Grain Boundary Effects Research Articles

Hierarchical architecture and mechanical behavior of K418 Ni-based superalloys manufactured by laser powder bed fusion

Laser powder bed fusion (LPBF) has great potential in the manufacturing of difficult-to-cut Ni-based superalloys with complex geometries. The approach endows the alloys with multi-scale microstructure hierarchy, while its formation mechanism and contribution to mechanical properties are poorly understood. In this work, the hierarchical architecture in LPBF-fabricated K418 Ni-based superalloys, in terms of 3D interlocked grains, low-angle grain boundaries (LAGBs), cellular structure, inter-cellular segregation, and nano-sized γ′ precipitates and carbides, is comprehensively investigated. The formation of cellular structure is governed by the rapid solidification condition, and the 3D interlocked grain structure is associated with the cell growth behavior regulated by locally varied temperature gradient in the melt pool. The LPBF K418 alloys show a higher tensile strength but a slightly lower ductility at both room temperature and 600 °C than their conventional cast counterparts. The superior strength of LPBF alloys is dominantly attributed to the strengthening effect of interlocked grain boundaries and cellular microstructure. This work demonstrates the capability of LPBF to manufacture Ni-based superalloys with hierarchical microstructures and high performance for the application of hot-section gas turbine components.

Phase-field model for anisotropic grain growth

Grain boundary properties may depend on the five macroscopic crystallographic degrees of freedom of the boundary, these being the misorientation between crystals and the inclination of the boundary plane. An orientation-field model is developed which uses a single-well potential and allows the grain boundary energy and mobility to depend on all five macroscopic degrees of freedom. The model reproduces the analytical equilibrium dihedral angles at triple junctions, has no anomalous trijunction drag, and Wulff shapes for cubic grain boundary energy anisotropy. Polycrystalline simulations in two dimensions demonstrate a pronounced effect of low angle grain boundaries on the microstructure evolution, demonstrating the importance of accounting for misorientation-dependent energy, particularly in highly textured materials. This model serves as a simple, computationally efficient, and physics-based method for simulating grain growth phenomena. With the ability to tune the grain boundary energy and mobility, the model shows promise for use in large scale simulations of grain coarsening in a wide range of systems.

Kinetic Modeling of Grain Boundary Diffusion: Typical, Bi-Modal, and Semi-Lamellar Polycrystalline Coating Morphologies

Polycrystalline coatings and materials are widely used in engineering applications. Therefore, it is important to know their kinetics and mass transport mechanisms. The effect of grain boundaries (GBs) on diffusion in thin films with different morphologies lacks understanding. Numerical studies are necessary to study GB kinetics but are limited to simplified cases. The present work addresses the lack of diffusion studies in more complex morphologies. Diffusion in two-dimensional polycrystalline coatings of typical, bi-modal, and semi-lamellar morphologies was modeled and the influence of the microstructure on the diffusion regimes and the overall rate was identified. Different morphologies with similar diffusion coefficients provided different regimes. The regime depends not only on the total diffusivity and grain/GB diffusivities, but also on the morphological features of the surface. While the fast diffusion pathways of GBs accelerated diffusion, the level of acceleration depends on the morphology since fast pathways and flux areas are limited to GBs. GB distribution is important to the mass transfer process, as GBs accelerate diffusion locally. The overall diffusion rate is generally dependent on the diffusion coefficients ratio. Nevertheless, the level of this dependence relies on the morphology.

A multiscale high cycle fatigue approach based on shakedown and cohesive zone theories—Application for additive-manufactured 316L steel

A high cycle fatigue (HCF) approach based on shakedown, crystal plasticity and cohesive zone theories is developed for metals. To take into account the effect of localized plasticity in grains and grain boundaries, crystal plasticity and elastoplastic cohesive zone models are considered to calculate the mechanical responses of grains and grain boundaries at the mesoscopic scale, respectively. The Papadopoulos fatigue criterion is used to gauge the fatigue property of the grain, and a new mesoscopic fatigue criterion, based on the shakedown theory as in the Dang Van criterion, is proposed to assess the fatigue performance on the grain boundary. Based on these two fatigue criteria, the effects of dislocation slip and grain boundaries, as dominant crack initiation features in 316L steel, are explored. To validate the approach, comparison between the fatigue properties of additive-manufactured (AM) 316L steel manufactured with the different process parameters has been carried out. The simulation results show very good correlation with the experimental ones obtained by the self-heating method. The proposed approach can be used to optimize the fatigue properties of AM steel with respect of the process parameters.

