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In this work, we report a counter-intuitive activation of abnormal grain growth (AGG) in randomly-oriented fine grains (FGs) in an L-PBF Al–Mg–Sc–Zr alloy, which is unlikely from perspectives of curvature-driven grain growth, crystallographic texture or strain-induced boundary migration. The observed spontaneous AGG is understood in light of the growth instability of an arbitrary grain in an evolving particle-pinned polycrystalline aggregation. A generalized criterion for AGG is hence proposed, which efficaciously explains the observed susceptibility of FGs to AGG, the presence of multiple AGG activation sites, and the asynchrony of AGG activation. [ABSTRACT FROM AUTHOR]

In this work, we report a counter-intuitive activation of abnormal grain growth (AGG) in randomly-oriented fine grains (FGs) in an L-PBF Al–Mg–Sc–Zr alloy, which is unlikely from perspectives of curvature-driven grain growth, crystallographic texture or strain-induced boundary migration. The observed spontaneous AGG is understood in light of the growth instability of an arbitrary grain in an evolving particle-pinned polycrystalline aggregation. A generalized criterion for AGG is hence proposed, which efficaciously explains the observed susceptibility of FGs to AGG, the presence of multiple AGG activation sites, and the asynchrony of AGG activation.

The presence of secondary particles to polycrystalline alloys results in kinetic stabilization of the grain boundaries, which maintains desirable fine microstructures. In some instances, secondary particles trigger abnormal grain growth. The mechanisms influencing abnormal grain growth are still a subject of conjecture. As dispersed fine particles can contribute to abnormal grain growth, it is necessary to clarify the governing mechanism by which this occurs. The current work employs a multiphase field modeling approach to shed light onto abnormal grain growth. Particular attention is placed on understanding the role of grain boundary–particle interactions on abnormal grain growth. The results show that, in the presence of particles, normal grain growth occurs until a pinned state is achieved. In the pinned state, some grains overcome the pinning pressure exerted by some particles by piercing through the particles, which results in abnormal grain growth. The piercing events appear to be entirely random and not related to the size of the interacting particles. None‐the‐less, a bimodal particle size distribution is observed to lead to abnormal grain growth. A pinning parameter is introduced as a metric to identify the transition from normal to abnormal grain growth. [ABSTRACT FROM AUTHOR]

Bi2Te3‐based alloys have great market demand in miniaturized thermoelectric (TE) devices for solid‐state refrigeration and power generation. However, their poor mechanical properties increase the fabrication cost and decrease the service durability. Here, this work reports on strengthened mechanical robustness in Bi2Te3‐based alloys due to thermodynamic Gibbs adsorption and kinetic Zener pinning at grain boundaries enabled by MgB2 decomposition. These effects result in much‐refined grain size and twofold enhancement of the compressive strength and Vickers hardness in (Bi0.5Sb1.5Te3)0.97(MgB2)0.03 compared with that of traditional powder‐metallurgy‐derived Bi0.5Sb1.5Te3. High mechanical properties enable excellent cutting machinability in the MgB2‐added samples, showing no missing corners or cracks. Moreover, adding MgB2 facilitates the simultaneous optimization of electron and phonon transport for enhancing the TE figure of merit (ZT). By further optimizing the Bi/Sb ratio, the sample (Bi0.4Sb1.6Te3)0.97(MgB2)0.03 shows a maximum ZT of ≈1.3 at 350 K and an average ZT of 1.1 within 300–473 K. As a consequence, robust TE devices with an energy conversion efficiency of 4.2% at a temperature difference of 215 K are fabricated. This work paves a new way for enhancing the machinability and durability of TE materials, which is especially promising for miniature devices. [ABSTRACT FROM AUTHOR]

In this study, microstructure, recrystallization texture, and mechanical properties of Al/WO3/SiC hybrid nanocomposite was investigated by electron backscatter diffraction (EBSD), analysis of orientation distribution function (ODF), and uniaxial tensile test during accumulative roll bonding (ARB) process. Microstructural observations show that the recrystallized grains are elongated in the rolling direction (RD) due to the Zener-pinning of nanoparticles at high angle grain boundaries and therefore growth is inhibited in the normal direction (ND) during the ARB process. The ODF investigation confirmed that after 5 cycles of ARB process, recrystallization is associated with nucleation of Goss, Q, and P components. When the number of ARB cycle was increased, Goss and Q recrystallization textures were eliminated, but on the other hand, the P, B and B* texture components were strongly developed. The ND-Cube and RT-Goss recrystallization texture is also formed with low intensity at the last stages. Also, the A and A* shear textures which formed in the fifth cycle, shifted towards the Dillamor and Cu textures with increasing the number of ARB cycles. Furthermore, the samples were heated using DSC under Argon atmosphere with four different heating rates. The Kissinger, Ozawa, Boswell, and Starink methods were used to determine the recrystallization kinetics. It can be seen that recrystallization temperature and thereby activation energy (Ea) decreases with increasing the number of ARB cycles. Furthermore, the tensile strengths and elongation of the hybrid nanocomposite increased and decreased by increasing the number of ARB cycle and reached to a maximum value of 204.5Mpa and 6.1% at the end of 9th cycle, respectively. [ABSTRACT FROM AUTHOR]

