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    Comparing Glass Formability of Cu-Zr and Ni-Zr Systems by Molecular Dynamics Simulation

    , M.Sc. Thesis Sharif University of Technology Ghaemi, Milad (Author) ; Tavakoli, Rohollah (Supervisor)
    Abstract
    Metallic glasses (MG) are a group of materials that have unique properties such as high yielding strength and high elastic deformation. Because of limitation in the production of these materials, a few industrial materials of MG have been made so far. A few of binary alloys, like binary alloys made of Cu-Zr, have high Glass Forming Ability (GFA) in medium cooling rates. Between two binary systems of Cu-Zr and Ni-Zr, although constituent elements in aspect of atomic radius, electronic structure, physical properties, phase diagrams and intermetallic compounds are so similar to each other, but GFA between them are very different. For example, in Cu-Zr system there are compounds, such as... 

    Study of Structural and Mechanical Properties of Zr-, Ti- and La-based bulk Metallic Glasses

    , Ph.D. Dissertation Sharif University of Technology Asadi Khanouki, Mohammad Taghi (Author) ; Aashuri, Hossein (Supervisor) ; Tavakoli, Rouhollah (Supervisor)
    Abstract
    Bulk metallic glasses (BMGs), in contrast to conventional crystalline materials, are defined as metals with an amorphous and disordered atomic-scale structure. Due to the absence of dislocations and grain boundaries, BMGs have considerably unique mechanical properties such as high strength and elastic strain, high wear resistance and desirable corrosion resistance. However, they generally suffer from poor plasticity caused by an inhomogeneous deformation which leads to catastrophic failure by localization of strain into narrow regions, known as shear bands. This factor has extremely restricted their application as advanced structural materials. Furthermore, the recently discovered phenomenon... 

    Molecular Dynamic Study of Short-and Medium-Range order Structures in Bulk Metallic Glasses

    , Ph.D. Dissertation Sharif University of Technology Foroughi, Alireza (Author) ; Aashuri, Hosien (Supervisor) ; Tavakoli, Roholah (Supervisor)
    Abstract
    In this work, structures of Cu-Zr bulk metallic glasses at atomic scale were studied by molecular dynamics simulation. Bulk metallic glasses have high glass form ability, which makes it possible to more effectively examine the relationship between structure and properties in glassy materials. Due to this reason, this family of materials has been selected in this research. Voronoi tessellation method, coordination number analysis, short-range order examination, glass transition temperature and pair distribution function have been selected to investigate the structure in atomic scale. Results show that full icosahedron (with the highest five-fold symmetry) and some distorted icosahedra have... 

    Molecular Dynamics Study of Tungsten and Iron Addition on the Glass Formability of Cobalt-based Electrodeposited Coatings

    , M.Sc. Thesis Sharif University of Technology Bayati, Mohammad Baher (Author) ; Tavakoli, Rouhollah (Supervisor)
    Abstract
    Recent research suggests that the electrochemical deposition of cobalt-based binary alloys, such as cobalt-tungsten and cobalt-iron, can lead to the formation of amorphous structures in the coatings. Theoretical studies suggest that these structures are equivalent to structures created by the melt cooling of the alloy at speeds of about 〖10〗^10 Kelvin per second. The formation of a glass phase can significantly increase the abrasion resistance and corrosion resistance of these coatings. Empirical studies show that the ability of the glass to become glassy, or more precisely, the transition from crystalline to amorphous, is largely a function of the chemical composition. Simulation of... 

    Investigation of Heat Treatment on Microstructure of Bulk Metallic Glass By Molecular Dynamics

    , M.Sc. Thesis Sharif University of Technology Naseri Karimi, Vahid (Author) ; Ashouri, Hossein (Supervisor) ; Tavakoli, Rouhulla (Supervisor)
    Abstract
    Metallic glasses have been applied in industry owing to their attractive mechanical, physical, and chemical properties. such as high electrical resistivity, soft-type superconductivity, good magnetic softness, high strength and large elastic strain. These unusual properties make the amorphous alloys potentially useful for various applications, which originate from the intrinsic random atomic configuration. but their plasticity is low and this disadvantage limites their applicable range as a engineering material. In this study we try to improve plasticity of metallic glasses by structural rejuvenation. Structural rejuvenation in metallic glasses by a thermal process (i.e. through recovery... 

