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Bi 1.6 Pb 0.4 Sr 2 Co 2 O 8 samples have been textured by the Laser Floating Zone (LFZ) process using Nd:YAG, and CO 2 laser radiation. Using different wavelengths resulted in significant structural and microstructural modifications. Powder XRD patterns showed that the thermoelectric phase is the major one in both cases. Microstructural studies revealed that all samples presented the same phases but with much lower content of secondary ones in those processed with the CO 2 laser. Electrical resistivity showed different behavior for the two types of samples, being in general, lower for the CO 2 grown rods. Seebeck coefficient is lower for the CO 2 grown samples up to 300 °C, and higher in the high-temperature range, reaching 240 μV/K at 650 °C, which is one of the highest values obtained so far in these compounds. Moreover, thermal conductivity at 600 °C for these samples (0.93 W/K m) is among the lowest reported in the literature. As a consequence, ZT values at 600 °C reached 0.42 in CO 2 textured materials, about two times higher than the obtained in Nd:YAG ones. This value is among the highest reported so far in the literature, and is comparable to the performance attained for the same composition containing nanoparticles addition. All these properties, combined with the fact that the processed materials can be directly integrated into thermoelectric modules, render them highly attractive for industrial production. [ABSTRACT FROM AUTHOR]

LiNi x Co y Mn 1-x-y O 2 (NCM) has been regarded as promising cathode materials for high energy density lithium ion battery. Apart from the capacity deterioration during cycling process, NCM particle microcracks' generation resulting from anisotropic strain in crystal lattice is also regarded as the crucial issue from mechanical scale. As the original materials of synthesizing NCM, NCMOH precursor decides much of the prepared NCM materials in both electrochemical and mechanical scale while mechanical degrading mechanism inside crystal lattice is missing in related literature. In this study, the LiNi 0.7 Co 0.1 Mn 0.2 O 2 single crystalline particles are calcined from NCMOH precursors with difference in mechanical stacking morphology and physical property (specific surface area and tapping density) through the same appropriate sintering condition. The evolution of lattice microstrain calculation and elastic modulus for NCM materials through structure and mechanical evaluation has been studied. It is indicated that larger size and higher rate capability (fast charging) of single crystalline LNCMO materials can be obtained from precursor with smaller specific surface area and higher tapping density while the larger microstrain and less elastic modulus degradation are determined during electrochemical charging process. Stronger structure maintaining ability of NCM lattice under large electric field force (during fast charging process) is beneficial to the capacity performance related with higher-rate capability. This might provide a novel insight of understanding the mechanical effect of NCMOH precursors on the synthesis and electrochemical performance of Ni-rich LiNi x Co y Mn 1-x-y materials. [ABSTRACT FROM AUTHOR]

During thermal deformation, grain coarsening due to grain growth and grain refinement resulting from dynamic recrystallization (DRX) collectively influence the deformed grain size. To investigate the separative and comprehensive effects of the two mechanisms in the Ni-38Cr-3.8Al alloy, grain growth experiments and isothermal compression tests were conducted. Kinetics models for grain growth and DRX behaviors were established based on the experimental data, which were integrated with finite element (FE) techniques to simulate the evolution of grain size throughout the entire thermal compression process. The effects of grain coarsening and grain refinement during this process were separated and quantified based on the simulation data. The results revealed that grain coarsening predominated during the heating and holding stages, with a longer holding time and higher holding temperatures intensifying this effect. However, during the compression stage, grain coarsening and grain refinement co-existed, and their competition was influenced by deformation parameters. Specifically, grain refinement dominated at strain rates exceeding 0.1 s−1, while grain coarsening dominated at lower strain rates (<0.1 s−1) and higher deformation temperatures (>1373 K). The simulated grain sizes closely matched the experimental observations. [ABSTRACT FROM AUTHOR]

Annealed ASTM A510/A853 hypoeutectoid steel wire is a very useful and versatile material that finds its applications in the construction industry; this is mainly due to its mechanical properties, since this product can reach high ductility. In order to achieve the sought quality and homogeneity in this material, it is necessary to have a suitable control during operation. Important operational control parameters in batch annealing are: heating and cooling rates, holding temperature and cycle time. These parameters have a direct effect on the process, and it is essential to elucidate the way they impact in the final product. This work concerns the development of a mathematical model targeted at the prediction of mechanical properties in terms of the operational parameters, in the outer spirals of American Iron and Steel Institute/Society of Automotive Engineers (AISI/SAE) steel drawn wire coils after they have been subjected to batch annealing. The model addresses non-isothermically the involved phenomena that take place during the annealing process: recovery, recrystallization, and grain growth below and above Ac1. It predicts grain size, tensile strength, and yield strength values with very good accuracy. [ABSTRACT FROM AUTHOR]

The grain growth behavior during an AC flash event was examined in 8 mol% yttria‐stabilized cubic zirconia (8Y‐CSZ) polycrystals. The effects of current/power densities on the grain growth behavior were investigated in 8Y‐CSZ samples with different specific surface areas at a constant sample temperature and applied field strength. The grain growth rate of flash‐treated 8Y‐CSZ was 300 times faster than that of heat‐treated 8Y‐CSZ at the same sample temperature in the absence of an electric current/field, suggesting that the promoted grain growth cannot be ascribed only to a thermal effect but also to an athermal effect occurring during the AC flash event. Moreover, the grain growth during the flash treatment strongly depends on the applied current/power densities and grain size; in particular, the grain growth showed enhancements with increasing applied current/power densities and for relatively small grain sizes. This result suggests that the grain boundary diffusivity of cations, which are regarded as the rate‐controlling species for grain growth, could be accelerated by tuning the current/power densities during the flash event. The grain growth mechanism was characterized using a grain growth exponent (n) value of 4.8 for the flash treatment at high current/power densities and using a conventional value of n = 3 under normal heat treatment conditions. The dependence of the grain growth behavior on the AC current/power density suggests that because the cation diffusivity is accelerated due to the formation of numerous point defects during the AC flash event, the grain growth mechanism might depend on the current/power densities and differ from that of conventional grain growth. [ABSTRACT FROM AUTHOR]

