Grain Boundaries Research Articles

Learning grain boundary segregation behavior through fingerprinting complex atomic environments

Although continuum-scale segregation is a well-documented behavior in multi-species materials, detailed site-specific behavior remains largely unexplored. This is partially due to the complexity of analyzing materials at the requisite time and length scales for describing segregation with full atomic accuracy. Here, we better evaluate the segregation behavior of disordered grain boundary (GB) atomic environments through leveraging a set of Strain Functional Descriptors (SFDs) to generate an atomic descriptor (i.e., fingerprint). Using this atomic fingerprint, we resolve key relationships between atomic structure and segregation energy. Machine learning (ML) techniques are utilized in concert with this SFD fingerprint to elucidate complex relationships relating segregation potential to changes in specific features of the local Gaussian density captured by the SFDs. Finally, we identify relationships that indicate both individual and joint structure-property correlations. Linking atomic segregation energy to key structural features demonstrates the value of higher-order descriptors for uncovering complex structure-property relationships at an atomic scale.

Enhanced comprehensive mechanical properties of laser additive manufactured TC17 by design of new heat treatment based on continuous cooling transition

Heat treatment is critical for enhancing the mechanical properties of high strength titanium alloys, especially for exploiting the potential of laser additive manufactured titanium alloys. In this work, the influence of the cooling rate of continuous cooling transition on microstructure evolution and mechanical behavior was investigated in TC17 titanium alloy fabricated by laser directed energy deposition (LDED) technology. It was found that the number density and orientation characteristics of grain boundary α phases (αGB) are jointly influenced by the cooling rate and the structure of β/β grain boundaries (GBs). The average number density (λavg) of αGB has a consistent trend with the degree of variant selection (DVS) for precipitated α clusters subsequently, which is attributed to the autocatalytic effect of the pre-existing α on the post-precipitated α phases. The largest λavg of αGB and the highest DVS of α clusters could be simultaneously obtained at a suitable cooling rate (4 °C/min). In that case, plenty of α/β phase interfaces and dominant variant type ensure high strength, meanwhile, the combinations of activated multi-slip systems and varied crack propagation paths extend work-hardening to maintain greater plastic deformation. This paper provides a novel thought for designing customized heat treatments of LDEDed high strength titanium alloys, and more importantly, promotes the engineering applications of large and complex components prepared by additive manufacturing technology.

Negative effect of lanthanum tungstate on the mechanical properties of W-La2O3 alloys

With the rapid development of the photovoltaic industry, tungsten alloys have been selected to replace the high‑carbon steel to fabricate the busbars for diamond wire saws due to their superior mechanical properties, and the La2O3 doped tungsten alloy has been widely used. However, the yield of eligible W alloy wires is not very high due to serious wire breakage problems during the production process, especially in the solid-liquid mixing route. To discover the possible reason, the commonly produced lanthanum tungstate (La30W17O96) in the solid-liquid mixing route has been systematically studied in the work, and its possible influence on the mechanical properties is also explored. It is found that the La30W17O96 leads to the formation of brittle second phases at grain boundaries (GBs), which weakens the intergranular bonding, resulting in a significant decrease in the compressive strength of tungsten alloys, manifested as intergranular fracture. Transmission electron microscopy (TEM) analysis reveals that these second phases are consisted of polycrystalline La6W2O15, La2W3O12 and amorphous states, and the La30W17O96 can decompose into a more stable structure during sintering. This work highlights the destructive effect of La30W17O96 on the intrinsic properties of tungsten alloys and provides a new perspective for solving the problem of tungsten wire breakage.

