Modelling and Simulation in Materials Science and Engineering

Moradi, Ali; Heidari, Ali; Amini, Kamran; Aghadavoudi, Farshid; Abedinzadeh, Reza

doi: 10.1088/1361-651X/ac03a3pmid: N/A

Shot peening is a surface treatment process that is used for the improvement of the mechanical properties of metallic alloys. The effects of sphere particle diameter and impact velocity on residual stress and mechanical properties of Ti-6Al-4V as titanium alloy are investigated in this study using molecular dynamics simulation. In this research, titanium atoms are simulated as a surface layer and the carbon steel atomic structure is modeled as the impacting particle in the shot peening process. Two types of molecular potential functions including embedded atom method (EAM) and Lennard-Jones (LJ) are used for molecular modeling of colliding atoms and the effect of these force fields on residual stress, hardness, surface roughness is investigated. Moreover, the effects of impacting particle velocity and diameter on these parameters are studied. The results show the amount of residual stress in the titanium surface layer increased by increasing the particle diameter and velocity of particles. The diameter and velocity of particles in the shot peening process has a significant effect on the mechanical behavior of the simulated titanium surface layer. The value of maximum compressive residual stress is −413 MPa which occurs in depth of 10 Å from the surface layer for 1 Å fs−1 velocity and 10 Å diameter of shot particle. Furthermore, the results show that the Vickers hardness of the titanium is increased by increasing the size and velocity of the carbon steel particle in both EAM and LJ potential functions.

Chen, ShiJie; Xie, Yaoping; Feng, He; Guo, Haibo

doi: 10.1088/1361-651x/ac0618pmid: N/A

Eu-doped rare earth sesquioxides RE2O3:Eu (RE = Lu, Y, Sc) exhibit subtle difference in scintillation processes, for which impurity levels play an important role but are difficult to assess computationally due to localized f-electrons and itinerant spd-electrons in the materials. Unpaired electrons of dopant Eu3+ ions also render complexity to these systems. In this study we calculated band structures of pure and Eu-doped RE2O3 using quasi-particle GW approximation with collinear spin polarization based on density functional theory. The band structures were similar across the three host material systems, varying with spin component and dopant’s lattice site. Only the majority-spin states are responsible for the scintillation process, whereas the minority-spin states are entangled with the hosts’ low-energy conduction bands. The materials’ afterglow behavior was explained in terms of the positions of 4f orbitals and electron traps, as well as thermodynamics of the doping systems.

Florez, Sebastian; Fausty, Julien; Alvarado, Karen; Murgas, Brayan; Bernacki, Marc

doi: 10.1088/1361-651x/ac0ae7pmid: N/A

The parallelization of algorithms is an essential step towards the optimization of large-scale computations. The modeling of evolving multi-domain problems is not an exception to this rule, specifically when it is applied to the context of microstructural evolutions. A new method for the simulation of evolving microstructures has been introduced in a previous work, consisting on a modified front-tracking approach where the main originality is that not only interfaces between domains are discretized but also their bulks. This new model has obtained promising results in terms of accuracy and numerical performance, however, it has been implemented in a sequential environment and is not readily usable in modern HPC units for parallel computations. This article proposes a parallel implementation for the new model using a distributed-memory approach developed with the standard protocol ‘message passing interface’. The new parallel methodology has been tested in an HCP station with up to 140 cores for problems involving motion by curvature flow in polycrystals, i.e. by considering pure grain growth. Good results were obtained in terms of CPU-time and speed-up for large polycrystals (with up to 560 000 initial grains), showing that this model can lead to fast and/or large computations of microstructural evolutions in a full-field context.

