INTERACTION OF H 2O AND O 2 WITH INTERMETALLIC ALLOYS

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The influence of atomic order on H2-induced environmental embrittlement of Ni3Fe intermetallics

The different sensitivity to H₂-induced environmental embrittlement for the ordered and disordered Ni₃Fe alloys has been investigated. The results show no environmental embrittlement in disordered Ni₃Fe in gaseous H₂ when tested at room temperature. However, the H₂-induced environmental embrittlement for the ordered Ni₃Fe becomes severer as the degree of order increases. The results of testing on simultaneous hydrogen charging show that disordered Ni₃Fe embritted as hydrogen atoms are forced into the material, implying that the embrittlement of ordered Ni₃Fe in gaseous H₂ is due to the acceleration of the kinetics of catalytic reaction to produce more atomic hydrogen. Further more, the hydrogen adsorption test of Ni₃Fe powder shows that the amount of chemically adsorbed hydrogen in the ordered state at room temperature is significantly larger than that adsorbed by the disordered materials, indicating that the more sensitive to H₂-induced embrittlement in the ordered Ni₃Fe is essentially due to accelerated catalytic reaction to produce more atomic hydrogen.     

Effect of atomic ordering on the hydrogen-induced environmental embrittlement of Ni4Mo intermetallics

Hydrogen induced environmental embrittlement of a Ni₄Mo alloy in different degree of ordered conditions was investigated by tensile tests in various atmosphere. The results show that the disordered Ni₄Mo alloy is not susceptible to embrittlement in hydrogen gas, but very susceptible to embrittlement in hydrogen charging. However, for the ordered Ni₄Mo alloy, there is similar deterioration in ductility when the environment changes from oxygen to hydrogen gas and simultaneous hydrogen charging. It indicates that the atomic ordering does not influence the dynamic hydrogen charging-induced environmental embrittlement, but has a considerable effect on the gaseous hydrogen-induced environmental embrittlement. In addition, hydrogen absorption and desorption of the Ni₄Mo alloy with disordered and ordered structures were also investigated using gas chromatographic analysis. The results show that the atomic ordering can promote gaseous hydrogen absorption at room temperature. This suggests that the atomic ordering accelerates the kinetics of the catalytic reaction for the dissociation of molecular hydrogen into atomic hydrogen due to the change of the outer layer electron structure and therefore exacerbates the hydrogen gas-induced environmental embrittlement.     

First-principles study of the influence of lattice misfit on the segregation behaviors of hydrogen and boron in the Ni–Ni3Al system

A first-principles method is employed to investigate the segregation behaviors of hydrogen and boron in Ni-based and Ni₃Al-based alloys using two models. Chemical binding energy analysis shows that both boron and hydrogen are able to segregate to the interstices in the Ni phase, Ni₃Al phase and Ni/Ni₃Al interface. Boron, however, is bound to its neighbor atoms more tightly than hydrogen in both models and its stable state exists over a broader lattice misfit range compared with hydrogen. The bond order analysis we have proposed reveals the origin of the boron-induced ductility and hydrogen-induced embrittlment at the Ni/Ni₃Al interface with different lattice misfit. The calculations indicate that hydrogen causes more severe embrittlement at the Ni/Ni₃Al interface in Ni₃Al-based than in Ni-based alloys. Furthermore, it is found that the boron-induced ductility and hydrogen-induced embrittlement are changed, and thus controllable, by the lattice misfit. Our results provide a quantitative explanation of many experimental phenomena caused by the addition of boron and hydrogen to Ni-based and Ni₃Al-based alloys.     

Hydrogen diffusivity in B-doped and B-free ordered Ni3Fe alloys

The hydrogen diffusion coefficient of the ordered Ni₃Fe–B alloys with and without boron additions was measured by a method of the cathodical precharging with hydrogen. The apparent hydrogen diffusion coefficient decreases with increasing the boron concentration doped in the ordered Ni₃Fe alloy. Comparing with the B-free ordered Ni₃Fe alloy, the activation energy of hydrogen diffusion for the ordered B-doped Ni₃Fe alloy increases by as high as 42% when the boron content is sufficient. The doping boron in the Ni₃Fe alloy is effective in reducing the hydrogen diffusion at the grain boundary.     

