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[Display omitted] • A grain growth model during material extrusion-based AM was developed. • Anisotropic sintering behavior was accounted to predict the grain size. • Grain growth kinetics parameters were calibrated to apply for other sintering profiles. 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. [ABSTRACT FROM AUTHOR]

Abstract The mechanisms during pressureless sintering of TiB 2 with 0–50 wt.% Ni were analysed by differential dilatometry after mechanical alloying with hard metal beads. Three main stages of sintering could be identified by means of the changing dilatation rate. The first interval (200–600 °C) comprises reactions with oxygen and moisture (H 3 BO 3 , B 2 O 3 formation, melting, evaporation). The second stage (600–700 °C) is governed by solid state sintering and reactions of Ni and/or Co with TiB 2 yielding Ni 3 B, Ni 2 B, the corresponding Co-compounds and the ternary ω-phase of W 2 NiB 2 -type. The third main shrinkage interval is due to liquid phase sintering (987–1640 °C) and is accompanied by a significant weight-loss. Both precipitation of ω-phase as well as a (Ti,W)B 2 solid solution in form of a core-shell structure are responsible for a grain growth retardation and the homogeneous microstructure. The results explain well all features reported in the corresponding literature while encountering the role of WC, too. [ABSTRACT FROM AUTHOR]

Grain growth in polycrystalline materials is commonly achieved with thermal annealing. Here, we explore an alternate athermal route through a synergy of electrical current and tensile strain at or near room temperature. Our experimental approach involves passing electrical current in 100 nm thick freestanding palladium films with about 5 nm initial grain size. The resulting Joule heating increases the temperature up to 470 K (homologous temperature of 0.25) at the middle section of the specimen. The massive heatsinks at the two ends of the specimen constrain them at the room temperature. At current density of 7 × 105 A/cm2, in-situ transmission electron microscopy shows more than two orders of magnitude grain growth in the high temperature regions and little growth at the room temperature regions. However, application of elastic strain (about 0.1%) dramatically increased the grain size to about 100 nm in a few seconds at the room temperature regions. Our finding suggests that the synergy of elastic strain energy and electrical current density may achieve grain growth in metallic materials even at the room temperature. Molecular dynamics simulation of this phenomenon reveals that the externally applied strain is localized at the grain boundaries in nanocrystalline metals, which promotes the effects of electron wind force on the grain boundary atoms. We conclude that a synergy of two or more stimuli can achieve grain growth at room or even lower temperatures. Unlabelled Image • Grain growth in metals typically requires very high temperatures and longer times. • A room or low temperature electro-mechanical metal annealing process is proposed. • In-situ TEM shows grain growth from 5 nm to 100 nm at 25 °C in <5 min. • MD simulation suggests synergy of electron wind force and elastic strain energy. [ABSTRACT FROM AUTHOR]

Abstract Two-step sintering (TSS) was applied on the M-type Sr-hexaferrite to control grain growth and form a desirable microstructure during the final stage of sintering. The process involved T 1 (temperature in the first sintering stage), t 1 (holding time at T 1), T 2 (temperature in the second sintering stage), and t 2 (holding time at T 2). The effects of each setting parameter on the density, grain growth, degree of orientation and magnetic properties were systematically investigated. The results revealed that T 1 was the main factor that primarily controlled microstructure and magnetic properties. Compared with conventional sintering (CS) process, the magnets with smaller average grain size were achieved by TSS, which suggested that two-step sintering could improve the coercivity of Sr-hexaferrites. It was shown that the high integrated magnetic properties for Sr-hexaferrites, including B r = 440.4 mT, H cj = 328.2 kA/m and (BH) max = 37.6 kJ/m3, were obtained under the optimum processing condition: T 1 = 1200 °C, T 2 = 1000 °C, t 1 = 5 min, and t 2 = 2 h. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Continuous transition between vertex-driven and curvature-driven models. • Extensive set of statistics computed. • Several choices of grain boundary energy considered, including the Read-Shockley energy. Abstract We consider a variational model for two-dimensional grain growth that allows for finite mobilities of boundaries and boundary junctions. We use a collocation method to describe the grain boundaries and posit evolution equations that ensure energy dissipation. The resulting algorithm is amenable to parallelization and a GPU implementation described in this work leads to significant reduction in computational time. We benchmark our code against known numerical and analytical solutions in some special cases. We also conduct an extensive analysis of statistics that develop for various parameter regimes. [ABSTRACT FROM AUTHOR]

Abstract Highly-oriented (104) polycrystalline α-Al 2 O 3 transparent ceramics were prepared by a templated grain growth (TGG) method using A-plane single-crystal sapphire as the template. This is the first report, to best of our knowledge, on polycrystalline α-Al 2 O 3 ceramics oriented to non-optical axes. XRD and SEM results indicate that samples with high oriented grains and high-density structure are prepared. Compared to random samples, the oriented samples exhibited an increase in the real in-line transmittance (at 650 nm) from ∼23% to ∼62%. Moreover, the transmission remains a high level as the wavelength shifts toward the UV range (< 300 nm). This method can be extended to other uniaxial materials without optical axes in order to produce polycrystalline ceramics with excellent optical transparency. [ABSTRACT FROM AUTHOR]

