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Normal grain growth (NGG) of a (111) textured Ni film on c-sapphire and abnormal grain growth (AGG) of (100) grains at the expense of this (111) texture has been studied as a function of temperature with and without a capping layer. The grain boundaries (GBs) in the Ni film are controlled by the preferred orientation relationships (ORs) adopted by the Ni grains on the sapphire substrate. The 2 variants of a single OR, Ni(111)< 1 1 ¯ 0 >//Al 2 O 3 (0001)< 1 1 ¯ 00 >, form a (111) mazed bicrystal with Σ3 GBs. The (100) grains have a single OR, Ni(100)<010>//Al 2 O 3 (0001)< 1 1 ¯ 00 > with 3 variants; their GBs within the (111) grains have the (111)< 1 1 ¯ 0 >//(100)<010> misorientation. (100) AGG within the (111) mazed bicrystal of the 100 nm Ni film takes place above 1023 K. The orientation transition is driven by the biaxial elastic modulus anisotropy which favors the growth of (100) grains over (111) grains, as this reduces the elastic strain energy induced by the thermal mismatch between Ni and sapphire. (100) AGG is suppressed and the NGG of the (111) texture is slowed down when the film is covered by a 10 nm amorphous alumina layer aimed at inhibiting surface diffusion. Thus, it is proposed that as long as the surface can act as a sink for the point defects diffusing along the GBs, the movement of the GBs is correlated to the diffusivity of atoms and vacancies, which is a function of their misorientation and crystallographic GB structure. [Display omitted] [ABSTRACT FROM AUTHOR]

Grain growth in polycrystalline materials can be impeded by grain boundary (GB) stagnation. Using atomistic simulations, we observed that during GB migration, the disruptive jumps of a few GB atoms can disturb the original ordered collective movement of GB atoms, leading to the stagnation of the entire GB. These disruptive atomic jumps can be activated by both high driving forces and high temperatures, with even jumps of a few atoms capable of causing the stagnation of an entire GB. This mechanism also explains the non-Arrhenius behavior observed in some GBs. Additionally, a large model size could increase the rate of disruptive atomic jumps, and a clear transition in thermal behavior is observed with the increase of the GB size in GBs exhibiting clear thermally activated stagnation. Our further investigation shows that the disruptive atoms involved in these jumps do not differ from other GB atoms in terms of atomic energy, volume, density, local entropy, or Voronoi tessellation, and no "jam transition" was observed in the energy barrier spectra. This fact makes those disruptive jumps challenging to detect. To address this issue, we propose a displacement vector analysis method that effectively identifies these subtle disruptive jumps. [Display omitted] [ABSTRACT FROM AUTHOR]

Zirconium alloys are widely used in nuclear reactors as fuel cladding materials. Fuel cladding is used to contain the nuclear fuel and cladding tubes are typically sealed using welds. Welding of zirconium alloys can result in changes in the local microstructure, with the potential to grow so called 'blocky α' grains in the welded region during subsequent thermal processing and these blocky α grains have the potential to be detrimental to the integrity of the component. In this work, complimentary heating experiments with ex situ and in situ electron backscatter diffraction (EBSD) analysis are used to aid understanding of the blocky α grain growth within the weld region. These experiments reveal that blocky α grain growth is related to the prior-β (high temperature) microstructure, as grains grow adjacent to a prior-β grain boundary and the orientation of the growing α grain can be explained using neighbourhood orientations from this prior-β grain. This growth mechanism is explained via a simple mechanism which is related to the α grain orientations, grain boundary structures and local stored energy. Ultimately, our findings indicate that the likely grain growth (size and morphology) across the weld region can now be predicted from the initial as-welded microstructure. [Display omitted] [ABSTRACT FROM AUTHOR]

Grain boundaries play a significant role during deformation and can exhibit both beneficial and adverse effects on deformation behaviour during irradiation. For instance, macroscopic irradiation growth is increased by the presence of a high density of grain boundaries. Grain boundaries can act as sinks for point defects and barriers to dislocation motion. Thus, a mechanistic assessment of the impact of grain boundaries is essential for better understanding irradiation-induced deformation. The interplay between irradiation-induced microstructure and mechanical properties has been an area of research for many decades, but many uncertainties remain. In the present work, the deformation fields adjacent to grain boundaries in pure zirconium are investigated by using high angular resolution electron backscatter diffraction (HR-EBSD) after a period of irradiation growth. It is observed that the extent of grain boundary deformation is associated with the crystallographic orientation difference between adjacent grains. High-angle grain boundaries function as a deformation constraint, whereas a significant grain boundary migration occurs at some low-angle grain boundaries, which likely act as an active diffusion path for the irradiation-induced point defects. In addition, the change in residual strain fields associated with the irradiation damage is quantified with respect to a non-irradiated reference. A net change in the state of residual elastic strain in the order of ∼ 10 − 3 has been estimated for ∼8 dpa proton irradiation. Furthermore, the origin of residual elastic strain and lattice misorientation concentration along (0002) traces are discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

There has long been a question concerning the existence of a single vs. multiple asymptotic grain size distributions in normal grain growth. The goal of this study has been to address this topic by investigating the underlying mechanisms by which grain face and volume distributions evolve from transient to asymptotic states during grain growth. This investigation employs 3D Monte Carlo simulations to quantify the topological mechanisms of this transition, i.e., grain separation, encounter and loss, and examine their mutual evolution with the distributions from a wide range of initial states towards their multiple final asymptotic states. In the initial transient growth stage, the topological event rates fluctuate markedly, seeking balance among themselves and with the face/volume distributions. In the asymptotic stage this balance is indicated by asymptotic behavior of the ratios of the rates, as well the mean face class 〈 f〉 , and normalized widths, CV , of the face and size distributions. It was found that a moderate range of initial distributions evolved through relatively short transient periods towards asymptotic states less spread from each other but with those same relative orders of width. Initial distributions much wider or narrower than these evolved through longer transient periods with trajectories crossing the paths of the more moderate ones, with the widest initial distributions becoming the narrowest and the initially narrowest becoming the widest. These behaviors are explained by their interactive evolution with the topological event rates. Topological event rate ratios indicate the transient and asymptotic growth stages of grain growth [Display omitted] [ABSTRACT FROM AUTHOR]

