Grain Boundary Migration Research Articles

Effect of Alcohol on the Mechanical and Electrical Properties of Ultrasonic Spot Welded Cu/Cu Joints

In order to improve the Cu/Cu joint quality, the bottom Cu sheet surface to be welded was dampened by a drop of absolute ethyl alcohol before the welding. Then, the ultrasonic spot welding (USW) was utilized to join a 0.5 mm thick Cu sheet to a 1.0 mm thick Cu sheet. The results demonstrated that, due to the action of the alcohol, obvious changes of the welding interface temperature, effective thickness, bond density, interface microstructure, joint resistance, micro-hardness, lap shear strength and fracture mode occurred in comparison with the joint without alcohol. Discontinuous dynamic recrystallization took place at the welding interface and facilitated the migration of grain boundaries across the contact interface, leading to the formation of the metallurgical bonding between the two Cu sheets.

Revealing Morphology Evolution of Lithium Dendrites by Large‐Scale Simulation Based on Machine Learning Force Field

AbstractSolving the dendrite growth problem is critical for the development of lithium metal anode for high‐capacity batteries. In this work, a machine learning force field model in combination with a self‐consistent continuum solvation model is used to simulate the morphology evolution of dendrites in a working electrolyte environment. The dynamic evolution of the dendrite morphology can be described in two stages. In the first stage, the energy reduction of the surface atoms induces localized reorientation of the originally single‐crystal dendrite and the formation of multiple domains. In the second stage, the energy reduction of internal atoms drives the migration of grain boundaries and the slipping of crystal domains. The results indicate that the formation of multiple domains might help to stabilize the dendrite, as a higher temperature trajectory in a single crystal dendrite without domains shows a higher dendrite collapsing rate. Several possible modes of morphological evolutions are also investigated, including surface diffusion of adatoms and configuration twists from [100] exposed surfaces to [110] exposed surfaces. In summary, reducing the surface and grain boundary energy drives the morphology evolution. Based on the analysis of these driving forces, some guidelines are suggested for designing a more stable lithium metal anode.

Atomistic simulation of shear deformation at bcc-Fe grain boundary and precipitation strengthening by Cr23C6

We investigate the temperature and the tilt angle dependence of the shear deformation behavior at a body-centered cubic (bcc)-Fe grain boundary (GB) by utilizing classical molecular dynamics simulations. We found that the shear deformation at a [001] axial bcc-Fe GB accommodates GB migration. The critical shear stress needed to induce GB migration was found to decrease with an increase in the temperature or molecular dynamics simulation time, because GB migration is a thermally activated process. The tilt angle dependence of the critical shear stress is successfully explained by the presence of a specific structure unit composed of under-coordinated atoms. It was suggested that Cr 23 C 6 precipitation within the bcc-Fe crystal decreases the critical stress required to generate dislocations, because of the interface between the precipitation and parent phases. On the other hand, Cr 23 C 6 precipitation on a [001] axial bcc-Fe GB was found to increase the critical shear stress, while the precipitation didn’t change the shear deformation mechanism. The degree of strengthening was verified by theoretical equation to evaluate the shear strengthening by the ordered precipitates.

Effect of grain size on Lüders-type deformation and plastic deformation of V nanowires/NiTi composites

In this paper, the grain size effect on Lüders-type deformation and plastic deformation of V nanowires/NiTi composites was investigated. The NiTi matrix grains size along radial direction was smaller than that along axial direction. The different grain boundary migration constant indicates that the V nanowires suppress the migration of grain boundary. When grain size increases, the deformation mode of annealed samples changed from a homogeneous deformation to a Lüders-type deformation. The plastic deformation of V nanowires led to a drop of lattice strain ε V 110 during the late loading stage. • V nanowires suppressed the grain growth of NiTi matrix along radial direction. • When grain size increases, the samples changed to a Lüders-type deformation. • The plastic deformation of V nanowires led to a drop of lattice strain ε V 110 .