Unraveling impacts of polycrystalline microstructures on ionic conductivity of ceramic electrolytes by computational homogenization and machine learning

The ionic conductivity at the grain boundaries (GBs) in oxide ceramics is typically several orders of magnitude lower than that within the grain interior. This detrimental GB effect is the main bottleneck for designing high-performance ceramic electrolytes intended for use in solid-state lithium-ion batteries, fuel cells, and electrolyzer cells. The macroscopic ionic conductivity in oxide ceramics is essentially governed by the underlying polycrystalline microstructures where GBs and grain morphology go hand in hand. This provides the possibility to enhance the ion conductivity by microstructure engineering. To this end, a thorough understanding of microstructure–property correlation is highly desirable. In this work, we investigate numerous polycrystalline microstructure samples with varying grain and grain boundary features. Their macroscopic ionic conductivities are numerically evaluated by the finite element homogenization method, whereby the GB resistance is explicitly regarded. The influence of different microstructural features on the effective ionic conductivity is systematically studied. The microstructure–property relationships are revealed. Additionally, a graph neural network-based machine learning model is constructed and trained. It can accurately predict the effective ionic conductivity for a given polycrystalline microstructure. This work provides crucial quantitative guidelines for optimizing the ionic conducting performance of oxide ceramics by tailoring microstructures.

Intergranular fracture, grain-boundary structure, and dislocation-density interactions in FCC bicrystals

A dislocation-density based crystalline plasticity (DCP) and nonlinear finite element (FE) analysis were used to predict, and fundamentally understand how and why fracture nucleation and propagation are related to the interrelated microstructural mechanisms of dislocation-density pileups, GB structure, orientation, and total and partial dislocation density interactions within and adjacent for a random low angle grain boundary (LAGB) and a random high angle GB (HAGB). The GB orientations and structures were obtained from micropillar experiments, such that LAGBs and the HAGBs can be accurately represented and used for the modeling predictions. The normal stress, density of pileups, and dislocation-density accumulation along and within the GB were higher for the low angle GB bicrystal. These interrelated phenomena delineate how fracture for high angle GBs nucleate and propagate at lower nominal strains than the lower angle GB bicrystal case. These predictions underscore how fundamental mechanisms can be identified and used to understand how failure nucleates and propagates for different GB structures and orientations.

A polycrystal plasticity-cellular automaton integrated modeling method for continuous dynamic recrystallization and its application to AA2196 alloy

Continuous dynamic recrystallization usually dominates the microstructural evolution in hot working of aluminum alloys, in which the high-angle grain boundaries of new grains mainly originate from the gradual increase in subgrain misorientation angles. In this work, an integrated computational method is proposed to simulate continuous dynamic recrystallization process of aluminum alloys by coupling three-dimensional cellular automaton and visco-plastic self-consistent models. The stress response, dislocation accumulation and recovery, and evolution of crystal orientations are computed in the context of polycrystal plasticity; the formation and rotation of subgrains, followed by stored energy and curvature-driven boundary migration, are captured and visualized by cellular automaton. The non-octahedral slip mode {110}<110> is additionally introduced to capture the 〈001〉 texture during hot compression. A universal cell topology deformation method is adopted to achieve an effective track of grain morphology evolution during plastic deformation. The proposed simulation framework is validated through simulating the isothermal uniaxial compression process of AA2196 alloy under different temperatures and strain rates. The orientation dependence of CDRX during compression is numerically reproduced by correlating the subgrain formation and rotation process with the activation state of slip systems. The simulated macroscopic flow stress, 3D microstructure and inherent microstructural characteristics such as subgrain size, subgrain boundaries and textures are in good agreement with the experimental results. The proposed method provides an effective and efficient tool for multi-scale simulation of hot forming process of aluminum alloys.

The texture evolution near the shear band and corresponding impact on the microvoid nucleation and crack propagation of Ti6242s alloys under high strain rates

This study investigates the texture evolution near the adiabatic shear bands (ASBs) and its impact of on the adiabatic shear failure mechanism, focusing on microvoid nucleation and crack propagation in Ti6242s alloy with an equiaxed α microstructure under high strain rates, using a split Hopkinson pressure bar (SHPB) for the experiments. To achieve this, the initial microstructure was used to enable the texture with the c-axis perpendicular to the loading direction. After compression, the initial texture transformed into two main texture components through different mechanisms. One texture component, aligned parallel to the loading direction, is strongly associated with the {10 1‾ 2} tensile twins, which were widely distributed near the ASBs with their c-axis are 85° to the parent grains in the initial microstructure. The other texture component, with its c-axis perpendicular to the ASB, is formed by the basal slip activation during adiabatic shear deformation. Besides, it should be noted that twinning resulted in the formation of hard grains (with the c-axis of α-grains parallel to the loading direction), increasing the number of hard-soft grain boundaries. A key finding of this paper is the presence of numerous microvoids near the ASB mainly initiated at soft-hard grain boundaries (approximately 65 %), attributed to stress concentration at the boundaries of soft-hard grains, making these boundaries potential nucleation sites for microvoids. Additionally, basal slip bands in the equiaxed α grains provide rapid channels, accelerating the crack propagation rate. This study provides new insights into refining the adiabatic shear failure mechanism under dynamic loading.