High entropy perovskite oxides (HEPOs) have garnered significant attention due to their superior performance compared to conventional perovskite oxides. However, there is a limited understanding of the sintering behavior of HEPOs. This study aims to obtain a mechanistic understanding of how the sintering of the manganite-based perovskite oxides varies with the mixed entropy. Based on systematic sintering experiments, detailed microstructural characterization, and molecular dynamics simulations, we found that the densification process of manganite-based perovskite oxides with increasing mixed entropies follows a similar pattern to conventional perovskite oxides. The mixed entropy significantly influences the final and middle stages of densification but has negligible effects in the initial stage. More importantly, the densification becomes easier and faster with the increased mixed entropy, while the grain growth is delayed to the higher relative density and the corresponding activation energy for grain growth is also significantly enhanced. Consequently, this creates a more ideal situation for achieving denser materials with smaller grain sizes during sintering, shedding light on rational optimization of the sintering and resulting properties of high entropy perovskite oxide. [Display omitted] [ABSTRACT FROM AUTHOR]

The solute drag effect of phosphorus (P) in the single-phase austenite (γ-Fe) region was studied under isothermal annealing conditions at temperatures of 1050 °C, 1150 °C, 1250 °C and 1350 °C. High-temperature laser scanning confocal microscopy (HT-LSCM) was used to observe and quantify in situ grain growth on the surface of three different samples containing 0.026 wt.-% P, 0.044 wt.-% P and 0.102 wt.-% P. The dependence of the arithmetic mean grain sizes on time and temperature were modeled mathematically using a simple ordinary differential equation (ODE) according to normal grain growth (NGG) theory. As no other major effects, i.e., Zener-pinning by precipitates, occurred under the selected experimental conditions, grain growth interference was only considered by grain boundary (GB) segregation of P. Thus, the total grain boundary mobility (M) was directly determined depending on the P content and isothermal annealing temperature. The fitted GB mobility values enabled the determination of an average binding energy value between impurity P atoms and grain boundaries (E 0 = -1.1 - -0.6 eV) in the system. Finally, the GB segregation of P in γ-Fe was derived from the observed grain growth kinetics. The results showed reasonable agreement with calculations using segregation enthalpies from the literature. [Display omitted] [ABSTRACT FROM AUTHOR]

The low strength of soft magnetic alloy limits the ability to operate high speed rotors in aero-generators. Strengthening of soft magnetic alloys remains a challenge, as most methods that enhance strength lead to the deterioration of magnetic properties. In this work, a rolled strip of FeCo-1.5V0.5Nb0.4W (wt.%) alloy with high yield strength and low coercivity was developed. We demonstrated that minor additions of Nb and W has a little effect on the magnetic properties. Due to the microalloying has a negligible effect on magnetocrystalline anisotropy, and low content of the secondary phase. Additionally, solute atoms are enriched at the grain boundaries, which reduces the grain boundary energy and promotes the nucleation and growth of Laves-Co 2 Nb phase at the grain boundary by grain boundary diffusion. These lead to almost stagnation of grain growth during isothermal annealing at 1123 K for 3 h and stabilization at ∼6 μm. Therefore, based on the reduction of dislocation density by high-temperature annealing to restore the magnetic properties of the alloy, fine-grain strengthening coupled with solid solution strengthening, and a certain amount of Orowan bypass effect and dislocation strengthening contribution, provide for the high strength of the alloy. The yield strength of the rolled strip reached 742 MPa under the conditions of no significant decrease in the saturation magnetization of 2.39 T and an extremely low coercivity of 152 A/m. These results are of great significance for understanding the relationship between the strengthening mechanisms of metallic materials and further developing high-performance FeCo-based soft magnetic alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

A theory is formulated that relates the rates of the three topological events to the evolution of the face distribution function. Implementation of the theory requires introduction of the concept of participation probabilities which reflect the face distribution of those grains that are incident upon the events. The prediction of the evolving face distribution is tested with a 3-D Monte Carlo simulation of the grain growth process that permits evaluation of the topological event rates by face class and the participation probabilities. The theory also develops a relation between the average number of faces per grain and the topological rates as well as the asymptotic distribution function; these results are also tested using the simulation. The transient period prior to normal grain growth is explained in terms of the evolution of the event rates that control the creation of small grains and evolve the face distribution to its final shape. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The phase-field method is increasingly used for studying grain growth in metallic alloys. Such an approach offers a thermodynamically consistent framework for studying microstructure evolution during grain growth without the need to explicitly track the interface positions. However, the numerical solution of the models requires a grid spacing much smaller than the interface width. This leads to computationally very intensive simulations, especially when a large number of grains is required to accurately measure statistical quantities. The recently proposed S-PFM approach provides a new inherently discrete formulation of phase field models where the interface width can be as small as the grid spacing, thus drastically improving the numerical performances of the method. Here, we show that this approach can be extended to a multi-phase field model for ideal grain growth. Then, we perform two dimensional simulations to analyse in detail the kinetics of both grain boundaries and triple junctions. We compare our model to a classical phase field formulation and we demonstrate that, for a prescribed accuracy, the memory requirement and simulation times are both reduced by a factor of 4 D , where D is the space dimension. Finally, we perform a large-scale simulation of grain growth in two dimensions and show that the method is able to reveal the specific shape of the grain size distribution in the scale-invariant regime displaying two peaks around the mean grain size and quantitative measurements of the underlying topological classes are performed. [ABSTRACT FROM AUTHOR]

