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Multiscale Assessment of Deformation Induced by Hydrogen Environment-Assisted Cracking in a Peak-Aged Ni-Cu Superalloy
Abstract
A multiscale approach leveraging electron backscatter diffraction (S-EBSD), high resolution EBSD (HR-EBSD), and transmission electron microscopy (TEM) was employed to assess the deformation induced proximate to the crack path by hydrogen environment-assisted cracking (HEAC) in peak-aged Monel K-500. Kernel average misorientation (KAM) results calculated from S-EBSD indicate that the deformation pertinent to HEAC is localized to within 25 μm of the crack path. Geometrically necessary dislocation (GND) density maps calculated from HR-EBSD confirm this localization. The evaluation of the deformation distribution in three separate grains along the crack path using complementary HR-EBSD and TEM suggest a qualitative similarity in dislocation density between the two techniques, though HR-EBSD is unable to spatially resolve the fine dislocation structures observed via TEM. However, non-negligible differences in dislocation patterning were observed in the three evaluated grains, highlighting the importance of a multiscale approach for characterizing deformation to understand the governing microstructural and mechanical factors.
... Meanwhile, thanks to cutting-edge techniques such as thin foil extraction by Focussed Ion Beam (FIB) milling for the analysis of deformation substructures beneath the fracture surface, the interpretations about environment effect on the plastic activity at the crack-tip can be revisited. For instance, Harris et al. [13], using Kernel Average Misorientation (KAM) and Geometrically-Necessary Dislocation (GND) mapping with Electron Back Scattered Diffraction (EBSD), have recently evidenced during hydrogen environment-assisted cracking in a Monel K-500 alloy plastic strains highly localized within a distance of 25 µm of the crack path and characterized by a high dislocation density. ...
... Nevertheless, it was noticed that a higher ERP value induces a reduction in the thickness of the cell walls and slip band thickness too, which again supports a lower plastic strain accumulation at the crack-tip during environmentally assisted fatigue crack growth. Harris et al. [13] observed a similar influence on dislocations substructures by means of KAM analysis on foils extracted from fracture surface in a peak-aged Ni-Cu superalloy. Based on Fig. 7 to 9,the higher strain observed at the crack-tip is very localized in the case where hydrogen assists crack propagation in a 7075-T651 alloy. ...
... This finding is consistent with the conclusion of Bowles about the contribution of hydrogen lattice diffusion [28]. Harris et al. [13] report however a penetration depth which is about one order of magnitude higher in the case of a Ni-Cu precipitation-hardened alloy. ...
... It is possible that the more uniform grain structure suppresses localization of deformation, further hindering the onset of SIM and improving the hydrogen resistance. This would be consistent with the need for higher plastic strains in the HIP condition to obtain the same '-martensite fraction present in the ANN material, but targeted near-crack characterization efforts are necessary to confirm this hypothesis [73,74]. ...
... Cartographies de la texture EBSD-TC (Texture Component) (a) de la lame sollicitée avec un ERP de 2,1x10 -5 Paxs et (b) une lame avec un ERP de 93600 PaxsHarris et al.[173] ont aussi souligné, via des analyses KAM, que la déformation est très localisée dans le cas où l'hydrogène assiste la propagation de fissures dans un alliage 7075-T651, ce qui plaide, là encore, en faveur d'un processus de type FPH pour un ERP de 93600 Pa x s. Ces observations soulèvent la question des rôles respectifs de la plasticité et de l'environnement, voire plus probablement dans leur interaction, dans la différence avérée de rotation du réseau, voire d'une recristallisation en pointe de fissure pour ces deux conditions d'ERP très élevé et très faible.En confrontant les mesures d'interstries avec les analyses de texture menées sur ces lames, comme illustré enFigure V. 27, on constate dans le cas d'un ERP de 93600 Pa x s une rotation du réseau très importante qui se produit systématiquement au niveau de la pointe de strie et dont la périodicité coïncide de ce fait avec l'interstrie (Figure V. 27(b)). ...
Dans une quête de recherche d’amélioration du niveau de confiance dans la prédiction des durées de vie de pièces d’aéronefs sujettes à la fatigue, la compréhension des mécanismes induits par les interactions environnement-fatigue est fondamentale. Ce projet de thèse cible l’étude des effets des environnements représentatifs des conditions de vols en altitude et au sol sur la tenue en fatigue d’un alliage d’aluminium particulièrement utilisé pour les pièces de renforts du fuselage. La compréhension de l’effet de la température, de l’humidité et des basses fréquences sur la durée de vie en fatigue mais surtout sur la propagation de fissures d’un alliage 7175 T7351 est donc indispensable pour améliorer la prédictibilité de pièces soumises à des sollicitations de fatigue sous divers environnements et temps d’exposition. La première partie de ces travaux consiste, à partir de résultats macroscopiques d’endurance et de propagation de fissure, d’établir un ordre de prédominance de chacun des paramètres sur la durée de vie totale ainsi que sur la résistance à la propagation. Il s’avère que ce matériau à l’état sur-revenu ne présente pas de sensibilité significative de la durée de vie totale du matériau aux divers environnements explorés. En outre la diminution de la fréquence entraine une augmentation des cinétiques de propagation, alors que la diminution importante de l’humidité entraine une augmentation de la résistance à la propagation. Quel que soit l’environnement considéré, d’inerte à air très humide, le mode de propagation correspond systémiquement au stade II avec un mode de rupture de type quasi clivage. Les analyses de faciès de rupture révèlent néanmoins un effet de l’environnement sur le marquage des stries, clairement visibles sous air humide alors qu’elles présentent un aspect « froissé » et sont difficilement détectables sous environnement inerte. Par ailleurs, une augmentation de l’humidité dans l’air et l’abaissement de la fréquence jusqu’à 10-4Hz conduisent à des cinétiques de fissuration globalement similaires. Néanmoins, cette même diminution de fréquence sous air sec serait probablement responsable d’une diminution importante de la résistance à la propagation par rapport à ce qui est observé sous air humide. Des analyses complémentaires ont été menées notamment pour évaluer l’effet de l’exposition à l’environnement sur l’activité plastique en pointe de fissure et estimer la quantité d’hydrogène piégé. La confrontation de ces résultats met en relation une augmentation d’hydrogène piégé avec importante rotation du réseau et particulièrement localisée en pointe de fissure dans un cas de taux d’exposition élevé à l’air humide. Cette analyse plaide en faveur d’une action de l’hydrogène lors du couplage environnement fatigue du matériau sous air humide de type HELP (Hydrogen Enhanced Localized Plasticity) et/ou AIDE (Adsorption Induced Dislocations Emissions).