Microstructural evolution and mechanical properties of micro-deformation diffusion bonding Inconel 617 superalloy

This study explicates the microstructural evolution and mechanical properties of the Inconel 617 joint, prepared through micro-deformation diffusion bonding. The joints were fabricated with a uniaxial deformation less than 2%. An interface without interfacial voids was achieved when the bonding temperature was increased from 1100 to 1150 °C. The microstructure of the joint devolved at 1150 °C under a holding time of 60 min, comprised of grain boundaries (GB), Cr-rich M 23 C 6 carbides, and α-Al 2 O 3 oxides. The similar microstructure was also observed in the joint prepared at 1150 °C with an incubation time of 180 min. As the bonding temperature was increased to 1180 °C, a continuous network of the M 23 C 6 carbide was nucleated along the bonding interface, accompanied by the grain coarsening, up to 3 times that of pre-bond substrate. The formation of the M 23 C 6 and Al 2 O 3 particles was governed by the diffusion and segregation of Cr, C, Al, and O along the bonding interface. The pinning effect of interfacial precipitates, the M 23 C 6 and Al 2 O 3 particles, inhibited the migration of GB across the bonding interface. The cracks were initiated and propagated along the bond line. The optimum combinations of strength, plasticity, and impact toughness were obtained for the two joint configurations, prepared at 1150 °C with the holding times of 60 and 180 min, respectively. A quasi-cleavage mode of fracture was revealed, consisting of cleavage planes as well as ductile dimples. • The joints of Inconel 617 superalloy were prepared by micro-deformation diffusion bonding. • The microstructural evolution was comprehensively investigated, and its evolution mechanism was elucidated. • The mechanical properties and fracture characteristics of the joints were evaluated.

The Interaction between He Bubble and Migrating Grain Boundary Induced by Shear Loading

This work reveals the interaction mechanism between He bubble and grain boundary (GB) in bicrystal copper under shear loading via molecular dynamics simulations. The influences of He/vacancy ratio RHe/V, temperature T0, and bubble diameter D0 on the interaction mechanism are clarified. Specifically, two interaction modes, i.e., the GB traverses or is pinned on He bubble, are observed by changing the initial RHe/V, T0, and D0. As RHe/V increases, the influence of He bubble on GB migration shows a decrease–increase trend. Different He bubble evolutions are demonstrated by comparing their shapes, pressure, and volume. In the cases of low RHe/V, the medium temperatures (10–300 K) are found to accelerate the GB migration, but higher temperatures (600–900 K) will lead to the change in interaction mode and deteriorate the interaction process. Furthermore, a more noticeable bubble-drag effect on GB migration is observed in the samples with larger He bubble.

Unraveling the electric field effect on the grain‐boundary migration in alumina through phase field modeling

AbstractIt is experimentally demonstrated that the electric field drives the grain‐boundary (GB) migration in ceramics, but this has not been interpreted mechanistically. This work develops a phase field model to study the GB migration in alumina (Al2O3) and validate through the comparison with previous experiments. Results show that the GBs move to the small grain domain. Under an electric field in the positive bias direction, GB migration is enhanced, whereas the migration to the small grain domain is inhibited under the electric field in the negative bias direction. The enhancement or inhibition effect becomes more pronounced with increasing the electric field. The high negative bias induces decrease in the GB migration velocity even with the migration direction altering. It is revealed that GB migrations are dominated by the competitive effect between the curvature and electric field driving forces, and an analytical expression of the critical electric field is derived.

Shape functions and kinetics of migrating grain boundaries in nanocrystalline materials

Grain boundary (GB) shapes determine driving force of curvature-driven GB migration and therefore is critical to grain growth kinetics. GB shape functions in nanocrystalline materials are hardly known due to experimental difficulties. This study represents the first one with regard to investigating shape functions of steadily migrating GBs at nanometer scale by using molecular dynamics. It is found that two types of GB migration kinetics occur depending on grain size and temperature, both differing from the one in coarse grain counterparts. The two kinetics correspond to different GB shapes. For the grains in a range of about 10–25 nm (the upper limit is expected to extend to sub-micron) at relatively low temperature, new shape functions have been successfully derived, while for the finest nanograins of around several nm at relatively high temperature their shape functions coincide with the ones developed for coarse grains. In addition, GB faceting occur for some GBs, which causes deviation of boundary shape from the functions proposed. The findings in the present study imply that the boundary shapes and migration kinetics may contribute to the specific grain growth kinetics in nanocrystalline materials.

Atomic-scale observation of dynamic grain boundary structural transformation during shear-mediated migration.

