Grain Boundary Mobility Research Articles

The Role of Grain Boundary Diffusion in the Solute Drag Effect.

Molecular dynamics (MD) simulations are applied to study solute drag by curvature-driven grain boundaries (GBs) in Cu–Ag solid solution. Although lattice diffusion is frozen on the MD timescale, the GB significantly accelerates the solute diffusion and alters the state of short-range order in lattice regions swept by its motion. The accelerated diffusion produces a nonuniform redistribution of the solute atoms in the form of GB clusters enhancing the solute drag by the Zener pinning mechanism. This finding points to an important role of lateral GB diffusion in the solute drag effect. A 1.5 at.%Ag alloying reduces the GB free energy by 10–20% while reducing the GB mobility coefficients by more than an order of magnitude. Given the greater impact of alloying on the GB mobility than on the capillary driving force, kinetic stabilization of nanomaterials against grain growth is likely to be more effective than thermodynamic stabilization aiming to reduce the GB free energy.

Comparative Study and Limits of Different Level-Set Formulations for the Modeling of Anisotropic Grain Growth.

In this study, four different finite element level-set (FE-LS) formulations are compared for the modeling of grain growth in the context of polycrystalline structures and, moreover, two of them are presented for the first time using anisotropic grain boundary (GB) energy and mobility. Mean values and distributions are compared using the four formulations. First, we present the strong and weak formulations for the different models and the crystallographic parameters used at the mesoscopic scale. Second, some Grim Reaper analytical cases are presented and compared with the simulation results, and the evolutions of individual multiple junctions are followed. Additionally, large-scale simulations are presented. Anisotropic GB energy and mobility are respectively defined as functions of the mis-orientation/inclination and disorientation. The evolution of the disorientation distribution function (DDF) is computed, and its evolution is in accordance with prior works. We found that the formulation called “Anisotropic” is the more physical one, but it could be replaced at the mesoscopic scale by an isotropic formulation for simple microstructures presenting an initial Mackenzie-type DDF.

Inclination-governed deformation of dislocation-type grain boundaries

Grain boundaries (GBs) in polycrystalline materials are frequently curved, which differ from the well-documented planar GBs in terms of structure and dynamics. However, the physical origin of curvature-controlled GB deformation remains unclear. Here, combining in situ transmission electron microscopy (TEM) nanomechanical testing and atomistic simulation, we rationalize the fundamental influences of GB inclination on the deformation of curved dislocation-type GBs in face-centered cubic metals. Non-uniform motion of curved GB is revealed and attributed to the inclination-dependent dislocation configurations, which simultaneously change the energy and mobility of GBs. An inclination-governed GB model extending from the classic dislocation theory is further established via geometric analyses, where a universal inclination threshold of 35° is deduced to precisely predict the deformation behaviors of curved GBs. These findings enhance our mechanistic understanding of GB-mediated plasticity, shedding light on the structural design of metallic materials via precise GB engineering.

The spectrum of atomic excess free volume in grain boundaries

The grain boundary (GB) excess volume is an important structural factor that is strongly correlated with various thermodynamic and kinetic properties of GBs such as GB energy, GB mobility, GB diffusivity, and GB segregation energy, etc. However, the excess volume is usually reported as an average value of the entire GB. Such simplification does not consider the spectral nature of the excess volume in a GB, which cannot be used to describe the atomic mechanisms of some kinetic process, such as GB migration, that involves only a few atoms at a time. Here, we explore the spectrum of atomic excess volume in representative nanocrystalline Ni and Al samples as well as 388 Ni bicrystals based on the Olmsted dataset by using atomistic simulations. It is found that the nanocrystalline Ni and Al models show a skew-normal distribution in the spectrum of both the atomic excess volume and the atomic excess energy in the GBs, which show a weak inverse correlation between them. This is in stark contrast to the widely reported positive correlation between GB energy and excess volume based on the average value. We further show based on the statistical analysis that the correlation between the atomic excess volume and excess energy strongly depends on the GB type and a universal trend between them does not exist. While low ∑ Ni GBs generally shows a strong inverse linear correlation between these two properties, such correlation is weak for high ∑ Ni GBs. Moreover, we find that the spectrum of the excess volume shows characteristics distribution in some special Ni GBs. For example, twist GBs generally show a symmetrical unimodal distribution while most surveyed ∑3 Ni GBs with anti-thermal behavior show an apparent bimodal distribution. Nevertheless, a strong correlation is found between the atomic excess volume and the segregation energy based on the nanocrystalline Al model with Mg impurity, which implies a possible universal trend between the two properties. The current study thus shows that the excess volume provides useful insights in revealing the elemental structure–property correlations in GBs, which may be used as a structural variable in future thermodynamic modeling of GBs.

