Rapid Grain Research Articles

A phase field crystal model for real-time grain boundary formation and motion in complex concentration alloy

The complex concentration alloys exhibit the excellent mechanical property due to the dual roles of their heterogeneous compositions and microstructures. However, the formation and motion of grain boundaries to significantly regulate the mechanical properties remain unknown at several microsecond and nanoscale in the complex concentration alloys. Here, we utilize a phase field crystal model to investigate the real-time grain boundary formation and motion in complex concentration alloys, specifically, in comparison to traditional alloys, using the example of AlCu alloys. Our findings reveal that the low concentration alloys exhibit segregation primarily at grain boundaries with minimal compositional fluctuations over a long period of grain growth, and the complex concentration alloys display a high concentration gradient within the grains to cause rapid grain rotation and merging in a short time frame. In the complex concentration alloys, the large component fluctuations not only cause the local lattice distortion to hinder dislocation movement during the deformation process, but also lead to the occurrence from dislocation locking to self-unlocking, which is beneficial to improve the strength and ductility. The present work provides a real-time atomic-scale understanding of grain boundary evolution using in situ atomic-resolution simulations.

Interface migration and grain growth in NbC-Ni cemented carbides with secondary carbide addition

Cemented carbides are widely used for cutting and drilling tools. They usually combine a WC hard carbide phase and a Co-based ductile binder. NbC-Ni materials are considered as a possible alternative, especially for wear applications. The advantageous economic situation for raw materials sourcing, their interesting mechanical properties and low density have raised a new interest for these materials. However, mechanical properties can be limited by the rapid grain growth during liquid phase sintering, as compared to WC-Co. Grain growth can be controlled by the addition of secondary carbides. In this paper, a quantitative EBSD analysis of grain growth is performed for NbC-12 vol%Ni materials sintered at 1450 °C with controlled addition of Mo2C and WC. The average grain size decreases continuously with Mo2C addition. The results are discussed based on a more detailed interface characterization and on a previous model for the cooperative migration of phase boundaries and grain boundaries.

Microstructural stability and precipitate evolution of thermally treated 7075 aluminum alloy fabricated by cold spray

In this work, the effects of post-deposition heat treatment on microstructural stability of cold sprayed AA7075 aluminum coatings were thoroughly characterized. The as-atomized powder and annealed coatings were investigated using scanning electron microscopy, electron back-scattered diffraction, and microhardness measurements. Scanning transmission electron microscopy was further employed to evaluate grain/subgrain structure near the particle interface region. Precipitation in the coating was studied through differential scanning calorimetry and X-ray diffraction. The feedstock powder revealed dendritic/cellular-like structure accompanied by microsegregations of the main alloying elements at dendrite boundaries. The high-pressure Cold Spray process led to formation of lamellar subgrains and UFG grains via dynamic recovery/recrystallization mechanisms. GP zones present in the AA7075 powder continuously reprecipitated into more stable η´/η phases owing to propellant gas heating and extensive plastic strains induced during cold spraying. The applied annealing resulted in progressive softening of the AA7075 coatings and gradual transformation of LAGBs into HAGBs. The solute-rich grain boundary network observed in powder microstructure was fully retained after CS deposition but decomposed into coarse η and S precipitates upon high-temperature annealing. The CS AA7075 coating showed decent thermal stability up to 300 °C and rapid grain growth at 400 °C. The relationship between precipitate size and thermal stability was further discussed in terms of the Zener pinning theory.