Study on electrical activity of grain boundaries in silicon through systematic control of structural parameters and characterization using a pretrained machine learning model

We report on the effects of grain boundary (GB) structures on the carrier recombination velocity at GB (vGB) in multicrystalline Si (mc-Si). The fabricated artificial GBs and an originally developed machine learning model allowed an investigation of the effect of three macroscopic parameters, misorientation angle α for Σ values, asymmetric angle β, and deviation angle θ from the ingot growth direction. Totally, 13 GBs were formed by directional solidification using multi-seeds with controlled crystal orientations. vGB was evaluated directly from photoluminescence intensity profiles across GBs using a pre-trained machine learning model, which allowed a quantitative and continuous evaluation along GBs. The evaluation results indicated that the impact of θ on vGB would be relatively large among the three macroscopic parameters. In addition, the results for the Σ5 and Σ13 GBs suggested that the minimum vGB would be related to the GB energy. These results were discussed in terms of the complexity of the local reconstruction of GB structures. The deviation would make a more complex reconstructed GB structure with local distortion, resulting in an increase in the electrical activity of GBs. The obtained knowledge will contribute to improving various polycrystalline materials through a comprehensive understanding of the relationship between GB structures and their properties.

Nanocrystalline embedded In2Se3 amorphous thin film investigation and optical enhancement characterization for photo-sensing application

In this investigation, highly adherent and unique advantageous properties of In2Se3 films were prepared for different studies and applications using a thermal evaporation technique under a high vacuum on different substrates (including glass, quartz, and a p-Si single crystal). X-ray diffraction results show that the prepared In2Se3 powder has a polycrystalline nature with α-phase. The average size of the crystallite, microstrain, and dislocation density was measured and found to be 273.6 nm, 4.9 × 10–3, and 1.34 × 10–5 nm−2, respectively. Due to the long-range array, the deposited In2Se3 films have an amorphous nature. SEM images of the film surface were measured to examine the RMS roughness features and grain-boundary effect. Spectrophotometer measurements of transmittance and reflectance were used to estimate the optical constants of In2Se3 films. The relationship between absorption coefficient and photon energy was tested, and the results revealed that the optical transition is directly allowed with an energy gap of 2.25 eV. The dispersion and oscillator energies were measured using the single oscillator model employing a relation of Wemple–DiDomenico and found to be 3.8 and 1.7 eV. The characteristics of the In2Se3-based junction showed promising candidates for photosensor applications under illumination.Graphical abstract

Grain size and temperature mediated twinning ability and strength-ductility correlation in pure titanium

The influence of grain size and temperature mediated twinning ability on mechanical properties of pure Ti is investigated in this study. A remarkable work-hardening ability was found in the uniaxial tensile stress-strain curve obtained at liquid nitrogen temperature, leading to a higher ultimate tensile stress and improved total ductility compared with the room temperature values. Evidence of microstructure and misorientation evolvement during tensile testing was provided to prove that twinning was prevalent in larger grains at lower temperature. The generation of twinning can provide additional boundary strengthening and work hardening capability by adjusting the parent grain orientation. The transition from traditional grain-boundary strengthening to dynamic twinning strengthening contributes to the outstanding strength-ductility combination, which follows a modified Hall-Petch relationship showing a strong synergistic effect of grain boundary and twinning.

