Grain Boundary Dislocations Research Articles

Sensitivity of Dislocation-GB interactions to simulation setups in atomistic models

Dislocation-grain boundary (GB) interactions play an integral role in strengthening of crystalline materials, and are dependent on external loading conditions and atomic arrangements at the grain boundary. While molecular dynamics (MD) simulations can provide critical insights into relationships between these parameters and the exact interaction, the outcomes are sensitive to the computational setup. In this work, we explore the effect of computational setup (system size, GB orientation and boundary conditions) on stress-controlled dislocation-grain boundary (DGB) reactions using MD simulations for a well-studied copper symmetric 〈112〉 tilt boundary. The study demonstrates that the DGB reactions (pinning, absorption, transmission, etc.) are sensitive to the system size and the orientation of the GB to the applied stress. Additionally, the current work shows that there is a critical system size for the dislocation-grain boundary reaction stresses to converge, one which was found to be much larger than the setups in comparable MD studies. This was attributed to the specific stress states and the influence of surfaces, fixed or free, as these computational setups are modified. Finally, a stress-controlled setup with minimal model artifacts is examined, and the effect of coupled Schmid and non-Schmid stress components on the dislocation-GB reactions are discussed.

The evolution of pit morphology and growth kinetics in aluminum during atmospheric corrosion

Assessing the lifetimes of alloys in humid, corrosive environments requires growth kinetic information regarding individual instances of damage, e.g. pit growth rates. Corrosion rates measured at the continuum scale using mass change convolute the rate of pit nucleation and growth, providing limited information on local kinetics. The current study used in-situ X-ray computed tomography to measure growth rates of individual pits in aluminum over 100 h of exposure in a humid, chloride environment. While pits grew at relatively constant rates over the first hours after nucleation, significant growth-rate nonlinearities subsequently occurred. These were linked to both droplet spreading, which altered the cathode size, and changes in the mode of pit growth. Pit morphology appeared to influence the dominant growth mode and the duration of pit growth. Post-mortem serial sectioning revealed pits preferentially attacked grain-boundary triple junctions and dislocation boundaries.

Direct Visualization of Localized Vibrations at Complex Grain Boundaries.

Grain boundaries (GBs) are a prolific microstructural feature that dominates the functionality of a wide class of materials. The functionality at a GB results from the unique atomic arrangements, different from those in the grain, that have driven extensive experimental and theoretical studies correlating atomic-scale GB structures to macroscopic electronic, infrared optical, and thermal properties. In this work, a SrTiO3 GB is examined using atomic-resolution aberration-corrected scanning transmission electron microscopy and ultrahigh-energy-resolution monochromated electron energy-loss spectroscopy, in conjunction with density functional theory. This combination enables the correlation of the GB structure, nonstoichiometry, and chemical bonding with a redistribution of vibrational states within the GB dislocation cores. The new experimental access to localized GB vibrations provides a direct route to quantifying the impact of individual boundaries on macroscopic properties.

Nano Mechanical Characterization and Physical Modeling of Plastic Deformation Chapter 1: Dislocation-Grain Boundary Interaction as a Strengthening Factor

Mechanical behaviors of metallic materials in a small scale are characterized and physically modeled based on experimental measurements and computational simulation. Local plasticity in the vicinity of a grain boundary is considered in terms of an interaction between dislocation and grain boundary. It is shown experimentally that nanoindentation technique can detect a resistance to dislocation slip transfer by a grain boundary with a geometrical or a chemical factor. Molecular dynamic simulation revealed that the geometrical effect depends on not only a misorientation but also a combination with a dislocation character. TEM in-situ straining has a great potential to measure directly a critical stress upon slip transfer as well as a dislocation reaction at a grain boundary. Another potential reaction of a dislocation absorption at a grain boundary was discussed based on both the experimental results by TEM in-situ observation and the atomistic simulation in bcc metals.

A nucleation rate limited model for grain boundary creep

The motion of grain boundary (GB) dislocations, via a combination of glide and climb, mediates GB plasticity during processes such as creep and GB stress relaxation. Creep within the low stress regime, where non-Newtonian GB creep occurs, remains poorly understood. Models for GB dislocation nucleation rate limited creep, however, have not been sufficiently developed previously in the literature. Herein, a model for nucleation rate limited kinetics is developed and demonstrated to describe existing creep data reasonably well, using physically realistic materials properties as inputs to the model.

