Strain-induced Vacancies Research Articles

Molecular dynamics study on the effect of hydrogen on the plastic behavior of α-Fe crack tip

In this paper, the unilateral tension of α-Fe in the local region of the crack tip are simulated by molecular dynamics method, the plastic deformation behaviors and hydrogen embrittlement mechanism of the crack tip under different hydrogen concentration and hydrogen positions are analyzed. The results show that for the hydrogen-containing models, there exists a critical hydrogen concentration c0 that can make the plasticity of the α-Fe material reduce greatly. For the crack-free model, the main hydrogen embrittlement mechanism is adsorption-induced dislocation emission (AIDE), and the critical hydrogen concentration c0 is 0.3%. When the hydrogen concentration cH<c0, the plastic deformation mechanism is mainly dominated by phase transformation; when cH≥c0, the plastic deformation mechanism is phase transformation at low strains and dislocation motion at high strains. For the cracked model, the main mechanism of hydrogen embrittlement is hydrogen-enhanced local plasticity at the crack tip, with a critical c0 of 1.7%. When cH<c0, the plastic deformation mechanism of the model is a combination of phase transformation and dislocation; when cH≥c0, the plastic deformation mechanism is controlled by phase transformation. When the hydrogen is far away from the crack tip, the local mechanical behavior of the crack tip does not change significantly, while when hydrogen gathered at the crack tip, the addition of a small amount of hydrogen atoms will make the fracture strain decrease rapidly, which shows a mechanism of hydrogen-enhanced strain-induced vacancies (HESIV). A saturation value of hydrogen concentration at the crack tip is confirmed in this paper, when the local hydrogen concentration exceeds this saturation value, the plastic behaviour tends to be stable. The influence of hydrogen atoms distribution on the crack initiation behavior was determined by atomic configuration analysis.

Damage Accumulation and Recovery Involving Vacancy-Type Defects Enhanced by Hydrogen in Tempered Martensitic Steel Showing Quasi-Cleavage Fracture

Hydrogen embrittlement (HE) is increasingly becoming a critical issue for using high-strength steels in the automotive and infrastructure industries. To overcome the risk posed by HE of structural components under a hydrogen uptake environment in long-term service, it is necessary to clarify the mechanism of HE. In the present study, the presence of hydrogen-enhanced strain-induced vacancies (HESIVs)—one type of defect associated with proposed HE mechanisms—was validated by low-strain-rate tensile tests with in-situ electrochemical hydrogen charging for tempered martensitic steel showing quasi-cleavage fracture with a tensile strength. The effect HESIVs on the mechanical properties of tempered martensitic steel was also studied. The combined use of low-temperature thermal desorption spectroscopy and tensile tests led to the following observations: (i) hydrogen enhanced the accumulation of vacancy-type defects under plastic strain, (ii) accumulated vacancy-type defects adversely affected the ductility of the tempered martensitic steel after hydrogen release, and (iii) aging at 150 °C after applying a given plastic strain with hydrogen charging decreased the amount of newly formed vacancy-type defects and resulted in recovery of ductility.

A coupled diffusional-mechanical model accounting for hydrogen enhancements of strain-induced dislocations and vacancies

Informed by the current understanding of hydrogen-dislocations/vacancies interactions, a coupled diffusional-mechanical model accounting for hydrogen-enhanced strain-induced dislocations and vacancies is developed. This model is applied to capture hydrogen diffusion and trapping in the uniaxially tensioned column and type-I loaded crack specimens of nickel alloy, with special attentions paid to the influence of hydrogen-vacancies interaction on them, which is seldom discussed in previous studies. The results show that the hydrogen-enhanced hardening rate and the hydrogen-promoted vacancy enrichment for the nickel alloys observed in experiments can be addressed well by the present model. The strain-induced vacancies rather than dislocations dominate the hydrogen trapping behavior, due to their higher hydrogen binding energies for the nickel alloy. The hydrogen-saturated state of vacancies can heavily impact hydrogen diffusion/trapping, which could be characterized by the effective trap concentration. The supersaturated vacancies caused by hydrogen-enhanced strain-induced vacancies can promote void growth through vacancy condensation, thus accelerating plastic localization or interface rupture. For the type-I loaded blunt crack specimen, the hydrogen-enhanced dislocation multiplication contributes to intergranular cracking in the pressure valley, while hydrogen-enhanced vacancy generation facilitates void growth in the plastic zone, both of which can exacerbate plastic localization ahead of crack tip. A quantitative model for plasticity localization that accounts for the influences of hydrogen-induced IG cracking and void growth would be helpful in physically based modeling of hydrogen embrittlement.