Moisture-induced embrittlement mechanism for (Ni,Fe)Ti alloys

Recent experimental studies showed that the ductility of NiTi is not affected by moisture, while addition of iron beyond 9 a/o in NiTi leads to moisture-induced embrittlement. To explore the nature of this embrittlement, we studied the chemical interaction between water vapor and (Ni,Fe)Ti(110) surfaces with 5 a/o and 10 a/o Fe. Temperature-programmed desorption and X-ray photoelectron spectroscopy show that decomposition of water to produce atomic hydrogen occurs on both surfaces. Activation energy for surface diffusion was calculated by density functional theory, showing that addition of Fe decreases H surface diffusivity, in agreement with experiment. Together with the observation that addition of 9 a/o Fe increases the strength of NiTi, this indicates that moisture-induced embrittlement in higher strength NiTi alloys is not due to faster H surface diffusion, but lower critical hydrogen concentration required for embrittlement.     

The effect of atomic ordering on the hydrogen absorption and desorption of Ni4Mo alloy

Hydrogen absorption and desorption of disordered and ordered Ni₄Mo alloys were investigated. The results show that the atomic ordering can promote gaseous hydrogen absorption at room temperature and therefore exacerbates the hydrogen gas-induced environmental embrittlement.     

Surface diffusivity of atomic deuterium on Ni3（Al,Ti）（110）surface with and without boron

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Welding of unique and advanced ductile intermetallic alloys for high-temperature applications

Intermetallic alloys represent a unique class of materials with atomic arrangements that are different from those of conventional disordered alloys. Among them are alloys based on Ni₃Al, Fe₃Al, and TiAl. Intermetallic alloys have unique properties, such as high melting point, low density, high-temperature strength, and high-temperature corrosion and oxidation resistance. Their only disadvantage is the lack of ductility at room temperature and at elevated temperatures. However, they can be ductilised by micro- and macroalloying. Application of intermetallic alloys for structural use at elevated temperature depends on their ability to be welded using conventional welding procedures. This paper focuses on the development of these alloys, their behaviour when subjected to weld thermal cycles, and their weldability. Most intermetallic alloys are susceptible to cracking during or after welding, but some can be modified to have good weldability. The paper discusses welding and weldability of Ni₃Al-, Fe₃Al-, and TiAl-based intermetallic alloys. In addition, the weldability of other long-range ordered alloys, of the type (Fe, Ni)₃V and (Fe, Co)₃V, are briefly discussed.     

Advances in processing of Ni3Al-based intermetallics and applications

The intermetallic-based alloys for structural applications have been an active field of research around the world for the last 20 years. Several major breakthroughs have occurred in this field during this time period. These breakthroughs include: (1) the dramatic effects of boron on ductility improvement for Ni₃Al at ambient and high temperatures, (2) effect of chromium addition for intermediate temperature ductility improvement of Ni₃Al, and (3) identification of an environmental effect from hydrogen generated by the reduction of moisture in air by aluminum in the aluminides. The knowledge of the compositional effects has led to the development of Ni₃Al-based alloys, which allowed them to be taken from laboratory-size melts to commercial applications. This paper will describe the advances in melting practice, casting practices, solidification modeling as it applies to static and centrifugal castings and weld repairs, and welding of castings. This paper will also describe various applications of Ni₃Al-based alloys and their current status of commercialization.     

Energetics and atomic transport at liquid metal/Al2O3 interfaces

The objective is to study interfacial mass transport mechanisms and to estimate interfacial energies for metal/Al₂O₃ systems. Experiments have been performed with molten drops of Ni, Cu, or Au on pure, polycrystalline alumina at oxygen partial pressures for which no adsorption is expected and with Al to determine the effect of extremely low p(O₂). After removing the metal drops, grain boundary grooves at the interface and oxide surface have been analyzed using AFM and SEM. Several sources of error are assessed, and corrections are proposed for large systematic errors that occur for root angles. These experiments yield higher grain boundary energies and lower M/Al₂O₃ interfacial energies than previously reported. Transport rates near the metal/ceramic interface are two to four orders of magnitude faster than on the oxide surface and the results suggest that diffusion through the liquid metal is usually the main atomic transport mechanism. Experiments with Al indicate that, at the far lower oxygen activities, transport is faster at both the interface and alumina surface and that the interfaces are more anisotropic and have lower energy.     

EFFECT OF TESTING ENVIRONMENT ON FRACTURING BEHAVIOR OF Fe3Si BASED ALLOY

The mechanical behavior of Fes Si based alloy with B2 structure was studied by tension and fracture toughness test in various testing media. The fracture strength σb of Fe3Si alloy decreased in the following order: oxygen, air and hydrogen respectively. The fracture toughness in different testing environment showed that KIC in oxygen is 11.5±0.3MPa·m1/2, and is 8.6±0.4MPa·ml/2 in distilled water. The reduction of fracture toughness is contributed to the environmental reaction of Si with water. Addition of Al element in Fe3Si is not beneficial to improve the intrinsic ductility of Fe-14Si-3Al alloy. The scattering phenomenon of fracture strength was found, and explained by fracture mechanics. It was found by means of SEM that the fracture mode changed from transgranular in oxygen to intergranular in hydrogen gas and distilled water.     

