Shear-coupled Migration Research Articles

Shear response of 〈110〉 asymmetric tilt grain boundaries in BCC-Fe

The present study investigated the effect of shear load on the 110 asymmetric tilt grain boundaries of body-centered cubic iron with molecular dynamics simulations. Various mechanisms like grain boundary (GB) sliding, shear-coupled migration, GB migration with rotation, GB decomposition, and GB deformation with dislocations emission were observed. Low-angle tilt GBs migrated with the help of two dislocation mechanisms, gliding and climbing. Two types of twin nucleation mechanisms were noticed in high-angle tilt GBs with varying tilt angles (η). For 16° ≤ η ≤ 112° and η ≥ 130° cases, dynamic self-adjustment of the atoms at the GBs caused GBs decomposition with the formation of twin boundaries (TBs). Newly nucleated TBs migrated under further applied shear load. For 112° < η < 130° case, TBs nucleated from the GB on either side of the misfit dislocations due to their localized core structure on the GB plane.

Dislocation-mediated migration of the α/β interfaces in titanium

Interphase boundaries are essential in the deformation and phase transformations in titanium (Ti) alloys. While static structures of semicoherent α/β interfaces in various Ti alloys have been carefully examined, their migration behavior at atomic scales is far less clear. In this study, we employed molecular dynamics simulations to investigate the migration of the semicoherent α/β interface in pure Ti. The interface migration behavior shows a shear-coupled feature with the interface dislocation glide and a macroscopic shear. The simulation reveals that both the glide direction of the dislocations with respect to the interface and the dislocation spacing strongly influence the migration rate, and the low-index glide plane of the interface dislocation plays a minor role. The dependence of interface mobility on temperatures confirms the critical role of thermal activation during the interface migration, especially for activating the interface dislocation glide. Furthermore, the shear-coupled interface migration driven by element partition is simulated using a newly developed Ti-Mo potential, consistent with the displacive-diffusional features previously observed in the surface precipitates. The simulated interface migration mode is validated by comparing it with the crystallography features of surface precipitates in a Ti-Cr alloy. The interface energy and mobility obtained from simulations further explain why the distinctive crystallographic features of the surface precipitates observed experimentally are favored over other candidate interfaces. The present study has explored an approach for systematically examining thermodynamic and kinetic factors governing the development of phase transformation crystallography at different temperatures and chemical driving forces.

Revealing shear-coupled migration mechanism of a mixed tilt-twist grain boundary at atomic scale

Shear-coupled grain boundary (GB) migration greatly influences the plasticity and creep resistance of nanocrystalline materials. However, the atomistic mechanisms underlying the shear-coupled migration of general mixed tilt-twist GBs (MGBs) remain largely elusive to date. Here, using in-situ high-resolution transmission electron microscopy and molecular dynamics simulations, we uncover the atomic-scale migration behavior of a typical MGB, i.e., 〈001〉{200}/〈01¯1〉{1¯11} GB, during the room-temperature shear deformation of Au nano-bicrystals. Two distinct migration patterns showing the opposite signs of shear-coupling factor were observed and further revealed to be mediated by the motion of GB disconnections with different crystallographic parameters and exhibit different lattice correspondence relations, i.e., 〈001〉{020}-to-〈01¯1〉{200} and 〈001〉{020}-to-〈01¯1〉{111}. Simulation results confirm that the two distinct migration patterns could be activated under different stress/strain states. Moreover, excess GB sliding and GB plane reorientation were found to accommodate the GB migration in both experiments and simulations, likely due to the necessity of establishing a point-to-point lattice correspondence during GB migration. These findings provide atomic-scale experimental evidence on the disconnection-mediated migration of MGBs and elaborate on the hitherto unreported complex shear response of MGBs, which have valuable implications for optimizing the ductility of metallic nanocrystals through controlling GB migration.

