Non-hydrogenated Alloys Research Articles

Effect of hydrogen on the fretting wear mechanism of a high Nb-TiAl alloy

In this paper, the effect of hydrogen placement on the microdynamic wear mechanism of high niobium-titanium-aluminum alloys is investigated. Corresponding decreases and increases in loading force and displacement amplitude cause the microslip behavior of the alloy to change from partial slip to mixed slip. Slip type in mixed fire complete slip when you, the friction coefficient fluctuates. The average friction coefficient of hydrogen-placed alloys is small compared with that of non-hydrogen-placed alloys, the maximum wear marks are not obvious, and the oxidative wear is weak. The main wear mechanisms of non-hydrogenated alloys are adhesive wear, abrasive wear and oxidative wear. The main wear mechanisms of hydrogen-placed alloys are adhesive wear and abrasive wear.

Interface microstructure evolution and bonding mechanism of low-temperature diffusion bonding of hydrogenated Zr-4 alloy

Zr-4 alloy is widely used as fuel cladding in the nuclear industry. And welding is one of the most significant procedures during the manufacturing of fuel assembly. Bonding at high temperature risks safe operation of nuclear reactors because high temperature deteriorates the properties of Zr-4 alloy. Herein, Zr-4 alloy was hydrogenated to a hydrogen content of 0.2 wt.% before diffusion bonding. Direct diffusion bonding of hydrogenated Zr-4 alloy was performed at various bonding temperatures. The microstructural evolution of the bonding interface and the interface bonding mechanism were systematically investigated. The bonding temperature of the hydrogenated Zr-4 alloy was reduced by 150 °C compared with the corresponding nonhydrogenated alloy. Furthermore, the shear strength of the joints was as high as 257 MPa (4.6 × higher than corresponding joints comprised of the nonhydrogenated alloy) at a bonding temperature of 700 °C. Transmission electron microscopy and electron backscatter diffraction indicate that plastic deformation of the joints increased because of a phase transition of the hydrides (to the β-Zr phase) during bonding, leading to high-density dislocations that accumulated at the bonding interface. The migration of the interface grain boundary was facilitated by high-density dislocations of sufficiently high energy. The bonding interface was no longer evident, enabled by dynamic recrystallization induced by migration of the interface grain boundary. Furthermore, the fracture mode of the joints was initially brittle but became ductile/brittle mixed mode with increasing temperature.

Specific Features of Deformation of the Nitinol Alloy After Electrolytic Hydrogenation

Specific features of the effect of hydrogenation on the susceptibility of a Ni–Ti alloy with shape memory to deformation are determined with the use of metallographic, electrochemical, and mechanical studies. Three sections are detected in the tensile curves of the specimens of nickel–titanium alloy in the initial state. The first section is linear due to the elastic deformation of the alloy with initial austenitic structure. The second section is nonlinear and associated with pseudoelastic structural transformations of the original austenitic structure into a martensitic structure. The third section is also linear and caused by the elastic deformation of martensite formed in the course of deformation of austenite. After hydrogenation of the Ni–Ti alloy, the pseudoelastic structural transformation starts at a somewhat lower level of stresses than without hydrogenation. In this case, the specimens are destroyed after the termination of this transformation for a much lower level of plasticity than in the nonhydrogenated alloy. It is assumed that the electrolytic hydrogenation of the alloy promotes the formation of a very brittle hydride phase on the surface of Ti-type inclusions revealed in the structure of alloy in the initial state. Its thickness is determined by the duration of the process of hydrogenation rather than by the current used for hydrogenation.

