Boundary Decohesion Research Articles

Unexpected thermal aging effect on brittle fracture and elemental segregation in modern dissimilar metal weld

A full-scale dissimilar metal weld safe-end mock-up, precisely replicating a critical component of a modern nuclear power plant, was investigated. The brittle fracture behavior, carbide evolution and nanoscale elemental segregation in the heat-affected zone (HAZ) of low alloy steel (LAS) were analyzed under both post-weld heat-treated and thermally-aged conditions (400 °C for 15,000 h, equivalent to 90 years of operation) using analytical electron microscopy and atom probe tomography. The observed increase in grain boundary (GB) decohesion and intergranular cracking on the fracture surface and the decrease of fracture toughness are primarily attributed to P and Mn segregation to GBs and the coarsening of carbides upon long-term thermal aging. The direct observations of significant elemental segregation to GBs and the consequent reduction in fracture toughness in the HAZ are unexpected for modern low-phosphorus LASs, highlighting potential concerns for evaluating the structural integrity of modern nuclear power plants.

Comparative study on mechanism of hydrogen embrittlement of Fe–18Mn-0.6C twinning-induced plasticity (TWIP) steel subjected to friction-stir welding (FSW) and tungsten inert-gas (TIG) welding

In present study, we investigated the effect of welding technique on resistance of hydrogen (H) embrittlement in Fe–18Mn-0.6C (wt.%) twinning-induced plasticity (TWIP) steel by using an electrochemical H-charging, thermal desorption spectroscope, and electron microscope. The friction-stir welding (FSW) specimen was less sensitive to H embrittlement relative to base metal and tungsten inert-gas (TIG) welding specimens by differences in microstructure and deformation mechanism. During H-charging of the FSW specimen, numerous dislocations/Σ3 boundaries reduced H diffusion into specimen interior, causing shallowest depth of brittle fracture. In contrast, the depth of brittle fracture in the H-charged TIG specimen was much larger due to rapid H diffusion by decreased grain boundaries including Σ3 annealing twin boundaries. During tensile deformation, the H-charged FSW specimen underwent the reduction in stress concentration by inactive TWIP as well as strong resistance of boundary decohesion. It was because of alleviation of H-enhanced localized plasticity (HELP) mechanism, leading to suppression of H-induced crack growth. Conversely, the H-charged base metal and TIG specimens revealed large stress concentration by active TWIP and weak boundaries owing to strong effects of HELP + H-enhanced decohesion (HEDE) mechanisms, exhibiting rapid H-induced crack propagation.

Exploring high-temperature deformation and damage behaviour in high-performance ferritic (HiperFerSCR) steel with Laves phase particles

The mechanical behaviour at high temperatures and the corresponding microstructural characteristics of a newly developed high-performance ferritic, salt-corrosion-resistant (HiperFerSCR) steel containing Laves phase particles were studied to investigate the active deformation mechanisms and the evolution of damage. A series of tensile tests were conducted at both room temperature and elevated temperatures (550, 600, and 650 °C), along with detailed microstructural analyses using scanning electron microscopy and electron backscatter diffraction. The tensile flow characteristics of HiperFerSCR at room temperature exhibit significant strain hardening, which is due to the Laves phase particles that encourage planar slip, demonstrated by the formation of numerous slip bands and low-angle grain boundaries. The intersections of these slip bands with grain boundaries appear to be the initiation sites for cracks, which then propagate intergranularly as well as within grain interiors along the slip bands. At high temperatures, the flow characteristics show strain softening, which is attributed to a dynamic recovery (DRV) mechanism, evident in the formation of subgrain boundaries decorated with Laves phase particles. During high-temperature deformation, damage initiates in the particle-free zone (PFZ), which expands at higher temperatures, leading to strain localisation, void formation along the grains, and eventually, grain boundary decohesion.