Gapped and Rotated Grain Boundary Revealed in Ultra-Small Au Nanoparticles for Enhancing Electrochemical CO2 Reduction.

Although gapped grain boundaries have often been observed in bulk and nanosized materials, and their crucial roles in some physical and chemical processes have been confirmed, their acquisition at ultrasmall nanoscale presents a significant challenge. To date, they had not been reported in metal nanoparticles smaller than 2 nm owing to the difficulty in characterization and the high instability of grain boundary (GB) atoms. Herein, we have successfully developed a synthesis method for producing a novel chiral nanocluster Au78(TBBT)40 (TBBT=4-tert-butylphenylthiolate) with a 26-atom gapped and rotated GB. This nanocluster was precisely characterized using single-crystal X-ray crystallography and mass spectrometry. Additionally, an offset atomic defect linked to the peripheral Au(TBBT)2 staple was found in the structure. Comparing it to similarly face-centered cubic-structured Au36(TBBT)24, Au44(TBBT)28, Au52(TBBT)32, Au92(TBBT)44, and ~5 nm nanocrystals, the bridging Au78(TBBT)40 nanocluster exhibited higher catalytic activity in the electroreduction of CO2 to CO. This enhanced activity was explained through density functional theory calculations and X-ray photoelectron spectroscopy analysis, which highlight the impact of GBs and point defects on the surface properties of metal nanoclusters in balancing intermediate adsorption and product desorption.

Thermal deformation behavior of a Ni-Cr-Fe superalloy and its effect on corrosion resistance: A comprehensive analysis of strain rate and temperature effects

Thermal deformation behavior of a Ni-Cr-Fe superalloy billet were investigated by hot compression experiments under temperatures of 900–1200 °C and strain rates of 0.001–1 s−1, and the corrosion properties of the superalloy after thermal deformation were also evaluated by electrochemical tests. Compression experiments determined that lower strain rates and higher temperatures reduce the superalloy's stress levels. The Arrhenius-type model accurately described the thermal deformation behavior of the superalloy. Furthermore, the optimal hot working domain was identified between 1120 and 1170 °C and strain rates of 0.001–0.01 s−1. Microstructure observations showed that the dynamic recrystallization (DRX), characterized by grain boundary bending nucleation and consumption of numerous dislocations is the main mechanism leading to grain refinement during thermal deformation. The results of the electrochemical corrosion tests proved that the corrosion predominantly occurred near the grain boundaries (GBs) of deformed grains, worsening with increased deformation temperatures.

Low cycle fatigue and cycle hardening behavior of heterogeneous Ni2Co1Fe1V0.5Mo0.2 medium entropy alloy

Low-cycle fatigue behavior of high-entropy alloy (HEAs) and medium-entropy alloys (MEAs) have rarely been reported in literature though it is critical for industrial applications. In our previous work, a non-equiatomic Ni2Co1Fe1V0.5Mo0.2 MEA was designed by adding V and Mo elements with bigger atomic size to heighten solution strengthening effect. Due to larger atomic size mismatch and more severe lattice distortion, the Ni2Co1Fe1V0.5Mo0.2 MEA exhibits stronger strain hardening effect than that in CoCrFeMnNi HEA. In present work, the low-cycle fatigue behavior of the non-equiatomic Ni2Co1Fe1V0.5Mo0.2 MEA with heterogeneous grain structures was further investigated. The heterogeneous Ni2Co1Fe1V0.5Mo0.2 MEA exhibits high fatigue resistance at 0.25 % strain amplitude (51285 N), attributed to the pronounced dislocation planar slip and formation of stacking faults. At 0.3 % and 0.5 % strain amplitudes, dislocation interactions (including tangles and microbands) induced by extensive dislocation cross-slip result in obvious cyclic hardening but reduced lifetime. The findings assess the effect of solid solution strengthening on the fatigue behavior of MEAs. The fatigue cracks form either along slip bands in large grains or grain boundaries of small grains.