Hydrogen embrittlement prompt fracture in Ni-based single crystal superalloy

Hydrogen-fueled and hydrogen-hybridized aircraft engines are a new trend in the aviation industry for environmental reasons. Single crystalline Ni-based superalloys are the most commonly used engine materials and their hydrogen embrittlement properties need urgent investigation. In this study, the hydrogen embrittlement behavior and underlying fracture mechanism of a second-generation Ni-based single crystal superalloy with electrochemical hydrogen pre-charge were investigated. The superalloy showed tremendous susceptibility to hydrogen embrittlement with reduced strength and ductility. A large number of micropores and cracks on the fracture surface are found in hydrogen-charged specimens, leading to embrittlement and ultimate cracking. More dislocations, stacking faults and DSBs are observed in specimens with hydrogen uptake. Hydrogen-induced micropores first form at the γ/γ′ interface and then propagate into the γ′ phase, leading to cracking, which was analyzed using in situ environmental studies with a transmission electron microscope. Hydrogen reduces the cohesive strength between the γ- and γ′-phase and accelerates crack propagation along the voids. Hydrogen embrittlement fracture in Ni-based single crystal superalloys is due to synergistic hydrogen-enhanced local plasticity, strain-induced vacancies and decohesion in the hydrogen-induced cracking process.

Hydrogen embrittlement fracture mechanism of 430 ferritic stainless steel: The significant role of carbides and dislocations

The effect of electrochemical hydrogen charging on the mechanical properties and the fracture mechanism of AISI430 ferritic stainless steel were studied by tensile test. Electrochemical hydrogenation experiments of 2, 4, 8 and 12 h on the tensile specimen with a current density of 50 mA/cm2. Hydrogen has a significant impact on the mechanical properties of the material: the elongation of the material is reduced from 27.8% to 13.0%, the yield strength is increased from 325 MPa to 380 MPa, and the tensile strength is increased from 500 MPa to 575 MPa. The obvious increase in tensile strength is attributed to two aspects: (1) the hydrogen-induced dislocations pinning; (2) the influence of the second phase carbide particles on the critical shear stress of dislocation glide. As the hydrogen outgassing time increases, the hardness measurement proves the softening effect of hydrogen. Since the reduction of the binding energy of the carbide and ferrite matrix interface is attributed to the HEDE mechanism, the crack nucleation in the vicinity of the carbide leads to the generation of the cleavage (C) region. Under low diffusible hydrogen content, the samples are mainly C mode and F-MVC mode guided by the synergistic effect of hydrogen-enhanced decohesion (HEDE), hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced strain-induced vacancies (HESIV) mechanisms and under high diffusible hydrogen content, the transgranular (TG) mode replaces the F-MVC mode, and the HEDE mechanism is more dominant than the HELP mechanism.

Modeling the Creep of Nickel

Abstract Many metals and alloys have a stress exponent for the creep rate that is considerably higher than the value of three that is typically predicted by creep recovery models. One example is pure Ni. Creep data from Norman and Duran that are analyzed in the paper give a stress exponent of about seven in the temperature range 0.3–0.55 of the melting point. It has recently been shown that the high creep exponent of Al and Cu in the power-law breakdown regime can be explained by the presence of strain-induced vacancies. By applying a creep recovery model that does not involve adjustable parameters, it is shown that strain-induced vacancies can also explain the high-stress exponent of pure nickel.

