The Advanced Computation and Artificial Intelligence (ACAI) Lab

The Advanced Computation and Artificial Intelligence (ACAI) Lab

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Concurrent atomistic and continuum simulation of strontium titanate

Published in Acta Materialia, 2013

This paper presents a concurrent atomistic–continuum methodology (CAC) to simulate the dynamic processes of dislocation nucleation and migration as well as crack initiation and propagation in complex crystals. The accuracy and efficiency of the method is tested with respect to the molecular dynamics (MD) method through simulations of the dynamic fracture processes in strontium titanate under a combination of tension and shear loading and the dislocation behavior under nanoindentation. CAC simulation results demonstrated a smooth passage of cracks and dislocations through the atomistic–continuum interface without the need for additional constitutive rules or special numerical treatment. Although some accuracy is lost in CAC simulations as a consequence of a 98.4% reduction in the degrees of freedom, all the CAC results are qualitatively and quantitatively comparable with MD results. The stacking fault width and nanoindentation hardness measured in the CAC simulations agrees well with existing experimental data. Criteria for cleavage and slip in ionic materials are verified. The need to include the internal degrees of freedom of atoms in concurrent atomistic–continuum methods for polyatomic crystalline materials is confirmed.

Recommended citation: S. Yang, L. Xiong, Q. Deng, Y. Chen. "Concurrent atomistic and continuum simulation of strontium titanate. " Acta Materialia. 2013: 61(1), 89-102. https://doi.org/10.1016/j.actamat.2012.09.032

Phonon thermal transport through tilt grain boundaries in strontium titanate

Published in Journal of Applied Physics, 2014

In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO3. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO3 contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies.

Recommended citation: Z. Zheng, X. Chen, B. Deng, A. Chernatynskiy, S. Yang; L. Xiong, Y. Chen. "Phonon thermal transport through tilt grain boundaries in strontium titanate. " Journal of Applied Physics. 2014: 116, 073706. https://doi.org/10.1063/1.4893648

Concurrent atomistic and continuum simulation of bi-crystal strontium titanate with tilt grain boundary

Published in Proceedings of The Royal Society A, 2015

In this paper, we present the development of a concurrent atomistic–continuum (CAC) methodology for simulation of the grain boundary (GB) structures and their interaction with other defects in ionic materials. Simulation results show that the CAC simulation allows a smooth passage of cracks through the atomistic–continuum interface without the need for additional constitutive rules or special numerical treatment; both the atomic-scale structures and the energies of the four different [001] tilt GBs in bi-crystal strontium titanate obtained by CAC compare well with those obtained by existing experiments and density function theory calculations. Although 98.4% of the degrees of freedom of the simulated atomistic system have been eliminated in a coarsely meshed finite-element region, the CAC results, including the stress–strain responses, the GB–crack interaction mechanisms and the effect of the interaction on the fracture strength, are comparable with that of all-atom molecular dynamics simulation results. In addition, CAC simulation results show that the GB–crack interaction has a significant effect on the fracture behaviour of bi-crystal strontium titanate; not only the misorientation angle but also the atomic-level details of the GB structure influence the effect of the GB on impeding crack propagation.

Recommended citation: S. Yang, N. Zhang, Y. Chen. "Concurrent atomistic and continuum simulation of bi-crystal strontium titanate with tilt grain boundary. " Proceedings of The Royal Society A. 2015: 471, 20140758. https://doi.org/10.1098/rspa.2014.0758

Concurrent atomistic–continuum simulation of polycrystalline strontium titanate’ mechanical properties

Published in Philosophical Magazine, 2015

This paper presents the new development of a concurrent atomistic–continuum (CAC) method in simulation of the dynamic evolution of defects in polycrystalline polyatomic materials. The CAC method is based on a theoretical formulation that extends Kirkwood’s statistical mechanical theory of transport processes to a multiscale description of crystalline materials. It solves for both the deformation of lattice cells and the internal deformation within each lattice cell, making it a suitable method for simulations of polyatomic materials. The simulation results of this work demonstrate that CAC can simulate the nucleation of dislocations and cracks from atomistically resolved grain boundary (GB) regions and the subsequent propagation into coarsely meshed grain interiors in polycrystalline strontium titanate without the need of supplemental constitutive equations or additional numerical treatments. With a significantly reduced computational cost, CAC predicts not only the GB structures, but also the dynamic behaviour of dislocations, cracks and GBs, all of which are comparable with those obtained from atomic-level molecular dynamics simulations. Simulation results also show that dislocations tend to initiate from GBs and triple junctions. The angle between the slip planes and the GB planes plays a key role in determining the GB-dislocation reactions.