Automated analysis framework of strain partitioning and deformation mechanisms via multimodal fusion and computer vision

Simultaneously investigating strain partitioning and the underlying deformation mechanisms for both the grain interior and the grain boundary (GB) is essential for understanding the complex plastic deformation of hexagonal close-packed metals. To this end, an automated analysis framework based on high-resolution digital image correlation (HRDIC) and electron backscatter diffraction (EBSD) data fusion and computer vision, integrating nanoscale resolution and a large field of view, is proposed. This framework consists of: (1) HRDIC-EBSD data fusion; (2) Segmenting the strain field into individual grains each with a core and a mantle; (3) Data clustering of the Matrix and slip bands (SBs) for each grain; (4) Full slip system (SS) identification and SS assignment to the SBs. The capabilities of this framework were demonstrated on Mg-10Y during compression. The strain field data, which was segmented into different clusters, including grain mantle, grain core, Matrix, and SBs, was analyzed statistically and quantitatively. The pixel-based slip activity, which considers the SB morphology, was obtained from a statistical perspective. Inter-granular accommodating mechanisms, including GB strain, slip transfer, and GB sliding, were quantitatively analyzed. Overall, this analysis framework, which can be applied to other materials, can automatically and statistically evaluate both nanoscale strain fields and underlying intra- and inter-granular deformation mechanisms grain-by-grain. This work provides valuable experimental insights into plastic deformation and accommodation mechanisms for polycrystals.

Revealing Real Active Sites in Intricate Grain Boundaries Assemblies on Electroreduction of CO2 to C2+ Products

AbstractAlthough intricate structural assemblies contribute to enhancing the activity of electrocatalytic CO2 reduction (ECR) to C2+ products, blindly coupling multiple design strategies may not yield the expected results, and even inhibit the activity of intrinsic catalytic sites. Therefore, elucidating the promoting or inhibitory effects of each design strategy on the CO2‐to‐C2+ conversion to clarify the real active sites and dynamic oxidation processes is of paramount importance. Here, commonly used grain boundaries (GBs), oxidation states, and alloying strategies are focused on, constructing four different types of catalysts structures: original Cu GBs, oxygen‐enriched grain boundary oxidation (GBO), Ag‐enriched GBO, and Cu/Ag GBs. Multiple operando characterizations reveal that GBs and GBO strengthen the resistance of the oxidative Cu species to the electrochemical reduction. The in situ generated strongly oxidative hydroxyl radicals alter the local reaction environment on the catalyst surface, inducing and stabilizing oxidative Cuδ+ species. Catalytic activity comparisons indicate that the oxidation state of Cu plays a decisive role in the CO2‐to‐C2+ conversion, and the nanoalloy effect tends to favor the CH4 production in intricate GBs assemblies. Theoretical calculations suggest that weak CO adsorption on GBO structures facilitates hydrogenation, promoting C–C coupling toward C2+ products.

Amphipathic Astaxanthin Additive for Low Voltage-loss Perovskite Solar Cells With Enhanced Quasi-Fermi Level Splitting and Solar Hydrogen Production Application.