The effects of interstitial hydrogen and carbon atoms and aging temperature on annihilation behavior of hydrogen-enhanced strain-induced vacancies in iron

The effects of interstitial hydrogen and carbon atoms and aging temperature on the annihilation behavior of hydrogen-enhanced strain-induced vacancies (HESIVs) in iron were examined using low-temperature thermal desorption spectroscopy (L-TDS) that can raise the temperature from −200 °C. Specimens containing HESIVs were aged under various conditions and charged with tracer hydrogen for detecting lattice defects. Atmospheric aging at 30 °C reduced small vacancies such as single vacancies and divacancies, which were thermally unstable, with increasing aging time, however, clustered vacancies remained after 7 days. In contrast, aging at −196 °C hardly caused any vacancy diffusion, aggregation, or annihilation. Aging in the presence of hydrogen and carbon at 30 °C reduced the rate of vacancy annihilation and left more vacancies, respectively. These findings indicate that interstitial hydrogen and carbon atoms in iron suppress the diffusion and annihilation of vacancies, resulting in vacancy stabilization.

Quantities and distribution of strain-induced vacancies and dislocations enhanced by hydrogen in iron

The role of hydrogen in the formation behavior of various lattice defects induced by plastic strain in pure iron specimens has been investigated using low-temperature thermal desorption spectroscopy (L-TDS), which can start measurement from −200 °C. Specimens were subjected to various plastic strains within a uniform elongation range in the absence and presence of hydrogen. Strain-induced lattice defects were quantitatively evaluated using L-TDS for specimens saturated with hydrogen as a probe for detecting lattice defects, not only vacancies but also dislocation cores. The L-TDS spectra contained two hydrogen desorption peaks: a low-temperature peak associated with dislocations and a high-temperature peak associated with vacancies. The tracer hydrogen content corresponding to each peak increased with plastic strain. However, when compared at the same plastic strain of 25%, the low-temperature peak, i.e., dislocations, was not enhanced by hydrogen, whereas the high-temperature peak, i.e., vacancies, was enhanced by hydrogen, increasing by about six times at 25% plastic strain and by seven times at 40% plastic strain. Analyses conducted by electron channeling contrast imaging (ECCI) and electron backscattered diffraction (EBSD) revealed that strain was localized along the vicinity of the ferrite grain boundaries at 25% plastic strain in the presence of hydrogen. Hence, the role of hydrogen in the formation behavior of plastic strain-induced lattice defects is presumably to enhance vacancy formation and the localization of the dislocation configuration near the grain boundaries without affecting the amount of dislocations formed.

ДИФФУЗИОННОЕ ЗАРОЖДЕНИЕ ПОР В СТЫКАХ ЗЕРЕН СУБМИКРОКРИСТАЛЛИЧЕСКИХ МАТЕРИАЛОВ

The conditions of diffusional cavity nucleation in submicrocrystalline materials processed by the methods of intensive plastic deformation (equal-channel angular pressing, multiaxial forging, high pressure torsion, etc.) are analyzed. To date, the question of the mechanism of nucleation of cavities in such materials remains debatable due to the fact that the processing of materials by the methods of intensive plastic deformation is carried out at high hydrostatic pressures that prevent the appearance of pores. The possibility of diffusive nucleation of nanopores in the region of triple junctions of grains containing negative strain-induced wedge disclinations, generating high tensile stresses in the vicinity of triple junctions, comparable in magnitude to external hydrostatic pressure, is shown. Such junction disclinations inevitably occur at the grain junctions due to the heterogeneity of the plastic deformation through the ensemble of polycrystal grains. It is shown that an important condition for the nucleation of cavities is not only the presence of high internal tensile stresses from junction disclinations, but also an extremely high concentration of nonequilibrium strain-induced vacancies characteristic of submicrocrystalline metals, comparable in values to the vacancy concentration, at temperatures close to solidus. The influence of the strength of junction disclinations, the value of external hydrostatic pressure and the degree of supersaturation of the material by nonequilibriumstrain-induced vacancies on the rate of diffusional nucleation and the volume of critical pore nuclei is analyzed. It is established that in order to effectively suppress the process of pore formation in the grain boundary triple junctions, it is necessary to apply an external hydrostatic pressure that compensates for internal elastic fields from junction disclinations.

