Lithium-sulfur (Li/S) batteries constitute a promising, next-generation energy storage technology... more Lithium-sulfur (Li/S) batteries constitute a promising, next-generation energy storage technology due to their high theoretical energy density and low cost. To increase sustainability, processability, and battery performance, conducting organic polymers have become a focus of research for the development of better cathode materials. Here, we investigate the solvation structure of the conjugated poly(4-thiophen-3-yl) benzenethiol) (PTBT) polymer as a high-potential macromolecular candidate for cathodes in Li/S batteries. Using molecular dynamics (MD) simulation with newly optimized force-field parameters, we examine the effects of polymer length and various molar fractions of the popular dimethoxyethane (DME) and dioxolane (DOL) solvents on the structure of the PTBT polymer at a temperature of 300 K. We characterize basic polymeric properties as well as the composition-dependent solvent adsorption 1 structure and thermodynamics. Importantly, we find an interesting co-solvency effect, namely that a solvent comprised of about 25% DME and 75% DOL leads to maximum swelling (best solvent quality) behavior, which should be important for optimizing cathode fractality and permeability in applications. Our study thus reveals intriguing polymer-solvent correlations and serves as a first step for further MD studies of realistic polymeric cathode structures and processes, e.g., toward charge transport in vulcanized (S-linked) network topologies.
Theoretical and Applied Fracture Mechanics, Oct 1, 2016
The interaction between segregated alloying elements (Al, Li, Sn, Y, and Ca) and three crack syst... more The interaction between segregated alloying elements (Al, Li, Sn, Y, and Ca) and three crack systems is investigated in magnesium simple crystal under mode I loading condition. Using molecular statics in the framework of the (modified) embedded-atoms method, the effect of segregated alloying elements on both the fracture behavior and the intrinsic fracture toughness were identified, and analyzed quantitatively in the framework of the Rice's theory for dislocation emission at a crack tip (Rice, 1992. J. Mech. Phys. Solids 40 239-271). In addition, the variations of the critical stress intensity factor as a function of the misfit strain between the alloying element and magnesium are discussed. The results revealed the existence of a transition from cleavage to dislocation emission at a crack tip. Such transition from cleavage to dislocation emission at the crack tip depends on (i) the orientation of the crack system, and (ii) the nature of the alloying element segregated at the crack tip. Furthermore, when analyzed as a function of the misfit strain between the alloying element and magnesium, our data suggest a direct relation between the fracture toughness and the misfit strain, independently of crack orientation. While alloying element with compressive misfit strain enhances the intrinsic fracture toughness of magnesium single crystal, alloying element with tensile misfit strain worsens the intrinsic fracture toughness of magnesium single crystal.
Modelling and Simulation in Materials Science and Engineering, Aug 1, 2016
Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the se... more Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the second nearest-neighbors modified embedded-atom method (MEAM). The validity and the transferability of the Ag potential were obtained by calculating physical, mechanical, thermal, and dislocation related properties. Since the {1 1 1}-generalized stacking fault energy curves obtained from first-principle calculations was used to develop the Ag potential, the critical resolved shear stress to move screw dislocations in Ag single crystal is in good agreement with the experimental data. By combining the ability of the potential to predict the surface energies with its accuracy in describing dislocation properties, the potential is thought to be a predictive model for analyzing the fracture properties of Ag. In addition, the performance of the potential was tested under dynamics conditions by predicting the melting temperature, where a good agreement with experimental value was found. The Ag-MEAM potential was then coupled to an existing Mg-MEAM potential to describe the properties of the binary system MgAg. While the heat of formation, the elastic constants, and the (1 1 0) γ-surface of the MgAg compound in the B2 phase were used to parameterize the potential, heat of formation for MgAg alloys with different stoichiometry, thermal properties of the B2-MgAg compound, as well as dislocation related properties in B2-MgAg compound were tested to validate the transferability of the potential. The heat of formation of Mg5Ag2, MgAg, and MgAg3, the elastic constants and the thermal properties of B2-MgAg obtained with the proposed potential align with first-principles and experimental data. In addition, the core structure of both 〈0 0 1〉 and 〈1 1 1〉 dislocations in {1 1 0} are in agreement with theoretical predictions, and the magnitudes of the critical resolved shear stress obtained at 0 K for both slip systems partially validate the slip behavior of B2-MgAg. Furthermore, the interaction between silver solute element and dislocations from the basal plane is correctly captured by the potential.
Modelling and Simulation in Materials Science and Engineering, May 23, 2017
Knowledge of the deformation mechanisms of (Mg,Fe) 2 SiO 4 olivine is important for the understan... more Knowledge of the deformation mechanisms of (Mg,Fe) 2 SiO 4 olivine is important for the understanding of flow and seismic anisotropy in the Earth's upper mantle. We report here a numerical modelling at the atomic scale of dislocation structures and slip system properties in Mg 2 SiO 4 forsterite. Our study focuses on screw dislocations of [100] and [001] Burgers vectors. Computations are performed using the so-called THB1 empirical potential set for Mg 2 SiO 4. Results of dislocation core structures highlight the primary importance of the (010) plane for [100] slip dislocations. For [001] dislocations, we confirm the occurrence of a stable narrow core that evolves into transient planar configurations to glide in (100) and (010). Such configurations suggest a locking-unlocking mechanism.