... Prior studies of the nearcrack region suggest that hydrogen exposure results in a more localized deformation distribution proximate to the crack for a given driving force. 81,[133][134] However, considering the current alloys, how this hydrogen-enabled localization is affected by the widespread sub-μm porosity, oxide inclusions, and carbides present in the AM 17-4PH is unclear. Moreover, prior reports have documented fundamental differences in the general deformation behavior between conventional and AM materials 135 as well as in hydrogen-charged vs. hydrogen-free precipitation-hardened alloys. ...
As a high-strength corrosion resistant alloy, stress corrosion cracking (SCC) behavior is a key consideration for the conventional, wrought form of 17-4PH stainless steel. With the increasing popularity of the additively manufactured (AM) form of 17-4PH, understanding the SCC behavior of AM 17-4PH will be similarly critical for its presumed, future applications. The current study quantifies and compares the SCC behavior of both the wrought form, as a baseline, and AM form of 17-4PH at peak aged (~1200 MPa) and overaged (~1050 MPa) strength levels. The laser powder bed fusion (LPBF) technique followed by post-process hot isostatic press (HIP), solution annealing, and aging heat treatments is used to produce AM 17-4PH with similar microstructures and strength levels to wrought 17-4PH and facilitate the comparison. SCC behavior is quantified using fracture mechanics-based rising (dK/dt = 2 MPa√m/hr) and constant (dK/dt = 0 MPa√m/hr) stress intensity tests in neutral 0.6 M NaCl at various applied potentials. Limited SCC susceptibility was observed at open circuit and anodic potentials for both forms of 17-4PH. At cathodic applied potentials, AM consistently underperforms wrought with up to 5-fold faster crack growth rates and 200-400 mV wider SCC susceptibility ranges. These results are interrogated through microstructural and fractographic analysis and interpreted through a decohesion-based hydrogen-assisted crack model. Initial analyses show that (1) increased oxygen content, (2) porosity induced by argon processing, and (3) slow cooling (310 °C/hr) during conventional HIP processing might contribute to degraded SCC performance in AM 17-4PH.
... A multiscale electron microscopy-based approach was employed to study the crack initiation mechanism and to study the effects of dispersoids and grain boundaries on the localized deformation behavior. Multiscale electron microscopy-based approaches are seeing increased application, with highly automated mesoscale characterization techniques providing statistical information linking microstructure to failure initiation sites and site specific high resolution analysis providing insight into the underlying defect-based mechanisms driving failure [20][21][22][23]. This approach is similar to multiscale correlative characterization, which investigates the same region of interest using a range of imaging techniques and length scales [24]. ...
The microstructural origins and early behavior of crack formation in AA6451 samples under three-point bending has been investigated using a multiscale electron microscopy-based approach. Two different heat treatments were investigated: solution treated and naturally aged (T4) and artificially aged (T6). Electron backscatter diffraction and cross-sectional scanning electron microscopy imaging showed that, prior to crack formation, grain boundary ledges formed. These ledges formed adjacent to grain boundaries whose surface traces were oriented parallel to the bend axis and their planes forming angles below 70° with the sample surface. The grain boundary misorientation angle did not show any correlation with grain boundary ledge or crack formation. The sub-surface deformation fields associated with the grain boundary ledges were strongly affected with the precipitate state, with grain refinement and ultrafine grain formation occurring in artificially aged samples in the precipitate free zone and more diffuse deformation structures forming in the non-aged samples. In both cases, constituent particles were not found to have a significant influence on damage nucleation processes.
... Site-specific TEM sample preparation techniques such as focused ion beam (FIB) machining are essential tools in these techniques as they link the scales. Examples of this approach include relating defect processes to strain localization in additive manufactured metals [14], understanding the role of second phase particles in defect accumulation during deformation of AA6xxx [15], characterizing the defect characteristics associated with hydrogen embrittlement processes [16], and characterizing the defect structures formed during fatigue crack growth [17]. As multiscale electron microscopy-based techniques have become more common, advances in quantitative characterization techniques at the nano and microscales, similar to the development that has occurred previously at the mesoscale, have also seen rapid improvements. ...
The influence of dispersoids and grain boundary characteristics on the crack propagation behavior in AA3xxx during deep drawing has been investigated using scanning transmission electron microscopy, energy-dispersive spectroscopy, and the dictionary indexing method applied to transmission Kikuchi diffraction (TKD) patterns. Observations showed that cracks formed on the surface of the material at the earliest stages of deformation and lengthened with increasing deformation. Subsurface analysis showed that the cracks propagated from the surface and bifurcated as they extended deeper into the material, forming branches. All of the observed cracks followed intergranular paths with bifurcations correlating with triple junction locations, but no strong trends were found relating grain boundary characteristics to the crack propagation pathway. Dispersoids located at the grain boundaries were found where cracks terminated, suggesting that dispersoids arrest crack propagation and divert crack growth pathways. This paper demonstrates the first practical application of combining the dictionary indexing method with TKD analysis.Graphic abstract
... In the latter, hydrogen will reduce the cohesive forces at interfaces, e.g. grain boundaries and particle-matrix interfaces, and thus weakening the material. A crucial drawback here is that both of these concepts have not been experimentally proven [6,7], which in turn complicates the interpretation of the environmentally ductile-to-brittle transition by using a single mechanism. For this reason, recent development of these concepts is suggesting a synergetic interplay of the two common micro-mechanical mechanisms, discussed in great detail in the suggested literature [8][9][10]. ...
In this work, an experimental-numerical screening method for studying the elastic-plastic properties in high strength steel subjected to environmentally assisted degradation due to hydrogen is proposed. The experiments were performed on single-edge-notch bend specimens loaded with a monotonic constant displacement rate, and the specimens were electrochemically hydrogen pre-charged and/or in-situ. A systematic investigation was conducted of the influence of current density, pre-charging time and loading rate on the fracture mechanical properties. It was found that the loading rate had the greatest effect on the J-R curves, and that the environmental ductile-to-brittle transition region was obtained in a less than a day of experimental time. In this transition region it was found from the fractography that the dominating mode of failure changed from dimple to dominating intergranular fracture.
It is known that hydrogen can influence the dislocation plasticity and fracture mode transition of metallic materials, however, the nanoscale interaction mechanism between hydrogen and grain boundary largely remains illusive. By uniaxial straining of bi-crystalline Ni with a Σ5(210)[001] grain boundary, a transgranular to intergranular fracture transition facilitated by hydrogen is elucidated by atomistic modeling, and a specific hydrogen-controlled plasticity mechanism is revealed. Hydrogen is found to form a local atmosphere in the vicinity of grain boundary, which induces a local stress concentration and inhibits the subsequent stress relaxation at the grain boundary during deformation. It is this local stress concentration that promotes earlier dislocation emission, twinning evolution, and generation of more vacancies that facilitate nanovoiding. The nucleation and growth of nanovoids finally leads to intergranular fracture at the grain boundary, in contrast to the transgranular fracture of hydrogen-free sample.
