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Continuous grain boundary α (CGB-α) usually causes degradation of mechanical properties in titanium alloys. Here, we propose to employ dislocation network in an additively manufactured titanium alloy for GB engineering to suppress CGB-α. A C-bearing titanium alloy was selectively laser melted to produce homogeneously distributed TiC flakes and a dislocation network that links them to GBs. Upon solution treatment, the dislocation network promotes dissolution of intragranular carbides and formation of discrete GB TiC particles. These GB carbides suppress the formation of CGB-α during ageing treatment, which together with the formation of high-density intragranular α leads to excellent strength-ductility synergy. [ABSTRACT FROM AUTHOR]

Manipulating the grain boundary and chiral structure of enantiomorphic inorganic thermoelectric materials facilitates a new degree of freedom for enhancing thermoelectric energy conversion. Chiral twist mechanisms evolve by the screw dislocation phenomenon in the nanostructures; however, contributions of such chiral transport have been neglected for bulk crystals. Tellurium (Te) has a chiral trigonal crystal structure, high band degeneracy, and lattice anharmonicity for high thermoelectric performance. Here, Sb‐doped Te crystals are grown to minimize the severe grain boundary effects on carrier transport and investigate the interface of chiral Te matrix and embedded achiral Sb2Te3 precipitates, which induce unusual lattice twists. The low grain boundary scattering and conformational grain restructuring provide electrical‐favorable semicoherent interfaces. This maintains high electrical conductivity leading to a twofold increase in power factor compared to polycrystal samples. The embedded Sb2Te3 precipitates concurrently enable moderate phonon scattering leading to a remarkable decrease in lattice thermal conductivity and a high dimensionless figure of merit (zT) of 1.1 at 623 K. The crystal growth and chiral atomic reorientation unravel the emerging benefits of interface engineering as a crucial contributor to effectively enhancing carrier transport and minimizing phonon propagation in thermoelectric materials. [ABSTRACT FROM AUTHOR]

Grain boundary engineering (GBE) was carried out on a nickel-based alloy (GH3535, Ni-16Mo-7Cr-4Fe), which intrinsically has many strings of primary molybdenum carbides. The strings induce inhomogeneous grain size distributions and increase the difficulties in achieving a GBE microstructure. In this work, the effects of the primary carbide distribution on the grain boundary network (GBN) evolution were investigated. A higher proportion of Σ3n grain boundaries (GBs) associated with extensive multiple twinning events was achieved in the specimen with more dispersive and finer primary carbides, which are the results of cross-rolling, i.e., cold rolling with a changed direction. In a starting microstructure with many strings of primary carbides, the dense and frequent occurrence of particle-stimulated nucleation (PSN) around the carbides induced more general high-angle GBs into the GBN, and the inhibition of GB migrations by the carbide strings suppressed the formation of large-sized highly twinned grain clusters. As a consequence, the Σ3n GBs could not be effectively enhanced. [ABSTRACT FROM AUTHOR]

Continuous grain boundary α (CGB-α) usually causes degradation of mechanical properties in titanium alloys. Here, we propose to employ dislocation network in an additively manufactured titanium alloy for GB engineering to suppress CGB-α. A C-bearing titanium alloy was selectively laser melted to produce homogeneously distributed TiC flakes and a dislocation network that links them to GBs. Upon solution treatment, the dislocation network promotes dissolution of intragranular carbides and formation of discrete GB TiC particles. These GB carbides suppress the formation of CGB-α during ageing treatment, which together with the formation of high-density intragranular α leads to excellent strength-ductility synergy.

The 304HCu stainless steel is a candidate material for superheater and reheater tubes in advanced ultra-supercritical power plants due to its excellent creep and oxidation resistance. However, these operating conditions involve exposure to steam at high pressure and temperature on the steam-side and hot coal-ash products on the fireside. In this study, the role of grain boundary character distribution (GBCD) on oxidation and fireside corrosion behavior of 304HCu steel is investigated. The GBCD was modified through grain boundary engineering (GBE) via optimized strain-annealing treatment on the as-received (AR) specimen. The air oxidation, steam oxidation (pressure ~ 243 bar) and fireside corrosion studies were conducted at 973 K for up to 1000 h, in custom-designed setups precisely simulating the operating conditions. Following GBE, the grain size (excluding twins) and coincident site lattice boundary (Σ ≤ 29) fraction increased from 21 ± 1 to 60 ± 12 μm and from 62 ± 4 to 74 ± 3%, respectively, resulting in disruption of the random high angle grain boundary networks through the introduction of twins. Evaluation of oxidation behavior revealed that the GBE specimens have lower oxidation resistance (i.e., higher weight gain and oxide scale thickness) in both air and steam, while the same specimen displayed improved fireside corrosion resistance (lower percolation depth) as compared to the AR specimen. From a detailed analysis of the oxidation/fireside corrosion products and cross-sectional microstructures of the oxide layers, the above responses could be correlated with the GBCD and grain size, and the possible mechanisms operative during the air/steam oxidation and fireside corrosion are also presented. [ABSTRACT FROM AUTHOR]

The effects of grain boundary engineering (GBE) coupled with subsequent stabilization heat treatment on the intergranular corrosion (IGC) resistance of a 321 stainless steel were studied with the boiling nitric-acid immersion corrosion tests. The proportion of low-ΣCSL grain boundaries was enhanced to more than 70% through GBE, most of which are inter-connected Σ3 and its variant grain boundaries forming highly-twinned large-size grain-clusters. After stabilizing at 900 °C, copious needle-like TiC carbides with the length directions parallel to the < 001 > directions of the matrix precipitated in grain interiors of the GBE specimen, leaving precipitate-free zones along grain boundaries. The specimen after the GBE and subsequent thermal stabilization shows a much less weight loss during the corrosion test than that adopted only GBE, stabilization or neither of them. The morphology of the stabilized GBE specimens after corrosion tests revealed that the enhancement of IGC resistance was attributed not only to the high proportion of special-structured grain boundaries, which constituted large size of highly-twinned grain-clusters, but also to the substantially reduced extent of Cr depletion at the grain boundaries by the formation of copious TiC carbides in the grain interiors. [ABSTRACT FROM AUTHOR]

