Types Of Grain Boundaries Research Articles

Revealing Different Types of Grain Boundaries in Perovskite Films by Intensity-dependent Fluorescence Lifetime Imaging Microscopy

Revealing Different Types of Grain Boundaries in Perovskite Films by Intensity-dependent Fluorescence Lifetime Imaging Microscopy

Hierarchical zero- and one-dimensional topological states in symmetry-controllable grain boundary

Structural imperfections can be a promising testbed to engineer the symmetries and topological states of solid-state platforms. Here, we present direct evidence of hierarchical transitions of zero- (0D) and one-dimensional (1D) topological states in symmetry-enforced grain boundaries (GB) in 1T′–MoTe2. Using a scanning tunneling microscope tip press-and-pulse procedure, we construct two distinct types of GBs, which are differentiated by the underlying symmorphic and nonsymmorphic symmetries. The GBs with the nonsymmorphic rotation symmetry harbor first-order topological edge states protected by a nonsymmorphic band degeneracy. On the other hand, the edge state of the symmorphic GBs attains a band gap. More interestingly, the gapped edge state realizes a hierarchical topological phase, evidenced by the additional 0D boundary states at the GB ends. We anticipate our experiments will pioneer the material platform for the hierarchical realization of first-order and higher-order topology.

Exploring variations in toughness contributions across different types of high-angle grain boundaries in bainitic steels

Generally, the toughness of steel is greatly influenced by the density of high-angle grain boundaries (HAGBs). However, in bainitic steel, various types of boundaries can exhibit high angles, potentially leading to differences in their contributions to toughness. In this research, two steel samples with same chemical composition, similar strength and elongation, and comparable HAGBs densities demonstrated varying absorption energies in Charpy impact tests conducted at 20 °C and −40 °C. Crystallographic analysis revealed that although both samples have similar density of HAGBs, prior austenite boundaries and packet boundaries contribute more to impact absorption energy by enhancing plasticity compared to block boundaries. The discrepancy was particularly notable due to the high proportion of V1/V2 variant boundaries within the block boundaries. The findings underscore the significance of understanding the nuanced contributions of different HAGB types to the toughness of bainitic steels.

An understanding of the segregation and migration mechanism of point defects in tungsten grain boundaries: An atomic scale simulation

In this study, molecular dynamics (MD) simulations are employed to investigate the interactions between point defects and grain boundaries (GBs) in tungsten. Firstly, the segregation energy of vacancies (Vs) and self-interstitial atoms (SIAs) to GBs is examined. The results indicate that both Vs and SIAs tend to segregate towards GBs. Notably, the different types of GBs exhibit varying attraction to Vs and SIAs. The interaction radius is applied to investigate the degree of segregation, which is a significant index of GBs’ attraction to Vs and SIAs. The segregation radius of SIAs is typically larger than that of Vs. High-angle grain boundaries (HAGBs) show strong segregation for SIAs, facilitating the aggregation at GBs. Additionally, elastic theory is applied to qualitatively discuss the factors influencing the segregation energy distribution. The differences in atomic free volume caused by hydrostatic stress and the lattice distortion induced by point defects lead to Vs and SIAs occupying different sites. Finally, the nudged elastic band (NEB) method is employed to study the migration of Vs and SIAs near GBs, revealing that migration energy barriers for Vs and SIAs in GBs are much lower than in bulk. GBs facilitate the migration of point defects. Low-angle grain boundaries (LAGBs) present a higher energy barrier for V migration. Vs tend to occupy the compressive region, while SIAs tend to occupy the tensile region. Particularly, in comparison to Vs, SIAs are more likely to segregate to GBs due to higher binding energy, wider interaction radius, and extremely lower diffusion energy barrier. This provides some insights into the segregation and migration of point defects in GBs.