Abstract The change of mechanical properties resulting from the addition of Ca alloying elements ... more Abstract The change of mechanical properties resulting from the addition of Ca alloying elements in nanopolycrystalline Mg-based alloys was investigated using molecular dynamics in the framework of the modified embedded-atom method. Using an energetic approach, it was demonstrated that grain boundaries and their vicinity are the favorable locations for substitutional Ca atoms in nanopolycrystalline Mg with average grain diameter in the order of 12 nm. Based on the McLean theory, a concentration around 8 at% of Mg can be substituted by Ca atoms at the grain boundaries. Thus, by increasing the concentration of substitutional Ca atoms from 1 at% to 5 at% in a nanopolycrystalline Mg sample, we demonstrated that the addition of Ca in Mg-based alloys leads to a systematical decrease of the yield surface. Moreover, a microstructural analysis was performed to identify the relation between the deformation mechanisms and the change in yield surface revealed by the addition of substitutional Ca atoms. It was found that the addition of substitutional Ca leads to (i) a decrease of the activity of 〈 a 〉 dislocations, (ii) an increase of the activity of non-basal dislocations, and (iii) an enhancement of the intergranular fracture.
Lithium-sulfur (Li/S) batteries constitute a promising, next-generation energy storage technology... more Lithium-sulfur (Li/S) batteries constitute a promising, next-generation energy storage technology due to their high theoretical energy density and low cost. To increase sustainability, processability, and battery performance, conducting organic polymers have become a focus of research for the development of better cathode materials. Here, we investigate the solvation structure of the conjugated poly(4-thiophen-3-yl) benzenethiol) (PTBT) polymer as a high-potential macromolecular candidate for cathodes in Li/S batteries. Using molecular dynamics (MD) simulation with newly optimized force-field parameters, we examine the effects of polymer length and various molar fractions of the popular dimethoxyethane (DME) and dioxolane (DOL) solvents on the structure of the PTBT polymer at a temperature of 300 K. We characterize basic polymeric properties as well as the composition-dependent solvent adsorption structure and thermodynamics. Importantly, we find an interesting co-solvency effect, n...
A set of modified embedded-atom method (MEAM) potentials for the interactions between Al, Si, Mg,... more A set of modified embedded-atom method (MEAM) potentials for the interactions between Al, Si, Mg, Cu, and Fe was developed from a combination of each element's MEAM potential in order to study metal alloying. Previously published MEAM parameters of single elements have been improved for a better agreement to the generalized stacking fault energy (GSFE) curves when compared with ab initio generated GSFE curves. The MEAM parameters for element pairs were constructed based on the structural and elastic properties of element pairs in the NaCl reference structure garnered from ab initio calculations, with adjustment to reproduce the ab initio heat of formation of the most stable binary compounds. The new MEAM potentials were validated by comparing the formation energies of defects, equilibrium volumes, elastic moduli, and heat of formation for several binary compounds with ab initio simulations and experiments. Single elements in their ground-state crystal structure were subjected to...
The Mg–Y binary system's potential has been developed within the framework of the second-near... more The Mg–Y binary system's potential has been developed within the framework of the second-nearest-neighbor modified embedded-atom method (MEAM) based on a potential for pure Mg. The potential fitting is done on a range of physical properties, either experimental or computed by first-principles methods. These include the Y interaction energy with basal and pyramidal stacking faults and properties of the B2 Mg–Y intermetallic phase. Using this model, one can make predictions generally in reasonable agreement with experiments and or DFT, but differences remain for subtle but important aspects of Y solutes in Mg.
In this study, the structure, the energetic, and the strength of a ሼ101 ത 1ሽ〈112 ത 0〉 symmetric t... more In this study, the structure, the energetic, and the strength of a ሼ101 ത 1ሽ〈112 ത 0〉 symmetric tilt grain boundary in magnesium and magnesium binary alloys were analyzed in the framework of (semi-)empirical potentials. Following a systematic investigation of the transferability and accuracy of the interatomic potentials, atomistic calculations of the grain boundary energy, the grain boundary sliding energy, and the grain boundary strength were performed in pure magnesium and in binary MgX alloys (X = Al, Ca, Gd, Li, Sn, Y, Ag, Nd, and Pb). The data gained in this study were analyzed to identify the most critical material parameters controlling the strength of the grain boundary, and their consequence on atomic shuffling motions occurring at the grain boundary. From the methodology perspective, the role of in-plane and out-of plane relaxation on the grain boundary sliding energy curves was investigated. In pure magnesium, the results showed that in-plane relaxation is critical in activating ܾ ଶ ሼଵଵ ഥ ଵሽ twinning dislocation resulting in grain boundary migration. In the alloy systems, however, grain boundary migration was disabled as a consequence of the pinning of the grain boundary by segregated elements. Finally, while the grain boundary energy, the shape of the grain boundary sliding energy curves, and the grain boundary sliding energy are critical parameters controlling the grain boundary strength in pure magnesium, only the grain boundary energy and the segregation energy of the alloying elements at the grain boundary were identified as critical material parameters in the alloys system.
