Nikolai Priezjev | Howard University (original) (raw)

Videos by Nikolai Priezjev

Visualization of the yielding transition in a binary glass periodically deformed for 60 shear cyc... more Visualization of the yielding transition in a binary glass periodically deformed for 60 shear cycles. Snapshots of one million atoms with large nonaffine displacements after a full cycle. Colorcode = D^2.

https://doi.org/10.3390/met10030300
Metals 10, 300 (2020).

Online presentation: https://youtu.be/92F8kajDvbk

Papers by Nikolai Priezjev

Research paper thumbnail of Rapid cooling across the glass transition temperature at finite strain rate

Research paper thumbnail of Fatigue behavior of Cu-Zr metallic glasses under cyclic loading

Metals, 2023

The effect of oscillatory shear deformation on the fatigue life, yielding transition, and flow lo... more The effect of oscillatory shear deformation on the fatigue life, yielding transition, and flow localization in metallic glasses is investigated using molecular dynamics simulations. We study a well-annealed Cu-Zr amorphous alloy subjected to periodic shear at room temperature. We find that upon loading for hundreds of cycles at strain amplitudes just below a critical value, the potential energy at zero strain remains nearly constant and plastic events are highly localized. By contrast, at strain amplitudes above the critical point, the plastic deformation is gradually accumulated upon continued loading until the yielding transition and the formation of a shear band across the entire system. Interestingly, when the strain amplitude approaches the critical value from above, the number of cycles to failure increases as a power-law function, which is consistent with the previous results on binary Lennard-Jones glasses.

Research paper thumbnail of Fatigue failure of amorphous alloys under cyclic shear deformation

Computational Materials Science, 2023

The accumulation of plastic deformation and flow localization in amorphous alloys under periodic ... more The accumulation of plastic deformation and flow localization in amorphous alloys under periodic shear are investigated using molecular dynamics simulations. We study a well-annealed binary mixture of one million atoms subjected to oscillatory shear deformation with strain amplitudes slightly above a critical value. We find that upon approaching a critical strain amplitude from above, the number of shear cycles until the yielding transition is well described by a power-law function. Remarkably, the potential energy at the end of each cycle as a function of the normalized number of cycles is nearly independent of the strain amplitude, which allows for estimation of the fatigue lifetime at a given strain amplitude. The analysis of nonaffine displacements of atoms elucidates the process of strain localization, including irreversible rearrangements of small clusters until the formation of a system-spanning shear band.

Online presentation: https://youtu.be/92F8kajDvbk

Research paper thumbnail of Numerical simulation of micro-filtration of oil-in-water emulsions

This study addresses the issue of oil removal from water using hydrophilic porous membranes. The ... more This study addresses the issue of oil removal from water using hydrophilic porous membranes. The effective separation of oil-in-water dispersions involves high flux of water through the membrane and, at the same time, high rejection rate of the oil phase. The effects of transmembrane pressure and crossflow velocity on rejection of oil droplets and thin oil films by pores of different cross-section are investigated numerically by solving the Navier-Stokes equation. We found that in the absence of crossflow, the critical transmembrane pressure, which is required for the oil droplet entry into a circular pore of a given surface hydrophilicity, agrees well with analytical predictions based on the Young-Laplace equation. An analytical expression for the critical pressure in terms of geometric parameters of the pore cross-section is validated via numerical simulations for a continuous oil film on elliptical and rectangular pores. With increasing crossflow velocity, the shape of the oil droplet is strongly deformed near the pore entrance and the critical pressure of permeation increases. We determined numerically the phase diagram for the droplet rejection, permeation, and breakup depending on the transmembrane pressure and shear rate. The critical pressure of permeation is identified as the line separating permeation and rejection regions. Using a novel method for computing the critical pressure, we investigated the effect of various physical and geometrical parameters on the critical pressure of permeation and breakup of droplets under shear flow. It is demonstrated numerically that the critical pressure of permeation increases with shear rate, viscosity ratio, surface tension coefficient, contact angle, and droplet size. On the LIST OF FIGURES .

Research paper thumbnail of Relaxation dynamics in amorphous alloys under asymmetric cyclic shear deformation

Journal of Non-Crystalline Solids

The influence of cyclic loading and glass stability on structural relaxation and yielding transit... more The influence of cyclic loading and glass stability on structural relaxation and yielding transition in amorphous alloys was investigated using molecular dynamics simulations. We considered a binary mixture cooled deep into the glass phase and subjected to cyclic shear deformation where strain varies periodically but remains positive. We found that rapidly cooled glasses under asymmetric cyclic shear gradually evolve towards states with lower potential energy and finite stress at zero strain. At the strain amplitude just below a critical value, the rescaled distributions of nonaffine displacements converge to a power-law decay with an exponent of about-2 upon increasing number of cycles. By contrast, more stable glasses yield at lower strain amplitudes, and the yielding transition can be delayed for hundreds of cycles when the strain amplitude is near a critical value. These results can be useful for the design of novel thermo-mechanical processing methods to improve mechanical and physical properties of metallic glasses.

