Addressing the discrepancy of finding equilibrium melting point of silicon using MD simulations

We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid–liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B 72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon

Addressing the discrepancy of finding equilibrium melting point of silicon using MD simulations

We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid–liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B 72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon

The concurrent atomistic-continuum method: Advancements and applications in plasticity of face-centered cubic metals

Metal plasticity is a multiscale phenomenon that is manifested by irreversible microstructure rearrangement associated with nucleation, multiplication, interaction, and migration of dislocations. Long range elastic interactions between dislocations and other crystal defects are important to describe, along with the nonlocal, nonlinear dislocation core field. These requirements necessitate multiscale modeling techniques which (i) describe certain lattice defects and their interactions using fully resolved atomistics, (ii) preserve the net Burgers vector and associated long range stress fields of curved mixed character dislocations in a sufficiently large continuum domain in a fully three-dimensional model, and (iii) employ the same governing equations and interatomic potentials in both atomistic and continuum domains to avoid the usage of phenomenological parameters/criteria and ad hoc procedures for passing dislocation segments between the two domains. One such approach is the concurrent atomistic-continuum (CAC) method. Unlike many other concurrent multiscale approaches, the continuum domain in CAC admits motion of dislocations and intrinsic stacking faults through a lattice without necessity of adaptive mesh refinement while employing an underlying interatomic potential as the only constitutive relation and is thus a suitable tool for dislocation-mediated metal plasticity phenomena. In this dissertation, the CAC method is advanced in multiple aspects and applied in a series of problems in plasticity of face-centered cubic (FCC) metals. First, four significant advancements in the CAC method have been made: (i) new types of finite elements are developed which yields a more accurate stacking fault energies and core structure in coarse-grained atomistic descriptions of dislocations, (ii) zero temperature, quasistatic CAC approaches are formulated to enable the constrained multiscale optimization for a sequence of non-equilibrium dislocation configurations in metals, (iii) mesh refinement schemes for both dynamic fracture and curved dislocation migration are implemented, and (iv) the code efficiency is improved using parallelized object-oriented programming. Subsequently, this enhanced CAC method is employed to study multiple plasticity problems in a variety of FCC metals, including screw dislocation cross-slip in Ni, edge dislocation bowing out from a row of collinear obstacles in Al, dislocation multiplication from Frank-Read sources in Cu, Ni, and Al, as well as sequential slip transfer of curved dislocations across a Σ3{111} coherent twin boundary and a Σ11{113} symmetric tilt grain boundary in Cu, Al, and Ni. This work makes significant contributions to the fields of mechanics of materials and multiscale modeling. It is anticipated that the finding in this dissertation will improve physical understanding of dislocation-mediated plastic deformation processes in FCC metals and may assist in formulating constitutive laws and rules used in computational techniques at higher length scales.Ph.D

Validation of the Concurrent Atomistic-Continuum Method on Screw Dislocation/Stacking Fault Interactions

Dislocation/stacking fault interactions play an important role in the plastic deformation of metallic nanocrystals and polycrystals. These interactions have been explored in atomistic models, which are limited in scale length by high computational cost. In contrast, multiscale material modeling approaches have the potential to simulate the same systems at a fraction of the computational cost. In this paper, we validate the concurrent atomistic-continuum (CAC) method on the interactions between a lattice screw dislocation and a stacking fault (SF) in three face-centered cubic metallic materials—Ni, Al, and Ag. Two types of SFs are considered: intrinsic SF (ISF) and extrinsic SF (ESF). For the three materials at different strain levels, two screw dislocation/ISF interaction modes (annihilation of the ISF and transmission of the dislocation across the ISF) and three screw dislocation/ESF interaction modes (transformation of the ESF into a three-layer twin, transformation of the ESF into an ISF, and transmission of the dislocation across the ESF) are identified. Our results show that CAC is capable of accurately predicting the dislocation/SF interaction modes with greatly reduced DOFs compared to fully-resolved atomistic simulations

Effects of ferromagnetism in ab initio calculations of basic structural parameters of Fe-A (A = Mo, Nb, Ta, V, or W) random binary alloys

Density functional theory (DFT) calculations are performed to study the effects of ferromagnetism on basic structural parameters including lattice parameters and elastic constants in 45 body-centered cubic (BCC) Fe-based random binary alloys. Each binary consists of Fe and one of the five pure BCC metals, including Mo, Nb, Ta, V, and W. To provide references, six pure metals are also studied. It is found that (i) the effects of ferromagnetism are more pronounced for elastic constants than for lattice parameter, (ii) the effects of ferromagnetism increase with the Fe concentration in the binary, (iii) when ferromagnetism is neglected in DFT calculations, pure Fe is elastically unstable, while most Fe-based alloys are stable, and (iv) relatively good estimates of the structural parameters of alloys can be provided via the simple rule of mixtures only when the ferromagnetism is included

Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study

Sequential slip transfer across grain boundaries (GB) has an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall–Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalised in terms of dislocation pile-ups against GBs. In spite of extensive studies in modelling individual phases and grains using atomistic simulations, well-accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long-range fields of the dislocation pile-up come into play. In this work, concurrent atomistic-continuum simulations are performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on Σ3 coherent twin boundary (CTB) in Cu and Al, with dominant leading screw character at the site of interaction. A Frank-Read source is employed to nucleate dislocations continuously. It is found that subject to a shear stress of 1.2 GPa, screw dislocations transfer into the twinned grain in Cu, but glide on the twin boundary plane in Al. Moreover, four dislocation/CTB interaction modes are identified in Al, which are affected by (1) applied shear stress, (2) dislocation line length, and (3) dislocation line curvature. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-GB reactions and highlight the importance of directly modeling sequential dislocation slip transfer reactions using fully 3D models.This article is published as Xu, Shuozhi, Liming Xiong, Youping Chen, and David L. McDowell. "Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study." npj Computational Materials 2, no. 1 (2016): 1-9. DOI: 10.1038/npjcompumats.2015.16. Copyright 2016 Shanghai Institute of Ceramics, Chinese Academy of Sciences/Macmillan Publishers Limited. Attribution 4.0 International (CC BY 4.0). Posted with permission

Validation of the Concurrent Atomistic-Continuum Method on Screw Dislocation/Stacking Fault Interactions

Dislocation/stacking fault interactions play an important role in the plastic deformation of metallic nanocrystals and polycrystals. These interactions have been explored in atomistic models, which are limited in scale length by high computational cost. In contrast, multiscale material modeling approaches have the potential to simulate the same systems at a fraction of the computational cost. In this paper, we validate the concurrent atomistic-continuum (CAC) method on the interactions between a lattice screw dislocation and a stacking fault (SF) in three face-centered cubic metallic materials—Ni, Al, and Ag. Two types of SFs are considered: intrinsic SF (ISF) and extrinsic SF (ESF). For the three materials at different strain levels, two screw dislocation/ISF interaction modes (annihilation of the ISF and transmission of the dislocation across the ISF) and three screw dislocation/ESF interaction modes (transformation of the ESF into a three-layer twin, transformation of the ESF into an ISF, and transmission of the dislocation across the ESF) are identified. Our results show that CAC is capable of accurately predicting the dislocation/SF interaction modes with greatly reduced DOFs compared to fully-resolved atomistic simulations

Shear stress- and line length-dependent screw dislocation cross-slip in FCC Ni

Screw dislocation cross-slip is important in dynamic recovery of deformed metals. A mobile screw dislocation segment can cross slip to annihilate an immobile screw dislocation segment with opposite Burgers vector, leaving excess dislocations of one kind in a crystal. Previous studies have found that the cross-slip process depends on both the local stress state and dislocation line length, yet a quantitative study of the combined effects of these two factors has not been conducted. In this work, we employ both dynamic concurrent atomistic-continuum (CAC) [L. Xiong, G. Tucker, D.L. McDowell, Y. Chen, J. Mech. Phys. Solids 59 (2011) 160–177] and molecular dynamics simulations to explore the shear stress- and line length-dependent screw dislocation cross-slip in face-centered cubic Ni. It is demonstrated that the CAC approach can accurately describe the 3-D cross-slip process at a significantly reduced computational cost, as a complement to other numerical methods. In particular, we show that the Fleischer (FL) [R.L. Fleischer, Acta Metall. 7 (1959) 134–135] type cross-slip, in which a stair-rod dislocation is involved, can be simulated in the coarse-grained domain. Our simulations show that as the applied shear stress increases, the cross-slip mechanism changes from the Friedel-Escaig (FE) [B. Escaig, J. Phys. 29 (1968) 225–239] type to the FL type. In addition, the critical shear stress for both cross-slip mechanisms depends on the dislocation line length. Moreover, the cross-slip of a screw dislocation with a length of 6.47 nm analyzed using periodic boundary conditions occurs via only the FL mechanism, whereas a longer dislocation with length of 12.94 nm can cross-slip via either the FE or FL process in Ni subject to different shear stresses.This is a manuscript of an article published as Xu, Shuozhi, Liming Xiong, Youping Chen, and David L. McDowell. "Shear stress-and line length-dependent screw dislocation cross-slip in FCC Ni." Acta Materialia 122 (2017): 412-419. DOI: 10.1016/j.actamat.2016.10.005. Copyright 2016 Acta Materialia Inc. Posted with permission

Validation of the Concurrent Atomistic-Continuum Method on Screw Dislocation/Stacking Fault Interactions

Dislocation/stacking fault interactions play an important role in the plastic deformation of metallic nanocrystals and polycrystals. These interactions have been explored in atomistic models, which are limited in scale length by high computational cost. In contrast, multiscale material modeling approaches have the potential to simulate the same systems at a fraction of the computational cost. In this paper, we validate the concurrent atomistic-continuum (CAC) method on the interactions between a lattice screw dislocation and a stacking fault (SF) in three face-centered cubic metallic materials—Ni, Al, and Ag. Two types of SFs are considered: intrinsic SF (ISF) and extrinsic SF (ESF). For the three materials at different strain levels, two screw dislocation/ISF interaction modes (annihilation of the ISF and transmission of the dislocation across the ISF) and three screw dislocation/ESF interaction modes (transformation of the ESF into a three-layer twin, transformation of the ESF into an ISF, and transmission of the dislocation across the ESF) are identified. Our results show that CAC is capable of accurately predicting the dislocation/SF interaction modes with greatly reduced DOFs compared to fully-resolved atomistic simulations.This article is published as Xu, Shuozhi, Liming Xiong, Youping Chen, and David L. McDowell. "Validation of the concurrent atomistic-continuum method on screw dislocation/stacking fault interactions." Crystals 7, no. 5 (2017): 120. DOI: 10.3390/cryst7050120. Copyright 2017 by the authors. Attribution 4.0 International (CC BY 4.0). Posted with permission