The hydrogen environment-assisted cracking (HEAC) behavior of a compositionally complex Co–Ni–Cr–Mo–Fe–Ti–Al alloy (MP98T) proposed for marine fastener applications was evaluated via slow-rising stress intensity testing while immersed in 0.6 M NaCl at applied potentials ranging from −1.0 to −1.2 VSCE (vs. saturated calomel electrode). Results indicate that MP98T is resistant to HEAC at applied potentials representative of cathodic protection. In particular, minimal crack growth was observed across the tested applied potential range, which represented conditions known to promote aggressive HEAC in legacy Ni-based fastener alloys, such as Monel K-500. Measurement of the diffusible hydrogen concentration (CH,Diff) of MP98T as a function of applied potential in 0.6 M NaCl suggests this improved susceptibility may be linked to a reduced efficacy for hydrogen uptake in MP98T at hydrogen overpotentials where Monel K-500 is susceptible. Specifically, the CH,Diff of MP98T at a given applied potential was consistently observed to be less than 50% of that measured for Monel K-500.
Uniaxial tensile experiments on hydrogen-charged and non-charged polycrystalline nickel deformed to a true plastic strain of ~0.1 at room temperature and 77 K reveal that hydrogen has no measurable effect on texture evolution. Texture measurements were completed on samples of each condition, as well as an as-received reference specimen, using electron backscatter diffraction data collected from >20,000 individual grains. Crystal plasticity simulations support the conclusion that the similarity is attributable to the insensitivity of small-strain deformation texture evolution to potential hydrogen-induced modifications in deformation mechanisms.
The onset of sub-critical crack growth during slow strain rate tensile testing (SSRT) is assessed through a combined experimental and modeling approach. A systematic comparison of the extent of intergranular fracture and expected hydrogen ingress suggests that hydrogen diffusion alone is insufficient to explain the intergranular fracture depths observed after SSRT experiments in a Ni–Cu superalloy. Simulations of these experiments using a new phase field formulation indicate that crack initiation occurs as low as 40% of the time to failure. The implications of such sub-critical crack growth on the validity and interpretation of SSRT metrics are then explored.
The deformation fields near fatigue crack tips grown in hydrogen and in air were measured using high-energy x-ray diffraction. A larger magnitude of elastic strain was observed in the hydrogen case compared to the air case. The magnitude of elastic strain was quantified through an effective crack tip stress intensity factor. The dislocation profile ahead of the crack was probed via x-ray line broadening and electron back-scatter diffraction was used to assess the crack path (intergranular vs. transgranular). Ahead of the crack tip grown in hydrogen, an order of magnitude lower dislocation density, compared to a baseline density far from the crack, was observed. This decrease in dislocation density was not observed in the air case. These differences are discussed in terms of two leading hydrogen embrittlement mechanisms, Hydrogen Enhanced Localized Plasticity (HELP) and Hydrogen Enhanced Decohesion (HEDE). We have observed a decrease in transgranular cohesion (transgranular HEDE), as well as an increase in intergranular fracture. The measurements of dislocation activity support a model of a decrease in intergranular cohesion (intergranular HEDE) which is likely facilitated by the HELP mechanism. This suggests that the increase in fatigue crack growth rate is due to a sum of the two effects of hydrogen, in which the crack grows faster in the transgranular fracture mode and faster due to an increase in a new mode of intergranular fracture.
The influence of hydrogen on the mechanical response of an FCC equi-molar solid-solution alloy, FeNiCoCr, was investigated. Hydrogen caused a reduction in ductility in terms of total elongation of only a few percent in the FeNiCoCr alloy, but the local reduction in area after necking was reduced, and the fracture mode transitioned from transgranular ductile microvoid coalescence to predominantly intergranular. This result demonstrates unequivocally that this equi-molar alloy is susceptible to hydrogen embrittlement. Specifically, hydrogen reduced the elongation of the grains in the loading direction, increased the out-of-plane distortion of grains on the specimen surface, increased the orientation deviations with respect to the mean of individual grains, and induced a more advanced microstructural state in the form of dislocation tangles and dislocation cells. Hydrogen transport by dislocations caused a redistribution of the hydrogen increasing the hydrogen concentration on the grain boundary. Hydrogen accommodated in dislocation cell walls and on dislocations, effectively locked the microstructure in a specific configuration that inhibited strain transfer across grain boundaries. Consequently, an incompatibility constraint across the grain boundary was introduced. Together these effects explain why the hydrogen-charged alloy did not exhibit necking and transitioned to a grain boundary failure mode.
A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAFCG. The FCG in gaseous hydrogen showed two distinguishing regimes corresponding to the unaccelerated regime at a relatively low stress intensity factor range, ΔK, and the accelerated regime at a relatively high ΔK. The fracture surface in the unaccelerated regime was covered by ductile transgranular and intergranular features, while mainly quasi-cleavage features were observed in the accelerated regime. The EBSD and ECCI results demonstrated considerably lower amounts of plastic deformation, i.e., less plasticity, around the crack path in the accelerated regime. The TEM results confirmed that the dislocation structure immediately beneath the crack in the accelerated regime showed significantly lower development and that the fracture surface in the quasi-cleavage regions was parallel to the {100} plane. These observations suggest that the HAFCG in pure iron may be attributed to “less plasticity” rather than “localized plasticity” around the crack tip.
In order to study the influence of hydrogen on plastic deformation behavior in the vicinity of the fatigue crack-tip in a pure iron, a multi-scale observation technique was employed, comprising electron channeling contrast imaging (ECCI), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The analyses successfully demonstrated that hydrogen greatly reduces the dislocation structure evolution around the fracture path and localizes the plastic flow in the crack-tip region. Such clear evidence can reinforce the existing model in which this type of localized plasticity contributes to crack-growth acceleration in metals in hydrogen atmosphere, which has not yet been experimentally elucidated.
Positron annihilation spectroscopy and thermal desorption spectroscopy experiments were combined to ascertain the role of hydrogen on generation of vacancies and vacancy clusters in Ni alloys. The effects of grain size and deformation temperature are emphasized for pure Ni single crystals and polycrystalline Ni-201 alloy samples with two grain sizes that were thermally pre-charged with 3000 appm hydrogen. Variation in positron lifetime and intensity suggests that hydrogen enhances and stabilizes vacancies and vacancy clusters. Additionally, grain boundaries and the regions adjacent to them are preferential sites for vacancy and cluster formation. Hydrogen-altered vacancies and vacancy clusters are manifest in yield behavior differences: uniform vacancy distributions augment strength increases after hydrogen charging; enhanced yield strength during cryogenic deformation is ascribed to an ‘Orowan type’ strengthening mechanism while cross-slip restriction dominates hardening behavior at room temperature.