Numerical Interpretation of Hydrogen Thermal Desorption Spectra for Iron with Hydrogen-Enhanced Strain-Induced Vacancies

We obtained thermal desorption spectra of hydrogen for a small-size iron specimen to which strain was applied during charging with hydrogen atoms. In the spectra, a shoulder-shaped peak in the high-temperature side was enhanced compared with the spectra of the specimen to which only strain was applied. We also observed that the peak almost disappeared by aging processes at ≥ 373 K. Then, assuming that the shoulder-shaped peak results from hydrogen atoms released by vacancies, we simulated the thermal desorption spectra using a model incorporating the behavior of vacancies and vacancy clusters. The model considered up to vacancy cluster {{V_9}}, which is composed of nine vacancies, and employed the parameters based on atomistic calculations, including the H trapping energy of vacancies and vacancy clusters that we estimated using the molecular static calculation. As a result, we revealed that the model could, on the whole, reproduce the experimental spectra, except two characteristic differences, and also the dependence of the spectra on the aging temperature. By examining the cause of the differences, the possibilities that the diffusion of clusters of {V_2} and {V_3} is slower than the model and that vacancy clusters are generated by applying strain and H charging concurrently were indicated.

Analysis of Conditions for the Nucleation of Grain Boundary Nanopores in Submicrocrystalline Materials under Intensive Plastic Deformation

A model describing the nucleation of grain boundary nanopores in submicrocrystalline materials under intensive plastic deformation is proposed. The effect of internal stress fields from planar mesodefects, external hydrostatic pressure, and the degree of oversaturation with nonequilibrium strain-induced vacancies in a material on the pore nucleation rate is analyzed.

Critical Assessment 38: Assessment of the intrinsic susceptibility to hydrogen embrittlement for qualification of steels

ABSTRACT Exact simulations of engineering practice are often difficult to accomplish with test methods of materials performance against hydrogen embrittlement (HE). Instead, it is proposed that assessment of the intrinsic susceptibility to HE be adaptable to diverse usage environments and loading modes. This notion is based on recent findings concerning the predominant role of strain-induced vacancies and their involvement in characteristic features of HE such as microstructural effects and dependence on strain rates and temperatures. The function of hydrogen in enhancing the generation of strain-induced vacancies operates throughout the entire process of fracture, and the density of vacancies is detectable using hydrogen as a tracer. A method is proposed here for using as the parameter the difference in the amounts of tracer-hydrogen between specimens given cyclic stressing with and without hydrogen.

Review of Hydrogen Embrittlement in Metals: Hydrogen Diffusion, Hydrogen Characterization, Hydrogen Embrittlement Mechanism and Prevention

Hydrogen dissolved in metals as a result of internal and external hydrogen can affect the mechanical properties of the metals, principally through the interactions between hydrogen and material defects. Multiple phenomena such as hydrogen dissolution, hydrogen diffusion, hydrogen redistribution and hydrogen interactions with vacancies, dislocations, grain boundaries and other phase interfaces are involved in this process. Consequently, several hydrogen embrittlement (HE) mechanisms have been successively proposed to explain the HE phenomena, with the hydrogen-enhanced decohesion mechanism, hydrogen-enhanced localized plasticity mechanism and hydrogen-enhanced strain-induced vacancies being some of the most important. Additionally, to reduce the risk of HE for engineering structural materials in service, surface treatments and microstructural optimization of the alloys have been suggested. In this review, we report on the progress of the studies on HE in metals, with a particular focus on steels. It focuses on four aspects: (1) hydrogen diffusion behavior; (2) hydrogen characterization methods; (3) HE mechanisms; and (4) the prevention of HE. The strengths and weaknesses of the current HE mechanisms and HE prevention methods are discussed, and specific research directions for further investigation of fundamental HE mechanisms and methods for preventing HE failure are identified.