Grain boundary (GB) structural change is commonly observed during and after stress-driven GB migration in nanocrystalline materials, but its exact atomic scale transformation has not been explored experimentally. Here, using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we observed the dynamic GB structural transformation stemming from reversible facet transformation and GB dissociation during the shear-mediated migration of faceted GBs in gold nanocrystals. A reversible transformation was found to occur between (002)/(111) and Σ11(113) GB facets, accomplished by the coalescence and detachment of [Formula: see text]-type GB steps or disconnections that mediated the GB migration. In comparison, the dissociation of (002)/(111) GB into Σ11(113) and Σ3(111) GBs occurred via the reaction of [Formula: see text]-type steps that involved the emission of partial dislocations. Furthermore, these transformations were loading dependent and could be accommodated by GB junctions. This work provides atomistic insights into the dynamic structural transformation during GB migration.

Microstructures and mechanical properties of Mg–5Li–4Sn–2Al–1Zn alloy after hot extrusion

In this work, Mg–5Li–4Sn–2Al–1Zn deformed magnesium alloy was prepared and the effects of the extrusion process on its microstructure, texture evolution, and mechanical properties were investigated. The microstructures of as-homogeneous and as-extruded alloys were characterized by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the alloy underwent dynamic recrystallization (DRX) after hot extrusion processes. Drastic refinement occurred in grain size, and the weakening of the basal fiber texture was obvious after the Forward-Parallel Channel Extrusion process, and the microstructure was more uniform. As a result, the mechanical properties of the alloy were superior after the Forward-Parallel Channel Extrusion process, and the best combination of strength and plasticity was obtained after the 105° FPE process. YS, UTS, and EL were 187 MPa, 275 MPa, and 15%, respectively. The increase in strength is mainly attributed to grain boundary strengthening and the pinning effect of shattered particles on the migration of dislocations and grain boundaries. The increase in plasticity can be explained by the activation of non-basal slip and the weakening of the basal fiber texture, the fine grains can coordinate the plastic deformation well and improve the plasticity of the alloy to some extent.

An In Situ Resistance-Based Method for Tracking the Temporal Evolution of Recovery and Recrystallization in Ni-Base Single-Crystal Superalloy at Super-Solvus Temperatures

This study explores the influence of thermo-mechanical behavior and microstructure on recovery and recrystallization in single-crystal superalloys during casting and subsequent solution heat treatment, using miniature testing. Here, the temporal evolution of resistance was measured using in situ electrothermal mechanical testing (ETMT) to track the process of recovery and recrystallization (RX). It was found that recrystallization is dominant only when recovery is incomplete and is dependent on both the history dependence of the strain path as well as the magnitude of the accumulated plastic strain. A precursor to recrystallization is the occurrence of subgrains and deformation twins on the sample surface, where a characteristic butterfly-type morphology of γ′ precipitates is always observed in highly strained regions. The migration of RX grain boundaries is accompanied by the elimination of lattice curvature associated with the density of geometrically necessary dislocations. Homogenized samples provide the most reliable results, while interpreting resistivity changes with recovery and recrystallization is more challenging when inhomogeneity (microsegregation, local variation of mechanical properties) in as-cast material prevails.

Significant improvement in tensile properties of an advanced PM Ni based superalloy joint by HIP treatment

Tensile properties of an advanced powder metallurgy (PM) Ni based superalloy diffusion bonded joints were significantly improved by hot isostatic pressing (HIP) and the improvement mechanism was studied. Results showed that HIP treatment eliminated the weakly bonded areas and microvoids at the interface. In addition, HIP treatment also significantly promoted the migration of grain boundaries and decreased the dislocation density at the interface. After HIP treatment, the ultimate tensile strength of the joint bonded using a 2-μm-thick Ni coating reached 98.63% of the base alloy following the same thermal cycle.

On cyclic plasticity of nanostructured dual-phase CoCrFeNiAl high-entropy alloy: An atomistic study

In this study, we have employed molecular dynamics simulations to examine plastic deformation mechanisms of a “supra-nanometer-sized dual-phase glass-crystal” (SNDP-GC) high-entropy alloy (HEA) composite under cyclic loadings. This composite is produced by embedding CoCrFeNi HEA crystalline grains into a softer CoCrFeNiAl2 HEA glass. Cyclic loadings of different amplitudes are applied on one polycrystalline CoCrFeNi sample and two SNDP-GC HEA samples mentioned above. For the polycrystalline sample, dislocation motion and grain boundary (GB)-mediated deformation control the plastic deformation process, and the sample loses its original structure after a few cycles of the set amplitude due to GB migration. However, for the two SNDP-GC samples, as the grain boundary is replaced by the metallic glass (MG) phase, the dominant plastic deformation mechanism has changed to concentrating shear transformation in the MG phase. Our results also show that structural stability is highly dependent on the MG phase thickness. For the sample with a thinner MG layer, MG cannot accommodate sufficient deformation—so voids are generated in it. However, for the sample with a thicker MG phase, MG can store adequate deformations, thus dislocation initiations in crystalline grains are reduced and void generation is prevented in the MG phase.