Effect of Ag agglomeration-driven nanovoids formation on fatigue reliability of Cu–Ag alloy flexible interconnects

Fatigue failure under cyclic deformation remains longstanding challenge in the flexible interconnect for the long term application. In this study, we design Cu–Ag alloy interconnects with excellent fatigue resistance by adopting nanovoid structures acquired by a simple post-annealing process. Nanovoids are generated during Ag agglomeration in the Cu matrix because of the low solubility of Ag in Cu, which enhances the fatigue property of a Cu–Ag alloy deposited on a polyimide film. The effect of high-density nanovoids located at the triple junction of the Cu and Ag grain boundaries on the mechanical fatigue lifetime is investigated by microstructure analysis (HAADF, HR-TEM and ASTAR™-coupled TEM). The fatigue damage of the Cu–Ag alloy has been observed to originate from grain boundary decohesion, unlike typical long fatigue cracks and extrusions. This can be attributed to the uniform distribution of nanovoids, which leads to stress concentration relief and consequently reduces the level of deformation.

Improved resistance to hydrogen embrittlement in the nugget zone of friction stir welded medium Mn steel via post-welding annealing

In this work, medium-Mn steel was friction stir welded and then intercritically annealed. Hydrogen diffusion and hydrogen embrittlement (HE) behavior in the nugget zone (NZ) were investigated. The as-welded NZ exhibited the highest HE index (HEI) of 79.2%, and hydrogen‑induced cracks propagated along the grain boundary, probably depending on grain boundary decohesion caused by H segregation. However, this condition was improved by post-welding annealing since fine lath reversed austenite, acting as a strong hydrogen trap, was introduced into the ferritic matrix, inhibiting the segregation of H to grain boundaries. Ultimately, the lowest HEI of 10.7% was obtained in the as-annealed NZ.

The effect of friction-stir welding in hydrogen embrittlement of Fe-17Mn alloy

In present study, we suggest the friction-stir welding (FSW) is an effective welding technique for improving the resistance of hydrogen (H) embrittlement in Fe-17Mn (wt%) alloy with a dual phase of γ austenite and ε martensite. The FSW specimen exhibited the enhanced resistance of H embrittlement during H-charging/tensile deformation despite active α′ martensitic transformation which is apt to H embrittlement, relative to base metal. During H-charging, numerous ε phase and higher initial dislocation density, as potential sites for H trapping, after FSW reduced H diffusion into specimen, causing the shallow H-related brittle fracture. In addition, the increased γ/ε interface and boundary in the FSW specimen alleviated H accumulation. During tensile deformation, the grain rotation and higher cε/aε ratio by FSW reduced Schmid factor of finer ε grains, leading to stronger resistance of boundary decohesion and the suppression of H-induced crack growth.

Hydrogen absorption and embrittlement of martensitic medium-Mn steels

In the present study, the H absorption and the H embrittlement (HE) resistance of medium (3–7 wt%) Mn martensitic steels were investigated. When the specimens were electrochemically H-charged, 7 wt% Mn specimen had the highest diffusible H content because of the highest reversible H trap density. When the diffusible H content was identical, HE resistance deterioration arose due to the Mn addition, probably caused by grain boundary decohesion and H segregation into the grain boundaries. However, this condition was improved by B addition due to the enhanced grain boundary cohesion and the suppression of H segregation into the grain boundaries.

Effect of Mg/Si Ratio on the Bendability and Anisotropic Bend Behavior of Extruded 6000-Series Al Alloy.

Extruded Al-Mg-Si profiles applied in the automotive industry are required to achieve an appropriate combination of strength and bendability. In order to investigate the effect of Mg/Si ratio on the bendability and anisotropic bending behavior, AA6005 and AA6061C were extruded to 2 mm thick plates. More Goss texture and anisotropic particle clusters exist in AA6005 alloys with a low Mg/Si ratio, which leads to a high tendency of surface roughing and cracking and to strong anisotropy in their bendability. However, more low-angle grain boundaries, cubic texture and comparatively random distribution of particles exists in AA6061C alloys with a high Mg/Si ratio, which blunts the surface roughing and crack process. The surface undulation is the outcome of the strain-intense localization of several layers of grains in the vicinity of the outer elongated surface. The strain localization and surface undulation lead to shear band initiation near the valleys. Several cooperating micro-mechanisms in AA6005, including grain clusters with Goss and Cubic orientation, heterogeneously nucleated particles and grain boundary spatial arrangements, lead to the grain boundary decohesion along a shear direction. AA6005 shows for predominately intergranular fractures in nature, with some areas exhibiting grain boundary decohesion during bending in the TD. However, AA6061C shows a predominately transgranular in nature, with some areas exhibiting intergranular fracture, which is affected by shear band development.