Abstract The microstructural stability during long-term storage and tensile deformation have been investigated for nanograined (NG) pure Cu and pure Zr produced by severe plastic deformation. Abnormal and uniform grain growth behaviour is observed in the NG Cu after long-term storage and tensile deformation at different strain rates, respectively. The abnormal grain growth during long-term storage is related to the relaxation of the fluctuating microstrain in the deformed microstructure and can be accelerated by increasing the temperature. The uniform grain growth during tensile deformation is dominated by grain boundary (GB) migration due to the absorption of dislocations. The degree of GB migration in the case of a low strain rate is relatively large owing to the dynamic equilibrium between the formation and disappearance of dislocations. However, the GB migration at a high strain rate is suppressed by the piled-up dislocations. In comparison, the NG Zr exhibits relatively high microstructural stability with no obvious grain growth during long-term storage and tensile deformation. This is attributed to the difference in the crystal structure between Cu and Zr. [ABSTRACT FROM AUTHOR]

Abstract Significant grain growth assisted by a martensitic transformation near or below room temperature was found in a nanocrystalline Ti-50.8Ni (at.%) alloy. Significant grain growth was observed when thermally- or stress-induced B2 ↔ R ↔ B19′ (B2: parent phase, R and B19′: two types of martensite phases) martensitic transformation is repeated while not when B2 ↔ R transformation is repeated. This implies that martensitic transformation to and from the B19′ martensite phase assist the grain growth. Transmission electron microscope observations indicate that migration of grain boundaries could be the main mechanism for the grain growth. Graphical abstract Growth of B2 parent phase grain assisted by martensitic transformation in a nanocrystalline Ti-50.8Ni (at.%) shape memory alloy. Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract The effects of a small addition of nanosized TiC particles (TiC p) on the microstructure evolution of the Al-Mg-Si (6061) alloy throughout the entire fabrication process including casting, cold rolling and heat treatments were investigated. TiC p impeded the recrystallization and grain growth, refined the grain structure, and evidently increased the tensile strength without obvious decrease on ductility. The ultimate tensile strength and yield strength of the 1.0 wt% TiC p /6061 composite were 330 MPa and 275 MPa in T6 state and 438 MPa and 426 MPa in the as-rolled state, an increase of 42 MPa and 38 MPa, 41 MPa and 52 MPa, respectively, compared to the matrix 6061 alloy. The strength increase is attributed to strengthening effect of TiC p , dislocation accumulation and precipitate strengthening. [ABSTRACT FROM AUTHOR]

Abstract Employing CaCO 3 as sintering additive, highly transparent aluminum oxynitride (AlON) ceramics were pressurelessly fabricated from AlON powder at 1870 °C during150 min. The transmittance of the AlON doped with 0.3–0.4 wt% CaCO 3 is up to 83–85% at ~3700 nm for 2 mm thickness samples, and their transmittances are consistently higher than that of the AlON doped with the ideal amount of Y 2 O 3 at wavelength ranging from 200 nm to 6000 nm. The AlON doped with CaCO 3 exhibits the transmittance of 71% at 4800 nm (typical band for infrared targeting), which is higher by 6% than that of the doped Y 2 O 3. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Oxygen content in n-type Bi 2 Te 3 -based alloys plays a crucial role in determining the thermoelectric properties. We aimed to control the residual oxygen that originated from the ball milling process; for this, sintering processes such as spark plasma sintering (SPS) and plasma activated sintering (PAS) were utilized. The reduction of oxygen content was successfully achieved with PAS, leading to improved electrical properties. With the removal of residual oxygen, the carrier concentration increased, resulting in reduced electrical resistivity. The phonon thermal conductivity was reduced due to the grain size of this sample, which was smaller than that of the hot-pressed (HP) samples. An enhanced value of the dimensionless figure of merit (ZT) of 0.83 at 473 K, an increase of 25.2% compared with that of the HP samples, was achieved. The selection of the appropriate sintering process should be considered to remove oxygen content, as well as to minimize grain growth. [ABSTRACT FROM AUTHOR]

Coarse grain growth has conventionally been induced by high temperature and pressure during the sintering of ZrC ceramic. Even under the harsh sintering conditions, the microstructure contained inhomogeneous grain sizes and residual pores. To reduce the agglomeration of grains during sintering, we focused on inducing lattice strain. Mechanical milling was carried out to induce high strain in the lattice of ZrC, with the amount of strain varying during milling. This could slow down the grain growth, eventually producing homogeneous grain sizes of sintered ZrC ceramic. The sintered grain size decreased at the maximum strain, with 10 h of milling, while it started to increase significantly after 10 h. This study demonstrates that the sintering conditions could be optimized to obtain the lattice strain for fully dense sintered ceramic. [ABSTRACT FROM AUTHOR]

The volumetric growth rate of individual grains as a function of their number of sides, as well as the average normalized volumetric growth rate for a given topological class are studied based on large-scale phase-field simulations of normal grain growth. For all cases, where different initial grain size distributions are considered, the same averaged normalized volumetric growth rate for a given topological class was obtained with the averaged normalized volumetric growth rate approximately zero for grains with F o ≈ 15 sides. The normalized average grain size as a function of topological class becomes constant within the transient regime and the relation in the transient and steady-state regime is similar and independent of initial grain structure. The relation does however not match Mullins' analytical model [W. Mullins, Acta Metall. 37 (1989) 2979–2984]. The effect of the average number of sides of the neighbouring grains on the average volumetric growth rate for a given topological class is analyzed, as well. Moreover, tracking of the evolution of four grains suggests that the normalized volumetric growth rate of a grain is strongly linked with the number of sides of the grain at the given time. [ABSTRACT FROM AUTHOR]