    Effect of Chemical Composition on Atomic Structure and Mechanical Properties of Mg-Zn Glass Alloys Using Molecular Dynamics Simulation

    , M.Sc. Thesis Sharif University of Technology Asl Roosta, Pouneh (Author) ; Tavakoli, Rohollah (Supervisor)
    Abstract
    In this research, Mg-Zn bulk metallic glasses have been investigated using molecular dynamics simulation. In order to study the structure of this alloy, coordination number, short and medium order of chemical range, Voronoi analysis, glass conversion temperature and atomic pair distribution function have been used. According to the results, the full icosahedral structure with IS connection type in comparison with other structures has an essential role in the stability of the system during cooling and creating medium-scale areas. The results show that the ability to form glass depends on the chemical composition and cooling rate. In this research, the cooling rate is considered to be 1011 K/s... 

    Simulation of the Effect of Cooling Rate and Chemical Composition on the Atomic Structure of Bulk Metallic Glass Iron-Phosphorus and Nickel-Phosphorus

    , M.Sc. Thesis Sharif University of Technology Hosseinzadeh, Danial (Author) ; Tavakoli, Rouhollah (Supervisor)
    Abstract
    bulk metallic glass Unlike metal materials that have a crystalline structure, amorphous glass is an amorphous material with an irregular atomic structure that simultaneously has the same physical properties as metals. Due to the absence of crystal defects such as dislocation and grain boundaries, these materials show unique mechanical properties such as high strength and elastic strain, abrasion resistance and proper corrosion. However, their plastic deformation is heterogeneous, concentrated, and accompanied by sudden failure. Unlike metals, these materials do not have a long-range crystalline order, and their atomic structure includes short-range and mid-range order. The short-range order... 

    Recent advancements in bulk metallic glasses and their applications: A review

    , Article Critical Reviews in Solid State and Materials Sciences ; Volume 43, Issue 3 , 2018 , Pages 233-268 ; 10408436 (ISSN) Khan, M. M ; Nemati, A ; Rahman, Z.U ; Shah, U. H ; Asgar, H ; Haider, W ; Sharif University of Technology
    Taylor and Francis Inc  2018
    Abstract
    Bulk metallic glasses (BMGs), that display extraordinary properties of high strength, corrosion resistance, polymer-like formability, and excellent magnetic properties, are emerging as modern quintessential engineering materials. BMGs have garnered significant research enthusiasm owing to their tremendous technological and scientific standing. In this article, the recent advancements in the field of BMGs and their applications are put in a nutshell. Novel state-of-the-art production routes and nano/microimprinting strategies with salient features capable of circumventing the processing related complexities as well as accelerating modern developments, are briefly summarized. Heterogeneous BMG... 

    Effect of temperature on the fracture surface morphology of Ti- and Zr-based bulk metallic glasses: exploring correlation between morphology and plasticity

    , Article Journal of Materials Science ; Volume 53, Issue 14 , July , 2018 , Pages 10372-10382 ; 00222461 (ISSN) Asadi Khanouki, M. T ; Tavakoli, R ; Aashuri, H ; Sharif University of Technology
    Springer New York LLC  2018
    Abstract
    According to previous studies on the fracture surface morphologies of bulk metallic glasses, the stable crack growth region width and vein pattern size increase with the plasticity at room temperature. In the present work, the fracture surface morphologies of Ti- and Zr-based bulk metallic glasses bent over a wide temperature range (0.1–0.8 glass transition temperature) are systematically analyzed. According to our finding, the stable crack growth region width increases while the vein pattern size decreases as the ductility improves by varying temperature. This observation is in contrast to the common thought that the ductility is proportional to the stable crack growth region width and vein... 

    On the origin of intermediate temperature brittleness in La-based bulk metallic glasses

    , Article Journal of Alloys and Compounds ; Volume 770 , 2019 , Pages 535-539 ; 09258388 (ISSN) Asadi Khanouki, M. T ; Tavakoli, R ; Aashuri, H ; Sharif University of Technology
    Elsevier Ltd  2019
    Abstract
    The effect of strain rate on the ductility of a La-based bulk metallic glass (BMG) over a wide temperature range, and the correlation between ductility and relaxation processes within this alloy are investigated in the present work. The three point bending test and dynamic mechanical analysis are employed to study these phenomena. It is found that the activation energies of the nearly constant loss (NCL) relaxation and intermediate temperature brittleness are almost identical. This observation reveals that the NCL relaxation, as a locally confined or caged dynamics, contributes as the main source of intermediate temperature brittleness in La-based BMGs. © 2018 Elsevier B.V  

    On the glass-forming ability of (Zr0.5Cu0.5)100−xAlx ternary alloys: A molecular dynamics study

    , Article Materials Today Communications ; Volume 31 , 2022 ; 23524928 (ISSN) Abbasi, M. H ; Shabestari, S. G ; Tavakoli, R ; Sharif University of Technology
    Elsevier Ltd  2022
    Abstract
    In this research, the atomic scale local structures in (Zr0.5Cu0.5)100−xAlx (x = 0,2,4,6,8,10,12) bulk metallic glass was studied using molecular dynamics simulation method. The pair distribution function, Voronoi analysis and mean squared displacement (MSD) were adopted for investigation of the local structures. It was found that Cu- and Al-centered full icosahedra possess the most frequency accompanied by the most changes during the glass transition process in the supercooled liquid region temperature. Moreover, it was observed that the Al-centered full icosahedra (Al-FI) and Cu-centered full icosahedra (Cu-FI) clusters with 2.5% and 1.9% increase (relative to total atoms), respectively,... 