Recommended citation: S. Yang, N. Zhang, Y. Chen. "Concurrent atomistic–continuum simulation of polycrystalline strontium titanate. " Philosophical Magazine. 2015: 95(24), 2697-2716. https://doi.org/10.1080/14786435.2015.1076178

Ballistic-diffusive phonon heat transport across grain boundaries

Published in Acta Materialia, 2017

The propagation of a heat pulse in a single crystal and across grain boundaries (GBs) is simulated using a concurrent atomistic-continuum method furnished with a coherent phonon pulse model. With a heat pulse constructed based on a Bose-Einstein distribution of phonons, this work has reproduced the phenomenon of phonon focusing in single and polycrystalline materials. Simulation results provide visual evidence that the propagation of a heat pulse in crystalline solids with or without GBs is partially ballistic and partially diffusive, i.e., there is a co-existence of ballistic and diffusive thermal transport, with the long-wavelength phonons traveling ballistically while the short-wavelength phonons scatter with each other and travel diffusively. To gain a quantitative understanding of GB thermal resistance, the kinetic energy transmitted across GBs is monitored on the fly and the time-dependent energy transmission for each specimen is measured; the contributions of coherent and incoherent phonon transport to the energy transmission are estimated. Simulation results reveal that the presence of GBs modifies the nature of thermal transport, with the coherent long-wavelength phonons dominating the heat conduction in materials with GBs. In addition, it is found that phonon-GB interactions can result in reconstruction of GBs.

Recommended citation: X. Chen, W. Li, L. Xiong, Y. Li, S. Yang, Z. Zheng, D. McDowell, Y. Chen. "Ballistic-diffusive phonon heat transport across grain boundaries. " Acta Materialia. 2017: 136, 355-365. https://doi.org/10.1016/j.actamat.2017.06.054

First-order interfacial transformations with a critical point: breaking the symmetry at a symmetric tilt grain boundary

Published in Physical Review Letters, 2018

First-order interfacial phaselike transformations that break the mirror symmetry of the symmetric ∑5 (210) tilt grain boundary (GB) are discovered by combining a modified genetic algorithm with hybrid Monte Carlo and molecular dynamics simulations. Density functional theory calculations confirm this prediction. This first-order coupled structural and adsorption transformation, which produces two variants of asymmetric bilayers, vanishes at an interfacial critical point. A GB complexion (phase) diagram is constructed via semigrand canonical ensemble atomistic simulations for the first time.

Recommended citation: S. Yang, N. Zhou, H. Zheng, S. Ong, J. Luo "First-order interfacial transformations with a critical point: breaking the symmetry at a symmetric tilt grain boundary. " Physical Review Letters. 2018: 120, 085702. https://doi.org/10.1063/5.0177062

Role of disordered bipolar complexions on the sulfur embrittlement of nickel general grain boundaries

Published in Nature Communications, 2018

Minor impurities can cause catastrophic fracture of normally ductile metals. Here, a classic example is represented by the sulfur embrittlement of nickel, whose atomic-level mechanism has puzzled researchers for nearly a century. In this study, coupled aberration-corrected electron microscopy and semi-grand-canonical-ensemble atomistic simulation reveal, unexpectedly, the universal formation of amorphous-like and bilayer-like facets at the same general grain boundaries. Challenging the traditional view, the orientation of the lower-Miller-index grain surface, instead of the misorientation, dictates the interfacial structure. We also find partial bipolar structural orders in both amorphous-like and bilayer-like complexions (a.k.a. thermodynamically two-dimensional interfacial phases), which cause brittle intergranular fracture. Such bipolar, yet largely disordered, complexions can exist in and affect the properties of various other materials. Beyond the embrittlement mechanism, this study provides deeper insight to better understand abnormal grain growth in sulfur-doped Ni, and generally enriches our fundamental understanding of performance-limiting and more disordered interfaces.

