Modelling and Simulation in Materials Science and Engineering

Takaki, Tomohiro; Sakane, Shinji; Ohno, Munekazu; Shibuta, Yasushi

doi: 10.1088/1361-651X/ab1a17pmid: N/A

Due to the recent acceleration of phase-field simulations using a graphics processing unit (GPU), the mechanism of competitive growth among columnar grains has been better understood. In this study, the effects of natural convection, caused by the gravity driven buoyancy force, on the competitive growth of columnar grains during directional solidification of bi-crystal and polycrystal binary alloys are investigated by performing two-dimensional large-scale phase-field simulations using a GPU supercomputer. As a result, the downward flow accelerates and the upward flow decelerates the selection speed in competitive growth. It is also confirmed that these phenomena are caused by a characteristic liquid flow pattern at the converging and diverging grain boundaries.

Shibuta, Yasushi; Sakane, Shinji; Miyoshi, Eisuke; Takaki, Tomohiro; Ohno, Munekazu

doi: 10.1088/1361-651X/ab1d28pmid: N/A

The micrometer-scale polycrystalline microstructure is directly obtained from a 10 billion atom molecular dynamics (MD) simulation of the nucleation and growth of crystals from an undercooled melt, which is performed on a graphics processing unit-rich supercomputer. The grain size distribution in the as-grown microstructure obtained from the MD simulation largely deviates from that resulting from steady-state growth in ideal grain growth, whereas the distribution of the disorientation angle between grains in contact with each other basically agrees with a random distribution. The atomistic configuration of the polycrystalline microstructure is then converted into a phase-field profile (diffuse interface description) of a phase-field model (PFM) and the subsequent grain growth is examined by multi-phase-field (MPF) simulation. A significant achievement in this study is direct mapping of the atomistic configuration into the phase-field profile used in the MPF simulation since only representative parameters for larger-scale model (e.g. interatomic potentials for MD and interfacial parameters for PFM) are extracted from a smaller‐scale simulation in conventional multi-scale modeling. Our new achievement supported by high-performance supercomputing can be regarded as an evolution of multi-scale modeling, which we call inter-scale modeling to differentiate it from conventional multi-scale modeling.

Miyoshi, Eisuke; Takaki, Tomohiro; Ohno, Munekazu; Shibuta, Yasushi; Sakane, Shinji; Aoki, Takayuki

doi: 10.1088/1361-651X/ab1e8bpmid: N/A

In this study, assuming an ideal system free from thermal grooving and anisotropy in grain boundary properties, we analyze thin-film grain growth via three-dimensional (3D) phase-field simulations with approximately one million initial grains. The large-scale simulations accelerated by multiple graphics processing units allow for the reliable statistical investigation of grain growth behaviors in films with various thickness. Over the transition from 3D to two-dimensional (2D) growth modes, variations in the averages and distributions of grain sizes are quantified and compared for different regions of the films. Furthermore, we propose a comprehensive scaling law of thin-film grain growth, by which the 3D–2D transition behaviors and grain growth kinetics can be described in a unified manner independent of film thickness.

Sakane, Shinji; Takaki, Tomohiro; Ohno, Munekazu; Shibuta, Yasushi; Aoki, Takayuki

doi: 10.1088/1361-651X/ab20b9pmid: N/A

Thermosolutal convection inevitably occurs during the solidification of alloys owing to the nonuniform distribution of temperature and/or solute concentration, and this can drastically alter the resulting solidification microstructures. In this study, we present a large-scale simulation scheme for the phase-field lattice Boltzmann model, which can express dendrite growth upon considering the solute, heat transport, and liquid flow. A multiple mesh and time step method was employed to reduce computational costs, where different mesh sizes and time steps are used to solve the phase-field equation, the advection–diffusion equations of heat and solute, and the lattice Boltzmann equations for fluid flow. Furthermore, we implemented parallel computations using multiple graphics processing units (GPUs) to accelerate the large-scale simulation. Through the application of the multiple mesh and time step method, the computation was accelerated by approximately one hundred times compared to the case using a constant mesh and time step for all equations. Moreover, we confirmed that the developed parallel-GPU computation combined with the multiple mesh and time step method could achieve good acceleration and scaling through increasing the number of GPUs. We also confirmed that the developed method could simulate multiple dendrite growth with thermosolutal convection.

Bachurina, O V

doi: 10.1088/1361-651X/ab17b7pmid: N/A

Discrete breather (DB) is a time-periodic, spatially localized vibrational mode in a perfect nonlinear lattice. In this study, two new types of DBs are reported in fcc metals (Al, Cu and Ni), based on the molecular dynamics simulations using standard embedded atom method interatomic potentials. All calculations are performed at a zero temperature in a three-dimensional computational cell with the use of periodic boundary conditions. A plane DB is excited in a single (111) atomic plane by displacing the atoms from their equilibrium lattice sites according to a specific pattern corresponding to a delocalized vibrational mode in a two-dimensional triangular lattice. This plane DB is delocalized in two dimensions and localized in one dimension, normal to the excited (111) plane. It is shown that in all studied metals the plane DBs have maximal lifetimes of 17–22 ps in the range of initial amplitudes of 0.15–0.30 Å. Herewith the studied mode demonstrates a hard type of nonlinearity, i.e. its frequency increases with amplitude. The second new type of DB is the plane-radial DB obtained by imposing a radial localizing function on the plane DB. This disk-type DB is localized in all three dimensions. The time evolution of the atomic displacement amplitudes and the kinetic energy are studied. The DBs of this type can exist for 9 and 4 ps in Cu and Ni, respectively, and then they decay by dissipating their vibrational energy onto neighboring atoms. The long-lived plane-radial DB in Al is not found.