Zhou, Henan; Dickel, Doyl E; Baskes, Michael I; Mun, Sungkwang; Zaeem, Mohsen Asle

doi: 10.1088/1361-651x/ac095cpmid: N/A

A semi-empirical interatomic potential for the post-transition metal, bismuth, is developed based on the second nearest-neighbor modified embedded-atom method (MEAM). The potential reproduces a range of physical properties, such as the lattice constant, cohesive energy, elastic constants, vacancy formation energy, surface energy, and the melting point of pure bismuth. The calculations are done for the rhombohedral ground state of Bi. The results show good agreement with density functional theory and experimental data. The developed MEAM potential for bismuth is useful for material and mechanical behavior studies of the pure material at different conditions and sets the stage for the development of interatomic potentials for bismuth alloys or other bismuth compounds.

Strachota, P; Wodecki, A; Beneš, M

doi: 10.1088/1361-651x/ac0f55pmid: N/A

We investigate a family of phase field models for simulating dendritic growth of a pure supercooled substance. The central object of interest is the reaction term in the Allen–Cahn equation, which is responsible for the spatial distribution of latent heat release during solidification. In this context, several existing forms of the reaction term are analyzed. Inspired by the known conclusions of matched asymptotic analysis, we propose a new variant (the ‘ΣP1-P’ model) that is simple enough to allow mathematical and numerical analysis and robust enough to be applicable to solidification under very large supercooling. The important component of the model (the Σ limiter) can also be incorporated into the original models to extend the range of their applicability. The individual models are tested in a number of numerical simulations focusing on mesh-dependence and model parameter settings. When the phase interface thickness is kept large with respect to the microscopic capillary length to make numerical computations feasible, the parameters of the Σ limiter can be tuned to improve agreement with previous models. The results obtained using the ΣP1-P model exhibit a good quantitative agreement with experimental data from rapid solidification of nickel melts.

Sun, Weizhao; Shan, Feihu; Zong, Nanfu; Dong, Hongbiao; Jing, Tao

doi: 10.1088/1361-651x/ac0c23pmid: N/A

The β grain of Ti–6Al–4V during wire laser additive manufacturing (WLAM) is notorious for its coarse grains and thus reducing the mechanical performance. In this research, to predict and further control the microstructure, three-dimensional (3D) β grain evolution is simulated by 3D cellular automaton method coupled with the transient thermal profile calculated from processing modeling. Simulations are validated by experiments. The competitive growth between grains shows that the grain growth rate is less than the isotherm moving velocity, leading to a flat S–L interface, which in turn makes the orientation plays a more important role than the undercooling. It interprets the phenomena that Ti–6Al–4V grains with ⟨001⟩ orientation along the building direction are preferred during WLAM. Effects of processing parameters on microstructure, such as the deposited layer, the laser power, the inter-layer time and the preheating temperature of substrate, are also investigated. Results show that the grain size at the horizon plane increases with the increase of the deposited layer, the laser power and the preheating temperature, while inter-layer time has few effects on the β grain evolution. 3D grain simulation of additive manufacturing shows promising prospect in predicting the microstructure, revealing underlying mechanisms and optimizing processing parameters.

Paliwal, Bhasker; Picu, Catalin R

doi: 10.1088/1361-651x/ac07f4pmid: N/A

Cyclotetramethylene tetranitramine (HMX) is a molecular crystal used as explosive in a broad range of civilian and military application. Its mechanical behavior is important as it relates to the initiation of the decomposition reaction though the formation of hot spots. However, the mechanics of the HMX crystal is complex and insufficiently understood. In particular, the pressure sensitivity of the mechanical behavior was largely ignored in the past, despite indications that material properties are strongly dependent on pressure. In this work we develop a pressure-sensitive crystal plasticity model for single crystals of β-HMX which accounts for the dependence on pressure of both the elastic constants and the yield surface, and calibrate it based on results from molecular simulations reported in the literature. The model is implemented in a finite element solver and is used to represent nanoindentation in β-HMX single crystals using a Berkovich tip. It is observed that accounting for pressure sensitivity is required if predictions close to experimental indentation results are to be obtained. The pressure sensitivity of the yield surface has a larger impact on predictions than that of the elastic constants. While these effects are demonstrated here in the context of quasi-static indentation, it is suggested that they become essential under shock loading conditions when stress states with large pressure components result.