Research on a Novel Heat Treatment Process for Boron Steel Used for Soil-Engaging Components of Tillage Machinery

To address the issue of high fracture and wear failure rates caused by the lack of toughness and abrasion resistance in the steel used for soil-engaging components of tillage machinery, a novel composite heat treatment process, “normalizing and intercritical quenching and tempering (NIQT)”, is proposed. By regulating the austenitizing heating temperature in the intercritical area (ferrite/austenite two-phase area), the type, content, and distribution of phases in the 27MnCrB5 test sample could be precisely controlled, which further influenced the mechanical properties of the material. The results demonstrated that a multiphase composite microstructure, predominantly consisting of martensite and ferrite, could be obtained in the 27MnCrB5 steel treated by the NIQT process. The results of an EBSD test indicated that the predominant type of grain boundary following the NIQT heat treatment was a high-angle grain boundary (approximately 59.5%), which was favorable for hindering crack propagation and improving the impact toughness of the material. The results of the mechanical tests revealed that, when the quenching temperature was set to 790 °C, the 27MnCrB5 steel attained excellent comprehensive mechanical properties, with a tensile strength of 1654 MPa, elongation of 10.4%, impact energy of 77 J, and hardness of 530 HV30. Compared with conventional heat treatment processes for soil-engaging components, this novel process has the potential to enhance the performance of soil-engaging components and prolong their service life.

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.

Cluster dynamics modeling of hydrogen retention and desorption in tungsten with saturation and multi-trapping effect of sinks

Hydrogen (H) retention and desorption in tungsten (W)-based plasma-facing materials are still not well understood, largely due to the limitations of ex-situ observations in experimental detection methods like thermal desorption spectroscopy (TDS). In order to reveal the fundamental mechanisms behind H retention and desorption, we developed a cluster dynamics model, IRadMat-TDS, for theoretical modeling of depth distribution and TDS of deuterium (D) in polycrystalline W. The model newly includes the saturated absorption and emission of D in inherent sinks like grain boundaries (GBs), as well as the multi-trapping effect of D in various types of GBs with different trapping energies. The simulated TDS spectra are in agreement with experimental ones. For polycrystalline W under D ion irradiation within keV-energy range, two typical thermal desorption peaks in TDS at around 490 and 550 K are explicitly attributed to D emission from GBs and vacancies, respectively. And GBs play a major role in D retention. Moreover, the broad peaks in TDS come from the convolution of multi-trapping of D in sinks with different types of trapping sites rather than a single-site approximation.

Effect of fretting wear regimes on stress corrosion cracking of Alloy 690TT in high-temperature pressurized water

Stress corrosion cracks initiation and propagation of Alloy 690TT under different fretting regimes were investigated. The results indicated that mixed regime had the highest susceptibility to stress corrosion cracking, followed by gross slip regime, and then partial slip regime. Crack initiation depended on the distribution of residual strain induced by fretting regimes. In partial slip and mixed regimes, cracks were mainly initiated at the edge of the wear scar, while in gross slip regime, cracks were randomly distributed on the wear scar. Crack propagation was related to the type of grain boundaries and geometrically necessary dislocation density induced by fretting loading. In partial slip and gross slip regimes, cracks usually tended to propagate intergranularly. While in MR, cracks mainly propagated in a transgranular manner.