Sintering phenomena, densification, and grain growth are crucial for proper control of the microstructure for good mechanical strength. Here, materials and processing parameters, the addition of La2O3, and two‐step sintering (TSS) were combined to lead to higher strength in the porous alumina prepared by freeze‐casting. Based on grain growth and densification behaviors with the La2O3 doping, a beneficial thermal profile was designed for the TSS. As a result, higher relative density levels and smaller grain size were obtained compared with the results with conventional sintering (CS): 38.56% and 0.82 µm at 1500°C by CS; 40.78% and 1.78 µm at 1600°C by CS; 41.43% and 0.87 µm by TSS. The microstructural benefits provided ∼1.4 times higher compressive strength (5.46 MPa) from TSS than from the CS sample (3.92 MPa), highlighting the synergetic effect of La2O3 doping and the TSS strategy. [ABSTRACT FROM AUTHOR]

In this work, the influence of Mo in the CoNiFeMn system during heat treatment after prior hot and cold rolling was investigated. At present, relatively few studies on static recrystallization and grain growth kinetics in high entropy alloys are available. This paper focuses on static recrystallization and grain growth kinetics as well as the influence of molybdenum on these phenomena. This work compares two alloys, CoNiFeMn and (CoNiFeMn)95Mo5, in relation to the formations of the brittle µ phase at room- and high-temperature plastic deformation regimes due to its negative affect on material ductility. Microstructures were characterized by energy dispersive X-ray spectroscopy analysis and by scanning electron microscopy, whereas the mechanical properties were assessed by tensile testing. The effect of recrystallization and grain growth behaviours on the microstructural evolution and the final mechanical properties was assessed. It was found that Mo addition into the CoNiFeMn system has a strong effect on both the static recrystallization and grain growth kinetics as well as the final mechanical properties. [ABSTRACT FROM AUTHOR]

This study investigates the development of freestanding thick films (FSFs) of lead-free (Ba,Ca)(Zr,Ti)O3 and the role of grain growth on the electromechanical response. During deposition, room temperature powder aerosol deposition rapidly produces thick films with a nano-grain structure that limits the electromechanical properties. In this study, the films are removed from the substrate using a sacrificial buffering layer to avoid thermal treatment and allow for an initial as-processed state. Following this, FSFs were thermally treated at various annealing temperatures from 800 °C to 1400 °C to induce grain growth, which was characterized with scanning and transmission electron microscopy. X-ray diffraction revealed an increase in the crystallite size consistent with an increase in grain size and a decrease in internal residual stress. The temperature-dependent dielectric behavior and the large-field ferroelectric response were also characterized, revealing significant differences of the FSFs from the bulk properties. [ABSTRACT FROM AUTHOR]

It has been a common method to improve the mechanical properties of metals by manipulating their microstructures via static recrystallization, i.e., through heat treatment. Therefore, the knowledge of recrystallization and grain growth is critical to the success of the technique. In the present work, by using in-situ high temperature EBSD, the mechanisms that control recrystallization and grain growth of an extruded pure Mg were studied. The experimental results revealed that the grains of priority for dynamic recrystallization exhibit fading competitiveness under static recrystallization. It is also found that grain boundary movement or grain growth is likely to show an inverse energy gradient effect, i.e., low energy grains tend to swallow or grow into high energy grains, and grain boundaries of close to 30° exhibit superior growth advantage to others. Another finding is that {10–12} tensile twin boundaries are sites of hardly observed for recrystallization, and are finally swallowed by adjacent recrystallized grains. The above findings may give comprehensive insights of static recrystallization and grain growth of Mg, and may guide the design of advanced materials processing in microstructural engineering. [ABSTRACT FROM AUTHOR]

Kinetics of antimony production via carbothermal reduction of Sb2O3–carbon powder–NaCl mixture using microwave and conventional heating was investigated to identify the dominant controlling mechanism. Results of conventional heating revealed the temperature range of conventional carbothermal reduction reaction is 500°C to 800°C, with the average activation energy of each stage being 81.97 kJ/mol (α = 0.1–0.5), 65.17 kJ/mol (α = 0.5–0.75), and 69.86 kJ/mol (α = 0.75–1.0), respectively. In the microwave field, the carbothermal reduction reaction of raw materials can be completed at 600°C to obtain antimony, and the weight loss data of the carbothermal reduction process were recorded for the first time. The above results show that the microwave field enhanced the interfacial chemical effect, accelerated the interfacial diffusion from the metal phase to the oxide phase, and reduced the activation energy of the carbon thermal reduction process to 6.85 kJ/mol. The growth index of antimony grain growth process is estimated to be 4.33, controlled by the surface diffusion. These data provide a reliable theoretical basis for studying the reduction reactions of minerals in microwave fields. [ABSTRACT FROM AUTHOR]

Ferrite materials are urgently demanded in high-frequency miniaturized power electronics for their high electromagnetic properties and high electrical resistivity. However, traditional NiZn ferrites are faced with low resistivity and high loss at low-temperature sintering, which severely restrict their applications. In this study, CdO was substituted into Ni 0.22 Cu 0.2 Zn 0.58 Fe 2 O 4 ferrites at a presintered stage to refine the grain size and to achieve superior performance at low-temperature sintering. Interestingly, the electrical, magnetic and gyromagnetic properties of the NiCuZn ferrites exhibited a strong dependence on the concentration of Cd2+ ions. The Cd2+ ions entered into the lattice and substituted for Fe3+ at the octahedral site. SEM image results showed that CdO-substituted NiCuZn ferrites for the first sintering stage possess inhibition of abnormal grain growth, which can effectively solve the problem of shrinkage and grain compaction of ferrite materials in LTCC technology. Furthermore, to promote the grain growth at low sintering temperature, the optimized Bi 2 O 3 additives were added at final sintering stage. The resistivity of the optimized Cd2+ ions is significantly enhanced from 2.41 × 108 Ω cm to 8.51 × 108 Ω cm. Finally, the NiCuZnCd (x = 0.15)-1.2 wt % Bi 2 O 3 ferrites with the highest densification possess ρ = 8.51 × 108 Ω cm, ε′ = 18, tan δ ε = 6.0 × 10−3, μ′ = 81, f r = 117 MHz, tan δ μ = 4.8 × 10−2 and ΔH = 281 Oe. This work provides a new doping method for the design of the high frequency LTCC device. [ABSTRACT FROM AUTHOR]