Non-metallic contamination is a practically unavoidable byproduct of commercial synthesis processes; however, non-metallic impurities are often overlooked when considering material design and performance of nanostructured materials. Importantly, there is disputing evidence as to whether non-metallic contamination stabilize or destabilize nanocrystalline materials against grain growth. Furthermore, it is unclear if non-metallic contamination has a significant impact on hardness of nanocrystalline systems. In this work, two nanocrystalline Ni-28at%W alloys with different impurity concentrations were produced via mechanical alloying to directly evaluate the effect of contamination on thermal stability and hardness of nanostructured materials. The alloys were isothermally annealed at several temperatures, and the microstructures were compared by applying atom probe tomography and aberration-corrected scanning transmission electron microscopy. It was determined that impurity concentrations can be substantially reduced by using specific milling media and pre-milling reduction processes. Furthermore, the clean and contaminated alloys exhibited similar thermal stabilities against grain growth, but the clean alloy displayed a 100% improvement in hardness despite a similar grain size and distribution of second phases. Impurity phases including CrOx, Ni 6 W 6 C, and trapped Ar pores were also identified and likely contributed to the thermal stability and mechanical properties observed in this study. Overall, this work suggests that impurities my not always be detrimental to thermomechanical behavior of nanocrystalline systems. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We have investigated the microstructural and crystallographic evolution of nanocrystalline zirconia under heavy ion irradiation using in-situ transmission electron microscopy (TEM) and have studied the atomic configurations of defect clusters using aberration-corrected scanning transmission electron microscopy (STEM). Under heavy ion irradiation the monoclinic-ZrO 2 is observed to transform into cubic phase, stabilised by the strain induced by irradiation-induced defect clusters. We suggest that the monoclinic-to-cubic transformation is martensitic in nature with an orientation relationship identified to be (100) m ∥(100) c and [001] m ∥[001] c. By increasing the damage dose, both the formation of voids and irradiation-induced grain growth were observed. A model for the formation of voids is proposed, taking defect interactions into consideration. The study has also demonstrated that high resolution orientation mapping by transmission Kikuchi diffraction (TKD) combined with in-situ irradiation in a TEM is a powerful method to probe the mechanisms controlling irradiation-induced processes, including grain boundary migration, phase transformations and texture evolution. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Applying pressure during sintering of ceramics is a common practice, allowing for significant reduction in processing temperature, and fabrication of dense specimens with fine-grained microstructures. However, sintering under relatively high pressure can accentuate stress-related phenomena that occur during the final stage of sintering. The present study describes transparent alumina microstructure evolution during the dwell period of high-pressure spark plasma sintering (HP-SPS) at 1100 °C under 500 MPa. It was observed that the grain size, pore morphology and transparency changed rapidly with dwell time and varied between the center and periphery of the samples. Microstructural evidence suggests that microstructure evolution is related to creep during the dwell period under relatively high pressure and low temperature. Temperature measurements in the HP-SPS setup revealed a small temperature gradient (Δ T =~10 °C) in the opposite direction to the grain size gradient. Based on grain growth kinetics at different regions, we propose that a stress gradient exists and the stress at the sample center is roughly twice as high than at the periphery. Thus, the applied pressure is the dominant factor governing densification and grain growth and is responsible for non-uniform microstructure evolution during HP-SPS. Furthermore, analysis of optical in-line transmittance allowed to decouple the scattering effects of porosity and birefringence. The analysis revealed significant pore growth between dwell time of 5 to 15 min, though, birefringence remained the main scattering source. Understanding these aspects of pressure-assisted sintering will help gain better control of final microstructures, which determine physical, mechanical, and optical properties of ceramics. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Tailoring microstructures by optimizing fabrication or forming processes is a challenge for metal industries. However, predicting microstructure evolution implies to develop models at the scale of the polycrystal, which is incompatible with large scale simulations of processes. In this context, we propose an energetic upscaling strategy to model anisotropic grain growth at large scale without loosing detailed grains statistics. Thus, a fast mesoscopic model is necessary to establish a large database of computations in order to develop a macroscopic model whose state variables contain statistical descriptors of the microstructure. This paper focuses on a fast mesoscopic model based on Voronoi-Laguerre tessellations, which are updated at each time step to capture grain growth. Several energetic contributions are considered at different scales. The grain boundary energy is obtained as a function of misorientation from molecular dynamics, and the dissipated power is obtained from crystal plasticity theory. The evolution law at the mesoscopic scale is obtained by considering all energetic contributions in the representative volume element, and from thermodynamic laws and approximate mass conservation. This upscaling approach reaches short computation time, which is essential to establish the database underlying the macroscopic model. Basic grain statistics are validated by comparison to classical models. Moreover, a good agreement is observed with an experiment conducted on pure iron. The model is then used to analyze the evolution of detailed statistics. To capture grain growth at macroscopic scale, it is necessary to consider couplings between means and standard deviations of various distributions (e.g., size, shape, misorientation etc.) [ABSTRACT FROM AUTHOR]

Secondary-phase particles are routinely dispersed in metals and ceramics to prevent grain growth and take full advantage of the small grain size in the mechanical properties of polycrystals. Somewhat surprisingly, the preferential or abnormal growth of a few grains is observed in particle-containing systems at relatively high temperature, which will limit the lifetime of the material. The origins and mechanisms of particle-assisted abnormal grain growth are widely contested. Here, we employ integrated three-dimensional X-ray imaging to throw new light on the complex interactions between grain boundaries and particles in an Al–Cu alloy as a model system. We observe abnormal grain growth in the presence of a highly non-random distribution of particles. The incipient grain size is set by the local distribution of particles such that the larger grains come from particle-poor regions. Subsequently, grains with a size advantage may "run away" from the grain size distribution, in agreement with predictions from an analytical model that takes into account the competing capillary and particle pinning pressures. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Determining a process–structure–property relationship is the holy grail of materials science, where both computational prediction in the forward direction and materials design in the inverse direction are essential. Problems in materials design are often considered in the context of process–property linkage by bypassing the materials structure, or in the context of structure–property linkage as in microstructure-sensitive design problems. However, there is a lack of research effort in studying materials design problems in the context of process–structure linkage, which has a great implication in reverse engineering. In this work, given a target microstructure, we propose an active learning high-throughput microstructure calibration framework to derive a set of processing parameters, which can produce an optimal microstructure that is statistically equivalent to the target microstructure. The proposed framework is formulated as a noisy multi-objective optimization problem, where each objective function measures a deterministic or statistical difference of the same microstructure descriptor between a candidate microstructure and a target microstructure. Furthermore, to significantly reduce the physical waiting wall-time, we enable the high-throughput feature of the microstructure calibration framework by adopting an asynchronously parallel Bayesian optimization to exploit high-performance computing resources. Case studies in additive manufacturing and grain growth are used to demonstrate the applicability of the proposed framework, where kinetic Monte Carlo (kMC) simulation is used as a forward predictive model, such that for a given target microstructure, the target processing parameters that produced this microstructure are successfully recovered. [ABSTRACT FROM AUTHOR]