Research paper thumbnail of Relaxation dynamics in amorphous alloys under asymmetric cyclic shear deformation

Journal of Non-Crystalline Solids

The influence of cyclic loading and glass stability on structural relaxation and yielding transit... more The influence of cyclic loading and glass stability on structural relaxation and yielding transition in amorphous alloys was investigated using molecular dynamics simulations. We considered a binary mixture cooled deep into the glass phase and subjected to cyclic shear deformation where strain varies periodically but remains positive. We found that rapidly cooled glasses under asymmetric cyclic shear gradually evolve towards states with lower potential energy and finite stress at zero strain. At the strain amplitude just below a critical value, the rescaled distributions of nonaffine displacements converge to a power-law decay with an exponent of about-2 upon increasing number of cycles. By contrast, more stable glasses yield at lower strain amplitudes, and the yielding transition can be delayed for hundreds of cycles when the strain amplitude is near a critical value. These results can be useful for the design of novel thermo-mechanical processing methods to improve mechanical and physical properties of metallic glasses.

Research paper thumbnail of Two local slip modes at the liquid-liquid interface over liquid-infused surfaces

Physics of Fluids, 2022

A liquid-liquid interface (LLI) at liquid-infused surfaces (LISs) plays a significant role in pro... more A liquid-liquid interface (LLI) at liquid-infused surfaces (LISs) plays a significant role in promoting slip flow and reducing frictional drag. By employing the transverse many-body dissipative particle dynamics simulations, the behavior of local and effective slip at a flat LLI for shear flows over periodically grooved LISs has been studied. With increasing viscosity ratio between the working fluid and lubricant fluid, two local slip modes are identified. For a small viscosity ratio, the local slip length remains finite along the LLI, while a hybrid local slip boundary condition holds along the LLI for large viscosity ratios, i.e., the local slip length is finite near the groove edge and unbounded in the central region of the LLI. The vortical flow inside the groove can be enhanced by increasing viscosity ratio due to the change in the local slip mode from the finite state to the hybrid one. Moreover, the results suggest two scenarios for the variation of the effective slippage. Fo...

Research paper thumbnail of External and intrinsic anchoring in nematic liquid crystals: A Monte Carlo study

Physical Review E, 2003

We present a Monte Carlo study of external surface anchoring in nematic cells with partially diso... more We present a Monte Carlo study of external surface anchoring in nematic cells with partially disordered solid substrates, as well as of intrinsic anchoring at free nematic interfaces. The simulations are based on the simple hexagonal lattice model with a spatially anisotropic intermolecular potential. We estimate the corresponding extrapolation length b by imposing an elastic deformation in a hybrid cell-like nematic sample. Our estimates for b increase with increasing surface disorder and are essentially temperature independent. Experimental values of b are approached only when both the coupling of nematic molecules with the substrate and the anisotropy of nematic-nematic interactions are weak.

Research paper thumbnail of Mechanical annealing and yielding transition in cyclically sheared binary glasses

Journal of Non-Crystalline Solids

The effect of cyclic shear deformation on structural relaxation and yielding in binary glasses wa... more The effect of cyclic shear deformation on structural relaxation and yielding in binary glasses was examined using molecular dynamics simulations. We studied a binary mixture slowly cooled from the liquid phase to about half the glass transition temperature and then periodically deformed at small strain amplitudes during thousands of cycles. We found that the potential energy decays logarithmically upon increasing number of cycles. The analysis of nonaffine displacements revealed that the process of mechanical annealing proceeds via intermittent plastic rearrangements whose spatial extent decreases upon reaching lower energy states. We also probed the yielding behavior for glasses with different degrees of annealing by adjusting strain amplitude near the critical value. Interestingly, in contrast to zero-temperature amorphous solids, the critical strain amplitude remains unchanged for glasses with initially different energy levels. The formation of a shear band at the yielding transition correlates well with the sharp increase of the number of atoms with large nonaffine displacements.

Research paper thumbnail of Molecular dynamics simulations of the mechanical annealing process in metallic glasses: Effects of strain amplitude and temperature

Journal of Non-Crystalline Solids, 2018

Molecular dynamics simulations are performed to examine the dynamic response of amorphous solids ... more Molecular dynamics simulations are performed to examine the dynamic response of amorphous solids to oscillatory shear at finite temperatures. The data were collected from a poorly annealed binary glass, which was deformed periodically in the elastic regime during several hundred shear cycles. We found that the characteristic time required to reach a steady state with a minimum potential energy is longer at higher temperatures and larger strain amplitudes. With decreasing strain amplitude, the asymptotic value of the potential energy increases but it remains lower than in quiescent samples. The transient decay of the potential energy correlates well with a gradual decrease in the volume occupied by atoms with large nonaffine displacements. By contrast, the maximum amplitude of shear stress oscillations is attained relatively quickly when a large part of the system starts to deform reversibly.

Research paper thumbnail of Alternating Shear Orientation During Cyclic Loading Facilitates Yielding in Amorphous Materials

Journal of Materials Engineering and Performance, 2020

Research paper thumbnail of Yielding transition in stable glasses periodically deformed at finite temperature

Computational Materials Science, 2021

The effect of glass stability on the yielding transition and mechanical properties of periodicall... more The effect of glass stability on the yielding transition and mechanical properties of periodically deformed binary glasses is investigated using molecular dynamics simulations. We consider a binary mixture first slowly cooled below the glass transition temperature and then mechanically annealed to deeper energy states via smallamplitude oscillatory shear deformation. We show that upon increasing glass stability, the shear modulus and the yielding peak during startup continuous deformation increase towards plateau levels. It is found that during the strain amplitude sweep, the yielding transition occurs at higher amplitudes and it becomes more abrupt in deeply annealed glasses. The processes of initiation and formation of a shear band are elucidated via the spatiotemporal analysis of nonaffine displacements of atoms. These results are important for thermo-mechanical processing of highly stable amorphous alloys.

