Grain Boundary Motion Research Articles

Improved ductility by coupled motion of grain boundaries in nanocrystalline B2-FeCo alloys

Embedded-atom method (EAM) potential for the FeCo binary systems has been improved to reproduce the antiphase boundary (APB) energies, crucial to the mechanical properties for ordered alloys. Using the improved potential, we further carried out tensile simulations on nanocrystalline FeCo models to investigate the effect of disordered grain boundaries (GBs) on the intergranular fracture. We found that the large concentrated stress at GBs ascribed to the poor dislocation behavior and plastic-deformation compatibility caused the occurrence of microcracks. As the degree of order at GBs was reduced, the GB motion can be enhanced contributed to the smaller GB migration energy barrier. The GB-dominant plasticity can improve the plastic-deformation compatibility and release the concentrated stress at GBs to prevent the generation of microcracks and intergranular fracture. Collectively, the enhancement of coupled GB motion by disordered GBs opens more pathways to improve the mechanical properties for both nanostructured and conventional polycrystalline alloys with ordered superlattice structures.

Effects of Sn addition on dynamic grain growth in a binary Mg-1Ca alloy

The possibility of dynamic grain growth of a binary Mg-1Ca alloy with minor tin contents was investigated at various conditions of temperature and strain rate. Mg-1Ca, Mg-1Ca-1Sn and Mg-1Ca-2Sn alloys were cast and homogenized at 410 °C followed by hot-rolling at 400 °C with equal strains. Calcium contributed to refining of grains, with an additional support from tin. In the as-cast alloys, tin was found to promote dendrite formation. Though presence of Ca2Sn phase was predicted by thermodynamic modelling, microstructurally a ternary phase MgCaSn was predominantly found. This ternary phase was also comparatively coarser in size. Uniaxial tensile test of rolled samples was carried out at temperatures 300, 350, 400 and 450 °C, and at two different strain rates 5 × 10−4 s−1 and 1 × Variation of grain distribution was also measured at the grip and gauge regions at different locations. Samples were taken from six different locations of the deformed samples. Even at faster deformation, the grains were not elongated. At slower strain rate condition, extent of dynamic grain growth was higher compared to that of the higher strain rate condition. Mg2Ca particles were not effective in restricting movement of grain boundaries. Mg2Sn particles were dissolved in the matrix at the elevated temperatures, and the grain growth limiting was profoundly dominated by the presence of MgCaSn phase.

Lattice transformation in grain boundary migration via shear coupling and transition to sliding in face-centered-cubic copper

Migration of symmetric tilt grain boundaries (GBs) via shear coupling has been studied extensively in experiments and simulations. It was reported that shear coupling transitioned to GB sliding at high temperatures, but how such transition occurs at low temperatures has not been investigated. Lattice transformation on the atomic scale during shear coupling has not been fully understood. In this work, mode of motion of symmetric tilt GBs with [001] tilting axis in face centered cubic copper under a shear strain parallel to the boundary plane at 100 K was carefully characterized by tracking the positions of the corresponding planes in atomistic simulations and new features of GB motion were observed. The results show that the angles between the two low-index planes, (110) and (100), and the boundary plane can be used to define a nominal magnitude of shear s. Approximately, if one of these two planes has a value of s < 0.5, shear coupling occurs with this plane being the active invariant plane; if 0.5 < s < 0.6, GB moves by shear coupling + sliding, i.e. a hybrid mode by which shear coupling transitions to sliding; if both planes have a value of s> 0.6, only GB sliding occurs. Careful structural analyses show that, for all the GBs that undergo shear coupling, some GB atomic planes remain invariant, very similar to the first invariant plane in deformation twinning, whereas the other GB atomic planes swap their positions in the GB normal direction through highly coordinated and complex atomic shuffles. This behavior allows identification of transformation units that are reoriented toward the neighboring grain. Rate-limiting factors are identified for lattice transformation and can be used to infer a kinetics model for shear coupling.