Misorientation-dependent transition between grain boundary migration and sliding in FCC metals

Grain boundaries (GBs) play a crucial role in the mechanical behaviors of nanocrystalline materials. The dynamics of GBs depend on a variety of intrinsic and extrinsic factors, including GB misorientation, inclination, curvature, metal type and loading history. Controlling GB geometry and deformation behavior thus provides effective means to tailor the mechanical properties of nanostructured materials. However, the relationship between deformation mechanism and GB geometry is still lacking, largely due to the vast amount of metastable GB structures and deformation paths. Here, the relation between GB misorientation (θ, a critical degree of freedom for GB structure) and GB deformation mechanisms was studied by large-scale molecular dynamic simulation in face-centered cubic (FCC) metals. With increasing GB misorientation, the GB deformation mechanism transitions from migration to sliding and further to migration again. The critical transition thresholds (critical misorientations) vary with metal types. An energetic model that links deformation mechanism with GB structure was further developed with a focus on predicting the critical GB misorientation at which GB sliding supersedes GB migration. The influence of metal type on the transition threshold is well captured. We finally extend this model to general GBs with complex atomic structures and compare the theoretical predictions with data reported in the literatures and obtained from the present work. Our findings shed lights on the misorientation-dependent GB deformation mechanisms and can be utilized in GB engineering that pursues high-performance nanocrystalline metals.

On the discontinuous precipitation during recrystallization in a multicomponent alloy

Discontinuous precipitation (DP) reactions are among the most important solid-state phenomena that occur during the migration of grain boundaries (GBs) in many engineering alloys. Migration involves solute redistribution across the moving GB and the solute diffusion assumed to occur only along it. In this work, we show the atomic scale compositional analysis across a migrating GB during recrystallization in a multicomponent alloy that reveals evidence of bulk/lattice diffusion in a precipitate free buffer zone ahead of the RF, and solute segregations at local interfaces that aid to determine the rate-limiting solutes for DP reaction kinetics.

Migration of solidification grain boundaries and prediction

Solidification processing is essential to the manufacture of various metal products, including additive manufacturing. Solidification grain boundaries (SGBs) result from the solidification of the last liquid film between two abutting grains of different orientations. They can migrate, but unlike normal GB migration, SGB migration (SGBM) decouples SGBs from solidification microsegregation, further affecting material properties. Here, we first show the salient features of SGBM in magnesium-tin alloys solidified with cooling rates of 8−1690 °C/s. A theoretical model is then developed for SGBM in dilute binary alloys, focusing on the effect of solute type and content, and applied to 10 alloy systems with remarkable agreement. SGMB does not depend on cooling rate or time but relates to grain size. It tends to occur athermally. The findings of this study extend perspectives on solidification grain structure formation and control for improved performance (e.g. hot or liquation cracking during reheating, intergranular corrosion or fracture).

Influences of grain size, temperature, and strain rate on mechanical properties of Al0.3CoCrFeNi high–entropy alloys

This study investigates the mechanical properties and deformation behaviours of Al0.3CoCrFeNi high-entropy alloys (HEA) under tension tests by using molecular dynamics (MD) simulations. The effect of different grain sizes (d), temperatures, and strain rates are examined. By varying the grain size from 4.0 nm to 20.0 nm, the shift from mechanical softening to strengthening was observed for the simulated samples. The result shows that the critical grain size of the inverse Hall – Petch and Hall – Petch regime is 12.0 nm. The average flow stress varies slightly increases from 3.02 to 3.19 GPa (5.3%) with the decrease of grain size from 20.0 to 12.0 nm in the Hall – Petch relation. While in the inverse Hall – Petch regime, the average flow stress significantly reduces from 3.19 to 2.7 GPa (15.3%) with grain size reducing from 12.0 to 4.0 nm. The migration of grain boundary (GB), and the rotation of grains are the dominant deformation mechanism in the inverse Hall – Petch regime. Meanwhile, in the Hall-Petch region, the dislocation activities are the main contribution to the deformation mechanism of polycrystalline. The GB plays as a barrier to hinder the dislocation propagation. Therefore, in the Hall - Petch regime, the fraction of GB increases as the grain size decreases, resulting in higher barrier densities for samples with small grain sizes and thereby causing the flow stress to increase. The result points out that Young's modulus of the HEA specimen increased with an increase of the average d. In addition, the result also shows that the ultimate stress, flow stress, and Young's modulus increase with rising the strain rate or decreasing the temperature.