Abstract Transitions of pyramidal ⟨ c + a ⟩ dislocations to sessile structures contribute to po... more Abstract Transitions of pyramidal ⟨ c + a ⟩ dislocations to sessile structures contribute to poor ductility in pure Mg. Mg-3 wt% Rare Earth (RE) alloys have good ductility, possibly due to ⟨ c + a ⟩ dislocation stabilization upon addition of RE solutes. Here, ⟨ c + a ⟩ stability is investigated in a model Mg-3 at.%Y random solid solution alloy using molecular dynamics simulations. Favorable fluctuations of Y solutes lower all dislocation energies and have no appreciable effects on the transition mechanism, energy barrier, or time. Enhanced ⟨ c + a ⟩ activity and improved ductility in Mg-3 wt%RE alloys are thus not likely associated with solute-stabilization of pyramidal ⟨ c + a ⟩ dislocations.
Modelling and Simulation in Materials Science and Engineering, 2016
Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the se... more Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the second nearest-neighbors modified embedded-atom method (MEAM). The validity and the transferability of the Ag potential were obtained by calculating physical, mechanical, thermal, and dislocation related properties. Since the {1 1 1}-generalized stacking fault energy curves obtained from first-principle calculations was used to develop the Ag potential, the critical resolved shear stress to move screw dislocations in Ag single crystal is in good agreement with the experimental data. By combining the ability of the potential to predict the surface energies with its accuracy in describing dislocation properties, the potential is thought to be a predictive model for analyzing the fracture properties of Ag. In addition, the performance of the potential was tested under dynamics conditions by predicting the melting temperature, where a good agreement with experimental value was found. The Ag-MEAM potential was then coupled to an existing Mg-MEAM potential to describe the properties of the binary system MgAg. While the heat of formation, the elastic constants, and the (1 1 0) γ-surface of the MgAg compound in the B2 phase were used to parameterize the potential, heat of formation for MgAg alloys with different stoichiometry, thermal properties of the B2-MgAg compound, as well as dislocation related properties in B2-MgAg compound were tested to validate the transferability of the potential. The heat of formation of Mg5Ag2, MgAg, and MgAg3, the elastic constants and the thermal properties of B2-MgAg obtained with the proposed potential align with first-principles and experimental data. In addition, the core structure of both 〈0 0 1〉 and 〈1 1 1〉 dislocations in {1 1 0} are in agreement with theoretical predictions, and the magnitudes of the critical resolved shear stress obtained at 0 K for both slip systems partially validate the slip behavior of B2-MgAg. Furthermore, the interaction between silver solute element and dislocations from the basal plane is correctly captured by the potential.
The interaction between segregated alloying elements (Al, Li, Sn, Y, and Ca) and three crack syst... more The interaction between segregated alloying elements (Al, Li, Sn, Y, and Ca) and three crack systems is investigated in magnesium simple crystal under mode I loading condition. Using molecular statics in the framework of the (modified) embedded-atoms method, the effect of segregated alloying elements on both the fracture behavior and the intrinsic fracture toughness were identified, and analyzed quantitatively in the framework of the Rice's theory for dislocation emission at a crack tip (Rice, 1992. J. Mech. Phys. Solids 40 239-271). In addition, the variations of the critical stress intensity factor as a function of the misfit strain between the alloying element and magnesium are discussed. The results revealed the existence of a transition from cleavage to dislocation emission at a crack tip. Such transition from cleavage to dislocation emission at the crack tip depends on (i) the orientation of the crack system, and (ii) the nature of the alloying element segregated at the crack tip. Furthermore, when analyzed as a function of the misfit strain between the alloying element and magnesium, our data suggest a direct relation between the fracture toughness and the misfit strain, independently of crack orientation. While alloying element with compressive misfit strain enhances the intrinsic fracture toughness of magnesium single crystal, alloying element with tensile misfit strain worsens the intrinsic fracture toughness of magnesium single crystal.