Pure formamidinium lead triiodide (α-FAPbI 3) organic-inorganic halide perovskite (OIHP) semiconductor is very attractive for use as light absorber in the new thin-film perovskite solar cells (PSCs) technology. This is primarily because of its superior thermal stability, more suitable bandgap, and compositional simplicity. However, the existence of the photo-inactive non-perovskite δ-FAPbI 3 polymorph ('yellow' phase) is a major hurdle in the path towards the development of α-FAPbI 3 -based PSCs. Also, there is general consensus that the fine-grained nature of OIHP thin films is detrimental to the environmental stability and performance of the resulting PSCs. In this context, here we take advantage of the polymorphism in FAPbI 3 , and use solvent-vapor-assisted δ-to-α phase transformation to induce exceptional grain coarsening (up to 50-fold) in 0.3-μm thickness FAPbI 3 thin films, resulting in an unprecedented average grain size of up to ~9 μm. The underlying mechanisms are elucidated based on the results from a combination of some key experiments, which involve studying systematically the effects of time, temperature, initial grain size, and solvent polarity index (PI). The ultra-coarse-grained α-FAPbI 3 thin films show dramatically improved environmental stability over their medium-grained counterparts, which is explained based on grain-boundary density arguments. PSCs made using the ultra-coarse-grained α-FAPbI 3 thin films have improved photovoltaic (PV) performance, but it is somewhat modest. This is attributed to the underestimation of the effective grain size relevant to photocarrier dynamics. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The equilibrium crystal shape is a convex shape bound by the lowest energy interfaces. In many polycrystalline microstructures created by grain growth, the observed distribution of grain boundary planes appears to be dominated at low driving forces (after long grain growth times) by the planes present in the equilibrium crystal shape. However, at earlier stages of grain growth, it is expected that kinetic effects will play an important role in grain boundary motion and morphology. Analogous to the equilibrium crystal shape, the kinetic crystal shape of seed crystals growing from a liquid at higher supersaturations is bound by the slowest growing orientations. This study presents an equivalent construction for grain boundaries in polycrystals and uses it to determine the kinetic crystal shape for strontium titanate as a function of temperature. Relative grain boundary mobilities for strontium titanate for the low energy crystallographic orientations from seeded polycrystal experiments are used to calculate the kinetic crystal shapes as a function of temperature and annealing atmosphere. The kinetic crystal shapes are then compared to the morphologies and orientations of the interfaces of the growing seed crystals, and to the equilibrium crystal shapes, as well. The conclusions are that (1) the kinetic crystal shape is extremely anisotropic and displays significant transitions as a function of temperature that do not mirror changes in equilibrium crystal shape, (2) the kinetic shapes observed in the microstructures are dominated by the growing side of the interface (single crystal) and not by the dissolving side (polycrystalline matrix), and (3) faster growing orientations break up into macroscopic facets composed of slower growing orientations. The implications for grain growth underscore the applicability of crystal growth models to grain growth in polycrystals. In particular, in strontium titanate, the anisotropy of the grain boundary mobility as represented in the kinetic crystal shape is expected to be reduced from five macroscopic parameters to two (interface normal) allowing for incorporation of growth rate anisotropy in simulations of microstructure evolution at the earliest stages of grain growth, i.e. at the highest driving forces. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Most metals, ceramics, semiconductors and rocks are composed of small crystals known as grains. When annealed, this polycrystalline structure coarsens, thus allowing the properties of a material to be tailored for a particular application. The mobility of grain boundaries is thought to be determined by the crystallography of the adjacent crystals, but experimental validation in bulk polycrystalline materials is lacking. Here we developed a novel fitting methodology by direct comparison of a time-resolved three-dimensional experimental data to simulations of the evolution of 1501 grains in iron. The comparison allows reduced mobilities of 1619 grain boundaries to be determined simultaneously. We find that the reduced mobilities vary by three orders of magnitude and in general exhibit no correlation with the boundary's five macroscopic degrees of freedom, implying that grain growth is governed by other factors. [ABSTRACT FROM AUTHOR]

The role of inclination dependence of grain boundary energy on the microstructure evolution and the orientation distribution of grain boundary planes during grain growth in polycrystalline materials is investigated by three-dimensional phase-field simulations. The anisotropic grain boundary energy model uses the description of the faceted surface structure of the individual crystals and constructs an anisotropic energy of solid-solid interface. The energy minimization occurs by the faceting of the grain boundary due to inclination dependence of the grain boundary energy. The simulation results for a single grain show the development of equilibrium shapes (faceted grain morphologies) with different families of facets which agrees well with the theoretical predictions. The results of grain growth simulations with isotropic and anisotropic grain boundary energy for cubic symmetry show that inclination dependence of grain boundary energy has a significant influence on the grain boundary migration, grain growth kinetics and the grain boundary plane distribution. It has been shown that the model essentially reproduces the experimental studies reported for NaCl and MgO polycrystalline systems where the anisotropic distribution of grain boundary planes has a peak for the low-index {100} type boundaries. [ABSTRACT FROM AUTHOR]

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

Attaining high densification without grain growth is one of the main objectives of the sintering optimization in ceramic materials. For dental implant applications, achieving this objective has a decisive impact on the mechanical resistance, the duration and the translucency of the implant. To improve these sintering outcomes a long experimental explorative study is generally required. In this work, we developed a combined experimental/modeling approach allowing a rapid identification of the optimal sintering conditions. The determination of the model densification and grain growth kinetic constitutive parameters has been done experimentally. We found that the sintering/grain growth kinetics have a detrimental acceleration above a critical temperature level. The pressure-less sintering model able to predict the sintering stress, powder densification and grain growth has been used for the determination of the optimal sintering trajectory. We utilized the two step sintering method to approach the critical temperature without an undesirable grain growth. We obtained translucent sintered specimens with a very limited grain growth. Image, graphical abstract • Densification and grain growth kinetics' identification for free sintering. • Sintering modeling accounting for grain growth. • Optimized sintering cycles for highly dense and fine grain ceramics. [ABSTRACT FROM AUTHOR]

In this work, we present a physically-parameterized microstructure evolution model for the Yttria-doped alumina system. Yttria-doped alumina is a well-known ceramic system which undergoes first-order phase-like transitions at grain boundaries, which can radically alter interface properties. The change in interfacial properties in turn can radically change microstructure outcomes during processing, including the induction of abnormal grain growth modes. In this work, we develop a simulation that evolves alumina microstructure as a function of yttria concentration and temperature. In the window studied, we achieve strong agreement with reviewed experimental results in identifying the windows for large grains, small grains, abnormal grain growth, and complexion transition kinetics (as measured by JMAK analysis). We then apply the model to study and demonstrate how the possible inclusion of second-phase particles or uneven solute distribution profiles will impact microstructure evolution. It is found that particles do not significantly affect abnormal grain growth in the window studied (but do lead to reduced grain size through pinning effects). It is found that even modest amounts of solute inhomogeneity will result in substantial changes in microstructure outcomes, frequently leading to clusters of abnormal grains. This model largely corroborates the expectations and hypotheses made from recent experimental studies in oxide-doped alumina systems. Further, it is found that there exists a peak transition fraction for the system at which abnormal grain size tends to be maximized. Image, graphical abstract [ABSTRACT FROM AUTHOR]

