An electron microscopic study of hydrogen embrittlement in vanadium—II

Hydrogen embrittlement in vanadium was studied by in situ observation with a high-voltage electron microscope to reveal the formation of non-Maxwellian atmosphere of hydrogen atoms around a propagating crack-tip, the formation of hydride from the atmosphere by a displacive transformation and the microscopic processes of crack growth.The mechanism of formation of the non-Maxwellian atmosphere in a heterogeneously stressed system and the modes of crack growth of two types were discussed theoretically. A resistive force due to the atmosphere was considered to classify the crack growth into the above two types. The results of calculation on significant quantities related to the hydrogen embrittlement were compared satisfactorily with experimental results obtained.     

Effect of Mo content on porous Ni3Al–Mo electrodes for hydrogen evolution reaction

Abstract

Porous Ni₃Al–Mo electrodes were obtained by reactive synthesis methods with Mo contents of 2, 5, 10, 15 and 20 wt-% respectively. The microstructure, surface morphology and composition of the prepared porous Ni₃Al–Mo electrodes were analysed by X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM). Hydrogen evolution reaction (HER) on the porous Ni₃Al–Mo electrodes in 6 mol L^?1 KOH solution was investigated by polarisation measurements and electrochemical impedance spectroscopy (EIS) technique. It was shown that the hydrogen evolution activity of the porous Ni₃Al–Mo electrodes was related with the Mo content. The porous Ni₃Al–Mo electrode with optimum catalytic activity for the HER was found to contain the maximum Mo content of 10 wt-%.     

Dynamic embrittlement of boron-doped Ni3Al alloys at 600°C

Boron-doped Ni₃Al alloys, with and without 0.5at.%Hf, were tensile tested in vacuum and in oxidizing environments at 600°C. Tensile ductility was found to be strongly dependent on test environment, with much lower ductilities observed in air than in vacuum. The loss in ductility is accompanied by a change in fracture mode from transgranular to intergranular. The severity of this environmental effect on elevated-temperature ductility is also affected by preoxidation in air as well as by the aluminum content of the aluminide. Tests of preoxidized specimens indicate that the embrittlement is due to a dynamic effect simultaneously involving localized stress concentrations, elevated temperature, and gaseous oxygen. The oxygen embrittlement becomes less severe with a decrease in aluminum concentration from 24 to 21 at.%.     

Anomalous strain rate dependence of the serrated flow in NiH and NiCH alloys

Serrated flow in NiH, NiCH, and NiC alloys was studied over a wide range of temperature and strain rates. The results for C related serrated flow in NiC or NiCH alloys were in excellent agreement with previous results and were consistent with dislocation pinning by C solutes at the dislocation cores. Hydrogen related serrated yielding was observed in NiH and NiHC alloys. Solute C had only a small effect on the temperature range of this H related serrated flow. The results could be interpreted on the basis of hydride formation at the dislocation cores and diffusion of H in these hydrides.     

Effect of strain rate and temperature on the flow stress of β-phase titanium- hydrogen alloys

Compression tests were carried out on seven titanium-hydrogen alloys containing hydrogen concen-trations up to 31 at. pct. All the experiments were performed within the β-phase field at strain rates of 0.001 to 1.0s ^~1 The dependences of the steady-state flow stress on strain rate, temperature, and hydrogen concentration were determined. The strain rate sensitivity increases with temperature but decreases with hydrogen concentration. The experimental activation energy of deformation decreases when the flow stress or strain rate is increased. At a fixed strain rate, it decreases when the hydrogen concentration is increased. However, when measured at a fixed steady-state stress, the activation energies are nearly the same for all the alloys. The steady-state flow stress increases with hydrogen concentration as can be expressed by both linear and quadratic dependences. The flow behavior of the alloys can also be described in terms of thermally activated glide and the relation $$\dot \varepsilon = K\sigma ^4 \exp \left( { - \frac{{\Delta H_0 }}{{kT}}} \right)\exp \left( {\frac{{v\sigma }}{{kT}}} \right)$$ where the constantsv and ΔH₀ are independent of hydrogen concentration, while the parameterK decreases exponentially when the hydrogen concentration is increased.     

The order-disorder transformation in Ni3Al and Ni3AlFe alloys—II. Phase transformations and microstructures

From a combination of dilatometric and micrographie evidence, the precipitation of the β′ phase at temperatures above the metastable ordering transition, T_tm, is demonstrated for those NiAlFe alloys containing at least 19 at.% Al. Ni-AlFe alloys which have been heated above T_tm and then slow-cooled are found to contain a network of disordered γ phase which outlines the boundaries of antiphase domains in the ordered γ′ phase. The implication of these observations is that the formation of equilibrium γ phase from off-stoichiometric γ′ requires antiphase domain boundaries as nucleation sites: the T_tm determined by dilatometry represents an upper absolute instability limit at which the ordered phase is converted into disordered phase of the same composition, whereas the equilibrium disordered phase would have a different composition. The microstructure of a series of binary Ni-Al alloys are examined: at 22–23 at.%Al, the γ′ phase consists of grains which partly contain fine antiphase domains (with γ at the APD boundaries) and partly very coarse APD's; microanalysis established that this is due to microségregation, and that the coarse APD's are in regions with excess Al so that T_tm is locally above the freezing temperature. The structural distinction between directly and sequentially ordered alloys is thereby made clear: fine APD's form only in alloys which freeze in disordered form.     