Recommended citation: T. Hu, S. Yang (Co-First Author), N. Zhou, Y. Zhang, J. Luo. "Role of disordered bipolar complexions on the sulfur embrittlement of nickel general grain boundaries. " Nature Communications. 2018: 9, 2764. https://doi.org/10.1038/s41467-018-05070-2

Atomistic Study of the Effect of Magnesium Dopants on the Strength of Nanocrystalline Aluminum

Published in JOM, 2019

Atomistic simulations have been used to study the deformation mechanisms of nanocrystalline pure Al and Al-Mg binary alloys. Voronoi tessellation was used to fully create a three-dimensional polycrystalline model with a grain size of 10 nm, while hybrid Monte Carlo and molecular dynamic simulations were used to achieve both mechanical and chemical equilibriums in nanocrystalline Al-5 at.%Mg. The results of tensile tests show an improved strength, including the yield strength and ultimate strength, through doping 5 at.%Mg into nanocrystalline aluminum. The results of atomic structures clearly reveal the multiple strengthening mechanisms related to doping in Al-Mg alloys. At the early deformation stage, up to an applied strain of 0.2, the strengthening mechanism of the dopants exhibits as dopant pinning grain boundary (GB) migration. However, at the late deformation stage, which is close to failure of nanocrystalline materials, dopants can prohibit the initiation of intergranular cracks and also impede propagation of existing cracks along the GBs, thus improving the flow stress of Al-Mg alloys.

Recommended citation: A. Kazemi, S. Yang. "Atomistic Study of the Effect of Magnesium Dopants on the Strength of Nanocrystalline Aluminum. " JOM. 2019: 71, 1209-1214. https://doi.org/10.1007/s11837-019-03373-3

Phonon Transport Across Coherent and Incoherent Interfaces

Published in JOM, 2019

Phonon-mediated heat transport in two-dimensional superlattices with coherent and incoherent interfaces is simulated by using the concurrent atomistic-continuum method. The energy transmission across superlattices with incoherent interfaces is found to be an order of magnitude lower than that with coherent interfaces. The simulation results provide a direct visualization of the transient processes of phonon propagation and scatterings, which facilitates an improved mechanistic understanding of phonon transport across multiple interfaces. This work finds that heat conduction in superlattices with coherent interfaces is dominated by coherent phonons, while the existence of defects that comprise the incoherent interfaces destroys the wave interference and changes the phonon transport nature from coherent to diffusive. In addition, phonon scattering by interface defects becomes stronger with decreasing phonon wavelength, and the phonon coherence is destroyed for phonons with 5 nm wavelength.

Recommended citation: W. Li, X. Chen, S. Yang. "Phonon Transport Across Coherent and Incoherent Interfaces. " JOM. 2019: 71, 3885-3891. https://doi.org/10.1007/s11837-019-03731-1

Effects of magnesium dopants on grain boundary migration in aluminum-magnesium alloys

Published in Computational Materials Science, 2021

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.

Recommended citation: X. Zhou, T. Li, Y. Cui, M. Meyerson, J. Weeks, C. Mullins, S. Yang, Y. Liu, L. Zhu "Effects of magnesium dopants on grain boundary migration in aluminum-magnesium alloys. " Computational Materials Science. 2021: 188, 110130. https://doi.org/10.1016/j.commatsci.2020.110130

Lithium trapping in germanium nanopores during delithiation process

Published in Applied Materials Today, 2021

Irreversible capacity loss is a critical problem in high capacity anode materials of Li-ion batteries, such as silicon, germanium, and tin. In addition to solid electrolyte interface formation and active material loss, Li trapping in high capacity anode materials during cycling has been considered a new mechanism of capacity loss but received less attention. In this study, we used single particle battery-based in situ focused ion beam-scanning electron microscopy, transmission electron microscopy (TEM), and scanning TEM to investigate the microstructure and composition of germanium nanopores formed at the end of delithiation. Our results show that a significant amount of Li accumulates inside the nanopores.

Recommended citation: X. Zhou, T. Li, Y. Cui, M. Meyerson, J. Weeks, C. Mullins, S. Yang, Y. Liu, L. Zhu "Lithium trapping in germanium nanopores during delithiation process. " Applied Materials Today. 2021: 24, 101140. https://doi.org/10.1016/j.apmt.2021.101140

Phase-field-lattice Boltzmann method for dendritic growth with melt flow and thermosolutal convection–diffusion