Grain growth kinetics under the anisotropic grain boundary properties is investigated by large-scale and long-time molecular dynamics (MD) simulations of contentious processes of nucleation, solidification and grain growth in a submicrometer-scale system. Microstructures obtained via homogeneous nucleation from undercooled melt iron consists of approximately 1500 grains and the number of grains decreases to one tenth of the number via the grain growth process. The grain growth exponent obtained from the MD simulation deviates from the ideal value since anisotropic effects in the grain boundary properties are inherently included in MD simulations. It is confirmed that the decrease of the reduced mobility (i.e., the product of the intrinsic grain boundary mobility and the grain boundary energy) is a dominant factor for the deviation from the ideal grain growth. The deviation from the Mackenzie function for the distribution of the disorientation angle between neighboring grains implies that the preferential selection of grain boundaries with small grain boundary energies occurs during the grain growth. This enhances the anisotropy in grain boundary properties and therefore decreases the reduced mobility of the grain boundary. Moreover, a multi-phase-field simulation starting from a MD configuration results in an ideal grain growth when a constant value of the reduced mobility is employed, which validates the discussion on the reduced mobility. The new insight in this study is achieved for the first time owing to a multi-graphics processing unit (GPU) parallel computation over 50 days for one case using 128 GPUs on the GPU-rich supercomputer. [ABSTRACT FROM AUTHOR]

6H-SiC porous SiC ceramics accompanied by the formation of large clubbed and tabular grains were successfully fabricated at 1800 °C under an applied pressure of 50 MPa for 5 min by spark plasma sintering technique without introducing any pore-creating agent. The phase composition, microstructure and properties of porous SiC ceramics were investigated. The measured porosity of the specimens was 35.7% and the mean pore size varied in the range of 0.3–3 μm. The sintered specimen had a strength of 103 MPa, a fracture toughness of 2.89 MPam 1/2 , and a density of 1.85 g/cm 3 . The mechanisms of porous structure and grain growth were elucidated. [ABSTRACT FROM AUTHOR]

In this study, a solution chemical route for the synthesis of ultrafine WC-15 wt %Co cemented carbides was proposed with WC-6Co aggregates as raw materials. The chemical Co was uniformly distributed around the WC-6Co aggregates and the Co layers in the raw materials had been well preserved. During sintering, the chemical Co could diffuse along the Co layers and thus avoid the aggregation and growth of WC grains. Additionally, the homogeneous distribution of Co phase and the existent of Co layers promoted the WC reprecipitations on the same crystal during the solution/re-precipitation process. The fine WC grains and the homogeneous of WC and Co phases resulted in the high hardness and transverse rupture strength. [ABSTRACT FROM AUTHOR]

An electrical current upon flash sintering experiments modifies the defect chemistry of corundum and causes the formation of color centers (F-centers), consisting of discharged oxygen vacancies. This phenomenon interacts with the diffusion kinetics in the anodic region and promotes abnormal growth of some grains which reach several hundreds of micrometers in just few minutes, the grains being strongly oriented orthogonally to the current flow. [ABSTRACT FROM AUTHOR]

‘Abnormally’ large grains, whose sizes are greater than twice the critical radius ( 2 R c ), are known to alter the isotropic grain growth phenomena. In the present work, phase-field simulations of 2-dimensional microstructures are extensively analysed to elucidate the deviations from the normal grain growth introduced by the presence of abnormal grains. Polycrystalline microstructures that are ’artificially’ made to resemble physical structures, by governing the distribution and the sizes of the abnormal grains, is employed to analyse the grain growth in the presence of large grains. This study unravels that the abnormal grains induce a period of transition during which its grain size distribution is shifted and confined within 2 R c , indicating a complete disappearance of the abnormality in the microstructure. Furthermore, it is identified that this transition period establishes a bimodal distribution, which subsequently evolves into a unimodal time-invariant distribution. This behaviour noticeably reveals the misconception that the disappearance of the abnormality signifies the onset of normal grain growth. Moreover, despite the apparent disappearance of the abnormal grains, a continued increase in the volume-fraction of these ‘pre-existing’ abnormal grains is recognized, and in the steady-state condition, it is observed that the microstructure predominantly consists of these pre-existing abnormal grains. Influence of the factors like initial volume-fraction of the abnormal grains F o and degree of abnormality U ‾ max on the duration of the transient period is quantified by investigating close to hundred microstructures with unique F o and U ‾ max . [ABSTRACT FROM AUTHOR]

The thermodynamic dissipation-based approach, being one of the approved effective methods for treatment of grain growth, is refined by accounting for dissipation due to translational motion of individual grains during grain growth. The present model introduces an interaction function correlating the velocity of translational motion and the rate of growth for individual grains. The comparison of results of simulations based on the present model with existing phase-field simulations indicates that about 1/4 to 1/3 of the driving force is dissipated by translational motion of grains lowering the grain growth kinetics and changing of the steady state grain radii distribution function. [ABSTRACT FROM AUTHOR]

The grain growth behaviors of a nickel-based superalloy after full recrystallization are investigated by hot compression test plus annealing treatment. Electron backscattered diffraction technology (EBSD) is utilized to analyze the grain boundary features. It is found that the annealing parameters greatly affect the average grain size, and annealing twin boundaries, including ∑3, ∑9, and ∑27 boundaries. The fully-recrystallized grains can rapidly grow up at relatively high annealing temperatures with the increase of annealing time. Meanwhile, large fractions of annealing twin boundaries form at the initial stage of annealing treatment. However, with the further increase of annealing time, the content of annealing twins slightly drops due to the migration of grain boundaries or twin boundaries. When the annealing temperature is increased, the shape of annealing twin is apparently lengthened while the number of annealing twin sharply decreases. In addition, the kinetics model for grain growth after full recrystallization is established and validated by the experimental observations. [ABSTRACT FROM AUTHOR]