Nickel-based superalloys are used in high strength, high-value applications, such as gas turbine discs in aero engines. In these applications the integrity of the disc is critical and therefore understanding crack initiation mechanisms is of high importance. With an increasing trend towards powder metallurgy routes for discs, sometimes unwanted non-metallic inclusions are introduced during manufacture. These inclusions vary in size from ∼10 μm to 200 μm which is comparable to the grain size of the nickel-based superalloys. Cracks often initiate near these inclusions, and the precise size, shape, location and path of these cracks are microstructurally sensitive. In this study, we focus on crack initiation at the microstructural length scale using a controlled three-point bend test, with the inclusion deliberately located within the tensile fibre of the beam. Electron backscatter diffraction (EBSD) is combined with high spatial resolution digital image correlation (HR-DIC) to explore full field plastic strain distributions, together with finite element modelling, to understand the micro-crack nucleation mechanisms. This full field information and controlled sample geometry enable us to systematically test crack nucleation criteria. We find that a combined stored energy and dislocation density provide promising results. These findings potentially facilitate more reliable and accurate lifing prediction tools to be developed and applied to engineering components.
Finite element analysis of stress about a blunt crack tip, emphasizing finite strain and phenomenological and mechanism-based strain gradient plasticity (SGP) formulations, is integrated with electrochemical assessment of occluded-crack tip hydrogen (H) solubility and two H-decohesion models to predict hydrogen environment assisted crack growth properties. SGP elevates crack tip geometrically necessary dislocation density and flow stress, with enhancement declining with increasing alloy strength. Elevated hydrostatic stress promotes high-trapped H concentration for crack tip damage; it is imperative to account for SGP in H cracking models. Predictions of the threshold stress intensity factor and H-diffusion limited Stage II crack growth rate agree with experimental data for a high strength austenitic Ni-Cu superalloy (Monel®K-500) and two modern ultra-high strength martensitic steels (AerMet™100 and Ferrium™M54) stressed in 0.6 M NaCl solution over a range of applied potential. For Monel®K-500, KTH is accurately predicted versus cathodic potential using either classical or gradient-modified formulations; however, Stage II growth rate is best predicted by a SGP description of crack tip stress that justifies a critical distance of 1 μm. For steel, threshold and growth rate are best predicted using high-hydrostatic stress that exceeds 6 to 8 times alloy yield strength and extends 1 μm ahead of the crack tip. This stress is nearly achieved with a three-length phenomenological SGP formulation, but additional stress enhancement is needed, perhaps due to tip geometry or slip-microstructure.
High-resolution, cross-correlation-based, electron backscatter diffraction (HR-EBSD) is an emerging technique capable of measuring elastic strains, lattice rotations, and defect populations in crystalline materials. Here we briefly review development of the technique and the fundamental method. Application of HR-EBSD to metallic samples is illustrated with three examples: nickel-superalloy matrix/carbide interactions during cyclic deformation; interaction of a slip band and grain boundary; and patterning of stress and dislocation storage in deformed copper. These three examples highlight the ability of HR-EBSD to deliver new science by revealing new insights into the fundamental nature of deformation, as well as validating existing models. Application of the technique is now commonplace, and emergence of the technique is opening it up to the wider materials science community to tackle grand challenges.
A variety of features broadly classed as deformation microstructure elements are created during the plastic deformation of polycrystalline metals. While virtually all elements of deformation microstructure are composed of dislocations, describing the creation and evolution of larger-scale elements in terms of interactions between individual dislocations is a goal that has not yet been achieved. A hierarchical approach is thus favored in which structure creation and evolution are described at a range of length scales, from the nanometer to millimeter scale.
The effect of hydrogen (H) on the deformation behavior of Monel K-500 in various isothermal heat treatment conditions (non-aged, under-aged, peak-aged, and over-aged) was assessed via uniaxial mechanical testing. H-charged and non-charged specimens were strained to failure to facilitate a comparison of ductility, fracture surface morphology, strength, and work hardening behavior. For all examined heat treatment conditions, H charging leads to a significant reduction in ductility, which is accompanied by a consistent change in fracture surface morphology from ductile microvoid coalescence to brittle intergranular fracture. While H charging led to a systematic enhancement in the yield strength of all heat treatments, the three age-hardened conditions exhibited a more than 2-fold increase relative to the non-aged heat treatment. This suggests that H modifies the dislocation–precipitate interactions, which also manifest themselves through changes in work hardening metrics related to the dislocation storage and recovery rates. In particular, the H-charged peak-aged specimen exhibited a significant increase in initial hardening (dislocation storage) rate relative to the H-charged under-aged specimen. Transmission electron microscopy of these samples revealed the onset of widespread dislocation looping in the H-charged peak-aged sample, in addition to the planar slip bands characteristic of the non-charged condition. This result suggests that hydrogen induces the particle shearing-to-looping transition at smaller particle sizes. Possible mechanistic explanations for this observed behavior are presented.
The role of hydrogen in tensile ductility loss and on the fracture behaviours of Ni-based superalloy 718 was investigated via tensile tests under hydrogen-charged conditions (internal hydrogen) or in gaseous hydrogen environments (external hydrogen), in combination with post-mortem analyses of fractured samples using electron microscopy techniques. Whereas intergranular fracture was responsible for material degradation under external hydrogen, the failure modes under internal hydrogen conditions were primarily dominated by cracking along slip planes or twin boundaries. The mechanisms of crack initiation and propagation are extensively discussed in terms of hydrogen distribution, intrinsic deformation character of the material and hydrogen-modified dislocation behavior.
The effect of isothermal heat treatment on hydrogen environment-assisted cracking (HEAC) susceptibility in a single heat of Monel K-500 was evaluated via slow-rising stress intensity (K) testing while immersed in 0.6 M NaCl solution and exposed to applied potentials ranging from −1000 to −1200 mVSCE (vs. saturated calomel electrode). Four heat treatments, corresponding to the non-aged, under-aged, peak-aged, and over-aged conditions, conducted at a constant aging temperature of 923 K, were examined in this study. Crack growth kinetics and fractography demonstrate that alloys heat-treated to the under-aged and peak-aged conditions exhibit increased susceptibility to HEAC relative to alloys treated to the non-aged and over-aged conditions, respectively. Microstructural variables that are expected to vary under this isothermal heat treatment protocol are grain size, grain boundary character, grain boundary impurity concentration, yield strength, strain hardening, and precipitate morphology. A detailed assessment of grain size, grain boundary character, and grain boundary impurity concentration evolution with aging time suggests these variables do not explain the observed variation in HEAC susceptibility of Monel K-500. Differences in the crack tip hydrostatic stress field induced by differences in yield strength and strain hardening were directionally consistent with the observed HEAC susceptibility, but do not capture the enhanced susceptibility of the under-aged heat treatment relative to the peak-aged condition. However, transmission electron microscopy of each heat treatment revealed the presence of planar slip in the under-aged and peak-aged specimens, as compared to wavy slip in the non-aged and over-aged specimens, suggesting that the propensity for strain localization correlates well with HEAC susceptibility.
