Grain Boundary Cracks Research Articles

Atomic-scale insights into grain boundary-mediated plasticity mechanisms in a magnesium alloy subjected to cyclic deformation

Sustaining grain boundary (GB) plasticity is important to prevent the premature GB cracking during plastic deformation of hexagonal-close-packed (HCP) materials. Here, we investigated atomic structures of GBs and their deformation modes in the form of motion, twin nucleation, dislocation emission, and dissociation in a HCP magnesium alloy subjected to cyclic deformation, according to atomic-resolution observations and crystallographic analyses. Besides migration and sliding, symmetric tilt GBs tended to move to form local facets, resulting in generation of asymmetric tilt low-angle GBs (LAGBs) parallel to {101¯2} lattice plane or serrated asymmetric tilt high-angle GBs (HAGBs). Crystallographic analyses indicated that local facets of the resultant asymmetric tilt GBs were often parallel to {101¯2} plane, basal and/or prismatic planes of adjacent grains. Interestingly, abundant {101¯2} twin embryos and recrysatllized nanograins nucleated from local facets of the asymmetric tilt GBs, accompanying with emission of arrays of basal 〈a〉 dislocations. A {101¯2} twin lamella along a tilt LAGB could be formed either by the coalescence of {101¯2} twin embryos, or by the combination of recrystallized nanograins, followed by {101¯2} twin nucleation from triple junctions of GBs and twin growth. Furthermore, the twin lamella could be formed through dissociation of one tilt LAGB into two tilt HAGBs, followed by {101¯2} twin nucleation from the HAGBs and triple junctions of GBs. Importantly, activation of GB deformation modes mentioned above can release local stress concentrations at GBs and sustain the ability of GB-mediated plasticity, playing important roles in plastic deformation and mechanical properties of HCP materials.

Investigation of Microstructure and Mechanical Properties of the Repaired Precipitation-Strengthened Ni-Based Superalloy via Laser Melting Deposition

In this study, a typical γ′ phase precipitation-strengthened Ni-based superalloy DZ411 was repaired using an LMD-based repairing technique with an IN738LC superalloy, and crack-free samples were acquired. The mechanical properties and microstructure of different areas inside the repair sample were investigated, including the IN738LC deposit, the DZ411 substrate, and the interface between these two parts. The differences in mechanical properties between different areas were explained via analyzing fractography and KAM maps. It was found that the coarse carbides of the DZ411 substrate might lead to rapid cracking of grain boundaries, resulting in the worst mechanical properties of the DZ411 substrate. The IN738LC deposit demonstrated significantly superior mechanical properties in comparison to the DZ411 substrate. Its tensile strength exceeded that of the substrate by over 250 MPa, while its relative elongation after fracture was twice as great as that of the substrate. The excellent mechanical properties of the IN738LC deposit could be attributed to its fine microstructure, which resisted rapid cracking and generated a large number of GNDs during the plastic deformation process. For the interface between the deposit and substrate, although its hardness before the tensile test was low, it could also generate many GNDs during the plastic deformation process, hence exhibiting commendable mechanical properties. The research results show that using an LMD-based repairing technique with IN738LC superalloy to repair γ′ phase precipitation-strengthened Ni-based superalloy DZ411 is a feasible solution.

Revealing the Grain-Boundary-Cracking Induced Capacity Decay of a High-Voltage LiCoO2 at 4.6 V.

During a practical battery manufacture process, the LiCoO2 (LCO) electrodes are usually rolled with high pressure to achieve better performance, including reducing electrode polarization, increasing compact density, enhancing mechanical toughness, etc. In this work, a high-voltage LCO (HV-LCO) is achieved via modulating a commercialized LCO with an Al/F enriched and spinel reinforced surface structure. We reveal that the rolling can more or less introduce risk of grain-boundary-cracking (GBC) inside the HV-LCO and accelerate the capacity decay when cycled at 3-4.6 V vs Li/Li+. In particular, the concept of interface structure is proposed to explain the reason for the deteriorated cycle stability. As the GBC is generated, the interface structure of HV-LCO alters from a surface spinel phase to a hybrid of surface spinel plus boundary layer phases, leading to the exposure of some the nonprotective layer phase against the electrolyte. This alternation causes serious bulk structure damage upon cycles, including expanding GBC among the primary crystals, forming intragranular cracks and inactive spinel phases inside the bulk regions, etc., eventually leading to the deteriorated cycle stability. Above all, we realize that it is far from enough to achieve a eligible high-voltage LCO via only applying surface modification. This work provides a new insight for developing more advanced LCO cathodes.