Improved Photocatalytic Hydrogen Evolution on Tantalate Perovskites CsTaO3 and LiTaO3 by Strain-Induced Vacancies

Tantalate perovskites are potential candidates for photocatalytic hydrogen production without cocatalyst addition; however, these oxides have large bandgaps, which result in their low photocatalyti...

Environmental Degradation Effect of High-Temperature Water and Hydrogen on the Fracture Behavior of Low-Alloy Reactor Pressure Vessel Steels

Structural integrity of reactor pressure vessel (RPV) in light water reactors (LWR) is of highest importance regarding operation safety and lifetime. The fracture behaviour of low-alloy RPV steels with different dynamic strain aging (DSA) & environmental assisted cracking (EAC) susceptibilities in simulated LWR environments was evaluated by elastic plastic fracture mechanics tests (EPFM) and by metallo- and fractographic post-test analysis. Exposure to high temperature water (HTW) environments at LWR temperatures revealed only moderated reductions in the fracture initiation and tearing resistance of low alloy RPV steels with high DSA or EAC susceptibility, accompanied with a moderate, but clear change in fracture morphology, which indicates the potential synergies of hydrogen/HTW embrittlement with DSA and EAC under suitable conditions. The most pronounced degradation effects occurred in a) RPV steels with high DSA susceptibility, where the fracture initiation and tearing resistance reduction increased with decreasing loading rate and were most pronounced in hydrogenated HTW and b) high sulphur steels with high EAC susceptibility in aggressive occluded crevice environment and with preceding fast EAC crack growth in oxygenated HTW. The moderate effects are due to the low hydrogen availability in HTW together with high density of fine-dispersed hydrogen traps in RPV steels. Stable ductile transgranular tearing by microvoid coalescence was the dominant failure mechanism in all environments with additional varying few % of secondary cracks, macrovoids and quasi-cleavage in HTW. The observed behavior suggests a combination of plastic strain localisation by the Hydrogen-enhanced Local Plasticity (HELP) mechanism, in synergy with DSA, and Hydrogen-enhanced Strain-induced Vacancies (HESIV) mechanism with additional minor contributions of Hydrogen-enhanced Decohesion Embrittlement (HEDE) mechanism.

Simulation of the Elastoplastic Behavior of Grade-4 Ti in the ECAP-C Process

Amodel of the elastoplastic behavior of commercial titanium is developed with allowance for the evolution of the microstructure of Ti Grade-4 Ti under equal channel angular pressing with application of the Conform process. The evolution of the mean dislocation density, of the dislocation density in the body and on the boundaries of grains, and of the density of mobile dislocations is studied. The concentrations of strain-induced vacancies, the speeds of gliding dislocations, and the angles of off-orientation between neighbor grains are determined, which widens substantially the understanding of the mechanisms of the elastoplastic behavior of Ti Grade-4 under severe plastic deformation.

Quantitative measurements of dynamic precipitation during fatigue of an Al–Zn–Mg–(Cu) alloy using small-angle X-ray scattering

The room temperature cyclic deformation behaviour of an Al–Zn–Mg–(Cu) alloy was examined in the under-aged and peak-aged states. The tests were performed under constant plastic strain amplitude loading in tension–compression mode, and the effect of cumulative strain (number of cycles), strain rate and plastic strain amplitude was examined. Pronounced cyclic hardening was observed in the under-aged states, and characterization of the microstructure using small-angle X-ray scattering illustrated that this was due to profuse dynamic precipitation at room temperature during cycling. Regardless of deformation conditions, the radius of the dynamically formed GP zones remained constant at 4–5Å, and only the volume fraction of the zones increased during cycling. The strain, strain rate and plastic strain amplitude dependence of the evolution of the volume fraction of zones forming dynamically was interpreted in terms of a nucleation-dominated reaction, and it is shown that the behaviour can be satisfactorily rationalized when the role of strain-induced vacancies is considered.