Atomistic simulations are a powerful complement to experimental probes for understanding the nanoscale processes associated with the effects of hydrogen (H) on plasticity and fracture that are the underlying causes of hydrogen embrittlement (HE). Current experimental techniques provide quantitative measures of the macroscopic effects of H on plastic flow and fracture but are unable to determine the nanoscale distribution of H atoms in equilibrium nor, more importantly, as a function of time. Conversely, atomistic simulations can provide information on the nanoscale distribution of H around important lattice defects (vacancies, dislocations, grain boundaries, cracks) and probe the mechanical behavior of these defects in the presence and absence of H. Thus, in principle, atomistic simulations can test fundamental theories and conjectures that arise in attempting to rationalize experimental features of HE. However, atomistic simulations have a range of limitations that must be well-recognized. Accurate ab initio simulations are limited to small numbers of atoms and cannot capture necessary time evolution. Molecular simulations using semi-empirical interatomic potentials can handle more atoms and longer time scales, but are limited by accuracy of the potentials and time scales that remain far smaller than experimental time scales. The value of atomistic simulations thus lies primarily in creating targeted simulations to assess the energetics of specific configurations or specific mechanisms of deformation or fracture, along with theoretical models to estimate realistic time scales that remain inaccessible in simulations. Because of their limitations, atomistic simulations may not be definitive, but they nonetheless provide considerable insight by supporting or contradicting conjectures and concepts proposed to rationalize experiments. Here, the above issues are discussed in more detail and several examples, mainly from the work of the current authors, and including previously-unpublished studies on the effects of H on the bowout of the edge dislocations in α-Iron and predictions of solute-drag by H in nickel, serve to demonstrate how atomistic simulations can be used to reveal important features of the behavior of H in metals.
A multi-scale experimental approach was used to determine the fundamental mechanisms responsible for the hydrogen-induced transition in failure mode from ductile transgranular to intergranular in polycrystalline Ni during uniaxial loading. Hydrogen accelerated the evolution of the deformation microstructure, producing smaller dislocation cells and microbands, and causing significantly different orientation deviations to develop in neighboring grains, while inducing less evolution of texture, less grain rotations, less elongation of the grains parallel to the tensile axis, and greater out-of-surface distortion of the grains. These observations are explained in terms of the hydrogen-enhanced plasticity mechanism, which results in a redistribution of hydrogen that stabilizes the deformed microstructure and increases the hydrogen coverage on the grain boundaries. The stabilization of the microstructure manifests as a reduced ability of grains to cooperatively accommodate evolving deformation structures, which introduces an additional compatibility constraint across grain boundaries. The combination of this compatibility constraint across grain boundaries, the locking of the microstructure in a specific configuration by hydrogen, and the hydrogen-weakening of the grain boundaries drives the hydrogen-induced intergranular failure.
In this study, a multiscale electron microscopy-based approach is applied to understanding how different aspects of the microstructure in a notched AA6061-T6, including grain boundaries, triple junctions, and intermetallic particles, promote localized dislocation accumulation as a function of applied tensile strain and depth from the sample surface. Experimental measurements and crystal plasticity simulations of dislocation distributions as a function of distance from specified microstructural features both showed preferential dislocation accumulation near intermetallic particles relative to grain boundaries and triple junctions. High resolution electron backscatter diffraction and site-specific transmission electron microscopy characterization showed that high levels of dislocation accumulation near intermetallic particles led to the development of an ultrafine sub-grain microstructure, indicative of a much higher level of local plasticity than predicted from the coarser measurements and simulations. In addition, high resolution measurements in front of a crack tip suggested a compounding influence of intermetallic particles and grain boundaries in dictating crack propagation pathways.
Despite recent substantial advances in understanding of hydrogen embrittlement (HE), many important aspects of this widespread phenomenon remain a subject of debates. Particularly, remarkably different opinions have been expressed on the nature of hydrogen-assisted cracking (HAC), which produces quasi-cleavage (QC)fracture surfaces in iron and low-carbon steels. Basically, two conflicting groups of theories are distinguished concerning causes the QC phenomenon: brittle cleavage-like models and ductile models exploiting localized ductile micro- or nanovoid concepts at the core. The present study was aimed at gaining a new insight into the QC mechanism through the detailed investigation of the HAC path by the microscopic observations of the side and fracture surfaces of the annealed low-carbon steel, which was tensile tested in the ex- and in-situ cathodic hydrogen charged conditions. Using a combination of the scanning electron microscopy, confocal laser scanning microscopy and electron-backscattering diffraction, it was found that the in-situ charging is more suitable for the investigation of HAC by side surface observations because it provokes QC cracking on the side surface and suppresses normal ductile fracture. As opposes to this, HAC after ex-situ charging occurs mainly internally. In this condition, HAC interferes with the normal ductile fracture on the side surface. It is established that the path of secondary QC cracks in the in-situ charged specimen is determined predominantly by the local stress distributions. This holds even on the scale of individual grain. Large deviations of the quasi-cleavage cracks paths from the specific crystallographic planes are found and explained. Nano-voids are regularly observed at the crack tips. Thus, the nano-voids coalescence is supposed to be an integral part of QC HAC.
Elasto-plastic fracture toughness tests of a commercially pure iron were performed in air and in hydrogen gas at two different pressures. Some unique characteristics of hydrogen-enhanced cracking were exhibited at both the macroscopic and microscopic length scales, based on the observation of fracture surface, fracture plane, plasticity distribution and dislocation structure. The possible mechanisms responsible for the hydrogen-induced degradation of fracture toughness are discussed.
The effect of hydrogen (H) on the fatigue behavior is of significant importance for metallic structures. In this study, the hydrogen-enhanced fatigue crack growth rate (FCGR) tests on in-situ electrochemically H-charged ferritic Fe-3wt%Si steel with coarse grain size were conducted. Results showed strong difference between the H-charged and the non-charged conditions (reference test in laboratory air) and were in good agreement with the results from literature. With H-charging, the fracture morphology changed from transgranular (TG) type to “quasi-cleavage” (“QC”), with a different fraction depending on the loading frequency. With the help of electron channeling contrast imaging (ECCI) inside a scanning electron microscope (SEM), a relatively large area in the failed bulk specimen could be easily observed with high-resolution down to dislocation level. In this work, the dislocation sub-structure immediately under the fracture surfaces were investigated by ECCI to depict the difference in the plasticity evolution during fatigue crack growth (FCG). Based on the analysis, the H-enhanced FCG mechanisms were discussed.
