@article{ji_phan_chen_mcdowell_xiong_2024, title={An atomistic-to-microscale characterization of the kink-controlled dislocation dynamics in bcc metals through finite-temperature coarse-grained atomistic simulations}, volume={262}, ISSN={["1873-2453"]}, DOI={10.1016/j.actamat.2023.119440}, abstractNote={Adopting bcc tungsten (W) as a model material, we characterize the temperature and stress dependence of kink dynamics on a dislocation line with length L ranging from 60 nm to 1 μm using finite-temperature coarse-grained (FT-CG) atomistic simulations. The main novelty of this work is to accommodate major salient aspects, namely the motion of μm-long dislocation lines, the atomic-scale kink dynamics, and the full spectrum of phonon dynamics, all in one single FT-CG model. At a fraction of the cost of molecular dynamics (MD) calculations, the FT-CG simulation predicts: (a) a dislocation-induced degeneration of the phonon density of states (PDoS) of W; (b) the kink-induced dislocation core structure transition from a “soft” (non-planar, compact) to a “hard” configuration (planar, split); and (c) the crossover from the line tension (LT) regime to the elastic interaction (EI) regime in the temperature dependence of the flow stress. Several findings arise from the simulations: (1) the kink activation stress, σf, not only depends on the temperature, T, but also exhibits a sensitivity to the dislocation line length, L. For μm-long dislocations, it approaches experimental results but σf for kink activation on nm-long dislocations does not; (2) upon an increase of T, the σf reduction for the sample containing μm-long dislocations is significantly larger than that for the one with nm-long dislocations; (3) based on data extracted from FT-CG simulations of “temperature jump tests”, the L- dependence of the kink activation enthalpy, ΔH, is characterized. It can be as high as ∼3 eV for a dislocation with a length of tens of nm but reduces to an experimentally comparable level of ∼1.5 eV when L is 0.3 μm or longer. This suggests an easier kink activation on a longer dislocation. Such a dislocation line length dependence of ΔH can be further amplified at an even lower applied stress; (4) the entropic kink activation barrier, ΔHT, is linearly proportional to T. The slope of the ΔHT – T relation, however, will be largely underestimated in nanoscale MD simulations, but can be comparable with that from experiments when L is ∼ 0.3 μm or longer. These findings highlight the limitations of nanoscale MD models in simulating kink-controlled dislocation dynamics. The knowledge gained here can support the development of mobility laws that incorporate the stress-, temperature-, and line length-dependence all into one formulation for understanding plasticity in bcc metals and other high-Peierls-stress alloys.}, journal={ACTA MATERIALIA}, author={Ji, Rigelesaiyin and Phan, Thanh and Chen, Youping and Mcdowell, David and Xiong, Liming}, year={2024}, month={Jan} }
 @article{peng_ji_phan_chen_zhang_xu_bastawros_xiong_2023, title={Effect of a Long-Range Dislocation Pileup on the Atomic-Scale Hydrogen Diffusion near a Grain Boundary in Plastically Deformed bcc Iron}, volume={13}, ISSN={["2073-4352"]}, DOI={10.3390/cryst13081270}, abstractNote={In this paper, we present concurrent atomistic-continuum (CAC) simulations of the hydrogen (H) diffusion along a grain boundary (GB), nearby which a large population of dislocations are piled up, in a plastically deformed bi-crystalline bcc iron sample. With the microscale dislocation slip and the atomic structure evolution at the GB being simultaneously retained, our main findings are: (i) the accumulation of tens of dislocations near the H-charged GB can induce a local internal stress as high as 3 GPa; (ii) the more dislocations piled up at the GB, the slower the H diffusion ahead of the slip–GB intersection; and (iii) H atoms diffuse fast behind the pileup tip, get trapped within the GB, and diffuse slowly ahead of the pileup tip. The CAC simulation-predicted local H diffusivity, Dpileup−tip, and local stresses, σ, are correlated with each other. We then consolidate such correlations into a mechanics model by considering the dislocation pileup as an Eshelby inclusion. These findings will provide researchers with opportunities to: (a) characterize the interplay between plasticity, H diffusion, and crack initiation underlying H-induced cracking (HIC); (b) develop mechanism-based constitutive rules to be used in diffusion–plasticity coupling models for understanding the interplay between mechanical and mass transport in materials at the continuum level; and (c) connect the atomistic deformation physics of polycrystalline materials with their performance in aqueous environments, which is currently difficult to achieve in experiments.}, number={8}, journal={CRYSTALS}, author={Peng, Yipeng and Ji, Rigelesaiyin and Phan, Thanh and Chen, Xiang and Zhang, Ning and Xu, Shuozhi and Bastawros, Ashraf and Xiong, Liming}, year={2023}, month={Aug} }