RESISTANCE OF FERRITE-PERLITE STEEL TUBING BRANCH PIPE METAL TO DESTRUCTION IN HYDROGEN SULFIDE MEDIUM

This paper discusses the problems of resistance of the metal of the tubing branch pipe (tubing) to the process of its destruction in a medium containing hydrogen sulfide and when exposed to external forces. A 73х73 tubing nozzle made of 38G2SFA steel was selected as the material for research. The tubing fragment was destroyed during production well operation. To establish the reasons for the failure of the tubing nozzle, a visual inspection was carried out, tests were performed to assess the hardness and mechanical properties, the microstructure and microtexture of the metal were analyzed using scanning electron microscopy (SEM) methods. To analyze the nature of the distribution of introduced defects, the mutual orientation of structural components and crystallographic texture, studies were carried out by the method electron backscatter diffraction (EBSD). Studies of the suspended branch pipe, which does not have chemical protection, showed that the cause of its destruction was hydrogen sulfide stress corrosion cracking (SSCC) as a result of hydrogen embrittlement and the action of external tensile forces exceeding the yield strength of the metal. It was established that as a result of the interaction of hydrogen sulfide of the distilled product with the metal of the branch pipe, hydrogen embrittlement of the material of the branch pipe occurred. The process of hydrogen embrittlement of the fragment was proved by detecting hydrogen blisters in the metal microstructure and establishing the process of coalescence of neighboring inclusions. In addition, the results of the test to measure the hardness of the metal along the wall thickness further supported the hypothesis of hydrogen embrittlement of the tubing. EBSD analysis showed that the threaded part of the branch pipe is characterized by a small size of structural elements and an increased density of introduced defects (geometrically necessary dislocations), which are a promising site for the deposition of molecular hydrogen. It is concluded that reaching the limit concentration of molecular hydrogen reduces the binding energy of metal atoms and this process initiates the formation of pores. It has been established that the development of cracks along large-angle grain boundaries is facilitated, and small-angle and coincidence site lattice (Σ3, Σ5, Σ11 and Σ13b) grain boundaries are active barriers to crack propagation. It has been shown that by optimizing the elemental composition of steel, controlling the microstructure, crystallographic texture, as well as the type and state of grain boundaries, new pipeline steels can be produced that are more resistant to SSCC.

Linear dependence of grain boundary energy on structural orderliness in FCC metals

Establishing the correlation between grain boundary (GB) structure and energy is crucial for advancing the fundamental understanding of polycrystalline materials, yet this correlation remains elusive. In this study, systematic atomic simulations are performed to comprehensively explore numerous GBs in different FCC metals and alloys. The findings unveil a universal linear relationship between the GB energy and GB structural orderliness parameter (GB excess centrosymmetry or excess bond orientational order parameters). Remarkably, the identified linear relationship proves universally applicable across different metal systems, GB types, and length scales, rooted in the linear correlation between atomic potential energy and atomic structural orderliness. Furthermore, the ratio between GB energy and orderliness parameter emerges as a potent indicator for gauging the extent of GB relaxation, thereby providing valuable insights to inform the design of nanocrystalline materials with enhanced GB stability.

Delayed recrystallization behavior and deformation mechanism of IN738LC alloy prepared by laser powder bed fusion

The design and development of heat treatment regimes adapted to the laser powder bed fusion technology for specific service conditions have become a hot topic of research in the field of additive manufacturing. This paper systematically studies the microstructure changes of the L-PBFed IN738LC alloy, solution heat treated at 1250 °C for various durations (1 h, 3 h, 5 h) and then aged at 850 °C for 24 h. The delayed recrystallization behavior was revealed, and the selective precipitation pattern of the M23C6 carbide on different types of grain boundaries was discussed. Additionally, the tensile deformation mechanism of the alloy before and after heat treatment is clarified. The results indicate that grain boundary migration and transformation are impeded under 1250 °C/1 h conditions due to carbide particle pinning and solute atom segregation during the initial stage of solution treatment, which accounts for the delayed recrystallization behavior, and the microstructure is mainly undergoing recovery. At 1250 °C/5 h, M23C6 carbides selectively precipitate at twin boundaries, accompanied by recrystallization. The samples treated at 1250 °C/1 h exhibit the highest mechanical property (σYS=1305 ± 21 MPa, σUTS=1462 ± 11 MPa, εEL=5.4 ± 0.7 %) owing to the precipitation of γ′ phase and the presence of sub-grain boundaries. The deformation mechanism shows that the as-built samples mainly experience planar dislocations slipping, while the heat-treated samples undergo stacking faults shearing and anti-phase boundary shearing. The dominant mechanism among these is anti-phase boundary shearing.