Electropulsing rapidly refined the microstructure and enhanced the strength and plasticity of Cu-3.5Ti alloy

Recrystallization annealing is a necessary process for refining the grain of processed metallic materials and eliminating work hardening. However, it can cause a large amount of precipitation and coarsening of the second phase in supersaturated solid solution alloys, which is detrimental to the subsequent processing and aging processes aimed at controlling the precipitation phase. This article compared the effects of electropulsing (EPT) and conventional heat treatment (CHT) processes on the microstructure and mechanical properties of cold-rolled Cu-3.5Ti alloy. The results indicated that compared with CHT, EPT achieved full recrystallization of the alloy at a lower temperature (650 °C, 100 °C lower than CHT) and in a shorter time (10 s, 1/360 of CHT), and effectively inhibited the precipitation of β-Cu4Ti phase, resulting in a fine and uniform supersaturated solid solution. This process significantly improved both the strength and plasticity of the alloy. The yield strength, tensile strength and elongation to failure of the EPT-650 °C/10 s sample were 570 MPa, 598 MPa and 21.2 %, respectively, which were 27.8 %, 25.9 % and 19.1 % higher than those of the CHT-750 °C/1 h sample. EPT significantly increased the recrystallization nucleation rate of the alloy, while its rapid heating method inhibited the precipitation of β-Cu4Ti phase, ensuring the recrystallization process was not affected by the precipitated phase, which was the main reason for rapid recrystallization and grain refinement of Cu-3.5Ti alloy by EPT at lower temperatures and shorter time.

An order-disorder core-shell strategy for enhanced work-hardening capability and ductility in nanostructured alloys

Nanocrystalline metallic materials have the merit of high strength but usually suffer from poor ductility and rapid grain coarsening, limiting their practical application. Here, we introduce a core-shell nanostructure in a multicomponent alloy to address these challenges simultaneously, achieving a high tensile strength of 2.65 GPa, a large uniform elongation of 17%, and a high thermal stability of 1173 K. Our strategy relies on an ordered superlattice structure that excels in dislocation accumulation, encased by a ≈3 nm disordered face-centered-cubic nanolayer acting as dislocation sources. The ordered superlattice with high anti-phase boundary energy retards dislocation motions, promoting their interaction and storage within the nanograins. The disordered interfacial nanolayer promotes dislocation emission and effectively accommodates the plastic strain at grain boundaries, preventing intergranular cracking. Consequently, the order-disorder core-shell nanostructure exhibits enhanced work-hardening capability and large ductility. Moreover, such core-shell nanostructure exhibits high coarsening resistance at elevated temperatures, enabling it high thermal stability. Such a design strategy holds promise for developing high-performance materials.

Molecular dynamics study of sintering of faceted cubic boron nitride nanoparticles at high temperatures

The sintering mechanisms and temperature dependence of coalescence of colliding cubic boron nitride (c-BN) nanoparticles are investigated using classical molecular dynamics (MD) simulation. Particle-particle collisions of 2.55-nm octahedral c-BN nanoparticles, consisting solely of the most stable {111} facets, with half of the surface terminations being boron and the other half nitrogen, are analyzed statistically and evaluated to assess the initial temperature range (2500 K – 3100 K) for sintering and its effect on grain growth. At these temperatures, the collision process maximizes contact surface area through interfacial sliding, thereby minimizing free energy and accommodating dangling bonds. Moreover, the exothermic formation of bonds of the coalescing nanoparticles increases the temperature. The alignment of the {111} orientation of the two collided nanoparticles occurs at a temperature slightly above the melting point, and rapid grain growth happens when the temperature is a few hundred degrees higher than that. However, phase separation also takes place at the corners away from the collision plane of the merging nanoparticles. Between 3100 K and 3250 K, crystalline alignment occurs, which aids the sintering process and allows for the formation of a well-structured nanocluster. However, above 3300 K, phase separation dominates and drives the melting of the entire sintered nanocluster.

Spatial Variations of Dust Opacity and Grain Growth in Dark Clouds: L1689, L1709, and L1712

