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Öğe The characterisation of atomic structure and glass-forming ability of the Zr-Cu-Co metallic glasses studied by molecular dynamics simulations(Taylor & Francis Ltd, 2018) Celtek, M.; Sengul, S.In the present work, the glass formation process and structural properties of Zr50Cu50-xCox (0x50) bulk metallic glasses were investigated by a molecular dynamics simulation with the many body tight-binding potentials. The evolution of structure and glass formation process with temperature were discussed using the coordination number, the radial distribution functions, the volume-temperature curve, icosahedral short-range order, glass transition temperature, Voronoi analysis, Honeycutt-Andersen pair analysis technique and the distribution of bond-angles. Results indicate that adding Co causes similar responses on the nature of the Zr50Cu50-xCox (0x50) alloys except for higher glass transition temperature and ideal icosahedral type ordered local atomic environment. Also, the differences of the atomic radii play the key role in influencing the atomic structure of these alloys. Both Cu and Co atoms play a significant role in deciding the chemical and topological short-range orders of the Zr50Cu50-xCox ternary liquids and amorphous alloys. The glass-forming ability of these alloys is supported by the experimental observations reported in the literature up to now.Öğe Crystallization analysis and determination of Avrami exponents during isothermal annealing and the effect of cooling rate on the evolution of the atomic structure of Pd78Si16Cu6 alloy: A molecular dynamics simulation study(Elsevier, 2023) Guder, V.; Celtek, M.; Celik, F. A.; Sengul, S.In this study, molecular dynamics simulations have been carried out to investigate the effect of cooling rate on the atomic structure of the Pd78Si16Cu6 alloy and the isothermal annealing process on the evolution of its glassy structure. It is observed that the atomic structure, glass transition temperature and crystallization temperature of the system are affected significantly by the cooling rate. While the liquid system cooled at slower cooling rates than 1 K/ps transitions to a crystalline structure, a glassy structure is observed for faster cooling rates. The crystallization kinetics of the annealing process are explained by the Johnson, Mehl and Avrami model using Honeycutt-Andersen crystalline-type bonded pairs. The findings show that the system transitions from a metastable phase to a stable crystalline phase during isothermal annealing of the amorphous Pd78Si16Cu6 alloy, and increasing the annealing time for lower temperature increases the thermal stability of the amorphous phase.Öğe Dynamical and structural properties of metallic liquid and glass Zr48Cu36Ag8Al8 alloy studied by molecular dynamics simulation(Elsevier, 2021) Celtek, M.; Sengul, S.; Domekeli, U.; Guder, V.The structure and dynamics of the Zr48Cu36Ag8Al8 alloy were investigated by molecular dynamics simulation method using the embedded atom method (EAM) and tight-binding (TB) potentials. Total structure factors and pair distribution functions calculated from both potentials are in good agreement with the reported experimental data. The glass transition temperature was determined more easily by using the Wendt-Abraham parameter, which we modified with small changes. The fraction of 1551-bonded pairs and Zr-, Cu-, Ag- and Al-centered icosahedral-like clusters increased with decreasing temperature. High fractions of Cu-, Ag- and Al-centered perfect icosahedra and Zr-centered icosahedra-like clusters were detected in both supercooled liquids and glasses. The critical temperature was determined to be similar to 760.65 K from mode coupling theory. Although the results for the EAM and TB potentials were largely consistent with the experiment, aggregations were detected in the Ag and Al atoms at low temperatures for the EAM potential.Öğe Effects of Ag or Al addition to CuZr-based metallic alloys on glass formation and structural evolution: A molecular dynamics simulation study(Elsevier Sci Ltd, 2021) Celtek, M.; Domekeli, U.; Sengul, S.; Canan, C.The local environment of Cu47.5Zr47.5M5 (M = Ag and Al) metallic liquids and glasses has been studied in detail using the embedded atom method combined with molecular dynamics simulations. The effects of the addition of Al and Ag separately to CuZr-based alloys on the microstructure have been comprehensively discussed in terms of structural functions, bond angle distributions, coordination numbers and common neighbor analysis. The simulated structure factors and coordination numbers are in good agreement with the experimental results the literature. According to different analysis results, it has been observed that Ag prefers Zr to Cu and Al prefers Cu to Zr as the neighboring atom. Our results show that Al is much more effective than Ag in the development of the icosahedral short range order, although the short range order of both glass-forming liquids and glasses shows a strong dependence on Ag and Al additions. In addition, the atomic radii of Ag and Al atoms are very close to each other, but results have shown that Ag atoms (12 = CN <= 15) tend to form interestingly larger clusters than Al atoms (12 = CN <= 14).