hydrogen-induced fragmentation is enhanced. The lattice parameter after hydrogenation increases in Ta ss , but not in Ta 2 Ni. Nanosized clusters with Moire patterns are observed in a HRTEM image of hydrogenated Ta ss , and Debye rings corresponding to tantalum hydride β-TaH appear in the associated diffraction pattern. It is suggested that the hydrogen fragmentation is attributed to the absorption of a large amount of hydrogen, the interfacial strain introduction by lattice expansion and hydride formation, and crack formation at brittle constituent phase(s) and hydride.

Hydrogen absorption of Nb–Al alloy bulk specimens

Abstract Pulverization by hydrogenation is applicable to powder fabrication for refractory intermetallic alloys. Hydrogen absorption of various Nb–Al alloys containing 0–28 mol% Al was investigated using bulk specimens in a Sieverts-type apparatus at test temperatures of 313–373 K under hydrogen pressure of 0–3.4 MPa. The lattice parameter of Nb solid solution (Nb ss ) was determined precisely as a function of Al concentration. It was found that (1) the lattice parameter of Nb ss is linearly reduced with increasing Al content, and (2) hydrogen absorption of Nb ss is also remarkably reduced by Al addition. The atomic radius of Al in Nb ss is determined to be 139.2 pm. Large hydrogen absorption more than 0.5 mass% H is observed only for pure Nb and Nb 3 Al-based two-phase alloys. However, pulverization and the large hydrogen absorption at a constant pressure (plateau) take place only in the latter alloys. This plateau does not stand for equilibrium hydride formation, but for rapid absorption through increased surface area by pulverization. These aspects are discussed in connection with hydrogen diffusivity.

Fine Precipitation in the Channel Region of Two-Phase Ni3Al and Ni3V Intermetallic Alloys Containing Mo and W

Fine precipitation was observed in the channel region of the Ni-based, two-phase Ni3Al and Ni3V intermetallic alloys containing Mo and W, which are soluble in the A1 phase at high temperatures but less soluble in the two intermetallic phases at low temperatures. The fine precipitates were identified as Mo- or W-rich phases (Mo solid solution with a body-centered-cubic (bcc) structure or Ni4W with a tetragonal structure) accompanied by a rigid orientation relationship and a habit plane with the constituent phases in the channel region. Fine precipitation was induced when added elements were substituted for Ni, thus, stabilizing the two intermetallic phases; however, it was not induced when the two intermetallic phases were destabilized by the addition of elements to substitute for Al and V. Fine precipitation was induced to a greater extent when Nb was concomitantly added to the alloys with Mo or W and when the two intermetallic phases were stabilized. Annealing at temperatures below the eutectoid temperature was necessary to induce fine precipitation in the alloys containing W. Accordingly, age hardening by annealing did not occur in the alloys containing Mo but did occur in the alloys containing W, whose behavior was correlated with the difference in the diffusivity of the Mo and W elements in the two-phase eutectoid microstructures (phases).