Stress-induced amorphization triggers deformation in the lithospheric mantle.

The mechanical properties of olivine-rich rocks are key to determining the mechanical coupling between Earth's lithosphere and asthenosphere. In crystalline materials, the motion of crystal defects is fundamental to plastic flow1-4. However, because the main constituent of olivine-rich rocks does not have enough slip systems, additional deformation mechanisms are needed to satisfy strain conditions. Experimental studies have suggested a non-Newtonian, grain-size-sensitive mechanism in olivine involving grain-boundary sliding5,6. However, very few microstructural investigations have been conducted on grain-boundary sliding, and there is no consensus on whether a single or multiple physical mechanisms are at play. Most importantly, there are no theoretical frameworks for incorporating the mechanics of grain boundaries in polycrystalline plasticity models. Here we identify a mechanism for deformation at grain boundaries in olivine-rich rocks. We show that, in forsterite, amorphization takes place at grain boundaries under stress and that the onset of ductility of olivine-rich rocks is due to the activation of grain-boundary mobility in these amorphous layers. This mechanism could trigger plastic processes in the deep Earth, where high-stress conditions are encountered (for example, at the brittle-plastic transition). Our proposed mechanism is especially relevant at the lithosphere-asthenosphere boundary, where olivine reaches the glass transition temperature, triggering a decrease in its viscosity and thus promoting grain-boundary sliding.

Stability of nanocrystalline metals: The role of grain-boundary chemistry and structure

Nanocrystalline metals are transitioning from laboratory curiosities to engineering materials, in large part due to advances in improving their stability, making their exceptional properties more predictable and accessible. Nanoscale grains typically have a very strong innate tendency to coarsen, but the grain-boundary structure can be designed and tuned to lower its excess energy, reducing both the driving force for coarsening and the grain-boundary mobility. This article reviews two major strategies for achieving low-energy grain boundaries in nanocrystalline structures. First, grain-boundary alloying is discussed, including grain-boundary segregation and its energetic competition with the formation of second phases; with sufficient grain-boundary segregation tendency it is possible to stabilize nanostructures to high temperatures. Second, methods of achieving low-energy crystallographic grain-boundary structures are discussed, including the formation of nanotwinned structures and relaxing grain boundaries into low-energy structures through their interactions with partial dislocations. Both of these strategies have led to effective and implementable stable nanocrystalline materials, and point to many directions for future advancements.

Texture Selection Mechanisms during Recrystallization and Grain Growth of a Magnesium-Erbium-Zinc Alloy

Binary and ternary Mg-1%Er/Mg-1%Er-1%Zn alloys were rolled and subsequently subjected to various heat treatments to study texture selection during recrystallization and following grain growth. The results revealed favorable texture alterations in both alloys and the formation of a unique ±40° transvers direction (TD) recrystallization texture in the ternary alloy. While the binary alloy underwent a continuous alteration of its texture and grain size throughout recrystallization and grain growth, the ternary alloy showed a rapid rolling (RD) to transvers direction (TD) texture transition occurring during early stages of recrystallization. Targeted electron back scatter diffraction (EBSD) analysis of the recrystallized fraction unraveled a selective growth behavior of recrystallization nuclei with TD tilted orientations that is likely attributed to solute drag effect on the mobility of specific grain boundaries. Mg-1%Er-1%Zn additionally exhibited a stunning microstructural stability during grain growth annealing. This was attributed to a fine dispersion of dense nanosized particles in the matrix that impeded grain growth by Zener drag. The mechanical properties of both alloys were determined by uniaxial tensile tests combined with EBSD assisted slip trace analysis at 5% tensile strain to investigate non-basal slip behavior. Owing to synergic alloying effects on solid solution strengthening and slip activation, as well as precipitation hardening, the ternary Mg-1%Er-1%Zn alloy demonstrated a remarkable enhancement in the yield strength, strain hardening capability, and failure ductility, compared with the Mg-1%Er alloy.