There is a growing interest in field‐assisted fast ceramic processing and long‐term service of electrochemical devices under harsh operation conditions. These extreme redox conditions can make some conventionally thought slow ions highly mobile in electrochemical materials and devices and greatly accelerate the microstructural evolution. Experimental observations for enhanced slow ion diffusion that controls mass transport include accelerated grain growth in zirconia and ceria under atmospheric and electrochemical reduction above 1000°C and lattice cavitation of high‐voltage Li‐rich layered cathodes of lithium‐ion batteries at room temperature. These observations cannot be explained by altered defect concentrations under defect chemistry arguments. Instead, they support hugely enhanced slow ion mobility. My colleagues and I recently proposed a new mechanism resulted from strong electron–phonon interactions and dynamic electronic relaxations at the saddle point of an ion hopping event, which significantly lowers the ion migration barrier. In this perspective article, the theory and its experimental and computational supports shall be discussed. Such an electronic effect holds even in nominally ionic ceramic materials with wide bandgaps and is expected to be general in many transition metal oxides. It rationalizes a variety of intriguing experimental observations and offers an electronic perspective on the redox enhancement of slow ion kinetics in structural, functional, and energy ceramics. [ABSTRACT FROM AUTHOR]

The role of anisotropic grain boundary energy in grain growth is investigated using textured microstructures that contain a high proportion of special grain boundaries. Textured and untextured Ca‐doped alumina was prepared by slip casting inside and outside a high magnetic field, respectively. At 1600°C, the textured microstructure exhibits faster growth than the untextured microstructure and its population of low‐angle boundaries increases. Atomic force microscopy (AFM) is employed to measure the geometry of thermal grooves to assess the relative grain boundary energy of these systems before and after growth. In the textured microstructure, the grain boundary energy distribution narrows and shifts to a lower average energy. Conversely, the energy distribution broadens for the untextured microstructure as it grows and exhibits abnormal grain growth. Further analysis of the boundary networks neighboring abnormal grains reveals an energy incentive that facilitates their growth. These results suggest that coarsening is not the only dominant grain growth mechanism and that the system can lower its energy effectively by replacing high energy boundaries with those of low energy. The faster growth of lower energy boundaries suggests that isotropic simulations do not adequately account for anisotropic grain growth mechanisms or anisotropic mobility. [ABSTRACT FROM AUTHOR]

Copyright of BHM Berg- und Hüttenmännische Monatshefte is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Mechanical stability of freeze-casted porous alumina can be achieved by the stimulation of densification with control of the grain growth during sintering. To tailor the porous microstructure beneficially, in this study TiO2 was added and its segregation effect on the densification and its grain growth behaviors were investigated. As a result, abnormal grain growth was observed with faceted interfaces and explained by grain boundary structure dependent grain growth theory. In terms of densification, a higher bulk density was obtained with the addition of 1.5 wt% TiO2 (41.76 %TD) compared to that without doping (36.02 %TD). The benefit was evident at 1400 °C, whereas the bulk density decreased even with an addition of TiO2 above 1500 °C. This temperature dependence could be understood by a change in the dominant densification mechanism at 1500 °C, solving the puzzle of the enhanced densification only at low temperature.

It has been a common method to improve the mechanical properties of metals by manipulating their microstructures via static recrystallization, i.e., through heat treatment. Therefore, the knowledge of recrystallization and grain growth is critical to the success of the technique. In the present work, by using in-situ high temperature EBSD, the mechanisms that control recrystallization and grain growth of an extruded pure Mg were studied. The experimental results revealed that the grains of priority for dynamic recrystallization exhibit fading competitiveness under static recrystallization. It is also found that grain boundary movement or grain growth is likely to show an inverse energy gradient effect, i.e., low energy grains tend to swallow or grow into high energy grains, and grain boundaries of close to 30° exhibit superior growth advantage to others. Another finding is that {10–12} tensile twin boundaries are sites of hardly observed for recrystallization, and are finally swallowed by adjacent recrystallized grains. The above findings may give comprehensive insights of static recrystallization and grain growth of Mg, and may guide the design of advanced materials processing in microstructural engineering.

Abstract Ceramic composites based on Ce-TZP/Al2O3/H6A were sintered in order to promote grain growth and to study the effects of ZrO2 grain size on the properties of this material. A mixture of ZrO2-CeO2-Al2O3 powders was sintered at 1450 °C-2 h, following the manufacturer’s recommendations. Then, the samples were further treated at 1500 or 1600 °C, for 0, 2, 8, or 24 h. The sintered specimens were characterized by X-ray diffraction, relative density, and grain size distribution. Vickers nanohardness, Young’s modulus (E), and fracture toughness were measured. The materials showed complete densification for all sintering conditions studied. t-ZrO2, α-Al2O3, and cerium hexaluminate (H6A) were observed. The alumina and hexaluminate grains had average grain sizes of 0.7 and 4.5 μm (AR>3), respectively, without significant variations during the additional heat treatments. However, significant growth took place for the ZrO2 grains with increasing temperature and holding time, increasing the average grain size from 0.6~1.4 μm, when sintered at 1500 °C-2 h to 1600 °C-24 h, respectively. The materials exhibited Vickers nanohardness of 1800 HV and E=241±15 GPa. On the other hand, the considerable grain growth of the ZrO2 grains as a function of holding time reflected in a reduction of the fracture toughness, which decreased from 8.5 to 5.7 MPa.m1/2 for samples sintered at 1500 °C-2 h to 1600 °C-24 h, respectively.

