Mechanism Of Grain Boundary Migration Research Articles

Investigation of Precipitation Behavior of a Novel Ni-Fe-Based Superalloy during High-Temperature Aging Treatment.

The precipitation behavior of a novel Ni-Fe-based superalloy developed for advanced ultra-supercritical (A-USC) coal-fired power plant applications during high-temperature aging treatment was investigated. The results showed that the major precipitates in the novel alloy were randomly distributed MC carbides, M23C6 carbides at grain boundaries, and the γ'-Ni3 (Al, Ti) phase in grain interiors after aging. MC remained relatively stable during both short-term and long-term aging. M23C6 quickly precipitated and exhibited a discrete distribution at grain boundaries during short-term aging, and partly developed into continuous films during long-term aging. After uniform precipitation, the shape of γ' remained spherical, and the size kept increasing with aging time according to the Lifshitz-Slyozov-Wagner (LSW) model. The hardness of the novel alloy was mainly associated with the precipitation behavior of γ'; as γ' gradually precipitated, the hardness steadily increased; after complete precipitation, as the size of γ' increased, the hardness first increased and then decreased, reaching the peak hardness when the average radius of γ' achieved the critical size. In addition, the novel alloy exhibited abnormal coarsening behavior at grain boundaries during both short-term and long-term aging. The coarsened grain boundaries were actually precipitate-free zones (PFZs) and the coarsened and elongated rod-like particles inside were identified as γ' precipitates. The mechanism of strain-induced grain boundary migration and the discontinuous coarsening reaction is proposed for the formation of PFZs. Furthermore, PFZs were considered to be potential crack sources during the creep rupture test, leading to earlier failure of the material.

Plastic deformation-driven grain coarsening in nanocrystalline Gd2Zr2O7 ceramics by nanoindentation

Unique deformation mechanisms are responsible for the superior mechanical properties of nanocrystalline materials. However, unlike in nanocrystalline metals, questions remain regarding the underlying microstructural mechanisms governing plastic deformation in nanocrystalline ceramics. In the present study, nanoindentation and postmortem electron microscopy analysis were carried out on the transparent nanocrystalline Gd2Zr2O7 ceramic. Apart from shear bands led from grain boundary (GB) sliding, experimental observations confirm that deformation-driven grain coarsening is controlled by grain rotation accompanying the GB migration mechanism.

Densification and grain growth of UO2 and MnO-UO2 during pressureless sintering

Large-grain UO2 has attracted much attention due to its advantage of suppressing fission gas release by reducing the specific surface area of grain boundaries. In this study, a single grain growth additive MnO is selected to modify UO2 by conventional pressureless sintering method. The effect of MnO modification on UO2 densification and grain growth at temperatures between 1200 and 1700 °C was investigated in detail. 0.5 wt%MnO-UO2 pellets can reach nearly 95% T.D. when sintered at 1200 °C for 24 h. An average grain size greater than 300 µm can be obtained at 1700 °C for 24 h. A series of sintering experiments were designed and conducted to verify the specific mechanism of MnO on the grain growth of UO2. The classical grain growth kinetics indicates that the grain growth exponent of MnO modified UO2 in the early stage is 3, which corresponds to the lattice diffusion under pore control, while that of UO2 is 2 and conforms to the grain boundary migration control. The SEM results show that there is solute segregation of MnO at the grain boundaries, followed by nucleation of lamellar structure in the form of piles and plates on the boundaries after high temperature treatment and traces of liquid phase in some region. Diffusion sintering experiments show that the mechanism of MnO-promoted UO2 grain boundary migration is based on solid phase diffusion. The XRD results of UO2 modified with different amounts of MnO demonstrate that the growth degree of UO2 grains can be related to the variation of lattice parameters resulted from the MnO modification. This is due to the enhanced mobility of UO2 grain boundaries because of the large distortion of surrounding lattice caused by Mn modification. These results can provide new ideas for the manufacture of UO2 ceramic fuels.

