EFFECT OF COMPOSITION ON HYDROGEN EMBRITTLEMENT IN Ni-Fe FCC ALLOYS

The ductility loss and threshold stress intensity,K_(IH)during hydrogen charging were measured for pure Ni and four Ni-Fe fcc alloys.The results show that ductility loss in 40Ni60Fe alloy and K_(IH)a 50Ni50Fe alloy have a minimum value.The variations of the amounts of hydride, hydrogen evolution and dislocation structure with composition have been investigated.The va- riation of hydrogen embrittlement susceptibility with composition measured by ductility loss and by K_(IH)or K_(IH)/K_C can be explained by means of the synthetical effects of amount of hydride,solutionized hydrogen and the extent of dislocation planarity on hydrogen embrittlement susceptibility.

Relation between Gamma Decomposition and Powder Formation of <i>γ</i>-U8Mo Nuclear Fuel Alloys via Hydrogen Embrittlement and Thermal Shock

Gamma uranium-molybdenum alloys have been considered as the fuel phase in plate type fuel elements for material and test reactors (MTR), due to their acceptable performance under irradiation. Regarding their usage as a dispersion phase in aluminum matrix, it is necessary to convert the as cast structure into powder, and one of the techniques considered for this purpose is the hydration-dehydration (HDH). This paper shows that, under specific conditions of heating and cooling, γ-UMo fragmentation occurs in a non-reactive predominant mechanism, as shown by the curves of hydrogen absorption/desorption as a function of time and temperature. Our focus was on the experimental results presented by the addition of 8% weight molybdenum. Following the production by induction melting, samples of the alloys were thermally treated under a constant flow of hydrogen for temperatures varying from 500°C to 600°C and for times of 0.5 to 4 h. It was observed that, even without a massive hydration-dehydration process, the alloys fragmented under specific conditions of thermal treatment during the thermal shock phase of the experiments. Also, it was observed that there was a relation between absorption and the rate of gamma decomposition or the gamma phase stability of the alloy.

Effect of Ordering on Embrittlement of Ni4Mo Alloy in Hydrogen Gas

The fracture behavior of disordered and ordered Ni4Mo alloy was investigated by tensile tests in hydrogen gas or during hydrogen charging. The results show that the ductility of the disordered alloy decreased slightly with the hydrogen pressure increasing, while that of the ordered alloy decreased rapidly with the hydrogen pressure increasing. However, the ductility of both disordered and ordered alloys reduced similarly seriously with the charging current density increasing. Therefore, the mechanism of order-induced embrittlement of Ni4 Mo alloy in hydrogen gas is supposed to be that atomic order accelerates the kinetics of the catalytic reaction for the dissociation of molecular H2 into atomic H.

Hydrogen embrittlement and failure mechanisms of multi-principal element alloys:A review

Multi-principal element alloys exhibit excellent physical,chemical and mechanical properties,and they are used as novel structural materials for potential applications in nuclear energy,hydrogen energy,and petrochemical fields.However,exposing components made of the alloys to service conditions related to the mentioned applications may induce hydrogen embrittlement(HE)as one of the typical failure mechanisms.In this review,we report and summarize the progress in understanding HE in multi-principal element alloys,with a particular focus on high-entropy alloys(HEAs).The review focuses on four aspects:(1)hydrogen migration behavior(hydrogen dissolution,hydrogen diffusion,and hydrogen traps);(2)factors affecting HE(hydrogen concentration,alloy elements and microstructure);(3)tensile mechanical properties in the presence of hydrogen and micro-damage HE mechanisms;(4)the design concept for preventing hydrogen-induced mechanical degradation.The differences in the HE behavior and failure mechanisms between HEAs and traditional alloys are compared and discussed.Moreover,specific research directions for further investigation of fundamental HE issues and a strategy for a simultaneous improvement in strength and HE resistance are identified.

Effects of laser powder bed fusion process parameters on microstructure and hydrogen embrittlement of high-entropy alloy

Hydrogen embrittlement behavior, micro-deformation, and crack propagation mechanism of CoCrFeNiMn high-entropy alloy (HEA) fabricated by laser powder bed fusion (LPBF) under different parameters were investigated by slow strain rate tensile tests (at room temperature) with/without electrochemical hydrogen pre-charging. The LPBF CoCrFeNiMn HEA shows excellent resistance to hydrogen embrittlement. Unsuitable LPBF parameters are accompanied by many microcracks and holes, resulting in a slight decrease in the hydrogen embrittlement resistance of the material. The electron backscatter diffraction (EBSD), electron channeling contrast image (ECCI) techniques, and transmission electron microscope (TEM) were carried out to research the main influencing factors of hydrogen on the deformation mechanism and crack propagation. Compared with un-charged samples, a larger number of deformation twins (DTs) appear in the deformation process of hydrogen-charged LPBF CoCrFeNiMn, attributing to the reduction of stacking fault energy (SFE) due to the ingress of hydrogen. The nano DTs and crossing twin system contribute to the extra work hardening, and a strain hardening platform is observed for all hydrogen-charged samples, resulting in the increase of strain hardening rate or the mitigation of the loss of strain hardening. Although unsuitable process parameters will trigger fabrication defects and reduce mechanical properties, the cellular structure can bring a hydrogen-induced strain hardening platform for LPBF CoCrFeNiMn to reduce the damage caused by hydrogen embrittlement.