Research paper thumbnail of A delayed yielding transition in mechanically annealed binary glasses at finite temperature

Journal of Non-Crystalline Solids, 2020

The influence of strain amplitude, glass stability and thermal fluctuations on shear band formati... more The influence of strain amplitude, glass stability and thermal fluctuations on shear band formation and yielding transition is studied using molecular dynamics simulations. The model binary mixture is first gradually cooled below the glass transition temperature and then periodically deformed to access a broad range of potential energy states. We find that the critical strain amplitude becomes larger for highly annealed glasses within about one thousand shear cycles. Moreover, upon continued loading at a fixed strain amplitude, the yielding transition is delayed in glasses mechanically annealed to lower energy states. It is also demonstrated that nucleation of a small cluster of atoms with large nonaffine displacements precedes a sharp energy change associated with the yielding transition. These results are important for thermal and mechanical processing of amorphous alloys with tunable mechanical and physical properties.

Research paper thumbnail of Accelerated rejuvenation in metallic glasses subjected to elastostatic compression along alternating directions

Journal of Non-Crystalline Solids, 2021

The influence of static stress and alternating loading direction on the potential energy and mech... more The influence of static stress and alternating loading direction on the potential energy and mechanical properties of amorphous alloys is investigated using molecular dynamics simulations. The model glass is represented via a binary mixture which is first slowly annealed well below the glass transition temperature and then subjected to elastostatic loading either along a single direction or along two and three alternating directions. We find that at sufficiently large values of the static stress, the binary glass becomes rejuvenated via collective, irreversible rearrangements of atoms. Upon including additional orientation of the static stress in the loading protocol, the rejuvenation effect is amplified and the typical size of clusters of atoms with large nonaffine displacements increases. As a result of prolonged mechanical loading, the elastic modulus and the peak value of the stress overshoot during startup continuous compression become significantly reduced, especially for loading protocols with alternating stress orientation. These findings are important for the design of novel processing methods to improve mechanical properties of metallic glasses.

Research paper thumbnail of Accessing a broader range of energy states in metallic glasses by variable-amplitude oscillatory shear

Journal of Non-Crystalline Solids, 2021

Research paper thumbnail of Shear band healing in amorphous materials by small-amplitude oscillatory shear deformation

Journal of Non-Crystalline Solids, 2021

The effect of small-amplitude periodic shear on annealing of a shear band in binary glasses is in... more The effect of small-amplitude periodic shear on annealing of a shear band in binary glasses is investigated using molecular dynamics simulations. The shear band is first introduced in stable glasses via large-amplitude periodic shear, and then amorphous samples are subjected to repeated loading during thousands of cycles at strain amplitudes below the yield strain. It was found that with increasing strain amplitude, the glasses are relocated to deeper potential energy levels, while the energy change upon annealing is not affected by the glass initial stability. The results of mechanical tests indicate that the shear modulus and yield stress both increase towards plateau levels during the first few hundred cycles, and their magnitudes are largest when samples are loaded at strain amplitudes close to the yield strain. The analysis of nonaffine displacements reveals that the shear band breaks up into isolated clusters that gradually decay over time, leading to nearly reversible deformation within the elastic range. These results might be useful for mechanical processing of metallic glasses and additive manufacturing.

Research paper thumbnail of Cooling under Applied Stress Rejuvenates Amorphous Alloys and Enhances Their Ductility

Metals, 2020

The effect of tensile stress applied during cooling of binary glasses on the potential energy sta... more The effect of tensile stress applied during cooling of binary glasses on the potential energy states and mechanical properties is investigated using molecular dynamics simulations. We study the three-dimensional binary mixture that was first annealed near the glass transition temperature and then rapidly cooled under tension into the glass phase. It is found that at larger values of applied stress, the liquid glass former freezes under higher strain and its potential energy is enhanced. For a fixed cooling rate, the maximum tensile stress that can be applied during cooling is reduced upon increasing initial temperature above the glass transition point. We also show that the amorphous structure of rejuvenated glasses is characterized by an increase in the number of contacts between smaller type atoms. Furthermore, the results of tensile tests demonstrate that the elastic modulus and the peak value of the stress overshoot are reduced in glasses prepared at larger applied stresses and ...

Research paper thumbnail of Spatiotemporal Analysis of Nonaffine Displacements in Disordered Solids Sheared Across the Yielding Point

Metallurgical and Materials Transactions A, 2020

The time evolution and spatial correlations of nonaffine displacements in deformed amorphous soli... more The time evolution and spatial correlations of nonaffine displacements in deformed amorphous solids are investigated using molecular dynamics simulations. The three-dimensional model glass is represented via the binary mixture, which is slowly annealed well below the glass transition temperature and then sheared at a constant strain rate. It is shown that with increasing strain, the typical size of clusters of atoms with large nonaffine displacements increases, and these clusters remain spatially homogeneously distributed, until the yielding point when mobile atoms become localized within a system-spanning shear band. Furthermore, the yielding transition is associated with an abrupt change in the spatial correlation of nonaffine displacements, which varies from exponential to power-law decay. We also find that the height of the first peak in the pair correlation function of small atoms exhibits a distinct increase at the yielding strain. These results are discussed in relation to the yielding transition in amorphous materials under cyclic loading.