Atomic insights into the effect of rapid heating pretreatment on the mechanical stability of ⟨100⟩ symmetrical tilt GBs in nanocrystalline materials

Nanograined materials possess ultrahigh strength, while their processing and technological applications are constrained by inherent thermal and mechanical instability. Existing experiments show that the stability of Cu nanograins can be enhanced by performing a rapid heating pretreatment that reduces the grain boundary (GB) energy by changing the GB structure. The variation in the GB structure inevitably affects the migration mechanism of GBs. However, the effect of the pretreatment-induced variation in migration mechanisms on stability remains unclear. Here, the shear deformation of a series of ⟨100⟩ symmetrical tilt GBs after rapid heating pretreatment is systematically investigated by molecular dynamics simulations. The simulations of unheated GBs are also included for comparison. Our results show that the rapid heating pretreatment does not improve the mechanical stability of GBs with tilt angles larger than 36.87° but rather enhances the mechanical stability of those with tilt angles less than 36.87° by the transformation of migration behavior from the normal ⟨110⟩ mode to (i) a ⟨100⟩ mode; (ii) an inhomogeneous mixed one that reconciles the ⟨110⟩ and ⟨100⟩ modes; and (iii) an inhomogeneous ⟨110⟩ mode. The former leads to an increase in the critical shear stress that is required to initiate the migration, whereas the latter two result in a decrease in the migration distance. The variation in the GB migration mechanism is attributed to the change in the GB structure from an ordered kite structure to a disordered one. The research gives an atomic insight into the stabilizing mechanism of nanocrystalline materials with rapid heating pretreatment.

Quasi-in-situ observation and analysis of grain boundary evolution of GH4169 nickel-based superalloy during the micro-strain stage of thermal deformation

A systematic study was conducted to analyze the grain boundary microstructure evolution of GH4169 nickel-based superalloy during the micro-strain stage of thermal deformation. A quasi-in-situ EBSD (electron backscatter diffraction) approach was used for observation. Moreover, at a temperature of 1100 °C and a strain rate of 0.1 s−1, a continuous strain uniaxial compression experiment was carried out. We observed the development of the grain boundary structure and the grain boundary deformation. The experimental findings revealed that GH4169 underwent locally inhomogeneous thermal deformation during the micro-strain stage, and the simultaneous occurrences of grain boundary slip, grain rotation, and grain boundary migration, exhibiting microstructural diversity. The grain boundary structure changed continuously as a result of the random movement of high-angle grain boundaries. Twin grains displayed a stable structure during the migration of grain boundaries, and their content indicated a considerable decrease as the strain increased. This provides a basis for directing the design of grain boundary engineering since it suggests that twin boundaries are easier to migrate during thermal deformation. In addition, no evidence of dynamic recrystallization was found throughout the entire micro-strain stage and a <101>/<111> micro-texture was observed, which is compatible with common dislocation slips in face-centered cubic crystals. Then, on the basis of the transmission electron microscope images of the microstructure, the numerous forms of grain boundary deformation were rationally explained from the standpoint of the relation mechanisms among grain boundaries and dislocations. Finally, the synergistic mechanism of grain boundary slip and migration is established as the primary mechanism of thermal deformation in the micro-strain stage.

Grain Boundary Migration in Polycrystals

Grain boundaries in polycrystalline materials migrate to reduce the total excess energy. It has recently been found that the factors governing migration rates of boundaries in bicrystals are insufficient to explain boundary migration in polycrystals. We first review our current understanding of the atomistic mechanisms of grain boundary migration based on simulations and high-resolution transmission electron microscopy observations. We then review our current understanding at the continuum scale based on simulations and observations using high-energy diffraction microscopy. We conclude that detailed comparisons of experimental observations with atomistic simulations of migration in polycrystals (rather than bicrystals) are required to better understand the mechanisms of grain boundary migration, that the driving force for grain boundary migration in polycrystals must include factors other than curvature, and that current simulations of grain growth are insufficient for reproducing experimental observations, possibly because of an inadequate representation of the driving force.