Segregation-enhanced grain boundary embrittlement of recrystallised tungsten evidenced by site-specific microcantilever fracture

Tungsten stands a prime candidate for plasma-facing applications in fusion reactors, attributed to its capacity to withstand high temperatures and intensive particle fluxes. The operational heat flux, however, can induce recrystallisation of the initial microstructure, increasing the brittle-to-ductile transition temperature. Although such a phenomenon is thought to result from impurity segregation to grain boundaries, direct evidence of impurity-induced grain boundary embrittlement has not yet been reported. Addressing this, our study employs microcantilever testing, coupled with local chemical analysis via atom probe tomography, to unveil the impact of impurity segregation on the fracture toughness of recrystallised tungsten with a purity of 99.98 at.%. The in situ fracture toughness measurements were performed with the notch placed directly at random high-angle grain boundaries, revealing brittle failure regardless of grain boundary misorientation or grain orientation. Notably, both single-crystalline microcantilevers and the as-received material exhibited significant plasticity before failure, with instances without crack propagation. In contrast, recrystallised grain boundaries displayed a fracture toughness of 4.7 ± 0.4 MPa·√m, determined using a linear elastic approach - notably lower than for cleavage plane fracture in tungsten microcantilevers. Local atom probe analysis of the high-angle grain boundaries exposed phosphorous segregation exceeding 2 at.% at the recrystallised interfaces, stemming from recrystallisation. Atomistic simulations confirmed the role of phosphorous in embrittling high-angle grain boundaries in tungsten, while additionally revealing mechanisms of crack-grain boundary interactions and their dependence on phosphorous segregation.

Grain boundary microstructure-property relationships in the cast & wrought Ni-based superalloy René 41 with boron and carbon additions

Cast & wrought Ni-based superalloys are required for manufacturing high-performance components in gas turbine engines. Their excellent high-temperature properties and corrosion resistance are brought about by their complex microstructure of a γ-matrix, intra- and intergranular γ' precipitates, carbides, and borides. The most highly-alloyed superalloy grades provide the high-temperature strength required for the design of next generation gas turbine engines but their processability remains challenging due to grain boundary (GB) cracking. A common mitigation approach is the addition of B and C to improve GB cohesion and strength. However, their detailed roles in GB segregation and precipitation and the link to GB properties during processing remains largely unexplored.We provide a systematic study on decorated GBs in the cast & wrought Ni-based superalloy René 41 with B and C additions. Tensile testing reveals that B additions reduce its ductility due to GB cracking but slow down grain growth during heat treatments more effectively, than C additions. GB-M2B, M6C, M23C6, and GB-γ' precipitates decorating GBs are sequentially precipitated. Their structures and compositions are shown to be unaffected by B and C additions. However, significant differences in the interfacial segregation of solutes are revealed. B is captured by GB-M2B and therefore unavailable to improve GB cohesion. C depletion and enrichment of solutes at interphase boundaries are identified as origin for the reduced ductility and thus, GB cracking. The findings are summarized as microstructural model and advance the detailed understanding of the processing – microstructure – properties relationships of cast & wrought Ni-based superalloys.

Atomic-scale understanding of solute interaction effects on grain boundary segregation, precipitation, and fracture of ultrahigh-strength maraging steels

Understanding the fundamental mechanisms of embrittlement and de-embrittlement is crucial for the development of strategies toward advanced steels with improved performance. In this study, the solute interaction effects on grain boundary (GB) segregation, precipitation, and fracture of Fe–Ni–Ti–(Mo) maraging steels were systematically investigated through a combination of experimental and theoretical techniques, including scanning transmission electron microscopy, atom probe tomography, and first-principles calculations. Our results reveal that the Fe–Ni–Ti maraging steel suffers from serious intergranular embrittlement and the mechanism is related to the formation of coarse Ni3Ti precipitates and associated precipitate-free zones (PFZs) at GBs, which facilitate the crack initiation and growth along the GBs. Interestingly, adding Mo to the maraging steel effectively suppresses the intergranular embrittlement, thereby substantially improving the ductility. Atomistic analyses reveal that Mo de-embrittles the GBs by reducing the segregation of Ni and Ti, which substantially inhibits the formation of coarse Ni3Ti precipitates and PFZs at the GBs, thereby alleviating their harmful impact on the GB cracking. In addition, the Mo segregation enhances the GB cohesion, which may play a minor role in suppressing the GB fracture.