Published in Computer Methods in Applied Mechanics and Engineering, 2021

We propose a new phase-field model formulated within the system of lattice Boltzmann (LB) equation for simulating solidification and dendritic growth with fully coupled melt flow and thermosolutal convection–diffusion. With the evolution of the phase field and the transport phenomena all modeled and integrated within the same LB framework, this method preserves and combines the intrinsic advantages of the phase-field method (PFM) and the lattice Boltzmann method (LBM). Particularly, the present PFM/LBM model has several improved features compared to the existing phase-field models including: (1) a novel multiple-relaxation-time (MRT) LB scheme for the phase-field evolution is proposed to effectively model solidification coupled with melt flow and thermosolutal convection–diffusion with improved numerical stability and accuracy, (2) convenient diffuse interface treatments are implemented for the melt flow and thermosolutal transport which can be applied to the entire domain without tracking the interface, and (3) the evolution of the phase field, flow, concentration, and temperature fields on the level of microscopic distribution functions in the LB schemes is decoupled with a multiple-time-scaling strategy (despite their full physical coupling), thus solidification at high Lewis numbers (ratios of the liquid thermal to solutal diffusivities) can be conveniently modeled. The applicability and accuracy of the present PFM/LBM model are verified with four numerical tests including isothermal, iso-solutal and thermosolutal convection–diffusion problems, where excellent agreement in terms of phase-field and thermosolutal distributions and dendritic tip growth velocity and radius with those reported in the literature is demonstrated. The proposed PFM/LBM model can be an attractive and powerful tool for large-scale dendritic growth simulations given the high scalability of the LBM.

Recommended citation: N. Wang, D. Korba, Z. Liu, R. Prabhu, M. Priddy, S. Yang, L. Chen, L. Li "Phase-field-lattice Boltzmann method for dendritic growth with melt flow and thermosolutal convection–diffusion. " Computer Methods in Applied Mechanics and Engineering. 2021: 385, 114026. https://doi.org/10.1016/j.cma.2021.114026

First-principles study of vacancy interaction with grain boundaries of tungsten under tensile strains

Published in Computational Materials Science, 2021

A large number of vacancies can form in tungsten (W) when subjected to high-energy plasma particle fluxes in future fusion reactors. Based on comprehensive first-principle calculations, the present study investigates the effects of tensile strain on vacancy interaction with two different grain boundaries (GBs) in W. We find that as the applied tensile strain increases, the vacancy formation energies increase monotonically vacancies for the sites at Σ3(1 1 2)[1 1 0] GB, but increases and then decreases those at the Σ5(3 1 0)[1 0 0] GB. The strain-induced change of average bond length was found to be an important factor in determining the vacancy formation energies at GBs under tensile loading. We also find that the vacancy formation energy at GBs and fracture strengthen of GBs are greatly affected by vacancy positions at GBs. For both GBs, the vacancy formation energy first decreases and then increases to a plateau when the vacancy position moves away from the GB plane. The results show that the vacancy position with the lowest formation energy was on the first layer from the GB, rather than on the GB plane. The underlying mechanism for this phenomenon was shown to be closely correlated with the lengths and energies of the bonds surrounding the vacancy position.

Recommended citation: Q. Han, Y. Wang, Y. Zhang, S. Yang. "First-principles study of vacancy interaction with grain boundaries of tungsten under tensile strains. " Computational Materials Science. 2021: 200, 110760. https://doi.org/10.1016/j.commatsci.2021.110760

Integrating uncertainty into deep learning models for enhanced prediction of nanocomposite materials’ mechanical properties

Published in APL Machine Learning, 2024

This paper is about developing an innovative approach to integrate uncertainty quantification into deep learning models.

Recommended citation: Y. Wang, G. Lin, S. Yang. "Integrating uncertainty into deep learning models for enhanced prediction of nanocomposite materials’ mechanical properties. " APL Machine Learning. 2024: 2, 016112. https://doi.org/10.1063/5.0177062

Rapid prediction of grain boundary network evolution in nanomaterials utilizing a generative machine learning approach

Published in Extreme Mechanics Letters, 2024

We developed a deep learning framework based on a conditional generative adversarial network (cGAN) to predict the evolution of grain boundary (GB) networks in nanocrystalline materials.

Recommended citation: Y. Wang, A. Kazemi, T. Jing, Z. Ding, L. Li, S. Yang. "Rapid prediction of grain boundary network evolution in nanomaterials utilizing a generative machine learning approach. " Extreme Mechanics Letters. 2024: 70, 102172. https://doi.org/10.1016/j.eml.2024.102172

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Design of Mechanism

Undergraduate course, Department of Mechanical Engineering, 2024

ME 372 Design of Mechanisms