Shear-coupled Grain Boundary Migration Research Articles

Effect of irradiation defects on the plastic slip of {112} grain boundary: Atomic scale study

Σ3{112} grain boundary (GB) is known to exhibit plastic slip in bcc metals, which overall contributes to the good ductility and toughness of such metals as iron, matrix element for the steels applied in nuclear industry. During nuclear operation, the neutron irradiation causes formation of point defects and their clusters which lead to the non-equilibrium mass transport and agglomeration of nanoscale defects inside grains and near the grain boundaries. Due to the effect of chemical segregation, GBs can encounter sessile obstacles preventing the GB plastic slip. Understanding the influence of the irradiation defects segregated at the grain boundaries is important for the development of structural steels for nuclear fusion and fission applications. The present paper studies the influence of the irradiation defects, such as Cr precipitates, He bubbles and voids on the plastic slip of the GB interface. The studied defects are found to act as strong obstacles to the glide of the elementary disconnections responsible for the shear-coupled GB migration. The interaction between a single elementary disconnection and a series of them provided by a source has been studied. As a result of the interaction with impenetrable defects the formation of residual defects such as loops is observed.

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.

Shear-coupled migration of grain boundaries: the key missing link in the mechanical behavior of small-grained metals?

Grain size reduction is a very efficient way to block dislocation movements and therefore create very strong metals and alloys. Not only grain boundaries are known obstacles for dislocations, but when reaching nanometer dimensions, crystallites usually become dislocation free, which imposes an additional constraint to develop plasticity. A recent effort to understand grain boundaries-based deformation mechanisms has therefore emerged. These mechanisms can be manifold, involving conservative and diffusive processes that are very poorly understood. A first approach consisting in downscaling mechanisms that are documented at large scale such as Coble creep, proved very limited. On the other hand, stress-assisted grain growth or shear-coupled grain boundary migration, that were recently observed in small-grained materials at room or low temperature may provide a crucial step to fully understand dislocation-less plasticity in nanocrystals. As this is a completely new field with many more degrees of freedom, a continuous research effort has to be carried out to link the mechanical properties of nanocrystals to these mechanisms specifically linked to grain boundaries.

Role of sessile disconnection dipoles in shear-coupled grain boundary migration

Shear-coupled grain boundary (GB) migration has been evidenced as an effective plastic mechanism in absence of the usual intragranular dislocation activity. GB migration occurs through the nucleation and further motion of disconnections. For perfect GBs, the activation barrier for migration is dominated by the disconnection nucleation. In this study, we examine the effects of a dipole of sessile disconnections on the shear-coupled GB migration using molecular dynamics simulations on a $\mathrm{\ensuremath{\Sigma}}41[001](540)$ symmetric tilt GB in aluminum. The first effect is observed on disconnection nucleation: we show that the disconnection dipole can operate as a source of mobile disconnections. The corresponding yield stress is weakly affected, but the GB migration energy barrier can be reduced by $35%$. The second effect is seen on the disconnection mobility: the sessile disconnection dipole impedes the disconnection motion by repulsing or attracting it. We conclude that the influence of such dipole on the GB migration favors the nucleation of mobile disconnections on one hand but slows down their motion on the other hand.

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.

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.

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

Abstract

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.

Atomic processes of shear-coupled migration in {112} twins and vicinal grain boundaries in bcc-Fe

Tilt ${112}$ grain boundaries (GBs) in bcc metals perform shear-coupled grain-boundary migration by the creation and glide of disconnections. Disconnection dipoles may be created at the pristine GB at high stresses or may be generated at the core of a GB dislocation that acts as a source of disconnections. We characterize this source in terms of its Burgers vector, denoted ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$, and describe the mechanism that allows the source to move conservatively with the GB. The ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$ grain-boundary dislocation is created, for instance, during the absorption of a crystal dislocation by the ${112}$ grain boundary. In addition, ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$ accommodates ${112}$ vicinal grain boundaries that are formed by segments of ${112}$ planes separated by ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$ grain-boundary dislocations. The presence of these ${\stackrel{P\vec}{b}}_{1/\ensuremath{-}1}$ dislocations facilitates the conservative displacement of both the pristine and the vicinal GBs. We show that the creation of disconnections is the key for the absorption of edge and screw dislocations by the GB and the drag of mixed dislocations by the GB during its migration. These conservative processes are efficient ways to accommodate plastic deformation by the growth and shrink of ${112}$ twins, and shear-coupled motion of the ${112}$ GB and its vicinal GBs.