The far-infrared (FIR) opacity of dust in dark clouds within the Ophiuchus molecular cloud is investigated through multiwavelength infrared observations from UKIDSS, Spitzer, and Herschel. Employing the infrared color excess technique with both near-infrared and mid-infrared photometric data, a high-resolution extinction map in the K band (A K ) is constructed for three dark clouds: L1689, L1709, and L1712. The derived extinction map has a resolution of 1′ and reaches a depth of A K ∼ 3 mag. The FIR optical depths τ 250 at a reference wavelength of 250 μm are obtained by fitting the Herschel PACS and SPIRE continuum data at 100, 160, 250, 350, and 500 μm using a modified blackbody model. The average dust opacity per unit gas mass at 250 μm, r κ 250, is determined through a pixel-by-pixel correlation of τ 250 with A K , yielding a value of approximately 0.09 cm2 g−1, which is about 2–3 times higher than the typical value in the diffuse interstellar medium. Additionally, an independent analysis across 16 subregions within the Ophiuchus cloud indicates spatial variations in dust opacity, with values ranging from 0.07 to 0.12 cm2 g−1. Although the observed trend of increasing dust opacity with higher extinction implies grain growth, our findings indicate that rapid grain growth has clearly not yet occurred in the dark clouds studied in this work.

Decomposition and dewetting of super-saturated Cu-15 at. % Co solid solution film

Nano-structured copper thin films present numerous applications for miniaturization of electronic devices. Their long term stability is key for reliable functionality. This work explores the thermal stability and solid state dewetting of a Cu-Co solid solution thin film and compares it to pure Cu. As according to the phase diagram Co is practically immiscible in Cu at room temperature, a metastable solid solution of Cu-15 at. % Co was deposited as thin film on a sapphire wafer. The microstructural evolution of the Co super-saturated film at temperatures ranging from 673 to 1073 K was evaluated using X-ray diffraction and high resolution microscopy techniques such as scanning electron and transmission electron microscopy. Interestingly, there is a competition between grain growth and phase separation. Co precipitates at grain boundaries acting as pinning sites preventing rapid grain growth during heat treatment at intermediate annealing temperature. However, grain growth occurs very quickly at higher temperatures, once the elemental phases of Cu and Co are formed. The competition between grain growth and phase separation and their consequence for dewetting are discussed.

Abnormal grain growth of tetragonal tungsten bronze structured ferroelectric ceramics

Microstructures of tetragonal tungsten bronze-structured ferroelectric ceramics vary owing to abnormal grain growth. Control of the grain microstructure stability is a critical aspect in the electrical properties. In this study, the grain growth behavior was meticulously explored through a combination of experiments and numerical simulations. Effects of the liquid phase and initial particle-size distribution on the microstructural evolution were investigated under different grain boundary energy anisotropy conditions. The grain area and edge number were calculated to investigate the grain growth kinetics and shape variation. The results revealed that the presence of a large amount of the liquid phase contributed to rapid grain growth and a duplex structure formation. The grain-size inhomogeneity can be eliminated by using initial particles with a bimodal particle-size distribution. The grain shape was mainly determined by the degree of anisotropy of the grain boundary energy and was nearly unaffected by the process parameters. The findings of this study afford theoretical guidance for microstructure regulation and stable preparation of high-performance ferroelectric ceramics.

A rapid, low-cost wheat spike grain segmentation and counting system based on deep learning and image processing

Spike grain number is a crucial parameter when estimating wheat yield. However, most methods predominantly focus on a single period, lacking universality. To achieve rapid and intelligent wheat spike grain counting from the filling stage to the mature stage, this study used different smartphones to obtain wheat ear images of different varieties and constructs the spike grain dataset required for deep learning segmentation model through image normalization and data enhancement. Then we propose a segmentation model called Multi-Attention TransUNet (MATransUNet) and a spike grain counting model (SGCountM) to achieve wheat spike grain counting. Our results indicate that, comparing with other models, MATransUNet achieves the best segmentation performance with strong robustness and generalization capabilities, achieving a mean intersection over union(mIoU) of 85.93%. Through the comparative analysis with manual counting results, our Method I, doubling the number of grains on one side of the ears, results in a root mean squared error (RMSE) of 4.38 and a coefficient of determination(R2) of 0.85; while the Method II, adding the grains on both sides of the ears, yields a RMSE of 3.13 and a R2 of 0.93. The counting accuracy significantly improves compared to not using the SGCountM. Moreover, transfer learning notably enhances the accuracy of the segmentation and counting model, enabling accurate spike grain counting from the filling stage to the mature stage. Finally, we integrate MATransUNet and SGCountM to design and implement a wheat spike grain segmentation and counting system (WeChat mini program). In practical tests, the mini program can effectively, accurately segments and counts wheat spike grain, demonstrating its feasibility and effectiveness. This research provides valuable insights into the intelligent application of wheat spike grain counting.