Site-independent strong phonon-vacancy scattering in high-temperature ceramics ZrB2 and HfB2

Similar effects of metal and boron vacancies on phonon scattering and lattice thermal conductivity $({\ensuremath{\kappa}}_{l})$ of ${\mathrm{ZrB}}_{2}$ and ${\mathrm{HfB}}_{2}$ are reported. These defects challenge the conventional understanding that associates larger impacts to bigger defects. We find the underlying reason to be a strong local perturbation caused by boron vacancy that substantially changes the interatomic force constants. In contrast, a long ranged but weaker perturbation is seen in the case of metal vacancy. We show that these behaviors originate from a mixed metallic and covalent bonding nature in the metal diborides. The thermal transport calculations are performed in a complete ab initio framework based on Boltzmann transport equation and density functional theory. Phonon-vacancy scattering is calculated using ab initio Green's function approach. Effects of natural isotopes and grain boundaries on ${\ensuremath{\kappa}}_{l}$ are also systematically investigated, however we find an influential role of vacancies to explain large variations seen in the experiments. We further report a two-order of magnitude difference between the amorphous and pure-crystal limits. Our results outline significant material design aspects for these multifunctional high-temperature ceramics.

Electric-field-induced metal-insulator transition and quantum transport in large-area polycrystalline MoS2 monolayers

Monolayer ${\mathrm{MoS}}_{2}$ is an attractive candidate for developing future electronics and exploring fundamental physics because of its unique quantum properties. High-performance ${\mathrm{MoS}}_{2}$ transistors with superior transport originating from spin-valley coupling have been reported. However, these transistors have used single-crystalline flakes. In most large-area polycrystalline ${\mathrm{MoS}}_{2}$, these excellent transport properties have not been realized because of the effects of grain boundaries, and their fundamental low-temperature transport has not been studied in detail. Here, we apply electrolyte-gating methods using chemically grown centimeter-scale polycrystalline monolayers. Owing to the high carrier density, the resistance is systematically tuned from insulating to metallic conduction. Importantly, we observed metallic transport at temperatures down to 1.9 K and a high mobility of $>100\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\mathrm{n}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Moreover, the magnetotransport exhibits an electric-field-induced crossover from weak localization to weak antilocalization, indicating that high carrier density dramatically suppresses the grain boundary effects to enable intrinsic quantum transport. The results reveal that polycrystalline ${\mathrm{MoS}}_{2}$ monolayers have significant potential for use in large-area nanoelectronics.

Effect of Σ3 grain boundaries on microstructure and properties of oriented Fe-6.5 wt%Si in twin-roll strip casting

Oriented Fe-6.5 wt%Si alloy with excellent magnetic properties was prepared by twin-roll strip casting (TRSC). We studied the abnormal Σ3 grain boundaries in the as-cast strip. The experimental results demonstrated that the Σ3 grain boundary (GB) in oriented Fe-6.5 wt%Si steel had a good thermal stability, which could refine the microstructure by inhibiting growth of structure in as-cast strip. Moreover, the Σ3 GB in Fe-6.5 wt%Si as-cast strip could improve the toughness of matrix and restrain the propagation of cracks, which was beneficial to its plastic working. The Σ3 GB was relatively stable during warm rolling and could be inherited partly after annealing. At the same time, the low migration rate of Σ3 GB in primary recrystallization structure could stabilize the matrix and help to obtain the abnormal growth of Goss grains. The finished Fe-6.5 wt%Si steel had excellent magnetic properties, B800 and P10/50 were 1.76 T and 0.59 W/kg, respectively.

Phonon thermal transport in copper: The effect of size, crystal orientation, and grain boundaries

In electronic devices at the micro- and nanoscale, thermal management is vital. At such small sizes, crystal orientation, grain boundaries, and even the size itself can play an important role in the thermal transport and need to be taken into careful consideration when devices are designed. In this article, we perform computational experiments using non-equilibrium molecular dynamics simulations to evaluate the effect of size, orientation, and grain boundaries on the phonon thermal transport of copper. In addition, we compare the results obtained from the rescale and Langevin thermostat procedures. We find that the contribution of phonons to the total thermal conductivity in copper increases as the size decreases. Furthermore, the Σ5(210)[001] twist grain boundary is found to have a significant effect on the thermal transport of a bi-crystalline copper system when the grains are 15 nm. No such effect is found for the Σ3(112)[110] twin boundary. The effect of crystal orientation on the thermal conductivity is also studied, and no discerned effect can be observed. It is found that the Langevin thermostat leads to an over-estimation of the thermal conductivities at smaller scales and should be used with caution.