Interaction of precipitate with shear–coupled grain boundary migration

Shear-coupled grain boundary migration (SCGBM) has already been observed in extensive experiments and serves as an effective mechanism for the grain growth in nanocrystalline materials. Meanwhile, it is well known that precipitates strongly inhibit the motion of GBs. However, the effect of precipitates on SCGBM is not clear. In this paper, a theoretical model is established to investigate the role of precipitates on SCGBM. The effects of the characteristics of the precipitate and the GB are studied in detail. It is found that precipitates can significantly inhibit SCGBM. Interesting, the results show that the pinning effect exerted by precipitates on SCGBM is strongly relevant to their positions. Moreover, the resistance for SCGBM hindered by precipitates can be controlled by tailoring the characteristics of both the precipitates and GBs. In addition, it is discussed that a possible recrystallization occurs when the GB is hindered and it is found that the SCGBM process also facilitates it.

A synergetic grain growth mechanism uniting nanograin rotation and grain boundary migration in nanocrystalline materials

A theoretical model has been developed to analyze the energetic characteristic of a cooperative grain growth process in nanocrystalline materials. We proposed that the process can be realized by rotating the adjoining grains first and then migrating the boundary that separates the grains, or vise versa, or by enabling both rotation and migration simultaneously. It is revealed that the synergetic process is energetically more favorable than the pure individual growth processes, i.e., nano-grain rotation or shear-coupled migration of grain boundaries. The critical stress that is required to initiate the growth process is also considerably reduced. The findings suggest a new general mode of grain growth in nanocrystalline metals.

In situ atomistic observation of disconnection-mediated grain boundary migration

Shear-coupled grain boundary (GB) migration is of general significance in the deformation of nanocrystalline and polycrystalline materials, but comprehensive understanding of the migration mechanism at the atomic scale remains largely lacking. Here, we systematically investigate the atomistic migration of Σ11(113) coherent GBs in gold bicrystals using a state-of-art in situ shear testing technique combined with molecular dynamic simulations. We show that shear-coupled GB migration can be realised by the lateral motion of layer-by-layer nucleated GB disconnections, where both single-layer and double-layer disconnections have important contributions to the GB migration through their frequent composition and decomposition. We further demonstrate that the disconnection-mediated GB migration is fully reversible in shear loading cycles. Such disconnection-mediated GB migration should represent a general deformation phenomenon in GBs with different structures in polycrystalline and nanocrystalline materials, where the triple junctions can act as effective nucleation sites of GB disconnections.

Shear-coupled grain-boundary migration dependence on normal strain/stress

In specific conditions, grain boundary (GB) migration occurs in polycrystalline materials as an alternative vector of plasticity compared to the usual dislocation activity. The shear-coupled GB migration, the expected most efficient GB based mechanism, couples the GB motion to an applied shear stress. Stresses on GB in polycrystalline materials have however seldom a unique pure shear component. This work investigates the influence of a normal strain on the shear coupled migration of a Σ13(320)[001] GB in a copper bicrystal using atomistic simulations. We show that the yield shear stress inducing the GB migration strongly depends on the applied normal stress. Beyond, the application of a normal stress on this GB qualitatively modifies the GB migration: while the Σ13(320)[001] GB shear-couples following the 110 migration mode without normal stress, we report the observation of the 010 mode under a sufficiently high tensile normal stress. Using the Nudge Elastic Band method, we uncover the atomistic mechanism of this 010 migration mode and energetically characterize it.

Uniaxial stress-driven grain boundary migration in Hexagonal Close-packed (HCP) metals: Theory and MD simulations

Stress-driven grain boundary (GB) migration is an important plastic deformation mechanism in nanopolycrystalline metals. Recently, Liu et al. (2014) and Zong et al. (2015) have shown a new mechanism for GB migration in a Hexagonal Close-packed (HCP) bicrystal with 90° GB misorientation under uniaxial stress loading conditions, which is different from shear-coupled GB migration. In real polycrystals, GBs have random orientations. To investigate the effects of GB misorientation and loading directions, we propose a mechanics model for uniaxial stress-driven GB migration in which the GB misorientation varies from 0° to 90°. Two types of imposed uniaxial loadings with loading directions parallel and perpendicular to the GB are considered. The model prediction is compared with that by molecular dynamics (MD) simulations. The results reveal that the uniaxial stress-driven GB migration can only occur in some range of GB misorientation under both loading conditions. The critical strain to trigger the GB migration varies with GB misorientation in a non-monotonic way. The proposed model provides new insights into the GB migration in HCP metals.