The high temperature grain growth behaviour of Y 2 O 3 partially stabilised ZrO 2 and ZrO 2 -CNT (0.5 and 2 vol%) composites was investigated in a series of isothermal experiments carried out between 1500 and 1800 °C. The grain growth activation energy was found to be 417 ± 20 kJ mol − 1 for the ZrO 2 , which increased with the addition of CNTs to 509 ± 41 and 497 ± 17 kJ mol − 1 for the 0.5 and 2 vol% composites respectively. A sintering exponent of 2 was observed for the ZrO 2 , corresponding to normal grain growth, whilst the nanocomposites showed a CNT drag dominated sintering mechanism, with exponents of 2.7 and 3.2 for the 0.5 and 2 vol% composites respectively. This work confirms and quantifies the direct influence of CNTs on the grain growth mechanism and activation energy of ZrO 2 -CNT composites. [ABSTRACT FROM AUTHOR]

CeO 2 films with mixed doping of gadolinium and zirconium were deposited on NiW substrates by chemical solution deposition. The influences of mixed doping on the epitaxial growth and microstructure of films were systematically investigated. The morphological evolution of films with different dwell times was also discussed. The phase formation, texture and surface morphology of (Ce 0.8 Gd 0.2 ) 1- x Zr x O 2-δ (CGZO) films were characterized by X-ray diffraction and atomic force microscopy. It was noticed that Zr 4+ doping had stronger effect on the crystal structure of CeO 2 than Gd 3+ doping. The increasing doping content of Zr 4+ could be favorable for the self-limited grain growth of the CGZO films and promote the lattice match with substrate. Moreover, the sharp texture and smooth surface of CGZO film could be easily acquired by the mixed doping. The results indicated that the (Ce 0.8 Gd 0.2 ) 1- x Zr x O 2-δ film was beneficial for the subsequent film growth, which could be a candidate buffer layer for the coated conductors. [ABSTRACT FROM AUTHOR]

In this paper, the significance and contribution of SiC particle size and spark plasma sintering (SPS) parameters on the mean TiB 2 grain size and Vickers hardness of TiB 2 –20 vol% SiC ceramic composites were studied. An L9 orthogonal array was used to design of experiments by the Taguchi method to investigate the effects of four processing parameters including the SPS temperature, SPS soaking time, SPS pressure and SiC particle size. Three levels were selected for each parameter and the analysis of variance (ANOVA) was employed for optimization of processing parameters. The SPS temperature was identified as the most effective parameter on the as-sintered grain size and hardness of TiB 2 –SiC ceramics, but, the SPS soaking time does not have an obvious influence on these characteristics. The presence of SiC together with the in-situ formed nano-sized TiC phases acted as grain growth inhibitors which was helpful to obtain a fine-grained composite microstructure. The ANOVA disclosed that the sintering temperature of 1800 °C, the soaking time of 15 min, the pressure of 30 MPa and the SiC particle size of 200 nm are the optimal processing conditions. A Vickers hardness of 28.5 GPa was predicted at such optimized conditions which is near to the experimentally measured value of 27.4 GPa as the confirmation test. [ABSTRACT FROM AUTHOR]

We studied the microstructures of the U–5.5 wt%Nb (U–5.5Nb) alloy with different thickness reductions induced by cold rolling and different heat treatments. The recrystallization and grain growth behavior of the U–5.5Nb alloy with 50% deformation were analyzed. Samples showed the typical rolling microstructure after plastic deformation. Microstructure analysis showed that increasing the deformation, annealing temperature, or annealing time can promote recrystallization. The size of recrystallized grains decreases with the increasing deformation. The isothermal kinetics curve of recrystallization shows a typical S -type behavior. The recrystallization index n is 1.58, which is lower than the ideal Johnson Mehl Avrami Kolmogorov (JMAK) value, i.e., 3. This indicates that the isothermal annealing of U–5.5Nb alloy at 750 °C is a one-dimensional recrystallization process. The recrystallization activation energy of U–5.5Nb alloy after annealing at 750 °C is between 314.47 and 555.84 kJ/mol. The grain growth index at 700 °C and 750 °C was calculated to be 2.24 and 2.21, respectively, which are slightly higher than the ideal value of 2. Values of grain growth activation energy for U–5.5Nb alloy after annealing for 30 min and 60 min between 700 and 900 °C are Q 30 min = 211.72 kJ/mol and Q 60 min = 186.56 kJ/mol, respectively. Both values are higher than the diffusion activation energy of niobium atoms in γ uranium, which is Q diff = 138 kJ/mol. It may be the “solute drag effect” that led to the increase in the activation energy of grain growth with the decrease in the annealing time. [ABSTRACT FROM AUTHOR]

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd 2 O 3 -Y 2 O 3 was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K) −1 after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon. [ABSTRACT FROM AUTHOR]

This paper demonstrates the successful fabrication of a graded steel with successive 15 layers by direct laser metal deposition. The phase evolution, microstructure and grain growth of the as-built graded steel were investigated by X-ray diffraction (XRD), optical microscopy (OM), and electron back-scatter diffraction (EBSD), respectively. Experimental results showed that the crystal structures of the graded steel evolved from FCC (Austenite) in the first two grades to BCC (Ferrite) with a small trace of Austenite phase. Columnar dendrites are observed in the first and second grade with the growth orientation horizontal to the direction of the laser deposition. The third grade has a finer microstructure with equiaxed grains. Preferred texture was dominant in the microstructure for the first two grades with low-angle grain boundaries (about 2°). No preferred texture was observed in the third grade. Large-angle grain boundaries are available in the third grade with distribution from 50° to 60°. Our findings suggest that a graded steel with microstructure and grain growth orientation evolution can be obtained by direct laser metal deposition. The evolution of microstructure and grain growth was not changed by different scanning rate. [ABSTRACT FROM AUTHOR]