    Thermally induced failure mechanism transition and its correlation with short-range order evolution in metallic glasses

    , Article Extreme Mechanics Letters ; Volume 9 , 2016 , Pages 215-225 ; 23524316 (ISSN) Jafary Zadeh, M ; Tavakoli, R ; Srolovitz, D. J ; Zhang, Y. W ; Sharif University of Technology
    Elsevier Ltd 
    Abstract
    The effect of temperature on the short-range order (SRO) structures, deformation mechanisms and failure modes of metallic glasses (MGs) is of fundamental importance for their practical applications. However, due to lack of direct structural information at the atomistic level from experiments and the absence of previous molecular dynamics (MD) simulations to reproduce experimental observations over a wide range of temperature, this issue has not been well understood. Here, by carefully constructing the atomistic models of Cu64Zr36 and Fe80W20 MGs, we are able to reproduce the major deformation modes observed experimentally, i.e. single shear banding (SB) at low temperatures, multiple... 

    Molecular dynamics study of structural formation in Cu50–Zr50 bulk metallic glass

    , Article Journal of Non-Crystalline Solids ; Volume 432 , 2016 , Pages 334-341 ; 00223093 (ISSN) Foroughi, A ; Tavakoli, R ; Aashuri, H ; Sharif University of Technology
    Elsevier B. V  2016
    Abstract
    In this work, the evolution of the local structure in Cu50–Zr50 bulk metallic glass during glass formation was studied by molecular dynamics simulation. The pair distribution function and Voronoi analysis were adopted to characterize local structures in this alloy. The stability of icosahedral clusters and the role of other local clusters in the formation of icosahedra were evaluated. It was found that the (0,2,8,2) polyhedron is not only the dominant cluster in this alloy, but also the most prone cluster to convert into an icosahedron in the course of cooling. Moreover, it acts as an intermediate state during the icosahedron formation. The onset of stability of icosahedra emerges at the... 

    Temperature dependence of serrated flow, strain-rate sensitivity and activation volume in a Ti-based bulk metallic glass

    , Article Journal of Non-Crystalline Solids ; Volume 553 , 2021 ; 00223093 (ISSN) Asadi Khanouki, M. T ; Tavakoli, R ; Aashuri, H ; Sharif University of Technology
    Elsevier B. V  2021
    Abstract
    In this study, the influence of temperature on serrated flow behavior, strain rate sensitivity (SRS) and effective activation volume in a Ti-based bulk metallic glass (BMG) bent at different strain rates has been investigated. Results reveal that the fundamental origin of shear deformation is related to the time-dependent structural relaxations, which occur during deformation at special temperatures and strain rates. Furthermore, the localized structural relaxation processes at intermediate temperatures cause an inelastic shuffling within cages and mitigate the activation of STZs. The main reason of negative SRS at intermediate temperatures is due to the insufficient time for structural... 

    Effect of chemical composition and affinity on the short- and medium-range order structures and mechanical properties of Zr-Ni-Al metallic glass

    , Article Journal of Non-Crystalline Solids ; Volume 456 , 2017 , Pages 68-75 ; 00223093 (ISSN) Jafary Zadeh, M ; Tavakoli, R ; Koh, J. J ; Aitken, Z. H ; Zhang, Y. W ; Sharif University of Technology
    Elsevier B.V  2017
    Abstract
    Previous studies have shown that a small variation in the chemical composition of metallic glasses (MGs) can drastically alter their strength and ductility. However, the underlying structural origin and atomistic mechanisms remain unclear. Using large-scale molecular dynamics simulations, we studied the effect of chemical composition and affinity on the microstructure and deformation behaviour of Zr50Ni50 − xAlx MG by varying the value of x in the range of 5 ≤ x ≤ 25 (at.%). We show that an increase in x is able to strengthen and embrittle the material. In particular, a ductile (homogeneous deformation) to brittle (shear banding) transition occurs at x ~ 15. To reveal the structural origin,... 