The main bottleneck in obtaining high‐performance thermoelectric (TE) materials is identified as how to decouple the strong interrelationship between electrical and thermal parameters. Herein, a precise interface modification approach based on the powder atomic layer deposition (ALD) technology is presented to enhance the performance of CuNi alloys. ZnO and Al2O3 layers as well as their combinations are deposited on the surface of powders, typically in 10–100 ALD cycles, and their effects on the TE performance of bulks is thoroughly investigated. The enhancement of the Seebeck coefficient, caused by the energy filtering effect, compensates for the electrical conductivity deterioration due to the low electrical conductivity of oxide layers. Furthermore, the oxide layers may significantly increase the phonon scattering. Therefore, to reduce the resistivity of coating layer, a multilayer structure is deposited on the surface of powders by inserting Al2O3 into ZnO. The accurate microstructure characterization shows that the Al atoms diffused into ZnO and realized the doping effect after pressing. Al diffusion has the potential to increase the electrical conductivity and complexity of coating layers. Compared to pure CuNi, zT increases by 128% due to the decrease in resistivity and stronger phonon scattering in phase boundaries. [ABSTRACT FROM AUTHOR]

RuO2 has been considered as the most likely acidic oxygen evolution reaction (OER) catalyst to replace IrO2, but its performance, especially long‐term stability under harsh acidic conditions, is still unacceptable. Here, we propose a grain boundary (GB) engineering strategy by fabricating the ultrathin porous RuO2 nanosheet with abundant of grain boundaries (GB‐RuO2) as an efficient acid OER catalyst. The involvement of GB induces significant tensile stress and creates an unsaturated coordination environment, effectively optimizing the adsorption of intermediates and stabilizing active site structure during OER process. Notably, the GB‐RuO2 not only exhibits a low overpotential (η10=187 mV) with an ultra‐low Tafel slope (34.5 mV dec−1), but also steadily operates for over 550 h in 0.1 M HClO4. Quasi in situ/operando methods confirm that the improved stability is attributed to GB preventing Ru dissolution and greatly inhibiting the lattice oxygen oxidation mechanism (LOM). A proton exchange membrane water electrolysis (PEMWE) using the GB‐RuO2 catalyst operates a low voltage of 1.669 V at 2 A cm−2 and operates stably for 100 h at 100 mA cm−2. [ABSTRACT FROM AUTHOR]

Abstract Manipulating the grain boundary and chiral structure of enantiomorphic inorganic thermoelectric materials facilitates a new degree of freedom for enhancing thermoelectric energy conversion. Chiral twist mechanisms evolve by the screw dislocation phenomenon in the nanostructures; however, contributions of such chiral transport have been neglected for bulk crystals. Tellurium (Te) has a chiral trigonal crystal structure, high band degeneracy, and lattice anharmonicity for high thermoelectric performance. Here, Sb‐doped Te crystals are grown to minimize the severe grain boundary effects on carrier transport and investigate the interface of chiral Te matrix and embedded achiral Sb2Te3 precipitates, which induce unusual lattice twists. The low grain boundary scattering and conformational grain restructuring provide electrical‐favorable semicoherent interfaces. This maintains high electrical conductivity leading to a twofold increase in power factor compared to polycrystal samples. The embedded Sb2Te3 precipitates concurrently enable moderate phonon scattering leading to a remarkable decrease in lattice thermal conductivity and a high dimensionless figure of merit (zT) of 1.1 at 623 K. The crystal growth and chiral atomic reorientation unravel the emerging benefits of interface engineering as a crucial contributor to effectively enhancing carrier transport and minimizing phonon propagation in thermoelectric materials.

Introducing coherent twin boundaries is an effective way to develop submicron/nano-sized grains with improved thermal stability in polycrystals, which exhibit enhanced mechanical properties of metals, such as high strength, high hardness and high fracture toughness. However, crystalline defects, such as dislocations and disclinations, usually arise at twin boundary junctions, which lead to local stress concentration and property degradation. Here we show that a large number of high-index coherent twin boundaries (e.g., Σ11 and Σ19a) can be produced through a well-controlled deformation twinning process. By using metastable β-Ti alloys as an example and analyzing the broken symmetry associated with deformation twinning, we demonstrate that the abnormal high-index twins result from intercorrelated twinning pathways and twin-twin reactions. The formation mechanism of self-organized multi-twin structures has been analyzed systematically through phase field modeling and simulations. It has been found that, under well designed thermo-mechanical condition, regularly distributed high-index twin boundaries and geometrically-compatible twin boundary junctions (i.e., dislocation/disclination-free) can be produced. The stress/strain condition required to obtain self-organized twins has been quantitatively determined, which provides a theoretical guideline for the development of self-organized submicron/nano-twin metals for unprecedented properties.

The effect of special grain boundary development via the grain boundary engineering process (GBE) was explored in a bimodal microstructured 316L austenitic stainless steel with an average grain size of 1.92 μm. The GBE process was carried out by the rolling-annealing method via two routes of low and medium applied strain, followed by a short annealing period in a single-step and iterative manner. The grain boundary character distribution was obtained by Electron Backscatter Diffraction (EBSD) analysis, and the intergranular corrosion resistance and the tensile behavior of the samples were examined. Microstructural characterization showed that applying low strain repetitively increased the coincidence site lattice (CSL) and Σ3 boundaries percentage and created large twin-related domains (TRD). The reduced sensitization was attributed to a high proportion of CSLs and large-sized TRDs by applying a low-strain route. In the early stages of the low-strain route, the grain boundary character distribution showed a high number of primary twin nuclei in coarse grains and a continuous network of single twin nuclei in smaller grains, creating a high percentage of Σ3-grain boundaries. The low applied strain and the absence of the necessary driving force for the recrystallization and GBs migration led to the formation of twin nuclei, which would assist with the reduction of microstructure energy. A high percentage of Σ3 boundaries and an increase in the percentage of triple points consisting of low energy boundaries were found to be influential factors in increasing elongation.