With high resistance against interdiffusion, electromigration, creep and fatigue, single-crystal films of refractory metals are highly desired for applications in opto- and micro-electronic devices. Their fabrication is nevertheless very challenging, primarily due to their high melting points. Requiring no external thermal input, ion-irradiation presents an alternative route for single-crystal films by converting from their polycrystalline counterparts through irradiation-induced selective grain growth. The incomplete poly- to single-crystal conversion is however a common problem which limits the application of this method. Here we use refractory W thin films, the element with the highest melting point, as a model system. We systematically investigate the influences of film microstructure (texture and grain size) and irradiation temperature (77 K and room temperature) on the evolution of selective grain growth under 4.5 MeV Au+ ion-irradiation. We find that the driving force for selective grain growth can be increased by narrowing the texture spread of the films, refining the grain size and decreasing the irradiation temperature. Following this guidance, a complete conversion of a polycrystalline W film into a single crystal is achieved. This study therefore provides insights into the mechanism of selective grain growth and proves that it is an effective technique for microstructure engineering in thin film materials. The concepts can be applied to all crystalline metal films. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The challenge of producing dense ultrafine-grained (UFG) tungsten is hereby addressed by a simple pressureless two-step sintering method. It provides a uniform microstructure with ~99% theoretical density and ~700 nm grain size, which is among the best sintering practice of pure tungsten reported in the literature. Benefitting from the finer and more uniform microstructures, two-step sintered samples show better mechanical properties in bending strength and hardness. While parabolic grain growth kinetics is verified, a transition in the nominal grain boundary mobility was observed at 1400 °C, above which the effective activation enthalpy is ~6.1 eV, below which grain boundary motion is rapidly frozen with unusually large activation enthalpy of ~12.9 eV. Such highly nonlinear behavior in activation parameters with respect to temperature suggests that activation entropy and maybe collective behavior play a role in grain growth. We believe the as-reported two-step sintering method should also be applicable to other refractory metals and alloys, and may be generalized to multi-step or continuous-cooling sintering design using machine learning. A simple pressureless two-step sintering method has been successfully applied to pure tungsten, producing dense ultrafine-grained samples with uniform microstructure and improved mechanical properties. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Nanocrystalline metals are promising radiation tolerant materials due to their large interfacial volume fraction, but irradiation-induced grain growth can eventually degrade any improvement in radiation tolerance. Therefore, methods to limit grain growth and simultaneously improve the radiation tolerance of nanocrystalline metals are needed. Amorphous intergranular films are unique grain boundary structures that are predicted to have improved sink efficiencies due to their increased thickness and amorphous structure, while also improving grain size stability. In this study, ball milled nanocrystalline Cu-Zr alloys are heat treated to either have only ordered grain boundaries or to contain amorphous intergranular films distributed within the grain boundary network, and are then subjected to in situ transmission electron microscopy irradiation and ex situ irradiation. Differences in defect density and grain growth due to grain boundary complexion type are then investigated. When amorphous intergranular films are incorporated within the material, fewer and smaller defect clusters are observed while grain growth is also limited, leading to nanocrystalline alloys with improved radiation tolerance. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Densification of ZnO-PTFE (polytetrafluoroethylene) composites is permitted by the Cold Sintering Process, having no effect on the stability of both materials. Highly dense samples can be obtained by this technique at extremely low temperatures in just a few minutes. Interestingly, the obtained samples show an anisotropy impacting: crystalline, microstructural and electrical properties. While the Wurztite ZnO crystals show a preferential growth along (00l) direction, microstructure observations show a grain growth along the in-plane (perpendicular to pressure application direction) up to 240%. Electrical conductivity is also influenced and is related to microstructure. In this situation, the addition of PTFE insulating phase allows to increase the conductivity in plane compared to the pure cold sintered ZnO sample. A mechanism is proposed to explain this phenomenon which involves PTFE transient distribution competing with the transient liquid driving densification and grain growth associated with cold sintering. This is further confirmed by the observation of a curvature of microstructure direction while approaching die edges. These observations offer a large variety of designs for further orientation driven properties. [ABSTRACT FROM AUTHOR]

We employ atomistic computer simulations to study the structure and migration behavior of a Σ11 symmetrical tilt grain boundary in a 4-component model FCC high-entropy alloy (HEA) (CuNiCoFe). The results are compared to grain boundaries in elemental metals and a so-called 'average-atom' sample. We find that the repeating structural units characterizing the static grain boundary structure show the same repeating structural units for all samples, while the high temperature equilibrium grain boundary structure is most strongly influenced by presence of stacking faults. Under an applied synthetic driving force, this GB migrates by a mechanism assisted by partial dislocations in all materials. For this reason the grain boundary mobilities and stacking fault energies are directly related. Moreover, the HEA sample and the average-atom sample show almost identical mobilities suggesting that local chemical fluctuations play a minor role. Solute segregation to the GB in the HEA suppresses GB migration up to very high temperatures and might be the main cause for reduced grain growth in FCC HEAs. [ABSTRACT FROM AUTHOR]

The top surface of a crystal–melt interface was observed directly during directional solidification of polycrystalline GaSb at a constant cooling rate, and the effect of {111}Σ3 twin boundary formation on the rate of grain growth parallel to the 〈111〉 direction was investigated. A {111}Σ3 twin boundary was generated by the decomposition of another {111}Σ3 twin boundary with formation of a Σ9 grain boundary. The growth of the crystal–melt interface in the direction parallel to the <111>B direction was stalled during the nucleation and propagation of the new {111}Σ3 twin boundary. The growth rate of the crystal–melt interface after twin formation was approximately half of that before. This is considered to be due to {111} polarity reversal by twinning. The dangling bond density on the {111}A plane of GaSb is almost ten times of that on the {111}B plane, and the dangling bond density is related to the attachment energy of atoms of the plane; therefore, the growth rate in the <111>A direction would be significantly lower than that in the <111>B direction. These results indicate different growth rates in opposite 〈111〉 directions and a significant effect of the polarity on the competition of grain growth during the directional solidification of polycrystalline GaSb. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The first Si-Fe electrical steel was produced in 1905, and the grain-oriented steel was discovered in 1930 after Goss demonstrated how optimal combinations of heat treatment and cold rolling could produce a texture giving Si-Fe strip good magnetic properties when magnetised along its rolling direction. This technology has reduced the power loss in transformers greatly and remains the basis of the manufacturing process today. Since then many postulations reported on the mechanism on abnormal grain growth (AGG) which is the key for Si-Fe superior magnetic properties, however, none have provided a concrete understanding of this phenomenon. Here, we established and demonstrated a new theory that underlines the fundamental mechanistic approach of abnormal grain growth in 3% Si-Fe steel. It is demonstrated, that the external heat flux direction applied during annealing and Si atom positions in the solid solution disordered α-Fe cube unit cell that cause lattice distortions and BCC symmetry reduction are the most influential factors in the early stage of Goss AGG than what was previously thought to be dislocation related stored energy, grain boundary characteristics and grain size/orientation advantages. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Microstructure affects the physical properties and behavior of materials. While metallurgists have long studied microstructure characterization and evolution, thermo-mechanical material processing to achieve a desired microstructure remains largely experience-based. This paper presents a distributed thermal control methodology for the microstructure evolution. We consider the problem of achieving a uniform microstructure, starting from a non-uniform initial distribution. This is a common goal in material processing, as uniform microstructure implies consistent macroscopic properties. To illustrate the approach, we consider an example process with a multi-zone micro-heater array controlling the grain growth of a copper thin film. Cascaded temperature and grain-growth models characterize the process dynamics – finite element method (FEM) models the temperature field in response to the heater input, which in turn drives the microstructure evolution through a biased Monte-Carlo (MC) model. The high order combined FEM/MC model is used as the validation "truth" model. For the control design and analysis, a simplified model is developed to only capture the essential trend in the full model. Using the simplified model and dividing the copper thin film into multiple spatial zones with measurable grain statistics in each zone, we obtain a nonlinear multi-input/multi-output control design model. Using the simplified model, this paper presents the development and comparison of three control methods: 1. Direct output feedback from the measured mean local grain sizes to the heater current. 2. Model predictive control (MPC) using a finite horizon optimization to compute the required heat input at each control step. 3. Inner-outer loop control with temperature as the surrogate input for the outer loop and using the heater current to achieve the required temperature in the inner loop. All three methods achieve uniform microstructure in grain growth in the higher order FEM/MC simulation. Direct output feedback is the simplest to implement, but has the slowest convergence. MPC shows fast convergence but requires model-dependent on-line optimization. Inner-outer loop demonstrates good compromise between model-dependence and rate of convergence. [ABSTRACT FROM AUTHOR]