    Comparing short–range and medium–range ordering in Cu–Zr and Ni–Zr metallic glasses – Correlation between structure and glass form ability

    , Article Journal of Non-Crystalline Solids ; Volume 499 , 2018 , Pages 227-236 ; 00223093 (ISSN) Ghaemi, M ; Tavakoli, R ; Foroughi, A ; Sharif University of Technology
    Elsevier B.V  2018
    Abstract
    According to recent studies, Cu–Zr and Ni–Zr binary alloying systems are very similar based on known glass-forming ability criteria, however, they exhibit significant difference in glass-forming ability in practice. In this work, local atomic structures of Cu–Zr and Ni–Zr metallic glasses are studied by molecular dynamics simulation to explain the source of mentioned difference. The total and partial distribution functions, coordination number and voronoi analysis are utilized to characterize the local atomic structures around Zr, Ni and Cu atoms. It was found that the local environment around Zr atoms is almost similar in both systems. The difference in the atomic structure in these systems... 

    Medium range order evolution in pressurized sub-Tg annealing of Cu64Zr36 metallic glass

    , Article Journal of Non-Crystalline Solids ; Volume 481 , 2018 , Pages 132-137 ; 00223093 (ISSN) Foroughi, A ; Tavakoli, R ; Aashuri, H ; Sharif University of Technology
    Elsevier B.V  2018
    Abstract
    Molecular dynamics (MD) simulations have been widely used to study the structure of metallic glasses (MGs) at atomic scale. However, ultrafast cooling rates in MD simulations create structures that are substantially under-relaxed. In this study, we introduce long-term pressurized annealing up to 1 μs slightly below the glass-transition temperature, Tg, in MD simulation, which effectively relaxes the structure of Cu64Zr36 MG toward experimental conditions. It is found that applying hydrostatic pressure up to 2 GPa relaxes the MG to low-energy states whereas higher pressures retard relaxation events. In the sample annealed at 2 GPa pressure, equivalent cooling rate reaches to 3.7 × 107 K/s,... 

    Serrated flow during inhomogeneous bending deformation of bulk metallic glasses: From self-organized critical to chaotic dynamics

    , Article Journal of Non-Crystalline Solids ; Volume 505 , 2019 , Pages 62-71 ; 00223093 (ISSN) Asadi Khanouki, M. T ; Tavakoli, R ; Aashuri, H ; Sharif University of Technology
    Elsevier B.V  2019
    Abstract
    In this research, the dynamic behavior of serrated flows observed in the flexural stress-deflection curves during three-point bending (3 PB) tests on Zr-, Ti- and La-based bulk metallic glasses (BMGs) is statistically analyzed. Furthermore, the effect of strain rate on the serrations is studied. Based on the ductility of alloy, two distinct types of dynamics are detected. For more ductile alloys (Zr- and Ti-based BMGs) the stress drop magnitude of serrations obeys a power-law distribution of shear avalanches, which is an indicator of the self-organized critical (SOC) state in dynamics. In this case, the size of avalanches has no characteristic scale. In contrast, the size of serrations... 

    Topological and chemical short-range order and their correlation with glass form ability of Mg-Zn metallic glasses: A molecular dynamics study

    , Article Computational Materials Science ; Volume 180 , 2020 Foroughi, A ; Tavakoli, R ; Sharif University of Technology
    Elsevier B.V  2020
    Abstract
    Mg-based metallic glasses are promising materials for biodegradable implants. Understanding atomistic mechanism behind glass formation in these glasses plays a critical role in developing them for future applications. In the present work, we perform a set of molecular dynamics simulations to study structural origin behind glass form ability of Mg-based Mg-Zn metallic glasses at a wide range of compositions. Pair distribution function, Voronoi tessellation and dynamical analysis were adopted to characterize local structures in these glasses. Structural analysis was performed considering both topological and chemical short-range orders. It was found that structure of Mg-Zn metallic glasses... 

    Universal correlation between the thermodynamic potentials and some physical quantities of metallic glasses as a function of cooling rate during molecular dynamics simulation

    , Article Journal of Non-Crystalline Solids ; Volume 536 , 2020 Ghaemi, M ; Tavakoli, R ; Sharif University of Technology
    Elsevier B.V  2020
    Abstract
    The existence of universal semi-log linear relation between potential energy and cooling rate during the molecular dynamics simulations of metallic glasses for Cu-Zr system was formerly reported. For this purpose, different classes of metallic glasses are considered and the validity of this correlation is studied. It is shown that it holds for other thermodynamics potentials and some physical quantities like density and Wendt and Abraham parameter, too. This observation enables us to economically construct atomic scale metallic glass structures equivalent to very low cooling rates, more consistent to experimental works, not accessible using classic molecular simulations. © 2020 Elsevier B.V