Dependency of grain boundary dislocation network configuration on generalized stacking fault energy surface in FCC metals

We use a microscopic phase-field (MPF) model, incorporating the generalized stacking fault energy (GSFE) surfaces (i.e., the γ-surface) as inputs, to investigate systematically the evolution of dislocation network configurations at low-angle pure twist grain boundaries (GBs) in six face-centered cubic (FCC) metals (Ag, Cu, Rh, Ir, Pd and Pt) that have different GSFE surfaces. The equilibrium configurations of GB dislocation networks are obtained via the interplay between the crystalline energy and the elastic strain energy during the energy minimization process. It was found that for {111} twist GBs, though the geometrically necessary dislocations (GNDs) of the screw type are observed, they can be either full or partial ones depending on the GSFE surfaces of the above metal systems. The areas of the staking faults regions formed between partial dislocations are quantitatively characterized as a function of the magnitude of the stacking fault energy, the elastic modulus and the GB misorientation angle. A fast-acting model for predicting the geometric characteristics of the GB dislocation networks is developed based directly on the material properties and GB misorientation, which is validated by its applications to another two metal systems (Au and Ni) with the same GB.

Experimental investigation of dislocation-grain boundary interaction in coarse-grained high‑manganese steels using quasi in situ electron channelling contrast imaging

Experimental investigation of dislocation-grain boundary interaction in coarse-grained high‑manganese steels using quasi in situ electron channelling contrast imaging

Energetic and atomic structural analyses of the screw dislocation absorption at tilt grain boundaries in BCC-Fe.

The dislocation-grain boundary (GB) interaction plays an important role in GB-related plasticity. Therefore, an atomistic investigation of the interaction provides a deeper understanding of the strength and fracture of polycrystalline metals. In this study, we investigated the absorption of a screw dislocation with a Burgers vector perpendicular to the GB normal and the corresponding symmetric tilt grain boundaries (STGBs) in BCC-Fe based on molecular static simulations focusing on the STGB-dislocation interaction energy and atomistic structural changes at GB. The STGB-screw dislocation interaction depends on the energetical stability of the STGB against the GB shift along the Burgers vector direction. When the interaction exhibited a large attractive interaction energy, the dislocation dissociation and the GB shift along the Burgers vector direction occurred simultaneously. The interaction energy reveals that the interaction depends on the energetical stability of the STGB in terms of the GB shift in addition to the geometrical descriptor of the GB type, such as the Σ value. The same behavior was also obtained in the reaction when the second dislocation was introduced. We also discuss the screw dislocation absorption and rearrangement of the GB atomistic structure in STGB from an energetic viewpoint.

Interaction of an edge dislocation with a 〈110〉 tilt boundary in nickel: molecular dynamics simulation

The interaction of a lattice edge dislocation with a 〈110〉 tilt boundary in nickel was studied by the molecular dynamics method in the case when the dislocation glide plane is parallel to the grain tilt axis. The dependence of the local threshold stress on the grain misorientation angle is obtained. It is found that with increasing misorientation angle, the local threshold stress increases, but the growth rate for low- and high-angle boundaries is different: for high-angle boundaries, this growth is slower. When studying the mechanism of overcoming the low-angle 〈110〉 tilt boundary by an edge dislocation in the considered orientation, it was found that the dislocation, in fact, does not pass through the boundary, but exchanges places with a grain-boundary dislocation, which are ordinary perfect edge dislocations in the low-angle 〈110〉 tilt boundaries. When studying the interaction of a dislocation with high-angle boundaries, a peculiarity was noticed, which consists in pushing two or three grain-boundary dislocations out of the boundary at once.

Shear-coupling of graphene grain boundaries: Elementary mechanisms, effects of topology, and role of buckling

Engineering grain boundary structure is a promising method for rational control of the microstructure and mechanical properties of two-dimensional materials. In bulk materials, shear stresses can drive grain boundary migration through the dislocations in grain boundaries. However, while shear coupling of grain boundaries has been studied in bulk materials like nanocrystalline copper, its translation to two-dimensional materials where out-of-plane deformation can relieve in-plane shear is not yet established. We investigate how the low flexural rigidity of graphene effects shear coupled grain boundary motion using atomic scale simulations of flat and buckled grain boundaries. We define the coupling shear strain as the strain at which a grain boundary has advanced by one Burgers vector and is at equilibrium and the critical shear strain as the strain at which migration of the first dislocation in the grain boundary becomes thermodynamically favorable. We show that the out-of-plane deformation does not influence the coupling shear strain and is governed only by the grain boundary topology. While the critical shear strain is altered somewhat by the low flexural rigidity due to buckling induced softening, it is also still dominated by the grain boundary topology. Our atomic scale results are synthesized into two models that predict the coupling and critical shears.