Research paper thumbnail of Structural relaxation in amorphous materials under cyclic tension-compression loading

Journal of Non-Crystalline Solids, 2020

The process of structural relaxation in disordered solids subjected to repeated tensioncompressio... more The process of structural relaxation in disordered solids subjected to repeated tensioncompression loading is studied using molecular dynamics simulations. The binary glass is prepared by rapid cooling well below the glass transition temperature and then periodically strained at constant volume. We find that the amorphous system is relocated to progressively lower potential energy states during hundreds of cycles, and the energy levels become deeper upon approaching critical strain amplitude from below. The decrease in potential energy is associated with collective nonaffine rearrangements of atoms, and their rescaled probability distribution becomes independent of the cycle number at sufficiently large time intervals. It is also shown that yielding during startup shear deformation occurs at larger values of the stress overshoot in samples that were cyclically loaded at higher strain amplitudes. These results might be useful for mechanical processing of amorphous alloys in order to reduce their energy and increase chemical resistivity and resistance to crystallization.

Visualization of the yielding transition in a binary glass periodically deformed for 60 shear cyc... more Visualization of the yielding transition in a binary glass periodically deformed for 60 shear cycles. Snapshots of one million atoms with large nonaffine displacements after a full cycle. Colorcode = D^2.

https://doi.org/10.3390/met10030300
Metals 10, 300 (2020).

Online presentation: https://youtu.be/92F8kajDvbk

Research paper thumbnail of Rapid cooling across the glass transition temperature at finite strain rate

Research paper thumbnail of Fatigue behavior of Cu-Zr metallic glasses under cyclic loading

Metals, 2023

The effect of oscillatory shear deformation on the fatigue life, yielding transition, and flow lo... more The effect of oscillatory shear deformation on the fatigue life, yielding transition, and flow localization in metallic glasses is investigated using molecular dynamics simulations. We study a well-annealed Cu-Zr amorphous alloy subjected to periodic shear at room temperature. We find that upon loading for hundreds of cycles at strain amplitudes just below a critical value, the potential energy at zero strain remains nearly constant and plastic events are highly localized. By contrast, at strain amplitudes above the critical point, the plastic deformation is gradually accumulated upon continued loading until the yielding transition and the formation of a shear band across the entire system. Interestingly, when the strain amplitude approaches the critical value from above, the number of cycles to failure increases as a power-law function, which is consistent with the previous results on binary Lennard-Jones glasses.

Research paper thumbnail of Fatigue failure of amorphous alloys under cyclic shear deformation

Computational Materials Science, 2023

The accumulation of plastic deformation and flow localization in amorphous alloys under periodic ... more The accumulation of plastic deformation and flow localization in amorphous alloys under periodic shear are investigated using molecular dynamics simulations. We study a well-annealed binary mixture of one million atoms subjected to oscillatory shear deformation with strain amplitudes slightly above a critical value. We find that upon approaching a critical strain amplitude from above, the number of shear cycles until the yielding transition is well described by a power-law function. Remarkably, the potential energy at the end of each cycle as a function of the normalized number of cycles is nearly independent of the strain amplitude, which allows for estimation of the fatigue lifetime at a given strain amplitude. The analysis of nonaffine displacements of atoms elucidates the process of strain localization, including irreversible rearrangements of small clusters until the formation of a system-spanning shear band.

Online presentation: https://youtu.be/92F8kajDvbk

Research paper thumbnail of Numerical simulation of micro-filtration of oil-in-water emulsions

This study addresses the issue of oil removal from water using hydrophilic porous membranes. The ... more This study addresses the issue of oil removal from water using hydrophilic porous membranes. The effective separation of oil-in-water dispersions involves high flux of water through the membrane and, at the same time, high rejection rate of the oil phase. The effects of transmembrane pressure and crossflow velocity on rejection of oil droplets and thin oil films by pores of different cross-section are investigated numerically by solving the Navier-Stokes equation. We found that in the absence of crossflow, the critical transmembrane pressure, which is required for the oil droplet entry into a circular pore of a given surface hydrophilicity, agrees well with analytical predictions based on the Young-Laplace equation. An analytical expression for the critical pressure in terms of geometric parameters of the pore cross-section is validated via numerical simulations for a continuous oil film on elliptical and rectangular pores. With increasing crossflow velocity, the shape of the oil droplet is strongly deformed near the pore entrance and the critical pressure of permeation increases. We determined numerically the phase diagram for the droplet rejection, permeation, and breakup depending on the transmembrane pressure and shear rate. The critical pressure of permeation is identified as the line separating permeation and rejection regions. Using a novel method for computing the critical pressure, we investigated the effect of various physical and geometrical parameters on the critical pressure of permeation and breakup of droplets under shear flow. It is demonstrated numerically that the critical pressure of permeation increases with shear rate, viscosity ratio, surface tension coefficient, contact angle, and droplet size. On the LIST OF FIGURES .