Atomistic dynamics of disconnection-mediated grain boundary plasticity: A case study of gold nanocrystals

• Representative nucleation, propagation and interaction dynamics of grain boundary (GB) disconnections are visualized at the atomic scale. • Sources and barriers of different disconnection nucleation dynamics in Σ11(113) GB are quantitatively compared. • The universal interlinks and transitions among different disconnection dynamics are systematically elucidated, which dominate GB migration. • A conceptual map of disconnection-mediated GB plasticity is established. Grain boundary (GB) mediated deformation is a vital contributor to the plasticity of polycrystalline materials, where the disconnection model has become a widely recognized approach to depict the GB dynamics. However, experimental understanding of the atomistic disconnection dynamics remains scarce. In this case study of gold nanocrystals, atomistic disconnection dynamics governing the shear-coupled migration of flat GBs have been systematically investigated via in situ transmission electron microscopy nanomechanical testing supported by molecular dynamics simulations. Specifically, the site-dependent nucleation, shear-driven propagation, and diverse interactions associated with distinct GB disconnections are systematically elucidated and quantitatively compared. Moreover, the disconnection-mediated GB plasticity proves to prevail among different tilt and mixed GBs in gold. Eventually, a conceptual map of disconnection-mediated GB dynamics is established, which would furnish a unified understanding of GB plasticity in metallic materials.

In situ observation of atomic-scale processes accomplishing grain rotation at mixed grain boundaries

A detailed monitoring of atomic-scale processes accomplishing grain rotation is important for understanding the deformation mechanisms in nanocrystalline metals. However, direct observations have been rare thus far, such that our knowledge about grain rotation has to rely heavily on hypothetical models and simulations. Here, we present in situ atomic-resolution evidence, indicating that the atomic processes accomplishing grain rotation in nanocrystalline Pt depend on the type of grain boundary (GB) separating the rotating grains. The grains around general (mixed) GBs rotate via the Frank–Bilby dislocation activities together with atomic shuffling and disconnection activities, the former being the generation, climb, glide and reaction of GB dislocations, whereas the latter leading to GB migration accompanying the grain rotation. While for the grains with a tilt GB in between: their rotation is accomplished almost entirely via the Frank–Bilby dislocation activities. We also discover that the GB dislocation climb, glide, and reaction often involve the formation and destruction of Lomer-like dislocations.

Understanding Micro and Atomic Structures of Secondary Phases in Cu-Doped SnTe.

Highly efficient thermoelectric materials require, including point defects within the host matrix, secondary phases generating positive effects on lowering lattice thermal conductivity (κL ). Amongst effective dopants for a functional thermoelectric material, SnTe, Cu doping realizes the ultra-low κL approaching the SnTe amorphous limit. Such effective κL reduction is first attributed to strong phonon scattering by substitutional Cu atoms at Sn sites and interstitial defects in the host SnTe. However, other crystallographic defects in secondary phases have been unfocused. Here, this work reports micro- to atomic-scale characterization on secondary phases of Cu-doped SnTe using advanced microscopes. It is found that Cu-rich secondary phases begin precipitation ≈1.7 at% Cu (x= 0.034 where Sn1- x Cux Te). The Cu-rich secondary phases encapsulate two distinct solids: Cu2 SnTe3 ( ) has semi-coherent interfaces with SnTe ( ) such that they minimize lattice mismatch to favor the thermoelectric transport; the other resembles a stoichiometric Cu2 Te model, yet is so meta-stable that it demonstrates not only various defects such as dislocation cores and ordered/disordered Cu vacancies, but also dynamic grain-boundary migration with heating and a subsequent phase transition ≈350 °C. The atomic-scale analysis on the Cu-rich secondary phases offers viable strategies for reducing κL through Cu addition to SnTe.