Abstract Bi2Te3‐based alloys have great market demand in miniaturized thermoelectric (TE) devices for solid‐state refrigeration and power generation. However, their poor mechanical properties increase the fabrication cost and decrease the service durability. Here, this work reports on strengthened mechanical robustness in Bi2Te3‐based alloys due to thermodynamic Gibbs adsorption and kinetic Zener pinning at grain boundaries enabled by MgB2 decomposition. These effects result in much‐refined grain size and twofold enhancement of the compressive strength and Vickers hardness in (Bi0.5Sb1.5Te3)0.97(MgB2)0.03 compared with that of traditional powder‐metallurgy‐derived Bi0.5Sb1.5Te3. High mechanical properties enable excellent cutting machinability in the MgB2‐added samples, showing no missing corners or cracks. Moreover, adding MgB2 facilitates the simultaneous optimization of electron and phonon transport for enhancing the TE figure of merit (ZT). By further optimizing the Bi/Sb ratio, the sample (Bi0.4Sb1.6Te3)0.97(MgB2)0.03 shows a maximum ZT of ≈1.3 at 350 K and an average ZT of 1.1 within 300–473 K. As a consequence, robust TE devices with an energy conversion efficiency of 4.2% at a temperature difference of 215 K are fabricated. This work paves a new way for enhancing the machinability and durability of TE materials, which is especially promising for miniature devices.

If variety is the spice of life, then abnormal grain growth (AGG) may be the materials processing equivalent of sriracha sauce. Abnormally growing grains can be prismatic, dendritic, or practically any shape in between. When they grow at least an order of magnitude larger than their neighbors in the matrix—a state we call extreme AGG—we can examine the abnormal/matrix interface for clues to the underlying mechanism. Simulating AGG for various formulations of the grain boundary (GB) equation of motion, we show that anisotropies in GB mobility and energy leave a characteristic fingerprint in the abnormal/matrix boundary. Except in the case of prismatic growth, the morphological signature of most reported instances of AGG is consistent with a certain degree of GB mobility variability. Open questions remain, however, concerning the mechanism by which the corresponding growth advantage is established and maintained as the GBs of abnormal grains advance through the matrix. [ABSTRACT FROM AUTHOR]

To study the microstructural evolution of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (fPer) between 0.1 and 0.8. Additionally, single‐phase samples of both Ol and Per were deformed under the same experimental conditions to facilitate comparison of the microstructures in two‐phase and single‐phase materials. Each sample was deformed in torsion at T = 1523 K, P = 300 MPa at a constant strain rate up to a final shear strain of γ = 6 to 7. Microstructural developments, analyzed via electron backscatter diffraction (EBSD), indicate differences in both grain size and crystalline texture between single‐ and two‐phase samples. During deformation, grain size approximately doubled in our single‐phase samples of Ol and Per but remained unchanged or decreased in two‐phase samples. Zener‐pinning relationships fit to the mean grain sizes in each phase for samples with 0.1 ≤ fPer ≤ 0.5 and for those with 0.8 ≥ fPer ≥ 0.5 demonstrate that the grain size of the primary phase is controlled by phase‐boundary pinning. Crystallographic preferred orientations, determined for both phases from EBSD data, are significantly weaker in the two‐phase materials than in the single‐phase materials. Plain Language Summary: Many rocks on Earth are made up of multiple mineral types. When these rocks are deformed, such as at the boundaries between tectonic plates, the different mineral types interact with each other affecting the properties of the crystal grains that make up the rock. Importantly, these microstructural effects are different than would be expected for rocks made up of only one mineral. To study the microstructural evolution of rocks that contain multiple minerals, we carried out high‐temperature deformation experiments on rocks that contained two different mineral types. We observed that grains of both minerals were mixed with each other during deformation, resulting in smaller grain sizes than found in rocks made up of only one mineral type and changing the way crystal grains of each mineral type orient themselves during deformation. Key Points: Samples of olivine plus ferropericlase were deformed to high strain to test how the amount of each phase effects microstructural evolutionPhase mixing is accompanied by pinning during deformation, resulting in grain sizes smaller than predicted for single‐phase materialsImportant for understanding microstructural developments during deformation along shear zones on Earth [ABSTRACT FROM AUTHOR]