Research paper thumbnail of Relaxation dynamics in amorphous alloys under asymmetric cyclic shear deformation

Journal of Non-Crystalline Solids

The influence of cyclic loading and glass stability on structural relaxation and yielding transit... more The influence of cyclic loading and glass stability on structural relaxation and yielding transition in amorphous alloys was investigated using molecular dynamics simulations. We considered a binary mixture cooled deep into the glass phase and subjected to cyclic shear deformation where strain varies periodically but remains positive. We found that rapidly cooled glasses under asymmetric cyclic shear gradually evolve towards states with lower potential energy and finite stress at zero strain. At the strain amplitude just below a critical value, the rescaled distributions of nonaffine displacements converge to a power-law decay with an exponent of about-2 upon increasing number of cycles. By contrast, more stable glasses yield at lower strain amplitudes, and the yielding transition can be delayed for hundreds of cycles when the strain amplitude is near a critical value. These results can be useful for the design of novel thermo-mechanical processing methods to improve mechanical and physical properties of metallic glasses.

Research paper thumbnail of Relaxation dynamics in amorphous alloys under asymmetric cyclic shear deformation

Journal of Non-Crystalline Solids

The influence of cyclic loading and glass stability on structural relaxation and yielding transit... more The influence of cyclic loading and glass stability on structural relaxation and yielding transition in amorphous alloys was investigated using molecular dynamics simulations. We considered a binary mixture cooled deep into the glass phase and subjected to cyclic shear deformation where strain varies periodically but remains positive. We found that rapidly cooled glasses under asymmetric cyclic shear gradually evolve towards states with lower potential energy and finite stress at zero strain. At the strain amplitude just below a critical value, the rescaled distributions of nonaffine displacements converge to a power-law decay with an exponent of about-2 upon increasing number of cycles. By contrast, more stable glasses yield at lower strain amplitudes, and the yielding transition can be delayed for hundreds of cycles when the strain amplitude is near a critical value. These results can be useful for the design of novel thermo-mechanical processing methods to improve mechanical and physical properties of metallic glasses.

Research paper thumbnail of Two local slip modes at the liquid-liquid interface over liquid-infused surfaces

Physics of Fluids, 2022

A liquid-liquid interface (LLI) at liquid-infused surfaces (LISs) plays a significant role in pro... more A liquid-liquid interface (LLI) at liquid-infused surfaces (LISs) plays a significant role in promoting slip flow and reducing frictional drag. By employing the transverse many-body dissipative particle dynamics simulations, the behavior of local and effective slip at a flat LLI for shear flows over periodically grooved LISs has been studied. With increasing viscosity ratio between the working fluid and lubricant fluid, two local slip modes are identified. For a small viscosity ratio, the local slip length remains finite along the LLI, while a hybrid local slip boundary condition holds along the LLI for large viscosity ratios, i.e., the local slip length is finite near the groove edge and unbounded in the central region of the LLI. The vortical flow inside the groove can be enhanced by increasing viscosity ratio due to the change in the local slip mode from the finite state to the hybrid one. Moreover, the results suggest two scenarios for the variation of the effective slippage. Fo...

Research paper thumbnail of External and intrinsic anchoring in nematic liquid crystals: A Monte Carlo study

Physical Review E, 2003

We present a Monte Carlo study of external surface anchoring in nematic cells with partially diso... more We present a Monte Carlo study of external surface anchoring in nematic cells with partially disordered solid substrates, as well as of intrinsic anchoring at free nematic interfaces. The simulations are based on the simple hexagonal lattice model with a spatially anisotropic intermolecular potential. We estimate the corresponding extrapolation length b by imposing an elastic deformation in a hybrid cell-like nematic sample. Our estimates for b increase with increasing surface disorder and are essentially temperature independent. Experimental values of b are approached only when both the coupling of nematic molecules with the substrate and the anisotropy of nematic-nematic interactions are weak.

Research paper thumbnail of Mechanical annealing and yielding transition in cyclically sheared binary glasses

Journal of Non-Crystalline Solids

The effect of cyclic shear deformation on structural relaxation and yielding in binary glasses wa... more The effect of cyclic shear deformation on structural relaxation and yielding in binary glasses was examined using molecular dynamics simulations. We studied a binary mixture slowly cooled from the liquid phase to about half the glass transition temperature and then periodically deformed at small strain amplitudes during thousands of cycles. We found that the potential energy decays logarithmically upon increasing number of cycles. The analysis of nonaffine displacements revealed that the process of mechanical annealing proceeds via intermittent plastic rearrangements whose spatial extent decreases upon reaching lower energy states. We also probed the yielding behavior for glasses with different degrees of annealing by adjusting strain amplitude near the critical value. Interestingly, in contrast to zero-temperature amorphous solids, the critical strain amplitude remains unchanged for glasses with initially different energy levels. The formation of a shear band at the yielding transition correlates well with the sharp increase of the number of atoms with large nonaffine displacements.

Research paper thumbnail of Molecular dynamics simulations of the mechanical annealing process in metallic glasses: Effects of strain amplitude and temperature

Journal of Non-Crystalline Solids, 2018

Molecular dynamics simulations are performed to examine the dynamic response of amorphous solids ... more Molecular dynamics simulations are performed to examine the dynamic response of amorphous solids to oscillatory shear at finite temperatures. The data were collected from a poorly annealed binary glass, which was deformed periodically in the elastic regime during several hundred shear cycles. We found that the characteristic time required to reach a steady state with a minimum potential energy is longer at higher temperatures and larger strain amplitudes. With decreasing strain amplitude, the asymptotic value of the potential energy increases but it remains lower than in quiescent samples. The transient decay of the potential energy correlates well with a gradual decrease in the volume occupied by atoms with large nonaffine displacements. By contrast, the maximum amplitude of shear stress oscillations is attained relatively quickly when a large part of the system starts to deform reversibly.