The effects of Ni3Al binder content on the electrochemical response of melt-infiltration processed TiC–Ni3Al cermets

TiC–Ni₃Al samples were successfully fabricated with varying amounts of the Ni₃Al intermetallic binder (alloy IC-50), ranging from 10 to 40?wt-%, through a simple melt-infiltration method. Each sample was then tested to determine the degree of resistance of that composition to electrochemical corrosion in an aqueous solution containing 3.5?wt-% NaCl, using a range of testing procedures including open-circuit potential, potentiodynamic polarisation and cyclic polarisation. Results indicate that the lowest binder content results in greater potential to resist corrosion. It is demonstrated that the Ni₃Al binder undergoes dissolution for the examined conditions, which was confirmed through the high amount of Al and Ni in the electrolyte solutions following testing. It was also confirmed from the electrochemical experiments and the SEM that localised corrosion was visible.     

Effects of changes in strain path on work-hardening in CP aluminium and an AlCuMg alloy

Effects of abrupt changes in the direction of plastic deformation on work-hardening behaviour have been investigated in two-stage stretching of sheets of AA1050 and a heat-treated 2014 aluminium alloy aged at various temperatures up to 300°C. The results show that reorganisation of dislocation distribution after a change in strain path can result in transient changes in work-hardening behaviour of two kinds. Changes of the first kind, which tend to increase the hardening rate in early stages of the second mode of deformation, are associated with reorientation of internal stresses. Changes in the second kind, which tend to cause transient reductions in hardening rate, are believed to be associated with partial dissolution of the original dislocation substructure. The relative magnitudes and strain dependencies of these two kinds of change depend on the deformation sequence and on material variables. The change in hardening rate of CP aluminium after a change in strain path is dominated by changes of the second kind which, after moderate prestrains, cause reductions in the limits of stable elongation. In similar tests on overaged conditions of the 2014 alloy the overall changes in hardening rate are dominated by changes of the first kind, so that the limit of uniform elongation is increased by a change in strain path. When dynamic ageing is active in the 2014 alloy changes of the second kind can be suppressed so that reductions in the hardening rate do not occur.     

Electron microscopy study of the formation of Ni5Al3 in a Ni62.5Al37.5 B2 alloy—I. Precipitation and growth

The formation and growth of Ni₅Al₃ precipitates found inside an annealed B2 matrix in Ni_62.5Al_37.5 samples is described on the basis of conventional and high resolution electron microscopy images and electron diffraction patterns. Short anneals introduce three-pointed star shaped precipitates consisting of twin related segments with different variants of the Ni₅Al₃ structure. Longer anneals result in separate plates growing from these wings and developing microtwinning in order to accommodate stress built-up at the interfaces with the surrounding matrix. Two different growth mechanisms were observed and are discussed. The lattice distortions are found to be much higher in the small precipitates than in the large microtwinned plates.     

A Novel In Situ (Al3Ni + Al3Ti)/Al Composite Inoculant and Its Effects on the Microstructure,Damping and Mechanical Properties of Zn–Al Eutectoid Alloy

Metallurgical and Materials Transactions A - A novel in situ (Al3Ni + Al3Ti)/Al composite inoculant was fabricated, and its effects on the microstructure, damping and mechanical properties of the...     

The effect of temperature on strain rate sensitivity in an AlMgSi alloy

The effect of temperature on strain rate sensitivity has been studied in an AlMgSi alloy in the range of dynamic strain ageing. Two regimes of behaviour were observed which depended on the temperature. At low temperatures (T <- 303 K) it was possible to correlate all strain rate sensitivity measurements with a single strain/strain rate/temperature parameter. In the high temperature regime the temperature and strain rate dependence of the strain rate sensitivity deviated significantly from that measured at low temperatures. The two regimes of behaviour were found to correspond with the normal and inverse Portevin-Le Chatelier effect.     