A mean-field model has been developed to simulate curvature-driven grain growth by exploring the evolution of grain size distribution under arbitrary thermal histories. The model was integrated into precipitation module TC-PRISMA, so that the pinning effect of the concurrently precipitated particles on the growing grains can be considered by a modified, location-specific Zener model. The developed model was validated against analytical calculations, and then applied to real alloy systems, fed with assessed grain boundary energy and mobility data. Its capabilities, limitations, and directions to improvements have been discussed. [ABSTRACT FROM AUTHOR]

In this study, phase-field models were employed to simulate the effects of second-phase particles (SPPs) on grain growth of the AZ31 Mg alloy, under realistic spatial and temporal scales, at 350 °C, during annealing. The particle sizes ranged from 0 to 7 μm, and the particles with large volume fractions were used in the paper. The results reveal that the volume fractions and sizes of the SPP affect grain growth and that the volume fractions and sizes of the SPP on pinning exhibited critical values. When the SPP volume fraction is f = 5%, the SPP is at the maximum critical size, r μ m m a x ; when the SPP size is r = 1   μ m , the SPP minimum critical volume fraction is fmin = 0.25% and the maximum critical volume fraction is fmax = 20%. The critical values increase with the increase of the sizes or volume fractions of the second-phase particles. Finally, the average grain size, particle size, and particle volume fraction obtained from the simulation were fitted according to the Zener relationship, and the obtained results showed that the fitting indices were in the range of 0.33–0.50. The results were compared with the experimental results. The simulation results obtained in this study will provide an important academic reference for understanding the mechanism and law of grain growth, an important reference for accurate control of grain size and properties of the material, a reference for the development of the annealing treatment process of Mg alloy, and a theoretical guide for the use of recrystallization process to control the microstructure of Mg alloy and improve the plastic-forming properties. [ABSTRACT FROM AUTHOR]

In the present research, an Al7150 alloys-based hybrid nanocomposite was fabricated by incorporating boron carbide and graphene nanoparticles through a novel fabrication method of double ultrasonic two-stage stir casting. Initially, the Al-B 4 C nanocomposite was optimized based on better microstructural and mechanical properties, and then a hybrid nanocomposite was prepared by incorporating graphene nanoparticles into the optimized Al-B 4 C nanocomposite. Homogeneous dispersion of graphene and boron carbide nanoparticles in Al7150 matrix was successfully achieved due to the double ultrasonic effect analyzed by Field Emission Scanning Electron Microscopy (FESEM) and High Resolution Transmission Electron Microscopy (HRTEM) analysis. Significant dispersion of nanoreinforcements with enhanced interfacial bonding and dislocation strengthening improved the microstructural and mechanical properties of the Orovan reinforced hybrid nanocomposite. The Vickers microhardness and ultimate tensile strength were improved by 19 % and 47 %, respectively, for the optimized Al-B 4 C nanocomposite compared to base metal. However, the Vickers microhardness and ultimate tensile strength were significantly improved by 36 % and 57 %, respectively, for the optimized hybrid nanocomposite compared to the base material (BM). The hard AlB 2 , Al 3 BC and Al 4 C 3 phases are formed due to the reaction between the matrix and reinforcement during solidification and act as reinforcement within the matrix and resist dislocation movements, resulting in a significant improvement in the mechanical properties of the nanocomposite. Fractography analysis by SEM also confirms the enhanced bonding of graphene nanoparticles as it deforms/slips during fracture and supports the effect of double ultrasonication on the tearing of boron carbide nanoparticles. [Display omitted] • Homogeneous dispersion of graphene and boron carbide nanoparticles in Al7150 matrix. • Dislocation density, Hall-Petch relationship, and Orowan strengthening mechanisms play a dominant role. • Graphene nanoparticles control the grain boundary migration through the Zener pinning effect. • Intermetallic hard phases Al 4 C 3 , Al 3 BC and AlB 2 are significantly enhanced the properties. • Interconnected network of graphene slides during deformation and resist the fracture. [ABSTRACT FROM AUTHOR]

Friction stir processing and post process artificial ageing was successfully carried out on AA7075 with and without reinforcement of SiC particles producing defect free processed zone with uniform distribution of filler material. Effect of SiC particle reinforcement and artificial ageing times on the microstructural modifications was characterized using optical and electron microscopy, electron backscattered diffraction and X-Ray diffraction. Hardness, impact and wear tests were carried out to investigate mechanical behaviour before and after processing. Reinforcement of SiC particles during FSP and subsequent age hardening treatment brought about nearly twofold increase in hardness and impact toughness values by the combined effect of grain refinement, Zener pinning, dispersion strengthening and precipitation hardening. Significant improvement in wear resistance in terms of wear loss was also observed after processing compared to the reference material AA7075-T6. Fractured surface of post FSP age hardened AA7075 alloy exhibited features of ductile fracture during Charpy impact test.