Ge-Sb-Te (GST) thin films are emerging materials for various non-volatile memory applications. The compounds contain a huge number of vacancies that play important roles in structural and electronic transitions. Control of disorder and understanding the structural transformations in GST thin films are of great importance for the design and reliability of phase change memory devices. In this work, in situ heating atomic-resolution transmission electron microscopy is used to observe structural transformations from vacancy disordered (fcc) to layered ordered (vacancy-ordered (vo)) and van der Waals-bonded trigonal (vdW t)) structures. Starting from 160 °C, randomly distributed vacancies in fcc grains gradually accumulate into vacancy layers. Further heating to 220 and 240 °C results in the formation of vdW t-domains within the vo-GST grains and the formation of large <001>-oriented t-grains, in a good agreement with outcomes on in situ XRD heating of epitaxial vo-GST thin film. Moreover, vo and fcc GST structures can coexist with vdW t-structure in one grain. Full transformation to 〈 11 2 ¯ 0 〉 -oriented t-grains occurs at 280 °C. Unlike other grains, the 〈001〉 t-grains show abnormal grain growth behaviour, while the electron beam irradiation accelerates the growth. Detailed characterizations of vo-grains reveal transient structures that indicate shear transformation mechanism and different defects as well as chemical changes during the transformations. Overall, this study provides new insights to understand the phase transition mechanisms and to optimize the microstructure in thin GST layers. [Display omitted] [ABSTRACT FROM AUTHOR]

High energy x-ray diffraction microscopy was used to image the microstructure of α-Fe before and after a 600 °C anneal. These data were used to determine the areas, curvatures, energies, and velocities of approximately 40,000 grain boundaries. The measured grain boundary properties depend on the five macroscopic grain boundary parameters. The velocities are not correlated with the product of the mean boundary curvature and grain boundary energy, usually assumed to be the driving force. The total migration of a boundary consists of both translations approximately normal to the boundary and lateral changes in area. Through the lateral changes in area, low energy boundaries tend to expand in area while high energy boundaries shrink, reducing the average energy through grain boundary replacement. The driving force for this process is not related to curvature and might disrupt the expected curvature – velocity relationship. [Display omitted] [ABSTRACT FROM AUTHOR]

"Field-assisted sintering techniques" (FAST) are one of the landmarks for ceramists nowadays. From the beginning of their discovery, their physical mechanisms are a pending question. In this paper, the growth of a collective of 3D ceramic grains (with or without an electric field) has been modelled through phase-field. The model allows switching off thermal effects from pure non-thermal electrodynamical ones. The results show that strong electric fields affect the sintering process, but, contrary to usual statements, they retard grain growth. This effect weakens as temperature rises. Therefore, the origin of FAST techniques must have a thermal component A hypothesis is proposed: local non-homogeneities of transport properties at the grains are demonstrated to induce intense temperature gradients even in stationary conditions, and it can act as driving force for flash. The intrinsic cause of electric field-induced inhomogeneities must be related to local atomistic processes, which are still unknown. [Display omitted] [ABSTRACT FROM AUTHOR]

Building on the recent discovery of tough nanocrystalline Cu-Zr alloys with amorphous intergranular films, this paper investigates ternary nanocrystalline Cu-Zr-Hf alloys with a focus on understanding how alloy composition affects the formation of disordered complexions. Binary Cu-Zr and Cu–Hf alloys with similar initial grain sizes were also fabricated for comparison. The thermal stability of the nanocrystalline alloys was evaluated by annealing at 950 °C (>95% of the solidus temperatures), followed by detailed characterization of the grain boundary structure. All of the ternary alloys exhibited exceptional thermal stability comparable to that of the binary Cu-Zr alloy, and remained nanocrystalline even after two weeks of annealing at this extremely high temperature. Despite carbide formation and growth in these alloys during milling and annealing, the thermal stability of the ternary alloys is mainly attributed to the formation of thick amorphous intergranular films at high temperatures. Our results show that ternary alloy compositions have thicker boundary films compared to the binary alloys with similar global dopant concentrations. While it is not required for amorphous complexion formation, this work shows that having at least three elements present at the interface can lead to thicker grain boundary films, which is expected to maximize the previously reported toughening effect. Image 1 [ABSTRACT FROM AUTHOR]

Nickel carbonyl vapor deposition (CVD) is a high-efficiency process used to produce nickel shell molds with high yield strength, reasonable ductility, and strong corrosion resistance. Such advantageous properties arise from the nanocrystals and nanotwins inside CVD nickel. However, the nanotwins do not persist at high temperatures, transforming into dislocation cells after 40-min annealing at 800 °C. Using experimental examinations and computational simulations, we investigated the kinetics of the annealing-induced detwinning in CVD nickel. TEM examinations showed that detwinning is realized by incoherent twin boundary (ITB) migration; meanwhile, plentiful dislocations are generated from coherent twin boundaries (CTBs). Our theoretical analysis revealed that these dislocations are necessary for the formation of the ITBs. Next, using molecular dynamics simulations, we found that the dislocations nucleated from CTBs during annealing are intrinsic grain boundary dislocations (IGBDs). Driven by the internal stress intensified by grain growth in the nanocrystalline regime, the IGBDs can separate from CTBs due to creep at 800 °C , resulting in a higher dislocation density inside the twin lamella than that of the outside. These dislocations can trigger the formation of ITBs. Overall, unlike grain growth, stress is necessary for detwinning, so a monolithic nanotwin structure should be more stable than the nanotwins inside a nanocrystalline matrix. Image 1 [ABSTRACT FROM AUTHOR]

