Coupled Grain Boundary Motion Research Articles

Suppression of grain boundary migration at cryogenic temperature in an extremely fine nanograined Ni-Mo alloy

Microindentation creep tests on an electrodeposited extremely fine (4.9 nm) nanograined (ng) Ni-14.2 at.% Mo (Ni-14.2Mo) at both room temperature (RT) and liquid nitrogen temperature (LNT) demonstrated that lowering temperature retarded softening in the ng Ni-Mo alloy. The obtained strain rate sensitivity at LNT was one order of magnitude lower than that at RT. Microstructural characterization revealed that mechanically-driven grain boundary (GB) migration was greatly suppressed by lowering temperature, which might be ascribed to the presence of solute Mo atoms that significantly retarded coupled GB motion at LNT. Deformation was instead carried by shear bands.

Effects of H segregation on shear-coupled motion of 〈110〉 grain boundaries in α-fe

By using large-scale molecular dynamics (MD) simulations, the influence of solute hydrogen (H) segregation into several typical [11¯0] symmetric tilt grain boundaries (STGBs) on the shear response and coupled grain boundary (GB) migration of bicrystals in α-Fe is systematically examined. Depending on our geometric model of coupling for [11¯0] STGBs in body-centred cubic (BCC) metals, two different coupling branches (〈100〉 and 〈111〉) are predicted and further validated by the MD results. Our atomistic simulations show that solute H atoms impede coupled GB motion irrespective of the GB structure, which mainly stems from the fact that H considerably modifies the local atomic structures of the GBs. At high H concentrations, the response of GBs to shear deformation changes from GB coupling to pure GB sliding. In addition, it is found that H can facilitate the vacancy generation via enhancing the interaction of GB dislocations within the framework of the H-enhanced localized plasticity (HELP) mechanism. Although H-vacancy clusters are formed by solute H combining with nucleated vacancies, they cannot grow larger owing to the migration of GBs with extensive dislocation plasticity. This directly prevents the occurrence of H embrittlement (HE). These findings deepen our overall understanding of the role of solute H in GB-mediated plasticity process (GB migration) of metallic materials and provide a possible path to designing new materials with high resistance to HE.

Coupled motion of grain boundaries and the influence of microcracks: A phase-field-crystal study

With phase-field-crystal, the two-dimensional coupled motion of grain boundary (GB) in triangular lattice crystal is studied. The migration mechanism of symmetric and asymmetric GBs in bicrystals is explored. Considering microcracks can hardly be avoided in materials, it is necessary to study the influence of microcracks on coupled GB motion. As shown in our simulation results, the influence of microcracks can be classified into three categories: (I) GB passes microcracks and is not impeded, cracks maintain unchanged; (II) Microcracks hinder GB motion, cracks migrate along GB and shrink; (III) GB motion is blocked by microcracks, cracks extend along GB and enlarge. The three kinds of results have great dependence on the misorientation and inclination angles of GB. Moreover, coupled GB motion can also lead to grain growth. The shear-induced grain growth process is simulated. As cracks can impede coupled GB motion, the shear-induced grain growth can also be hindered by cracks.

Initiation, evolution, and saturation of coupled grain boundary motion in nanocrystalline materials

Coupled grain boundary (GB) motion in sheared nanocrystalline materials composed of Ni, Al, or Cu is investigated by atomistic simulation methods, and the effects of grain size and temperature are evaluated. Due to the pinning effect of triple junctions, saturation of coupled GB motion is observed in all the nanocrystals except Cu. The two components of coupled GB motion, normal migration (NM) and tangential motion (TM), initiate and saturate at nearly equivalent nominal shear strains. Accompanied with coupled GB motion, massive dislocations and stacking faults are found to form within some grains, and the elementary structure units in the observed GBs transform from in-order to out-of-order. Compared with nanocrystalline Ni, the coupled GB motion in nanocrystalline Al has a reduced shear strain threshold and saturated NM displacement. The effects of grain size and temperature are similar in both NM and TM, so that their influence on coupled GB motion is slight.

Coupled motion of grain boundaries in bcc tungsten as a possible radiation-damage healing mechanism under fusion reactor conditions

As a potential first-wall fusion reactor material, tungsten will be subjected to high radiation flux and extreme mechanical stress. We propose that under these conditions, coupled grain boundary (GB) motion, in some cases enhanced by interstitial loading, can lead to a radiation-damage healing mechanism, in which a large stress activates coupled GB motion, and the GB sweeps up the defects, such as voids and vacancies, as it passes through the material. The stress-induced mobility characteristics of a number of GBs in tungsten are examined to investigate the likelihood of this scenario.