Friction stir processing (FSP), a technique based on the principles of friction stir welding (FSW), is a solid-state method that was developed to enhance the properties of metallic materials. The possibility of the addition of hard second phase particles to matrix structure by distributing, subsequently caused that this method has also been used commonly in surface metal matrix composite (SMMC) manufacturing. By means of the advantages of being a simple, environmentally friendly, and cost-effective solid-state method, FSP has come into prominence more than various liquid and gas-phase methods that are used to manufacture SMMCs. In the FSP method, multiple mechanisms take part together in enhancing the microstructure and mechanical properties of the material during processing. These strengthening mechanisms in matrix structure obtained by hybridizing the specific properties gained to processed material by the FSP method and the enhanced properties by distributing the hard second phase particles in the matrix material. In this study, the effects of hard second phase particles, dynamic recrystallization (DRX), Zener pinning, and Orowan mechanism in the enhancement of mechanical properties of the SMMCs manufactured by FSP were investigated. [ABSTRACT FROM AUTHOR]

In this study, phase-field models were employed to simulate the effects of second-phase particles (SPPs) on grain growth of the AZ31 Mg alloy, under realistic spatial and temporal scales, at 350 °C, during annealing. The particle sizes ranged from 0 to 7 μm, and the particles with large volume fractions were used in the paper. The results reveal that the volume fractions and sizes of the SPP affect grain growth and that the volume fractions and sizes of the SPP on pinning exhibited critical values. When the SPP volume fraction is f = 5%, the SPP is at the maximum critical size, rμmmax; when the SPP size is r=1 μm, the SPP minimum critical volume fraction is fmin = 0.25% and the maximum critical volume fraction is fmax = 20%. The critical values increase with the increase of the sizes or volume fractions of the second-phase particles. Finally, the average grain size, particle size, and particle volume fraction obtained from the simulation were fitted according to the Zener relationship, and the obtained results showed that the fitting indices were in the range of 0.33–0.50. The results were compared with the experimental results. The simulation results obtained in this study will provide an important academic reference for understanding the mechanism and law of grain growth, an important reference for accurate control of grain size and properties of the material, a reference for the development of the annealing treatment process of Mg alloy, and a theoretical guide for the use of recrystallization process to control the microstructure of Mg alloy and improve the plastic-forming properties.

We recently reported that the columnar grain width of Cr2AlC MAX phase coatings increases during oxidation (4 h at 1120°C) by 80%, while for 0.7 at.% Si additions to Cr2AlC, coarsening of only 12% was observed. Here, we use nm scale compositional and microstructural investigations to identify significant differences between Cr2AlC and Cr2Al(Si)C. In particular, needle-shaped precipitates coarsen into globular Cr3Si precipitates upon oxidation in the Si-containing MAX phase. We infer that the presence of these precipitates, which are located predominantly along grain boundaries in the MAX phase, retards coarsening during oxidation by Zener pinning.

Zener pinning between a curved Cu grain boundary (GB) and a differently shaped and oriented Ag particle has been simulated via molecular dynamics. The computed magnitudes of the maximum pinning force agreed with theoretical predictions only when the force was small. As the force increased, discrepancy became obvious. Through careful inspection of the structures of the Cu–Ag interfaces, detailed interaction processes, and variation of the Cu GB during the interaction, the discrepancy is found to correlate with GB faceting, which very likely reduces the maximum pinning force and facilitates boundary passage. GB anisotropy and/or interface characteristics are also found to slightly contribute to the discrepancy. These findings suggest that the assumption of an isotropic GB with constant energy utilized in previous theoretical studies for deriving the maximum pinning force might be inappropriate and that an accurate maximum pinning force could not be predicted without knowing the effects of GB evolution together with detailed properties of both GBs and interfaces.

Nanostructured materials are potential candidates for future structural materials in advanced nuclear reactors. La-doped nanocrystalline 304 austenitic stainless steel (NC304-La) is an advanced nanostructured steel, in which excellent small grain size of ~45 nm is achieved and stabilized by the doped La element. We carried out irradiation using 3.3 MeV Fe ions to 108 dpa at room temperature and 500 °C, and then characterized the microstructural change in NC304-La by transmission electron microscopy, scanning transmission electron microscopy and atom probe tomography. We studied the detailed microstructural evolution and elemental behaviors in irradiated NC304-La, focusing on the effects of La element on the stability of nanograins and nanoparticles in NC304-La.