Study on Flow Behavior and Processing Maps of High‐Ti Low‐C Microalloyed Steel during Hot Compression

Deformation behavior of a high‐Ti low‐C microalloyed steel is studied by isothermal compression tests at temperatures of 800–1100 °C and strain rates of 0.1–10 s−1. The results indicate that the lower the strain rate and higher the temperature of steel, the more likely is the dynamic recrystallization. The stress–strain predicted by the constitutive model, which is established based on the experimental data, are in good agreement with the experimental results. It is noted from the processing maps of steel at various strains that there is instability zone which should be avoided during thermal processing at strain rate less than 1 s−1. The range of 850–1100 °C and 5–10 s−1 strain rate is more conducive to the processing of experimental steel. The movement of grain boundaries, subboundaries, and dislocations is hindered by precipitates in steel. In addition, the mean grain size of austenite at 0.1 s−1 is larger than 5 s−1. This is mainly because low strain rate provides more time for growth of grains.

Creep Behavior of Diffusion-Welded Alloy 617

Plate-type heat exchangers are anticipated to be used in the next-generation nuclear industry, and solid-state diffusion welding is a critical technology for building plate-type heat exchangers with high integrity. In this study, we manufactured a diffusion weldment and evaluated its creep behavior. Microscopic analysis revealed that Al-rich oxides were developed along the interface, significantly impeding grain-boundary movement across the interface. Oxide-containing planar grain boundaries resulted in premature brittle fracture at the interface with less than 9% creep strain under all test conditions. The time to rupture and time to 1% creep strain of the diffusion weldment were less than those of the as-received alloy, while the slopes in double-logarithmic plots were almost identical for both alloys. In a Larson–Miller parameter study, the stress to rupture of the diffusion weldment reached 95.59% of that of the as-received alloy, whereas the stress to 1% creep strain steeply decreased in the low-stress range.

Evolution of residual stress and its impact on Ni-based superalloy

Severe residual stresses generated by various preparative techniques can greatly affect the mechanical properties in the Ni-based superalloy parts. Limited data is currently available for deeply understanding the origin of residual stress at nanoscale and in particular, for establishing the relationship between the residual stress and mechanical property to accelerate the development of the Ni-based superalloys. Here, the effect of the heat treatment on the grain growth, residual stress and flow stress in Ni-based superalloy is investigated by using an atomic simulation combined with a physically-based theoretical model. The dissociation of grain boundary causes the coalescence of adjacent grains during annealing, which is observed by atomic simulation. However, this process is different from the previous work that the curvature-driven grain-boundary migration leads to the grain growth. The residual stress consists of the grain and grain boundary, where the average value from grain boundary is about 50% higher that from the grain based on the statistical result. The grain growth and local movement of grain boundary cause the stress relaxation, and in turn they reduce the residual stress. The evolution of the residual stress is in good agreement with the previous experiment. Further, an analytical theoretical model is proposed to qualitatively evaluate the variation trend of the residual stress with the annealing temperature. It is also proved that the residual stress decreases with the increase of the annealing temperature. The present work can be applied to determine the residual stress in the annealed structure, which greatly reduces the process flow, therefore provides a scientific basis to turn the technological process for the accelerated development of the new Ni-based alloys.

Equation of motion for grain boundaries in polycrystals

Grain boundary (GB) dynamics are largely controlled by the formation and motion of disconnections (with step and dislocation characters) along with the GB. The dislocation character gives rise to shear coupling; i.e. the relative tangential motion of two grains meeting at the GB during GB migration. In a polycrystal, the shear coupling is constrained by the presence of other grains and GB junctions, which prevents large-scale sliding of one grain relative to the other. We present continuum equations of motion for GBs that is based upon the underlying disconnection dynamics and accounts for this mechanical constraint in polycrystals. This leads to a reduced-order (zero-shear constrained) model for GB motion that is easily implemented in a computationally efficient framework, appropriate for the large-scale simulation of the evolution of polycrystalline microstructures. We validated the proposed reduced-order model with direct comparisons to full multi-disconnection mode simulations.