Simultaneously improving yield strength and formability potential of Mg alloy via introducing nano precipitates and texture-weakened heterogeneous grains

Simultaneously improving the yield strength (YS) and formability at room temperature of magnesium (Mg) alloys is one of the key issues to broaden the application further. In this work, we report a new hot-extruded + annealed alloy (i.e., Mg–2Sm–0.8Mn–0.6Ca–0.5Zn) having good combination of yield strength (YS: 167 MPa) and formability potential (UTS−YS: 115 MPa and elongation: 25%), mainly due to formation of nano precipitates and texture-weakened heterogeneous grains. The results indicate that introduction of a certain amount of nano precipitates can enhance the strength of the alloy with little detrimental effect on ductility. The formation of heterogeneous-grained structure with a weak basal texture is mainly related to Sm/Zn/Ca elements grain boundary (GB) segregation. Geometrically necessary dislocations would nucleate at grain boundaries due to uneven deformation behavior caused by heterogeneous-grained structure, obtaining additional hetero-deformation induced hardening and large work-hardening capacity. Formation of heterogeneous-grained structure and solid solution of Sm/Zn/Ca promote the activation of considerable non-basal slips and the suppression of twinning during the deformation process. Meanwhile, GB segregation is conducive to dislocation nucleation from GBs and suppression of the GB cracking. All of these contribute to the high elongation. Thus, we put forwards a new strategy for improving the YS and formability potential via introducing nano precipitates and texture-weakened heterogeneous grains.

Chemical inhomogeneity inhibits grain boundary fracture: A comparative study in CrCoNi medium entropy alloy

Grain boundary (GB) fracture is arguably one of the most important reasons for the catastrophic failure of ductile polycrystalline materials. It is of interest to explore the role of chemical distribution on GB deformation and fracture, as GB segregation becomes a key strategy for tailoring GB properties. Here we report that the inhomogeneous chemical distribution effectively inhibits GB fracture in a model CoCrNi medium entropy alloy compared to a so-called ‘average-atom’ sample. Atomic deformation kinematics combined with electronic behavior analysis reveals that the strong charge redistribution ability in chemical disordered CrCoNi GBs enhances shear deformation and thus prevents GB crack formation and propagation. Inspects on the GBs with different chemical components and chemical distributions suggest that not only disordered chemical distribution but also sufficient “harmonic elements” with large electronic flexibility contribute to improving the GB fracture resistance. This study provides new insight into the influence mechanism of GB chemistry on fracture behavior, and yields a systematic strategy and criterion, from the atoms and electrons level, forward in the design of high-performance materials with enhanced GB fracture resistance.

Developing a Mg alloy with ultrahigh room temperature ductility via grain boundary segregation and activation of non-basal slips

Poor room temperature (RT) ductility is an inherent disadvantage of magnesium (Mg) alloys, which limits their wide application. Grain refinement is an accepted method to improve ductility and strength simultaneously. However, obtaining ultrafine grains (about 1 μm and below) usually requires severe deformation processes, which is not conducive to be widely used. Here, by utilizing a distinctive design strategy, i.e., introducing beneficial solute to achieve grain boundary (GB) segregation and effectively reduce the critical resolved shear stress (CRSS) ratio between non-basal slips and basal slip, thus efficiently activating non-basal slips, we obtained a Mg-0.3Er (at%) alloy with moderate-fine grains (∼8 μm) having elongation near 50% at RT. Based on the results of in-situ electron back-scatter diffraction and slip trace analysis, and weak beam dark-field observation, we confirmed that minor Er element and appropriate fine grains would contribute to the activation of considerable non-basal dislocations and the suppression of twinning. In addition, through energy dispersive spectrometry element analyses for systematic investigation, we found that the GB segregation level of the alloy annealed at 300 °C is higher than that of the alloy annealed at 360 °C. The higher GB segregation level would impact the cohesion strength and be conducive to activate more non-basal dislocations to accommodate local stress, thus suppressing the GB cracking and finally improving the ductility. This study not only suggests an effective way for large-scale development of high ductility Mg alloys at RT, but also provides a new insight to elucidate the origins of ultrahigh ductility by revealing the fraction of non-basal slips and the level of GB segregation.