The challenge of preparing dense ultrafine-grained high-entropy ceramics through conventional pressureless sintering is hereby addressed by a simple two-step sintering method. Ultrafine-grained (Ce 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2) 2 Zr 2 O 7 high-entropy zirconate with 99.2 % theoretical density and 135 nm grain size has been fabricated by pressureless two-step sintering for the first time. Compared to the conventional sintering, two-step sintering provides finer grain sizes and better microstructural uniformity, which yield excellent comprehensive mechanical properties with high hardness of 16.0 GPa, large elastic modulus of 262 GPa and high fracture toughness of 2.8 MPa·m1/2. It is believe that pressureless two-step sintering can be utilized to prepare other high-entropy ceramics with fine grain size and high quality. [ABSTRACT FROM AUTHOR]

Core-rim structure has distinct advantage to improve the performances of KNN-based piezoceramics. Whereas the state of the core-rim structure is difficult to control during sintering. Here, the core-rim structured 0.96(K 0.51 Na 0.47 Li 0.02)(Nb 0.8 Ta 0.2)O 3 -0.04CaZrO 3 (KNLNT) ceramics were both obtained by conventional sintering (CS) and rapid sintering (RS). KNLNT ceramics prepared by rapid sintering exhibit more outstanding controllability on grain growth and core-rim structure and can hold the core/rim size ratio in a stable and favorable level due to extended effective range of grain boundary diffusion in densification. Benefiting from the controllable microstructure, the RS method prepared samples show excellent performance. The unipolar strain value of RS-1240 (S max =0.252%) is 2.07 times as much as CS-1110 (S max =0.122%). Large strain, low hysteresis and low dielectric permittivity features make the core-rim structured KNLNT ceramics a potential material for pulse drive applications and demonstrate that the manual precise control of core-rim structure could create many possibilities on materials design. [Display omitted] [ABSTRACT FROM AUTHOR]

The ability to control the particle size of titanium dioxide nanoparticles (TNPs) is crucial for their practical use. This study presents a method for preparing monodisperse amorphous and anatase TNPs with high surface areas and tunable grain sizes. The method combines a sol-gel process with a hydrothermal process, using benzoic acid (BA) as a structure-directing agent. By adjusting the hydrolysis and condensation rates of the titanium alkoxide, we can synthesize monodisperse amorphous TNPs with a controllable size range of 200–800 nm. Additionally, the amorphous TNPs synthesized through this method serve as raw material, enabling the morphology of anatase TNPs to be inherited from amorphous TNPs with a hydrothermal reaction time of 1 h. When the hydrothermal reaction time is extended to 10 h, the closely linked primary titanium dioxide is fully separated and replaced by ultra-fine anatase TNPs. This optimal scheme show promise for self-assembly and large-scale synthesis of monodisperse amorphous and anatase TNPs. [ABSTRACT FROM AUTHOR]

Two strategies have been extensively employed to develop advanced accident tolerant fuels (ATF): improving thermal conductivity and producing large grain sized pellets. However, there are few reports on the simultaneous utilization of both strategies. In this work, we fabricated Mo–Cr alloy reinforced UO 2 (UMC) composite pellet with both high thermal conductivity and large grain size by a simple in-situ alloying method for the first time. The average grain size of UO 2 increased from 6 μm to 113 μm. Thermal conductivity of the UMC pellet (with 2 vol% Mo) at 1200 °C increased by 36.46% compared with pure UO 2. The increase rate of thermal conductivity per 1 vol% dopant reached about 18%. The Mo–Cr alloy formed continuous micro-cell layer around UO 2 particles. Such continuous micro-cell Mo–Cr alloy layer and increase of UO 2 grain size both helped improve the thermal conductivity of UMC composite pellets. Thermal expansion coefficient of the UMC under the operating temperature decreased by 11.90% compared with pure UO 2. The oxidation resistance of the UMC pellet under high temperature steam was also improved. This work provides a new strategy towards fabricating ATF with both high thermal conductivity and large grain size. [ABSTRACT FROM AUTHOR]

The superplastic behavior and associated deformation mechanisms of a fine-grained Mg-10.1 Li-0.8Al-0.6Zn alloy (LAZ1011) with a grain size of 3.2 µm, primarily composed of the BCC β phase and a small amount of the HCP α phase, were examined in a temperature range of 473 K to 623 K. The microstructural refinement of this alloy was achieved by employing high-ratio differential speed rolling. The best superplasticity was achieved at 523 K and at strain rates of 10−4 -5 × 10−4 s−1, where tensile elongations of 550–600% were obtained. During the heating and holding stage of the tensile samples prior to tensile loading, a significant increase in grain size was observed at temperatures above 573 K. Therefore, it was important to consider this effect when analyzing and understanding the superplastic deformation behavior and mechanisms. In the investigated strain rate range, the superplastic flow at low strain rates was governed by lattice diffusion-controlled grain boundary sliding, while at high strain rates, lattice diffusion-controlled dislocation climb creep was the rate-controlling deformation mechanism. It was concluded that solute drag creep is unlikely to occur. During the late stages of deformation at 523 K, it was observed that grain boundary sliding led to the agglomeration of the α phase, resulting in significant strain hardening. Deformation mechanism maps were constructed for β-Mg-Li alloys in the form of 2D and 3D formats as a function of strain rate, stress, temperature, and grain size, using the constitutive equations for various deformation mechanisms derived based on the data of the current tests.