Microfabrics impact on rock salt geomechanical diversity – an experimental investigation

Geomechanical studies of rock salt serve as the foundation for designing and ensuring the long-term stability of storage caverns. Research conducted in recent years has shown that rock salt exhibit highly variable mechanical parameters. Even macroscopically similar rock salt samples from the same formation have very different mechanical parameters in standard strength tests. In this paper, the effect of microfabrics on the mechanical behaviour of rock salt was studied using a uniaxial compression device with simultaneous observation under a microscope. The tests were carried out on rock salt samples, previously tested on standard uniaxial compression (UCS). Tested samples showed different strength values despite macroscopic similarity and similar anhydrite content. The tests revealed that the microfabrics, such as fluid inclusions and anhydrite crystals distributed along the halite grain boundaries, play a significant role in the strength of a rock salt sample. Four types of grain boundaries were recognized in both group of samples: (1) dominated by anhydrite crystals with a small number of fluid inclusions (FIs), (2) with FIs prevailing over anhydrite crystals, (3) only with FIs, and (4) without anhydrite crystals or FIs. The experimental uniaxial compression tests in microscale (mUCS) showed that the damage process is initiated by FIs activity at the halite grain boundaries and consists of three main stages. The samples dominated by anhydrite crystals at halite grain boundaries were characterised by the highest mUCS. Conversely, if FIs prevail at the grain boundaries the lowest mUCS values were registered. Therefore, anhydrite crystals located along halite grain boundaries influenced the mechanical behaviour and deformation process. On the contrary, anhydrite crystals dispersed within halite grains had no impact on failure and mechanical parameters. The experiment showed that apart from impurity content, the distribution of these impurities within halite grains should be examined during geomechanical evaluation of rock salt samples. Thus, the overall strength of rock salt samples determined in standard test results from the local variations of petrological features in micro- and macro scale.

Fracture behaviour of bicrystalline graphene characterized via freestanding indentation

Using molecular mechanics (MM) simulations, the failure stress of bicrystalline graphene with different tilt angles is investigated using their free-standing indentation behaviour, in which two types of grain boundaries, including armchair (ac) and zigzag (zz) grain boundaries, and two types of indenter tip, including cylindrical and spherical tips are considered. For reference purposes, the corresponding results under inplane stretching are also examined. It is found that the failure stress of grain boundary (GB) in bicrystalline graphene decreases with the decrease of the GB tilt angle [Formula: see text], which is similar to that determined in inplane stretching. In indentation, the stress concentration under the indenter tip will decrease the failure stress of graphene; in addition, the out-of-plane deformation of graphene in indentation can partially release the pretension induced by the GB in bicrystalline graphene. For the GB with a small prestress, the effect of the former is larger than that of the latter; but for the GB with a larger prestress, the effect of the latter is larger than that of the former. Consequently, the effect of tilt angle [Formula: see text] on the GB strength of bicrystalline graphene is lower in indentation than that given by inplane stretching.

Transformation of Coherent Twin Boundary into Basal-Prismatic Boundary in HCP-Ti: A Molecular Dynamics Study.

The manufacturing process for wrought Ti alloys with the hexagonal close-packed (HCP) structure introduces a complicated microstructure with abundant intra- and inter-grain boundaries, which greatly influence performance. In the hexagonal close-packed (HCP) structure, two types of grain boundaries are commonly observed between grains with ~90° misorientation: the basal/prismatic boundary (BPB) and the coherent twin boundary (CTB). The mechanical response of the BPB and CTB under external loading was studied through molecular dynamic simulations of HCP-Ti. The results revealed that CTB undergoes transformation into BPB through the accumulation of twin boundary (TB) steps and subsequent emission of Shockley partial dislocations. When the total mismatch vector is close to the Burgers vector of a Shockley partial dislocation, BPB emits partial dislocations and further grows along the stacking faults. When a pair of CTBs are close to each other, severe boundary distortion occurs, facilitating the emission and absorption of partial dislocations, which further assists the CTB-BPB transformation. The present results thus help to explain the frequent observation of coexisting CTB and BPB in HCP alloys and further contribute to the understanding of their microstructure and property regulation.