Effect of superplastic deformation on precipitation behavior of sigma phase in 3207 duplex stainless steel

Owing to excellent strength, toughness and corrosion resistance, duplex stainless steels (DSS) are widely used in constructional and petrochemical applications. Sigma phase, which has detrimental impact on the properties, is readily precipitated during hot working of DSS. However, precipitation behavior of sigma phase during superplastic deformation, which is the most significant processing method of DSS, is still unclear. In the current study, the effect of superplastic deformation on the precipitation behavior of sigma phase was investigated in 3207 duplex stainless steel. The result shows that superplastic deformation could prevent sigma phase precipitation generally by increasing mobility of grain boundaries and decreasing misorientation of the sigma phase boundaries, resulting in some sigma phase precipitated on the twin boundaries. Most of the sigma phase precipitated on ferrite-austenite interface with misorientation of 20–25°, while it precipitated in ferrite or austenite with the misorientation of 40°–45°. The orientation relationship between sigma phase and matrix matched well in austenite and on the ferrite/austenite interfaces, while it showed a small misfit in ferrite. The prevention effect of the superplastic deformation on the sigma phase precipitation was beneficial to quasi stable deformation stage, resulting in longer elongation.

In-situ observation of abnormal grain growth in a low-alloyed carbon steel using SEM-EBSD

Because abnormal grain growth (AGG) degrades mechanical properties of industrial polycrystalline materials such as steel, understanding and controlling AGG are important. In this study, AGG of Al-Nb-microalloyed low-carbon steel was investigated at 1100°C using in-situ scanning electron microscopy-electron backscatter diffraction (SEM-EBSD). Owing to the pinning particles of AlN and Nb(C,N), fine austenite grains formed initially, and AGG appeared owing to the dissolution of the pinning particles at high temperatures. Relatively large grains invaded the surrounding smaller grains with a size ratio of approximately 0.3, resulting in AGG. We developed the in-situ observation method to investigate the AGG of the carburization process using a diffusion couple of high- and low-carbon steels. The carbon diffusion into the low-carbon steel from the high-carbon steel enhanced the grain growth of the low-carbon steel. Although the detailed mechanism is still unclear, we clearly showed that carburization can reduce the pinning force. When focusing on the misorientation of the grain boundary, grain boundaries with misorientation angles of 50–59° remained during AGG, which is explained by their lower mobility owing to their lower grain boundary energy. These results suggest that the difference in grain-boundary mobility can induce duplex grain growth with a bimodal distribution, resulting in AGG.

Mechanistic grain growth model for fresh and irradiated UO2 nuclear fuel

The purpose of this work is to develop a mechanistic model of grain growth in UO2 fuel during reactor operation. The model development builds on published experimental data on UO2 grain growth, as well as atomistic and mesoscale simulation results. We begin by developing new fits with temperature T (in K) for the average grain boundary (GB) energy and mobility in UO2 using literature data, where the GB energy γ¯=(1.56−5.87×10−4T)±0.3 J/m2 and the GB mobility M¯=(2.14±0.15×10−7)exp(−(290±22kJ/mol)/RT) m4/(Js). We then discuss the pinning of grain boundaries by porosity, including porosity left over after sintering and fission gas bubbles that form during operation. We present our mechanistic model and validate it using existing grain growth data. Finally, we implement the model in the BISON fuel performance code and quantify its impact for constant and transient power cases. Our model produces similar results to an empirical model for the constant power case, but it predicts more grain growth than the empirical model for the transient power case. We attribute this discrepancy to the new mechanistic model’s ability to account for the impact of the temperature and power history.

Enhancing the sinterability and electrical properties of BaZr0.1Ce0.7Y0.2O3‐δ proton‐conducting ceramic electrolyte

AbstractA novel strategy was proposed to enhance the sinterability and electrical properties of BaZr0.1Ce0.7Y0.2O3‐δ (BZCY) proton‐conducting electrolyte by adding 10 wt.% La0.9Sr0.1Ga0.8Mg0.2O3‐δ (LSGM) to form a 90 wt.% BZCY–10 wt.% LSGM (BL91) composite electrolyte. XRD patterns showed that no reaction occurred between the BZCY and LSGM electrolytes after sintering at 1400°C, 1450°C, 1500°C, and 1550°C for 10 h. The BL91 composite electrolyte exhibited higher relative densities and Vickers hardness and excellent electrical properties compared with those of the BZCY electrolyte. A combined approach of equivalent circuit model and distribution of relaxation time analysis was used to distinguish the bulk and grain‐boundary contributions to the total conductivity and electrode processes. The introduction of 10 wt.% LSGM serves as a grain‐boundary pinning phase, which can reduce the mobility of grain boundaries, thereby increasing sintered density and enhancing conductivity in BL91. A solid oxide fuel cell with proton‐conducting BL91 and BZCY membranes was tested, in which the former displayed higher power outputs than the latter. Ohmic and interfacial polarization resistances decreased by approximately 20%, thereby revealing the remarkable electrical properties of the BL91 electrolyte. Results demonstrated that BL91 composite is a development prospect proton‐conducting electrolyte.