The formation mechanism of stacking faults in ZrFe2 Laves phase in Zircaloy-4 alloy

The role of grain boundary (GB) migration in producing stacking faults (SFs) in nanosized C15 structured ZrFe2 Laves phase precipitate was studied by transmission electron microscopy (TEM). Results revealed that the interaction between the studied second phase particle (SPP) and the GB was occured, which result in the curvature change of GB near the SPP. The movement of GB was hindered by the SPP which changed the normal migration of GB to that leaving behind a large number of faulted stacking in the SPP. In addition, the SPP was embedded in the α-Zr grain matrix; therefore, a dragging force of GB was applied on the SPP. Further deformation engendered the stacking faults in the centre of SPP by virtue of the maximum dragging force on the GB.

Motion of Grain Boundaries with Dynamic Lattice Misorientations and with Triple Junctions Drag

Most technologically useful materials are polycrystalline microstructures composed of myriad small monocrystalline grains separated by grain boundaries. The energetics and connectivities of grain b...

Large time asymptotic behavior of grain boundaries motion with dynamic lattice misorientations and with triple junctions drag

Many technologically useful materials are polycrystals composed of a myriad of small monocrystalline grains separated by grain boundaries. Dynamics of grain boundaries play an essential role in defining the materials properties across multiple scales. In this work, we study the large time asymptotic behavior of the model for the motion of grain boundaries with the dynamic lattice misorientations and the triple junctions drag.

Prismatic Edge Dislocations in Graphite

Dislocations are a central concept in materials science, which dictate the plastic deformation and damage evolution in materials. Layered materials such as graphite admit two general types of interlayer dislocations: basal and prismatic dislocations, of which prismatic dislocations have been relatively less studied. Using density functional theory (DFT) calculations, we have examined different prismatic core structures in graphite and evaluated their structure, energetics and mobility. We find close energetic interplay between bonded and “free-standing” core structures in both zigzag and armchair directions, with a reconstructed stable zigzag core identified. We explore grain boundaries and prismatic dislocation pile-up, identifying metastable structures which may be important in energy storage. The role of interlayer stacking in core structure, dislocation glide and climb is also considered in-depth. Our calculations suggest that the prismatic dislocation core is stable up to high temperatures of approximately 1500K in bulk graphite. Above this temperature, the breaking of bonds in the dislocation core can facilitate climb, grain-boundary motion, and the annealing of damage through prismatic dislocation glide.

The Effects of a Trace Amount of Manganese and the Homogenization on the Recrystallization of Al-7Mg-0.15Ti Alloys.

The aim of this study is to explore the effects of Manganese addition and homogenization treatment on the microstructures and mechanical properties of the Al–7Mg–0.15Ti (B535.0) alloy. The optical microscopy, electrical conductivity measurements, transmission electron microscopy, scanning electron microscopy (SEM + EBSD), as well as Rockwell hardness and tensile tests, were exploited for this purpose. The main objectives are to refine the grain size, inhibit grain growth in the annealed state, and enhance the mechanical strength of the alloy. The results show that the addition of manganese to the Al–7Mg–0.15Ti alloys refined the as-cast and recrystallized grains of the alloys. During the homogenization process, Al4Mn high-temperature stable dispersoids were precipitated in the aluminum matrix. After annealing, the Al4Mn particles blocked the movement of grain boundaries during the growth of the recrystallized grains and inhibited grain growth. Consequently, the annealed alloys showed grain refinement and dispersion strengthening. The Al4Mn dispersoids of the alloys with manganese added were smaller and denser after a two-stage homogenization process compared to those that underwent a one-stage homogenization process. By contrast, for the alloys without the addition of manganese, the recrystallized grains showed normal growth after annealing, and different homogenization processes had no significantly different effects.