Gu, Chunping; Ye, Guang; Wang, Qiannan; Sun, Wei

doi: 10.1088/1361-651X/ab1838pmid: N/A

The chloride diffusivity of ultra-high performance concrete (UHPC) is the key parameter that determines the service life of UHPC structures in chloride environments. In this study, a multi-scale model, which considered the structural information at micro and meso-scale, was proposed to predict macro property, i.e. chloride diffusivity, of UHPC. At micro-scale, the chloride diffusivity of UHPC paste was predicted based on an extended HYMOSTRUC3D model and a finite element method. At meso-scale, Anm model and finite element method were applied to calculate the chloride diffusivity of UHPC mortar. At macro-scale, the chloride diffusivity of UHPC was determined with a two phase model. The chloride diffusivity at a lower scale was used as input to predict the chloride diffusivity at the higher scale. Non-steady chloride diffusion tests were performed to validate the proposed model. The results showed that the proposed multi-scale model gave a little higher chloride diffusivity of UHPC than experiment results. However, the proposed model is still helpful for the durability design of UHPC structure in chloride environments.

Mason, D R; Sand, A E; Dudarev, S L

doi: 10.1088/1361-651X/ab1a1epmid: N/A

We describe the development of a new object kinetic Monte Carlo (kMC) code where the elementary defect objects are off-lattice atomistic configurations. Atomic-level transitions are used to transform and translate objects, to split objects and to merge them together. This gradually constructs a database of atomic configurations- a set of relevant defect objects and their possible events generated on-the-fly. Elastic interactions are handled within objects with empirical potentials at short distances, and between spatially distinct objects using the dipole tensor formalism. The model is shown to evolve mobile interstitial clusters in tungsten faster than an equivalent molecular dynamics (MD) simulation, even at elevated temperatures. We apply the model to the evolution of complex defects generated using MD simulations of primary radiation damage in tungsten. We show that we can evolve defect structures formed in cascade simulations to experimentally observable timescales of seconds while retaining atomistic detail. We conclude that the first few nanoseconds of simulation following cascade initiation would be better performed using MD, as this will capture some of the near-temperature-independent evolution of small highly-mobile interstitial clusters. For the 20keV cascade annealing simulations considered here, we observe internal relaxations of sessile objects. These relaxations would be difficult to capture using conventional object kMC, yet are important as they establish the conditions for long timescale evolution.

El-Achkar, T; Weygand, D

doi: 10.1088/1361-651X/ab1b7cpmid: N/A

The dislocation structure formation under low-amplitude fatigue in fcc metals for multislip loading conditions is investigated using three-dimensional discrete dislocation dynamics. Tools based on graph analysis, a statistical description of stable dislocation arrangements such as dislocation dipoles and prismatic loops are developed and applied. Upon decreasing the loading amplitude one order of magnitude below the persistent slip band threshold, although qualitative microstructural differences are seen, the elementary features of the investigated defects are the same. A critical number of cycles is required to produce sessile Lomer junctions that stabilize the structure and result in dislocation clustering around them. The crystallographic orientation of the crystal with respect to the loading axis results in different patterns strongly linked to sessile junctions, which are analyzed using spatial correlation functions. The increase in irreversible bulk dislocation arrangements results in roughening of the free surface and increase in surface step heights. Furthermore the crystallographic orientation with respect to the free surface is shown to control the dislocation density evolution combined with the macroscopic Schmid factor.

Aphinyan, Suphanat; Ang, Elisa Y M; Yeo, Jingjie; Ng, Teng Yong; Lin, Rongming; Liu, Zishun; Geethalakshmi, K R

doi: 10.1088/1361-651X/ab1d43pmid: N/A

Nanodroplet formation is a critical process in the development of 3D nano-inkjet printing. We show that many-body dissipative particle dynamics (MDPD) can be used to predict nanodroplet formation in nanosized nozzles with good accuracy. A conversion methodology is also introduced to overcome the problem of large coarse-graining factor, which results in unphysical results when the simulation is scaled up to real units. Using our MDPD model and the new conversion methodology, insights into possible trends of physical quantities in nanodroplet formation of polymeric ultraviolet ink can be gained. It was found that higher temperature and applied pressure reduce droplet break-up time. In addition, higher temperature increases the droplets’ diameter while higher effective pressure reduces it. These findings suggest that the physical environment can be tuned to achieve the desired droplet properties for 3D nano-inkjet printing. Due to the technical challenges that impedes experimental testing, this work demonstrates that MDPD provides a low-cost alternative to study nanodroplet formation in 3D nano-inkjet printing.

Ansari, Mohammad Ali; Behnagh, Reza Abdi

doi: 10.1088/1361-651X/ab1ca7pmid: N/A

This present study investigates the application of the fully Lagrangian method, smoothed particle hydrodynamics to develop the three-dimensional numerical model for simulation of the friction stir welding (FSW) plunging stage. This method was initially used to model fluid motion due to several benefits over traditional grid-based techniques. Recently, its applications have been expanded to simulate the forming processes. The tool axial force, stress and strain fields during the plunging stage are predicted as characteristics for qualification of a FSW process. Also, the mass scaling technique with two different factors is investigated to find the converged model as well as reduce the CPU time. The developed model is validated by comparing the predicted welding force with experimental values. Two different rotational speed and three stages for plunging depth are also discussed to analyze the influence of the rotational speed and plunging depth on the stress and strain rate.