Influence of point defects on grain boundary mobility in bcc tungsten

Atomistic computer simulations were performed to study the influence of radiation-induced damage on grain boundary (GB) sliding processes in bcc tungsten (W), the divertor material in the ITER tokamak and the leading candidate for the first wall material in future fusion reactors. In particular, we calculated the average sliding-friction force as a function of the number of point defects introduced into the GB for a number of symmetric tilt GBs. In all cases the average sliding-friction force at fixed shear strain rate depends on the number of point defects introduced into the GB, and in many cases introduction of these defects reduces the average sliding-friction force by roughly an order of magnitude. We have also observed that as the number of interstitials in the GB is varied, the direction of the coupled GB motion sometimes reverses, causing the GB to migrate in the opposite direction under the same applied shear stress. This could be important in the microstructural evolution of polycrystalline W under the harsh radiation environment in a fusion reactor, in which high internal stresses are present and frequent collision cascades generate interstitials and vacancies.

Coupled motion of asymmetrical tilt grain boundaries: Molecular dynamics and phase field crystal simulations

Previous simulation and experimental studies have shown that some grain boundaries (GBs) can couple to applied shear stresses and be moved by them, producing shear deformation of the lattice traversed by their motion. While this coupling effect has been well confirmed for symmetrical tilt GBs, little is known about the coupling ability of asymmetrical boundaries. In this work we apply a combination of molecular dynamics and phase field crystal simulations to investigate stress-driven motion of asymmetrical GBs between cubic crystals over the entire range of inclination angles. Our main findings are that the coupling effect exists for most of the asymmetrical GBs and that the coupling factor exhibits a non-trivial dependence on both the misorientation and inclination angles. This dependence is characterized by a discontinuous change of sign of the coupling factor, which reflects a transition between two different coupling modes over a narrow range of angles. Importantly, the magnitude of the coupling factor becomes large or divergent within this transition region, thereby giving rise to a sliding-like behavior. Our results are interpreted in terms of a diagram presenting the domains of existence of the two coupling modes and the transition region between them in the plane of misorientation and inclination angles. The simulations reveal some of the dislocation mechanisms responsible for the motion of asymmetrical tilt GBs. The results of this study compare favorably with existing experimental measurements and provide a theoretical ground for the design of future experiments.

Variable-charge method applied to study coupled grain boundary migration in the presence of oxygen

One of the important differences between simulation and experiments in grain boundary (GB)-dominated metallic structures is the lack of impurities such as oxygen in computational samples. A modified variable-charge method [Elsener A, Politano O, Derlet PM, Van Swygenhoven H. Modell Simul Mater Sci Eng 2008;16:025006] based on the Streitz and Mintmire approach [Streitz FH, Mintmire JW. Phys Rev B 1994;50:11996] is used to study coupled GB motion in an Al bicrystal with a [112] symmetrical tilt GB in the presence of substitutional O, and compared with the stick–slip process identified by Cahn and Mishin [Cahn JW, Mishin Y, Suzuki A. Acta Mater 2006;54:4953]. It is found that the critical shear stress for migration of the GB increases linearly with the number of O atoms. These observations are then rationalized in terms of the internal stress signature of the O atoms in the vicinity of the boundary.

Duality of dislocation content of grain boundaries

The Frank–Bilby equation (FBE) can give many solutions for the dislocation content of a grain boundary (GB); most of them are considered to have little physical reality except in limited ranges of the angles which characterize low-angle GBs. We explore two such solutions, each of which is an accurate description for a different low-angle tilt GB with the same tilt axis. We develop a model that uses the two solutions to predict the coupling factor between normal motion and the shear strain produced by any (low- or high-angle) GB. Within this model, the two FBE solutions give rise to two possible modes of coupled GB motion which for the same GB under the same stress differ in direction of motion. Using molecular dynamics simulations we confirm our model. We find positive and/or negative coupled GB motion for all misorientation angles, and the model gives accurate predictions for the shear produced even by high-angle GBs where individual dislocations cannot be resolved. At low temperatures dual behaviour is observed for the same high-angle GB and applied stress: both coupling modes are found with a switch between them after some time. This switch signifies a change in the effective dislocation content. Dual behaviour indicates that both solutions for the dislocation content can be meaningful for the same GB. At higher temperatures only one mode is seen for each GB under the same shear stress, and the switch between the two modes seems to occur discontinuously at some high misorientation angle.