[en] Recent experiments on devitrification of Al₉₀Sm₁₀ amorphous alloys revealed an unusual polymorphic transformation to a complex cubic crystal structure called the ε-Al₆₀Sm₁₁ phase. Molecular dynamics simulations of the growth of the stoichiometric ε-phase seed in contact with an undercooled Al-10 at% Sm liquid are performed to elucidate the microscopic process of transformation. The as-grown crystal and undercooled liquid possess similar local order around Al atoms whereas a rigid network defined by the Sm sub-lattice develops during the growth. Using a template-cluster alignment method, we define an order parameter to characterize the structural evolution in the system.

[en] Three-dimensional discrete dislocation dynamics (DDD) simulations in combination with the phase-field method are performed to investigate the influence of different realistic Ni-base single crystal superalloy microstructures with the same volume fraction of ${\mathrm{\gamma <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ precipitates on plastic deformation at room temperature. The phase-field method is used to generate realistic microstructures as the boundary conditions for DDD simulations in which a constant high uniaxial tensile load is applied along different crystallographic directions. In addition, the lattice mismatch between the γ and ${\mathrm{\gamma <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ phases is taken into account as a source of internal stresses. Due to the high antiphase boundary energy and the rare formation of superdislocations, precipitate cutting is not observed in the present simulations. Therefore, the plastic deformation is mainly caused by dislocation motion in γ matrix channels. From a comparison of the macroscopic mechanical response and the dislocation evolution for different microstructures in each loading direction, we found that, for a given ${\mathrm{\gamma <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ phase volume fraction, the optimal microstructure should possess narrow and homogeneous γ matrix channels. (paper)

[en] In this paper, a crystal plasticity finite-element model (CPFEM) is developed to simulate the hot extrusion texture of the magnesium alloy AZ31. The crystal plasticity model is implemented in ABAQUS^™ via user interface VUMAT subroutine. The elasto-plastic self-consistent (EPSC) model is used as the basic polycrystal framework to simulate the slip and twinning during the extrusion. Furthermore, this framework is extended to account for the effects of the dynamically recrystallized (DRX) grains on the extrusion textures. Good agreement is found between the experimentally measured and simulated textures. The simulation results show that the presence of a secondary texture component around $⟨<!--\; ???\; -->11.0⟩<!--\; ???\; -->$ || extrusion direction (ED) can be attributed to the lattice rotation around the c-axis during the formation of the DRX grains. In addition, the shear strain imposed on the extruded material affects the resulting texture by enhancing the basal $⟨<!--\; ???\; -->a⟩<!--\; ???\; -->$ slip mode as the material passes through the extrusion opening. (paper)

[en] We propose a new scheme based on machine learning for the efficient screening in grain-boundary (GB) engineering. A set of results obtained from first-principles calculations based on density functional theory (DFT) for a small number of GB systems is used as a training data set. In our scheme, by partitioning the total energy into atomic energies using a local-energy analysis scheme, we can increase the training data set significantly. We use atomic radial distribution functions and additional structural features as atom descriptors to predict atomic energies and GB energies simultaneously using the least absolute shrinkage and selection operator, which is a recent standard regression technique in statistical machine learning. In the test study with fcc-Al [110] symmetric tilt GBs, we could achieve enough predictive accuracy to understand energy changes at and near GBs at a glance, even if we collected training data from only 10 GB systems. The present scheme can emulate time-consuming DFT calculations for large GB systems with negligible computational costs, and thus enable the fast screening of possible alternative GB systems. (paper)

[en] Laser shock peening (LSP), an innovative surface treatment technique, generates compressive residual stress on the surface of metallic components to improve their fatigue performance, wear resistance and corrosion resistance. To illustrate the dynamic response during LSP and residual stress fields after LSP, this study conducted FEM simulations of LSP in a Ti6Al4V alloy. Results showed that when power density was 7 GW cm⁻², a plastic deformation occurred at 10 ns during LSP and increased until the shock pressure decayed below the dynamic yield strength of Ti6Al4V after 60 ns. A maximum tensile region appeared beneath the surface at around 240 ns, forming a compressive-tensile-compressive stress sandwich structure with a thickness of 98, 1020 and 606 μ m for each layer. After the model became stabilized, the value of the surface residual compressive stress was 564 MPa at the laser spot center. Higher value of residual stress across the surface and thicker compressive residual stress layers were achieved by increasing laser power density, impact times and spot sizes during LSP. A ‘Residual stress hole’ occurred with a high laser power density of 9 GW cm⁻² when laser pulse duration was 10 ns, or with a long laser pulse duration of 20 ns when laser power density was 7 GW cm⁻² for Ti6Al4V. This phenomenon occurred because of the permanent reverse plastic deformation generated at laser spot center. (paper)