Recrystallization behavior and mechanical properties of AZ31B alloy during the hot-rolling process

In this work, microstructural characteristics, dynamic recrystallization and mechanical properties of the hot-rolled AZ31B were investigated. The results indicated that the different mechanisms of recrystallization are responsible for the microstructural evolution during the rolling deformation. It was found, as for the specimens with the rolling reduction of 30%, large number of twins are formed within the deformed matrix and the recrystallized grains are usually formed nearby the twins. This indicated that the mechanism of the twin-induced nucleation mainly dominates the recrystallization process of the specimens with the 30% reduction. As for the specimens with the rolling reduction of 75%, the recrystallized grains are nucleated on the bulged boundaries or within the deformed grains. This indicates that the mechanisms of the stress-induced grain boundary migration (SIBM) and the sub-grain growth mainly dominate the recrystallization process of the specimens with the reduction of 75%. The recrystallization process contributes to the grain refinement and to the improvement of mechanical properties of alloys. Eventually, the specimens with the rolling reduction of 75% obtain an excellent strength-ductility balance. The results of this work not only elaborate underlying knowledge on the correlation relationship between the recrystallization mechanisms and the mechanical properties, but provide an effective approach to optimize the microstructure-mechanical properties of alloys.

High ductility CrCoNi medium entropy alloy prepared by liquid nitrogen temperature rolling and short time annealing at moderate temperature

As-cast CrCoNi Medium Entropy Alloy (as-casted MEA) was obtained by vacuum arc melting. The as-casted MEA was rolled 3 times at the temperature of liquid nitrogen (77 K), the sum of the total deformation was 50%. Then, the rolled CrCoNi Medium Entropy Alloy (as-rolled MEA) were annealed with different parameters, the annealing parameters were 923 K/30min, 973 K/30min and 1073 K/30min respectively. The experimental samples were characterized by uniaxial tensile test at room temperature, electron back scatter diffraction (EBSD) and transmission electron microscopy (TEM). The tensile test results showed the CrCoNi MEA was deformed by Liquid Nitrogen rolling, the tensile strength increased from 421 MPa to 1312 MPa, but the elongation was only 7.4%. The as-rolled MEA annealed at moderate temperature and short time, the ductility of the CrCoNi was improved significantly. When the as-rolled MEA annealed at 1073 K/30 min, the elongation of CrCoNi increased to 65.3%, while the tensile strength maintained 800 MPa. The EBSD and TEM results showed that the CrCoNi MEA was deformed by Liquid Nitrogen rolling, there were a lot of high density dislocation regions in the alloy. At the same time, a large number of bunches of stacking faults (SFs), intersected SFs and deformation twins (DT) were found in the rolled MEA. The EBSD and TEM results also showed that when the CrCoNi MEA was annealed, a large number of type B and type C annealing twins and 9 R structures were found in the alloy. The growth of annealing twins were achieved by the mechanism of grain boundary migration caused by stacking faults annexation. After annealing, the transformation of FCC→9 R structure was occurred, and the 9 R structure caused more boundary nucleation of annealing twins. The annealing twins and 9 R structure played a key role in improving the ductility of the annealed CrCoNi MEA. Adopt the technology of moderate temperature combining short time annealing process, can effectively control the properties of CrCoNi MEA.

Interaction of ytterbium pyrosilicate environmental-barrier-coating ceramics with molten calcia-magnesia-aluminosilicate glass: Part II, Interfaces