Al alloys, despite their excellent strength-to-weight ratio, cannot be used at elevated temperatures because of microstructural instability owing to grain growth and precipitate coarsening, thus, leading to a drastic loss in their strength. In this work, we have attempted to address the issue of grain growth by introducing in-situ formed polymer derived ceramics in an Al-Mg alloy. A stable grain structure with minimal loss in hardness when exposed to 450°C and 550°C for 1 hour was obtained due to the particle pinning of the grain boundaries by the Zener mechanism. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Friction stir processing and post process artificial ageing was successfully carried out on AA7075 with and without reinforcement of SiC particles producing defect free processed zone with uniform distribution of filler material. Effect of SiC particle reinforcement and artificial ageing times on the microstructural modifications was characterized using optical and electron microscopy, electron backscattered diffraction and X-Ray diffraction. Hardness, impact and wear tests were carried out to investigate mechanical behaviour before and after processing. Reinforcement of SiC particles during FSP and subsequent age hardening treatment brought about nearly twofold increase in hardness and impact toughness values by the combined effect of grain refinement, Zener pinning, dispersion strengthening and precipitation hardening. Significant improvement in wear resistance in terms of wear loss was also observed after processing compared to the reference material AA7075-T6. Fractured surface of post FSP age hardened AA7075 alloy exhibited features of ductile fracture during Charpy impact test. [ABSTRACT FROM AUTHOR]

How soluble particles affect microstructural evolution during directional recrystallization was studied in a Ni-12Al alloy containing two different sizes of Ni 3 Al particles. Ni-12Al with small particles can form columnar grains, but at a higher annealing temperature compared to its particle-free Ni-3Al counterpart, whilst columnar grains were not formed in Ni-12Al with large γ' particles for the same annealing parameters. Columnar grains only formed after the γ' particles dissolved. γ' particle dissolution is not a trigger for columnar grain formation but is simply a prerequisite. Whether columnar grain can form or not was determined by the texture at the time of particle dissolution. If texture pinning exists after particle dissolution, abnormal grain growth occurs, and columnar grains form; if texture pinning is absent, columnar grains cannot form and normal grain growth occurs. How particles affect the columnar grain orientations, grain boundary character, and grain length/ width during directional recrystallization was also studied. Image, graphical abstract [ABSTRACT FROM AUTHOR]

AlN precipitates are frequently adopted to pin the austenite grain boundaries for the high-temperature carburization of special gear steels. For these steels, the grain coarsening criterion in the carburizing process is required when encountering the composition optimization for the crack-sensitive steels. In this work, the quantitative influence of the Al and N content on the grain size after carburization is studied through pseudocarburizing experiments based on 20Cr steel. According to the grain structure feature and the kinetic theory, the abnormal grain growth is demonstrated as the mode of austenite grain coarsening in carburization. The AlN precipitate, which provides the dominant pinning force, is ripened in this process and the particle size can be estimated by the Lifshitz−Slyosov−Wagner theory. Both the mass fraction and the pinning strength of AlN precipitate show significant influence on the grain growth behavior with the critical values indicating the grain coarsening. These criteria correspond to the conditions of abnormal grain growth when bearing the Zener pinning, which has been analyzed by the multiple phase-field simulation. Accordingly, the models to predict the austenite grain coarsening in carburization were constructed. The prediction is validated by the additional experiments, resulting in accuracies of 92% and 75% for the two models, respectively. Finally, one of the models is applied to optimize the Al and N contents of commercial steel.

We recently reported that the columnar grain width of Cr2AlC MAX phase coatings increases during oxidation (4 h at 1120°C) by 80%, while for 0.7 at.% Si additions to Cr2AlC, coarsening of only 12% was observed. Here, we use nm scale compositional and microstructural investigations to identify significant differences between Cr2AlC and Cr2Al(Si)C. In particular, needle-shaped precipitates coarsen into globular Cr3Si precipitates upon oxidation in the Si-containing MAX phase. We infer that the presence of these precipitates, which are located predominantly along grain boundaries in the MAX phase, retards coarsening during oxidation by Zener pinning. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Microstructural evolution studied for non-conserved and conserved systems. • Continuous transition between interface-controlled and diffusion-limited regimes observed. • Influence of volume fraction, interfacial energy and diffusivity on the coarsening kinetics is analyzed. • Topological events influence the local coarsening behavior. Abstract A thermodynamically consistent phase-field model is exploited to demonstrate the influence of relative volume fractions and bulk diffusivity on the grain growth phenomena in two-phase polycrystalline systems. For very small and high volume fractions, the simulated morphology consists of a dispersion of isolated minor phase grains embedded in the matrix of major phase grains. At intermediate fractions, the obtained microstructure resembles an interpenetrating network-like structure. The performed large-scale 2-D simulations elucidate the governing mechanisms for the concurrent two-phase growth at low and high volume fractions, and the continuous transition between interface-controlled and diffusion-limited regimes. While the slowest kinetics is observed for the 50 / 50 volume fraction case, irrespective of the diffusivity, the fastest kinetics is displayed by the pure systems, with a slight difference, which is due to relative interfacial energies. The relative growth rates of the individual phases and the maximum attainable grain size to mean size ratio are observed to follow the well-known trends for isotropic systems. The obtained results for the concurrent growth of a minor phase with various diffusivities, reveal that it is difficult to reconcile all observed behaviors with a universal Zener relation, in contradiction to the previously made claims in the literature. A need for the full-fledged growth law, which takes into consideration the role of the diffusivity, the volume fractions, and the relative interfacial energies in multiphase polycrystalline systems, is pointed out. [ABSTRACT FROM AUTHOR]