A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy is processed by solution treatment and high pressure torsion (HPT) at room temperature to produce a nanostructured light material with high hardness. The stability of this alloy is subsequently tested through isochronal annealing for 0.5 h at 373 K to 673 K. The results reveal a thermal stability that is vastly superior to that of conventional Mg-based alloys processed by severe plastic deformation: the grain size remains at around 50 nm on heating to 573 K, and as the temperature is increased to 673 K, grain growth is restricted to within 500 nm. The stability of grain refinement of the present alloy/processing combination allowing grain size to be limited to 55 nm after exposure at 573 K, appears to be nearly one order of magnitude better than for the other SPD processed Mg-RE type alloys, and 2 orders of magnitude better than those of SPD processed RE-free Mg alloys. This superior thermal stability is attributed to formation of co-clusters near and segregation at grain boundaries, which cause a thermodynamic stabilization of grain size, as well as formation of β-Mg5RE equilibrium phase at grain boundaries, which impede grain growth by the Zener pinning effect. The hardness of the nanostructured Mg-Gd-Y-Zn-Zr alloy increases with increasing annealing temperature up to 573 K, which is quite different from the other SPD-processed Mg-based alloys. The high hardness of 136 HV after annealing at 573 K is mainly due to solute segregation and solute clustering at or near grain boundaries.

This study investigated the effects of KCl treatment on microstructure and thermoelectric properties of n-type Bi2Te2.7Se0.3 (BTS) thermoelectric materials. The innovative KCl treatment was originally intended to introduce nanopores through selective dissolution of KCl from a mixture of thermoelectric materials and KCl. However, it unexpectedly induced substantial variations in material composition and microstructure during the subsequent spark plasma sintering (SPS) process. Hydroxyl groups adsorbed on the powder surface during the dissolution resulted in the emergence of a Bi2TeO5 secondary phase within the BTS matrix after the SPS process at 450 ℃. The concentration of Bi2TeO5 increased with an increase in the KCl content. Furthermore, a remarkable grain growth occurred at low KCl concentrations, likely due to the liquid-phase formation in a Te-rich composition during SPS. However, excessive Bi2TeO5 at higher KCl concentrations hindered grain growth. These variations in the microstructure had complex effects on electrical properties: The TeBi antisite defects increased the electron concentration, and Bi2TeO5 reduced electron mobility. Additionally, the lattice thermal conductivity decreased due to the presence of Bi2TeO5, from 0.8 W∙m−1∙K−1 at 298 K for the pristine BTS to 0.6 W∙m−1∙K−1 at 298 K for BTS treated with 1 wt% KCl. These insights allowed precise adjustments of the electrical and thermal conductivities, leading to an enhancement in the maximum value of figure-of-merit (ZT) from 0.76 to 0.96 through the selective dissolution of KCl approach. We believe that our observations potentially enable advances in thermoelectric materials by engineering microstructures.

Preparation of high-entropy ceramics (HECs) requires long duration at high temperatures, which limits their wide application. In this study, high-entropy (La 0.2 Ce 0.2 Nd 0.2 Sm 0.2 Eu 0.2) 2 Zr 2 O 7 ceramics was prepared at low temperatures of 762–885 °C in very short time of 60 s using the flash sintering. The effects of electric field strength and current limit on the densification and grain growth, as well as the phase transformation were systematically investigated. Both density and grain size were increased with the increase of the electric field strength and current limit. The phase transformation from defective fluorite to pyrochlore occurred at large current limit. The flash-sintered bulks exhibited good mechanical properties. This study demonstrates that flash sintering is an effective method for rapid preparation of HECs at low temperatures. Moreover, the microstructure of HECs can be controlled by adjusting the electric field strength and current limit. [ABSTRACT FROM AUTHOR]

Shrinkage during the sintering of powder compacts depends on numerous parameters, including green body characteristics such as particle size and green density. These parameters are also decisive for the initial microstructure and its evolution during sintering. In this study, a novel experimental setup is used to quantify the time-dependent microstructural evolution in water-atomised Astaloy 85Mo powder. Green bodies with different particle sizes and density levels were polished on the top surface and then subjected to an interrupted sintering procedure in a quenching dilatometer. Intermediate examinations of the microstructure by scanning electron microscopy revealed the pore morphology and the thermally etched austenite grain size. It was found that pore rounding relies solely on the local curvature only, whereas neck growth is in good agreement with analytical models. An increase in diffusivity was found on the macroscale and on the microscale due to the pre-deformation of the particles. [ABSTRACT FROM AUTHOR]

β‐silicon nitride (β‐Si3N4) ceramics with an additive oxide system of 1 wt% MgO and 3 wt% Re2O3 (Re = Gd, La, Y, and Yb) were fabricated by liquid phase sintering at 1950°C under nitrogen pressure of 748 kPa. Starting α‐Si3N4 powder with 10 vol% rod‐like β‐Si3N4 seed crystallites and an extended sintering time from 5 to 40 h resulted in the formation of bimodal microstructure composed of fine matrix grains and large grains. The 40 h‐sintered specimens of pseudo ternary β‐Si3N4‐MgO‐Gd2O3 system exhibited enhanced thermal conductivity of 127.2 ± 2.5 W m−1 K−1 associated with a degradation of the fracture strength from 1008.0 ± 38.0 to 491.7 ± 32.0 MPa, which was due to the formation of coarse‐grained aggregates that acted as both fracture origin as well as a thermal conductive pathway. The theoretical thermal conductivity was predicted for the sintered specimens by using equations based on a mean‐field micromechanics model to estimate the effective thermal conductivity of each component of binary composites. The calculation results suggested that the thermal conductivity of the large β‐Si3N4 grains (≥ 2 μm in diameter) was relatively high and estimated to be in the range of 175 to 191 Wm−1 K−1. The improved thermal conductivity of the 40 h‐sintered specimens was further discussed for the series of β‐Si3N4‐MgO‐Re2O3 systems based on the nanostructure characterization results obtained by the high‐resolution transmission electron microscopy and scanning transmission electron microscopy‐energy dispersive X‐ray spectrometry analyses. [ABSTRACT FROM AUTHOR]