Öğe Evolution of local structure during melting of Zr0.7Pd0.3 nanowires by molecular dynamics simulations(Pergamon-Elsevier Science Ltd, 2020) Sengul, S.The temperature dependence of the local structural characteristics of Zr0.7Pd0.3 alloy nanowires has been investigated by molecular dynamics simulations in conjunction with the many body type tight - binding potentials. To observe the size dependence of the melting temperature, five different size of cylindric nanowires ranging from 3078 to 27918 atoms have been considered in which the periodic boundary conditions have been applied through their length. The melting points of nanowires have been predicted by following the changes incaloric properties such as the total energy and heat capacity. In addition, the pair distribution functions, the number density distributions and the diffusion coefficients have been calculated to analyze the structural and thermal evolution during heating process.. The effects of pair potential on the surface morphology of Zr0.7Pd0.3 nanowires have been discussed. It has been concluded that the melting points of nanowires are less than the melting point of bulk system, and increase with the rising size of the nanowire. The obtained results reveal that the higher size provides more thermally stabilized nanowires.Öğe Glass formation and structural properties of Zr50Cu50-xAlx bulk metallic glasses investigated by molecular dynamics simulations(Elsevier Sci Ltd, 2017) Celtek, M.; Sengul, S.; Domekeli, U.Temperature effects on the structural evolution and the glass formation of Zr50Cu50-xAlx (x = 0, 10, 20, 30, 40, and 50) in the liquid and glassy states are studied by classical molecular dynamics simulations. In order to perform a comprehensive comparison and analysis, we consider the Honeycutt-Andersen indices, Voronoi analysis, radial distribution functions, coordination numbers, enthalpy, specific heat, and self-diffusion coefficients in our classical simulations in conjunction with the many body tight binding and embedded atom method potentials. The simulated structural properties were found to be in good agreement with available experimental data for Al poor concentration. We may conclude that the Al is a key element in glass transition and icosahedral ordering in considered systems, Zr-Cu-Al alloys have the best GFA until the concentration of Al in ternary alloy reaches the value of 20% and the parameters of TB model potentials for Al need to improve to explain the aggregation of Al atoms in ternary Zr-Cu-Al alloy. (C) 2017 Elsevier Ltd. All rights reserved.Öğe The melting mechanism in binary Pd0.25Ni0.75 nanoparticles: molecular dynamics simulations(Taylor & Francis Ltd, 2018) Domekeli, U.; Sengul, S.; Celtek, M.; Canan, C.The melting mechanism for Pd0.25Ni0.75 alloy nanoparticles (NPs) was investigated using molecular dynamics (MD) simulations with quantum Sutton-Chen many-body potentials. NPs of six different sizes ranging from 682 to 22,242 atoms were studied to observe the effect of size on the melting point. The melting temperatures of the NPs were estimated by following the changes in both the thermodynamic and structural quantities such as the total energy, heat capacity and Lindemann index. We also used a thermodynamics model to better estimate the melting point and to check the accuracy of MD simulations. We observed that the melting points of the NPs decreased as their sizes decreased. Although the MD simulations for the bulk system yielded higher melting temperatures because of the lack of a seed for the liquid phase, the melting temperatures determined for both the bulk material and the NPs are in good agreement with those predicted from the thermodynamics model. The melting mechanism proceeds in two steps: firstly, a liquid-like shell is formed in the outer regions of the NP with increasing temperature. The thickness of the liquid-like shell increases with increasing temperature until the shell reaches a critical thickness. Then, the entire Pd-Ni NP including core-related solid-like regions melts at once.Öğe Molecular dynamics simulations of glass formation and atomic structures in Zr60Cu20Fe20 ternary bulk metallic alloy(Pergamon-Elsevier Science Ltd, 2017) Sengul, S.; Celtek, M.; Domekeli, U.In this study, the atomic structures and the glass formation process of Zr60Cu20Fe20 ternary bulk metallic alloy were studied by molecular dynamics simulation using the many body tight-binding potentials. In order to investigate glass transition, the specific heat, enthalpy, radial distribution functions, and coordination number were used. The local atomic order in the ternary Zr60Cu20Fe20 was been analyzed using Honeycutt-Andersen pair analysis technique, the distribution of bond-angles and Voronoi analysis. The results show that Zr60Cu20Fe20 alloy begins to transform into a glass, the temperature decreased rapidly to -1000K, and Cu and Fe atoms play important role in the formation of icosahedra environment in alloy. (C) 2016 Elsevier Ltd. All rights reserved.