Recent studies on hydrogen enhancement of the strain-induced generation of vacancies, i.e. the HESIV mechanism, and its role in hydrogen embrittlement (HE) of steels are overviewed. A high density of vacancies and their clustering have been detected by newly developed low temperature hydrogen thermal desorption spectroscopy and positron lifetime measurements. The promotion of crack nucleation and growth is ascribed to the evolution of damage associated with intense strain localization, and characteristic features of HE such as dependence on the microstructures of steels and test conditions are consistent with the HESIV mechanism. The predominant and supplementary roles of vacancies and hydrogen, respectively, in premature fracture are demonstrated by experiments devised to separate each contribution. A constitutive relation for porous materials is applicable to describe mechanistic effects of the HESIV mechanism on crack growth resistance and strain localization. The sequential void nucleation, growth and link mechanism in nanoscale are suggested.
Fatigue crack growth (FCG) tests were performed with two types of metastable austenitic stainless steels having different austenite phase stabilities under hydrogen-precharged conditions (internal hydrogen) and in gaseous hydrogen environments (external hydrogen). The materials showed a peculiarly slower FCG rate with internal hydrogen than with external hydrogen even though the hydrogen concentration was much higher under the internal hydrogen conditions. The results are interpreted in terms of hydrogen-modified plastic deformation character comprising inhibited cross-slipping or enhanced deformation twinning in combination with the sequence of hydrogen penetration and strain-induced α′ martensite formation in the local region surrounding the fatigue crack tip.
Uniaxial mechanical testing conducted at room temperature (RT) and 77 K on hydrogen (H)-exposed nickel was coupled with targeted microscopy to evaluate the influence of deformation temperature, and therefore mobile H-deformation interactions, on intergranular cracking in nickel. Results from interrupted tensile tests conducted at cryogenic temperatures (77 K), where mobile H-deformation interactions are effectively precluded, and RT, where mobile H-deformation interactions are active, indicate that mobile H-deformation interactions are not an intrinsic requirement for H-induced intergranular fracture. Moreover, an evaluation of the true strain for intergranular microcrack initiation for testing conducted at RT and 77 K suggests that H which is segregated to grain boundaries prior to the onset of straining dominates the H-induced fracture process for the prescribed H concentration of 4000 appm. Finally, recent experiments suggesting that H-induced fracture is predominately driven by mobile H-deformation interactions, as well as the increased susceptibility of coherent twin boundaries to H-induced crack initiation, are re-examined in light of these new results.
The influence of internal hydrogen on the tensile properties of an equi-molar FeNiCoCrMn alloy results in a significant reduction of ductility, which is accompanied by a change in the fracture mode from ductile microvoid coalescence to intergranular failure. The introduction of 146.9 mass ppm of hydrogen reduced the plastic strain to failure from 0.67 in the uncharged case to 0.34 and 0.51 in hydrogen-charged specimens. The reduction in ductility and the transition in failure mode are clear indications that this alloy exhibits the classic signs of being susceptible to hydrogen embrittlement. The results are discussed in terms of the hydrogen-enhanced plasticity mechanism and its influence on hydrogen-induced intergranular failure. Furthermore, a new additional constraint that further promotes intergranular failure is introduced for the first time.
The fatigue-crack growth rate of a ferritic-pearlitic low carbon steel was faster when the tests were conducted in high-pressure H2 gas environments than in air. The predominant fracture feature changed from ductile fatigue striations with some “quasi-cleavage-like” regions when the test was conducted in air to mixed “quasi-cleavage” and “flat” facets when tested in a H2 gas environment. The microstructure beneath the fracture surfaces produced in air was sub-grains, and over a distance of 15 μm from the fracture surface, the dimensions of the sub-grains increased. With hydrogen, dense dislocation bands and refined dislocation cells existed beneath the “quasi-cleavage” and “flat” fracture surfaces. The cell size increased with distance from the fracture surface. The decrease in the dimensions of the key microstructural features as the fracture surface is approached is attributed to the propagation of the crack through an already deformed matrix. The differences in evolved dislocation structure are explained in terms of the hydrogen-enhanced localized plasticity mechanism, and the hydrogen-modified dislocation structure establishes the local conditions that promote the fracture mode transition from ductile fatigue striations to a mixture of “quasi-cleavage” and “flat” features, which directly leads to enhanced fatigue-crack growth.
In this work, the relative capabilities and limitations of electron channeling contrast imaging (ECCI) and cross-correlation electron backscattered diffraction (CC-EBSD) have been assessed by studying the dislocation distributions resulting from nanoindentation in body centered cubic Ta. Qualitative comparison reveals very similar dislocation distributions between the CC-EBSD mapped GNDs and the ECC imaged dislocations. Approximate dislocation densities determined from ECC images compare well to those determined by CC-EBSD. Nevertheless, close examination reveals subtle differences in the details of the distributions mapped by these two approaches. The details of the dislocation Burgers vectors and line directions determined by ECCI have been compared to those determined using CC-EBSD and reveal good agreement.
The influence of heat-to-heat variations on the hydrogen embrittlement susceptibility of age-hardened Monel K-500 (UNS N05500) was evaluated through detailed characterization of metallurgical attributes and hydrogen interactions, coupled with notched tensile specimen embrittlement metrics. Four nominally peak-aged material heats of Monel K-500 were assessed using slow strain rate tensile (SSRT) testing while immersed in 0.6 M NaCl solution and exposed to cathodic polarization levels ranging from −0.850 to −1.1 VSCE. Despite each of the four heats meeting the US Federal Procurement Specification QQ-N-286G, the hydrogen embrittlement susceptibility was found to vary extensively between the tested material heats. Characterization of microstructural features, composition, and hydrogen-metal interactions were performed to facilitate correlation between material property and susceptibility trends. Results suggest that subtle differences in grain boundary chemistry and H uptake behavior may contribute to heat-to-heat variations in hydrogen embrittlement susceptibility of Monel K-500. Conversely, parameters including yield strength, hydrogen diffusivity, hydrogen production rate, grain boundary character, and grain size do not independently control the observed variations in susceptibility. Based on these experimental results, a macroscale framework for assessing the degradation in fracture stress as a function of applied potential is proposed and possible avenues for framework improvement are suggested.