Hysteresis of the Grain-Boundary Mobility of the [10$$\overline{1}$$0] Tilt Boundaries in Zinc

The hysteresis of the temperature dependence of the grain-boundary mobility induced by grain-boundary phase transitions is studied. The dependence of the grain-boundary mobility on the reciprocal annealing temperature is experimentally found to be nonlinear and different upon heating and cooling. The curved segment formed by the temperature dependences of the mobility upon heating and cooling is considered as a hysteresis loop.

Hysteresis of the Grain-Boundary Mobility during Grain-Boundary Phase Transitions

The motion of two individual special [11 $$\bar {2}$$ 0] tilt boundaries in the form of a half-loop with a facet is experimentally studied. The temperature dependence of the grain-boundary mobility is found to deviate from the Arrhenius law. This behavior is interpreted as the manifestation of the hysteresis of the grain-boundary mobility due to a faceting–defaceting transition. The faceting–defaceting transition temperature and the parameters of the grain-boundary mobility hysteresis are determined experimentally for the individual boundaries under study.

Static Recrystallization Microstructure Evolution in a Cold-Deformed Ni-Based Superalloy during Electropulsing Treatment

The influence of electropulsing treatment (EPT) parameters on the static recrystallization (SRX) microstructure in a cold-deformed Ni-based superalloy was investigated. During EPT, both the volume fraction of SRX grains and the average grain size increased with the increasing EPT temperature, which was attributed to the thermal effect and athermal effect induced by EPT. The mobility of SRX grain boundaries was promoted at the higher temperature due to the thermal effect, while the nucleation rate would be increased by EPT through decreasing the thermodynamic barrier. The formation of parallel dislocations caused by electron wind force could also play an indirect role in promoting SRX process. Moreover, the volume fraction of SRX grains increased significantly with the extension of EPT time at 700 °C, while the EPT time had a trivial effect on the average grain size. In addition, the sufficient deformation was essential to the occurrence of SRX behavior during EPT, and the localized Joule heating effect could promote the SRX behavior in the samples with the larger strains. Besides that, the influence of twining and carbides on the SRX behaviors was also investigated.

Grain boundary migration and Zener pinning in a nanocrystalline Cu–Ag alloy

Grain boundary (GB) migration and Zener pinning, i.e. retardment of migrating GBs by second-phase particles, in a nanocrystalline Cu–Ag alloy have been investigated via molecular dynamics simulation. Among 86 GBs of different rotation axes and misorientation angles, about half of the GBs either did not move for an obvious distance within limited simulation time or underwent unsteady migration, and the other GBs exhibited steady migration. For the motionless and unsteady-migration GBs, boundary faceting was frequently observed. In particular, as found in experiments, all low angle GBs and several high angle GBs became faceted. For the steady-migration GBs, rotation-axis and misorientation dependences of GB energy, GB mobility and particle pinning effects have been systematically revealed. It is found that both the energy and mobility of low angle GBs were generally lower than those of high angle GBs, GBs of distinct rotation axes differed in mobility by as large as three orders of magnitude, and the maximum pinning force always agreed well with the theoretical prediction proposed by Ashby et al []. Moreover, pinning efficiency obtained via comparisons between boundary migration with and without the pinning indicates that the efficiency is not related to the GB mobility or the maximum pinning force and a nanosized second phase particle seems to have a limited effect on retarding GB motion in very fine nanograins.

Microstructural investigations in binderless tungsten carbide with grain growth inhibitors

The microstructure of binderless tungsten carbide, with small additions of Cr3C2 or VC, was investigated using transmission electron microscopy associated with X-ray energy dispersive spectrometry. The distribution of Cr and V elements was determined. In the material sintered with Cr3C2, numerous (Cr,W)2C grains were found, some of them displaying an epitaxy orientation relationship with the basal facet of adjacent WC grains, and Cr segregation was observed in all examined grain boundaries. In the carbide with VC additive, small V-rich carbides were found at triple junctions of WC grains. Unlike Cr, no V segregation was detected in grain boundaries. The grain growth inhibiting effect of Cr3C2 and VC is very likely different. For Cr3C2, it is supposed that both (Cr,W)2C grains and Cr segregation reduce the mobility of grain boundaries. For VC, probably the grain boundary triple junctions are pinned by small V-rich carbide grains.