Nitrogen solid solution can effectively enhance the strength of steels, but its effect on grain boundary engineering and the corrosion mechanism of steels is still unclear and systematic investigation is needed. In this study, 316LN steel specimens with five nitrogen concentrations were melted and subjected to compression and annealing. The grain boundary character distribution was investigated by electron backscatter diffraction. The corrosion behavior was investigated by the electrochemical polarization curves, electrical impedance spectroscopy, and X-ray photoelectron spectroscopy. The results showed that nitrogen promotes the formation of low-Σ boundaries when the pre-strain is not above 10%. The polarization resistance increases with increasing nitrogen content. The quantitative relationship between the nitrogen content and the pitting potential was obtained. Nitrogen favors the formation of oxides, hydroxides, and NH3/NH4+ on the passivated surface. The effect of planar slip on grain boundary engineering and the influence of NH4+ on the corrosion resistance are discussed.

Additive manufacturing of tungsten (W) is challenging due to its high melting point, high thermal conductivity, oxidation tendency, and brittleness from grain boundary (GB) oxides. In this study, the processing of W through laser powder bed fusion is investigated. Parts are fabricated under argon (Ar) and nitrogen (N2) atmospheres using the same processing parameters. The part produced in Ar has cracks with oxide precipitates decorating the fractured GBs. On the other hand, crack‐free W samples are produced under N2 atmosphere without any additional process modification. In both cases, the oxygen (O) content in the LPBF samples is similar to the starting powder. Interestingly, the analysis of the samples fabricated in nitrogen suggests that nitrogen is retained beyond the equilibrium solid solubility limit, while high‐resolution electron micrographs of fractured surfaces reveal reduced levels of oxides at GBs. Increased hardness for samples processed under N2 atmosphere is observed. Density Functional Theory (DFT) calculations performed to study the influence of interstitial nitrogen on oxygen diffusion in W indicated a hindrance to O diffusion from the presence of dissolved N. [ABSTRACT FROM AUTHOR]

In this study, the grain boundary character distribution (GBCD) of a B10 alloy was optimized, employing thermomechanical processing consisting of friction stirring processing (FSP) and annealing treatment. Using electron backscatter diffraction, the effects of rotational speed of FSP and annealing time on the evolution of GBCD were systematically investigated. The GBCD evolution was analyzed concerning various parameters, such as the fraction of low-Σ coincidence site lattice (CSL) boundaries, the average number of grains per twin-related domain (TRD), the length of longest chain (LLC), and the triple junction distribution. The experimental results revealed that the processing of a 1400 rpm rotational speed of FSP followed by annealing at 750 °C for 60 min resulted in the optimum grain boundary engineering (GBE) microstructure with the highest fraction of low-Σ CSL boundaries being 82.50% and a significantly fragmented random boundary network, as corroborated by the highest average number of grains per TRD (14.73) with the maximum LLC (2.14) as well as the highest J2/(1 − J3) value (12.76%). As the rotational speed of FSP increased from 600 rpm to 1400 rpm, the fraction of low-Σ CSL boundaries monotonously increased. The fraction of low-Σ CSL boundaries first increased and then decreased with an increase in annealing time. The key to achieving GBE lies in inhibiting the recrystallization phenomenon while stimulating abundant multiple twinning events through strain-induced boundary migration. [ABSTRACT FROM AUTHOR]

Grain boundary (GB) segregation of Al in ZnO plays an important role in lowering its thermal conductivity for thermoelectric applications. However, the effect of Al concentration on the GB complexions and their transition is not well understood. Herein, a genetic algorithm assisted multiscale modelling framework was used to study the role of GB concentration on the GB segregation of Al on five special twin GBs of ZnO. A critical concentration of 5–6 atoms/nm2 was determined for the complexion transition from single layer to multilayer. Calculated segregation energies were used in a phenomenological model to link GB concentration with the nominal concentration of dopants. The model was used to calculate the nominal solubility of Al in ZnO as a function of grain size, which was validated with the experimental data from the literature. The proposed framework provides a path for establishing GB-structure – property correlation and thereby, predictive dopant engineering of ceramics. [ABSTRACT FROM AUTHOR]

[Display omitted] Volatile Organic Compounds (VOCs) have long been a threat to human health. However, designing economical and efficient transition metal composite oxide catalysts for VOCs purification remains a challenge. Herein, this study demonstrates the enormous potential of grain boundary engineering in facilitating VOCs decomposition over ordered mesoporous composite oxide denoted as 3D-Mn x Co y (x, y = 1, 3, 5, 7, 9). Specifically, the three-dimensional (3D) Mn7Co1 catalyst shows 100% ethyl acetate removal efficiency for a continuous airflow containing 1000 ppm ethyl acetate over 60000 h−1 space velocity at 160 °C. Mechanism study suggests that the high catalytic performance originates from the lattice distortion caused by the introduction of heteroatoms, along with the size effect of nanopore walls, which leads to the formation of various grain boundaries on the catalyst surface. The presence of grain boundaries facilitates the generation of oxygen vacancies, thus promoting the migration and activation of oxygen species. Furthermore, the near-atmospheric pressure X-ray photoelectron spectroscopy (NAP- XPS) monitoring results reveal that the bimetallic synergy enhanced by grain boundary accelerates the catalytic reaction rate of VOCs through Mn3++Co3+↔Mn4++Co2+ redox cycle. This study may shed light on the great potential of ordered mesoporous bimetallic oxide catalysts in VOCs pollution control. [ABSTRACT FROM AUTHOR]