The influencing factors, distribution characteristics and evolution mechanisms of the typical low-energy grain boundaries in the WC-Co cemented carbides were demonstrated. Based on characterizations from various angles, the correlations of grain shape aspect ratio, WC contiguity and distribution of low-energy grain boundaries were analyzed for cemented carbides with addition of different grain growth inhibitors. It was found that the ∑2 grain boundaries have higher fraction in the cemented carbides with stable complexions, finer grain size and larger WC contiguity. The ∑13a grain boundaries are likely to form in the cemented carbides with anisotropic surface energy by coalescence of WC platelets with large shape aspect ratio. The results suggest that the fraction and distribution of low-energy grain boundaries in the cemented carbides can be modulated by matching grain growth inhibitor and sintering temperature of the initial WC-Co composite powder. The features of low-energy grain boundaries in the WC-Co cemented carbides were studied considering the influencing factors, distribution laws and formation and evolution mechanisms. The correlations of the distribution and fraction of low-energy grain boundaries with the shape aspect ratio, size, and contiguity of WC grains were disclosed. The principles to modulate the low-energy grain boundaries in the cemented carbides through matching the composition and sintering temperature of the initial powder were proposed, which may be applicable to a large variety of powder metallurgical materials. Image 1 [ABSTRACT FROM AUTHOR]

During sintering, a multitude of mechanisms act in different ways resulting in densification and coarsening. Since the material properties depend on the microstructure, the manufacturing of advanced ceramics requires a deep understanding of the sintering process. The present study focuses on the occurrence of grain growth during final stage sintering. In a porous microstructure, the pores exert a dragging force to grain boundary motion retarding the grain growth. However, as soon as the porosity becomes low enough, the grain growth exceeds the dragging force and the microstructure starts to coarsen. This interdependency of densification and grain growth is still not fully understood. The present study uses a 3D phase-field model to investigate grain growth in porous microstructures during final stage sintering. A model extension treats pore dynamics under consideration of pressure stability as well as pore coalescence. To account for the size effect of pores on its dragging force, a surface diffusion based mobility approache is incorporated. Large-scale 3D grain growth simulations are conducted to analyze the effect of different porosities and pore sizes with statistical relevance comparable to real microstructures. Depending on the porosity, two dominating effects on the reduction of grain growth rate are found. For low porosities, the growth rate depends on the number of pores whereas for higher porosities the pore size and their mobility dominate the process. The results show the need to consider the effects of the pore size and their distribution during final stage sintering. Image 1 [ABSTRACT FROM AUTHOR]

Strontium titanate is well-known for its non-Arrhenius grain growth, where grain growth coefficients decrease by orders of magnitude between 1350 °C and 1425 °C. This transition is assumed to be caused by the existence and coexistence of two grain boundary types and results in the formation of bimodal microstructures. So far, no quantified data on the transition behavior was available. The present study uses a comparison of experimental microstructures for various heating times and temperatures with simulated microstructures from phase-field simulations considering various fractions of fast-growing grains. The microstructures are compared by means of their grain size distributions. It is found that the fraction of fast-growing grains follows an anti-Arrhenius behavior. Evaluating the present findings with respective literature data, the grain growth transition could be related to a space charge transition where the fast and slow grain boundaries are associated with strong and weak space charge and segregation. Overall, the present study sheds light on general grain growth transitions observed in several perovskite ceramics. Image 1 [ABSTRACT FROM AUTHOR]

The cyclic deformation behaviour and microstructural stability of severe plastic deformation processed bulk nanostructured (ultrafine-grained, UFG) commercially pure cp-Ti were investigated by using in situ neutron diffraction combined with R = −1 high-cycle-fatigue (HCF) loading at room and cryogenic temperatures. The UFG microstructure was created by equal channel angular pressing (ECAP) and multi-direction forging (MDF). A considerable continuous grain growth was revealed by neutron diffraction for MDF cp-Ti fatigued at 25 °C, as opposed to that at −200 °C. The same HCF fatigue loading at 25 °C only caused very limited grain growth for ECAP cp-Ti. Transmission electron microscopy confirmed the grain growth. Further confirmation of the room-temperature HCF fatigue-induced grain growth was obtained by transmission Kikuchi diffraction based analysis. Novel insights into fatigue induced grain growth mechanism in UFG cp-Ti are thus provided: (i) the thermally activated process plays an important role in grain growth during the room-temperature HCF fatigue; (ii) Continuous dynamic recrystallisation is responsible for the grain growth and dislocation slip or twinning is not essential to trigger such a grain growth; (iii) the anisotropic grain growth behaviour in {0002} grain family can be reconciled by accepting that these grains accumulated highly stored energy during initial severe plastic deformation and the subsequent recrystallisation nucleation occurred at these highly deformed regions. Image 1 [ABSTRACT FROM AUTHOR]

The time change of the area of a single grain is calculated in 2D by applying Herring's analysis. The grain is a regular n-sided polygon with its corners representing triple junctions (TJs). The resulting change of grain area is compared to the Von Neumann–Mullins analysis, where grain boundaries (GB) are curved and angles at TJs are equilibrium angles. The rates of area change are similar with the largest deviation for triangles. For both cases of different angles at TJs polygons with n < 6 shrink and those with n > 6 grow. Pieces of evidence are provided for describing the GB-movement by disclination generation and motion due to thermal fluctuations. Thus besides energy considerations a stochastic element comes into play. Image 1 [ABSTRACT FROM AUTHOR]

The growth of multicrystalline silicon and the formation of a random angle grain boundary, as well as the dislocation generation and expansion is observed dynamically in situ, by Synchrotron X-ray imaging techniques. The focus is kept on a random angle grain boundary since its behavior is particularly important to better understand the HP mc-Si (High Performance Multi-crystalline Silicon) photovoltaic properties. Due to the process conditions and to the grain competition that occurs during the solidification, a facetted {111}/facetted {111} groove is formed by this random angle grain boundary at the solid/liquid interface. It is shown how the shape of the solid/liquid interface allows the change of the preferential {111} growth facet and affects the grain boundary propagation direction. In one of the groove configurations, the two adjacent {111} facets do not have the same growth velocity and as a consequence the corresponding grain boundary does not follow the bisector of the angle between the two facets. Indeed, the direction of the grain boundary is determined by the growth velocities of the facets which control the grain competition. Moreover, under these experimental conditions a clear relationship is observed between the existence of random angle grain boundaries and the local generation of dislocations as well as their expansion. By comparison, dislocation emission is not observed at the level of Σ3 {111} grain boundaries. Image 1 [ABSTRACT FROM AUTHOR]