A grain growth model to describe dopant effects on nanocrystalline ceramics is proposed by incorporating the dopant-segregation-dependent grain boundary (GB) energy and the GB mobility subjected to intrinsic drag and pore drag (both affected by dopant segregation) into the parabolic growth formula. The model addresses the common case of residual porosity in grain growth behavior. Taking near-fully dense nanocrystalline lanthanum doped Yttria stabilized Zirconia (La doped YSZ) as the system of study, the grain growth behavior was explored using the model. The substantially suppressed grain growth in La doped YSZ as compared to La-free YSZ could be attributed to the combined effect of thermodynamically reduced GB energy and kinetically reduced GB mobility. Contrary to previous assumptions, the model suggests that, relative to the GB energy overall effect, the effect of the dopant on the GB mobility plays a more significant role in reducing coarsening. Furthermore, model calculation shows that both intrinsic drag and pore drag makes certain contribution to the evolution of GB mobility during the grain growth. [ABSTRACT FROM AUTHOR]

The mechanisms of stress-assisted grain growth are explored using molecular dynamics simulations of nanoindentation in nanocrystalline Ni and Ni-1 at.% P as a function of grain size and deformation temperature. Grain coalescence is primarily confined to the high stress region beneath the simulated indentation zone in nanocrystalline Ni with a grain size of 3 nm. Grain orientation and atomic displacement vector mapping demonstrates that coalescence transpires through grain rotation and grain boundary migration, which are manifested in the grain interior and grain boundary components of the average microrotation. A doubling of the grain size to 6 nm and addition of 1 at.% P eliminates stress-assisted grain growth in Ni. In the absence of grain coalescence, deformation is accommodated by grain boundary-mediated dislocation plasticity and thermally activated in pure nanocrystalline Ni. By adding solute to the grain boundaries, the temperature-dependent deformation behavior observed in both the lattice and grain boundaries inverts, indicating that the individual processes of dislocation and grain boundary plasticity will exhibit different activity based on boundary chemistry and deformation temperature. [ABSTRACT FROM AUTHOR]

The effect of particle size and initial powder phase on the microstructure of porous SiC ceramics was investigated. When β-SiC powder was used as a starting material, porous SiC ceramics were formed by rapid grain growth of α-SiC during the β-α phase transformation. As the particle size of the initial powders decreased, β-α SiC phase transformation and rapid grain growth of α-SiC occurred earlier. Grain and pore size of SiC porous ceramics can be controlled by the addition of α-SiC seed. Grain and pore size decreased as the concentration of α-SiC seed increased. The bending strength of porous SiC ceramics did not depend on pore size. [ABSTRACT FROM AUTHOR]

Curvature driven grain growth proceeds by the motion of curved grain boundaries in polycrystals leading to a decrease in the total interfacial free energy. However, grain growth cannot occur without the so-called topological transitions. Indeed, it is a mathematical impossibility that a decrease in the number of grains per unit volume may take place without the topological transition that corresponds to the grain disappearance. Nonetheless, despite their importance, no previous work has studied the topological transitions in detail that take place during the transient as well as the self-similar state of coarsening. In the present work, the three classical topological changes are tracked during 2-d grain growth simulated by a Monte Carlo method. It is shown and discussed how topological transitions reach a self-similar state together with the grain size and the number of edges (faces) per grain distribution. [ABSTRACT FROM AUTHOR]

Oxidation of metals can be influenced by the presence of electric field, lattice strain, rearrangement of the oxide structure, and formation of cracks in an oxide. The understanding of the interplay of these factors is still incomplete. We focus on the scenario including the oxide-grain growth. The model used implies that the whole process is limited by diffusion of metal or oxygen atoms along the grain boundaries as it was originally proposed by Fehlner and Mott for macroscopic samples. For nanoparticles, the model predicts a transition from the power-law oxide growth at low conversion to slower growth at high conversion. [ABSTRACT FROM AUTHOR]

In this paper, the thermal stability and grain growth kinetics of nanocrystalline Cu, reinforced with SiC nanoparticles and obtained using a mechanical milling process, were investigated during isothermal annealing. The presence of the nanoparticles in the nanocrystalline copper matrix resulted in a significant decrease in grain growth, the formation of partially textured microstructure and twin boundaries at higher temperatures, and an increase in the volume fraction of recrystallized grains, as estimated by grain orientation spread, in comparison to unreinforced Cu during annealing. The lower volume fraction of recrystallized grains at higher temperatures was attributed to dynamic recovery. Normal grain growth was observed in the annealing range of 400–600 °C, and significant abnormal grain growth was observed at higher temperatures. An analysis of the grain growth kinetics in the temperature range of 400–600 °C revealed a time exponent of n ≈ 3.6 and activation energy of ≈ 34 kJ mol − 1 , based on the parabolic equation. The calculated activation energy for grain growth in the SiC dispersion strengthened Cu was found to be less than that of nanocrystalline Cu. The low activation energy and high thermal stability were attributed to high lattice strain and the retarding effect of nanoparticles by the Zener mechanism. [ABSTRACT FROM AUTHOR]