Fracture and wear mechanisms of FeMnCrNiCo + x(TiC) composite high-entropy alloy cladding layers

FeMnCrNiCo + x(TiC) composite high-entropy alloy (HEA) coatings were manufactured by laser cladding. Then the small punch and wear experiments at room temperature were conducted. The phases and microstructures of the composite HEA coatings were characterized using XRD, SEM/EDS, EBSD and TEM, and the strengthening, fracture and wear mechanisms of coatings were systematically analyzed and discussed. The results showed that small-sized particles and large-sized particles in coatings reduced the nucleation energy barrier of grain, and particles precipitated at grain boundaries hindered the movement of grain boundary. And the refined grain and increased dislocation density enhanced the capability of coatings to resist potential plastic deformation. As for the properties, the strength of the coating with 5 wt% TiC was increased, but cracks easily initiated and propagated around the precipitated particles at grain boundaries, which lowered the toughness. The excess TiC made the brittle and large-sized particles become cracking sources in coatings, thus the strength and toughness of coatings were deteriorated. Obvious fatigue wear tendency in these two coatings (5 wt% & 10 wt%) influenced their service life in the wear environment, although the coating with 10 wt% TiC had better surface wear performance.

Shear-assisted grain coarsening in colloidal polycrystals

Grain growth under shear annealing is crucial for controlling the properties of polycrystalline materials. However, their microscopic kinetics are not well understood because individual atomic trajectories are difficult to track. Here, we study grain growth with single-particle kinetics in colloidal polycrystals using video microscopy. Rich grain-growth phenomena are revealed in three shear regimes, including the normal grain growth (NGG) in weak shear melting-recrystallization process in strong shear. For intermediate shear, early stage NGG is arrested by built-up stress and eventually gives way to dynamic abnormal grain growth (DAGG). We find that DAGG occurs via a melting-recrystallization process, which naturally explains the puzzling stress drop at the onset of DAGG in metals. Moreover, we visualize that grain boundary (GB) migration is coupled with shear via disconnection gliding. The disconnection-gliding dynamics and the collective motions of ambient particles are resolved. We also observed that grain rotation can violate the conventional relation [Formula: see text] (R is the grain radius, and θ is the misorientation angle between two grains) by emission and annihilation of dislocations across the grain, resulting in a step-by-step rotation. Besides grain growth, we discover a result in shear-induced melting: The melting volume fraction varies sinusoidally on the angle mismatch between the triangular lattice orientation of the grain and the shear direction. These discoveries hold potential to inform microstructure engineering of polycrystalline materials.

Grain Boundary Motion in Two-Dimensional Hexagonal Boron Nitride.

An in-depth understanding and precise controlling of grain boundary (GB) motion at the atomic scale are crucial for grain growth and recrystallization in polycrystalline materials. So far, the reported studies mainly focus on the GB motion in the ideal bicrystal system, while the atomic mechanisms of GB motion in polycrystals remain poorly understood. Herein, taking two-dimensional (2D) hexagonal boron nitride (h-BN) as a model system, we experimentally investigated the atomic-scale mechanisms of the GB motion in 2D polycrystals. Since GB motion is directly related to the GB structures, this article is organized following the configurations of GBs, which can be divided into straight (including symmetric and asymmetric GBs) and curved GBs. We revealed that (I) for symmetric GBs, the shear-coupled motion alone is insufficient to drive the continuous GB motion in polycrystalline materials, and GB sliding is also needed. (II) For asymmetric GBs, GB motion follows a defaceting-faceting process, in which dislocation reactions are crucial. (III) For curved GBs, shear-coupled GB motion (during grain shrinking) leads to grain rotation, and the rotation direction highly depends on the misorientation angles. (IV) Finally, we will discuss the characteristics of binary lattice h-BN and find that partial dislocations participate in the GB motion at high misorientation angles (>38°). Our results build up the framework of the atomic-scale mechanisms of the GB motion in 2D polycrystalline materials and will be instructive for technological applications such as grain growth and GB engineering.

