Browsing by Author "Zhang, Youtian"

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    Self-supervised learning and prediction of microstructure evolution with convolutional recurrent neural networks
    (Elsevier, 2021) Yang, Kaiqi; Cao, Yifan; Zhang, Youtian; Fan, Shaoxun; Tang, Ming; Aberg, Daniel; Sadigh, Babak; Zhou, Fei
    Microstructural evolution is a key aspect of understanding and exploiting the processing-structure-property relationship of materials. Modeling microstructure evolution usually relies on coarse-grained simulations with evolution principles described by partial differential equations (PDEs). Here we demonstrate that convolutional recurrent neural networks can learn the underlying physical rules and replace PDE-based simulations in the prediction of microstructure phenomena. Neural nets are trained by self-supervised learning with image sequences from simulations of several common processes, including plane-wave propagation, grain growth, spinodal decomposition, and dendritic crystal growth. The trained networks can accurately predict both short-term local dynamics and long-term statistical properties of microstructures assessed herein and are capable of extrapolating beyond the training datasets in spatiotemporal domains and configurational and parametric spaces. Such a data-driven approach offers significant advantages over PDE-based simulations in time-stepping efficiency and offers a useful alternative, especially when the material parameters or governing PDEs are not well determined.
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    Stress Effects on Phase Morphological and Compositional Non-uniformity in Intercalation Compounds
    (2022-04-22) Zhang, Youtian; Tang, Ming
    Intercalation compounds have important applications in rechargeable batteries and hydrogen storage systems. Solute insertion into or extraction from these compounds frequently induces phase transformations, and the resultant microstructure evolution plays an important role in the (de)intercalation kinetics, performance and reliability of the systems. In this thesis, theoretical analyses and mesoscale simulations are carried out to elucidate and predict how coherency stress, which usually arises due to the solute concentration dependence of lattice parameter, impacts the evolution of phase morphology and solute composition distribution in intercalation compounds. In the first topic, the effect of coherency stress on the stability of the intercalation front in both isotropic and anisotropic systems is investigated by linear stability analysis and numerical simulations. Theoretical analysis shows that the misfit strain between solute-rich and solute-poor phases could cause the flat interface between them to become unstable and develop non-planar morphology during interface- and diffusion-limited intercalation processes, which is analogous to the well-known Asaro-Tiller-Grinfeld instability in epitaxial thin film growth. Predictions of the analysis is corroborated by phase-field simulations, which further reveals the phase morphology evolution at the late stage of interface instability development. Because this phenomenon leads to non-uniform solute intercalation and stress concentration, it is detrimental to the rate performance and reliability of intercalation compounds used as electrode materials in lithium- or sodium-ion batteries. For systems with strongly anisotropic misfit strains, it is discovered that the interface could be destablized by two different modes of perturbation, i.e. surface vs bulk mode. While the surface-mode interface perturbations could be suppressed when interface moves far away from particle surface, unstable bulk-mode perturbations persist throughout the (de)intercalation process. Our predictions provide satisfactory explanation to experimentally observed phase boundary morphologies in various battery electrodes. Furthermore, an interface stability diagram is derived from the analysis, which provides guidance in choosing the proper particle sizes, elastic anisotropy and/or (dis)charge conditions to avoid the instability. The second topic concerns the effect of coherency stress on the metastability of solid solution in nanoscale intercalation compounds that are increasingly employed in batteries. Previous theoretical works predict that stress can significantly suppress phase separation and extend the solid solution regime in nanosized electrode particles. However, these works do not consider how the solid solution stability is impacted by the recently discovered surface-driven coherent spinodal decomposition. We comprehensively analyze the phase separation kinetics in nanosized systems by considering both the bulk and surface modes of coherent spinodal decomposition within the linear stability theory. It shows that the stress effect on stabilizing solid solution is considerably weakened by stress relaxation near free surface. The dominant composition modulation pattern emerging from the spinodal decomposition is predicted as a function of particle size, solute supersaturation and misfit strain, which could be compared against experiments. An analytical expression is derived for the minimum particle size below which phase separation is suppressed in the presence of coherency stress. It is found to be only slightly larger than the critical particle size in the absence of stress. As a result, coherency stress only modestly improve the metastability of solid solution in nanoscale particles as against common beliefs. Lastly, the coupling between coherency stress and microstructure evolution in technically important battery electrode systems is investigated. Using micromechanical-microstructure modeling, we examine the interplay between coherency stress, particle size and morphology and its role in the lithium insertion dynamics in electrochromic Titanium dioxide nanocrystal ensembles, intragranular fracture in single-crystal nickel-rich layered oxides (NMC) and intergranular cracking in polycrystalline NMC particles.