Interfaces in as-processed β-Yb2Si2O7 environmental barrier coating (EBC) ceramics, and those after high-temperature (1500 °C) interaction with a calcia-magnesia-aluminosilicate (CMAS) glass from Part I are studied in further detail using high-resolution transmission electron microscopy (HRTEM), annular dark field scanning TEM (ADF STEM) and high-angle ADF (HAADF) STEM. Disconnections are detected, or inferred, at twin and general grain-boundaries and also at phase boundaries. Grain-boundary and facet planes are found to be aligned parallel to {110} planes of β-Yb2Si2O7. When detected, step planes are also found to be aligned parallel to β-Yb2Si2O7 {110} planes. At the same time, the dislocation component has major projections aligned parallel to {110} planes of β-Yb2Si2O7. As such, both the step and the dislocation components of the disconnections are anisotropic. The tendency of the grain-boundary and facet planes at interfaces in the system to align parallel to β-Yb2Si2O7{110} planes, both before and after CMAS-interaction, is discussed. The chemistry of grain-boundaries in the vicinity of the disconnections is found to be non-stoichiometric, but this by itself cannot account for the extended contrast variations along twin boundaries. Thus, contrast variations along grain boundaries are associated mainly with the presence of disconnections and associated strain, as well as long-range distortions in the unit cells in twin boundaries. These anisotropic disconnections are associated with the mechanisms of grain-boundary and phase-boundary migration, and thus, they have implications on the evolution of microstructures in this system. Combined insights from Part I and this Part II elucidate the nature of the interfaces in β-Yb2Si2O7 EBC ceramic and the mechanisms of CMAS glass penetration.

Atomistic migration mechanisms of [formula omitted] symmetric tilt grain boundaries in magnesium

Grain boundary (GB) is an important microstructure and plays a vital role in the mechanical properties of polycrystalline materials by GB migration and sliding. In this work, molecular dynamic (MD) simulations were performed to investigate the migration mechanisms of [12¯10] symmetric tilt grain boundaries (STGBs) in magnesium. A total of 15 STGBs with the rotation angle θ from 0° to 90° were studied under a pure shear loading. The results show that the GB migration mechanisms are significantly influenced by the GB structure. For small angle STGBs (θ<28°), the GB migration is mediated by twin nucleation from GB and subsequent twin growth. For large angle STGBs (θ>83°), the GB migration is achieved by the glide of GB dislocations. The medium angle STGBs (28°<θ<83°), which are the majority of studied STGBs, were observed to be transformed into twin boundary (TB) by emitting lattice dislocations/stacking faults (SFs) during migration. The migration mechanisms for medium angle STGBs can be explained by two rules: GB decomposition and emission of lattice dislocations/SFs. This work provides atomic mechanisms on the GB migration, which are important for understanding the GB behaviors and mechanical properties in magnesium.

Revealing the size effect mechanism of reversible grain boundary migration in nanocrystalline coppers: Molecular dynamics simulations and a refined disconnection model

Reversible grain boundary (GB) migration is of great significance for promoting cyclic deformability and has been observed in nanocrystalline metals. However, GBs as dislocation sources generally emit more dislocations under deformation with increasing grain sizes. The size effect mechanism of reversible GB migration is thus crucial but remains unknown. We demonstrate that the reversibility of the representative Σ11(113) GB migration in copper (Cu) bicrystals under cyclic shear strongly depends on their grain sizes using molecular dynamics (MD) simulations. Atomic observation of the GB plane reveals the transition mechanism from the fully reversible disconnection-mediated GB migration to the disconnection pair grooving (DPG) induced irreversible structural damage with increasing grain size (d), where the disconnections at the groove are proved to be screw disconnections with Burgers vectors of 1/22[332¯] using Burgers circuit and lattice analysis. A refined disconnection model considering grain size is developed to predict disconnection-mediated GB migration behavior and interpret the forming mechanism of DPG. When satisfying d > 1.6651Gb/πτ (shear modulus G, Burgers vector b, and shear stress τ), the DPG mode is triggered, while the single disconnection sliding provokes the reversible GB migration when d ≤ 1.6651Gb/πτ. This study holds implications for nanocrystalline metals towards endurable cyclic deformability by controllable grain sizes.

A taxonomy of grain boundary migration mechanisms via displacement texture characterization

Atomistic simulations provide the most detailed picture of grain boundary (GB) migration currently available. Nevertheless, extracting unit mechanisms from atomistic simulation data is difficult because of the zoo of competing, geometrically complex 3D atomic rearrangement processes. In this work, we introduce the displacement texture characterization framework for analyzing atomic rearrangement events during GB migration, combining ideas from slip vector analysis, bicrystallography and optimal transportation. Two types of decompositions of displacement data are described: the shear-shuffle and min-shuffle decomposition. The former is used to extract shuffling patterns from shear coupled migration trajectories and the latter is used to analyze temperature dependent shuffling mechanisms. As an application of the displacement texture framework, we characterize the GB geometry dependence of shuffling mechanisms for a crystallographically diverse set of mobile GBs in FCC Ni bicrystals. Two scientific contributions from this analysis include 1) an explanation of the boundary plane dependence of shuffling patterns via metastable GB geometry and 2) a taxonomy of multimodal constrained GB migration mechanisms which may include multiple competing shuffling patterns, period doubling effects, distinct sliding and shear coupling events, and GB self diffusion.