Extracting grain boundary motion in Cu–Al alloy from atomistic simulations

Abstract

Reevaluation of metal interconnectivity in a partially molten silicate matrix using 3D microtomography

Understanding metal-silicate differentiation in small rocky bodies that accreted early in solar system history requires quantification of the effects of variable amounts of silicate melt and molten metal on the connectivity of metal-rich liquids. To shed light on this question, the equilibrium geometry and textural ripening of metal grains in the vicinity of the metal interconnection threshold have been determined experimentally. High pressure and temperature experiments were performed in the three-phase system forsterite + silicate melt + nickel at conditions of 1 GPa and up to 2080 K using piston-cylinder and Paris-Edinburgh presses. Sample textures were analyzed by 3D X-Ray microtomography either in-situ at the PSICHE beamline of the SOLEIL synchrotron, or on quenched samples using a laboratory Computed Tomography scan. Although dihedral angles point to textural equilibrium at the scale of individual grains at the end of each experiment, the attainment of textural equilibrium at sample scale is not straightforward. Depending on the relative proportions of the phases, different states of textural maturation are revealed. A particularly important issue is that time-resolved in-situ microtomography data show that cold (subsolidus) compression at the beginning of the experiment leads to soft metal grains being squeezed between silicates, leading to a forced interconnectivity of nickel. High temperature experiments with metal contents ≤20 vol% resulted in disruption of these forced networks, while the networks persisted in time for metal contents ≥25 vol%. This is taken to indicate that the stable interconnection threshold of pure nickel in a partially molten silicate matrix lies between 20 and 25 vol%. Therefore, we conclude that care must be taken when defining the interconnection threshold: not only should there be existence of a network of touching grains, but this network must persist in time in the absence of external forces and at pressure-temperature conditions that permit grain-boundary movement (i.e. excluding kinetically arrested systems). Growth of silicate grains is identified as the process driving textural maturation, and may explain the variability of interconnection thresholds reported in the literature. In addition, these considerations shed light on the diversity of textures observed in natural meteoritic samples (e.g. carbonaceous and ordinary chondrites and primitive achondrites), providing textural arguments to constrain the processes that affected these meteorites.

Slip of low-angle tilt grain boundary (110) in FCC metals at perpendicular shear

Grain boundaries (GBs) make a significant contribution into the kinetics of plastic deformation of both polycrystalline and nanocrystalline metals. Among others, motion of GBs is an important phenomenon, which has to be taken into account in the consideration of the thermo-mechanical response of poly- and nanocrystals. In this work, we study motion of low-angle tilt GBs and the accompanying relaxation of shear stresses in the case, when the shear is applied perpendicular to GB. Using the molecular dynamics (MD) simulations, we consider bicrystals made of aluminum, copper and nickel as FCC metals with different stacking fault energies. MD data are used for construction and verification of a GB motion model, which is based on the previously proposed equation of motion of a solitary dislocation in a single crystal. Within this model, a low-angle tilt GB is presented as a wall of periodically located perfect edge dislocations. The main driving factors are the external shear stress and the local stress distribution due to the plastic relaxation in the trace of each dislocation creating an area of partially relaxed stresses behind the shifted GB. These factors define the average GB velocity during the main stage of motion, while dislocation-dislocation interactions between the walls and inside each wall define details of the motion, such as the curvature of GBs and their mutual acceleration at the late stage of interaction of two approaching opposite GBs.

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.

Direct atomistic modeling of solute drag by moving grain boundaries

We show that molecular dynamics (MD) simulations are capable of reproducing the drag of solute segregation atmospheres by moving grain boundaries (GBs). Although lattice diffusion is frozen out on the MD timescale, the accelerated GB diffusion provides enough atomic mobility to allow the segregated atoms to follow the moving GB. This finding opens the possibility of studying the solute drag effect with atomic precision using the MD approach. We demonstrate that a moving GB activates diffusion and alters the short-range order in the lattice regions swept during its motion. It is also shown that a moving GB drags an atmosphere of non-equilibrium vacancies, which accelerate diffusion in surrounding lattice regions.