Fine-tuning of mechanical properties of additively manufactured AlSi10Mg alloys by controlling the microstructural heterogeneity

Cell structures decorated with Si precipitates are key microstructures that endow AlSi10Mg alloys produced by laser powder bed fusion (LPBF) with excellent mechanical properties. This study investigated the effect of heat treatment on the cell structure and properties of LPBFed AlSi10Mg alloys. Heat-treatment was performed within the temperature range of 270–320 °C, which did not alter the crystallographic texture and grain structure of the alloys. When the heat-treatment temperature increased, the strain-hardening ability of the alloys decreased through a weakening of the cell structure, leading to a decrease in the strength and uniform elongation of the alloy. Contrarily, when the annealing temperature exceeded 300 °C, the spatial heterogeneity of the cell structure decreased, significantly improving post-elongation by delaying the melt pool boundary decohesion of the alloys. This demonstrated that the positive and negative effects of the cell characteristics competitively affect the mechanical properties of the alloys, and the strength and ductility of the alloys are governed by the spatial heterogeneity of the cell structure. This study offers guidelines for fine-tuning the performance by optimizing cell characteristics of LPBFed AlSi10Mg parts.

Atomistic investigation of the impact of phosphorus impurities on the tungsten grain boundary decohesion

In the present work, we have generated a new second-nearest neighbour modified embedded atom method potential (2NN-MEAM) for the W–P system to investigate the impact of P impurity segregation on the strength of symmetric 〈110〉 tilt coincident site lattice grain boundaries (GBs) in tungsten. By incorporating the impurity-induced reduction of the work of separation in the fitting strategy, we have produced a potential that predicts decohesion behaviour as found by ab initio density functional theory (DFT) modelling. Analysis of the GB work of separation and generalized stacking fault energy data derived from DFT and the 2NN-MEAM potential show that P-impurities reduce the resistance to both cleavage and slip. Mode I tensile simulations reveal that the most dominant mode of GB failure is cleavage and that pristine GBs, which are initially ductile, on most accounts change to brittle upon introduction of impurities. Such tendencies are in line with experimentally observed correlations between P-impurity content and reduced ductility.

Microstructure-based polycrystalline finite element modeling of Inconel X-750 irradiated in a CANDU reactor

Aging of Inconel X-750 spacers in Canada Deuterium Uranium (CANDU) reactors is managed through a combined program of in-service inspection, component removal for material surveillance testing, research and development, and fitness-for-service evaluation. Crush tests of ex-service spacer coils have shown that the load carrying capacities of Inconel X-750 spacers decrease with operating time. The spacers also exhibit reduced ductility and inter-granular failure that is typical of helium (He)-embrittlement. Examination of ex-service spacers confirmed that material interfaces (mainly grain boundaries) were perforated by He-bubbles. Assuming that the grain boundary bubble coverage dictates the grain boundary strength and thus the failure load of the polycrystalline component, we have simulated the degradation and inter-granular failure of the spacer coils at low and high operating temperatures.The observed inter-granular fracture has been simulated by a cohesive zone model that governs the grain boundary de-bonding process. The grain boundary strength is related to the perforation by He-stabilised cavities. The model of grain boundary traction separation consists of three parts: (i) a rate-theory calculation to predict grain boundary coverage; (ii) a local material degradation model to quantify helium induced grain boundary strength reduction; (iii) a traction separation model to simulate grain boundary de-cohesion. The results show that the accumulation of He-stabilised cavities on grain boundaries causes material failure at lower stresses and strains that are dependent on the irradiation temperature and the neutron exposure. The load carrying capacity of the X-750 spacers decreases non-linearly with irradiation and the rate of change is decreasing with increasing dose.

Computational modelling of hydrogen assisted fracture in polycrystalline materials

We present a combined phase field and cohesive zone formulation for hydrogen embrittlement that resolves the polycrystalline microstructure of metals. Unlike previous studies, our deformation-diffusion-fracture modelling framework accounts for hydrogen-microstructure interactions and explicitly captures the interplay between bulk (transgranular) fracture and intergranular fracture, with the latter being facilitated by hydrogen through mechanisms such as grain boundary decohesion. We demonstrate the potential of the theoretical and computational formulation presented by simulating inter- and trans-granular cracking in relevant case studies. Firstly, verification calculations are conducted to show how the framework predicts the expected qualitative trends. Secondly, the model is used to simulate recent experiments on pure Ni and a Ni–Cu superalloy that have attracted particular interest. We show that the model is able to provide a good quantitative agreement with testing data and yields a mechanistic rationale for the experimental observations.