The precipitation of carbide at the grain boundary is one of the key factors that influences the resistance of intergranular corrosion. The morphology and growth mechanism of the carbides precipitated at different grain boundaries in Alloy 600 aged at 715°C for different times were investigated by transmission electron microscopy. Two types of carbides, M23C6 and M7C3, can be observed in the sample. The grain boundary character significantly affects the precipitation and distribution of different carbides. The face-centred cubic structured fine M23C6 has a semi-coherent orientation relationship with the matrix, and mainly precipitates at Σ3 grain boundary in all the samples, and at Σ9 grain boundary in the short-term aged samples. The large M7C3 has a hexagonal crystal structure, which does not have typical orientation relationship with the matrix, and precipitates at Σ9 grain boundary in the long-term aged samples, and at Σ27 grain boundary and random grain boundary in all the samples. [ABSTRACT FROM AUTHOR]

A multi-physics simulation model has been established to investigate the influence of Laser powder bed fusion parameters on the spatial composition distribution and grain growth mechanism of the single-track printed dissimilar alloys. Our study shows that alloy composition gradient isosurfaces can be used to visualize the spatial distribution of alloy composition for miscible dissimilar alloys. When the melt pool aspect ratio changes from large to small, the grain growth transitions from the temperature gradient mode to composition gradient mode and then to the mixed mode. Our experimental observations show that in extreme cases, the curved grain angle can reach 272°.

Grain boundary engineering (GBE) was carried out on a nickel-based alloy (GH3535, Ni-16Mo-7Cr-4Fe), which intrinsically has many strings of primary molybdenum carbides. The strings induce inhomogeneous grain size distributions and increase the difficulties in achieving a GBE microstructure. In this work, the effects of the primary carbide distribution on the grain boundary network (GBN) evolution were investigated. A higher proportion of Σ3n grain boundaries (GBs) associated with extensive multiple twinning events was achieved in the specimen with more dispersive and finer primary carbides, which are the results of cross-rolling, i.e., cold rolling with a changed direction. In a starting microstructure with many strings of primary carbides, the dense and frequent occurrence of particle-stimulated nucleation (PSN) around the carbides induced more general high-angle GBs into the GBN, and the inhibition of GB migrations by the carbide strings suppressed the formation of large-sized highly twinned grain clusters. As a consequence, the Σ3n GBs could not be effectively enhanced.

In this work, a series of isothermal hot compression tests were performed at the temperature of 900–1150 °C with the strain rate ranging from 0.01 to 10s−1 in Inconel X-750 alloy (referred as Alloy X-750). Based on the flow stress data, the back-propagation artificial neural network model was utilized to accurately predict the flow stress at elevated temperatures. Subsequently, by constructing a modified hot processing map coupled with the variation of Σ3n (n = 1, 2, and 3) twin boundaries fraction, the optimal processing parameters were determined to be in the range of 1100–1150 °C and 1-10s−1 for Alloy X-750. Moreover, the possibility of grain boundary engineering via hot deformation were also evaluated by using some critical quantification descriptors of twin related domains (TRDs), such as the number of grains in the TRDs, the lengths of the longest chain, and the average TRDs size. All the results can further verify the validity of modified hot processing map. Finally, the high-temperature flow behavior combined with the relevant microstructure characterization showed that discontinuous dynamic recrystallization is considered as the predominant softening mechanism, while the continuous dynamic recrystallization and particle-stimulated nucleation also additionally contribute to the recrystallization nucleation during the hot deformation of Alloy X-750.

Grain boundary engineering (GBE) is designed for metals to optimize the grain boundary characteristics and improve their resistance to intergranular corrosion (IGC). In this study, the effect of various GBE processing on the fraction of low ΣCSL grain boundaries and random large-angle grain boundary connectivity in 304 stainless steel was investigated quantitatively with a fractal model. The corrosion weight loss rate, corrosion current density and sensitization degree of samples with various grain boundary characteristics were measured, and these corrosion behaviors were studied in term of the fractal dimension. The results showing the specimen after alternating directions cold rolling plus annealed at 1100 °C obtained the highest ΣCSL score that was 67.4%. And the value of fractal dimension for specimen after solid solution annealing, cold rolled 5% or cold rolled 20% reduction plus annealed at 1100 °C for 5 min in GBE was 1.75, 1.53 and 1.46, respectively. An indicator defined by fractal dimension for grain boundary characteristic on corrosion behaviors of 304 stainless steel was verified and clarified, and the mechanisms of GBE to improve corrosion resistance were confirmed with the observation of low-energy grain boundary fragments introduced by annealing twins and triple junctions in grain boundaries.

We report a novel electro-nano-pulsing (ENP) processing method to achieve localized engineering of grain boundary (GB) morphology in polycrystalline metallic materials. ENP is extraordinarily capable of generating intense nanopulse electric current with a current density greater than a few to several hundreds of 1010A/m2 and a pulse duration on the order of a few 100ns. Such a level of current density is ∼3–5 magnitudes higher than that is usually achieved during the Spark Plasma Sintering process. Using the Nichrome-80 superalloy as a model material, we observed a variety of GB roughening phenomena at multiple length scales, resulting in the generation of diverse forms of atomistic facets, nanoscale serrations, and nanoscale step-like GB morphologies after the ENP processing. We think that the excessive GB heat localization and electron wind force or stress are the main factors contributing to the GB morphological changes during the ENP processing. The ENP processing provides a new unique grain boundary engineering strategy to manipulate the GBs with the changes localized at the GB region, without altering its adjacent grains.