The influence of hydrogen on the evolution of the deformation microstructure in Ni at the same specific macroscopic strain has been determined for the first time. The evolution of the microstructure was accelerated by hydrogen as evidenced by the formation of smaller dislocation cells and dense dislocation walls compared to only dislocation cells in the absence of hydrogen. This enhanced evolution is described in terms of the hydrogen-enhanced localized plasticity mechanism.
The influence of microstructural variation on hydrogen environment-assisted cracking (HEAC) of Monel K-500 was evaluated using five nominally peak-aged lots of material tested under slow-rising stress intensity loading while immersed in NaCl solution under cathodic polarizations. Minimal variation in HEAC resistance among material lots was observed for an applied potential of −950 mVSCE (E
app, vs saturated calomel), whereas lot-to-lot variability in the fracture morphology demonstrates a significant difference in the HEAC resistance at the less negative potential of −850 mVSCE, suggesting that relatively severe H environments produce sufficient crack-tip H to minimize the impact of metallurgical differences. Sensitivity analyses accomplished by varying the inputs used in decohesion-based, micromechanical models imply significant variations in HEAC resistance are possible for realistic changes in grain boundary toughness, hydrogen uptake behavior, and yield strength. Grain size, impurity segregation (including the effects of gettering elements), grain boundary character/connectivity, and crack path tortuosity are also considered in the context of HEAC susceptibility. Yield strength, global hydrogen content, as well as impurity segregation to grain boundaries, especially boron and sulfur, are speculatively considered to be the dominant contributions in determining HEAC resistance. Modifications that would incorporate the effects of grain boundary segregation are proposed for the K
TH model; detailed validation of such changes require high-fidelity and quantitative inputs for the degree of grain boundary segregation. Regardless, fracture mechanics-based HEAC results, detailed microstructural characterization, and micromechanical modeling were successfully coupled to gain insights into the influences governing the microstructure-dependent HEAC susceptibility of Monel K-500.
A propagation approach to manage stress corrosion cracking (SCC) is justified for modern high-strength alloys that exhibit low-subcritical crack growth rates, and is enabled by maturing probabilistic fracture mechanics and mechanism-based hydrogen damage modeling. A computer program, SCCrack, is developed based on stress intensity (K) similitude and fast Monte Carlo analysis to predict SCC life distributions from input distributions of variable material crack growth rate (da/dt vs. K), starting crack size, environment, and stress. Growth rates are obtained using an accelerated experiment that exploits precision electrical potential measurement of crack propagation, small crack size, and slow-rising K loading. Data are leveraged by fundamental models of SCC threshold (KTH) and H-diffusion limited Stage II da/dt, built on the hydrogen environment assisted cracking mechanism. With these inputs, SCCrack enables multi-scale "What if?" simulations of the effects of alloy-environment-precorrosion-stress variables on component cracking. Examples are presented for SCC in modern ultra-high strength martensitic steel (AerMet™100 [UNS K92580] and Ferrium™M54 [UNS K91973]), a Ni-Cu superalloy (Monel K-500 [UNS N05500]), and a sensitized Al-Mg alloy (Alloy 5083-H131 [UNS A95083]), each stressed in chloride solution with varying cathodic polarization. Research is required to enhance accelerated measurement of very low da/dt, to capture the effects of atmospheric-environment spectra, to advance mechanism modeling, and to validate the approach for field-specific components.
This research improves H decohesion mechanism-based modeling of intergranular stress corrosion cracking in a Ni-Cu superalloy, Monel K-500. New cracking data plus improved model parameters lead to accurate predictions of the cathodic potential dependencies of K
TH and H diffusion-limited da/dt
II for Monel K-500 under slow-rising K in 0.6 M NaCl solution. Experiments and modeling demonstrate that IGSCC is eliminated for applied potentials more positive than a critical level between −900 and −840 mVSCE, but slow-subcritical cracking persists by a microvoid-based mechanism.
This chapter focuses on two kinds of environmental fracture, stress corrosion cracking (SCC) and hydrogen embrittlement, and discusses the role in such failures of a number of metallurgical variables. These include chemical composition; microstructural components such as precipitate type and structure, and grain size and shape; crystallographic texture; heat treatment and its effect on the foregoing variables; and processing, particularly the thermomechanical treatments (TMT), which are attracting increased attention for property optimization. These variables are expected to be of great importance in the development of new engineering materials to meet demanding service conditions.
The microstructure of deformed alloys which are susceptible to stress corrosion cracking has been investigated by transmission electron microscopy. It is found that the mode of failure is strongly related to the dislocation distribution. Alloys with a cellular arrangement of dislocation tangles have superior resistance to transgranular failure, whereas alloys containing planar groups of dislocations are generally more susceptible. In alloys, the active path for preferential electrochemical attack during transgranular stress corrosion cracking is associated with a continuous plane of disordered material which is created by the motion of dislocations through a matrix of short-range order. In superlattices, it is proposed that the active site for chemical attack is the antiphase boundary created during deformation by the accidental separation of superlattice dislocation pairs. The susceptibility of annealed, fully ordered single crystals to chemical embrittlement is predicted to depend on the continuity of the grown-in antiphase boundary structure.
Hydrogen environment-assisted cracking (HEAC) of Monel K-500 is quantified using slow-rising stress intensity loading with electrical potential monitoring of small crack propagation and elastoplastic J-integral analysis. For this loading, with concurrent crack tip plastic strain and H accumulation, aged Monel K-500 is susceptible to intergranular HEAC in NaCl solution when cathodically polarized at −800 mVSCE (E
A, vs saturated calomel) and lower. Intergranular cracking is eliminated by reduced cathodic polarization more positive than −750 mVSCE. Crack tip diffusible H concentration rises, from near 0 wppm at E
A of −765 mVSCE, with increasing cathodic polarization. This behavior is quantified by thermal desorption spectroscopy and barnacle cell measurements of hydrogen solubility vs overpotential for planar electrodes, plus measured-local crevice potential, and pH scaled to the crack tip. Using crack tip H concentration, excellent agreement is demonstrated between measurements and decohesion-based model predictions of the E
A dependencies of threshold stress intensity and Stage II growth rate. A critical level of cathodic polarization must be exceeded for HEAC to occur in aged Monel K-500. The damaging-cathodic potential regime likely shifts more negative for quasi-static loading or increasing metallurgical resistance to HEAC.