The equiatomic high entropy alloy (HEA) NiCoCrFePd crystalizes as a single face-centered cubic (FCC) phase with strong local lattice distortion due to large atomic size mismatch between Pd element and other constitute elements. To better understand this quinary alloy, a family of single FCC phase equiatomic alloys made from the constituent elements of the NiCoCrFePd HEA, including the binary NiPd alloy, medium entropy alloys (MEAs) of NiCoPd, NiCrPd, and NiFePd, and the quinary NiCoCrFePd HEA with fully-recrystallized microstructure was experimentally investigated to understand the chemical effects on grain growth kinetics and solid solution hardening. With the principal elements increasing from two to five, the grain growth was increasingly inhibited in the annealing temperature range of 800–900 °C, while at 1000 °C and above, the NiCrPd MEA showed the slowest grain growth, which may attribute to the higher melting temperature of Cr and negative mixing enthalpy between Cr and other constituent elements, increasing the activation energy of grain growth. Moreover, the hardness depending on the grain size complied with the Hall-Petch relationship, in which NiCoPd exhibited the lowest hardness, while NiPd had a comparable hardness with NiCrPd and NiFePd. The above results suggested that the number of alloying elements was not the sole factor determining the sluggish diffusion and hardness. Instead, the type of constituent elements in the Pd-containing multicomponent alloys played more critical role. Furthermore, it was concluded that the strength of MEAs and HEA should depend on the combination of atomic size and modulus mismatch and electronegativity difference. [ABSTRACT FROM AUTHOR]

Copyright of Journal of the Chinese Society of Rare Earths is the property of Editorial Department of Journal of the Chinese Society of Rare Earths and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Grain rotation is commonly observed during the evolution of microstructures in polycrystalline materials of different kinds, including metals, ceramics, and colloidal crystals. It is widely accepted that interface migration in these systems is mediated by the motion of line defects with step and dislocation character, i.e., disconnections. We propose a crystallography-respecting continuum model for arbitrarily curved grain boundaries or heterophase interfaces, accounting for the disconnections' role in grain rotation. Numerical simulations demonstrate that changes in grain orientations, as well as interface morphology and internal stress field, are associated with disconnection flow. Our predictions agree with molecular dynamics simulation results for pure capillarity-driven evolution of grain boundaries and are interpreted through an extended Cahn-Taylor model. [ABSTRACT FROM AUTHOR]

The abnormal grain growth of steel, which is occurs during carburization, adversely affects properties such as heat treatment deformation and fatigue strength. This study aimed to control abnormal grain growth by controlling the materials and processes. Thus, it was necessary to investigate the effects of microstructure, precipitation, and heat treatment conditions on abnormal grain growth. We simulated abnormal grain growth using the cellular automaton (CA) method. The simulations focused on the grain boundary anisotropy and dispersion of precipitates. We considered the effect of grain boundary misorientation on boundary energy and mobility. The dispersion state of the precipitates and its pinning effect were considered, and grain growth simulations were performed. The results showed that the CA simulation reproduced abnormal grain growth by emphasizing the grain boundary mobility and the influence of the dispersion state of the precipitate on the occurrence of abnormal grain growth. The study findings show that the CA method is a potential technique for the prediction of abnormal grain growth. [ABSTRACT FROM AUTHOR]

This study utilized high rotational speeds (5500, 8500, and 10,000 rpm) and various traverse speeds (300, 600, and 900 mm/min) to carry out micro-friction stir welding on 0.8 mm thick plates of 5052–0 and 5052-H32 aluminum alloys. The weld heterogeneity was assessed through microstructural analysis and tensile testing under various process parameters. It was found that high rotational speeds generate enough heat to provide grain growth within the stirring zone. Moreover, the stirring zone showed a sizeable population of equiaxed grains, measuring less than 5 μm, that resulted in an improved mechanical strength of the joint. The joint efficiencies in the 5052-O and 5052-H32 alloys were significantly improved. The joint efficiencies of yield strength for 5052-O and 5052-H32 ranged between 129 and 139%, and 118% and 124%, respectively. The joint efficiencies of tensile strength for 5052-O varied between 102 to 109%, while in 5052-H32 they ranged from 93 to 100%. A comparison of the micro friction stir welding parameter to conventional thick plate friction stir welding showed that the former provided superior properties. [ABSTRACT FROM AUTHOR]

Ultrafast high‐temperature sintering (UHS) is a joule heating method with an extremely high heating rate (∼103–104°C/min). In this work, commercial ZrB2 powders were rapidly densified by UHS to >90% relative density within 60 s in vacuum without pressure. Bulk density improved from 75% to 93% of relative density by increasing the sintering duration from 10 to 60 s. The final grain size increases from 2.7 ± 1.1 to 19.0 ± 8.4 μm when the sintering time increased from 10 to 60 s. X‐ray diffraction and energy‐dispersive spectroscopy show crystalline phase and compositional uniformity in ZrB2 after UHS. [ABSTRACT FROM AUTHOR]

Solid-state sintering of Y 2 O 3 is difficult and requires high-temperature heat treatment or the use of a sintering additive. In this study, liquid-phase sintering was utilized to produce dense Y 2 O 3 bodies with fine-grained microstructure. Al 2 O 3 and SiO 2 were added to a commercial nano-Y 2 O 3 powder using aluminum nitrate and tetraethylorthosilicate (TEOS) as the precursors; the samples were uniaxially pressed at 100 MPa and sintered at 1500 °C and 1650 °C. The addition of the sintering aids increased the relative density of the samples sintered at lower temperatures; also, the sintering aids suppressed grain growth. The dissolution of Y 2 O 3 into the liquid phase changes the solid-liquid interfacial energy resulting in the formation of isolated yttrium-silicate/yttrium-aluminate grains and accounts for the suppression of the growth of yttria grains. [Display omitted] [ABSTRACT FROM AUTHOR]