Development of new powder feedstocks using nanoparticles (NPs) has the potential to influence the microstructure of as-built parts and overcome the limitations of current powder-based additive manufacturing (AM) techniques. The focus of this study is to investigate the impact of NP-modified magnetic microparticle powder feedstock on the microstructure of suction-cast Nd-Fe-B-based alloys. This particular casting method has been recognized for its ability to replicate, to some extent, the melting and rapid solidification stages inherent to metal powder-based AM techniques such as powder bed fusion using a laser beam. Two types of NP materials, Ag and ZrB2, are used, and their effects on the grain size distribution and dendritic structures are evaluated after suction casting. Ag NPs result in smaller, more uniform grain sizes. ZrB2 NPs result in uniformly distributed grain sizes at much lower mass loadings. The results show that feedstock powder surface modification with low-melting-point metal NPs can improve permanent magnets' microstructure and magnetic properties, at below 1 vol%, equal to submonolayer surface loads. Herein, the potential of using NPs to develop new powder feedstocks for AM is highlighted, significantly improving the final part's properties. [ABSTRACT FROM AUTHOR]

A multi-physics simulation model has been established to investigate the influence of Laser powder bed fusion parameters on the spatial composition distribution and grain growth mechanism of the single-track printed dissimilar alloys. Our study shows that alloy composition gradient isosurfaces can be used to visualize the spatial distribution of alloy composition for miscible dissimilar alloys. When the melt pool aspect ratio changes from large to small, the grain growth transitions from the temperature gradient mode to composition gradient mode and then to the mixed mode. Our experimental observations show that in extreme cases, the curved grain angle can reach 272°. The study found that the composition gradient of dissimilar alloys can dominate grain growth, which is entirely different from the conventional temperature gradient-dominated grain growth mechanism. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) has provided new possibilities for improving the grain boundary properties of metallic components. However, effectively modifying the microstructure, particularly the grain boundary properties, of laser powder bed fusion (L-PBF) components remains a challenge. Post-processing methods have shown some success in adjusting grain boundary angles, but they have limitations when it comes to complex geometries and internal features. In this study, we propose an innovative in situ heat treatment to control the grain boundary properties of L-PBF components. A model is proposed to predict the thermal cycle at a single point, and it is validated through experiments on 2507 super duplex steel and 316L austenitic steel samples. The results demonstrate that, by applying controlled in situ heat treatment, the dynamic recovery processes can be influenced, and thereby the grain boundary properties of the manufactured parts can be controlled. This proposed method improves our understanding of the impact of in situ heat treatment on grain boundary properties and offers potential for designing and fabricating high-performance L-PBF components. The findings from this study lay the groundwork for the further exploration of grain boundary engineering in metallic components using L-PBF. By leveraging in situ heat treatment, future research can open up new avenues in additive manufacturing, facilitating the production of advanced and high-quality metallic components. [ABSTRACT FROM AUTHOR]

This study demonstrates atomic layer deposition (ALD) of an extremely thin Al 2 O 3 layer over n-type Bi 2 Te 2.7 Se 0.3 to alleviate the adverse effects of multiple boundaries on their thermoelectric performance. Multiple boundaries reduce thermal conductivity (κ), but generate electrons, deviating from the optimum carrier concentration. Only one Al 2 O 3 ALD cycle effectively suppresses Te volatilization at the grain boundaries, resulting in a decrease from 5.8 × 1019/cm3 to 3.6 × 1019/cm3 in the electron concentration. Concurrently, the one-cycle-Al 2 O 3 coating produces fine grains, thus inducing numerous boundaries, ultimately suppressing the lattice κ from 0.64 to 0.33 W/m·K. A further increase in the number of Al 2 O 3 cycles leads in a significant rise in the resistance, resulting in degradation of thermoelectric performance. Consequently, the ZT value is increased by 51 % as a result of Al 2 O 3 coating with a single ALD cycle. Our approach offers new insights into the simultaneous reduction of the κ and electron concentration in n-type Bi 2 Te 3 -based materials. • Boundary engineering strategy for thermoelectric materials. • Conformal and thin Al 2 O 3 layer over n-type Bi 2 Te 2.7 Se 0.3 thermoelectric materials. • Facile atomic layer deposition-based approach. • Suppression of Te volatilization at the grain boundaries. • Simultaneous reduction of lattice thermal conductivity and electron concentration. [ABSTRACT FROM AUTHOR]

The maximum permissiblse content of surface-active elements of carbon, nitrogen and boron in low-carbon Cr–Ni and Cr–Ni–Mo steels, which provide high resistance against intergranular corrosion (IGC) when tested in strongly and weakly oxidizing environments, was established. To ensure high resistance against IGC when tested in boiling 65% HNO3 (according to ISO 3651-1), the carbon content in 03Kh18N11 (304L) and 03Kh17N14M3 (316L) steels should not exceed 0.025% and 0.015%, respectively, and when tested in boiling H2SO4 (ISO 3651-2, method B) should not exceed 0.03%. The nitrogen amount up to 0.2% does not influence negatively the IGC of the studied steels, and the simultaneous increase of nitrogen and carbon gives a negative synergistic effect. The negative influence of 0.003% and, to a greater extent, 0.03% boron on the grain boundary structure and resistance against IGC of steels hardened at temperatures >1100°C was shown. Technologies for increasing the resistance against IGC of pipes made of the studied steels were developed, taking into account the principle of grain boundary engineering of polycrystalline materials. [ABSTRACT FROM AUTHOR]

Grain boundary engineering (GBE) is considered to be an attractive approach to microstructure control, which significantly enhances the grain-boundary-related properties of face-centered cubic (FCC) metals. During the twinning-related GBE, the microstructures are characterized as abundant special twin boundaries that sufficiently disrupt the connectivity of the random boundary network. However, controlling the grain boundary character distribution (GBCD) is an extremely difficult issue, as it strongly depends on diverse processing parameters. This article provides a comprehensive review of controlling GBCD during the twinning-related GBE of FCC materials. To commence, this review elaborates on the theory of twinning-related GBE, the microscopic mechanisms used in the optimization of GBCD, and the optimization objectives of GBCD. Aiming to achieve control over the GBCD, the influence of the initial microstructure, thermo-mechanical processing (TMP) routes, and thermal deformation parameters on the twinning-related microstructures and associated evolution mechanisms are discussed thoroughly. Especially, the development of twinning-related kinetics models for predicting the evolution of twin density is highlighted. Furthermore, this review addresses the applications of twinning-related GBE in enhancing the mechanical properties and corrosion resistance of FCC materials. Finally, future prospects in terms of controlling the GBCD during twinning-related GBE are proposed. This study will contribute to optimizing the GBCD and designing GBE routes for better grain-boundary-related properties in terms of FCC materials. [ABSTRACT FROM AUTHOR]