INVESTIGATION OF HYDROGEN INDUCED DUCTILE BRITTLE TRANSITION IN 7175 ALUMINUM ALLOY

Effects of hydrogen on the mechanical properties of differently aged 7175 aluminum alloys were investigated by using cathodic H-permeation, slow strain rate tension and so on. The results indicate that both the yield stress and the percentage reduction of area decrease with increasing hydrogen charging time, and the degree of reduction decreases as aging time increases for the same hydrogen charging time.

A new dynamic method for measuring hydrogen partial pressure in molten aluminum alloy

Hydrogen partial pressure is an important parameter to calculate hydrogen concentration levels in molten aluminum alloy. A new dynamic method for measuring hydrogen partial pressure in molten aluminum alloy is studied. Dynamic and rapid measurement is realized through changing the volume of the vacuum chamber and calculating the pressure difference ΔP between the theoretical and measured pressures in the vacuum chamber. Positive ΔP indicates hydrogen transmits from melt to vacuum chamber and negative ΔP means the reverse. When ΔP is equal to zero, hydrogen transmitted from both sides reached a state of dynamical equilibrium and the pressure in the vacuum chamber is equal to the hydrogen partial pressure in the molten aluminum alloy. Compared with other existing measuring methods, the new method can significantly shorten the testing time and reduce measuring cost.

The Synergetic Effects of Hydrogen and Oxygen on the Strength and Ductility of Vanadium Alloys

A V4Ti alloy and several V4Cr4Ti alloys with different oxygen contents were studied on their tensile properties with the effect of hydrogen concentrations. The ductility of the alloys showed a successive decrease in a varied rate with an increased hydrogen concentration, while the ultimate tensile strength remained unchanged or even decreased for the high oxygen content alloy in spite of the occurrence of hardening in the low oxygen content alloy. Oxygen in the alloy causes grain boundary weakening, increasing the possibility of intergranular fractures and thus enhancing the hydrogen embrittlement. V4Ti showed a higher resistance to the hydrogen embrittlement as compared to the V4Cr4Ti alloys on a similar oxygen content level.

Hydrogen Content and Porosity Behavior of Hypereutectic Aluminum-silicon Alloy with Phosphorus

By making castings that pick up gas from moisture in red sand molds,the porosity generated at different cooling rates was discussed during solidification of hypereutectic Al-25%Si alloy without and with phosphorus additions. The effect of phosphorus addition on hydrogen content in the melt was also studied. It was observed that the phosphorus addition made hydrogen content in alloy melts present a “see-saw' tendency.In addition to primary silicon refinement,the phosphorus promoted gas porosity formed not only in slowly cooled sections, but also in rapidly cooled sections. There was a small difference in density of full dense sample between P-refined and unrefined castings, with a larger density associated with phosphorous addition. The change of the surface tension seemed more reasonable to explain the mechanism of porosity behavior.

Electrochemical investigation on the hydrogen permeation behavior of 7075-T6 Al alloy and its influence on stress corrosion cracking

The hydrogen permeation behavior and stress corrosion cracking （SCC） susceptibility of precharged 7075-T6 A1 alloy were inves- tigated in this paper. Devanthan-Stachurski （D-S） cell tests were used to measure the apparent hydrogen diffusivity and hydrogen permeation current density of specimens immersed in 3.5wt% NaCl solution. Electrochemical experiment results show that the SCC susceptibility is low during anodic polarization. Both corrosion pits and hydrogen-induced cracking are evident in scanning electron microscope images after the specimens have been charging for 24 h.