An Ultralow Concentration of Cr2O3 Dopants‐Driven Lower Temperature Sintering ZnO‐Based Varistor Ceramics

Low working voltage‐driven ZnO‐based varistor ceramics play an important role in the multilayer chip varistors, which require a low sintering temperature of ZnO varistor for its low energy consumption. Herein, a remarkable reduction of the sintering temperature from the usual 1100–1300 °C to 950 °C is successfully achieves in the ZnO ceramics via a certain 0.05 mol% of Cr2O3 dopants. The underlying mechanism is found to be involved with the formation of basal‐plane inversion boundaries between the ZnO grains, which can promote the rapid grain growth within the ceramics. Furthermore, the ZnO varistors with 0.05 mol% Cr2O3 dopant exhibit excellent performance. A low breakdown voltage of 416 V mm−1, a high nonlinear coefficient of 39, and a low leakage current of 3.4 μA are obtained simultaneously. This work presents an effective and promising approach for the cost‐efficient preparation of high‐performance ZnO‐based varistors, which have particular significance for the application of multilayer chip varistors with low working voltage.

Effects of SmCo Co-substitution on the magnetic and electrical properties of low temperature sintering synthesized strontium hexaferrite

In order to mitigate the rapid grain growth, a sintered ferrite magnet exhibiting stable characteristics was successfully synthesized. M-type strontium ferrite Sr1-xSmxFe12-xCoxO19 (x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.10) was synthesized via a low-temperature co-firing method, incorporating an appropriate quantity of Bi2O3 and CuO as additives within the solid-phase reaction process. The magnetic and electrical properties of the resulting composites were meticulously examined, and X-ray diffraction (XRD) analysis was performed for structural characterization. The findings reveal that a pure hexagonal strontium phase is achieved consistently when the substitution level, x, remains below 0.5. With an increase in the substitution level, a heterogeneous phase α-Fe2O3 becomes apparent within the phase structure. Upon introducing doping elements, the subtle incorporation of SmCo (x ≤ 0.02) enhances the saturation magnetization, remanent magnetization, and coercivity of the composite material. Moreover, the resistances of grain and grain boundary (Rg, Rgb) have been downscaled from 1.856 × 106Ω and 3.61 × 106Ω to 3.182 × 104Ω and 4.37 × 105Ω respectively. Consequently, the Sr1-xSmxFe12-xCoxO19 ferrite sintered under low-temperature conditions demonstrates outstanding electromagnetic characteristics, presenting a viable option for materials in microwave device applications.

Pressureless densification and properties of high-entropy boride ceramics with B4C additions

High entropy boride ceramics have great potential as structural materials serving in extreme environments. However, their applications are limited by the difficulty of sintering. In the present study, dense (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 ceramics with B4C additions were prepared through pressureless sintering at as low as 1900 °C. Calculations based on the CALPHAD approach predict that (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 starts to melt at about 3315 °C whilst B4C additions reduce the temperature and broaden the temperature region where solid and liquid coexist. Results showed that the introduction of B4C could trigger the densification of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 at a lower temperature and promote their densification significantly. The relative density of samples with 5 wt% of B4C additions sintered at 1900 and 2000 °C was 97.7 % and 99.7 %, respectively. While the sintering temperature was further increased to 2100 °C, the liquid phase was reactively formed, leading to the rapid grain coarsening in samples with B4C additions. Strengthened by well-dispersed B4C grains, the sample with 5 wt% B4C sintered at 2000 °C exhibited excellent mechanical properties with the Vickers hardness, flexural strength, and fracture toughness of 21.07 ± 2.09 GPa, 547 ± 45 MPa, and 5.24 ± 0.14 MPa m1/2, which are comparable or even higher than counterparts sintered under pressure.