Abstract We studied the solid-state dewetting behavior of thin Ni 80 Fe 20 films deposited on basal plane oriented sapphire substrate and annealed in the range of temperatures of 1023–1323 K. All studied films exhibited strong <111> texture and maze bicrystal microstructure, with only two grains misoriented by 60° around the common <111> axis present in the film. The morphology of partially dewetted films changed from the one typical for polycrystalline thin films to the one typical for single crystalline heteroepitaxial films with increasing temperatures and annealing times. This change of dewetting behavior was associated with the fast grain growth in the films. The films of pure Ni of identical thickness, annealed under identical conditions exhibited significantly slower grain growth and lower thermal stability. Both the high-resolution X-ray diffraction and the cross-sectional high-resolution transmission electron microscopy observations revealed the phase separation of the Ni 80 Fe 20 films into two parallel layers of the face-centered cubic (adjacent to the substrate) and hexagonal close-packed (on the top of the film) phases of similar compositions. Our density functional theory (DFT) calculations indicated that this phase separation is driven by the decrease of the film surface and interface energy, leading to the thermodynamically equilibrium thickness of the metastable hexagonal close-packed phase. This phase exhibits higher surface anisotropy than its stable face-centered cubic counterpart and is instrumental in accelerating the grain growth in the film via suppression of grain boundary grooving. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract While interstitial alloying has been utilized to improve mechanical properties of multi-component high-entropy alloys (HEAs), its effectiveness depends on the interstitial content, microstructure and compositional homogeneity states. Here we present and discuss the influences of these factors on the mechanical behavior of interstitial equiatomic CoCrFeMnNi HEAs at room temperature. Interstitial HEAs containing carbon of 0.2, 0.5 and 0.8 at. % were processed to different compositional homogeneity states and grain sizes. We found that deformation of the various interstitial HEAs at early deformation stages is accommodated by dislocation slip whereas twinning occurs at the later stages. Upon an identical local strain at the later stages of deformation, nano-twin density decreases as the increase of carbon content due to the increased stacking fault energy. Also, the increase of C content leads to significantly higher energy barrier to recrystallization during annealing. Partially recrystallized (∼20 vol %) interstitial HEA with C content of 0.8 at. % shows more than five times higher yield strength compared to the as-homogenized coarse-grained (∼200 μm) reference material, suggesting the significant beneficial effect of interstitials enabled microstructural adjustment on performance of the interstitial HEAs. Further, the compositionally inhomogeneous coarse-grained (∼200 μm) interstitial HEAs exhibit lower work-hardening ability and ultimate strength compared to the homogenized reference material due to that the compositional inhomogeneity promotes the localized plasticity. Some more insights for the design and processing of interstitial HEAs are generalized and discussed. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Grain boundary (GB) properties and associated anisotropies due to the boundary's geometric degrees of freedom (DOF) greatly influence many of the salient features of polycrystalline aggregates, and as a consequence the observable properties of the material. Through theoretical analysis and atomistic simulations, we show that when considering the GB plane normal DOF, the GB interface stiffness tensor plays a paramount role in a wide range of GB dynamical processes. As a demonstration, we examine the interface stiffness tensor of Σ3 GBs in nickel and show that the stiffness can be much larger in magnitude and more anisotropic than the GB energy itself. Moreover, it is found that a wide range of inclinations exhibit negative stiffness values, a signature of a structural instability. This is the first study to consider the complete spatial description of the GB stiffness tensor, where both angular variations describing the interface plane normal are explored. In broad terms, our results highlight the integral role that the stiffness tensor plays in GB interfacial phenomena, such as curvature-driven flow and faceting instabilities. [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 in nanocrystalline vanadium films was studied at 600 and 700 °C in vacuum and in hydrogen atmosphere with partial pressures ranging from 1 to 1000 Pa. It is shown that grain growth is significantly increased in the presence of hydrogen. Thus the expected effect of retarding grain growth either by reducing grain boundary energy due to hydrogen segregation or by hydrogen drag on the moving boundary did not occur. Two explanations are given for the accelerated grain growth. First, hydrogen reduces the formation energy of ledges, assuming these ledges are required for initiating and advancing boundary motion. Second, grain growth requires the annihilation of excess volume which may be enhanced by reducing the formation energy of vacancies in the presence of hydrogen. [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]

In powder bed fusion (PBF) additive manufacturing, powder layers are locally melted with a laser or an electron beam to build a component. Hatching strategies and beam parameters as beam power, scan velocity and line offset significantly affect the grain structure of the manufactured part. While experiments reveal the result of specific parameter combinations, the precise impact of distinct parameters on the resulting grain structure is widely unknown. This knowledge is necessary for a reliable prediction of the microstructure and consequently the mechanical properties of the manufactured part. We introduce the adaption of a three-dimensional model for the prediction of dendritic growth for use with PBF. The heat input is calculated using an analytical solution of the transient heat conduction equation. Massively parallel processing on a high-performance cluster computer allows the computation of the grain structure on the scale of small parts within reasonable times. The model is validated by accurately reproducing experimental grain structures of Inconel 718 test specimens manufactured by selective electron beam melting. The grain selection zone within the first layers as well as the subsequent microstructure in several millimeters build height is modeled in unprecedented level of detail. This model represents the cutting-edge of grain structure simulation in PBF and enables a reliable numerical prediction of appropriate beam parameters for arbitrary applications. [ABSTRACT FROM AUTHOR]

Additives are widely used to control the microstructure of materials via their effect on defect chemistry during sintering. As the primary nuclear fuel, the properties of UO 2 are crucial for safe and efficient reactor operation. UO 2 has been manipulated by fuel vendors through doping to enhance grain size to provide improved fission gas retention and plasticity. In this work the common phenomenon that governs the effect of Mg, Ti, V, Cr, Mn, and Fe doping of UO 2 for enhanced grain growth is revealed, elucidating experimental observations. A combined density functional theory and empirical potential description of defect free energy is used to calculate the doped UO 2 defect concentrations as a function of temperature. At high (sintering) temperatures all dopants studied transition to a positively charged interstitial defect. Furthermore, a number of dopants (Ti, V, Cr, and Mn) do so in sufficiently high concentrations to greatly increase the negatively charged uranium vacancy concentration. High uranium vacancy concentrations can enhance grain growth and fission gas diffusion. Mg and Fe also enhance uranium vacancy concentrations but to a lesser extent, while Al has no impact. The enhanced uranium vacancy concentrations, associated with solution of dopants interstitially, is proposed as the mechanism responsible for the enlarged grains seen experimentally in (Ti/V/Cr/Mg)-doped systems. Mn- and V-doped UO 2 have been predicted to have higher uranium vacancy concentrations than the more widely used Cr-doped UO 2 , leading to higher grain growth and fission gas diffusivity. [ABSTRACT FROM AUTHOR]