Influence of Elemental Iron on Hydrogen Content in Superheated Aluminum-iron Melts

The hydrogen content in liquid binary aluminum alloys with 1,3,5 and 8 wt% iron has been determined in the temperature range from 973K to 1103K.The hydrogen content in molten Al-Fe alloys increases remarkably when the temperature of the melt rises to about 1053K.This work indicates that the alloying element iron plays an important role in hydrogen content in superheated Al-Fe alloy melts below about 1053K.The results make it clear that the hydrogen content in the melt aluminum reduces with the increasing element levels.A conclusion is drawn that the degree of gassing in molten Al-Fe alloys is bound up with the properties of oxide film of aluminum alloy melts.The element iron has no effect on the compact structure of oxide film in aluminum melts.The effects of alloying element are theoretically analyzed in terms of Wagner interaction parameter.According to the values of the first order interaction parameter,it is concluded that the interaction between iron atom and aluminum is much stronger than that between hydrogen atom and aluminum,and the addition of the alloying element decreases the affinity of liquid aluminum for hydrogen.

Effect of atomic ordering on environmental embrittlement of (Co, Fe)_3V alloy in gaseous hydrogen

The diffusible hydrogen contents in precharged (Co,Fe) 3V alloy were measured. It is found that atomic ordering can not promote hydrogen penetration in the (Co,Fe) 3V alloy. The ultimate tensile strength (UTS) and ductilities in various condition were also investigated. The results show that the UTS and elongation of disordered alloy are higher than that of ordered one with fixed diffusible hydrogen content and (Co,Fe) 3V alloy with ordered structure is highly susceptible to the embrittlement in hydrogen gas. The factor which may affect the susceptibility to the embrittlement of (Co,Fe) 3V alloy in hydrogen gas is mainly due to that the atomic ordering may accelerate the kinetics of the catalytic reaction for the dissociation of molecular hydrogen into atomic hydrogen. However, it can not be roled out that atomic ordering intensifies planar slip and restricts cross slip at the grain boundaries and enhances the susceptibility of the alloy to hydrogen embrittlement.

X-RAY TOPOGRAPHIC STUDY OF HYDROGEN DAMAGE IN Fe-3 wt-% Si ALLOY

The formation and aggravation of hydrogen damage in Fe-3 wt-％ Si alloy during cathodic charging Were studied by means of Lange transmission X-ray topography.Results showed that hydrogen damage did not form in the specimens charged in 0.5 mol/L H_2S0_4 so- lution without poison,and occurred with addition of 250 mg/L As_2O_3 even at very low charge current density.As charging at a certain current density,the size of the damage may enlarge up to a limit yet no more by prolonged time.The damage size increased with increase of charging current density,but not so apparent at high current density. An external tensile stress could promote the formation of hydrogen damage obviously. It seems that for charging in H_2SO_4 solution containing poison,the drop of hydrogen permeation curve against time prolongation is due to the formation of hydrogen damages.

Effect of Preparation Process on Compressive Strength and Hydrogenation Performance of Raney-Ni/Al2O3 Catalyst

In this study, Raney-Ni/Al_2O_3 catalysts were prepared from precursors which were calcined in different atmosphere(argon and air) and leached for different time. XRD, SEM, H_2-TPR and BET techniques were introduced to characterize the catalysts, and hydrogenation of unsaturated hydrocarbons, including vinyltoluene, dicyclopentadiene and indene, was used as probe reaction to evaluate the catalytic hydrogenation performance of the catalysts. The results demonstrated that the calcination atmosphere had noticeable effects on the compressive strength and hydrogenation performance of Raney-Ni/Al_2O_3. The catalyst calcined in argon exhibited better compressive strength and catalytic performance than the one calcined in air. The compressive strength and activity stability of catalyst were also determined by the leaching time. A too longer leaching time was not conducive to improving the compressive strength and catalytic performance of catalysts. When the precursor calcined in argon was treated in a 15%(by wt.) sodium hydroxide solution at 343 K for 5 h, a catalyst with higher compressive strength(at 26.10 N/mm) was obtained and the conversion of vinyltoluene, dicyclopentadiene(DCPD), and indene could amount to more than 87.5%, 99% and even 100%, respectively, under the reaction conditions covering a temperature of 493 K, a hydrogen pressure of 2.0 MPa, a LSHV of 3.0 h^(-1), and a hydrogen/oil ratio of 200:1.

Effect of AlN coating on hydrogen permeability and surface structure of VT6 alloy by vacuum arc ion plating

We study the absorption of hydrogen of metal by the permeability method. With the help of the gas reaction controller(GRC), the absorptive capacity of hydrogen, which is a function of time, temperature and pressure, can be recorded. The effect of the performance of the hydrogen permeability of AlN coating on the titanium alloy surface structure is studied.In the research, the AlN is selected to be added to the titanium alloy sample VT6, and the properties of the titanium alloy are investigated, and the hydrogen absorption rate of the coating is calculated by performing the hydrogen saturation of the test sample. The results show that under 600℃ the AlN film reduces the hydrogen absorption rate of titanium alloy and improves the surface properties of VT6 alloy.