Abstract The pre-mixed pure Ti and Cr 2 O 3 powder was consolidated by spark plasma sintering (SPS) and hot extrusion to fabricate α-titanium (Ti) materials with oxygen (O) and chromium (Cr) elements by powder metallurgy (PM) process. The Cr 2 O 3 particles were completely decomposed during SPS, and then O and Cr atoms were dissolved in α-Ti matrix. Oxygen atoms were remarkably improved the mechanical strength of PM Ti-O-Cr alloys by their solid solution hardening effect. The Cr solution and Ti 4 Cr precipitates had important roles to obstruct the grains coarsening behavior by the solute drag and Zener pinning effects, respectively. The solid solution strengthening effect by Cr atoms, however, was very limited due to a small Cr solubility of about 0.35 at% in α-Ti phase of Ti-O-Cr alloys. Since Ti 4 Cr precipitates with 10–20 µm diameters were not so fine, they hardly contributed to the precipitation hardening of Ti-O-Cr alloys. [ABSTRACT FROM AUTHOR]

Zener pinning refers to the dispersion of fine particles which influences grain size distribution via movement of grain boundaries in a polycrystalline material. Grain size distribution in polycrystals has a significant impact on their properties including physical, chemical, mechanical, and optical to name a few. We explore the use of Phase-field modeling and machine-learning techniques to understand and improve the control of grain size distribution via Zener pinning in polycrystalline materials. We develop a machine learning model that determines the relative importance of various parameters to exercise microstructure control via Zener pinning. Our workflow combines high-throughput phase-field simulations and machine learning to address the computational bottlenecks associated with large-scale simulations as well as identify features necessary for microstructure control in polycrystals. A random forest (RF) regression model was developed to predict grain sizes based on five Phase-field model parameters, achieving an average prediction error of 0.72 nm for the training data and 1.44 nm for the test data. The importance of the input parameters is analyzed using the SHapley Additive exPlanations (SHAP) approach which reveals that diffusivity, volume fraction, and particle diameter are the most important parameters in determining the final grain size. These findings will allow us to select the best second-phase particles, optimize grain size distributions and thus design microstructures with the desired properties. The developed method is a highly versatile and generalizable approach that can be used to assess the combined effects of individual features in the presence of multiple variables. [Display omitted] [ABSTRACT FROM AUTHOR]

Bi 2 Te 3 -based alloys have great market demand in miniaturized thermoelectric (TE) devices for solid-state refrigeration and power generation. However, their poor mechanical properties increase the fabrication cost and decrease the service durability. Here, this work reports on strengthened mechanical robustness in Bi 2 Te 3 -based alloys due to thermodynamic Gibbs adsorption and kinetic Zener pinning at grain boundaries enabled by MgB 2 decomposition. These effects result in much-refined grain size and twofold enhancement of the compressive strength and Vickers hardness in (Bi 0.5 Sb 1.5 Te 3 ) 0.97 (MgB 2 ) 0.03 compared with that of traditional powder-metallurgy-derived Bi 0.5 Sb 1.5 Te 3 . High mechanical properties enable excellent cutting machinability in the MgB 2 -added samples, showing no missing corners or cracks. Moreover, adding MgB 2 facilitates the simultaneous optimization of electron and phonon transport for enhancing the TE figure of merit (ZT). By further optimizing the Bi/Sb ratio, the sample (Bi 0.4 Sb 1.6 Te 3 ) 0.97 (MgB 2 ) 0.03 shows a maximum ZT of ≈1.3 at 350 K and an average ZT of 1.1 within 300-473 K. As a consequence, robust TE devices with an energy conversion efficiency of 4.2% at a temperature difference of 215 K are fabricated. This work paves a new way for enhancing the machinability and durability of TE materials, which is especially promising for miniature devices., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