Fracture mechanisms and suppressed temperature effects of YBCO superconducting materials dominated by grain boundaries

YBa2Cu3O7-δ (YBCO) superconducting materials are typical polycrystalline materials, and the presence of intrinsic grain boundaries (GBs) is the main reason that affects superconductivity, in which high-angle GBs lead to low critical current density. There is no doubt that GBs also play an important role in the mechanical properties and fracture mechanisms of YBCO superconducting materials. In this work, the uniaxial tensile molecular simulations have been performed to investigate the fracture mechanisms of YBCO superconducting materials dominated by different types of GBs. The (100) single crystal and (110) twinning boundary models have been employed as comparative references. The results show that the low-angle GBs of YBCO consist of discontinuous dislocation cores, while the high-angle GBs are composed of a series of structural unit arrangements. Meanwhile, Young's modulus and ultimate tensile strength of YBCO can be enhanced by twin boundaries, while the other GBs have different effects on Young's modulus and ultimate tensile strength. In addition, the expansion of dislocation cores and the deformation of characteristic structural units are the main fracture mechanisms. More significantly, a novel temperature effect has been found to be significantly suppressed under the high-angle GBs. These findings not only provide a deep understanding of the GB structures, but may also provide guidance for modulating the mechanical properties of YBCO superconducting materials.

Van der Waals Epitaxially Grown Molecular Crystal Dielectric Sb2O3 for 2D Electronics.

Two-dimensional (2D) semiconducting materials have attracted significant interest as promising candidates for channel materials owing to their high mobility and gate tunability at atomic-layer thickness. However, the development of 2D electronics is impeded due to the difficulty in formation of high-quality dielectrics with a clean and nondestructive interface. Here, we report the direct van der Waals epitaxial growth of a molecular crystal dielectric, Sb2O3, on 2D materials by physical vapor deposition. The grown Sb2O3 nanosheets showed epitaxial relations of 0 and 180° with the 2D template, maintaining high crystallinity and an ultrasharp vdW interface with the 2D materials. As a result, the Sb2O3 nanosheets exhibited a high breakdown field of 18.6 MV/cm for 2L Sb2O3 with a thickness of 1.3 nm and a very low leakage current of 2.47 × 10-7 A/cm2 for 3L Sb2O3 with a thickness of 1.96 nm. We also observed two types of grain boundaries (GBs) with misorientation angles of 0 and 60°. The 0°-GB with a well-stitched boundary showed higher electrical and thermal stabilities than those of the 60°-GB with a disordered boundary. Our work demonstrates a method to epitaxially grow molecular crystal dielectrics on 2D materials without causing any damage, a requirement for high-performance 2D electronics.

Atomic insights of grain boundary behaviors in TaWNbMo refractory high entropy alloys

Refractory high entropy alloys (RHEAs), such as TaWNbMo, demonstrate promising potential for high-temperature applications owing to their excellent mechanical properties. Given the pivotal role of grain boundaries (GBs) in determining the mechanical behavior of RHEAs, it is imperative to investigate their characteristics. In this work, the mechanical response of TaWNbMo RHEA and its components (Ta, W, Nb, and Mo) bicrystal samples with Σ3{112‾}<110> and Σ3{111‾}<110> GB under shear were performed using molecular dynamics simulations to investigate the motion characteristics of GBs and their influence on mechanical properties, as well as the variations brought by high entropy GBs. Both types of GBs were observed to migrate coupled with shear deformation, and the migration mechanism was analyzed based on the theory of interface defects. Additionally, twin nucleation and growth were observed in the samples with Σ3{111‾}<110> GB influenced by the high GB energy, and its mechanism was analyzed. Furthermore, a comparison was made between TaWNbMo and its component element metals, and the influence of high entropy GBs was analyzed. The multicomponent complexity invoked the local interfacial disconnection nucleation in HEA for both Σ3{112‾}<110> and Σ3{111‾}<110> GBs, the inhomogeneity of HEA also influenced the twin growth process from Σ3{112‾}<110> GB to proceed and thicken in a localized and non-uniform manner. These results contribute to a deeper understanding of the GB characteristics of TaWNbMo RHEA.