Addition of solutes such as lithium enhances ductility of hexagonal-close-packed (hcp) Magnesium ... more Addition of solutes such as lithium enhances ductility of hexagonal-close-packed (hcp) Magnesium (Mg). However, the atomistic underpinning of Li addition on individual deformation mechanisms remain unclear and is the focus of the present work. We compared the deformation mechanisms in nanocrystalline (NC) and single crystal simulation systems of pure Mg and Mg-Li hcp alloys. Five deformation modes are observed in the pure NC Mg with randomly oriented grains-one basal {0001} 1120 , one pyramidal type-I {1011} 1123 , and three twinning slip systems {1012} 1011 , {1013} 3032 , and {1011} 1012. Distributing 10 at.% Li randomly to this NC Mg decreased its compressive yield strength by 14.5%. This also increases the ductility by activating non-basal deformation modes and by reducing the plastic anisotropy. We benchmarked these results by comparing the effect of Li addition on these deformation modes in Mg single crystals. Finally, we present a formability parameter (F p) model based on unstable stacking fault energy, twin fault energy, and nucleation stress for dislocations (τ N S). Quantifying the changes in F p values for the Mg-Li alloys with respect to pure Mg in single crystal simulations explain the decrease in compressive yield strength and change in deformation mechanisms with Li additions. A sensitivity analysis study, comparing our CD-EAM results with a MEAM potential, shows that the effects of Li on the single deformation mechanisms are potential independent. Lastly, while results for Mg-10 at.% Li random alloy are presented here, similar conclusions can be drawn for other compositions of this hcp Mg-Li alloy.
Abstract The change of mechanical properties resulting from the addition of Ca alloying elements ... more Abstract The change of mechanical properties resulting from the addition of Ca alloying elements in nanopolycrystalline Mg-based alloys was investigated using molecular dynamics in the framework of the modified embedded-atom method. Using an energetic approach, it was demonstrated that grain boundaries and their vicinity are the favorable locations for substitutional Ca atoms in nanopolycrystalline Mg with average grain diameter in the order of 12 nm. Based on the McLean theory, a concentration around 8 at% of Mg can be substituted by Ca atoms at the grain boundaries. Thus, by increasing the concentration of substitutional Ca atoms from 1 at% to 5 at% in a nanopolycrystalline Mg sample, we demonstrated that the addition of Ca in Mg-based alloys leads to a systematical decrease of the yield surface. Moreover, a microstructural analysis was performed to identify the relation between the deformation mechanisms and the change in yield surface revealed by the addition of substitutional Ca atoms. It was found that the addition of substitutional Ca leads to (i) a decrease of the activity of 〈 a 〉 dislocations, (ii) an increase of the activity of non-basal dislocations, and (iii) an enhancement of the intergranular fracture.
Modelling and Simulation in Materials Science and Engineering, 2017
Knowledge of the deformation mechanisms of (Mg,Fe) 2 SiO 4 olivine is important for the understan... more Knowledge of the deformation mechanisms of (Mg,Fe) 2 SiO 4 olivine is important for the understanding of flow and seismic anisotropy in the Earth's upper mantle. We report here a numerical modelling at the atomic scale of dislocation structures and slip system properties in Mg 2 SiO 4 forsterite. Our study focuses on screw dislocations of [100] and [001] Burgers vectors. Computations are performed using the so-called THB1 empirical potential set for Mg 2 SiO 4. Results of dislocation core structures highlight the primary importance of the (010) plane for [100] slip dislocations. For [001] dislocations, we confirm the occurrence of a stable narrow core that evolves into transient planar configurations to glide in (100) and (010). Such configurations suggest a locking-unlocking mechanism.
Early experiences and results on parallelizing discrete dislocation dynamics simulations on multi... more Early experiences and results on parallelizing discrete dislocation dynamics simulations on multi-core architectures
from atomistic calculations in the framework of semi-empirical potentials depends on the accuracy... more from atomistic calculations in the framework of semi-empirical potentials depends on the accuracy and transferability of the potentials to capture the physics involved in the problem. In view of modeling the mechanical properties of a binary magnesium-based alloy using semi-empirical potentials, one has to validate the ability of the potentials to capture the physics governing the interactions between the alloying element and the micromechanisms carrying the inelastic behavior. In this chapter, we are reviewing the interaction between alloying elements and (i) stacking faults and (ii) <a> dislocations from the basal and prismatic slip systems.
Materials science simulations are among the leading applications for scientific supercomputing. D... more Materials science simulations are among the leading applications for scientific supercomputing. Discrete dislocation dynamics (DDD) is a numerical tool used to model the plastic behavior of crystalline materials using the elastic theory of dislocations. DDD simulations require very long running times to produce meaningful scientific results. This work presents early experiences and results on improving the running time of Micromegas, an application code for three-dimensional DDD simulations. We used open source profiling and tracing tools to analyze the behavior and performance, as well as to identify the performance bottlenecks of Micromegas. The major performance bottleneck of Micromegas, amounts to ∼73% of the total sequential run time and is parallelized using OpenMP. Evaluation and validation tests conducted on a Nehalem quad-core processor show ∼50% improvement in the simulation time for 3-D DDD over 30,000 time steps. The correctness and accuracy of the scientific data produc...