The densification kinetics and grain growth behavior of an undoped tungsten powder during spark plasma sintering (SPS) were investigated under the pressure of 40 MPa and constant heating rate of 100 °C min − 1 . Two stages of the sintering process were clearly identified: densification without grain growth at the low temperatures (1200–1450 °C) and grain growth without much further densification at higher temperatures (1500–2000 °C). A creep model was applied to determine the densification mechanisms involved in the densification stage, which can be elucidated by evaluating the stress exponent ( n ) and the apparent activation energy ( Q d ) from the densification rate law. It shows that a boundary diffusion governs the densification process at low effective compaction stresses ( n = 1.5, Q d = 140.57 ± 12 kJ mol − 1 ), while grain boundary diffusion and dislocation motion both operate at higher effective compaction stresses ( n = 3, Q d = 302.48 ± 24 kJ mol − 1 ), which is confirmed by transmission electron microscopy observation. During the final-stage of sintering, the fast grain growth mechanism was suggested as surface diffusion. [ABSTRACT FROM AUTHOR]

The study of grain growth kinetics in nano-grained UO 2 samples is reported. Dense nano-grained UO 2 samples with well-controlled stoichiometry and grain size were fabricated using the spark plasma sintering technique. To determine the grain growth kinetics at elevated temperatures, a synchrotron wide-angle X-ray scattering (WAXS) study was performed in situ to measure the real-time grain size evolution based on the modified Williamson-Hall analysis. The unique grain growth kinetics of nanocrystalline UO 2 at 730 °C and 820 °C were observed and explained by the difference in mobility of various grain boundaries. [ABSTRACT FROM AUTHOR]

The role of sodium (Na) in cadmium telluride (CdTe) thin film photovoltaic absorbers deposited on a metal-coated substrate is investigated. Introducing Na during the growth of the structure influences the morphological and crystallographic properties of the CdTe layer grown by metal-organic chemical vapour deposition (MOCVD). It is observed that the introduction of Na between the metal and CdTe layers induces a slight randomisation via promotion of (400) and (220) orientations. It is shown that the inclusion of Na between the substrate and the Mo back metal contact enlarges the CdTe grains following a CdCl 2 treatment by 50% but weakens the adhesion to the substrate. The introduction of Na between the Mo back contact and CdTe layer promotes the formation of large faceted grains for the as-grown films with an average grain size ten times larger than in the case of Na free deposition while maintaining good adhesion to the substrate. There is no further grain growth following CdCl 2 treatment however the CdTe grain size is still double that of the Na-free samples. [ABSTRACT FROM AUTHOR]

Compact, pinhole-free and PbI 2 -free perovskite films, are desirable for high-performance perovskite solar cells (PSCs), especially if large columnar grains are obtained in which the adverse effects of grain boundaries will be minimized. However, the conventional solid-state reaction methods, originated from the two-step method, failed to grow columnar grains of CH 3 NH 3 PbI 3 in a facile way. Here, we demonstrate a strategy for growing large columnar grains of CH 3 NH 3 PbI 3 , by less-crystallized nanoporous PbI 2 (ln-PbI 2 ) film enhanced solid-state reaction method. We demonstrated columnar grains were obtainable only when ln-PbI 2 films were applied. Therefore, the replacement of compact PbI 2 by ln-PbI 2 in the solid-sate reaction, leads to higher power conversion efficiency, better reproducibility, better stability and less hysteresis. Furthermore, by systematically investigating the effects of annealing temperature and duration, we found that an annealing temperature ≥120 °C was also critical for growing columnar grains. With the optimal process, a champion efficiency of 16.4% was obtained and the average efficiency reached 14.2%. Finally, the mechanism of growing columnar grains was investigated, in which a V P b ″ -assisted hooping model was proposed. This work reveals the origins of grain growth in the solid-state reaction method, which will contribute to preparing high quality perovskite films with much larger columnar grains. [ABSTRACT FROM AUTHOR]

The thickness of the lamellar band (LB) structure that is generated in an accumulative roll bonded nickel sheet represents the degree of structure refinement. The LBs reach a saturation in thickness after several roll bonding cycles due to adiabatic heating that balances the dislocations generated during rolling. The thermal restoration process starts with the rearrangement of dislocations and boundary migration. At the initial stages of thermal restoration, LBs with all the orientations in the deformation texture grow by the so-called continuous recrystallization process. However, in the latter stages, growth becomes orientation selective and dominated by S and copper orientations. Finally, grain growth after recrystallization results in the preferred growth of Cube-oriented grains to the full layer thickness of the ARB sheet, thereby generating a strong Cube texture. Thus, grain growth yields a final texture similar to that generated in conventionally rolled and annealed nickel sheets. [ABSTRACT FROM AUTHOR]

The structure and properties of welded and additively manufactured alloys are affected by the microstructural evolution in the fusion zone (FZ) and heat affected zone (HAZ). The motion of the liquid pool and the interdependence of grain growth in both the solid and liquid regions are important in the evolution of the final grain structure. Previous investigations of microstructure evolution have been limited to either the HAZ or the FZ and in many cases in idealized isothermal systems. Here we report the evolution of grain structure and topology in three dimensions in both the FZ and the HAZ considering the motion of the liquid pool. Temporal and spatial distributions of temperature obtained from a well-tested heat transfer and liquid metal flow calculation are combined with Monte Carlo and topology calculations in a computationally efficient manner. The computed results are tested against independent experimental data for arc welding of an aluminum alloy. The average size of the columnar grains in the FZ and the equiaxed grains in the HAZ are shown to decrease with increasing scanning speed. For a given weld, the size and aspect ratio of the columnar grains in the longitudinal and horizontal planes are shown to decrease with distance from the weld interface. It is further shown that the grain size distributions and topological class distributions in the HAZ are largely unaffected by the temporal and spatial variations of the temperature created by different welding parameters. [ABSTRACT FROM AUTHOR]