The relationship between evolution in grain boundary characteristics and intergranular corrosion properties of cold rolled 443 ultra-pure ferritic stainless steel was investigated. The results showed that low-angle (LA) grain boundaries, special grain boundaries (SGB), and high-angle (HA) grain boundaries had a high intergranular corrosion sensitivity after cold deformation in reducing acid medium. In oxidative acid medium, the LA and SGB boundaries had a high intergranular corrosion resistance in comparison to HA boundaries, and their inhibitory effect on the corrosion continuance of the dynamic recrystallization high-angle (RHA) grain boundaries can improve the corrosion resistance of the RHA grain boundaries.

Austenitic stainless steels 304 L (SS304) and 316 L (SS316) are additive manufactured under the same processing conditions to reveal two distinct microstructures. Particularly, the resulting grain morphology for SS304 is singular – there are subgrains dispersed across the sample; there is a wide range of grain size spanning nearly two orders of magnitude; and grain boundaries are convoluted, resembling a fractal object. The materials solidification pathway governed by chemical composition is responsible for the grain boundary fractality (ferrite-to-austenite solidification for SS304 and direct transformation to austenite for SS316). Operando X-ray diffraction studies at Cornell High Energy Synchrotron Source substantiate the solidification pathway of the materials. The findings from the study open up a new avenue for grain boundary engineering using additive manufacturing.

In this study, the grain boundary character distribution (GBCD) of a B10 alloy was optimized, employing thermomechanical processing consisting of friction stirring processing (FSP) and annealing treatment. Using electron backscatter diffraction, the effects of rotational speed of FSP and annealing time on the evolution of GBCD were systematically investigated. The GBCD evolution was analyzed concerning various parameters, such as the fraction of low-Σ coincidence site lattice (CSL) boundaries, the average number of grains per twin-related domain (TRD), the length of longest chain (LLC), and the triple junction distribution. The experimental results revealed that the processing of a 1400 rpm rotational speed of FSP followed by annealing at 750 °C for 60 min resulted in the optimum grain boundary engineering (GBE) microstructure with the highest fraction of low-Σ CSL boundaries being 82.50% and a significantly fragmented random boundary network, as corroborated by the highest average number of grains per TRD (14.73) with the maximum LLC (2.14) as well as the highest J2/(1 − J3) value (12.76%). As the rotational speed of FSP increased from 600 rpm to 1400 rpm, the fraction of low-Σ CSL boundaries monotonously increased. The fraction of low-Σ CSL boundaries first increased and then decreased with an increase in annealing time. The key to achieving GBE lies in inhibiting the recrystallization phenomenon while stimulating abundant multiple twinning events through strain-induced boundary migration.

In the present work, the thermomechanical processing consisting of rotary swaging deformation and annealing treatment was performed to optimize the grain boundary character distribution (GBCD) of 304 austenitic stainless steel. To systematically investigate the effect of GBCD evolution on intergranular corrosion (IGC), double loop electrochemical potentiokinetic reactivation and electrolytic oxalic acid etch tests were employed. The experimental results show that the fraction of low-Σ coincident site lattice (CSL) boundaries increased from 58.1% to 74.0% for the specimen swaged to 0.06 true strain and then annealed at 1050 °C for 5 min duration. By characterizing the evolution of GBCD as a function of strain level in terms of low-Σ CSL boundaries fraction, average twin-related domain (TRD) size, average number of grain per TRD and fractal dimension of the maximum random boundary connectivity, the grain boundary engineering (GBE) microstructure was realized by the occurrence of prolific multiple twinning events during strain-induced boundary migration while static recrystallization has a detrimental effect on optimizing GBCD for the prolific of new strain-free grains with random boundaries. The IGC resistance of the GBE-treated 304 austenitic stainless steel is enhanced by inhibiting the nucleation and propagation of IGC cracks, resulting from the increase in the fraction of low-Σ CSL boundaries, especially Σ3 boundaries and the disruption of random boundary network connectivity.

By using thermo-mechanical processing, 99.996 wt.% pure Ni with different grain boundary characteristics were fabricated (Sample #1, 700 ℃ × 10 h + cold rolling reduction 50%+ 650 ℃ × 2 h; Sample #2, 700 ℃ × 10 h + cold rolling reduction 50%+ 900 ℃ × 5 min). Hydrogen embrittlement sensitivity of the two samples was determined by using high-pressure hydrogen charging, low strain rate tensile test and SEM fractography. Compared with Sample #1, hydrogen embrittlement index of Sample #2 increased from 0.52 to 0.71. The effects of grain size, grain boundary type and grain boundary curviness on hydrogen embrittlement of pure Ni were discussed. The results indicate that: 1) special grain boundaries including twin grain boundaries may have a marginal effect on tailoring hydrogen embrittlement resistance in pure Ni; 2) increasing grain boundary curviness is an effective way to improve hydrogen embrittlement resistance of Ni.