Characterisation of the interplay between microstructure and opto-electronic properties of Cu(In,Ga)S2 solar cells by using correlative CL-EBSD measurements

Cathodoluminescence and electron backscatter diffraction have been applied to exactly the same grain boundaries (GBs) in a Cu(In,Ga)S2 solar absorber in order to investigate the influence of microstructure on the radiative recombination behaviour at the GBs. Two different types of GB with different microstructure were analysed in detail: random high angle grain boundaries (RHAGBs) and Σ3 GBs. We found that the radiative recombination at all RHAGBs was inhibited to some extent, whereas at Σ3 GBs three different observations were made: unchanged, hindered, or promoted radiative recombination. These distinct behaviours may be linked to atomic-scale grain boundary structural differences. The majority of GBs also exhibited a small spectral shift of about ±10 meV relative to the local grain interior (GI) and a few of them showed spectral shifts of up to ±40 meV. Red and blue shifts were observed with roughly equal frequency.

In-situ EBSD tensile revealing the evolution mechanism of high angle grain boundaries in Al–Zn–Mg alloy profile with heterogeneous structures

The evolution of grain boundary (GB) of an Al–Zn–Mg alloy profile with heterogeneous structures is studied based on in-situ EBSD tensile. This evolution is not only related to its grain boundary misorientation angle (GBMA) and slip transfer parameter (m'/(Δb/b)) but also to the behavior of the geometrically necessary dislocation (GND) tensor in the deformation. The effect of GBMA and m'/(Δb/b) for three types of high angle grain boundary (HAGB) on GND density and back stress has been revealed. The increase of GND density is not obvious near the HAGBs with 45°–55° GBMA of neighboring coarse grains or neighboring fibrous grains. The optimal grain boundary for forming back stress is the HAGBs between coarse grain and fibrous grain, which has 35°–45° GBMA and a low m'/(Δb/b). The back stress is hard to form near these HAGBs with 25°–35° GBMA and a low m'/(Δb/b). Combined with the neighboring grain type, slip transfer and GBMA, the new criterion is proposed to predict GND behavior near HAGBs.

Evolutionary Search and Theoretical Study of Silicene Grain Boundaries' Mechanical Properties.

Defects such as grain boundaries (GBs) are almost inevitable during the synthesis process of 2D materials. To take advantage of the fascinating properties of 2D materials, understanding the nature and impact of various GB structures on pristine 2D sheets is crucial. In this work, using an evolutionary algorithm search, we predict a wide variety of silicene GB structures with very different atomic structures compared with those found in graphene or hexagonal boron-nitride. Twenty-one GBs with the lowest energy were validated by density functional theory (DFT), a majority of which were previously unreported to our best knowledge. Based on the diversity of the GB predictions, we found that the formation energy and mechanical properties can be dramatically altered by adatom positions within a GB and certain types of atomic structures, such as four-atom rings. To study the mechanical behavior of these GBs, we apply strain to the GB structures stepwise and use DFT calculations to investigate the mechanical properties of 9 representative structures. It is observed that GB structures based on pentagon-heptagon pairs are likely to have similar or higher in-plane stiffness and strength compared to the zigzag orientation of pristine silicene. However, an adatom located at the hollow site of a heptagon ring can significantly deteriorate the mechanical strength. For all of the structures, the in-plane stiffness and strength were found to decrease with increasing formation energy. For the failure behavior of GB structures, it was found that GB structures based on pentagon-heptagon pairs have failure behavior similar to that of graphene. We also found that the GB structures with atoms positioned outside of the 2D plane tend to experience phase transitions before failure. Utilizing the evolutionary algorithm, we locate diverse silicene GBs and obtain useful information about their mechanical properties.