Revealing the influence of carbon on shear-coupled grain boundary migrationin α-iron via molecular dynamics simulations

Shear-coupled grain boundary (GB) migration is of importance theoretically on the mechanisms of GB behaviors and application oriented on the development of novel processing methods by plastic deformation. We demonstrate via molecular dynamics simulations that the interstitial species carbon may affect the shear-coupled migration in α-iron strongly. The impact of carbon selectively takes effect depending on the variety of grain boundaries. Specifically, the coupling factor of the Σ9(221) GB is increased in the presence of carbon. While the carbon effect is inactive for the Σ17(223) GB. The influence of carbon is revealed to be connected with the disconnection modes of grain boundaries. The results of the work implicate correlations between the carbon enrichment and the final microstructure after severe plastic deformation.

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.

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.

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.

Twin boundary reversibility characteristics in α-Fe

Understanding the grain boundary deformation dynamics is very crucial to designing materials with stable microstructures. With this quest, the deformation behavior of coherent twin boundary under cyclic shear loading has been studied in α-Fe using molecular dynamics simulations to understand the influence of strain amplitude and temperature. Twin boundary exhibited shear coupled migration along with cyclic irreversibility character at lower temperatures and gained almost perfect reversibility at higher temperatures of around 1500 K. TB exhibited more sliding than migration with an increase in temperature. The stress associated with the migration of twins was observed to fluctuate around its average value. This was caused by layer by layer propagation of twin boundary through the activity of 1/6 111 = 1/12 111 + 1/12 111 type edge partial dislocations on {112} twin plane. Complete twin reversibility and complex TB migration were noticed in the presence of multiple parallel twin nanowires. Twin migration was observed at the larger size nanowires with a proper combination of boundary conditions and strain rates; otherwise, it was absent. The influence of shear strain amplitude in offsetting the twin boundary was found to be minimal.

A molecular dynamics study of dislocation ejection and shear coupling associated with incoherent twin boundary migration

Nanotwin structures can enhance both strength and ductility, but are amenable to detwinning caused by incoherent twin boundary (ITB) migration. While previous research treated the ITB as an array of Shockley partial dislocations, ITBs can deviate from this ideal model by either introducing Frank partial dislocations or changing misorientation angles. Using molecular dynamics simulations, we found both deviations lead to new migration behaviors. The Frank partials can be ejected out of the ITB moving by dislocation slip under high driving forces; as the driving force decreases, the ITB curves around the Frank partials and then is flattened by boundary sliding. Beviating the misorientation angle of ITB from its ideal coincident-site-lattice relationship removes a portion of its original grain boundary dislocations, leading to shear-coupled migration, while the migration of the original ITB under the same condition is not shear-coupled. Our findings deepen our understanding of the structure-migration-mechanism relationship for ITBs.

On the migration of {3 3 2} 〈1 1 0〉 tilt grain boundary in bcc metals and further nucleation of {1 1 2} twin

{3 3 2} 〈1 1 0〉 tilt grain boundaries (GB) move conservatively under a shear stress by the creation and glide of disconnections. When crystal dislocations interact with the GB they are absorbed and transformed into GB dislocations (GBD). The behaviour of GBDs under shear stress depends on the orientation of the Burgers vector and sense of shear stress. There are two possible scenarios: a) the GBD moves together with the GB in a compensated climb, then plastic deformation is accommodated by shear-coupled GB migration; b) the GBD is sessile because it cannot undergo a compensated climb when interacting with the disconnections. If so, the sessile GBD is the nucleus of a {1 1 2} twin. The nucleation of the twin is produced by the pileup of disconnections at both sides of the GBD. Then, plastic deformation is accommodated by the combination of the motion of the {3 3 2} GB and the growth of {1 1 2} twins inside the grain.