Impurity effect on recrystallization and grain growth in severe plastically deformed copper

The impurity effect on commercial purity copper (4 N and 3 N) was investigated by comparing their annealing induced recrystallization and grain growth behavior after severe plastic deformation. Equal channel angular pressing (ECAP) with additional cryogenic rolling was applied to produce equiaxed ultra-fine grained (UFG) and nano-laminated (NL) coppers with high stored excess energy. The stored excess energy increased with decreasing purity from 4 N to 3 N, whereas the grain boundary velocity and recrystallized grain size decreased for both UFG and NL Cu. In particular, the NL structures were found to be more sensitive to impurities than equiaxed ultra-fine grained ones, and consequently the abnormal grain growth at the early stages of recrystallization in NL 4 N Cu was effectively suppressed in NL 3 N Cu. The analysis of the grain growth rate during recrystallization, using the microstructural path methodology (MPM), demonstrates that the correlation between the abnormal grain growth and the significantly enhanced grain boundary mobility cannot be solely explained by the high stored excess energy. Under all conditions, the impurity drag effect poses a kinetic constraint on the rate-controlling mechanism for grain boundary migration.

Multiphase field modeling of grain boundary migration mediated by emergent disconnections

Knowledge about grain boundary migration is a prerequisite for understanding and ultimately modulating the properties of polycrystalline materials. Evidence from experiments and molecular dynamics (MD) simulations suggests that the formation and motion of disconnections is a mechanism for grain boundary migration. Here, grain boundary migration is modeled using a multiphase field model based on the principle of minimum dissipation potential with nonconvex boundary energy, along with a stochastic model for thermal nucleation of disconnection pairs. In this model, disconnections arise spontaneously in the presence of an elastic driving force, and that their motion mediates boundary migration. The effect is due to the fact that the formation of the disconnections pairs results in a stress concentration, causing the elastic driving force to exceed the threshold value and driving the propagation of the disconnection along the interface. The model is applied to study the propagation/annihilation of single disconnection pairs, the relaxation of a perturbed interface, and shear coupling at various temperatures. The results are consistent with the current understanding of disconnections, and capture the effect of thermal softening.

Effects of magnesium dopants on grain boundary migration in aluminum-magnesium alloys

Atomistic simulations have been used to study the grain boundary (GB) migration in pure Al and Mg-doped Al binary alloys. The results of shearing simulations indicate that the effect of dopants on the GB migration depends on the character of the GB. Negligible influence is found on the migration of the coherent ∑3 GB and only a slight change was found in its strength due to the doping. For the other GBs considered in our study, GB migration was pinned by dopants at the GBs, which results in a strengthening effect. The atomic-level GB migration mechanisms are identified for both shear-coupled GB migration in pure Al and dopants pinning GB migration in Mg-doped Al alloys.

In situ atomistic observation of grain boundary migration subjected to defect interaction

Grain boundary (GB) migration associated with grain growth and recrystallization encounters lattice defects frequently, while the atomistic mechanisms of GB-defect interactions during these dynamic processes remain largely unclear. Here, we systematically investigate the atomistic mechanism of the interactions between migrating Σ11(113) symmetrical tilt GBs and different lattice defects, including full dislocation, stacking fault and nanotwin in Au, via the integrated in situ nanomechanical testing and molecular dynamics simulations. Experimental evidence demonstrated a conservative defect accommodation mechanism of Σ11(113) GB and a coupled GB-twin evolution, where localized atom displacement dominated the GB-defect interactions. Moreover, the shear-coupled GB migration was unperturbed by the interaction, due to the glissile residual disconnections left on the GB. These findings provide a comprehensive experimental understanding on the atomistic mechanism of GB migration under the influence of different lattice defects, which significantly advances the theoretical development of GB-dominated plasticity under more realistic circumstances.