The hydrogen embrittlement of a commercial-grade pure iron was examined by using repeated stress-relaxation tests under simulta- neous cathodic hydrogen charging. The hydrogen-charged iron, containing an estimated 25.8 appm H, fractured after repeated tran- sients, with a total strain of 5%. The fracture mode was intergranular. Thermal activation measurements show a decrease in activation volume and free energy, which is consistent with hydrogen enhancing the dislocation velocity. The microstructure beneath the intergranular facets displays a dislocation cell structure more complex than expected for intergranular fracture and this strain- to-failure. It is proposed that hydrogen accelerates the evolution of the dislocation microstructure through the hydrogen-enhanced plasticity mechanism and this work-hardening of the matrix along with the attendant hydrogen concentration at the grain boundaries are crucial steps in causing the observed hydrogen-induced intergranular failure.
The evolution of dislocation storage in deformed copper was studied with cross-correlation-based high-resolution electron backscatter diffraction. Maps of 500 mu m x 500 mu m areas with 0.5 mu m step size were collected and analysed for samples deformed in tension to 0%, 6%, 10%, 22.5% and 40% plastic strain. These. maps cover similar to 1500 grains each while also containing very good resolution of the geometrically necessary dislocation (GND) content. We find that the average GND density increases with imposed macroscopic strain in accord with Ashby's theory of work hardening. The dislocation density distributions can be described well with a log-normal function. These data sets are very rich and provide ample data such that quantitative statistical analysis can also be performed to assess the impact of grain morphology and local crystallography on the storage of dislocations and resultant deformation patterning. Higher GND densities accumulate near grain boundaries and triple junctions as anticipated by Ashby's theory, while lower densities are rather more spread through the material. At lower strains (<= 6%) the grain-averaged GND density was higher in smaller grains, showing a good correlation with the reciprocal of the grain size. When combined with a Taylor hardening model this last observation is consistent with the Hall-Petch grain size effect for the yield or flow stress.
In cold-rolled Pd and Pd–H alloys the presence of hydrogen enhances the multiplication of dislocations and leads to an increase in dislocation density as compared to pure Pd. Subsequent removal of H from Pd–H alloys does not influence the dislocation density. H was identified as a temporary alloying addition (defactant) which increases the dislocation density of cold-deformed Pd.
The microstructure associated with the hydrogen-induced features “flat” and “quasi-cleavage” on the fracture surface of a lath martensitic steel has been visualized in a transmission electron microscope by using focused-ion beam machining to extract samples perpendicular to the fracture surface. Beneath both hydrogen-induced fracture surfaces there is direct evidence, in the form of intense slip bands and destruction of lath boundaries, for significant plasticity. These observations are considered in terms of the fundamental hydrogen embrittlement mechanisms, and the conclusion is reached that the failure is driven by a hydrogen-enhanced and plasticity-mediated decohesion mechanism.
Using a combination of high-resolution scanning and transmission electron microscopy, the basic mechanisms of hydrogen-induced intergranular fracture in nickel have been revisited. Focused-ion beam machining was employed to extract samples from the fracture surface to enable the examination of the microstructure immediately beneath it. Evidence for slip on multiple slip systems was evident on the fracture surface; immediately beneath it, an extensive dislocation substructure exists. These observations raise interesting questions about the role of plasticity in establishing the conditions for hydrogen-induced crack initiation and propagation along a grain boundary. The mechanisms of hydrogen embrittlement are re-examined in light of these new results.
The dislocation structures produced in Ni during room-temperature deformation in hydrogen and in helium were observed by transmission electron microscopy. The objective of the study was to determine whether specimens tested under the same strain conditions in the two atmospheres would exhibit different dislocation structures. It is found that the dislocation cell structure and dislocation density are not strongly coupled to the change in flow stress caused by hydrogen. This conclusion is not inconsistent with the observed effect of hydrogen on the dislocation mobility and on the flow stress at low strains, since these effects result from different types of dislocation interactions.
Both internal and hydrogen environment assisted cracking continue to seriously limit high performance structural alloys and confound quantitative component prognosis. While intergranular H cracking assisted by impurity segregation can be minimized, other mechanisms promote IG cracking and transgranular H cracking modes have emerged; new alloys suffer serious H cracking similar to old materials. Micromechanical models of crack tip H localization and damage by decohesion predict important trends in threshold and subcritical crack growth rate behaviour. H diffusion appears to limit rates of cracking for monotonic and cyclic loading; however, uncertain%adjustable parameters hinder model effectiveness. It is necessary to better define conditions within 0.1-5 micronmeter of the crack tip, where dislocations and microstructure dominate continuum mechanics, and chemistry is localized. Nano-mechanics modeling and experimental results show very high levels of H accumulated in the crack tip fracture process zone, as necessary for interface decohesion. Contributing mechanisms include high crack tip stresses due to dislocation processes such as strain gradient plasticity, as well as powerful H production and trapping proximate to the electrochemically reacting crack tip surface. New sub- micrometer resolution probes of crack tip damage will better define features such as crack path crystallography (EBSD + Stereology) and surface morphology (high brightness, dual detector SEM), local H concentration (%IDS and NRA), and validate crack tip mechanics modelling (micro-Laue x-ray diffraction and EBSD).
The applicability of Johnson's equation for the potential drop to three specimen geometries is demonstrated. In addition, some remarks are made concerning the influence of crack length and specimen width on the resolution of the potential method.
Recent advances using cross-correlation analysis of full resolution high quality electron backscatter diffraction (EBSD) patterns have provided a method for quantitatively mapping the stored dislocation density at high spatial resolution. Larger areas could be mapped with image binning or coarser step sizes. We have studied the effects of image binning and step size on the recovery of GND density. Our results suggest that: (i) the measured lower bound GND density noise floor broadly agrees with Wilkinson and Randman's 2009 prediction, where a decrease in step size or an increase in misorientation uncertainty increases the noise floor; (ii) increasing the step size results in a lower GND density being recovered as some dislocations are now considered as statistically stored dislocations (SSDs); (iii) in deformed samples the average GND density stays relatively constant as the degree of pattern binning is increased up to 8×8. Pattern binning thus provides a means of increasing the data acquisition and analysis rate without unduly degrading the data quality.
Cross-correlation-based electron backscattering diffraction analysis has been used to map lattice rotations in Ti-6Al-4V polycrystals deformed by load-controlled fatigue and dwell fatigue including a hold at maximum load. The lattice curvatures were used to form lower bound estimates of the geometrically necessary dislocation (GND) density distributions. In all cases the density of < a >-type GNDs was much higher than for < c + a >-type GNDs. As for tensile deformation the GND density histograms were significantly skewed toward the high density side. Observations of interrupted fatigue tests suggested that the density of < a >-type GNDs decreases during continued cyclic loading, presumably due to the formation of tightly bound dipoles and multipole structures. As has been proposed in models of facet fatigue formation, an example is presented of the accumulation, within a soft grain, of GNDs in a diffuse pile-up against a grain boundary with a hard grain. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.