Nelasov, I V; Kartamyshev, A I; Boev, A O; Lipnitskii, A G; Kolobov, Yu R; Nguyen, Truong Khang

doi: 10.1088/1361-651x/ac0c22pmid: N/A

We present molecular dynamics simulation to study the α–ω phase transformation in titanium under different conditions simulating high-energy impacts. We employed the interatomic potential developed within the N-body method, which predicts the stability of the ω phase and the stacking fault energy in the α phase in excellent agreement with the experimental and theoretical data. The latter is crucial for the correct description of the deformation mechanisms. The dependence of the beginning and mechanism of the α–ω transition process on loading conditions are derived. In particular, at the uniaxial compression along the [0001] direction at 300 K, the ω phase is localized in deformation bands within the α phase, and the α–ω transition is observed at a pressure of more than 3 GPa. With this type of deformation, the residual inclusions of the α phase remain in the ω phase volume. A similar deformation at a temperature of 700 K does not lead to the formation of the ω phase. Meanwhile, at the hydrostatic compression, the α–ω transition is restrained and at a pressure of 20 GPa is not observed. In the case of anisotropic three-axis deformation along the α–ω transition pathways proposed by Trinkle et al at a constant pressure of 20 GPa, the transition mechanism includes the formation of dislocations, followed by the transformation of the regions between the dislocations into the ω phase. The simulation results demonstrate good agreement with the experimental data and confirm the applicability of the employed interatomic potential for simulating the deformation of titanium.

Jorgensen, Jeremy J; Hart, Gus L W

doi: 10.1088/1361-651x/ac13capmid: N/A

Density functional theory (DFT) codes are commonly treated as a ‘black box’ in high-throughput screening of materials, with users opting for the default values of the input parameters. Often, non-experts may not sufficiently consider the effect of these parameters on prediction quality. In this work, we attempt to identify a robust set of parameters related to smearing and tetrahedron methods that return numerically accurate and efficient results for a wide variety of metallic systems. The effects of smearing and tetrahedron methods on the total energy, number of self-consistent field cycles, and forces on atoms are studied in two popular DFT codes: the Vienna ab initio Simulation Package and Quantum Espresso. From nearly 40 000 computations, it is apparent that the optimal smearing depends on the system, smearing method, smearing parameter, and k-point density. The benefit of smearing is a minor reduction in the number of self-consistent field cycles, which is independent of the smearing method or parameter. A large smearing parameter—what is considered large is system dependent—leads to inaccurate total energies and forces. Blöchl’s tetrahedron method leads to small improvements in total energies. When treating diverse systems with the same input parameters, we suggest using as little smearing as possible due to the system dependence of smearing and the risk of selecting a parameter that gives inaccurate energies and forces.

Wu, Lingkang; Zhu, Yiying; Wang, Hao; Li, Mo

doi: 10.1088/1361-651x/ac13c9pmid: N/A

As a sequel to the previous paper on the calculation of the crystal–melt interface free energy (2021 Materialia 15 100962), here we report the results on the kinetic coefficients using molecular dynamics simulations performed on six fcc metals and four bcc metals with the intention to compare the crystal structural influence. We found that the calculated kinetic coefficients are well described by the model by Broughton, Gilmer and Jackson (1982 Phys. Rev. Lett. 49 1496), and in particular, they exhibit varying degrees of anisotropy. We reveal that the anisotropies are related to the fluctuation of the crystal–melt interfaces, which causes the increase of the actual interface area in melting or solidification. The kinetic coefficients always display asymmetry between the solidification and melting process, and the difference is much more pronounced for the (111) interfaces in fcc metals which have the highest anisotropy. We found that the atomic mechanisms of the kinetic behaviors of these interfaces are closely related to the formation of twin-crystal domains during solidification, which delays the solidification process and consequently causes a decrease in the calculated kinetic coefficients.