Significant change of local atomic configurations at surface of reduced activation Eurofer steels induced by hydrogenation treatments

Reduced-activation steels such as Eurofer alloys are candidates for supporting plasma facing components in tokamak-like nuclear fusion reactors. In order to investigate the impact of hydrogen/deuterium insertion in their crystalline lattice, annealing treatments in hydrogen atmosphere have been applied on Eurofer slabs. The resulting samples have been analyzed with respect to local structure and atomic configuration both before and after successive annealing treatments, by X-ray diffractometry (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and conversion electron Mössbauer spectroscopy (CEMS). The corroborated data point out for a bcc type structure of the non-hydrogenated alloy, with an average alloy composition approaching Fe0.9Cr0.1 along a depth of about 100nm. EDS elemental maps do not indicate surface inhomogeneities in concentration whereas the Mössbauer spectra prove significant deviations from a homogeneous alloying. The hydrogenation increases the expulsion of the Cr atoms toward the surface layer and decreases their oxidation, with considerable influence on the surface properties of the steel. The hydrogenation treatment is therefore proposed as a potential alternative for a convenient engineering of the surface of different Fe-Cr based alloys.

Local structure in Ti–Hf–Ni metallic glasses and its evolution with hydrogenation

The effect of hydrogenation on the local structure of aTi25Hf50Ni25 amorphous alloy was studied by extended x-ray absorption fine structure (EXAFS)measurements. The samples were loaded to different hydrogen-to-metal ratios, H/M, 1.2 and1.4. For the non-hydrogenated alloy, the local structure was found to be different from thatin the crystalline Ti–Hf–Ni phase. A new icosahedral ordering, similar to that in theTi2Ni crystal, is identified around the nickel atoms. Upon hydrogenation, little change is observedin the environment of the nickel atoms. As in the 3/2 phase and in the Ti–Zr–Ni alloys, theperturbation of the local structure with hydrogenation concerns mainly the environments ofhafnium (zirconium) and titanium atoms. This indicates that the hydrogen atoms sitpreferentially near hafnium (zirconium) and titanium neighbours. Moreover, a drasticdecrease of the disorder parameter for the Hf–Ti correlation could be explained by astiffening of the structure of the glass, possibly due to the formation of a hydrogen network.

An extended x-ray absorption fine structure study of hydrogen storage in Ti–Hf–Nialloys

The effect of hydrogenation on the local structure of aTi40Hf40Ni20 crystalline alloy was studied by means of the extended x-ray absorption finestructure. The samples were loaded to different hydrogen-to-metal ratios,H/M, between1.2 and 1.7. The desorption process was also investigated by studying the local order in an alloy chargedto H/M = 1.85 thendesorbed to H/M = 0.8.For the non-hydrogenated alloys, the local order was very similar in Ti–Hf–Ni andTi–Zr–Ni alloys, quasicrystal or crystal. With increasing hydrogen concentration,all of the mean interatomic distances increased, except for the Ni–Hf (Hf–Ni)ones, which show an anomalous behaviour, decreasing with increasingH/M.This leads to a remarkable inversion of the atomic subshells oftitanium and nickel in the first environment of hafnium atoms aroundH/M = 1.2.The increase of distances is a maximum for the Ti–Ti and the Ti–Hf correlations, which isconsistent with the hydrogen atoms sitting preferentially near titanium and hafnium atoms.Similar conclusions were obtained previously from a study of Ti–Zr–Ni alloys.

The Evolution of Short-Range Order in Ti-Hf-Ni Alloys with Hydrogenation

ABSTRACTThe evolution of the short-range order in a Ti-Hf-Ni 3/2 approximant phase, has been studied as a function of hydrogenation by means of extended X-ray absorption fine structure (EXAFS). The local structure of the non-hydrogenated alloy has been derived from that of 1/1 Ti-Zr-Ni. After hydrogenation, drastic changes are observed in the local structure of Ti-Hf-Ni. Preliminary results of the EXAFS analysis are also presented for an amorphous Ti-Hf-Ni alloy.