Hydrogen embrittlement behavior of Inconel 718 alloy at room temperature

Hydrogen embrittlement of Inconel 718 alloy was investigated. Multi-scale observation technique were employed, comprising slow strain rate tensile tests, scanning electron microscopy and transmission electron microscopy analysis. The results demonstrate that hydrogen charging deteriorates mechanical properties of the alloy. Inconel 718 alloy shows partial Portevin-Le Chatelier(PLC) effect at room temperature when hydrogen charging current density is 220 mA cm^(-2) and 590 mA cm^(-2). Moreover, plastic deformation features with dislocation cells are detected in hydrogen-induced brittle zone. Thus, it is concluded that dragging effect of hydrogen atoms on dislocations contributes to PLC effect.

Atomic study of hydrogen behaviors at∑3(111)grain boundary in equiatomic CoCrNi and CoCrNiFe alloys

High-entropy alloys are potential advanced structural materials for applications in nuclear energy due to their proved high irradiation performance.However,exposing components made of structure materials to service conditions under certain nuclear environments may induce hydrogen embrittlement(HE)as one of the typical failure mechanisms.In this work,we performed density functional theory(DFT)calculations to examine the role of specific element species in HE mechanism in CoCrNi and CoCrNiFe alloys.The solution energy,binding energy and diffusion barrier of H atoms at∑3 GBs(GBs is short for grain boundaries)are presented.Based on the DFT data,we found that Cr limits the H atom to a specific potential position,thus suppresses H segregation.The dipole moment interaction between H and Fe atoms weakens the binding of H atoms.The lattice distortion effect-induced trapping for H provides higher H diffusion barriers at∑3 GBs than that in pure Ni.

Atom probe tomographic observation of hydrogen trapping at carbides/ferrite interfaces for a high strength steel

A three-dimensional atom probe （3DAP） technique has been used to characterize the hydrogen dis- tribution on carbides for a high strength AISI 4140 steel. Direct evidence of H atoms trapped at the carbide/ferrite interfaces has been revealed by 3DAP mapping. Hydrogen is mainly trapped on car- bide/ferrite interfaces along the grain boundaries. Slow strain rate tensile （SSRT） testing shows that the AIS14140 steel is highly sensitive to hydrogen embrittlement. The corresponding ffactographic mor- phologies of hydrogen charged specimen exhibit brittle fracture feature. Combined with these results, it is proposed that the hydrogen trapping sites present in the grain boundaries are responsible for the hydrogen-induced intergranular fracture of AISI 4140. The direct observation of hydrogen distribution contributes to a better understanding of the mechanism of hydrogen embrittlement.

Effects of low melting point metals(Ga,In,Sn) on hydrolysis properties of aluminum alloys

Low melting point metals（Ga, In, Sn） as alloy elements were used to prepare Al-In-Sn and Al-Ga-In-Sn alloys through mechanical ball milling method. The effects of mass ratio of In to Sn and Ga content on the hydrolysis properties of aluminum alloys were investigated. X-ray diffraction（XRD） and scanning electron microscopy（SEM） with energy disperse spectroscopy（EDS） were used to analyze the compositions and morphologies of the obtained Al alloys. The results show that the phase compositions of Al-In-Sn ternary alloys are Al and two intermetallic compounds, In3 Sn and In Sn4. All Al-In-Sn ternary alloys exhibit poor hydrolysis activity at room temperature. Al-In-Sn alloy with the mass ratio of In to Sn equaling 1：4 has the highest hydrogen yield. After Ga is introduced to the ternary alloys, the hydrolysis activity of aluminum alloys at room temperature is greatly improved. It is speculated that the addition of Ga element promotes the formation of defects inside the Al alloys and Ga-In3Sn-In Sn4 eutectic alloys on the alloys surface. Al atoms can be dissolved in this eutectic phase and become the active spots during the hydrolysis process. The small size and uniform distribution of this eutectic phase may be responsible for the enhancement of hydrolysis activity.

Effects of hydrogen charging and deformation on tensile properties of a multi-component alloy for nuclear applications

In this study,the influence of hydrogen charging and deformation on the tensile behavior of a 60Fe-12Cr-10Mn-15Cu-3Mo multi-component alloy was investigated using electron microscopy and positron annihilation lifetime spectroscopy.The results show that hydrogen-induced vacancy clusters found in the electrochemically charged hydrogen specimens are responsible for crack initiation.Upon ingress to the microstructure,hydrogen promotes the formation of cell-structured dislocations that are beneficial for the improvement of tensile strength.In addition,hydrogen embrittlement can be mitigated by dislocations that can hinder hydrogen mobility in the deformed specimens.