A Zr-45Ti-54Al-3V alloy with excellent elevated temperature mechanical properties: Millimeter-scale coarse grains with initial subgrains

For improved elevated temperature mechanical properties, a Zr-45Ti-54Al-3V alloy with initial subgrains microstructure and millimeter-scale coarse grains is prepared herein via an electric field-assisted high-temperature compression process at temperatures of 1200 °C and 1400 °C. After the treatment at 1400 °C, the tensile strength and elongation of the alloy at 500 °C are increased by 25.2% and 21.7%, respectively, compared with those of the as-forged 47Zr. Both treatment temperatures lead to a compressive strength of 1400 MPa, along with good elevated temperature compression deformation ability. The rapid grain growth is attributed to the promotion of atomic diffusion by the pulsed current, which also results in a uniform distribution of each solute element. The dynamic recovery and continuous dynamic recrystallization during hot deformation are the main reason of elevated temperature plasticity improvement of EFHC sample. The elevated temperature strength of EFHC sample is higher than that of as-forged sample because it has more α phase and LAGB than as-forged sample

Effect of CaF2 on the preparation and optical property of MgAl2O4 transparent ceramics

To achieve optimal light transparency in MgAl2O4 transparent ceramics, a higher sintering temperature is required to eliminate pores. However, inevitable rapid grain growth occurs, leading to lower mechanical properties. To address this problem, calcium fluoride (CaF2) is utilized as a sintering additive to enhance the transparency of MgAl2O4 ceramics. The influence of CaF2 on the densification process and microstructure evolution is investigated. CaF2 demonstrates notable abilities in facilitating densification and lowering the sintering temperature. Moreover, it inhibits grain growth during the final sintering stage. However, the sintering temperature must be controlled because the secondary phases (calcium aluminate) precipitate at a higher temperature, thereby affecting the homogeneity of the ceramic microstructure. After optimizing the sintering process, ceramics (with a thickness of 5 mm) containing 0.05 wt% CaF2 exhibited the highest in-line transmittance of 81 % and 87 % in the visible and near-infrared wavelengths, respectively, after pre-sintering at 1505 °C for 3 h and subsequent hot isostatic pressing (HIPing) at 1550 °C for 3 h.

Synthesis of high-entropy germanides and investigation of their formation process.

High-entropy alloys and compounds have emerged as an attractive research area in part because of their distinctive solid-solution structure and multi-element compositions that provide near-limitless tailorability. A diverse array of reports describing high-entropy compounds, including carbides, nitrides, sulfides, oxides, fluorides, silicides, and borides, has resulted. Strikingly, exploration of high-entropy germanides (HEGs) has remained relatively limited. In this study, we present a detailed investigation into the synthesis of HEGs, specifically AuAgCuPdPtGe and FeCoNiCrVGe, via a rapid thermal annealing. The structural, compositional, and morphological characteristics of the synthesized HEGs were assessed using laboratory X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Complementing these post-synthesis analyses, we interrogated the formation and growth mechanisms using in situ heating XRD and TEM and determined that HEG formation involved initial decomposition of germanane (GeNSs) during the annealing, followed by gradual grain growth via atom diffusion at temperatures below 600 °C, and finally a rapid grain growth process at elevated temperatures.

SCGNet: efficient sparsely connected group convolution network for wheat grains classification.

Efficient and accurate varietal classification of wheat grains is crucial for maintaining varietal purity and reducing susceptibility to pests and diseases, thereby enhancing crop yield. Traditional manual and machine learning methods for wheat grain identification often suffer from inefficiencies and the use of large models. In this study, we propose a novel classification and recognition model called SCGNet, designed for rapid and efficient wheat grain classification. Specifically, our proposed model incorporates several modules that enhance information exchange and feature multiplexing between group convolutions. This mechanism enables the network to gather feature information from each subgroup of the previous layer, facilitating effective utilization of upper-layer features. Additionally, we introduce sparsity in channel connections between groups to further reduce computational complexity without compromising accuracy. Furthermore, we design a novel classification output layer based on 3-D convolution, replacing the traditional maximum pooling layer and fully connected layer in conventional convolutional neural networks (CNNs). This modification results in more efficient classification output generation. We conduct extensive experiments using a curated wheat grain dataset, demonstrating the superior performance of our proposed method. Our approach achieves an impressive accuracy of 99.56%, precision of 99.59%, recall of 99.55%, and an F 1-score of 99.57%. Notably, our method also exhibits the lowest number of Floating-Point Operations (FLOPs) and the number of parameters, making it a highly efficient solution for wheat grains classification.