Extended X-ray absorption fine-structure study of the hydrogen location in a Ti–Zr–Ni icosahedral quasicrystal and a 1/1 bcc crystal approximant

Abstract Extended X-ray absorption fine-structure measurements have been performed on Ti-Zr-Ni alloys using the Dispositif de Collision dans l'Igloo synchrotron radiation facility. Both a multigrained icosahedral quasicrystal and a 1/1 bcc polycrystalline approximant were studied. Alloys hydrogenated to a hydrogento-metal ratio of 1.7 for the icosahedral phase and 1.4 for the crystalline phase were also studied. The local atomic structures, which are different around the different types of atom, titanium, zirconium and nickel, are very similar in the non-hydrogenated alloys, crystal or quasicrystal. Drastic changes are observed in the local order in both hydrogenated alloys with modifications of distances and an increase in the disorder. Moreover, these modifications are slightly different in the 1/1 and icosahedral alloys after hydrogenation.

Calculation of the densities of states of a-Si 1 − yC y(:H) alloys and comparison with measured X-ray spectra

A simplified model based on the Tight-Binding Coherent-Potential Approximation method that does not account for topological disorder, is employed to calculate the s and p-symmetry densities of states of a-Si 1 − y C y (:H) alloys and crystalline SiC. Valence and conduction partial Si and C densities of states are obtained which allow us to observe that sp 3 hybridization is existing in these compounds. The calculations also show that in a-Si 1 − y C y alloys, the total band gap increases by about 0.7 eV when going from y=0.1 to y=0.287 due to a progressive shift of both valence and conduction edges below and above the top of the valence band, respectively. For crystalline SiC, the gap increases by about 1.5 eV with respect for example to a-Si 0.9C 0.1. In hydrogenated alloys, the calculations display a steepening of the valence and conduction band edges as compared to non-hydrogenated alloys confirming thus the `healing' effect with H incorporation. The soft X-ray Si Kβ emission spectra are simulated by convolution of the occupied Si 3p distribution with appropriate Lorentzian and Gaussian functions which account for the lifetime of the inner level involved in the X-ray transition and the instrumental function, respectively. The calculated spectra are in good agreement with experimental results on samples of similar nominal composition despite the fact that the theoretical model does not account for topological disorder.

Effect of hydrogen on the low-temperature mechanical properties of V-5 at % Ti alloys

The effects of grain size and hydrogen in solid solution or as hydrides on the strength and ductility of V-5 at % Ti was studied over the temperature range 15–448 K. Comparison of the strength and ductility characteristics of hydrogenated alloys where hydrides were not observed down to 78 K (1.8 and 1.9 at % H alloys) or where hydrides were observed to form near 230 K (3.8 and 3.9 at % H alloys) indicated that the presence of hydride precipitates had no apparent influence on the strength or ductility characteristics. It appears that the main consequence of hydride precipitation is that hydrogen is removed from solid solution making strengthening less effective than expected based on the total hydrogen content. Decreasing grain size from 31 μm to 8 μm had no apparent effect on ductility in the nonhydrogenated alloys (< 0.05 at % H) but it did increase the strength over most of the temperature range and especially at 15 K. In the hydrogenated alloys this decrease in grain size lowered the transition temperature about 10 K and it appreciably increased the degree of ductility return at 78 K and below. The ductility return below 78 K peaked near 50 K before decreasing below 30 K with the improvement in ductility being greatest in the alloys with the lower hydrogen contents.

Hydrogen embrittlement of MFR candidate vanadium alloys

Pure vanadium, V-3Ti-1Si and V-15Cr-5Ti alloys were charged with 0.3 at% H, and tested in tension between 77 and 520 K. The fracture surfaces of specimens were examined by SEM. Both non-hydrogenated alloys were appreciably solution-strengthened, but had good ductility even at 77 K. The specimen hydrogenated with 0.3 at% H exhibited a precipitous ductility loss at 150 K for V-3Ti-1Si, at 200 K for pure V and at 300 K for V-15Cr-5Ti. The V-3Ti-1Si alloy has a greater resistance to hydrogen embrittlement than pure vanadium. On the other hand, the V-15Cr-5Ti alloy was most severely deteriorated by hydrogen and always fractured in a brittle mode at temperatures lower than 300 K. Hydrogen embrittlement in both alloys was discussed in terms of the inhomogeneity of deformation on yielding and the solute effect on the diffusion coefficient and solubility of hydrogen.