Lithium-sulfur (Li/S) batteries constitute a promising, next-generation energy storage technology... more Lithium-sulfur (Li/S) batteries constitute a promising, next-generation energy storage technology due to their high theoretical energy density and low cost. To increase sustainability, processability, and battery performance, conducting organic polymers have become a focus of research for the development of better cathode materials. Here, we investigate the solvation structure of the conjugated poly(4-thiophen-3-yl) benzenethiol) (PTBT) polymer as a high-potential macromolecular candidate for cathodes in Li/S batteries. Using molecular dynamics (MD) simulation with newly optimized force-field parameters, we examine the effects of polymer length and various molar fractions of the popular dimethoxyethane (DME) and dioxolane (DOL) solvents on the structure of the PTBT polymer at a temperature of 300 K. We characterize basic polymeric properties as well as the composition-dependent solvent adsorption 1 structure and thermodynamics. Importantly, we find an interesting co-solvency effect, namely that a solvent comprised of about 25% DME and 75% DOL leads to maximum swelling (best solvent quality) behavior, which should be important for optimizing cathode fractality and permeability in applications. Our study thus reveals intriguing polymer-solvent correlations and serves as a first step for further MD studies of realistic polymeric cathode structures and processes, e.g., toward charge transport in vulcanized (S-linked) network topologies.
Theoretical and Applied Fracture Mechanics, Oct 1, 2016
The interaction between segregated alloying elements (Al, Li, Sn, Y, and Ca) and three crack syst... more The interaction between segregated alloying elements (Al, Li, Sn, Y, and Ca) and three crack systems is investigated in magnesium simple crystal under mode I loading condition. Using molecular statics in the framework of the (modified) embedded-atoms method, the effect of segregated alloying elements on both the fracture behavior and the intrinsic fracture toughness were identified, and analyzed quantitatively in the framework of the Rice's theory for dislocation emission at a crack tip (Rice, 1992. J. Mech. Phys. Solids 40 239-271). In addition, the variations of the critical stress intensity factor as a function of the misfit strain between the alloying element and magnesium are discussed. The results revealed the existence of a transition from cleavage to dislocation emission at a crack tip. Such transition from cleavage to dislocation emission at the crack tip depends on (i) the orientation of the crack system, and (ii) the nature of the alloying element segregated at the crack tip. Furthermore, when analyzed as a function of the misfit strain between the alloying element and magnesium, our data suggest a direct relation between the fracture toughness and the misfit strain, independently of crack orientation. While alloying element with compressive misfit strain enhances the intrinsic fracture toughness of magnesium single crystal, alloying element with tensile misfit strain worsens the intrinsic fracture toughness of magnesium single crystal.
Modelling and Simulation in Materials Science and Engineering, Aug 1, 2016
Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the se... more Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the second nearest-neighbors modified embedded-atom method (MEAM). The validity and the transferability of the Ag potential were obtained by calculating physical, mechanical, thermal, and dislocation related properties. Since the {1 1 1}-generalized stacking fault energy curves obtained from first-principle calculations was used to develop the Ag potential, the critical resolved shear stress to move screw dislocations in Ag single crystal is in good agreement with the experimental data. By combining the ability of the potential to predict the surface energies with its accuracy in describing dislocation properties, the potential is thought to be a predictive model for analyzing the fracture properties of Ag. In addition, the performance of the potential was tested under dynamics conditions by predicting the melting temperature, where a good agreement with experimental value was found. The Ag-MEAM potential was then coupled to an existing Mg-MEAM potential to describe the properties of the binary system MgAg. While the heat of formation, the elastic constants, and the (1 1 0) γ-surface of the MgAg compound in the B2 phase were used to parameterize the potential, heat of formation for MgAg alloys with different stoichiometry, thermal properties of the B2-MgAg compound, as well as dislocation related properties in B2-MgAg compound were tested to validate the transferability of the potential. The heat of formation of Mg5Ag2, MgAg, and MgAg3, the elastic constants and the thermal properties of B2-MgAg obtained with the proposed potential align with first-principles and experimental data. In addition, the core structure of both 〈0 0 1〉 and 〈1 1 1〉 dislocations in {1 1 0} are in agreement with theoretical predictions, and the magnitudes of the critical resolved shear stress obtained at 0 K for both slip systems partially validate the slip behavior of B2-MgAg. Furthermore, the interaction between silver solute element and dislocations from the basal plane is correctly captured by the potential.
Modelling and Simulation in Materials Science and Engineering, May 23, 2017
Knowledge of the deformation mechanisms of (Mg,Fe) 2 SiO 4 olivine is important for the understan... more Knowledge of the deformation mechanisms of (Mg,Fe) 2 SiO 4 olivine is important for the understanding of flow and seismic anisotropy in the Earth's upper mantle. We report here a numerical modelling at the atomic scale of dislocation structures and slip system properties in Mg 2 SiO 4 forsterite. Our study focuses on screw dislocations of [100] and [001] Burgers vectors. Computations are performed using the so-called THB1 empirical potential set for Mg 2 SiO 4. Results of dislocation core structures highlight the primary importance of the (010) plane for [100] slip dislocations. For [001] dislocations, we confirm the occurrence of a stable narrow core that evolves into transient planar configurations to glide in (100) and (010). Such configurations suggest a locking-unlocking mechanism.
