The evolution of grain boundary (GB) networks in polycrystalline metals depends not only on the movement of the constituent GBs but also on the migration of GB triple junctions (TJs), the latter would have strong influence on the mechanical properties of polycrystals. We investigate the migration mechanism of TJs in face-centered cubic metals using molecular dynamics simulations. The TJ migration is revealed to follow a typical stick-slip behaviour that involves TJ grooving and coordinated GB migration. Upon external shear loading, the TJ experiences a quick migration process, resulting in a grooved TJ configuration with curved GBs at the TJ. The accumulation of dislocations at the TJ facilitates the TJ grooving process, since it compensates the angle gap caused by the inconsistent shear-migration coupling factors between the GBs. After reaching a critical grooving depth, the TJ migration is stagnated, while the connecting GBs begin to migrate via disconnections emitted from the TJ. Based on the mechanism of accumulated dislocation-initiated TJ migration, we propose an energetic model that considers the extra energy barrier for the accumulation of dislocations, to quantify the effect of GB misorientation on TJ migration. These findings deepen our understanding of the physical origin of TJ migration and point out the importance of dislocation accumulation on coordinated GB motion in nanocrystalline metals. 

Migration of grain boundary triple junctions in nanocrystalline metals initiated by accumulated dislocations

Dislocation pileups in small grains

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

The oxide dispersion strengthening (ODS) high entropy alloy (HEA) exhibits the high elevated temperature performance and radiation resistance due to severe atomic lattice distortion and oxide particles dispersed in matrix, which is expected to become the most promising structural material in the next generation of nuclear energy systems. However, microstructure and damage evolution of irradiated ODS HEA under loading remain elusive at submicron scale using the existing simulations owing to a lack of atomic-lattice-distortion information from a micromechanics description. Here, the random field theory informed discrete dislocation dynamics simulations based on the results of high-resolution transmission electron microscopy are developed to study the dislocation behavior and damage evolution in ODS HEA considering the influence of severe lattice distortion and nanoscale oxide particle. Noteworthy, the damage behavior shows an unusual trend of the decreasing-to-increasing transition with the continuous loading process. There are two main types of damage micromechanics generated in irradiated ODS HEA: the dislocation loop damage in which the damage is controlled by irradiation-induced dislocation loops and their evolution, the strain localization damage in which the damage comes from the dislocation multiplication in the local plastic region. The oxide particle hinders the dislocation movement in the main slip plane, and the lattice distortion induces the dislocation sliding to the secondary slip plane, which promotes the dislocation cross-slip and dislocation loop annihilation, and thus reduces the material damage in the elastic damage stage. These findings can deeply understand atomic-scale damage mechanism and guide the design of ODS HEA with high radiation resistance. 

Unveiling deformation behavior and damage mechanism of irradiated high entropy alloys

Micro-beam bending of FCC bicrystals: A comparison between defect dynamics simulations and experiments

This work demonstrates how the structure of a grain boundary (GB) and its evolution due to slip transmission history influences subsequent dislocation transmission. First, a model for the evolution of stress fields within grain boundaries that accounts for the effects of coherent dislocation transmission is introduced. Starting with a disclination-based construct of GBs at minimum energy (equilibrium), the model describes the evolution of the GB stress field to a state characteristic of excess energy (non-equilibrium) due to the incorporation of residual Burgers vector content following sequential slip transmission events. Several essential features of this model are verified via molecular dynamics simulations of lattice dislocation absorption. Second, this model is implemented into a discrete dislocation dynamics (DDD) code and simulations are performed to understand the influence of Non-equilibrium GB stress fields, conditioned by the slip transmission history, on subsequent dislocation transmission. DDD simulations reveal that the critical resolved stress necessary for slip propagation can be reduced by ∼ 80 % with continued absorption of residual dislocation content. Moreover, DDD simulations prove that a comprehensive consideration of both the binding and driving stresses, and the evolution of the transmission configuration is necessary to quantify the influence of the mechanical state of the GB on slip propagation. Overall, this work provides important insights into the role of GB structure evolution, conditioned by prior deformation history, on intergranular plasticity. 

Disclination-dislocation based model for grain boundary stress field evolution due to slip transmission history and influence on subsequent dislocation transmission

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Disclination-dislocation based model for grain boundary stress field evolution due to slip transmission history and influence on subsequent dislocation transmission