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Decoding the Mechanics of Metals by Coarse-Grained Atomistics

Periodic Reporting for period 2 - COGRA (Decoding the Mechanics of Metals by Coarse-Grained Atomistics)

Reporting period: 2019-09-01 to 2021-02-28

Understanding why materials behave the way they do as well as creating novel materials with as-designed properties requires a careful investigation and a thorough understanding of the structure of materials. Limited by computational resources, there is a significant gap between the length and time scales of typical engineering applications and those of predictive simulations at the atomic level and below. Bridging across scales, i.e. exporting atomic-scale accuracy to macroscopic simulations – has remained a holy grail of multiscale modeling. This proposal aims for the seemingly impossible: the application of atomistic techniques to problems occurring over microns to millimeters and microseconds to minutes. Instead of relying on computational power, this will be achieved by a combination of new scale-bridging methodology and computational science strategies in order to produce new theory and an open-source computational toolset for long-term, large-scale simulations relying solely on atomic-scale input. Spatial upscaling (from nanometers to millimeters) and temporal upscaling (from femtoseconds to seconds) will be integrated with the description of heat and mass transfer in crystalline solids with a focus on metals. Once the new method is available, two representative open challenges will be investigated: recrystallization in magnesium such as during thermo-mechanical processing (promoting our understanding and usability of magnesium as a structural metal) and corrosion in steel by hydrogen embrittlement (a major thread to pressure vessels in industry). Both problems are of enormous technological and economic importance, but current modeling techniques are insufficient to bridge the gap between the macroscopic mechanical behavior and the underlying microstructural mechanisms at the atomic scale. Beyond these two target applications, this project will provide new techniques and numerical tools of wide applicability and of interest for science and technology – wherever problems require to bridge across length and time scales.
A new and powerful numerical codebase has been generated, which serves as the basis for all coarse-grained atomistic simulations in 3D (and which will be released in the final project phase). Rather than relying on a static finite-element mesh, this new code operates in the current, deformed configuration (using a so-called updated-Lagrangian description). This formulation allows us to more accurately refine the model where higher accuracy and resolution is needed, and it also lays the foundation for fully automatic adaptivity – allowing the code to autonomously maintain a high level of accuracy by introducing higher resolution where it is required within the simulation domain, while modeling other regions at low resolution to gain efficiency. In addition, we have equipped our new simulation tool with finite temperature and thermal effects, following the so-called hotQC approach of “thermalizing” atoms in a statistical-mechanics fashion. Rather than modeling temperature as lattice vibrations of high frequency (which usually dictate the small time steps of atomistic simulations), we separate their slow mean motion (which is of interest for the mechanical behavior of the material) from their statistical fluctuations (now represented by statistical measures rather than being fully resolved). The consequence is an efficient simulation technique that enables us to simulate atomic ensembles at finite temperature at, in principle, arbitrary time scales. Ongoing work focuses on heat transport, which requires a coarse-grained description of heat flow at the atomic scale, as well as on mass transport (the basis for which has already been laid within the numerical code). To understand the nature of atomic-level heat transport, we complement coarse-grained simulations with atomistic modeling using molecular dynamics applied to representative metallic systems. Towards the study of recrystallization, we have investigated grain boundaries in metals and assessed their mechanical performance as a function of temperature using both atomistics and coarse-grained atomistics. The integration of our (updated-Langrangian) spatial coarsening with temporal coarsening by hotQC for long-term finite temperature simulations and with multiple species for mass transport forms the basis for subsequent investigations of finite-temperature mechanical and diffusion processes.
Our simulation framework goes beyond existing techniques in various ways. It is the first coarse-grained atomistics environment that allows for fully-nonlocal automatic adaptivity in 3D simulations. Without requiring any a-priori knowledge about where atomistic accuracy is required within a simulation domain, our method increases and/or decreases resolution adaptively throughout a simulation, thus aiming for an optimal balance between efficiency and accuracy. Providing this capability in 3D without the need for a Lagrangian mesh is a novelty, paired with a computational implementation that is massively parallel to exploit the use of high-performance computing infrastructure. Further, combining the aforementioned spatial coarsening with the implementation of finite temperature without having to resolve thermal fluctuations of individual atoms is a key novelty. Finally, our ongoing theoretical work into heat and mass transport paves the road for unprecedented long-term simulations of thermal and diffusive effects and their impact on the mechanical behavior of materials. Until the end of the project, we will deploy the new simulation environment both for our own simulations (aiming for new understanding of grain nucleation and growth during recrystallization in magnesium and of hydrogen embrittlement of iron) and for the scientific community at large. Especially the addition of heat and mass transport across length and time scales are novel and go significantly beyond available state-of-the-art codes and tools.
Finite-temperature coarse-grained atomistic simulation of nanoindentation