To investigate the role of grain boundary pinning and the mechanisms by which phase mixing occurs during deformation of polymineralic rocks, we conducted high-strain torsion experiments on samples consisting of olivine plus 30 vol% ferropericlase. Experiments were performed in a gas-medium deformation apparatus at 1524K and 300 MPa. Samples were deformed to outer radius shear strains of up to γ(R)=14.1. The value of the stress exponent and the small grain sizes of our samples indicate that our two-phase material deformed by dislocation-accommodated grain boundary sliding. In samples deformed to 1<γ <7, elongated clusters of ferropericlase grains form thin layers in the olivine matrix, and small grains of ferropericlase appear at olivine grain boundaries and three- and four-grain junctions. By γ ≈14, a well-distributed mixture of small ferropericlase grains among the olivine grains developed. Microstructures exhibit similarities to both mechanical and chemical models proposed to describe the processes leading to phase mixing. Our results provide evidence for grain size reduction during phase mixing that results in a grain size significantly smaller than the value predicted by the single-phase recrystallization piezometer for olivine. Thus, phase mixing provides a mechanism for the persistent weakening of rocks that is important for developing and maintaining shear zones necessary for plate tectonics. [ABSTRACT FROM AUTHOR]

Abstract We have performed an in-depth characterisation of the microstructure evolution of 20Cr-25Ni Nb-stabilised austenitic stainless steel during isothermal annealing at 930 °C using scanning and transmission electron microscopy. This steel grade is used as cladding material in advanced gas-cooled fission reactors, due to its resistance to thermal creep and water corrosion. The initial deformed microstructure undergoes recrystallisation via a strain-induced boundary migration mechanism, attaining a fully recrystallised microstructure after 120 s of annealing. The transition from low-to-high grain boundaries has already occurred after 15 s, together with an increase in the cube grain orientation at the expense of the S texture component. After 120 s, the grain boundary migration induces the formation of new fine Nb(C,N) particles, whereas the pre-existing particles become enriched in Ni and Si. The resulting particle population limits the grain growth in the austenitic matrix, based on the Zener pinning model, resulting in relatively small recrystallised austenite grains and a high density of high-angle and special coincidence-lattice-site grain boundaries, together with a large number of particle/matrix interfaces. Graphical Abstract Unlabelled Image Highlights • At 930 °C recrystallisation of deformed 20/25 Nb steel is completed after 120 s. • Recrystallisation proceeds by strain induced boundary migration. • An increase of cube oriented grains at the expense of S oriented is observed. • The boundary migration induced the precipitation of niobium carbides. • The grain size is limited by the particle dispersion according to Zener pinning. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract A novel phase field model has been developed to study the effect of coherent precipitate on the Zener pinning of matrix grain boundaries. The model accounts for misfit strain between precipitate and matrix as well as the elastic inhomogeneity and anisotropy between them. The results show that increase in elastic misfit, elastic inhomogeneity, and elastic anisotropy increases the coarsening rate of the precipitates. Increased coarsening of precipitates in turn decreases the pinning of grain boundaries. Therefore, increase in misfit strain, elastic inhomogeneity and anisotropy mostly negatively affect the Zener pinning through coherent precipitate. This study shows elastic anisotropy gives rise to the needle shape precipitate. It has also been shown that these needle shaped precipitates are not very effective in Zener pinning. This study provides an understanding into the effect of coherent precipitate on the Zener pinning of matrix grain boundaries. To design a material with smallest possible grain size, coherent precipitate with least lattice misfit and highest elastic modulus will be most effective. [ABSTRACT FROM AUTHOR]

Highlights • The mean grain size in the stir zone increased with a decreasing tool traverse speed. • The boarder region between the stir zone and the base material is composed of shear bands with a distinct microstructure. • The post-weld heat treatment for 1 h at 1380 °C was able to induce abnormal grain growth in the SZs of all the welds. • The shear banded region contained a high density of particles and resisted abnormal grain growth during heat treatment. • Abnormal grain growth appears to initiate at the top of the weld close to a very fine-grained surface layer. Abstract We have characterized three friction stir butt welds of MA956 ODS steel produced using different traverse speeds of the welding tool, by a combination of micro-hardness testing and optical and electron microscopy. The welds were also given a high temperature heat treatment at 1380 °C for one hour to induce abnormal grain growth. The mean grain size at all measured locations increased with a lower welding speed, due to the increased thermal energy into the weld. The grain size changed gradually across the stir zone of the weld, with larger grains present towards the top of the weld and on the advancing side. This was accompanied by lower hardness values at those locations. Shear banding, in the thermo-mechanically affected zone, and a deformed region of the base material, was clearly observed for all welds at the weld border. The post-weld heat treatment was able to induce abnormal grain growth in all the welds, creating a very coarse microstructure with grain sizes in the order of hundreds of microns or millimetres. The coarse-grained structure seemed to develop from the top of the stir zone, close to the surface fine-grained layer, and progressed downwards until it generally covered the entirety of the welds' stir zone. Abnormal grain growth did not occur in the border region of the welds, most likely due to the observed local particle pile-up in that region. [ABSTRACT FROM AUTHOR]