Atomistic mechanism for vacancy-enhanced grain boundary migration

Mechanical behavior of polycrystalline materials is intimately connected to migration of grain boundaries, which in turn is dramatically impacted by the presence of defects. In this paper, we present atomistic simulations to elucidate the elementary mechanism that dictates the role of vacancies in enhancing grain boundary migration via shear-coupled normal motion. The minimum energy pathway and the associated energy barriers are calculated using the nudged elastic band method. Fully three-dimensional atomistic simulations provide excellent verification of the three-dimensional disconnection model and furnish quantitative evidence that vacancies facilitate grain boundary migration by weakening the line tension of a disconnection loop. It is also revealed that vacancies serve as energetically favorable sites for the nucleation of grain boundary disconnections, thereby inducing shear-coupled grain boundary migration.

Grain growth in Nio–MgO and its dependence on faceting and the equilibrium crystal shape

The impact of faceting on grain growth was approached by model experiments in NiO–MgO. Grain growth rates were found to be 10 times higher in NiO compared to MgO. As the self-diffusion acoefficients differ by a factor of 250, grain growth in NiO is unexpectedly slow compared to MgO. Recently, the movement of steps was identified as the atomic mechanism of grain boundary migration. According to the equilibrium crystal shape, grain boundaries in NiO are more faceted. The faceted grain boundaries of NiO have fewer steps at the grain boundaries resulting in a relatively lower mobility.

Heterogeneous disconnection nucleation mechanisms during grain boundary migration

Shear-coupled grain boundary (GB) migration has been evidenced as an efficient mechanism of plasticity in the absence of dislocation activity. The GB migration occurs through the nucleation and motion of disconnections. Using molecular simulations, we report a detailed study of the elementary mechanisms occurring during heterogeneous disconnection nucleation. We study the effect of a preexisting sessile disconnection in a symmetric $\mathrm{\ensuremath{\Sigma}}17(410)\phantom{\rule{4pt}{0ex}}[001]$ tilt GB on the GB migration mechanism. Shearing this imperfect GB induces its migration and reveals a new GB migration mechanism through the nucleation of a mobile disconnection from the sessile one. Energy barriers and yield stress for the GB migrations are evaluated and compared to the migration of a perfect GB. We show that the migration of the imperfect GB is easier than the perfect one and that a sessile disconnection can operate as a source of disconnection driving the GB migration. This GB migration mechanism has been observed on two other high-angle GBs.

Atomistic simulations of grain boundary migration under recrystallisation conditions

We present details of an approach to simulating the migration of grain boundaries using classical molecular dynamics (MD) in a regime in which the boundary motion is driven by the presence of dislocation defects resulting from plastic deformation. By simulating the shrinkage of spherical grains of deformed material embedded in a perfect crystal, we create conditions in which grain boundary motion takes place on time scales accessible to MD simulations, in which the defect free energy acts as a driving force additional to the capillary force due to excess grain boundary energy. This approach is particularly flexible in allowing arbitrary choices of misorientation axis and angle and providing an interface in which all grain boundary plane orientations are represented. This flexibility is at the cost of additional complexity in the system under study and we establish an approach suitable for analysing the excess grain boundary energy and the components of the force for boundary motion on a local basis over the surface of the shrinking grain. We demonstrate that this approach is able to resolve variation in the grain boundary energy related to the intrinsic grain boundary dislocation network in lower angle boundaries and to detect local maxima in the driving force due to the dislocation structure within the deformed grain. We show that the variation in measured grain boundary velocities is accounted for by the excess energy of the dislocations. We further show that the presence of dislocations in the shrinking grain has a tendency to reduce the anisotropy in grain boundary behaviour. Indicative evidence is presented that the dislocation network may act as a focal point for mechanisms of grain boundary migration.