Hydrogen embrittlement micromechanisms and direct observations of hydrogen transportation by dislocations during deformation in a carbon-doped medium entropy alloy

The hydrogen embrittlement micromechanisms and the effect of carbon interstitial on hydrogen distribution were characterized in Fe40Mn40Ni10Cr10 and Fe38Mn41Ni10Cr10C1 medium entropy alloy. Ex-situ microstructural observations revealed that the segregation of carbon on grain boundaries suppresses hydrogen from being trapped in the grain boundaries for carbon-doped alloy before deformation. However, the distribution of hydrogen was similar for both alloys after plastic strain so that the grain boundaries trapped a large fraction of hydrogen during deformation. The fully intergranular fracture mode in the hydrogen affected area of both alloys was explained by the synergy of grain boundary–dislocation reactions and hydrogen-enhanced grain boundary decohesion effects.

Hydrogen trapping and embrittlement in high-strength Al alloys

Ever more stringent regulations on greenhouse gas emissions from transportation motivate efforts to revisit materials used for vehicles1. High-strength aluminium alloys often used in aircrafts could help reduce the weight of automobiles, but are susceptible to environmental degradation2,3. Hydrogen ‘embrittlement’ is often indicated as the main culprit4; however, the exact mechanisms underpinning failure are not precisely known: atomic-scale analysis of H inside an alloy remains a challenge, and this prevents deploying alloy design strategies to enhance the durability of the materials. Here we performed near-atomic-scale analysis of H trapped in second-phase particles and at grain boundaries in a high-strength 7xxx Al alloy. We used these observations to guide atomistic ab initio calculations, which show that the co-segregation of alloying elements and H favours grain boundary decohesion, and the strong partitioning of H into the second-phase particles removes solute H from the matrix, hence preventing H embrittlement. Our insights further advance the mechanistic understanding of H-assisted embrittlement in Al alloys, emphasizing the role of H traps in minimizing cracking and guiding new alloy design.

Grain boundary decohesion in body-centered cubic Mg-Li-Al alloys

Body-centered cubic (BCC) Mg-Li alloy can be effectively strengthened by Al addition. However, brittle fracture degrades the ductility. Further improvement in mechanical properties requires a deep understanding of the fracture mechanism. In this work, we investigate the fracture behaviour of BCC Mg-Li-Al alloys and probe underneath mechanisms. Three factors are likely responsible for the grain boundary decohesion: narrow precipitation free zones provide dislocations initiation and slip pathway; shearing of intragranular precipitates by planar slip will cause large stress concentration to GBs at the end of slip bands; grain boundary precipitates of Al-rich ordered phases weaken the interatomic bonding among grains. Once the stress locally exceeds GB cohesion, cracking nucleates and propagates along/through GBs and intergranular/cleavage fracture takes place.

Creep anisotropy modeling and uncertainty quantification of an additively manufactured Ni-based superalloy

The advantages offered by additive manufacturing over traditional processes has driven a great deal of industrial and academic interest in recent years. However, the process is relatively new and requires additional investigation to become sufficiently mature for wide scale industrial adoption. Electron beam melting powder bed fusion is one technology that has shown promise for fabricating high temperature resistant materials such as nickel based superalloys. The resulting microstructures typically exhibit a strong fiber texture in the build direction giving rise to anisotropic time-dependent deformation behavior. In order to accelerate the qualification of these materials for industrial adoption accurate numerical models are needed for simulating their behavior. In this work a crystal plasticity model including non-Schmid effects is presented for capturing creep anisotropy observed in additively manufactured IN738LC. The model is calibrated via a probabilistic framework where model parameters are treated as random variables. An iterative sequential design strategy is utilized to efficiently identify the probability density of the unknown model parameters. As a case study the model is utilized to investigate the behavior of randomly oriented equiaxed grain clusters sometimes observed embedded in the additively manufactured columnar structure. A synthetic realization is simulated and uncertainty is propagated through to the full-field response. Results indicate that these features are the source of significant creep relaxation and strain accumulation which partially explains observed grain boundary decohesion at these locations.