Molecular dynamics study on the effect of electric current on electrically-assisted scratching for crystal copper

Investigation of the effect of electric current on the plastic deformation mechanism of metals during the electrically-assisted machining process is significant in further improving surface properties. In this paper, the molecular dynamics (MD) method is adopted to simulate the electrically-assisted scratching process of crystal copper, obtaining and analyzing the surface morphology, potential energy change, von Mises stress distribution, and crystal defect structure evolution. The MD simulation results show that the electric current effectively expands the dislocation slip range, resulting in a larger plastic deformation zone. Meanwhile, the combined action of the electron wind forces and Joule heating causes more dislocations to proliferate and increases the dislocation density limit, enhancing the plastic deformation ability of the single-crystal copper. Furthermore, the electric current strengthens the dislocation-grain boundary interactions and reduces the hindering effect of the grain boundaries on dislocations, promoting more dislocations to cross the grain boundaries. This work will be helpful for guiding the optimization of surface strengthening techniques to get better surface properties of metals.

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.

Dislocation exhaustion and ultra-hardening of nanograined metals by phase transformation at grain boundaries

The development of high-strength metals has driven the endeavor of pushing the limit of grain size (d) reduction according to the Hall-Petch law. But the continuous grain refinement is particularly challenging, raising also the problem of inverse Hall-Petch effect. Here, we show that the nanograined metals (NMs) with d of tens of nanometers could be strengthened to the level comparable to or even beyond that of the extremely-fine NMs (d ~ 5 nm) attributing to the dislocation exhaustion. We design the Fe-Ni NM with intergranular Ni enrichment. The results show triggering of structural transformation at grain boundaries (GBs) at low temperature, which consumes lattice dislocations significantly. Therefore, the plasticity in the dislocation-exhausted NMs is suggested to be dominated by the activation of GB dislocation sources, leading to the ultra-hardening effect. This approach demonstrates a new pathway to explore NMs with desired properties by tailoring phase transformations via GB physico-chemical engineering.

Arrangement and Decomposition of Grain Boundary Dislocations: Two-Mode Phase-Field Crystal Simulation

The grain-boundary dislocation arrangement and decomposition during constant-volume deformation of a nanoscale bi-crystal system in fcc-structured materials were studied by using the two-mode phase-field crystal (2PFC) method. The effects of different grain boundary misorientations (GBMs) and tensile deformation directions on the dislocation arrangement and decomposition are analyzed. In three different symmetrical tilt grain boundaries evaluated by PFC, the atomic density profile of grain boundaries changed periodically at equilibrium. The initial grain boundary dislocation arrangement of the three samples is almost the same when tensile deformation is applied to the samples in the x- or y- direction, and all are symmetrically arranged in a “bowknot ”structure. The stress at the grain boundary is concentrated with the increase of strain, and dislocation decomposition can effectively reduce the stress concentration. The time steps of dislocation decomposition at grain boundaries decreases with increasing strain rate. This work facilitates the application of PFC in the analysis of grain-boundary mechanics in an extended range of materials.

Studying crack propagation along symmetric tilt grain boundary with H segregation in Ni by MD simulation

Hydrogen ingression in metals generally causes catastrophic failure. The H-induced fracture surface mostly exhibits intergranular feature, suggesting that grain boundary (GB) is the dominant crack propagation path. In this paper, the grand canonical Monte Carlo method was first performed to obtain equilibrated H distribution around various symmetric tilt grain boundaries (STGBs) in nickel. Then, atomistic simulations were conducted to study the influence of H segregation on the Mode I crack propagation behavior. In the absence of hydrogen, significant crack tip blunting and transgranular fracture are observed, implyinga tendency toward ductile transgranular fracture. However, in the case of H segregation, it is found that dislocation emission from the crack tip is enhanced but the crack tip still remains highly sharp. Moreover, crack propagation is dominated by alternate GB dislocation emission and GB cleavage, implying a tendency of ductile–brittle alternating intergranular fracture behavior. The H-inhibited GB migration and H-enhanced GB dislocation emission are believed to be the key mechanisms governing the H-assisted crack propagation along the GB.

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.