On the basis of reiterating the concept of grain boundary engineering (GBE), the recent progress in the theoretical models and mechanisms of twin-related GBE optimization and its effect on the mechanical properties is systematically summarized in this review. First, several important GBE-quantifying parameters are introduced, e.g., the fraction of special grain boundaries (GBs), the distribution of triple-junctions, and the ratio of twin-related domain size to grain size. Subsequently, some theoretical models for the GBE optimization in face-centered cubic (FCC) metals are sketched, with a focus on the model of "twin cluster growth" by summarizing the in-situ and quasi-in-situ observations on the evolution of grain boundary character distribution during the thermal-mechanical process. Finally, some case studies are presented on the applications of twin-related GBE in improving the various mechanical properties of FCC metals, involving room-temperature tensile ductility, high-temperature strength-ductility match, creep resistance, and fatigue properties. It has been well recognized that the mechanical properties of FCC materials could be obviously improved by a GBE treatment, especially at high temperatures or under high cyclic loads; under these circumstances, the materials are prone to intergranular cracking. In short, GBE has tremendous potential for improving the mechanical properties of FCC metallic materials, and it is a feasible method for designing high-performance metallic materials. [ABSTRACT FROM AUTHOR]

The change in the grain boundary network during recrystallization and grain growth was studied in a 316L austenitic stainless steel subjected to 5% cold rolling reduction. The primary recrystallization rapidly developed upon heating to 1000 °C, resulting in the development of relatively coarse-grained microstructure with a grain size about 100 μm. The recrystallized microstructures contained large fractions of annealing twins with their ∑3n SCL boundaries. The latter ones served as interrupters of the ordinary grain boundary network. The fraction of ∑3n CSL boundaries increased with increasing the grain size during prolonged annealing. On the other hand, the number of interruptions per unit area remained nearly the same during annealing. Hence, the number of interruptions per a grain increased in accordance with a power law function of the grain size with an exponent of 2. The relationships obtained for the grain boundary network evolution can be used to predict the microstructure evolution in austenitic stainless steels during primary recrystallization followed by grain growth. [ABSTRACT FROM AUTHOR]

Grain boundary character distribution and disconnectivity of random boundaries were observed “in-situ” in the Hastelloy C-276 alloy sample through thermomechanical processing. The role of Σ3 boundaries in microstructural evolution of grain boundary character distribution was analyzed by electron back-scattered diffraction (EBSD) and the mechanisms of grain boundary engineering (GBE) during annealing were elucidated. The results showed that the fraction of Σ3n boundaries (including Σ3 boundaries) consistently increased in number with annealing time; however, the Σ3 boundaries had different effects on the disconnectivity of random boundaries because of their variants, i.e., coherent and incoherent Σ3 boundaries. At the initial annealing stage, new incoherent Σ3 boundaries were generated, according to the “Σ3 regeneration mechanism” model; Σ9 and Σ27 boundaries appeared and increased in number with increasing Σ3 boundary interactions. These incoherent Σ3 boundaries could disrupt the connectivity of the random grain boundaries and form large clusters. However, at the annealing stage, some incoherent Σ3 boundaries were annihilated, and new coherent ones emerged. As a result, the random grain boundary network evolved by obtaining significant connectivity with larger clusters, attributed to the reduction of the incoherent Σ3 boundaries. This implies that the incoherent Σ3 boundaries play an important role in the optimization of grain boundary engineering.

In order to modify the room-temperature plasticity of TiAl-based alloy, grain boundary character distribution (GBCD) in TiAl-based alloy was tuned through multidirectional isothermal forging (MDIF) combined with annealing. The experimental results showed that MDIF provided appropriate driving force for the regeneration of the coincidence site lattice (CSL) boundaries through recovery and recrystallization process. Combination of MDIF and annealing at 1100 °C for 90 min promoted the formation of a large number of annealing twins and increased the fraction of low-ΣCSL boundaries to 65.88% and the ratio of (Σ9+Σ27)/Σ3 to 15.12%, which contributed to the disruption of random boundary networks and enhanced the room-temperature plasticity of the TiAl-based alloy.

The microstructure, precipitates and grain boundary character of 15Cr-30Ni-2Ti-3Cu valve alloy after different solution treatments were investigated. The results show that the precipitates in the forged alloy are Laves phase, (Ti,Nb)C carbides and Sigma phase. The grain sizes of the alloy are 35.21 ± 12.17 μm (980 ℃/1 hour) and 53.14 ± 15.88 μm (1015 ℃/1 hour), respectively, both of which exhibit a single peak distribution, indicating that the austenite grains show normal growth mode. A two-stage grain growth model was established to predict austenite grain growth during solution. The average absolute relative error of the predicted values is 4.93 % and the correlation coefficient of linear fitting is 0.983. According to grain boundary engineering analysis, the ratio of special grain boundaries in the alloy after the solution treatment at 980 ℃ for 3 hours reaches the largest (63.1 %). The activation energy for (Ti,Nb)C carbide dissolution is 223.6 kJ/mol based on the modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model calculation. The microhardness of the alloy is the highest in the forged state. The microhardness decreases first and then increases with the extension of the soaking time, and reaches the maximum value of 238.7 ± 8.4 HV after the solution treatment at 1050 ℃ for 3 hours. • The evolution of microstructure and precipitates in 15Cr-30Ni-2Ti-3Cu valve alloy after solution treatments were studied. • The grain boundary characteristic distribution of the alloy after solution treatments was investigated. • The dissolution activation energy of (Ti,Nb)C is 223.6 kJ/mol based on the analysis of the modified JMAK model. • The mechanism of the variation of microhardness after solution treatment in the valve alloy investigated. [ABSTRACT FROM AUTHOR]