Stable cycling of high-density three-dimensional sintered LiCoO2 plate cathodes

High-density sintered plate-type cathodes composed of only an active material can achieve a high energy density of lithium-ion batteries (LIBs) beyond the current state of the art. However, slow lithium ion movement in the solid-state cathode plate and deterioration of the grain boundary of sintered cathodes over many charge/discharge cycles are obstacles to commercialization. Herein, we present a new three-dimensional high-density sintered LiCoO2 plate cathode with periodically aligned channels, which enable fast charging/discharging of high-energy-density LIBs. In addition, we investigate the grain-boundary cracking mechanism and introduce a conductive coating agent to LiCoO2 cathodes that prevents crack propagation, enhances electron transfer between sintered LiCoO2 grains, and prevents the formation of an undesirable cathode electrolyte interphase. In full-cell experiments, the cell with a sintered LiCoO2 cathode with industrial-level current density (3.5 mA cm−2) has a volumetric capacity (723 mAh cm−3) 21% higher than that of a conventional cathode (597 mAh cm−3). The cathode coated with the conductive agent shows a 30% higher cycle life after testing for 500 cycles compared to the bare cathode. High-density sintered cathodes are fabricated with an industrially acceptable cycle lifetime of 84% capacity retention over 500 cycles at 1 C.

The application of in situ TEM on the grain boundary brittleness of precipitation-strengthened Ni-based superalloys: Recent progress and perspective

Due to their superior strength and oxidation resistance at high temperatures, precipitation-strengthened Ni-based superalloys are widely used in load-bearing hot components in energy generation systems, such as gas/steam engines in aircraft or power plants. However, brittleness originating from a grain boundary (GB) in a certain temperature range is one of the greatest deadlocks, which desperately restricts their thermal-mechanical processing capabilities and also industrial applications. Experimental and theoretical investigations of the origin of GB brittleness with aims to overcome it still attract many research efforts in the high-temperature material field. It is desirable to understand the GB embrittlement mechanism by dynamically investigating the entire GB cracking process in real time under stress/temperature combination, which might be hardly revealed by a traditional experiment on precipitation strength/theoretical technologies. Current advances in high-temperature mechanical testing systems, which can be operated in a transmission electron microscope (TEM), provide unique opportunities for in situ exploration of the mechanistic origins of GB brittleness of superalloys with the resolution up to an atomic scale. Here, we first briefly give an overview of the phenomenon and current understanding on GB brittleness, followed by introducing the state-of-art techniques in an in situ TEM/mechanical testing system (MTS). In the end, we will also discuss the potential application of the in situ TEM/MTS on GB brittleness and a perspective overlook.

Dynamic failure behavior and damage evolution process of holed sandstone under impact loads

In this research, the damage process and dynamic failure behavior of some sandstone rocks, containing an elliptical hole, were evaluated. For this reason, a series of laboratory tests under both static and dynamic loads in 4 different orientations of the elliptical hole was performed. Modified Split Hopkinson Pressure Bar testing machine was employed to apply impact load. Dynamic fracture evolution was recorded by utilizing a high-speed camera to evaluate the macro damage evolution process. Scanning electron microscope analysis and thin section studies were also used to assess mineral's behavior regarding micro-crack evolution. Furthermore, the experimental tests were numerically simulated, and strain energy density and dynamic stress concentration factor were measured to evaluate the effect of elliptical hole inclination on dynamic response. The results indicated that when the largest diameter is perpendicular to core axis, the rock shows the lowest strength in both static and dynamic loading conditions. Also, with increasing hole inclination, both strain energy density and dynamic stress concentration factor decreased. Evaluation of fracture surface indicated that grain-boundary cracks are the dominant type of cracks and iron oxide cement distribution has a vital role in the development of the cracks.