Peak profile analysis, electrical, dielectric behaviour and defect mediated yellow photoluminescence of zinc oxide nanostructures

A simple chemical method has been used to synthesize pure and crystalline ZnO nanoparticles. XRD study revealed hexagonal unit-cell with crystallite size 18.32 nm. Non-Debye type relaxation mechanism leads to an additional constant-phase-contribution to the equivalent parallel RC-network. The grain-boundary effect to the conductivity is found to be very large. The dc conductivity was calculated to be 4.9 × 10−8 (Ω cm)−1. The high loss-tangent at lower frequency is contributed by the large grain boundary effect of the material. The material is highly transparent in the visible window of wavelength with wide and direct band gap of 3.53 eV. The highly intense photoluminescence at 583 nm is attributed to oxygen related defects (interstitials or vacancy).

Effect of Symmetric Tilt and Twist Grain Boundaries on the Void Nucleation, Growth and Spall in polycrystalline Al : Multiscale modelling

Multiscale modelling using molecular dynamics (MD) and hydrodynamics (HD) in sequence is carried out to obtain the dynamic spall strength of aluminum (Al). Specifically, we simulate the effect of symmetric tilt and twist grain boundaries (GB) on the void nucleation and growth in Al using MD and transfer the information to HD simulations of plate impact. MD is used to create bi-crystals mimicking 11 tilt and 12 twist grain boundaries (GB) along <100>, with orientation angles spanning 0° to 90° using an NPT ensemble. The grain boundary energy (GBE) is validated with published results. MD simulations of the tri-axial tensile deformation of the Al bi-crystals show that the voids nucleate at the grain boundaries and then grow and coalesce. The time history of the bulk pressure and void volume fraction are recorded for calculating the material specific parameters of a void nucleation and growth (NAG) fracture model. 1-D Hydrodynamic flyer plate impact simulations are performed using these NAG parameters to obtain the free surface velocity vs time curve. The spall strength and spall thickness of Al, estimated from the free surface velocity profile, show a good match with published experimental values of the dynamic spall strength (relative error of 2.2%–6%) and spall thickness (relative error of 2.4%–4%) for three impact velocities viz. 518, 1588, and 2275 m/s respectively.

Effect of the prior-β grain boundaries on mechanical behavior of Ti6Al4V alloy processed by laser directed energy deposition

Laser directed energy deposition (LDED) has a manufacturing advantage, however, the anisotropy of mechanical properties is a challenge. In this work, the tensile and fatigue properties of LDED-ed samples under different direction stresses were systematically investigated. The pore distribution and the microstructure of LDEDed samples were researched, moreover, the micro-hardness, grain orientation spread (GOS), and Schmidt factor of prior-β grain boundaries (αGB) were calculated by nanoindentation and electron backscatter diffraction analyses. It was found that the micro-hardness of αGB was much lower than that of prior-β grains, however, the GOS Value and Schmidt factor of αGB were higher than those of prior-β grains. Therefore, dislocation slip at αGB was easily initiated. The anisotropy of mechanical properties was dependent on the relationship between the prior-β grains and the stress direction. The crack initiation and propagation easily occurred in αGB during horizontal stress. Furthermore, the forming height greatly affected the pore distribution, and larger pores caused a deterioration of mechanical properties during vertical stress. The findings of this work can provide useful insights into the anisotropy of LDED-ed titanium alloys.

Dual-Functional Enantiomeric Compounds as Hole-Transporting Materials and Interfacial Layers in Perovskite Solar Cells.