Ultrafast high‐temperature sintering (UHS) is a joule heating method with an extremely high heating rate (∼103–104°C/min). In this work, commercial ZrB2 powders were rapidly densified by UHS to >90% relative density within 60 s in vacuum without pressure. Bulk density improved from 75% to 93% of relative density by increasing the sintering duration from 10 to 60 s. The final grain size increases from 2.7 ± 1.1 to 19.0 ± 8.4 μm when the sintering time increased from 10 to 60 s. X‐ray diffraction and energy‐dispersive spectroscopy show crystalline phase and compositional uniformity in ZrB2 after UHS. [ABSTRACT FROM AUTHOR]

• Superplasticity of fine-grained Mg-10Li alloy was studied. • Severe plastic deformation was used to achieve the fine grains. • Its deformation mechanisms for superplastic flow were analyzed. • Deformation mechanism maps were constructed in 2D and 3D. The superplastic behavior and associated deformation mechanisms of a fine-grained Mg-10.1 Li-0.8Al-0.6Zn alloy (LAZ1011) with a grain size of 3.2 µm, primarily composed of the BCC β phase and a small amount of the HCP α phase, were examined in a temperature range of 473 K to 623 K. The microstructural refinement of this alloy was achieved by employing high-ratio differential speed rolling. The best superplasticity was achieved at 523 K and at strain rates of 10−4 -5 × 10−4 s−1, where tensile elongations of 550–600% were obtained. During the heating and holding stage of the tensile samples prior to tensile loading, a significant increase in grain size was observed at temperatures above 573 K. Therefore, it was important to consider this effect when analyzing and understanding the superplastic deformation behavior and mechanisms. In the investigated strain rate range, the superplastic flow at low strain rates was governed by lattice diffusion-controlled grain boundary sliding, while at high strain rates, lattice diffusion-controlled dislocation climb creep was the rate-controlling deformation mechanism. It was concluded that solute drag creep is unlikely to occur. During the late stages of deformation at 523 K, it was observed that grain boundary sliding led to the agglomeration of the α phase, resulting in significant strain hardening. Deformation mechanism maps were constructed for β-Mg-Li alloys in the form of 2D and 3D formats as a function of strain rate, stress, temperature, and grain size, using the constitutive equations for various deformation mechanisms derived based on the data of the current tests. [ABSTRACT FROM AUTHOR]

Dual-phase (DP) steel has been widely used in automotive steel plates with a balance of excellent strength and ductility. Grain refinement in DP steel is important to improve the properties further; however, the factors affecting grain growth need to be well understood. The remaining problem is that acquiring data through experiments is still time-consuming and difficult to evaluate quantitatively. With the development of materials informatics in recent years, material development time and costs are expected to be significantly reduced through experimentation, simulation, and machine learning. In this study, grain growth behavior in DP steel was studied using two-dimensional (2D) and three-dimensional (3D) Monte Carlo modeling and simulation to estimate the effect of some key parameters. Grain growth can be suppressed when the grain boundary energy is greater than the phase boundary energy. When the volume fractions of the matrix and the second phase were equal, the suppression of grain growth became obvious. The long-distance diffuse frequency can promote grain growth significantly. The simulation results allow us to better understand the factors affecting grain growth behavior in DP steel. Machine learning was performed to conduct a sensitivity analysis of the affecting parameters and estimate the magnitude of each parameter's effects on grain growth in the model. Combining MC simulation and machine learning will provide one promising research strategy to gain deeper insights into grain growth behaviors in metallic materials and accelerate the research process. [ABSTRACT FROM AUTHOR]

Li 2 TiO 3 ceramic pebbles with uniform structure, high density and good mechanical property were successfully fabricated by wet process. The Li 2 TiO 3 pebbles sintered at 1000 °C for 4 h had satisfactory density (91% T.D.) and crush load (a.v. 27.1 N). The apparent activation energy of Li 2 TiO 3 grain growth was 201 ± 7 kJ/mol. Accelerated densification was observed in a reducing atmosphere, this might be attributed to the oxygen vacancies generated by the reduction of Ti4+ to Ti3+ in 0.1%H 2 +He atmosphere, which facilitate the diffusion of ions through the lattice. The grain growth was controlled by lattice diffusion under reducing atmosphere, and the apparent activation energy was 168 ± 6 kJ/mol. The mechanical results show that the crush load of Li 2 TiO 3 pebbles decreased with the increase of temperature, and the average crush load at 500 °C was 13.9 N. In addition, the thermal cycling and long-time annealing experiments suggested that Li 2 TiO 3 pebbles have good structural and performance stability. The Li 2 TiO 3 ceramics prepared by wet process were satisfactory tritium breeder materials with ideal application potential. [ABSTRACT FROM AUTHOR]

The present paper examines the mechanisms for simultaneous manipulation of grain size and γ′ dissolution leading to the microstructure that balance creep and fatigue properties in UDIMET 720Li. For sub (near) solvus heat treatments, grain growth and γ′ dissolution occur simultaneously. Grain size following heat treatment is determined largely by the primary γ′ particle dispersion, via Zener-Smith pinning. Remarkably, grain growth occurs only as a result of precipitate dissolution. The resulting grain size is dictated by a combination of initial particle dispersion in the as-received material and heat treatment temperature. It is likely to hold so long as the initial grain size is close to the Zener-Smith limit and where the kinetics of grain growth and dissolution share similar rates. [ABSTRACT FROM AUTHOR]