Research paper thumbnail of Alternating Shear Orientation During Cyclic Loading Facilitates Yielding in Amorphous Materials

Journal of Materials Engineering and Performance, 2020

Research paper thumbnail of Yielding transition in stable glasses periodically deformed at finite temperature

Computational Materials Science, 2021

The effect of glass stability on the yielding transition and mechanical properties of periodicall... more The effect of glass stability on the yielding transition and mechanical properties of periodically deformed binary glasses is investigated using molecular dynamics simulations. We consider a binary mixture first slowly cooled below the glass transition temperature and then mechanically annealed to deeper energy states via smallamplitude oscillatory shear deformation. We show that upon increasing glass stability, the shear modulus and the yielding peak during startup continuous deformation increase towards plateau levels. It is found that during the strain amplitude sweep, the yielding transition occurs at higher amplitudes and it becomes more abrupt in deeply annealed glasses. The processes of initiation and formation of a shear band are elucidated via the spatiotemporal analysis of nonaffine displacements of atoms. These results are important for thermo-mechanical processing of highly stable amorphous alloys.

Research paper thumbnail of A delayed yielding transition in mechanically annealed binary glasses at finite temperature

Journal of Non-Crystalline Solids, 2020

The influence of strain amplitude, glass stability and thermal fluctuations on shear band formati... more The influence of strain amplitude, glass stability and thermal fluctuations on shear band formation and yielding transition is studied using molecular dynamics simulations. The model binary mixture is first gradually cooled below the glass transition temperature and then periodically deformed to access a broad range of potential energy states. We find that the critical strain amplitude becomes larger for highly annealed glasses within about one thousand shear cycles. Moreover, upon continued loading at a fixed strain amplitude, the yielding transition is delayed in glasses mechanically annealed to lower energy states. It is also demonstrated that nucleation of a small cluster of atoms with large nonaffine displacements precedes a sharp energy change associated with the yielding transition. These results are important for thermal and mechanical processing of amorphous alloys with tunable mechanical and physical properties.

Research paper thumbnail of Accelerated rejuvenation in metallic glasses subjected to elastostatic compression along alternating directions

Journal of Non-Crystalline Solids, 2021

The influence of static stress and alternating loading direction on the potential energy and mech... more The influence of static stress and alternating loading direction on the potential energy and mechanical properties of amorphous alloys is investigated using molecular dynamics simulations. The model glass is represented via a binary mixture which is first slowly annealed well below the glass transition temperature and then subjected to elastostatic loading either along a single direction or along two and three alternating directions. We find that at sufficiently large values of the static stress, the binary glass becomes rejuvenated via collective, irreversible rearrangements of atoms. Upon including additional orientation of the static stress in the loading protocol, the rejuvenation effect is amplified and the typical size of clusters of atoms with large nonaffine displacements increases. As a result of prolonged mechanical loading, the elastic modulus and the peak value of the stress overshoot during startup continuous compression become significantly reduced, especially for loading protocols with alternating stress orientation. These findings are important for the design of novel processing methods to improve mechanical properties of metallic glasses.

Research paper thumbnail of Accessing a broader range of energy states in metallic glasses by variable-amplitude oscillatory shear

Journal of Non-Crystalline Solids, 2021

Research paper thumbnail of Shear band healing in amorphous materials by small-amplitude oscillatory shear deformation

Journal of Non-Crystalline Solids, 2021

The effect of small-amplitude periodic shear on annealing of a shear band in binary glasses is in... more The effect of small-amplitude periodic shear on annealing of a shear band in binary glasses is investigated using molecular dynamics simulations. The shear band is first introduced in stable glasses via large-amplitude periodic shear, and then amorphous samples are subjected to repeated loading during thousands of cycles at strain amplitudes below the yield strain. It was found that with increasing strain amplitude, the glasses are relocated to deeper potential energy levels, while the energy change upon annealing is not affected by the glass initial stability. The results of mechanical tests indicate that the shear modulus and yield stress both increase towards plateau levels during the first few hundred cycles, and their magnitudes are largest when samples are loaded at strain amplitudes close to the yield strain. The analysis of nonaffine displacements reveals that the shear band breaks up into isolated clusters that gradually decay over time, leading to nearly reversible deformation within the elastic range. These results might be useful for mechanical processing of metallic glasses and additive manufacturing.

Research paper thumbnail of Cooling under Applied Stress Rejuvenates Amorphous Alloys and Enhances Their Ductility

Metals, 2020

The effect of tensile stress applied during cooling of binary glasses on the potential energy sta... more The effect of tensile stress applied during cooling of binary glasses on the potential energy states and mechanical properties is investigated using molecular dynamics simulations. We study the three-dimensional binary mixture that was first annealed near the glass transition temperature and then rapidly cooled under tension into the glass phase. It is found that at larger values of applied stress, the liquid glass former freezes under higher strain and its potential energy is enhanced. For a fixed cooling rate, the maximum tensile stress that can be applied during cooling is reduced upon increasing initial temperature above the glass transition point. We also show that the amorphous structure of rejuvenated glasses is characterized by an increase in the number of contacts between smaller type atoms. Furthermore, the results of tensile tests demonstrate that the elastic modulus and the peak value of the stress overshoot are reduced in glasses prepared at larger applied stresses and ...