Even though the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is nearly approaching the Schottky-Queisser limit, low open-circuit voltage (Voc) and severe Voc loss problems continue to impede the improvement of PCEs. Astaxanthin (ASTA) additive is introduced in the formamidinium lead triiodide (FAPbI3)perovskite film as an additive, which can facilitate the transportation of charge carriers and interact with Pb2+ by its distinctive groupings. Furthermore, the addition of ASTA decreases the defect's active energy, regulates the deep-level defect by filling up the grain boundaries (GBs), and promotes the crystallization of perovskite film. Remarkably, an enhanced quasi-Fermi level splitting (QFLS) of 1.164eV and a reduced Voc loss of only 96mV are realized. The champion PCE of 24.56% is attained by ASTA-modified PSCs on the basis of 22.75% PCE. Moreover, the PSCs that underwent ASTA modification demonstrate improved operational stability, ensuring consistent output in real-world scenarios. Furthermore, PSCs with an active area of 1 cm2 are used for water electrolysis to produce hydrogen and exhibit a PCE of 22.41%. This work offers an environmentally benign solution to address the inherent issues of FAPbI3 PSCs and lays the groundwork for the development of a prospective solar hydrogen production application.

Size-dependent mechanical responses of twinned Nanocrystalline HfNbZrTi refractory high-entropy alloy

Atomistic simulations are performed to study the size-dependent mechanical responses of HfNbZrTi refractory high-entropy alloy (RHEA) containing ultrafine grains and highly oriented twin boundaries (TBs). The strength and flow stress of nanocrystalline RHEA (NC-RHEA) under tensile loadings are explored versus decreasing grain size d. The transition from classical Hall-Petch (HP) strengthening to inverse HP softening at a critical grain size dc = 5.91 nm is attributed to the change of plastic deformation mechanisms from dislocation emission and phase transformation to grain boundary (GB) activities. Besides, the intragranular TBs considerably enhance the strength of nanotwinned RHEA (NT-RHEA); the enhancing effect reduces with decreasing twin thickness λ. As the volume fraction of GB increases with decreasing d, GB activities dominate the plasticity of NT-RHEA and cause comparable mechanical properties with NC-RHEA. Moreover, the influences of dislocation glide, phase transformation and twinning on the mechanical properties of RHEA are quantified and separately analyzed to further verify our simulation results. Findings of this study not only promote insights into the nanostructure-property relation of HfNbZrTi, but also shed the light on performance enhancement through nanostructural design.

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.

The influence of the dopant concentration and sintering parameters on properties of antimony doped barium stannate ceramics

In this paper, the influence of antimony concentration and different sintering techniques on the structural, microstructural, and electrical properties of antimony-doped barium stannate, BaSn1-xSbxO3 (BSSO, x = 0.00, 0.04, 0.06, 0.08 and 0.10) was investigated. BSSO-based ceramic samples were obtained by conventional and spark plasma sintering. The XRD analysis confirmed the single-phase, cubic BaSnO3 lattice system in all conventionally sintered samples. Apart from the dominant cubic phase, the spark plasma sintering conditions led to the formation of a secondary phase, Ba2SnO4, in all samples. FESEM analysis revealed the presence of low angle grain boundaries (LAGBs) in BSSO samples with high antimony concentration (x = 0.08), independently of the sintering technique. However, the fraction of LAGBs is significantly higher in the BaSn0.92Sb0.08O3-SPS sample due to the simultaneous exposure of the conductive sample to the effects of high temperature and pressure during sintering process. These boundaries have low activation energy and allow free charge carrier transport through the grain boundary region. The high dopant concentration and the presence of large fraction of LAGBs in BaSn0.92Sb0.08O3-SPS sample reflected on its electrical properties through low and almost temperature-independent electrical resistivity in the temperature range of 25–150 °C.

Evolution of NbC during laser welding and its impacts on the performance of molybdenum alloy joint

Influences of addition of niobium carbide (NbC) in the fusion zone (FZ) on room-temperature and high-temperature mechanical properties of laser-welded joints of molybdenum (Mo) alloys were studied. Results show that after adding NbC powder, the average Vickers microhardness in the upper part of the FZ increases from 183.0 HV to 712.3 HV; the room-temperature tensile strength grows from 33.9 MPa to 323.1 MPa, reaching 45.4 % that of base metal (BM). In addition, the fracture mode of joints turns from intergranular fractures into transgranular fracture; at 1100 °C, the high-temperature tensile strength of joints added with NbC is 141.6 MPa, which is 65.9 % that of BM. Energy dispersive spectrometer (EDS) and electron backscattered diffraction (EBSD) results show that after adding NbC powder, the average grain size in the FZ diminishes from 40.3 μm to 32.9 μm, where the number of low-angle grain boundaries (LAGBs) increases; the FZ not only contains NbC phase but also a large quantity of Nb2O5 and Mo2C phases dispersed on grain boundaries (GBs), and the number of MoO2 phase on GBs decreases apparently. Therefore, physical mechanisms underlying significant improvement of room-temperature and high-temperature tensile strengths of laser-welded joints of Mo alloys added with NbC in the FZ mainly include fine-grain strengthening, GB purification, and GB strengthening.