Abstract The change of mechanical properties resulting from the addition of Ca alloying elements ... more Abstract The change of mechanical properties resulting from the addition of Ca alloying elements in nanopolycrystalline Mg-based alloys was investigated using molecular dynamics in the framework of the modified embedded-atom method. Using an energetic approach, it was demonstrated that grain boundaries and their vicinity are the favorable locations for substitutional Ca atoms in nanopolycrystalline Mg with average grain diameter in the order of 12 nm. Based on the McLean theory, a concentration around 8 at% of Mg can be substituted by Ca atoms at the grain boundaries. Thus, by increasing the concentration of substitutional Ca atoms from 1 at% to 5 at% in a nanopolycrystalline Mg sample, we demonstrated that the addition of Ca in Mg-based alloys leads to a systematical decrease of the yield surface. Moreover, a microstructural analysis was performed to identify the relation between the deformation mechanisms and the change in yield surface revealed by the addition of substitutional Ca atoms. It was found that the addition of substitutional Ca leads to (i) a decrease of the activity of 〈 a 〉 dislocations, (ii) an increase of the activity of non-basal dislocations, and (iii) an enhancement of the intergranular fracture.
Lithium-sulfur (Li/S) batteries constitute a promising, next-generation energy storage technology... more Lithium-sulfur (Li/S) batteries constitute a promising, next-generation energy storage technology due to their high theoretical energy density and low cost. To increase sustainability, processability, and battery performance, conducting organic polymers have become a focus of research for the development of better cathode materials. Here, we investigate the solvation structure of the conjugated poly(4-thiophen-3-yl) benzenethiol) (PTBT) polymer as a high-potential macromolecular candidate for cathodes in Li/S batteries. Using molecular dynamics (MD) simulation with newly optimized force-field parameters, we examine the effects of polymer length and various molar fractions of the popular dimethoxyethane (DME) and dioxolane (DOL) solvents on the structure of the PTBT polymer at a temperature of 300 K. We characterize basic polymeric properties as well as the composition-dependent solvent adsorption structure and thermodynamics. Importantly, we find an interesting co-solvency effect, n...
A set of modified embedded-atom method (MEAM) potentials for the interactions between Al, Si, Mg,... more A set of modified embedded-atom method (MEAM) potentials for the interactions between Al, Si, Mg, Cu, and Fe was developed from a combination of each element's MEAM potential in order to study metal alloying. Previously published MEAM parameters of single elements have been improved for a better agreement to the generalized stacking fault energy (GSFE) curves when compared with ab initio generated GSFE curves. The MEAM parameters for element pairs were constructed based on the structural and elastic properties of element pairs in the NaCl reference structure garnered from ab initio calculations, with adjustment to reproduce the ab initio heat of formation of the most stable binary compounds. The new MEAM potentials were validated by comparing the formation energies of defects, equilibrium volumes, elastic moduli, and heat of formation for several binary compounds with ab initio simulations and experiments. Single elements in their ground-state crystal structure were subjected to...
The Mg–Y binary system's potential has been developed within the framework of the second-near... more The Mg–Y binary system's potential has been developed within the framework of the second-nearest-neighbor modified embedded-atom method (MEAM) based on a potential for pure Mg. The potential fitting is done on a range of physical properties, either experimental or computed by first-principles methods. These include the Y interaction energy with basal and pyramidal stacking faults and properties of the B2 Mg–Y intermetallic phase. Using this model, one can make predictions generally in reasonable agreement with experiments and or DFT, but differences remain for subtle but important aspects of Y solutes in Mg.
In this study, the structure, the energetic, and the strength of a ሼ101 ത 1ሽ〈112 ത 0〉 symmetric t... more In this study, the structure, the energetic, and the strength of a ሼ101 ത 1ሽ〈112 ത 0〉 symmetric tilt grain boundary in magnesium and magnesium binary alloys were analyzed in the framework of (semi-)empirical potentials. Following a systematic investigation of the transferability and accuracy of the interatomic potentials, atomistic calculations of the grain boundary energy, the grain boundary sliding energy, and the grain boundary strength were performed in pure magnesium and in binary MgX alloys (X = Al, Ca, Gd, Li, Sn, Y, Ag, Nd, and Pb). The data gained in this study were analyzed to identify the most critical material parameters controlling the strength of the grain boundary, and their consequence on atomic shuffling motions occurring at the grain boundary. From the methodology perspective, the role of in-plane and out-of plane relaxation on the grain boundary sliding energy curves was investigated. In pure magnesium, the results showed that in-plane relaxation is critical in activating ܾ ଶ ሼଵଵ ഥ ଵሽ twinning dislocation resulting in grain boundary migration. In the alloy systems, however, grain boundary migration was disabled as a consequence of the pinning of the grain boundary by segregated elements. Finally, while the grain boundary energy, the shape of the grain boundary sliding energy curves, and the grain boundary sliding energy are critical parameters controlling the grain boundary strength in pure magnesium, only the grain boundary energy and the segregation energy of the alloying elements at the grain boundary were identified as critical material parameters in the alloys system.
