The Experts below are selected from a list of 8772 Experts worldwide ranked by ideXlab platform
Levente Vitos - One of the best experts on this subject based on the ideXlab platform.
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effect of temperature on the Stacking Fault energy and deformation behaviour in 316l austenitic stainless steel
2019Co-Authors: Xun Sun, David Molnar, Goran Engberg, Levente VitosAbstract:The Stacking Fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial disloc ...
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Stacking Fault energy of C-alloyed steels: The effect of magnetism
2017Co-Authors: Song Lu, Ruihuan Li, Kalevi Kokko, Staffan Hertzman, Krisztina Kádas, Yanzhong Tian, Hualei Zhang, Qing-miao Hu, Se Kyun Kwon, Levente VitosAbstract:First-principles calculations have been performed to study the effect of C on the Stacking Fault energy (SFE) of paramagnetic γ-Fe and Fe[sbnd]Cr[sbnd]Ni austenitic steel. In these systems, the local magnetic structure is very sensitive to the volume in both fcc and hcp structures, which emphasizes the importance of the magnetovolume coupling effect on the SFE. The presence of C atom suppresses the local magnetic moments of Fe atoms in the first coordination shell of C. Compared to the hypothetical nonmagnetic case, paramagnetism significantly reduces the effect of C on the SFE. In the scenario of C being depleted from the Stacking Fault structure or twin boundaries, e.g., due to elevated temperature, where the chemical effect of C is dissipated, we calculate the C-induced volume expansion effect on the SFE. The volume induced change in the SFE corresponds to more than ∼ 50% of the total C effect on the SFE obtained assuming uniform C distribution.
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Stacking Fault energy of face centered cubic metals thermodynamic and ab initio approaches
2016Co-Authors: Dongyoo Kim, Levente Vitos, Stephan Schonecker, Jijun Zhao, S K KwonAbstract:The formation energy of the interface between face-centered cubic (fcc) and hexagonal close packed (hcp) structures is a key parameter in determining the Stacking Fault energy (SFE) of fcc metals a ...
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Stacking Fault energies in austenitic stainless steels
2016Co-Authors: Jun Lu, Erik Holmstrom, Karin H Antonsson, Mikael Grehk, Levente Vitos, Lars Hultman, Wei Li, Ardeshir GolpayeganiAbstract:Abstract We measure the Stacking Fault energy of a set of 20 at% Cr-austenitic stainless steels by means of transmission electron microscopy using the weak beam dark field imaging technique and the isolated dislocations method. The measurements are analyzed together with first principles calculations. The results show that experiment and theory agree very well for the investigated concentration range of Mn (0–8%) and Ni (11–30%). The calculations show that simultaneous relaxation of atomic and spin degrees of freedom is important in order to find the global energy minimum for these materials. Our results clearly show the great potential of the weak beam dark field technique to obtain accurate measurements of the Stacking Fault energy of austenitic steels and that the reliable predictability of first principles calculations can be used to design new steels with optimized mechanical properties.
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Generalized Stacking Fault energy of γ-Fe
2016Co-Authors: Wei Li, Jan Y. Johnsson, Mikael Grehk, Song Lu, Qing-miao Hu, Se Kyun Kwon, Borje Johansson, Levente VitosAbstract:© 2016 Taylor and Francis.We investigate the generalized Stacking Fault energy (-surface) of paramagnetic-Fe as a function of temperature. At static condition, the face-centred cubic (fcc) lattice is thermodynamically unstable with respect to the hexagonal close-packed lattice, resulting in a negative intrinsic Stacking Fault energy (ISF). However, the unstable Stacking Fault energy (USF), representing the energy barrier along the-surface connecting the ideal fcc and the intrinsic Stacking Fault positions, is large and positive. The ISF is calculated to have a strong positive temperature coefficient, while the USF decreases monotonously with temperature. According to the recent plasticity theory, the overall effect of temperature is to move paramagnetic fcc Fe from the Stacking Fault formation regime (K) towards maximum twinning (K) and finally to a dominating full-slip regime (K). Our predictions are discussed in connection with the available experimental observations.
Alexander H. King - One of the best experts on this subject based on the ideXlab platform.
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effects of stable and unstable Stacking Fault energy on dislocation nucleation in nano crystalline metals
2016Co-Authors: Valery Borovikov, Alexander H. King, Mikhail I. MendelevAbstract:Dislocation nucleation from grain boundaries (GB) can control plastic deformation in nano-crystalline metals under certain conditions, but little is known about what controls dislocation nucleation, because when data from different materials are compared, the variations of many interacting properties tend to obscure the effects of any single property. In this study, we seek clarification by applying a unique capability of semi-empirical potentials in molecular dynamics simulations: the potentials can be modified such that all significant material properties but one, are kept constant. Using a set of potentials developed to isolate the effects of Stacking Fault energy, we show that for a given grain boundary, loading orientation and strain rate, the yield stress depends linearly on both the stable and unstable Stacking Fault energies. The coefficients of proportionality depend on the GB structure and the value of the yield stress is related to the density of the E structural units in the GB. While the impact of the stable Stacking Fault energy is easy to understand, the unstable Stacking Fault energy requires more elucidation and we provide a framework for understanding how it affects the nucleation and propagation process.
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effect of Stacking Fault energy on mechanism of plastic deformation in nanotwinned fcc metals
2015Co-Authors: Valery Borovikov, Alexander H. King, Mikhail I. Mendelev, Richard LesarAbstract:Starting from a semi-empirical potential designed for Cu, we developed a series of potentials that provide essentially constant values of all significant (calculated) materials properties except for the intrinsic Stacking Fault energy, which varies over a range that encompasses the lowest and highest values observed in nature. These potentials were employed in molecular dynamics (MD) simulations to investigate how Stacking Fault energy affects the mechanical behavior of nanotwinned face-centered cubic (fcc) materials. The results indicate that properties such as yield strength and microstructural stability do not vary systematically with Stacking Fault energy, but rather fall into two distinct regimes corresponding to low and high Stacking Fault energies.