Öğe Molecular dynamics simulations of glass formation, structural evolution and diffusivity of the Pd-Si alloys during the rapid solidification process(Elsevier, 2023) Celtek, M.; Sengul, S.; Domekeli, U.; Guder, V.The atomic structure, glass formation process and diffusion mechanism of Pd100-xSix (x = 10, 20, 30, 40 and 50) alloys during rapid solidification have been investigated by molecular dynamics simulations using embedded atom method potentials. The structure factors, total (or Pd90Si10) and partial pair distri-bution functions calculated for Pd80Si20 bulk metallic glass are in good agreement with the experimental/ other data. Bond angle distribution function, Honeycutt-Andersen index and Voronoi tessellation analysis have revealed that the increasing Si amount caused a decrease in the number of icosahedral-like clusters and an increase in the number of crystal-like clusters. The majority of icosahedral-like clusters in the sys-tems are mostly composed of Pd-centered clusters, suggesting that Pd has an effective role in the glass formation process in Pd-Si systems. It has been observed that the mobility of Si atoms decreases in envi-ronments with more Pd atoms. The critical temperature and T0 temperature for Pd-Si liquids have been determined from the self-diffusion coefficients using the mode-coupling theory and the Vogel-Fulcher-Tammann law, respectively. The present findings show us that the Pd70Si30 compound has a critical importance in terms of the glass formation process, which is consistent with experimental observations. We hope that the results will contribute to understanding the Si effect on the atomic local structure of Pd -Si systems and will encourage research on many properties of these systems, such as their mechanical properties.(c) 2022 Elsevier B.V. All rights reserved.Öğe Molecular dynamics simulations of liquid GeSe alloy: the first application of modified analytic embedded atom method (MAEAM)(Natl Inst Optoelectronics, 2007) Dalgic, S. Senturk; Sengul, S.We present the results of molecular dynamics (MD) simulations of liquid GeSe alloy near melting point using an effective pair potential based on modified analytic embedded atom method (MAEAM). The potential functions of MAEAM are parameterized by fitting to both solid and liquid state properties of pure Ge and Se. MD simulations have been performed to investigate the structure and the dynamical properties of liquid GeSe alloy. We have compared the total structure factor and pair correlation function, obtained in the MD simulations to the results of self consistent integral equation theory of Variational Modified Hypernetted Chain (VMHNC), experiment and other MD results. To describe the atomic dynamics in I-GeSe, we calculate the diffusion coefficient D which is directly related to the mean square atomic displacement. Viscosity of the liquid alloy is estimated by Stokes-Einstein formula. On the other hand, the MD results of dynamics in I-GeSe are compared with those obtained by the viscoelastic theory of fluids. The collective dynamic properties such as the intermediate scattering function and the dynamic structure factor have determined. We have analyzed the behaviour of self dynamic-structure factor computed in the frame of the viscoelastic model.Öğe Pressure dependent evolution of microstructures in Pd80Si20 bulk metallic glass(Elsevier, 2022) Guder, V; Sengul, S.; Celtek, M.; Domekeli, U.The effect of the pressure on the local microstructure in the amorphous Pd80Si20 alloy was investigated by coordination analyses and structural calculations using molecular dynamics simulations. We observed that the glass transition temperature increases monotonically as the pressure increases. Also, it was seen that the Pd atoms composing the face centered cubic (fcc)- and icosahedral-like order are the origin of the complexity of the local structure while the short-range order around Si atoms is characterized by a tri-capped trigonal prism. We found that the topological structure and local environment of the system which consists of icosahedral-, hexagonal- and fcc-like structural units, change significantly with the effect of pressure. We revealed that the short-range order of the system develops with pressure due to the increase of icosahedral and some defected icosahedral polyhedra, and Pd atoms are responsible for the formation of the amorphous state of the system.