Effects of CoAl2O4 inoculants on microstructure and mechanical properties of IN718 processed by selective laser melting

This work investigated the effects of CoAl2O4 inoculants on the microstructure and mechanical properties of a superalloy – Inconel 718 (IN718), processed by selective laser melting (SLM). IN718 powder was blended with 0.2 wt. % of CoAl2O4 particles. After the SLM process, the CoAl2O4 particles reacted with IN718 and formed a dispersion of Al-rich nano-oxide particles in the matrix. These inoculants were observed to facilitate the formation of fine, equiaxed grains and reduce the degree of crystallographic texture in the as-built microstructure. The presence of the nano-oxide particles in the microstructure served to restrict the mobility of grain boundaries during heat-treatment and promoted the formation of a bimodal grain structure with comparatively finer average size. Due to the reduction of crystallographic texture, the elastic anisotropy present in as-built specimens was greatly minimized with the addition of CoAl2O4 inoculants. Specimens containing inoculants were also found to possess improved tensile properties following heat-treatment at both room temperature and 650 °C. Furthermore, creep testing at 650 °C/650 MPa revealed that the steady strain rate of IN718 was lowered from 8.8×10−9 s−1 to 4.9×10−9 s−1, and the creep rupture life was extended by 52 hours in samples that were fabricated with CoAl2O4. These results strongly suggest that the decomposition of CoAl2O4 into Al-rich oxides during SLM processing not only contributes to grain refinement and a reduction in the degree of crystallographic texture but also forms an oxide dispersion that restricts the mobility of dislocations and grain boundaries in IN718.

Efficient calculation of the ECO driving force for atomistic simulations of grain boundary motion

A new formulation and implementation of the Energy Conserving Orientational (ECO) Driving Force (DF) for Molecular Dynamics (MD) simulations of grain boundary (GB) motion has been developed. While the original ECO-DF slows down MD simulations of GB motion by more than an order of magnitude, the new version is almost as efficient as the widely used Janssens DF (J-DF). In order to rate the new method in comparison to others, the velocity of a symmetric Σ5 〈001〉 36.87° tilt GB in nickel has been simulated using the new ECO-DF and the J-DF. The deviations are discussed, including the impacts on the AIRwalk method to evaluate the GB mobility. The temperature T, the DF magnitude p, and the boundary conditions were varied. Under fully periodic boundaries, all results consistently yield a GB velocity v(p,T) that is based on two subsequent or co-dependent mechanisms, shuffling and its initiation, just like for a symmetric Σ7 〈111〉 38.2° tilt GB investigated previously with the original ECO-DF. Under the so-called shrink-wrapped boundary conditions, a relative grain displacement occurs. The magnitude of this coupling effect strongly depends on temperature. From the comparison of simulations under different boundary conditions it appears probable that the initiation process under periodic boundary conditions is actually the initiation of GB sliding, which needs to undo fully coupled grain motion. The shuffling mechanism is the combined GB motion fully coupled with grain displacement.

The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys

A model of discontinuous precipitation of γ′ phase at grain boundaries (GBs) in polycrystalline Ni-based superalloys during continuous cooling from supersolvus temperature was developed. The model calculates the size and γ′ phase fraction in GB precipitates as a function of cooling rate and GB mobility. The model is based on the classical mechanism of diffusion-controlled precipitation. It considers fast diffusion of γ′-forming elements along the GB and the motion of the GBs under driving force induced by alloy decomposition. The model predicts either discrete GB gamma-prime particles and GB serration or the growth of fan-type structures depending on the cooling rate and GB mobility. We conclude from the model predictions that the GB mobility is the key factor controlling the type and morphology of GB precipitates. High GB mobility results in formation of fan-type structures, while the lower GB mobility leads to GB serration. The predicted dependence of the size of GB precipitates on the cooling rate is in good agreement with the experimental observations.

On the temperature dependence of grain boundary mobility

The grain boundary (GB) mobility relates GB velocity to the thermodynamic driving forces and is central to our understanding of microstructure evolution in polycrystals. Recent molecular dynamics (MD) and experimental studies have shown that the temperature-dependence of the GB mobility is much more varied than is commonly thought. GB mobility may increase, decrease, remain constant or show multiple peaks with increasing temperature. We propose a mechanistic model for GB migration, based on the formation and migration of line defects (disconnection) within the GB. We implement this model in a kinetic Monte Carlo and statistical mechanics framework; the results capture all of these observed temperature dependencies and are shown to be in quantitative agreement with each other and direct MD simulations of GB migration for a set of specific grain boundaries. Examination of the dependence of GB mobility on disconnection mode and temperature provides new insight into how GBs migrate in polycrystalline materials.