In this paper, we describe the application of the enantiomeric compounds YLC-1-YLC-4, each featuring a bulky spiro[fluorene-9,9'-phenanthren]-10'-one moiety, as both hole-transporting materials (HTMs) and interfacial layers in both n-i-p and p-i-n perovskite solar cells (PSCs). These HTMs contain an enantiomeric mixture and a variety of core units linked to triarylamine donors to extend the degree of π-conjugation. The n-i-p PSCs incorporating YLC-1(a) exhibited a power conversion efficiency (PCE) of 19.15% under AM 1.5G conditions (100 mW cm-2); this value was comparable with that obtained using spiro-OMeTAD as the HTM (18.25%). We obtained efficient and stable p-i-n PSCs having the dopant-free structure indium tin oxide (ITO)/NiOx/interfacial layer (YLC)/perovskite/PC61BM/BCP/Ag. The presence of the spiro-based compounds YLC-1 and YLC-2 efficiently passivated the interfacial and grain boundary defects of the perovskite and enhanced the sizes of its grains, more so than did YLC-3 and YLC-4. These spiro-based YLC derivatives packed densely and functioned as Lewis bases to coordinate Pb and Ni ions in the perovskite and NiOx layers, respectively. Together, the effects of smaller grain boundaries and defect passivation of the perovskite enhanced the optoelectronic properties of the PSCs. The photoinduced charge carrier extraction in the linearly increasing voltage (photo-CELIV) curves of NiOx/YLC-1(a) showed the faster carrier transport 3.3 × 10-3 cm2 V-1 s-1, which improved the carrier mobility, supporting the notion of defect passivation of the perovskite. The best-performing NiOx/YLC-1(a) device provided a short-circuit current density (JSC) of 22.88 mA cm-2, an open-circuit voltage (VOC) of 1.10 V, and a fill factor (FF) of 80.93%, corresponding to an overall PCE of 20.37%. In addition, the PCEs of the NiOx/YLC-1(a) and NiOx/YLC-4(b) PSC devices underwent decays of only 98.1 and 97.0% of their original values after 41 days under an Ar atmosphere. Thus, these YLC derivatives passivated the NiOx surface and optimized the film quality of perovskites, thereby leading to superior PCEs of their respective PSCs.

Actual Marine Atmospheric Pre-Corrosion Fatigue Performance of 7075-T73 Aluminum Alloy

Actual marine atmospheric pre-corrosion behavior and its effect on the fatigue performance of 7075-T73 aluminum alloy were studied by means of marine atmospheric outdoor exposure testing and fatigue testing. The surface and cross-sectional microstructures of aluminum alloy specimens after different numbers of days of exposure were analyzed. Localized pitting, and intergranular and exfoliation corrosion occurred during the outdoor exposure of aluminum alloy specimens in a marine atmosphere. The degree of severity of atmospheric corrosion increased with increasing duration of exposure. The effects of Fe-rich constituent particles (Al23CuFe4) and grain boundary precipitates (MgZn2) on the marine atmospheric corrosion behavior were discussed. In addition, when the exposure time was increased from 0 days to 15 days, the average fatigue life of aluminum alloy specimens decreased dramatically from about 125.16 × 104 cycles to 16.58 × 104 cycles. As the exposure time was further increased to 180 days, the average fatigue life slowly decreased to about 6.21 × 104 cycles. The fatigue fracture characteristics and the effect mechanism of marine atmospheric pre-corrosion on the fatigue life of 7075-T73 aluminum alloy were also analyzed.

Tubular glassy carbon microneedles with fullerene-like tips for biomedical applications.

Glassy carbon, in general, is made by the pyrolysis of polymeric materials and has been the subject of research for at least fifty years. However, as understanding its microstructure is far from straightforward, it continues to be an area of active research. Glassy carbon adopts different allotropes depending on the hybridizations of the C–C bond, that is, sp, sp2, or sp3. Furthermore, a variety of short-range ordering effects can interact with each other and this, along with the effects of microporosity, grain boundaries, and defects, render this a fascinating material. Following the nanoarchitectonics concept of bottom-up creation of functional materials, we use methane rather than a polymer to form glassy carbon. Here we show that tubular glassy carbon microneedles with fullerene-like tips form when methane undergoes pyrolysis on a curved alumina surface. X-ray diffraction of these glassy carbon tubules shows long-range order with a d-spacing of 4.89 Å, which is indicative of glassy carbon. Raman spectroscopy shows the material to be graphitic in nature, and SEM shows the fullerene-like structure of the material. This work provides new insights into the structure of glassy carbons relevant to the application of glassy carbons as a biomaterial, for example, as a new form of carbon-based microneedles. Since metallic needles can introduce toxic/allergenic species into susceptible subjects, this alternative carbon-based microneedle form has great potential as a replacement biomedical material for metallic needles in the field of neural engineering and as acupuncture needles.