Grain boundary (GB) solute segregation has been proposed as a route to mitigate grain growth in nanocrystalline (NC) metals and stabilize their grain structures. An interesting effect emerges in immiscible NC alloys due to the intertwined roles of GB segregation and bulk alloy phase separation. Based on a diffuse interface model, we examine grain growth dynamics in immiscible NC alloys, where both GB solute segregation and bulk phase separation act in conjunction. Analytical treatments identify regimes, where the reduction in GB energy due to segregation is significant. Simulation results reveal that microstructural evolution and enhanced stability are a manifestation of the competing effects of GB heat of segregation and that of bulk heat of mixing. More specifically, in systems with low GB segregation precipitation of solute-rich domains and associated GB (Zener) pinning effects lead to sluggish grain growth rates. In contrast, GB solute segregation plays a more dominant role as the heat of segregation increases in comparison with the bulk heat of mixing. On the whole, this modeling framework provides an avenue to explore the role of bulk alloy and interfacial effects on the microstructural evolution of NC metals. [ABSTRACT FROM AUTHOR]

A two-stage solution treatment composed of an initial soaking at 420 °C for 10 min and the second soaking at 500 °C for 10 min was applied to cold-worked AA6063 aluminum alloy samples after equal channel angular pressing (ECAP) for two and four passes. The microstructures and mechanical properties of the samples were compared with those of the samples after a routine one-stage solution treatment at 500 °C for 10 min. Abnormal grain growth (AGG) occurred to the samples during the one-stage solution treatment. However, no AGG was observed in the samples after the two-stage solution treatment. As a result of the prevention of AGG from occurring, the hardness, yield strength and ultimate tensile strength of the alloy after the two-stage solution treatment were significantly increased, while elongation to failure remained almost unchanged. [ABSTRACT FROM AUTHOR]

In the present study, the microstructure and texture evolution of pure magnesium during annealing was captured through ‘ex situ’ electron backscattered diffraction (EBSD). Pure magnesium was subjected to plastic deformation through CSM (Continuous Stiffness Measurement) indentation followed by annealing at 200 °C for 5 min and 30 min of soaking time respectively. The nucleation mechanism was attributed to the formation of sub-grains independent of the orientation of the grains. However, the growth of the grains was dependent on the difference in GOS values between the neighboring grains. When the difference in GOS values was more, growth of the grain with low GOS value was noticed during short term annealing. Otherwise, a strain free sub-grain was formed in the deformed microstructure which subsequently consumed the neighboring grain. A rotation of nearly 10–30° with respect to c-axis of parent grain was also noticed during the growth of the grains. Further, high mobility of contraction or double twin boundaries leading to grains > 10 μm were also observed during static recrystallization. [ABSTRACT FROM AUTHOR]

A new critical point termed ‘stabilisation stress’ ( σ stab ) is identified in high temperature stress-strain curves. σ stab represents the beginning of a stage in the hot working process where grain growth becomes the dominant microstructural phenomenon. This point is detected using an irreversible thermodynamics approach. Predicted correlations between σ stab and grain growth are experimentally validated for structural steels, enabling the delineation of dynamic recrystallization from grain growth based on stress-strain curves. [ABSTRACT FROM AUTHOR]

WC grains in WC–Co alloys are shaped as truncated triangular prisms, each with a basal plane of a (0001) truncated triangle and two {1 1 ̅ 00} prismatic planes, one narrower and one wider. This anisotropic shape is related to the polarity of the WC lattice, which comes from the difference in the spacing of the W and C layers in the 〈0001〉 projection. We directly determined the relationship between the two {1 1 ̅ 00} prismatic planes and the polarity of the WC lattice by using high-resolution scanning transmission electron microscopy (STEM). High-angle annular dark-field STEM and annular bright-field STEM revealed that the wider prismatic plane had W and C atomic layers with a shorter spacing, while the narrower plane had a longer spacing. The WC/Co interface on the wider prismatic side was atomically flat, while many steps formed on the surface of the narrower prismatic side. This difference in step formation at these interfaces could have come from the difference in how their W C bonds formed. [ABSTRACT FROM AUTHOR]

Grain boundary engineering (GBE) of nickel-based alloy 825 tubes was carried out with different cold drawing deformations by using a draw-bench on a factory production line and subsequent annealing at various temperatures. The microstructure evolution of alloy 825 during thermal-mechanical processing (TMP) was characterized by means of the electron backscatter diffraction (EBSD) technique to study the TMP effects on the grain boundary network and the evolution of grain boundary character distributions during high temperature annealing. The results showed that the proportion of ∑ 3 n coincidence site lattice (CSL) boundaries of alloy 825 tubes could be increased to > 75% by the TMP of 5% cold drawing and subsequent annealing at 1050 °C for 10 min. The microstructures of the partially recrystallized samples and the fully recrystallized samples suggested that the proportion of low ∑ CSL grain boundaries depended on the annealing time. The frequency of low ∑ CSL grain boundaries increases rapidly with increasing annealing time associating with the formation of large-size highly-twinned grains-cluster microstructure during recrystallization. However, upon further increasing annealing time, the frequency of low ∑ CSL grain boundaries decreased markedly during grain growth. So it is concluded that grain boundary engineering is achieved through recrystallization rather than grain growth. [ABSTRACT FROM AUTHOR]