 @article{peng_ji_phan_capolungo_levitas_xiong_2023, title={Effect of a micro-scale dislocation pileup on the atomic-scale multi-variant phase transformation and twinning}, volume={230}, ISSN={["1879-0801"]}, DOI={10.1016/j.commatsci.2023.112508}, abstractNote={In this paper, we perform concurrent atomistic–continuum (CAC) simulations to assess the contribution of the internal stress induced by the microscale dislocation pileup at an atomically structured interface to the atomic-scale phase transformations (PTs), reverse PTs, and twinning. The main novelty of this work is to unify the atomistic description of the interface and the coarse-grained (CG) description of the lagging dislocations away from the interface within one single framework. Our major findings are: (a) the interface dynamically responds to a pileup by forming steps/ledges, the height of which is proportional to the number of dislocations arriving at the interface; (b) the pileup-induced internal stress concentration profile follows neither the classical Eshelby model nor the super-dislocation model alone, but a combination of them; (c) when the pre-sheared sample is compressed, a direct square-to-hexagonal PT occurs ahead of the pileup tip and eventually grows into a wedge shape. The two variants of the hexagonal phases form a twin with respect to each other; (d) upon a further increase of the loading, part of the newly formed hexagonal phase transforms back to the square phase. The square product phase resulting from this reverse PT forms a twin with respect to the initial square phase. All phase boundaries (PBs) and twin boundaries (TBs) are stationary and correspond to zero thermodynamic Eshelby driving forces; and (e) the microscale dislocation pileup-induced internal shear stress and the structural change at the atomic-scale interface reduces the stress required for initiating a PT by a factor of 5.5, comparing with that in the sample containing no dislocations. This work is the first characterization of the behavior of PTs/twinning resulting from the reaction between a microscale dislocation slip and an atomically structured interface. The gained knowledge will advance our understanding of how the multi-phase material behaves in many complex physical processes, such as the synthesis of multi-phase high-entropy alloys or superhard ceramics under high-pressure torsion, deep mantle earthquakes in geophysics, and so on, which all involve dislocation slip, PTs, twinning, and their interactions across from the atomistic to the microscale and beyond.}, journal={COMPUTATIONAL MATERIALS SCIENCE}, author={Peng, Yipeng and Ji, Rigelesaiyin and Phan, Thanh and Capolungo, Laurent and Levitas, Valery I. and Xiong, Liming}, year={2023}, month={Oct} }
 @article{shuang_ji_xiong_gao_2023, title={Effect of periodic image interactions on kink pair activation of screw dislocation}, volume={228}, ISSN={["1879-0801"]}, DOI={10.1016/j.commatsci.2023.112369}, abstractNote={Periodic boundary condition along a dislocation line is commonly used in computing activation barriers or formation energies of kink pair of screw dislocation in BCC metals. Although the effect of periodic image interactions on the computation results is obvious, there had been no comprehensive analysis on such effect. In this work, we quantify it through combined nudged elastic band (NEB) simulations and theoretical analysis based on dislocation mechanics. The NEB calculation result demonstrates a non-negligible size dependence on the activation barrier at zero and low stresses. The theoretical analysis offers a practical approach to quantify such size effect without the need of time-consuming NEB simulations. Notably, a simple relationship between kink activation barrier and dislocation line length is derived at zero stress, offering a new approach to compute kink pair formation energy based on NEB simulation results.}, journal={COMPUTATIONAL MATERIALS SCIENCE}, author={Shuang, Fei and Ji, Rigelesaiyin and Xiong, Liming and Gao, Wei}, year={2023}, month={Sep} }
 @article{su_phan_xiong_kacher_2023, title={Multiscale computational and experimental analysis of slip-GB reactions: In situ high-resolution electron backscattered diffraction and concurrent atomistic-continuum simulations}, volume={232}, ISSN={["1872-8456"]}, DOI={10.1016/j.scriptamat.2023.115500}, abstractNote={In this paper, in situ high-resolution electron backscattered diffraction (EBSD) is combined with concurrent atomistic-continuum (CAC) simulations to study the interactions between dislocation-mediated slip and grain boundaries (GBs) in Ni. It is found that the local stress associated with slip-GB intersections first increases upon the pileup of dislocations, then remains high even after the nucleation of dislocations in the neighboring grain, only relaxing after the nucleated dislocations propagate away from the GB due to more incoming dislocations participating in the pileup. The local stress relaxation is accompanied by an atomic-scale GB structure reconfiguration, which affects not only the subsequent dislocation transmission, but also the configuration of those dislocations away from the GB. These findings demonstrate the importance of incorporating local stress history at higher length scale models, such as crystal plasticity finite element.}, journal={SCRIPTA MATERIALIA}, author={Su, Yang and Phan, Thanh and Xiong, Liming and Kacher, Josh}, year={2023}, month={Jul} }