The effect of solute atoms on the bulk and grain boundary cohesion in Ni: Implications for hydrogen embrittlement

Solute atoms dissolved in the Ni matrix can segregate to grain boundaries and have a pronounced effect on the performance of Ni-base alloys. Some elements, like, hydrogen lead to deterioration of interatomic bonding in the alloys resulting in a phenomenon known as hydrogen embrittlement. In this paper, we investigate the effect of 83 impurity atoms on the bulk and grain boundary decohesion resistance of Ni by calculating changes in the partial cohesive energy and in the grain boundary work of separation from density functional theory. The calculated results are used as training sets for two machine learning models. Both models are based on the use of the partial cohesive energies as one of the key features and posses very good predictive capability. The calculated data are analyzed in detail to identify the effect of all computed solutes on the grain boundary cohesive properties in Ni general and their effect on the overall resistance of Ni to hydrogen enhanced decohesion in particular.

Effect of grain size on fracture behavior of pure magnesium sheet: An in-situ study combined with grain-scale DIC analysis

In this work, a series of in-situ experiments were conducted to study the effect of grain size on fracture behavior of commercial-purity Mg polycrystalline aggregates. For this purpose, pure Mg rolled at 225 °C was annealed at 325 °C and 525 °C, each for 1 h, to acquire different grain structures. Micro specimens of the as-rolled and anneal-treated sheets were subjected to tension in the rolling direction using a micro-tensile loading stage integrated with an optical microscope to discover micro-mechanisms triggering failure at room temperature. Moreover, grain-scale DIC analysis was applied to the optical images taken during tension to quantify strain heterogeneity and plastic anisotropy coefficient in the specimens. It was found that the dominant failure mechanism changes by grain size. In the fine grain sample with an average grain size (AGS) of 13.3 μm, shear bands developed during tension played an important role in the damage. In the sample with intermediate grain size (AGS = 36.1 μm), intergranular fracture took place along surface steps that stemmed from slip-induced grain boundary sliding. In the sample with the largest grains (AGS = 137.2 μm), twin cracking as well as grain boundary decohesion were the primary mechanisms by which failure proceeded. Micro DIC analysis revealed that the formation of steps and twins was accompanied by heavy strain localizations in the microstructure, making them susceptible sites to fracture.

Effect of Mo doping on the gaseous hydrogen embrittlement of a CoCrNi medium-entropy alloy

By using slow-strain-rate tensile test and post-mortem characterization of microstructure and fractography, we found that Mo-doping can simultaneously improve the strength and hydrogen resistance of an equi-molar CoCrNi medium-entropy alloy. At a high hydrogen concentration of 55.51 mass ppm, CoCrNi exhibited brittle intergranular fracture. By contrast, (CoCrNi)97Mo3 possessed a mixed fracture morphology with lots of dimples and a small portion of intergranular feature. We attributed the enhanced hydrogen resistance to the Mo-promoted twinning-dominated deformation process, which can suppress the redistribution of hydrogen concentration by dislocation motion, and thus inhibit the hydrogen-induced grain boundary decohesion.

Hydrogen effect on the mechanical behaviour and microstructural features of a Fe-Mn-C twinning induced plasticity steel

The influences of hydrogen on the mechanical properties and the fracture behaviour of Fe-22Mn-0.6C twinning induced plasticity steel have been investigated by slow strain rate tests and fractographic analysis. The steel showed high susceptibility to hydrogen embrittlement, which led to 62.9% and 74.2% reduction in engineering strain with 3.1 and 14.4 ppm diffusive hydrogen, respectively. The fracture surfaces revealed a transition from ductile to brittle dominated fracture modes with the rising hydrogen contents. The underlying deformation and fracture mechanisms were further exploited by examining the hydrogen effects on the dislocation substructure, stacking fault probability, and twinning behaviour in pre-strained slow strain rate test specimens and notched tensile specimens using coupled electron channelling contrast imaging and electron backscatter diffraction techniques. The results reveal that the addition of hydrogen promotes planar dislocation structures, earlier nucleation of stacking faults, and deformation twinning within those grains which have tensile axis orientations close to <111>//rolling direction and <112gt;//rolling direction. The developed twin lamellae result in strain localization and micro-voids at grain boundaries and eventually lead to grain boundary decohesion.