Oxidation assisted recrystallization and cracking at grain boundaries in Nimonic 80 A during elevated temperature service

Oxidation and cracking at the grain boundary (GB) of wrought Nimonic 80 A alloy exposed to 550 °C for 2000 h and statically loaded to 490 MPa, is studied. The GB cracks and transforms to a hierarchical structure with a nanoscale recrystallized Ni layer in the center flanked by ambilateral NiCr2O4/NiCrO3 layers. Widths of both layers increases with increasing proximity to the crack and the recrystallized crack bearing Ni layer oxidizes. Formation of the recrystallized Ni layer is attributed to oxidation of Cr and externally applied stress. Oxidation mechanism and cracking of the GB in Nimonic 80 A is discussed in detail.

Crack initiation of neutron-irradiated 304 L stainless steel in PWR primary water

Stress corrosion crack (SCC) initiation of neutron-irradiated 304 L stainless steel was studied following irradiation to various doses. Tensile test results show that SCC susceptibility dependence of the alloy on dose is slight. Grain boundary (GB) oxidation was observed prior to crack initiation and DCs always proceeded before or accompanies crack initiation in the bend test. The DC-GB intersections were the predominant crack initiation sites due to the high local stress. Constant load tests resulted in greater GB oxidation and cracks initiated at applied stress as low as 50% of the irradiated yield strength due to the long exposure time.

Dynamic fracture mechanism and fragmentation analysis of fine grained Al2O3/SiC composite

The Al2O3/SiC composite has an improved mechanical property compared with alumina matrix, and it can be used in the lightweight armour system. In this study, the dynamic tensile and compressive strength, strain rate sensitivity, fracture mechanism and fragments scale of the composite were carried out by experimental and theoretical analysis. Results showed that the dynamic strength of the Al2O3/SiC composite was sensitive to the strain rate, and it was controlled by its microstructure. The microfracture mechanism of the composite varied with the locations and stress states of specimen, which consisted of second phase particles effects, the intergranular and transcrystalline fracture mode, fractured core area, grain-boundary crack, grain distortion and splitting, cleavage fracture zones, slip bands et al. In the dynamic compression experiments, the statistic analysis results of recycled fragments were more agreed with the prediction of the modified DID model.

Columnar grain-driven plasticity and cracking in nanotwinned FCC metals

The mechanisms of strengthening and plasticity in columnar-grained metals with preferentially oriented nano-sized twins have been examined traditionally by considering dislocation processes, but rarely from the perspective of grain boundary (GB) deformation. Here, the effects of GB strain accommodation on plastic deformation in four different columnar-grained nanocrystalline nanotwinned (nt) face-centered-cubic metals (Cu, Ag, Al, and Ni) were studied by large-scale molecular dynamics simulations. It is observed that in tensile deformation parallel to coherent twin boundaries (CTBs), the dislocation mechanisms in each metal are identical and associated with GB emissions of jog and threading dislocations at small and large CTB spacings, respectively. However, CTB strengthening effects are increasingly more pronounced in columnar-grained nt metals as their shear modulus increases, which is rationalized by the dependence of GB stress concentrations on twin size, metal type and strain rate. Also, while flow stresses in nt-Cu, nt-Ag, and nt-Al metals increase linearly with decreasing CTB spacing, a maximum strength limit is reached in nt-Ni below a critical CTB spacing of 6 nm. The strength limit in nt-Ni results from columnar GB cracking induced by prominent GB sliding. For columnar-grained microstructures, GB sliding is equivalent in nt-Ag and nt-Al and slightly lower in nt-Cu but markedly higher in nt-Ni. These findings underscore the importance of new GB deformation mechanisms on plasticity and fracture in columnar-grained nt metals and enrich our understanding of CTB strengthening in fcc metals synthesized in the literature.