Phase structure and hydrogen storage properties of REMg8.35Ni2.18Al0.21 （RE=La, Ce, Pr, and Nd） hydrogen storage alloys

REMg 8.35Ni2.18Al0.21 (RE=La, Ce, Pr, and Nd) alloys were prepared by induction melting and following annealing. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results showed that the alloys were composed of Mg2Ni, (La, Pr, Nd)Mg2Ni, (La, Ce)2Mg17 , (Ce, Pr, Nd)Mg12 and Ce2Ni7 phases. The above phases were disproportioned into Mg2NiH4 , MgH2 and REH x (x=2.51 or 3) phases in hydriding. CeH2.51 phase transformed into CeH2.29 phase in dehydriding, whereas LaH3 , PrH3 and NdH3 phases remained unchanged. The PrMg8.41Ni2.14Al0.20 alloy had the fastest hydriding kinetics and the highest dehydriding plateau pressure while the CeMg8.35Ni2.18Al0.21 alloy presented the best hydriding/dehydriding reversibility. The onset hydrogen desorption temperature of the CeMg8.35Ni2.18Al0.21 hydride decreased remarkably owing to the phase transformation between the CeH2.51 and the CeH2.29 .     

Anomalous coarsening behavior of small volume fractions of Ni3Al precipitates in binary NiAl alloys

The kinetics of coarsening of γ′ precipitates in binary NiAl alloys containing nominally 5.72, 5.74 and 5.78 wt% Al and aged at 630°C were investigated by transmission electron microscopy and magnetic analysis. The alloys were each pre-aged at a temperature just below its solvus prior to re-aging at 630°C. The volume fractions of γ′ were low between 0.02 and 0.03. the pre-aging treatment produced microstructures containing precipitates that nucleated heterogeneously on dislocations and grain boundaries, leaving empty matrix areas in which coherent γ′ precipitates subsequently nucleated and coarsened. Measurements were made only on these precipitates. The coarsening kinetics are anomalous in that the rate constant decrease with increasing volume fraction, in contradiction with all the theories of this process. Furthermore, the increase is quite large, exceeding a factor of four over a range of volume fractions that increased by less than 40%. Additionally, the rate constants exceed the values expected from the literature by factors of 10–40. The distributions of particles sizes were measured, and with few exceptions were found to agree with those of earlier investigations. It is suggested that elastic interactions among the γ′ precipitates play a pivotal role in decelerating the kinetics of coarsening, overpowering the expected accelerating effect of increasing volume fraction.     

Observations of plastic strain rate continuity in iron in and near the “athermal plateau”

The nature of dislocation slip in interstitial free Ti-gettered iron and type 304 stainless steel has been examined through tests for plastic strain rate continuity based on load relaxation experiments. Measurements have been made at temperatures from 77 to 773 K and as a function of the degree of strain hardening. The results for pure iron demonstrate that the plastic strain rate is continuous during an abrupt change in imposed stress rate under all of the conditions which were studied. These observations indicate that the rate of dislocation motion depends only on stress, temperature and internal structure. We conclude that slip is a thermally activated process, and that mechanical activation of strain is not observed in these experiments. The results are unchanged in the range of temperatures for which iron exhibits a so-called athermal plateau in yield stress versus temperature. The present findings indicate that it is not appropriate to associate this plateau with the level of the internal stress. Similar results for 304 stainless steel suggest that impurities in this alloy act as thermal obstacles to dislocation motion.     

Effects of trace ytterbium addition on microstructure,mechanical and thermal properties of hypoeutectic Al–5Ni alloy

Over the past decade, the cast aluminum alloys with excellent mechanical and conductivity properties have emerged as potential materials for thermal management. However, the traditional Al–Si based alloys are difficult to make significant breakthrough in conductivity performance. The hypoeutectic Al–5Ni alloy also possesses sound castability and is expected to be applied in thermal management applications. In this study, the effects of ytterbium (Yb element) at 0–0.5 wt% on the microstructures as well as the electrical/thermal conductivity and mechanical properties of the Al–5Ni alloy were systematically investigated. The experimental results indicate that the addition of Yb at a relatively low amount not only reduces the secondary dendrite arm spacing of the α-Al grains, but also modifies the morphology and distribution of eutectic boundary phase. Moreover, it is found that the dosage of Yb at 0.3 wt% in the Al–5Ni alloy can simultaneously improve the yield strength, ultimate tensile strength and electrical/thermal conductivity. The strengthening and toughening of the Al–5Ni alloy are mainly attributed to the decrease of secondary dendrite arm spacing and the improvement of eutectic phases. The transmission electron microscopy/selected area electron diffraction (TEM/SAED) analysis indicates that the ytterbium in Al–5Ni alloy will form Al₃Yb phase, which mainly agglomerates in the Al₃Ni phase region. This phase is helpful to decrease the solubility of impurity elements (e.g., Fe and Si) in the α-Al matrix, which is beneficial to electrical/thermal conductivity. The value of this study lays foundation for manufacturing Al–Ni alloys with high thermal conductivity and acceptable mechanical properties.     

Study on Degradation and Recovery of AB5—Type Hydrogen Storage Alloy Used in Ni—MH Batteries

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