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

Abstract

The discontinuous effect of pressure on twin boundary strength in MgO

MgO makes up about 20% of the Earth’s lower mantle; hence, its rheological behaviour is important for the dynamics and evolution of the Earth. Here, we investigate the strength of twin boundaries from 0 to 120 GPa using DFT calculations together with structure prediction methods. As expected, we find that the energy barrier and critical stress for shear-coupled migration of the 310/[001] interface vary strongly with pressure. However, what is surprising is that the twin boundary also exhibits sudden strong discontinuities in strength which can both weaken and strengthen the boundary with increasing pressure. Since twin boundary migration is a proposed mechanism for both deformation and seismic attenuation in MgO, these results may suggest that MgO can undergo sudden changes in rheology due to transitions in grain boundary structure. The multiplicity of interfaces, however, necessitates the need for further studies to examine the role that phase changes in grain boundary structure play in mediating polycrystalline plasticity in the Earth.

Dislocation-mediated migration of interphase boundaries

Faceted interphase boundaries (IPBs) are commonly observed in lath-shaped precipitates in alloys consisting of simple face-centred cubic (fcc), body centred-cubic (bcc) or hexagonal closed packed (hcp) phases, which normally contain one or two sets of parallel dislocations. The influence of these dislocations on interface migration and possible accompanying long-range strain field remain unclear. To elucidate this, we carried out atomistic simulations to investigate the dislocation-mediated migration processes of IPBs in a pure-iron system. Our results show that the migration of these IPBs is accompanied with the slip of interfacial dislocations, even in high-index slip planes, with two migration modes were observed: the first mode is the uniform migration mode that occurs only when all of the dislocations slip in a common slip plane. A shear-coupled interface migration was observed for this mode. The other interfaces propagate in the stick-slip migration mode that occurs when the dislocations glide on different slip planes, involving dislocation reaction or tangling. A quantitative relationship was established to link the atomic displacements with the dislocation structure, slip plane, and interface normal. The macroscopic shear deformation due to the effect of overall atomic displacement shows a good agreement with the results obtained based on the phenomenological theory of martensite crystallography. Our findings have general implications for the understanding of phase transformations and the surface relief effect at the atomic scale.

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.

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.

Atomistic insights into shear-coupled grain boundary migration in bcc tungsten

Shear-coupled grain boundary (GB) migration is an efficacious plasticity mechanism in nanocrystalline materials. However, the atomistic aspects of this kind of GB motion have received far less attention in bcc metals than that in fcc ones. In this work, we have investigated the shear-coupled migration (SCM) of ∑13[100](015¯) and ∑85[100](076¯)GBs in bcc tungsten using atomistic simulations. We demonstrate that the SCM of the two GBs proceeds via the collective glide of GB dislocations along the 〈100〉 and 〈111〉 directions, respectively. The magnitudes of the GB migration depend on system lateral dimension, temperature and GB dislocation character. Nudged elastic band calculations in combination with the dynamic simulations give the elementary processes of the SCM and show that the shear strength and thermal resistance of the 〈100〉 mode GB is much higher than the 〈111〉 one, consistent with the fact that the 〈100〉 dislocations are much more difficult to glide than the 1/2〈111〉 dislocations. This conclusion is further supported by the simulation results of GB random walk behavior under the free boundary condition. The present results demonstrate that the atomistic SCM process is fundamentally related to the character of GB dislocations.

Shear-coupled grain boundary migration assisted by unusual atomic shuffling.

Shear-coupled grain boundary (GB) migration can be an efficacious mechanism to accommodate plastic deformation when the grain size of polycrystalline materials goes small. Nevertheless, how this kind of GB motion comes into play at the atomic level has not been fully revealed. Here, we have investigated the shear-coupled migration (SCM) of typical [100] group symmetrical tilt GBs in bcc W using atomistic simulations. Depending on GB character, the SCM is found to proceed via dislocation slipping in the 〈100〉 or 〈110〉 mode with striking shear strength difference between them. We demonstrate that there exists an unusual atomic shuffling along the tilt axis, which greatly assists SCM to operate in the easier 〈110〉 mode instead of the 〈100〉 one. The present results highlight the significant role of GB character in the atomistic SCM process and contribute to the future design and fabrication of high-performance materials in GB engineering.

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.

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.

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.