Grain boundaries are Brownian ratchets.

We demonstrate that grain boundaries (GBs) behave as Brownian ratchets, exhibiting direction-dependent mobilities and unidirectional motion under oscillatory driving forces or cyclic thermal annealing. We observed these phenomena for nearly all nonsymmetric GBs but not for symmetric ones. Our observations build on molecular dynamics and phase-field crystal simulations for a wide range of GB types and driving forces in both bicrystal and polycrystalline microstructures. We corroborate these simulation results through in situ experimental observations. We analyze these results with a Markov chain model and explore the implications of GB ratchet behavior for materials processing and microstructure tailoring.

Induction assisted autogenous plasma arc welding of HSLA steel

In this work, a high frequency induction heating (HFIH) assisted plasma arc welding (IPAW) technique is proposed to weld 6 mm thick S690QL high strength low alloy steel plates in square butt joint configuration and without using any filler material. A 3D finite element based coupled electromagnetic-thermal analysis is carried out to ascertain the weld's thermal characteristics. Microstructure evolution and mechanical performance are investigated using scanning electron microscopy, electron back scattered diffraction, transmission electron microscopy, tensile test, Charpy impact test and micro hardness test. The results demonstrate that 15 s initial static heating using a co-directional current coil with magnetic flux concentrator predominantly improves the weld penetration by 48 % and joint efficiency reaches 90 % to the base plate strength. Microstructural study shows that the addition of HFIH promotes low angle grain boundaries (LAGB) with bainite ferrite and granular bainite micro-constituents in the fusion zone, which causes localised yielding and failure in this zone during tensile test. Further investigation reveals that the HFIH encourages the formation of BI-type bainite ferrite laths of 1.3–2.2 μm width, combined with slender martensite-austenite (M-A) island at the bainite ferrite lath boundaries. The results also reveal that the induction heating promotes the formation of M23C6 precipitates and B2 structured Cu-enrich nano precipitates in the fusion zone (FZ). The microhardness distribution indicates that the FZ and coarse grain heat affected zone are significantly reduced due to the assistance of the HFIH. The impact test result shows a lower energy absorption by the IPAW weldments due to dominance of the LAGB with unfavourable strain distribution.

Interfacial inhomogeneous plastic deformation during rotary friction welding of dissimilar AA2219-SS321 joint combination with AA6061 interlayer

This research investigates the inherent radial non-uniformity within the rotary friction welding process, particularly concerning microstructure attributes like grain size, grain boundaries, misorientation angles, and interlayer presence along the radial axis. SS321-AA2219 rotary friction welding was carried out with and without an AA6061 interlayer. The numerical thermal model suggests increase in temperatures from the center to the periphery, due to non-uniform heat generation. Also, dissimilar material across the interface resulted in an asymmetric temperature profile along axial direction. Plastic deformation on the Aluminum side suggests dynamic recrystallization and grain refinement, whereas pronounced low-angle grain boundary (LAGB) formation near the SS side interface validates dynamic recovery. A radial non-uniformity in microstructure is observed, with metrics such as average grain size, LAGB fraction, and misorientation showing an increase from the center towards the periphery. The insertion of an interlayer alters process dynamics, manifesting in reduced temperatures and heightened forces, resulting in a more consolidated joint by enhancing the strength by 31 %. Interdiffusion of elements across the interface formed Fe-Al intermetallic compounds (IMC) confirmed with X ray diffraction. Fractography analysis elucidates the presence of rubbing marks and facet surfaces in interlayer-less joints, while joints with interlayer display sticking and dimples.