Abstract Transitions of pyramidal ⟨ c + a ⟩ dislocations to sessile structures contribute to po... more Abstract Transitions of pyramidal ⟨ c + a ⟩ dislocations to sessile structures contribute to poor ductility in pure Mg. Mg-3 wt% Rare Earth (RE) alloys have good ductility, possibly due to ⟨ c + a ⟩ dislocation stabilization upon addition of RE solutes. Here, ⟨ c + a ⟩ stability is investigated in a model Mg-3 at.%Y random solid solution alloy using molecular dynamics simulations. Favorable fluctuations of Y solutes lower all dislocation energies and have no appreciable effects on the transition mechanism, energy barrier, or time. Enhanced ⟨ c + a ⟩ activity and improved ductility in Mg-3 wt%RE alloys are thus not likely associated with solute-stabilization of pyramidal ⟨ c + a ⟩ dislocations.
Modelling and Simulation in Materials Science and Engineering, 2016
Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the se... more Interatomic potentials for pure Ag and Mg–Ag alloy have been developed in the framework of the second nearest-neighbors modified embedded-atom method (MEAM). The validity and the transferability of the Ag potential were obtained by calculating physical, mechanical, thermal, and dislocation related properties. Since the {1 1 1}-generalized stacking fault energy curves obtained from first-principle calculations was used to develop the Ag potential, the critical resolved shear stress to move screw dislocations in Ag single crystal is in good agreement with the experimental data. By combining the ability of the potential to predict the surface energies with its accuracy in describing dislocation properties, the potential is thought to be a predictive model for analyzing the fracture properties of Ag. In addition, the performance of the potential was tested under dynamics conditions by predicting the melting temperature, where a good agreement with experimental value was found. The Ag-MEAM potential was then coupled to an existing Mg-MEAM potential to describe the properties of the binary system MgAg. While the heat of formation, the elastic constants, and the (1 1 0) γ-surface of the MgAg compound in the B2 phase were used to parameterize the potential, heat of formation for MgAg alloys with different stoichiometry, thermal properties of the B2-MgAg compound, as well as dislocation related properties in B2-MgAg compound were tested to validate the transferability of the potential. The heat of formation of Mg5Ag2, MgAg, and MgAg3, the elastic constants and the thermal properties of B2-MgAg obtained with the proposed potential align with first-principles and experimental data. In addition, the core structure of both 〈0 0 1〉 and 〈1 1 1〉 dislocations in {1 1 0} are in agreement with theoretical predictions, and the magnitudes of the critical resolved shear stress obtained at 0 K for both slip systems partially validate the slip behavior of B2-MgAg. Furthermore, the interaction between silver solute element and dislocations from the basal plane is correctly captured by the potential.
The interaction between segregated alloying elements (Al, Li, Sn, Y, and Ca) and three crack syst... more The interaction between segregated alloying elements (Al, Li, Sn, Y, and Ca) and three crack systems is investigated in magnesium simple crystal under mode I loading condition. Using molecular statics in the framework of the (modified) embedded-atoms method, the effect of segregated alloying elements on both the fracture behavior and the intrinsic fracture toughness were identified, and analyzed quantitatively in the framework of the Rice's theory for dislocation emission at a crack tip (Rice, 1992. J. Mech. Phys. Solids 40 239-271). In addition, the variations of the critical stress intensity factor as a function of the misfit strain between the alloying element and magnesium are discussed. The results revealed the existence of a transition from cleavage to dislocation emission at a crack tip. Such transition from cleavage to dislocation emission at the crack tip depends on (i) the orientation of the crack system, and (ii) the nature of the alloying element segregated at the crack tip. Furthermore, when analyzed as a function of the misfit strain between the alloying element and magnesium, our data suggest a direct relation between the fracture toughness and the misfit strain, independently of crack orientation. While alloying element with compressive misfit strain enhances the intrinsic fracture toughness of magnesium single crystal, alloying element with tensile misfit strain worsens the intrinsic fracture toughness of magnesium single crystal.