The effect of hydrogen on the strength and ductility of V-15at.%Cr-5at.%Ti

Abstract The effect of hydrogen in solid solution on the strength and ductility of a V-15at.%Cr-5at.%Ti alloy was investigated from 78 to 450 K. The non-hydrogenated alloy exhibited a ductile-to-brittle transition at a low temperature (210 K) which scanning Auger microprobe analyses indicated was due to segregation of sulfur and titanium to the grain boundaries. The addition of only 0.1 at.% H raised the ductile-to-brittle transition temperature above room temperature. Hydrogen produced little, if any, strengthening in this alloy. A comparison of hydrogen effects on ductility and strengthening between this alloy and V-20at.%Ti and V-20at.%Cr is presented.

Neutron-diffraction study of hydrogenated and deuterated CuTi amorphous alloys

Neutron-diffraction experiments have been performed on amorphous CuxTi1-x (x=0.67,0.50,0.35) alloys charged either with hydrogen or deuterium. The variation of the interference functions and pair-correlation functions have been studied as a function of copper and hydrogen or deuterium concentration. The results obtained of non-hydrogenated alloys confirm the existence of chemical order. First-neighbour distances are determined and can be well compared with those in corresponding crystalline phases. The good contrast between copper and titanium on the one hand and between hydrogen and deuterium on the other allows one to identify individual metal-hydrogen correlations. Their positions in the total pair-correlation functions are very close to those found in crystalline hydrides and show that Ti3 tetrahedra are preferred hydrogenation sites in the concentration range studied.

Crystallisation and hydrogen absorption in amorphous Cu60Zr40and Cu50Zr50

The crystallisation processes in as-quenched and hydrogenated amorphous alloys of composition Cu100-xZrx (x=40 and 50) have been studied using differential thermal analysis and X-ray diffraction measurements. The as-quenched samples with x=40 and 50 yield crystallisation temperatures of 761 and 716K, respectively. X-ray diffraction measurements of fully crystallised samples show that crystallisation products are Cu10Zr7 from Cu60Zr40, and both Co10Zr7 and CuZr2 from Cu50Zr50. Differential thermal analysis scans of electrolytically hydrogenated samples show four features: (1) a small exothermic peak at 460K; (2) a large exothermic peak at approximately 550K; (3) a cluster of exothermic peaks beginning at 710-748K; (4) a large endothermic peak at approximately 1050K. X-ray diffraction measurements show that the features may be identified as (1) relaxation of the amorphous structure, (2) partial crystallisation into ZrH2 and Cu, (3) complete crystallisation of the remaining amorphous phase into ZrH2 and Cu and (4) breakdown of the hydride and formation of crystalline products as found in the non-hydrogenated alloys.

Crystallisation of as-quenched and hydrogenated amorphous Cu100-xTixalloys

Amorphous alloys of the composition Cu100-xTix (x=40, 50, 60) were prepared by rapid quenching from the melt. The crystallisation processes in these alloys were characterised by differential thermal analysis (DTA) measurements. The alloy with x=50 shows a single crystallisation process and forms the single crystalline compound CuTi. Alloys with x=40 and x=60 both show two crystallisation processes and precipitate two crystalline compounds, CuTi and Cu3Ti for x=40, and CuTi and CuTi2 for x=60. DTA measurements for electrolytically hydrogenated alloys all show an exothermic peak at approximately 400K, a cluster of several exothermic peaks beginning at around 700K and a large endothermic peak at approximately 950K. X-ray measurements of annealed samples indicated that the lowest-temperature peak results from a decrease in volume of the amorphous structure but no formation of crystalline phases. The cluster of peaks at around 700K results from the formation of Cu and TiH2 and the endothermic peak results from the breakdown of the hydride and the formation of resulting crystalline products similar to those of the non-hydrogenated alloys.