Effects of the Sintering Process on Al2O3 Composite Ceramics Fabricated Using Material Extrusion and Photo-Polymerization Combined Process

The sintering process can improve the microstructure of Al2O3 composite ceramics and enhance their comprehensive properties, but the effects of the sintering process on Al2O3 composite ceramics are still unclear. Herein, a novel Al2O3 composite ceramic was printed using the material extrusion and photo-polymerization combined process, and the final ceramic was obtained using one-step sintering (TS) and two-step sintering technology (TSS). Based on the testing results, such as the relative density (Drel), average grain size (AGS), hardness, bending strength, and fracture toughness, TSS was suitable for the refinement of commercial Al2O3 ceramics. Moreover, the highest sintering temperature of the second step (T2) was at 1550 °C, while that of the shortest holding time (t) was at 4 h (TSS8), which was to ensure densification before rapid grain growth. The Drel and AGS of the best ceramics obtained via TSS8 were 97.65% and 1.52 µm, respectively. Their hardness, bending strength, and fracture toughness were also enhanced, and they were affected by T2, t, and the interaction. In sum, the TSS obtained better fracture toughness and bending strength, which had great potential in the application of the additive manufacturing field.

The coupling effect of particle and electric current on the grain refinement of Al–Cu–Li alloy by an improved thermo-mechanical treatment with electrically-assisted annealing

An improved thermo-mechanical treatment with electrically-assistant recrystallization annealing (EARA) was proposed for Al–Cu–Li alloy with the fine and equiaxed grains. The Al–Cu–Li alloy strips were subjected to the different overaging times, asymmetric rolling and EARA. The size and number of particles had a significant effect on the average grain size. The critical particle size for particle-stimulated nucleation (PSN) was determined. The introduction of electric current did not change the critical size, however, the efficiency of PSN triggered by coarse particles was improved by 3.6 times, compared with the conventional recrystallization annealing (CRA). By the optimized overaging time, the grains were refined and the texture was weakened. The recrystallization mechanism for the specimens containing coarse particles was the discontinuous static recrystallization (DSRX) by PSN and the continuous static recrystallization (CSRX). At the constant current density and annealing time, the critical temperature for rapid recrystallization for EARA decreased by 40 °C, compared with CRA. The results were due to the local Joule heating effect, the activation energy reduction and atomic diffusion rate increase by the athermal effect of electric current. The improved RI-ITMT with EARA would be an ideal candidate for rapid grain refinement in Al–Cu–Li alloy alloys.

Effect of B4C on microstructure evolution and high-temperature mechanical properties of TiAl matrix composites

This study synthesized and characterized the Ti-48Al-2Cr-2 Nb-based composite reinforced by B4C addition, which was produced by spark plasma sintering. The grain size of composites decreased significantly from 225 ± 5 µm to 51 ± 5 µm with the increase of B4C content, mainly due to the pinning effect of in-situ generated TiB2 and Ti2AlC particles at grain boundaries, which limited the rapid grain growth of the matrix. With the addition of B4C, the volume fraction of Ti2AlC increased from 3.0% to 7.4%, with an average size of approximately 5 µm. The ultimate tensile strength and total fracture elongation of the Ti-48Al-2Cr-2 Nb alloy were 482.9 MPa and 10.2% at 800 ℃, correspondingly. With 0.3 wt% addition of B4C, B4C/Ti-48Al-2Cr-2 Nb composite exhibited superior strength (579.5 MPa) and ductility (12.6%) at the same tensile temperature. The reinforced strength of the composite was mainly attributed to the grain refinement and precipitation strengthening.