The effect of scandium on the microstructure and texture of hot rolled and annealed binary Mg-Sc alloys has been studied. Hot rolled alloys develop weaker and more random basal texture than the corresponding texture of pure Mg after undergoing the same thermomechanical processing. The increase of Sc concentration leads to a systematic weakening of the texture, however texture randomization is not as prominent as in other Mg-RE alloys. The annealed texture of Mg-Sc alloys inherits features of the deformed texture, showing moderately weaker intensities of dominant components. The texture weakening is attributed to the heterogeneous deformation and nucleation of randomly oriented grains at micro shear bands. The microstructure and grain growth analysis suggest that Sc atoms have a low tendency to migrate towards grain boundaries and impose minimal drag effect. As a result, the grain growth is determined by grain boundary self-diffusion with an activation energy of ∼82 ± 8 kJ/mol. The results confirm the important role of solute segregation to grain boundaries in the development of annealed texture in Mg-based alloys. [ABSTRACT FROM AUTHOR]

Ultrafine grain tungsten is a promising candidate for plasma facing component applications in future fusion reactors. The conventional press-and-sinter powder metallurgy method is a preferred route for producing ultrafine grain tungsten from nanosize tungsten powders. However, controlling grain growth, while simultaneously attempting to achieve maximum densification, has been a challenge for making UGF tungsten by sintering. In this study, the effect of processing parameters on the densification and grain growth of nano-W powders was investigated. Near-fully densified tungsten was obtained at sintering temperatures between 1100 and 1300 °C, and both Ar and H 2 sintering atmospheres were investigated. The Ar sintering atmosphere was determined to more favorably promote densification and minimize grain growth. The nanosized tungsten powder compacts were subjected to reduction in H 2 as a part of the sintering cycle. The reduction temperature was found to have significant effects on the sintering of nano-W powder, primarily as a result of grain coarsening, which was seen at temperatures as low as 700 °C. [ABSTRACT FROM AUTHOR]

Important statistical descriptors of grain boundary networks are misorientation distribution functions (often referred to as grain boundary character distributions), which show relative measures of interfaces with given misorientation parameters. This paper is aimed at extending the Boltzmann-type kinetic modeling framework developed in the previous work (Yegorov et al., 2016) to the two-dimensional case of general textures with no restrictions on possible grain orientations, but under the assumption that the grain boundary energy density depends only on the disorientation variable. In order to avoid an enormous computational complexity of treating the general-texture case, the kinetic model for polycrystalline grain growth is constructed from the very beginning in a specific discretized form with respect to the disorientation variable. A significant aspect of the developed approach is that its formulation requires a collection of a priori distributions which describe possible disorientations of grain boundaries connected in a triple junction. A separate numerical algorithm for approximating such distributions is proposed as applied to cubic crystal lattices. The numerical results obtained by the constructed model are in good agreement with the corresponding large-scale simulation results from the related literature. [ABSTRACT FROM AUTHOR]

A detailed investigation was carried out to study the effect of volume fraction on microstructure evolution in segregation-free isotropic volume-conserved two-phase alloys. Large scale two-dimensional (2D) and three-dimensional (3D) simulations were performed for 10/90, 30/70 and 50/50 alloys with a modified quantitative phase-field model. The grain growth in the two-phase alloys was consistent with volume-diffusion controlled growth with growth exponent m ∼ 3 . The kinetic coefficient, mean grain size, mean number of faces, grain size distribution (GSD), grain topology distribution (GTD) and grain size/topological class relation were all influenced by the volume fraction of the minor phase ( α ) in the two-phase alloys. In the 10/90 alloy, a bimodal distribution in the GSD and GTD for the 2D total grain structure was found. The timelines of evolution of major-phase grains in 3D simulations for all alloys were examined. ‘Particle-like’ grain migration of isolated α -phase grains was observed in the 2D simulations. The formation and evolution of non-constant curvatures on interphase boundaries in the isotropic volume-conserved two-phase alloys is discussed. The microstructure evolution in the volume-conserved two-phase alloys was compared with that in non-conserved two-phase alloys ( m ∼ 2 ). Simulation results are found to be in good agreement with a wide range of previous experimental and computer simulation results. [ABSTRACT FROM AUTHOR]

Historically, metallography has been the two-dimensional characterization of materials microstructures by optical microscopy. Consequential problems have long been known: A two-dimensional section through a three-dimensional object gives us only a very poor idea about size and form of the object. Therefore, many attempts have been made to gain three-dimensional information experimentally. Nevertheless, in simulations and also analytical theories thin films are commonly still treated as two-dimensional objects making comparisons with 3D experimental data rather hard. In the present paper, grain growth in thin films is investigated for the first time by three-dimensional Potts model simulations focusing particularly on the transition from bulk-like growth to columnar microstructures. [ABSTRACT FROM AUTHOR]

The synergetic effects SiC particles and short carbon fibers (C sf ) as well as hot pressing parameters (sintering temperature, dwell time and applied pressure) on the grain growth of ZrB 2 -based composites were investigated. Taguchi methodology was employed for the design of experiments to study the microstructure and grain growth of ZrB 2 –SiC–C sf ceramic composites. Three hot pressing parameters and SiC/C sf ratio were selected as the scrutinized variables. The sintering temperature and SiC/C sf ratio were identified by ANOVA as the most effective variables on the gain growth of ZrB 2 -based samples. Removal of oxide impurities from the surface of starting particles by the reactant C sf , not only hindered the extraordinary grain growth of ZrB 2 matrix, but also improved the sinterability of the ceramics. A fully dense ceramic with an average grain size of 8.3 µm was obtained by hot pressing at 1850 °C for 30 min under 16 MPa through adding 20 vol% SiC and 10 vol% C sf to the ZrB 2 matrix. SEM observations and EDS analysis verified the in-situ formation of ZrC which can restrain the growth of ZrB 2 particles, similar to the role of SiC, by the pinning of grain boundaries as another stationary secondary phase. [ABSTRACT FROM AUTHOR]