Hydrogen embrittlement of austenitic stainless steels revealed by deformation microstructures and strain-induced creation of vacancies

Hydrogen embrittlement of austenitic stainless steels has been examined with respect to deformation microstructures and lattice defects created during plastic deformation. Two types of austenitic stainless steels, SUS 304 and SUS 316L, uniformly hydrogen-precharged to 30 mass ppm in a high-pressure hydrogen environment, are subjected to tensile straining at room temperature. A substantial reduction of tensile ductility appears in hydrogen-charged SUS 304 and the onset of fracture is likely due to plastic instability. Fractographic features show involvement of plasticity throughout the crack path, implying the degradation of the austenitic phase. Electron backscatter diffraction analyses revealed prominent strain localization enhanced by hydrogen in SUS 304. Deformation microstructures of hydrogen-charged SUS 304 were characterized by the formation of high densities of fine stacking faults and ε-martensite, while tangled dislocations prevailed in SUS 316L. Positron lifetime measurements have revealed for the first time hydrogen-enhanced creation of strain-induced vacancies rather than dislocations in the austenitic phase and more clustering of vacancies in SUS 304 than in SUS 316L. Embrittlement and its mechanism are ascribed to the decrease in stacking fault energies resulting in strain localization and hydrogen-enhanced creation of strain-induced vacancies, leading to premature fracture in a similar way to that proposed for ferritic steels.

Enhanced susceptibility to delayed fracture in pre-fatigued martensitic steel

Interaction between fatigue damage and hydrogen is the concern of the present study. The susceptibility of a high-strength martensitic steel to delayed fracture has been examined using as-heat-treated and pre-fatigued specimens. The pre-fatigued specimens showed a tendency to fail earlier, but annealing the pre-fatigued specimens at 200°C recovered nearly all of the delayed fracture life. Production of point defects during fatigue was detected by means of hydrogen thermal desorption analysis (TDA), using hydrogen as a probe of defects. Hydrogen absorption capacity increased in fatigued specimens, but it was reduced to the level of the as-heat-treated specimens when fatigued specimens were annealed at 200°C, implying that increased hydrogen-trapping defects, presumably vacancies, were produced during fatigue. Hydrogen TDA peak profiles showed alterations that imply agglomeration of vacancies associated with an increase in fatigue cycles. The involvement of vacancies created during fatigue in the enhanced delayed fracture is consistent with a model that proposes strain-induced vacancies and their agglomeration are the primary mechanism of hydrogen-related failure.

Formation of vacancy clusters in deformed thin films of Al–Mg and Al–Cu dilute alloys

In the present study, vacancy clusters in elongated Al–Mg and Al–Cu thin films (Mg/Cu concentration=0.05–1.70 at.%) were examined by electron microscopy. No dislocations were observed in these films. In Al–Mg thin films deformed at room temperature, a large number of stacking fault tetrahedra (sft) were observed alongside a few vacancy loops. The opposite was true for Al–Cu thin films, where well-grown loops predominated, and only a few sft were observed. The Al–Cu film results show that the majority of vacancies form loops larger than sft. We also deformed Al–0.05at.% (Mg or Cu) alloys in liquid nitrogen and cold-transferred to an electron microscope. In Al–Mg, a large number of dotted defects (possibly sft) were observed, while very few such defects were observed in Al–Cu. This indicates that loops observed in Al–Cu thin films deformed at room temperature, grew during/after deformation. The likely contribution of strain-induced vacancies in deformed Al thin films to the voiding in VLSI interconnect wires due to electro-migration were discussed.