Characterization of stacking faults on basal planes in intermetallic compounds La5Ni19 and La2Ni7

Stacking faults on basal planes as well as intergrowth structures in La₅Ni₁₉ and La₂Ni₇ have been characterized by high-resolution transmission electron microscopy (HREM) and electron diffraction. Most stacking faults observed in La₅Ni₁₉ are of the inter-block-layer type in which the faulting in the stacking sequence of block layers occurs without changing the number of unit layers in each block layer. In contrast, most stacking faults observed in La₂Ni₇ are of the intra-block-layer type in which the faulting occurs in the number of unit layers in block layers, although some stacking faults of the inter-block-layer type are also observed. Intergrowth structures are commonly observed in as-cast alloys with compositions corresponding to La₅Ni₁₉ and La₂Ni₇. The implication of these results for increasing the hydrogen absorption rate of LaNi₅-based alloys without significantly affecting hydrogen absorption/desorption properties described in terms of pressure-composition isotherms are discussed.     

Hydrogen solubility and diffusivity in amorphous La14Ni86 films

Hydrogen solubility and diffusivity in Pd-coated amorphous La₁₄Ni₈₆ films have been studied by thermal desorption spectroscopy (TDS). The TDS spectra were analyzed according to the statistical Harris and thermodynamic Kirchheim models. For H-loading pressures up to 100 mbar, hydrogen essentially occupies La₂Ni₂ and LaNi₃ tetrahedral sites at room temperature. Only hydrogen in LaNi₃ sites can be thermally desorbed before crystallization of the films takes place. Progressive filling of LaNi₃ sites with increasing hydrogen pressure occurs in agreement with the thermodynamic model. The density of LaNi₃ sites is well characterized by a Gaussian distribution with a mean energy of −0.44 eV. As regards diffusivity, hydrogen atoms jump from mean energy LaNi₃ sites with an activation enthalpy of 0.47±0.03 eV. Nonetheless, discrepancies in the width of the site energy distribution were found when it was independently evaluated from solubility and diffusivity analyses.     

Effects of electrochemical hydrogenation of Zr-based alloys with high glass-forming ability

Zr–(Ti)–Cu–Al–Ni metallic glasses exhibit a high thermal stability corresponding to a wide undercooled liquid region. Depending on their composition, the formation of metastable intermediate phases, e.g. a quasicrystalline phase is possible. The combination of early and late transition metals makes these alloys very interesting regarding their interaction with hydrogen. Amorphous Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₆₅Cu_17.5Al_7.5Ni₁₀ and Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ ribbons were prepared by melt spinning and their microstructure and thermal behaviour was checked by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in 1 vol.-% HF was investigated in 0.1 M NaOH by applying potentiodynamic polarisation techniques. Galvanostatically hydrogenated samples were characterised by XRD, DSC, TEM and thermal desorption analysis (TDA). For amorphous Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ samples an increase in electrochemical surface capacity by two orders of magnitude is observed after pre-etching. Compared to the quasicrystalline and crystalline alloy, the hydrogen reduction takes place at significantly lower overpotentials. Zr-based alloys cathodically absorb hydrogen up to H/M=1.65 while keeping the amorphous structure. Already small amounts of hydrogen cause a significant decrease of the thermal stability and changes in the crystallisation sequence. The hydrogen desorption is a two-stage process: (T<623 K) hydrogen desorption from high interstitial-site energy levels and (T>623 K) zirconium hydride formation and subsequent transformation under hydrogen effusion. Hydrogen suppresses the oxygen-triggered formation of metastable phases upon heating and supports primary copper segregation. At very high H/M ratios, severe zirconium hydride formation causes the crystallisation of new compounds.     

The effect of Nb addition on environmental embrittlement of a Ni3(Si,Ti) alloy

The effect of Nb addition on the moisture-induced embrittlement of a Ni₃(Si,Ti) alloy was investigated at room temperature by tensile test and SEM fractography. Embrittlement/ductility was assessed as functions of strain rate and environment. The Nb-containing second-phase dispersion was found to be effective in reducing the moisture-induced embrittlement of the Ni₃(Si,Ti) alloy, while Nb as a solute in the Ll₂ matrix was shown to enhance the moisture-induced embrittlement of the Ni₃(Si,Ti) alloy. Possible mechanisms accounting for the beneficial effect of the Nb-containing second-phase dispersion on the moisture induced embrittlement of the Ni₃(Si,Ti) alloy was discussed in terms of microstructural modification by the second-phase, hydrogen transportation kinetics and deformation properties in the constituent phases or L1₂ matrix/second-phase interface.     