Öğe Size dependence of melting process of ZnSe nanowires: molecular dynamics simulations(Natl Inst Optoelectronics, 2011) Sengul, S.; Dalgic, S. SenturkIt is known that the stable crystal structure of ZnSe nanowires depends on the nanowire diameter. For this reason, we have focused on the impact of size of ZnSe nanowires on their structural properties. The molecular dynamics (MD) simulations have performed to especially discuss consequences for size effect on melting process for ZnSe nanostructures with cylindrical shape. The interactions between the atoms in system have defined by an empirical model potential developed for semiconductor metal-chalcogenides. The nanowires studied in this work have a different number of diameters and have generated by assembling the zincblende unit cell. Periodic boundary conditions have applied only along c-axis. The size effect on melting of nanowires has investigated. Some structural and dynamic properties such as distribution functions, mean square displacements and diffusion coefficients have also calculated to get detailed information about the nature of melting process of ZnSe nanowires. Calculations show that melting temperatures of ZnSe nanowires are lower than that of bulk and highly related with the size of the nanowires.Öğe Structural and dynamical properties of liquid Mg. An orbital-free molecular dynamics study(Iop Publishing Ltd, 2009) Sengul, S.; Gonzalez, D. J.; Gonzalez, L. E.Several static and dynamic properties of liquid magnesium near melting have been evaluated by the orbital-free ab initio molecular dynamics method. The calculated static structure shows good agreement with recent experimental data, including an asymmetric second peak in the structure factor which has been linked to the existence of an important icosahedral short-range order in the liquid. As for the dynamic structure, we obtain collective density excitations with an associated dispersion relation which closely follows recent experimental results. Accurate estimates have also been obtained for several transport coefficients.Öğe The structural evolution and abnormal bonding ways of the Zr80Pt20 metallic liquid during rapid solidification under high pressure(Elsevier, 2020) Sengul, S.; Celtek, M.; Domekeli, U.The structural evolutions and abnormal bonding ways of the Zr80Pt20 binary alloy during rapid solidification under different pressures from 0 to 120 GPa have been investigated by classical molecular dynamics simulations in conjunction with the embedding atom method. The pair distribution function, the coordination number, the Warren-Cowley parameter, the bond length and the pair analysis technique are used to reveal the structural evolution of the Zr80Pt20 solidified under normal and high pressures. Persuasive evidence indicates that the applied pressure strongly affects the vitrification (for 0 <= P <= 20 and 90 <= P <= 120 GPa) and crystallization (for 30 <= P <= 80 GPa) processes of the metallic liquid and causes significant changes in the microstructure of the system. Interestingly, we have observed that the crystallization for the Zr80Pt20 system is associated with volume expansion between 50 and 80 GPa, in contrast to the volume contraction observed under 30 and 40 GPa. The results of the atomic structure analysis show that there is an unexpected shortening of Zr-Zr bonds under high pressures, which is related to the change of the atomic packing in the Zr80Pt20 alloy from loose to dense with increasing pressure. The results of the analysis show that the bonds between Zr-Zr and Pt-Pt pairs can be shortened more easily than the bonds between Zr-Pt pairs at high pressures and also the clustering behaviors of Zr-Zr or Pt-Pt bonds reveals the presence of composition segregation. This study presents encouraging findings for the experimental investigation of glass transition and crystallization processes in Zr-Pt metallic liquids during rapid cooling and under high pressure.Öğe Structure and atomic transport properties in liquid AsTe alloys using AMEAM based potentials(Natl Inst Optoelectronics, 2007) Dalgic, S. Senturk; Sengul, S.The static, dynamic structure and atomic transport properties of liquid AsTe alloys have been calculated using the integral equation theory with the effective pair potentials based on the analytic modified embedded atom method (AMEAM). The effective pair interactions are described with the potential functions recently proposed Hu and co-workers which are parametrized by fitting the cohesive energy, vacancy formation energy and equilibrium conditions of solid and liquid state properties of pure metals. In the structural calculations for liquid AsTe alloys, the thermodynamically self-consistent variational modified hypernetted chain (VMHNC) theory of liquids has been carried out to compute the partial static structure factors. The calculated single particle and collective dynamic properties have used to obtain the atomic transport properties, such as diffusion coefficients. The results have compared with available experimental data.