Addition of solutes such as lithium enhances ductility of hexagonal-close-packed (hcp) Magnesium ... more Addition of solutes such as lithium enhances ductility of hexagonal-close-packed (hcp) Magnesium (Mg). However, the atomistic underpinning of Li addition on individual deformation mechanisms remain unclear and is the focus of the present work. We compared the deformation mechanisms in nanocrystalline (NC) and single crystal simulation systems of pure Mg and Mg-Li hcp alloys. Five deformation modes are observed in the pure NC Mg with randomly oriented grains-one basal {0001} 1120 , one pyramidal type-I {1011} 1123 , and three twinning slip systems {1012} 1011 , {1013} 3032 , and {1011} 1012. Distributing 10 at.% Li randomly to this NC Mg decreased its compressive yield strength by 14.5%. This also increases the ductility by activating non-basal deformation modes and by reducing the plastic anisotropy. We benchmarked these results by comparing the effect of Li addition on these deformation modes in Mg single crystals. Finally, we present a formability parameter (F p) model based on unstable stacking fault energy, twin fault energy, and nucleation stress for dislocations (τ N S). Quantifying the changes in F p values for the Mg-Li alloys with respect to pure Mg in single crystal simulations explain the decrease in compressive yield strength and change in deformation mechanisms with Li additions. A sensitivity analysis study, comparing our CD-EAM results with a MEAM potential, shows that the effects of Li on the single deformation mechanisms are potential independent. Lastly, while results for Mg-10 at.% Li random alloy are presented here, similar conclusions can be drawn for other compositions of this hcp Mg-Li alloy.
Abstract The change of mechanical properties resulting from the addition of Ca alloying elements ... more Abstract The change of mechanical properties resulting from the addition of Ca alloying elements in nanopolycrystalline Mg-based alloys was investigated using molecular dynamics in the framework of the modified embedded-atom method. Using an energetic approach, it was demonstrated that grain boundaries and their vicinity are the favorable locations for substitutional Ca atoms in nanopolycrystalline Mg with average grain diameter in the order of 12 nm. Based on the McLean theory, a concentration around 8 at% of Mg can be substituted by Ca atoms at the grain boundaries. Thus, by increasing the concentration of substitutional Ca atoms from 1 at% to 5 at% in a nanopolycrystalline Mg sample, we demonstrated that the addition of Ca in Mg-based alloys leads to a systematical decrease of the yield surface. Moreover, a microstructural analysis was performed to identify the relation between the deformation mechanisms and the change in yield surface revealed by the addition of substitutional Ca atoms. It was found that the addition of substitutional Ca leads to (i) a decrease of the activity of 〈 a 〉 dislocations, (ii) an increase of the activity of non-basal dislocations, and (iii) an enhancement of the intergranular fracture.
Modelling and Simulation in Materials Science and Engineering, 2017
Knowledge of the deformation mechanisms of (Mg,Fe) 2 SiO 4 olivine is important for the understan... more Knowledge of the deformation mechanisms of (Mg,Fe) 2 SiO 4 olivine is important for the understanding of flow and seismic anisotropy in the Earth's upper mantle. We report here a numerical modelling at the atomic scale of dislocation structures and slip system properties in Mg 2 SiO 4 forsterite. Our study focuses on screw dislocations of [100] and [001] Burgers vectors. Computations are performed using the so-called THB1 empirical potential set for Mg 2 SiO 4. Results of dislocation core structures highlight the primary importance of the (010) plane for [100] slip dislocations. For [001] dislocations, we confirm the occurrence of a stable narrow core that evolves into transient planar configurations to glide in (100) and (010). Such configurations suggest a locking-unlocking mechanism.
Early experiences and results on parallelizing discrete dislocation dynamics simulations on multi... more Early experiences and results on parallelizing discrete dislocation dynamics simulations on multi-core architectures
from atomistic calculations in the framework of semi-empirical potentials depends on the accuracy... more from atomistic calculations in the framework of semi-empirical potentials depends on the accuracy and transferability of the potentials to capture the physics involved in the problem. In view of modeling the mechanical properties of a binary magnesium-based alloy using semi-empirical potentials, one has to validate the ability of the potentials to capture the physics governing the interactions between the alloying element and the micromechanisms carrying the inelastic behavior. In this chapter, we are reviewing the interaction between alloying elements and (i) stacking faults and (ii) <a> dislocations from the basal and prismatic slip systems.
Materials science simulations are among the leading applications for scientific supercomputing. D... more Materials science simulations are among the leading applications for scientific supercomputing. Discrete dislocation dynamics (DDD) is a numerical tool used to model the plastic behavior of crystalline materials using the elastic theory of dislocations. DDD simulations require very long running times to produce meaningful scientific results. This work presents early experiences and results on improving the running time of Micromegas, an application code for three-dimensional DDD simulations. We used open source profiling and tracing tools to analyze the behavior and performance, as well as to identify the performance bottlenecks of Micromegas. The major performance bottleneck of Micromegas, amounts to ∼73% of the total sequential run time and is parallelized using OpenMP. Evaluation and validation tests conducted on a Nehalem quad-core processor show ∼50% improvement in the simulation time for 3-D DDD over 30,000 time steps. The correctness and accuracy of the scientific data produc...
Uploads
Papers by Sebastien groh