• HPT-processed Mg-Gd-Y-Zn-Zr nanostructure exhibits an excellent thermal stability. • Mg-Gd-Y-Zn-Zr nanostructure exhibits an unique phase transformation upon annealing. • The formation of solute segregation and β phase retards grain growth during annealing. • A hardness peak is achieved by annealing due to the effects of solute segregation. A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy is processed by solution treatment and high pressure torsion (HPT) at room temperature to produce a nanostructured light material with high hardness. The stability of this alloy is subsequently tested through isochronal annealing for 0.5 h at 373 K to 673 K. The results reveal a thermal stability that is vastly superior to that of conventional Mg-based alloys processed by severe plastic deformation: the grain size remains at around 50 nm on heating to 573 K, and as the temperature is increased to 673 K, grain growth is restricted to within 500 nm. The stability of grain refinement of the present alloy/processing combination allowing grain size to be limited to 55 nm after exposure at 573 K, appears to be nearly one order of magnitude better than for the other SPD processed Mg-RE type alloys, and 2 orders of magnitude better than those of SPD processed RE-free Mg alloys. This superior thermal stability is attributed to formation of co-clusters near and segregation at grain boundaries, which cause a thermodynamic stabilization of grain size, as well as formation of β-Mg 5 RE equilibrium phase at grain boundaries, which impede grain growth by the Zener pinning effect. The hardness of the nanostructured Mg-Gd-Y-Zn-Zr alloy increases with increasing annealing temperature up to 573 K, which is quite different from the other SPD-processed Mg-based alloys. The high hardness of 136 HV after annealing at 573 K is mainly due to solute segregation and solute clustering at or near grain boundaries. [ABSTRACT FROM AUTHOR]

Most traditional powder melting-based additive manufacturing (AM) technologies generally yield high-performance parts at the expense of additional cost and energy consumption. As an economical and efficient alternative method, material extrusion (ME) that deploys polymer-based filaments with highly filled metal particles offers the possibility of scalable, low-cost fabrication of metal components. As a critical step, sintering governs the mechanical strength and geometry of the final parts. The grain growth behavior induced during sintering should be well controlled to achieve the parts with the desired mechanical performance. Due to the nature of AM, grain growth kinetics could also involve extremely complex grain boundary migration and atomic diffusion mechanisms caused by the heterogeneous pore distribution. In this work, an analytical model was developed to predict and understand the grain growth behavior of stainless steel (SS) 316L built by ME-based sintering-assisted AM. Such a model accounts for anisotropic viscosity parameters calibrated through a three-dimensional dilatometry test, enabling the prediction of grain size evolution during sintering. Grain growth kinetic parameters were further identified by the grain size data at the heating and holding stages. To validate and generalize the model, we also conducted the grain size evolution prediction under different sintering temperature profiles for as-built SS 316L specimens. This work will provide scientific insights into the grain growth behavior of metal parts built by this AM technique and its understanding can be readily transferred to other sintering-assisted AM processes like binder jetting and ink writing for metal structure creation.

This work presents a Monte Carlo Potts grain growth model in which the grain boundary (GB) energies depend on the GB inclination. The inclination is calculated using a linear smoothing approach developed by the authors. In bicrystal simulations with a shrinking grain, the grain changes shape to prefer low energy GB inclinations. However, in polycrystal simulations the preferred inclinations depend on the approach used to assign the triple junction (TJ) energies. Approaches that produce unimodal TJ energy distributions result in the expected behavior of preferring low energy GB inclinations. However, approaches that produce bimodal TJ energy distributions result in medium energy or even high energy inclinations being preferred. Overall, this study underscores the importance of TJs in anisotropic grain growth.

Recent research has focused on the ultrafast laser welding of metal and glass due to its diverse applications in aerospace, medicine, and optical sensing. However, the significant material property differences between metals and glass, along with strict surface requirements, pose challenges to the development of metal-glass welding and limit its industrial utility. Utilizing high-pulse-energy lasers has proven effective in mitigating these challenges. This study comprehensively analyzes a millijoule (mJ)-level femtosecond pulse laser welding technique applied to aluminum alloy and fused silica, with a primary emphasis on understanding the characteristics and mechanism of joint. During the welding process, aluminum and fused silica mutually diffuse, leading to the formation of five distinct zones at the joint's cross-section. Within the aluminum molten pool in rapid cooling, aluminum grains undergo continuous growth along a temperature gradient and then transform into smaller epitaxial grains with altered grain orientations and HAGBs. An amorphous Al2O3 phase coexists with aluminum, forming a lamellar structure. Besides, silicon nanoparticles and transitional γ-Al2O3 precipitate near the center of the weld. This study examines the underlying causes of these phenomena and provides a detailed discussion of the associated strengthening mechanisms, ultimately resulting in a maximum shear strength of 8.94 MPa.

Remelting has a significant effect on controlling microstructure and enhancing properties of components prepared by laser powder bed fusion. In order to reveal the mechanism of the effect of remelting time intervals, this study employed a dual-laser powder bed fusion equipment to ensure precise and controllable time intervals of remelting. The effects of intervals (2 ms, 5 ms, and 50 ms) on substructure morphology, microstructure and mechanical properties of 316L were analyzed. The results indicate that the grain size ranged from 13.03 μm to 26.55 μm due to the influence of initial temperature at different remelting intervals on grain size. Additionally, the composition ratio between columnar and cellular substructure varies with the time interval, which greatly influences mechanical properties. Longer intervals favor columnar substructures in high-temperature gradients, and dislocation motion is impeded, leading to an increased strength of up to 689.6 MPa. Shorter intervals promote cellular substructures in low-temperature gradients, and dislocations move smoothly along the boundaries of substructure, leading to an elongation of up to 49.3 %. Therefore, a novel method for controlling microstructure and properties is provided in this study utilizing the precise controllable remelting time intervals of dual-laser powder bed fusion.

Ultrafine/nano WC–Co cemented carbide is widely used because of its high hardness, strength, toughness, and other comprehensive properties. However, WC grain growth during preparation is a persistent issue that hinders the development of this type of cemented carbide. In this context, we review current research on the preparation of ultrafine/nano WC–Co composite powder and sintering methods of ultrafine/nano cemented carbide. The correlation between grain growth and densification during sintering is analyzed. The key technologies inhibiting WC grain growth are explored and discussed, and the synergism between transition metal carbides and rare-earth element mixtures is found to effectively inhibit the growth of WC grains and improve the general performance of ultrafine/nano WC–Co cemented carbide. Finally, future research directions and development trends of ultrafine/nano WC–Co cemented carbides are presented.