Orientation behavior in TRC-ZA21 magnesium alloy with fine grain size during nano-indentation process

The deformation behavior in magnesium alloy was greatly affected by grain orientation. It is crucial to analyze the relationship between deformation mechanism and grain orientation. In this study, the deformation behaviors in grains possessing different orientations for TRC-ZA21 alloy with fine grain size were investigated using the nano-indentation deformation coupling EBSD method. The effects of grain orientation and grain boundary on deformation behavior and propagation of deformation mechanism were discussed. The results illustrated that the irregular performance of the nano-mechanical properties could be attributed the inconsistency of grain orientation as well as the grain boundary misorientation angle. Grain orientation, followed by the criterion of Schmid factor, regulated the deformation mechanism of grains that were in direct contact with the nano-indenter. This signified the orientation dependence in plastic deformation. The deformation behavior in grains, induced by plastic deformation and indirectly in contact with the nano-indenter, was found to be influenced by the grain boundary. The orientation difference in adjacent grains determines the influence of grain boundary in plastic deformation, revealing the competitive and coordinative behavior associated with these deformation mechanisms. The c-axes angle between adjacent grains was the key factor to determine the influence of grain boundary on deformation propagation.

Galvanic-Cell Deposition Enables the Exposure of Bismuth Grain Boundary for Efficient Electroreduction of Carbon Dioxide.

Metallic bismuth (Bi) holds great promise in efficient conversion of carbon dioxide (CO2 ) into formate, yet the complicated synthetic routes and unobtrusive performance hinder the practical application. Herein, a facile galvanic-cell deposition method is proposed for the rapid and one-step synthesis of Bi nanodendrites. Compared to the traditional deposition method, it is found that the special galvanic-cell configuration can promote the exposure of low-angle grain boundaries. X-ray absorption spectroscopy, in situ characterizations and theoretical calculations indicate the electronical structures can be greatly tailored by the grain boundaries, which can facilitate the CO2 adsorption and intermediate formation. Consequently, the grain boundary-enriched Bi nanodendrites exhibit a high selectivity toward formate with an impressively high production rate of 557.2 µmol h-1 cm-2 at -0.94V versus reversible hydrogen electrode, which outperforms most of the state-of-the-art Bi-based electrocatalysts with longer synthesis time. This work provides a straightforward method for rapidly fabricating active Bi electrocatalysts, and explicitly reveals the critical effect of grain boundary in Bi nanostructures on CO2 reduction.

Impact characteristic and crack propagation mechanisms of large-size molybdenum alloy single crystal and polycrystal

The impact characteristic and crack propagation mechanism for large-size Mo3Nb single crystal with a diameter of about 30 mm have been investigated and disclosed comprehensively by Charpy impact test. The fracture analysis shows that the single crystal crack propagates radially, resulting in a multi-step distribution on the fracture surface. The fracture of Mo3Nb polycrystal is composed of cleavage and quasi-cleavage, exhibiting a trans-granular fracture feature. The impact toughness of single crystal shows orientation dependence, which is attributed to the crack propagation path. Compared with the straight path, the waved path can delay the crack propagation and absorb higher impact energy. The impact toughness of Mo3Nb single crystals is lower than that of Mo3Nb polycrystal at room temperature because the single crystal lacks the coordinated deformation of grain boundary. The analysis of dislocation arrangement for Mo3Nb single crystal shows the different crack propagation paths is caused by the mobility of screw segments, which are controlled by the double cross-slip multiplication mechanism. However, the waved fracture morphology of Mo3Nb polycrystal is caused by the effect of grain boundaries instead of the cross-slip of screw dislocation.