Precipitation is one of the most important influences on microstructural evolution during thermomechanical processing (TMCP) of micro-alloyed steels. Due to precipitation, pinning of prior austenite grain (PAG) boundaries can occur. To understand the mechanisms in detail and in relation to the thermomechanical treatment, a local characterization of the precipitation state depending on the microstructure is essential. Commonly used methods for the characterization, such as transmission electron microscopy (TEM) or matrix dissolution techniques, only have the advantage of local or statistically secured characterization. By using scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques, both advantages could be combined. In addition, in the present work a correlation of the precipitation conditions with the prior austenite grain structure for different austenitization states could be realized by Electron Backscatter Diffraction (EBSD) measurement and reconstruction methods using the reconstruction software Merengue 2. [ABSTRACT FROM AUTHOR]

The interaction between strain-induced precipitation, recovery and recrystallization governs the properties of the particle strengthened Mo-Hf-C alloy MHC. Complex, multi-staged thermomechanical processes have become the key approach for further improvement of the high temperature properties of MHC products within recent years. An investigation of those processes, including a wide field of parameters, usually turns out to be both difficult and time consuming. With multi-pass compression tests it is possible to analyze complex thermomechanical processes on small scale samples in a straightforward manner. The evaluation of the gathered information is demonstrated and the results are compared to microstructural investigations, conventional mechanical tests as well as relevant literature. Furthermore, the influence of different particle size distributions in MHC is interpreted. Precipitation between individual deformation steps resulted in minor additional hardening of the final material. Intermediate recrystallization during the thermomechanical processing showed high softening and ductilization of the final condition. [ABSTRACT FROM AUTHOR]

Nanocrystalline Cu-Ta alloys are emerging as a new class of structural materials preserving the nano-scale grain size up to the melting point of Cu. This extraordinary structural stability is caused by the strong pinning of grain boundaries (GBs) by Ta nano-clusters precipitating from the unstable solid solution after mechanical alloying. Many aspects of the Ta stabilization effect remain elusive and call for further experimental and simulation work. In previous atomistic computer simulations of stress-driven GB migration [ JOM 68 , 1596 (2016)], the GB–cluster interactions in Cu-Ta alloys have been studied for several different compositions and GB velocities. The results have pointed to the Zener pinning as the main mechanism responsible for the grain stabilization. This paper extends the previous work to the motion of individual GBs driven by capillary forces whose magnitude is similar to that in real nanocrystalline materials. Both the impingement of a moving GB on a set of Ta clusters and the GB unpinning from the clusters are studied as a function of temperature and alloy composition. The results demonstrate a quantitative agreement with the Zener pinning model and confirm the “unzip” mechanism of unpinning found in the previous work. In the random Cu-Ta solid solution, short-circuit Ta diffusion along stationary and moving GBs leads to the nucleation and growth of new GB clusters, which eventually stop the GB motion. [ABSTRACT FROM AUTHOR]

Interaction between coherent Ag particles and Cu grain boundaries (GBs) has been investigated by molecular dynamics simulations. Through measuring GB energy evolution as a function of GB position during the interaction, the maximum pinning force was directly determined and found to agree well with theoretical predictions. The relationship between the maximum pinning force and the pinning efficiency for different GBs has been discussed. It is concluded that the two parameters are irrelative to each other, as also observed in experiments. [ABSTRACT FROM AUTHOR]

The pinning effect of different shape second-phase particles on the grain growth in polycrystalline structures is numerical simulated by the phase-field method. Simulation results indicate that the average grain size is highly dependent on the shape and distribution of the second-phase particles, and the shape effect of particles on grain growth restraining is enhanced with increasing numbers of particles. In order to discuss the relation between the constraint grain growth and the second-phase particles, pinning forces induced by different shape particles are theoretically calculated via the Zener pinning theory. The calculated pining forces indicate that the maximum pinning force is highly dependent on the contact mode between grains and particles, and the distance between particles has a significantly influence on the pinning forces. Therefore, controlling the shape and distributions of second-phase particles in polycrystalline metals or ceramics might be an efficient way to achieve materials with specified microstructures. [ABSTRACT FROM AUTHOR]