Disconnections kinks and competing modes in shear-coupled grain boundary migration

The response of small-grained metals to mechanical stress is investigated by a theoretical study of the elementary mechanisms occurring during the shear-coupled migration of grain boundaries (GB). Investigating a model $\mathrm{\ensuremath{\Sigma}}17(410)$ GB in a copper bicrystal, both $\ensuremath{\langle}110\ensuremath{\rangle}$ and $\ensuremath{\langle}100\ensuremath{\rangle}$ GB migration modes are studied focusing on both the structural and energetic characteristics. The minimum energy paths of these shear-coupled GB migrations are computed using the nudge elastic band method. For both modes, the GB migration occurs through the nucleation and motion of disconnections. However, the atomic mechanisms of both modes qualitatively differ: While the $\ensuremath{\langle}110\ensuremath{\rangle}$ mode presents no metastable state, the $\ensuremath{\langle}100\ensuremath{\rangle}$ mode shows multiple metastable states, some of them evidencing some kinks along the disconnection lines. Disconnection kinks nucleation and motion activation energies are evaluated. Besides, the activation energies of the $\ensuremath{\langle}100\ensuremath{\rangle}$ mode are smaller than those of the $\ensuremath{\langle}110\ensuremath{\rangle}$ one except for very high stresses. These results significantly improve our knowledge of the GB migration mechanisms and the conditions under which they occur.

Bi-crystallographic lattice structure directs grain boundary motion under shear stress.

Shear stress driven grain boundary (GB) migration was found to be a ubiquitous phenomenon in small grained polycrystalline materials. Here we show that the GB displacement shift complete (DSC) dislocation mechanism for GB shear coupled migration is still functioning even if the geometry orientation of the GBs deviates a few degrees from the appropriate coincidence site lattice (CSL) GBs. It means that any large angle GB can have a considerable chance to be such a “CSL-related GB” for which the shear coupled GB migration motion can happen by the GB DSC dislocation mechanism. We conclude that the CSL-DSC bi-crystallographic lattice structure in GB is the main reason that GB can migrate under shear stress.

Effects of a disconnection dipole on the shear-coupled grain boundary migration

Effects of a preexisting disconnection dipole on the migration of a Σ5(3¯10)/[001] grain boundary (GB) at low shear rates have been investigated by using the minimized bulk dissipation phase field crystal model. Simulation results show that the two disconnections play different roles during the process of shear-coupled GB migration via different behaviors regarding heterogeneous nucleation and propagation of disconnections. Specifically, the different roles can be attributed to their structural deviation from the equilibrated crystallographic structure caused by shear loading: one disconnection contributes to the onset of GB migration by heterogeneous nucleation of disconnections, while the other one slows down the corresponding GB migration dynamics through the impediment of homogeneous nucleation in the flat GB plane.

A coupling crack blunting mechanism in nanocrystalline materials by nano-grain rotation and shear-coupled migration of grain boundaries

A theoretical model has been developed to investigate the effect of a special cooperative process between two main modes of stress-driven nano-grain growth, i.e., nano-grain rotation and shear-coupled migration of grain boundaries, on the emission of lattice dislocations from a semi-infinite crack in nanocrystalline materials. The results obtained show that the above-mentioned collaborative process is highly conducive to the process of dislocation emission, thus, rendering strong crack blunting in nanocrystalline materials.

The role of disconnections in deformation-coupled grain boundary migration

Grain boundary (GB) migration under stress has been recognized in recent years as an important plastic deformation mechanism especially in small-grained materials. It is believed to occur via the motion of disconnections along the interface. However, the origin of these disconnections is a key point for a deeper understanding of this mechanism. In this paper, we consider that GB migration under stress can occur both due to the motion of pre-existing disconnections and due to disconnections resulting from decomposition of lattice dislocations interacting with the GB. High-resolution transmission electron microscopy experiments carried out on an aluminum bicrystal with a Σ41〈001〉{540} GB indeed confirm the existence of different kinds of disconnections and pure steps prior to deformation. In situ straining experiments performed in the same bicrystal at room and high temperatures reveal the rapid decomposition of lattice dislocations in the GB plane. Theoretical investigation of the possible decomposition reactions shows that different types of disconnections with Burgers vector having both glide and climb components, i.e. parallel and perpendicular to the GB plane, can be produced. Disconnections with a small climb component are likely to move along the GB under stress and induce deformation parallel and perpendicular to the GB plane. Concomitant motion of disconnections with Burgers vectors at right angles to the GB plane is believed to produce GB migration coupled with grain rotation. It is also shown that disconnection interactions in the GB lead preferentially to purely glissile disconnections producing a coupling factor in agreement with the observed coupling factor measured in experiments on macroscopic bicrystals. The idea that shear-coupled GB migration can occur by the continuous feeding of lattice dislocations decomposing in the GB during the migration is also investigated. This process is thought to play a role during recrystallization.

Strong crack blunting by shear-coupled migration of grain boundaries in nanocrystalline materials

A theoretical model is proposed to illustrate the effect of shear-coupled migration of grain boundaries on the emission of lattice dislocations from a semi-infinite crack tip in nanocrystalline materials. The results obtained show that the shear-coupled migration process is able to considerably enhance the capability of the crack to emit dislocations, thus leading to strong crack blunting. Moreover, the combination of grain boundary migration and dislocation emission can serve as an effective toughening mechanism in nanocrystalline materials. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.