Microstructure and remarkably improved hydrogen storage properties of Mg2Ni alloys doped with metal elements of Al,Mn and Ti

Mg₂Ni_0.7M_0.3 (M=Al, Mn and Ti) alloys were prepared by solid phase sintering process. The phases and microstructure of the alloys were systematically characterized by XRD, SEM and STEM. It was found that Mg₃MNi₂ intermetallic compounds formed in Mg₂Ni_0.7M_0.3 alloys and coexisted with Mg and Mg₂Ni, and that radius of M atoms closer to that of Mg atom was more beneficial to the formation of Mg₃MNi₂. The hydrogen storage properties and corrosion resistance of Mg₂Ni_0.7M_0.3 alloys were investigated through Sievert and Tafel methods. Mg₂Ni_0.7M_0.3 alloys exhibited remarkably improved hydrogen absorption and desorption properties. Significantly reduced apparent dehydriding activation energy values of ?46.12, ?59.16 and ?73.15 kJ/mol were achieved for Mg₂Ni_0.7Al_0.3, Mg₂Ni_0.7Mn_0.3 and Mg₂Ni_0.7Ti_0.3 alloys, respectively. The corrosion potential of Mg₂Ni_0.7M_0.3 alloys shifted to the positive position compared with Mg₂Ni alloy, e.g. there was a corrosion potential difference of 0.110 V between Mg₂Ni_0.7Al_0.3 alloy (?0.529 V) and Mg₂Ni (?0.639 V), showing improved anti-corrosion properties by the addition of Al, Mn and Ti.     

Hydrogenation of Zr–Cu–Al–Ni–Pd metallic glasses by electrochemical means

Amorphous materials of Zr–Cu–Ni–Al systems have shown attractive electrochemical hydrogen absorption properties. A comparison between Zr₆₀Cu₁₅Al₁₀Ni₁₀Pd₅ and Zr₆₅Cu_17.5Al_7.5Ni₁₀ reveals that the palladium (Pd) increases the hydrogen absorption capacity. Charging melt-spun Zr₆₀Cu₁₅Al₁₀Ni₁₀Pd₅ ribbons electrochemically to different hydrogen-to-metal (H/M) ratios and following the effusion of hydrogen by thermal desorption analysis (TDA) reveals hydrogen desorption from interstitial sites of high energy levels at temperatures below 630 K. Zirconium hydrides are formed above 630 K. At higher temperatures partial desorption of hydrogen occurs. The thermal stability observed with differential scanning calorimetry (DSC) of the amorphous phase has been significantly deteriorated by hydrogen absorption. After hydrogenation, the crystallization behaviour shows suppression of the characteristic quasicrystalline phase and depends on the hydrogen content. Therefore, at low hydrogen concentrations H/M = 0.3, Cu and/or Cu-rich phases are primarily formed while at high hydrogen concentrations H/M ≥ 0.9 Zr-hydride phase(s) are mainly formed.     

H2-induced environmental embrittlement in ordered and disordered Ni3Fe: An electronic structure approach

The different behaviors in H₂-induced environmental embrittlement in ordered and disordered Ni₃Fe are associated with differences in their electronic structures. The experimental study on electronic structures of ordered and disordered Ni₃Fe has been carried out by electron energy-loss spectroscopy (EELS). The onset energy of Ni L_2,3 edges from ordered phase is 0.3 eV lower than that from disordered phase, while the 3d occupancy of Ni atoms in ordered phase is 0.07 electrons/atom less than that in disordered phase. Severe H₂-induced environmental embrittlement in ordered phase is attributed to rather negative dissociative adsorption energy of hydrogen at surfaces, which arises from upward shifting of the valence band center of Ni.     

Hydrogen desorption kinetics of amorphous Mg0.9Ti0. 1Ni1-xPdx （x=0, 0.05, 0.1 and 0.15） electrode alloys

1 Introduction Mg-based alloy is a kind of promising hydrogen storage materials used for fuel cell. It is also a potential candidate as cathode materials of Ni-MH rechargeable batteries due to its large discharge capacity[1]. However, its cyclic stabilit…