The Bi 2 Te 3 -based alloys with atomically layered structure are the best thermoelectric performance at the vicinity of room temperature. However, the inferior mechanical properties due to weak van der Waals interactions between layers are the obstacle to further promote the widespread commercialized application. Here, the grain refinement BiSbTe alloys prepared by ball milling and spark plasma sintering (grain boundary engineering) is reported. The microstructure characterization results indicate that ball milling can significantly reduce the grain size. The grain boundary engineering can significantly improve Seebeck coefficient about 23 %, maintain large hall mobility and reduce the carrier thermal conductivity about 48 %, and the lattice thermal conductivity is slightly decreased about 15 % by enhanced grain boundary phonon scattering. The highest average ZT reaches 1.2 in the range of 300–400 K for grain refinement sample, which is beneficial to improve the conversion efficiency of thermoelectric devices, and the Vickers hardness of the corresponding sample is increased by 58 %. The easy-to-operate strategy of grain refinement caused by ball milling provides a way to mass produce high performance and robust mechanical properties Bi 2 Te 3 -based thermoelectric devices. • The grain boundary engineering can significantly reduce thermal conductivity. • The coordinated regulation of electro-thermal-mechanical properties was achieved. • Our work provides a method to the large-scale production of Bi 2 Te 3 alloys. • The high Vickers hardness can promote the development of thermoelectric devices. [ABSTRACT FROM AUTHOR]

One of the primary challenges in commercializing nickel (Ni)-rich layered cathodes for lithium-ion batteries (LIBs) is addressing the problem of crack formation during cell cycling. These cracks quickly spread along grain boundaries (GBs), causing capacity and voltage degradation. GB engineering, a classical technique for strengthening materials that has been largely unexplored in LIBs, could potentially inhibit intergranular fractures and significantly extend battery life. In this study, we employed a synergistic approach that combined high-throughput first-principles calculations and an interpretable machine learning (ML) framework to assess the feasibility of enhancing GBs through segregation-induced strengthening via doping. As a result, we generated a comprehensive database containing 64 elements doped at 4 sites in ∑3 [100](012) GB of LiNiO 2 , representing the largest database for first-principle-aided GB engineering in LIBs to date. This database further enabled the implementation of an interpretable ML workflow to uncover the fundamental principles governing dopant segregation and GB strengthening. Based on the insights gained from this research, we provide practical and promising recommendations for dopant elements that can effectively anchor and reinforce GBs, such as Mg, Al, Si, Ti, Cr, Mn, Fe, Cu, Zn, Hf, and Ce, with some already demonstrated success in experiments. This work on GB engineering serves as a foundation for the development of advanced, doped cathode materials for next-generation batteries. [Display omitted] [ABSTRACT FROM AUTHOR]

X2CrNi12 ferritic stainless steel has a wide range of application prospects in the railway transportation, construction, and automobile fields due to its excellent properties. The properties of X2CrNi12 ferritic stainless steel can be further improved by cold-rolling and subsequent annealing treatment. The purpose of this work is to investigate the effect of cold-rolling reduction on the microstructure, texture and corrosion properties of the recrystallized X2CrNi12 ferritic stainless steel by using SEM, TEM, EBSD and electrochemical testing technology. The results show that the crystal orientation characteristics of the cold-rolled sheet could be inherited into the annealed sheet. The higher cold-rolling reduction could promote the deformed grains rotating into the {111} orientation, increasing storage energy and driving force for recrystallization, which could reduce the recrystallized grain size. The orientation densities of α-fiber and γ-fiber were low at 50% cold-rolling reduction. After recrystallization annealing, a large number of grains with random orientation could be produced, and the texture strength was weakened. When the cold-rolling reduction rose to 90%, the γ-fiber texture at {111}<110> was strengthened and the α-fibers, particularly the {112}<110> component, were weakened after recrystallisation annealing, which could improve the formability of the steels. The proportions of special boundaries, i.e., low-angle grain boundaries and low-Σ CSL boundaries, among the grain boundary distribution of the recrystallized X2CrNi12 stainless steel were higher when the reduction was 90%, especially when the annealing temperature was 770 °C. Additionally, the proportion of LAGBs and low-Σ CSL boundaries were 53% and 7.43%, respectively, which improves the corrosion resistance of the matrix, showing the best corrosion resistance. [ABSTRACT FROM AUTHOR]

Enhancement of thermoelectric properties by virtue of decreased electrical resistance through grain boundary engineering is realised in this study. A robust strategy of optimisation of the transport properties by tuning the energy filtering effects at the interfaces by decreasing the interfacial electrical resistance is achieved in LaCoO 3 (LCO). This is accomplished by the incorporation of multilayer graphene within the parent LCO matrix containing multi-scale nano/micro grains. The present work has attained a substantial increment in electrical conductivity from a value of 96 Scm-1 for bare LCO to ∼5300 Scm-1 at 750 K by incorporating 0.08 wt% multilayer graphene in LCO. No significant change in thermal conductivity is observed due to the presence of multilayer graphene in LCO. A zT of 0.33 at 550 K for 0.08 wt% multi-layer graphene incorporated LCO composite is achieved which is the highest thermoelectric figure of merit value for undoped LCO reported until now. [ABSTRACT FROM AUTHOR]

To forge ahead with the next generation of power batteries boasting superior energy density, nickel-rich layered oxides are regarded as some of the most promising cathode materials. However, challenges such as microcracks, which are attributed to the elevated nickel content of the materials, have posed impediments to their further development and application. Consequently, this article focuses on the understanding of the materials in the deep delithiation state, dissecting their degradation mechanisms through a dual lens of electrochemical and mechanical properties. The comprehensive analysis reveals that microcracks within the particles exhibit a degree of reversibility. However, with repeated Li + de-/intercalation, these microcracks progressively propagate and permeate the entire particle, ultimately leading to particle fragmentation. Therefore, this study employs Dy 2 O 3 as an inducer to facilitate the growth of primary crystal grains, reducing the internal porosity of the particles. This effectively enhances the conductivity and lithium-ion diffusion kinetics in deep lithium-ion deintercalation states of nickel-rich cathode materials. The modified material exhibits significant suppression of microcrack formation and growth during cycling, leading to notable improvements in its chemical-mechanical properties. These degradation mechanisms and modification strategies of Ni-rich cathodes offer valuable insights into the development of Ni-rich cathode materials tailored for electric vehicles.