Research paper thumbnail of Spatiotemporal Analysis of Nonaffine Displacements in Disordered Solids Sheared Across the Yielding Point

Metallurgical and Materials Transactions A, 2020

The time evolution and spatial correlations of nonaffine displacements in deformed amorphous soli... more The time evolution and spatial correlations of nonaffine displacements in deformed amorphous solids are investigated using molecular dynamics simulations. The three-dimensional model glass is represented via the binary mixture, which is slowly annealed well below the glass transition temperature and then sheared at a constant strain rate. It is shown that with increasing strain, the typical size of clusters of atoms with large nonaffine displacements increases, and these clusters remain spatially homogeneously distributed, until the yielding point when mobile atoms become localized within a system-spanning shear band. Furthermore, the yielding transition is associated with an abrupt change in the spatial correlation of nonaffine displacements, which varies from exponential to power-law decay. We also find that the height of the first peak in the pair correlation function of small atoms exhibits a distinct increase at the yielding strain. These results are discussed in relation to the yielding transition in amorphous materials under cyclic loading.

Research paper thumbnail of Structural relaxation in amorphous materials under cyclic tension-compression loading

Journal of Non-Crystalline Solids, 2020

The process of structural relaxation in disordered solids subjected to repeated tensioncompressio... more The process of structural relaxation in disordered solids subjected to repeated tensioncompression loading is studied using molecular dynamics simulations. The binary glass is prepared by rapid cooling well below the glass transition temperature and then periodically strained at constant volume. We find that the amorphous system is relocated to progressively lower potential energy states during hundreds of cycles, and the energy levels become deeper upon approaching critical strain amplitude from below. The decrease in potential energy is associated with collective nonaffine rearrangements of atoms, and their rescaled probability distribution becomes independent of the cycle number at sufficiently large time intervals. It is also shown that yielding during startup shear deformation occurs at larger values of the stress overshoot in samples that were cyclically loaded at higher strain amplitudes. These results might be useful for mechanical processing of amorphous alloys in order to reduce their energy and increase chemical resistivity and resistance to crystallization.

Research paper thumbnail of The effect of thermal history on the atomic structure and mechanical properties of amorphous alloys

Computational Materials Science, 2020

The influence of thermal processing on the potential energy, atomic structure, and mechanical pro... more The influence of thermal processing on the potential energy, atomic structure, and mechanical properties of metallic glasses is examined using molecular dynamics simulations. We study the three-dimensional binary mixture, which was first relaxed near the glass transition temperature, and then rapidly cooled deep into the glass phase. It was found that glasses prepared at higher annealing temperatures are relocated to higher energy states and their average glass structure remains more disordered, as reflected in the shape of the pair correlation function. The results of mechanical testing demonstrate that both the shear modulus and yielding peak increase significantly when the annealing temperature approaches T g from above. Moreover, the shear modulus becomes a strong function of strain rate only for samples equilibrated at sufficiently high temperatures. Based on the spatial distribution of nonaffine displacements, we show that the deformation mode changes from brittle to ductile upon increasing annealing temperature. These results can be useful for the design and optimization of the fabrication processes of bulk glassy alloys with improved plasticity.

Research paper thumbnail of Fatigue failure of amorphous alloys under cyclic shear deformation

The accumulation of plastic deformation and flow localization in amorphous alloys under periodic ... more The accumulation of plastic deformation and flow localization in amorphous alloys under periodic shear are investigated using molecular dynamics simulations. We study a well-annealed binary mixture of one million atoms subjected to oscillatory shear deformation with strain amplitudes slightly above a critical value. We find that upon approaching a critical strain amplitude from above, the number of shear cycles until the yielding transition is well described by a power-law function. Remarkably, the potential energy at the end of each cycle as a function of the normalized number of cycles is nearly independent of the strain amplitude, which allows for estimation of the fatigue lifetime at a given strain amplitude. The analysis on nonaffine displacements of atoms elucidates the process of strain localization, including irreversible rearrangements of small clusters until the formation of a system-spanning shear band.

Research paper thumbnail of Relaxation dynamics in amorphous alloys under asymmetric cyclic shear deformation

The influence of cyclic loading and glass stability on structural relaxation and yielding transit... more The influence of cyclic loading and glass stability on structural relaxation and yielding transition in amorphous alloys was investigated using molecular dynamics simulations. We considered a binary mixture cooled deep into the glass phase and subjected to cyclic shear deformation where strain varies periodically but remains positive. We found that rapidly cooled glasses under asymmetric cyclic shear gradually evolve towards states with lower potential energy and finite stress at zero strain. At the strain amplitude just below a critical value, the rescaled distributions of nonaffine displacements converge to a power-law decay with an exponent of about-2 upon increasing number of cycles. By contrast, more stable glasses yield at lower strain amplitudes, and the yielding transition can be delayed for hundreds of cycles when the strain amplitude is near a critical value. These results can be useful for the design of novel thermo-mechanical processing methods to improve mechanical and physical properties of metallic glasses.