A single grain boundary parameter to characterize normal stress fluctuations in materials with elastic cubic grains

A finite element analysis of intergranular normal stresses is performed in order to identify a possible statistical correlation between the intergranular normal stresses and the corresponding grain boundary type within a polycrystalline aggregate. Elastic continuum grains of cubic lattice symmetry are assumed in the analysis. Meaningful results are obtained by analyzing the first two statistical moments of grain boundary normal stresses obtained on several grain boundary types.Among the five macroscopic parameters (5D) describing a grain boundary, the orientation of the grain boundary plane relative to the two adjacent crystal lattices (4D) is found to be the most important property influencing the normal stresses. To account for it, a single new (1D) parameter E12 is introduced, which combines the geometrical aspect of grain boundary with its material properties and measures the average stiffness of grain boundary neighborhood along the grain boundary normal direction. It is demonstrated that E12, in combination with Zener elastic anisotropy index A, is able to accurately predict normal stress fluctuations on any grain boundary type in a material with cubic lattice symmetry. It is argued that largest normal stresses most likely form on grain boundaries whose normals are oriented along the stiffest direction in both adjacent grains (〈111〉 for crystals with A>1 or 〈001〉 for crystals with A<1). Moreover, it is shown that classification of grain boundaries according to their propensity to exhibit large normal stresses can be trivially reduced to the (analytical) calculation of the corresponding effective stiffness parameter E12.A few practical implications are discussed relevant to intergranular stress-corrosion cracking of Coincidence Site Lattice grain boundaries. For example, it is highlighted that in face-centered cubic materials the coherent Σ3 twin grain boundaries, which are known for their very high cracking resistance, nevertheless exhibit largest intergranular normal stresses, indicating that cracking resistance is associated with high grain boundary strength.

Crystal cracking of grain-gradient aluminum by a combined CPFEM-CZM method

Gradient nanostructured metals attract extensive attention due to their excellent mechanical properties, while they are prone to microcracks during forming and servicing process and eventually develop into a sudden fracture. Actually, experimental studies show that microcracks occurred in both the grain boundary (GB) and the intragrain, but most of the current reports only focus on the GB cracking, very limited work has studied the intragranular cracking. In this paper, the crystal plasticity finite element method (CPFEM) and cohesive zone model (CZM) are combined to study the cracking mechanism of polycrystalline aluminum (Al) with different grain-gradient structures under tensile load, where molecular dynamics (MD) method is used to determine the cohesive parameters of the intragrain and GB. The results not only show the crack initiation and propagation process of polycrystalline Al with different grain-gradient structures, but also reveal the mechanism of intragrain fracture, GB fracture and crack transgranular. The grain-gradient distribution with optimal comprehensive performance is obtained. Moreover, it is also found that the initial microcracks with different positions, numbers and angles have a great influence on the cracking mechanism and effective properties of the whole material. This study provides a solid theoretical basis for improving the quality and operation life of metal parts.

Investigation on the grain boundary strengthening effect of a nickel‐based superalloy

AbstractThe grain boundary strengthening effect (GBSE) was investigated for nickel‐based superalloy bicrystals with various misorientations under different temperatures and loading directions. The results show that the GBSE is enhanced with the increase of misorientation and insensitive to the temperature and the loading direction relative to the GB also has a strong effect on the GBSE. On the other hand, the GBSE is often accompanied by GB cracking. The trend of grain boundary (GB) cracking is heightened with the increase of the misorientation and temperature, and the transformation of the loading direction from vertical to parallel.

Synthesis routes to eliminate oxide impurity segregation and their influence on intergrain connectivity in K-doped BaFe2As2 polycrystalline bulks

The poor reproducibility of intergrain critical current density Jc in Fe-based superconductors is often believed to result from uncontrolled grain boundary (GB) connectivity degraded by extrinsic factors such as the local or global impurity concentration or GB porosity or cracks. Earlier we found that Ba and K can appear as oxide impurities at GBs, along with GB-wetting FeAs. In this study, we evaluated how the sample preparation environment and purity of the starting materials influence the polycrystalline Jc in K-doped BaFe2As2 (Ba122) bulks. Using a high-performance glovebox, the oxygen and water levels were significantly reduced, eliminating traces of FeAs. Oxide impurities and Ba (or K) segregation associated with oxygen in the starting materials were significantly reduced by using high purity starting materials. This combination essentially doubled the best Jc(4.2 K) values to 2.3 × 105 at self-field and 1.6 × 104A cm−2 at 10 T and analytical scanning transmission electron microscopy showed no GB or O segregation in the best samples, but did show dark Z-contrast and distinct nanoscale porosity. Our work shows that an inert synthesis environment and high purity K and Ba do reduce current-blocking oxygen impurity and GB impurity phases, allowing deeper exploration of the role of extrinsic and intrinsic GB blocking effects in controlling the Jc of polycrystalline Ba122.