[en] A representative all-atom molecular dynamics (MD) system of polycarbonate (PC) is built and conditioned to capture and predict the behaviours of PC in response to a broad range of thermo-mechanical loadings for various thermal aging. The PC system is constructed to have a distribution of molecular weights comparable to a widely used commercial PC (LEXAN 9034), and thermally conditioned to produce models for aged and unaged PC. The MD responses of these models are evaluated through comparisons to existing experimental results carried out at much lower loading rates, but done over a broad range of temperatures and loading modes. These experiments include monotonic extension/compression/shear, unilaterally and bilaterally confined compression, and load-reversal during shear. It is shown that the MD simulations show both qualitative and quantitative similarity with the experimental response. The quantitative similarity is evaluated by comparing the dilatational response under bilaterally confined compression, the shear flow viscosity and the equivalent yield stress. The consistency of the in silico response to real laboratory experiments strongly suggests that the current PC models are physically and mechanically relevant and potentially can be used to investigate thermo-mechanical response to loading conditions that would not easily be possible. These MD models may provide valuable insight into the molecular sources of certain observations, and could possibly offer new perspectives on how to develop constitutive models that are based on better understanding the response of PC under complex loadings. To this latter end, the models are used to predict the response of PC to complex loading modes that would normally be difficult to do or that include characteristics that would be difficult to measure. These include the responses of unaged and aged PC to unilaterally confined extension/compression, cyclic uniaxial/shear loadings, and saw-tooth extension/compression/shear. (paper)

[en] In order to simulate the flow behavior and microstructure evolution during the pass interval period of the multi-pass deformation process, models of static recovery (SR) and static recrystallization (SRX) by the cellular automaton (CA) method for the 7055 aluminum alloy were established. Double-pass hot compression tests were conducted to acquire flow stress and microstructure variation during the pass interval period. With the basis of the material constants obtained from the compression tests, models of the SR, incubation period, nucleation rate and grain growth were fitted by least square method. A model of the grain topology and a statistical computation of the CA results were also introduced. The effects of the pass interval time, temperature, strain, strain rate and initial grain size on the microstructure variation for the SRX of the 7055 aluminum alloy were studied. The results show that a long pass interval time, large strain, high temperature and large strain rate are beneficial for finer grains during the pass interval period. The stable size of the static recrystallized grain is not concerned with the initial grain size, but mainly depends on the strain rate and temperature. The SRX plays a vital role in grain refinement, while the SR has no effect on the variation of microstructure morphology. Using flow stress and microstructure comparisons of the simulated and experimental CA results, the established CA models can accurately predict the flow stress and microstructure evolution during the pass interval period, and provide guidance for the selection of optimized parameters for the multi-pass deformation process. (paper)

[en] The implementation of a comprehensive toolbox for generating microstructure models of periodic random particulate materials is outlined. The toolbox provides the functionality to create random configurations of overlapping or non-overlapping spherical particles with user-defined minimum inter-particle centre-to-centre distance and periodic boundary and export these random particle configurations for use in numerical simulations, either as lists of particle centre coordinates and radii or as Python-based scripts for generating solid geometric models or periodic hexahedral meshes (‘voxel meshes’). The random particle configurations can be used as microstructure models to investigate or predict the microstructure-property relation of various engineering materials such as particle-reinforced composites, structural alloys, (partially-)sintered ceramics, powders, porous media or granular matter, as demonstrated by several examples. The outlined toolbox is open-source, giving users the possibility to modify and adapt the code to suit specific needs. (paper)

[en] During powder based additive manufacturing processes, a component is realized layer upon layer by the selective melting of powder layers with a laser or an electron beam. The density of the consolidated material, the minimal spatial resolution as well as the surface roughness of the resulting components are complex functions of the material and process parameters. So far, the interplay between these parameters is only partially understood. In this paper, the successive assembling in layers is investigated with a recently described 2D-lattice Boltzmann model, which considers individual powder particles. This numerical approach makes several physical phenomena accessible, which cannot be described in a standard continuum picture, e.g. the interplay between capillary effects, wetting conditions and the local stochastic powder configuration. In addition, the model takes into account the influence of the surface topology of the previous consolidated layer on the subsequent powder layer. The influence of the beam power, beam velocity and layer thickness on the formation and quality of simple walls is investigated. The simulation results are compared with experimental findings during selective electron beam melting. The comparison shows that our model, although 2D, is able to predict the main characteristics of the experimental observations. In addition, the numerical simulation elucidates the fundamental mechanisms responsible for the phenomena that are observed during selective beam melting. (paper)

[en] Molecular dynamics simulations of dislocation/obstacle interactions are enhancing our physical understanding of plasticity. However, despite increasing computational power, the interaction between simulation cell boundaries and the long ranged fields of dislocations make spurious image effects inevitable. Here, these image effects are examined in detail, providing a general map of the spurious image stress as a function of simulation cell size, aspect ratio and bow-out for both nominally edge and screw dislocations. This is achieved using an approximate image solution of the resulting boundary value problem as well as an analytic model that captures most of the spurious image effects. A unique simulation cell shape is found to minimize spurious image effects for a fixed simulation volume (i.e. fixed total number of atoms) and specified initial dislocation line length. The results are used to estimate image stress effects in various literature studies involving dislocation bow-out. The image effects are non-negligible. Several case studies involving simulation cell dimensions are shown to converge due to a near-zero scaling of the image stress with respect to the simulation cell dimensions used. Finally, a direct comparison is made between a dislocation bow-out configuration under an applied load in a finite simulation cell and an image-free multiscale simulation of the same problem and the difference is shown to be consistent with our estimated image stresses. Overall, the results here provide guidance for both the development and interpretation of quantitative molecular dynamics studies involving curved dislocation structures. (paper)