Research paper thumbnail of MOLECULAR DYNAMICS SIMULATION STUDY OF A POLYMER DROPLET TRANSPORT OVER AN ARRAY OF SPHERICAL NANOPARTICLES

Molecular dynamics simulation study of a polymer droplet transport over an array of spherical nan... more Molecular dynamics simulation study of a polymer droplet transport over an array of spherical nanoparticles. This study uses molecular dynamics simulations to evaluate the dynamic behavior of a partially wetting polymer droplet driven over a nanostructured interface. We consider the bead-spring model to represent a polymeric liquid that partially wets a rough surface composed of a periodic array of spherical particles. Results show that at sufficiently small values of external force, the droplet remains pinned at the particle's surface, whereas above the threshold its motion consists of alternating periods of pinning and rapid displacements between neighboring particles. The latter process involves large periodic variation of the advancing and receding contact angles due to the attachment and detachment of the contact line. Finally, upon increasing the external force, the droplet's center of mass is steadily displaced, while at the same time the oscillation amplitude of the receding contact angle as well as the maximum contact angle hysteresis remain relatively unchanged. iv TABLE OF CONTENTS CHAPTER 1 INTRODUCTION…………………………..………………...…1 1.1 Literature Review…………………………………………………………...……...….1 1.2 Thesis Outline…….…………………………………………………………………...5 CHAPTER 2 NUMERICAL SIMULATIONS DETAILS……………………6 2.1 Details of Molecular Dynamics Simulations……………………………………….…6 CHAPTER 3 RESULTS …………………………………………...………….10 3.1 Results of molecular dynamics simulations…………….……………………………10 3.1.1 Droplet shape for various surface energies …………………………………..…..11 3.1.2 Apparent contact angle as a function of surface energy ………….……………....13 3.1.3 Fitting procedure for droplet and apparent contact angle………………………...15 3.1.4 Droplet Center of Mass (COM) for various values of external force………….…18 3.1.5 Variation of the Advancing and the receding contact angles and the velocity of Center of Mass (COM) …………………………………………………………..21 3.1.6 Attaching and Detaching process of a droplet …………………………….……..26 3.1.7 Fluid monomers in contact with surface atom of a spherical particle …………....29 CHAPTER 4 CONCLUSION…………………………..……………………..31 CHAPTER 5 BIBLIOGRAPHY………………………………...…………….32

Research paper thumbnail of Mechanical annealing and yielding transition in cyclically sheared binary glasses

The effect of cyclic shear deformation on structural relaxation and yielding in binary glasses wa... more The effect of cyclic shear deformation on structural relaxation and yielding in binary glasses was examined using molecular dynamics simulations. We studied a binary mixture slowly cooled from the liquid phase to about half the glass transition temperature and then periodically deformed at small strain amplitudes during thousands of cycles. We found that the potential energy decays logarithmically upon increasing number of cycles. The analysis of nonaffine displacements revealed that the process of mechanical annealing proceeds via intermittent plastic rearrangements whose spatial extent decreases upon reaching lower energy states. We also probed the yielding behavior for glasses with different degrees of annealing by adjusting strain amplitude near the critical value. Interestingly, in contrast to zero-temperature amorphous solids, the critical strain amplitude remains unchanged for glasses with initially different energy levels. The formation of a shear band at the yielding transition correlates well with the sharp increase of the number of atoms with large nonaffine displacements.

Research paper thumbnail of Yielding transition in stable glasses periodically deformed at finite temperature

The effect of glass stability on the yielding transition and mechanical properties of periodicall... more The effect of glass stability on the yielding transition and mechanical properties of periodically deformed binary glasses is investigated using molecular dynamics simulations. We consider a binary mixture first slowly cooled below the glass transition temperature and then mechanically annealed to deeper energy states via small-amplitude oscillatory shear deformation. We show that upon increasing glass stability, the shear modulus and the yielding peak during startup continuous deformation increase towards plateau levels. It is found that during the strain amplitude sweep, the yielding transition occurs at higher amplitudes and it becomes more abrupt in deeply annealed glasses. The processes of initiation and formation of a shear band are elucidated via the spatiotemporal analysis of nonaffine displacements of atoms. These results are important for thermo-mechanical processing of highly stable amorphous alloys.

Research paper thumbnail of Shear band healing in amorphous materials by small-amplitude oscillatory shear deformation

The effect of small-amplitude periodic shear on annealing of a shear band in binary glasses is in... more The effect of small-amplitude periodic shear on annealing of a shear band in binary glasses is investigated using molecular dynamics simulations. The shear band is first introduced in stable glasses via periodic shear at a strain amplitude slightly above the critical value, and then samples are subjected to repeated loading during thousands of cycles at smaller amplitudes. It was found that with increasing strain amplitude, the glasses are relocated to deeper potential energy levels, while the energy change upon annealing is not affected by the glass initial stability. The results of mechanical tests indicate that the shear modulus and yield stress both increase towards plateau levels during the first few hundred cycles, and their magnitudes are greater for samples loaded at larger strain amplitudes. The analysis of nonaffine displacements reveals that the shear band breaks up into isolated clusters that gradually decay over time, leading to nearly reversible deformation within the elastic range. These results might be useful for mechanical processing of metallic glasses and additive manufacturing.

Research paper thumbnail of Slip Flow Regimes and Induced Fluid Structure in Nanoscale Polymer Films: Recent Results from Molecular Dynamics Simulations Motivation: Nano-and Microfluidics

Motivation: Nano-and Microfluidics "Microflows & Nanoflows" Karniadakis (2005) • Control and mani... more Motivation: Nano-and Microfluidics "Microflows & Nanoflows" Karniadakis (2005) • Control and manipulation of fluids at submicron scales • The behavior of fluids at the microscale is different from 'macrofluidic' behavior (low Re, high S/V ratio)