Disconnection-mediated motion of 〈110〉 tilt grain boundaries in α -Fe

It is possible for $\ensuremath{\langle}110\ensuremath{\rangle}$ tilt grain boundaries (GBs) in bcc metals to perform a conservative displacement by the glide of intrinsic GB line defects, namely, disconnections. The paper presents the characteristic of these defects in ${112}$ and ${332}$ twin boundaries, their vicinal GBs, and the ${116}$ GB studied by molecular dynamics simulation. The sources of disconnections and their interaction with the other GB dislocations are described together with their role in the shear-coupled GB migration. The absence of gliding disconnections in the ${111}$ GB impedes the shear-coupled GB migration, but two pure shuffles inside the coincident site lattice unit facilitate the transformation of the ${111}$ GB into ${110}/{001}$ or ${110}/{112}$ facets in regions of concentration of stresses.

Hydrogen assisted intergranular cracking of alloy 725: The effect of boron and copper alloying

To overcome the Hydrogen embrittlement (HE) susceptibility of the standard Alloy 725 (Mod A), two alloys with minor alloying modifications with B (Mod B) and B+Cu (Mod C) were produced. Then, the intergranular cracking susceptibility was investigated on bi-crystal beams by electrochemical in situ micro-cantilever bending test. The atom probe tomography and first principles calculations were employed to capture and calculate the grain boundary (GB) segregation and its effect on the GB cohesion. Cross-sectional view of the bent beams showed the superior resistance of Mod B against HE by facilitating the GB dislocation transfer/generation. While bending Mod A sample in hydrogen environment leads to form a sharp intergranular cracking, Mod B showed some nano-voids/cracks mostly in dislocation slip bands and rarely in GB path. However, a reduction of strength was observed in load-displacement (L-D) curves of Mod B. The addition of Cu, although not participated in GB segregation, compromised the lost strength of Mod B. In Mod C, after bending in H-charged condition, the nano-voids were formed in GB, but no load drop in L-D curves nor crack propagation in post-deformation observations was detected. The micro-alloying proposed in this study could be an important contribution to the future developing of H resistant alloys via GB segregation engineering.

Atomic-level study on the interaction of plastic slip with Σ3{112} tilt grain boundary and {112} twins in bcc metals

The $\mathrm{\ensuremath{\Sigma}}3{112}$ tilt grain boundary (GB) is found in many grains in bcc polycrystalline metals due to its low energy and high stability. Moreover, it is the coherent boundary of the ${112}$ twin. This paper studies the interaction of a pileup of $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ dislocations with the ${112}$ GB, extendable to the coherent ${112}$ twin boundary (TB). The results are applied to the interaction of the pileup of dislocations with the ${112}$ twin. The interacting dislocation is transformed into a GB dislocation (or TB dislocation) that acts as a source of disconnections responsible for the shear-coupled GB migration leading to twin growth or shrinkage when the interface is a TB. While a single dislocation cannot be transmitted through the interface, the stress field of the pileup facilitates the transmission if the tensile part of the dislocation core is closer to the interface than the compression part. The ${112}$ twin is found to create barriers to the motion of $1/2\ensuremath{\langle}111\ensuremath{\rangle}$ crystal dislocations, and the strength of the barrier depends on crystallographic parameters. The results obtained in the slip-TB interaction prove that there is no transmission of dislocations through the twin. Thus, under twinning shear stress, all twins are strong obstacles for the glide of dislocations. Under antitwinnning shear stress, twins with thickness less than a few nanometers (5.6 nm in Fe) are annihilated by the interaction with a pileup of dislocations, contributing to softening, whereas thicker twins block the propagation of dislocations and confine dislocations inside the twin, which contributes to hardening.

Atomic-scale study of dislocation-grain boundary interactions in Cu bicrystal by Berkovich nanoindentation

The plastic behavior of metal materials is mainly dominated by dislocation movement, whereas for polycrystalline metals, dislocation-grain boundary (GB) interactions play a significant role in the mechanical response. In the present work, deformation mechanisms of single crystal and bicrystal Cu workpieces subjected to Berkovich nanoindentation are studied by experiments and molecular dynamics (MD) simulations. The numerical model considers crystallographic orientations of the individual grains determined by electron backscatter diffraction (EBSD) characterization of the designated location on the polycrystalline surface. Experimental results indicate as the distance between the indentation and the GB decreases, the hardness of the tested specimen decreases and Young's modulus increases. Meanwhile, the corresponding simulation results reveal the anisotropic nature of dislocation evolution under the indenter for different crystallographic orientations of the single crystal Cu. Furthermore, GB as a complicated factor to the dislocation motion leads to the unusual deform behavior of materials in the vicinity of the GB, and dislocation-GB interactions introduce different deformation mechanisms. For the GB with a high angle of [1-11] 59.8°, dislocation slip along the GB is believed to play an important role in the decrease of the hardness.