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Effect of Stacking Fault energy on mechanism of plastic deformation in nanotwinned FCC metals
2015Co-Authors: Valery Borovikov, Alexander H. King, Mikhail I. Mendelev, Richard LesarAbstract:Starting from a semi-empirical potential designed for Cu, we have developed a series of potentials that provide essentially constant values of all significant (calculated) materials properties except for the intrinsic Stacking Fault energy, which varies over a range that encompasses the lowest and highest values observed in nature. These potentials were employed in molecular dynamics (MD) simulations to investigate how Stacking Fault energy affects the mechanical behavior of nanotwinned face-centered cubic (FCC) materials. The results indicate that properties such as yield strength and microstructural stability do not vary systematically with Stacking Fault energy, but rather fall into two distinct regimes corresponding to ‘low’ and ‘high’ Stacking Fault energies.
Jörg Neugebauer - One of the best experts on this subject based on the ideXlab platform.
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Temperature dependence of the Stacking-Fault Gibbs energy for Al, Cu, and Ni
2018Co-Authors: Xi Zhang, Andrei V Ruban, Y. Gong, Roger C. Reed, Fritz Körmann, Blazej Grabowski, Tilmann Hickel, Jörg NeugebauerAbstract:The temperature-dependent intrinsic Stacking Fault Gibbs energy is computed based on highly converged density-functional-theory (DFT) calculations for the three prototype face-centered cubic metals ...
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impact of local magnetism on Stacking Fault energies a first principles investigation for fcc iron
2016Co-Authors: Ivan Bleskov, Jörg Neugebauer, Tilmann Hickel, Andrei V RubanAbstract:A systematic ab initio study of the influence of local magnetism on the generalized Stacking Fault energy (GSFE) surface in pure fcc iron at 0 K has been performed. In the calculations we considered ferro- and antiferro- (single- and double-layer) magnetic order of local moments as well as their complete disorder, corresponding to paramagnetic (PM) state. We have shown that local magnetism is one of the most important factors stabilizing austenitic structure in iron (with respect to more stable at 0 K hcp) and that the perturbation of magnetic structure by the formation of Stacking Fault is a short-range effect. Local magnetism also strongly influences the GSFE surface topology and, therefore, the material's plasticity by reducing the energetic barriers that need to be overcome to form the intrinsic Stacking Fault (ISF) or return from the ISF structure to fcc. The influence of atomic relaxations on such barriers is moderate and does not exceed 15%. In addition, a methodology to evaluate the PM ISF energy using a superposition of the ISF energies obtained for ordered magnetic structures is proposed to overcome computational impediments arising when dealing with disorder in the PM state. The complications of the proposed methodology together with the ways to overcome them are also discussed.
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The relation between ductility and Stacking Fault energies in Mg and Mg–Y alloys
2012Co-Authors: Stefanie Sandlöbes, Alexey Dick, Sang Bong Yi, Martin Friak, Stefan Zaefferer, Dietmar Letzig, Jörg Neugebauer, Dierk RaabeAbstract:The underlying mechanisms that are responsible for the improved room-temperature ductility in Mg–Y alloys compared to pure Mg are investigated by transmission electron microscopy and density functional theory. Both methods show a significant decrease in the intrinsic Stacking Fault I1 energy (I1 SFE) with the addition of Y. The influence of the SFE on the relative activation of different competing deformation mechanisms (basal, prismatic, pyramidal slip) is discussed. From this analysis we suggest a key mechanism which explains the transition from primary basal slip in hexagonal close-packed Mg to basal plus pyramidal slip in solid solution Mg–Y alloys. This mechanism is characterized by enhanced nucleation of hc + ai dislocations where the intrinsic Stacking Fault I1 (ISF1) acts as heterogeneous source for hc + ai dislocations. Possible electronic and geometric reasons for the modification of the SFE by substitutional Y atoms are identified and discussed.
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first principles investigation of the effect of carbon on the Stacking Fault energy of fe c alloys
2011Co-Authors: Afshin Abbasi, Tilmann Hickel, A Dick, Jörg NeugebauerAbstract:Abstract The intrinsic Stacking Fault energy (SFE) is a critical parameter that defines the type of plasticity mechanisms in austenitic high-Mn steels. We have performed ab initio investigations to study the effect of interstitial carbon atoms on the SFE of face-centred cubic (fcc) Fe–C alloys. Simulating the Stacking Fault explicitly, we observe a strong dependence of the SFE on the position of carbon atoms with respect to the Stacking-Fault layer and the carbon concentration. To determine the SFE for realistic carbon distributions we consider two scenarios, assuming (i) an equilibration of the carbon atoms in response to the Stacking Fault formation and (ii) a homogeneous distribution of the C atoms when creating the Stacking Fault (i.e. diffusion is suppressed). This distinction allows us to interpret two sets of apparently contradicting experimental data sets, where some find an almost negligible dependence on the carbon concentration while others report a large carbon dependence. In particular, our results for the second scenario show a significant increase in the SFE as a function of carbon concentration. These trends are consistently found for the explicit calculations as well as for the computationally much more efficient axial next-nearest-neighbour Ising approach. They will be decisive for the selection of specific plasticity mechanisms in steels (such as twin formation, martensitic transformations and dislocation gliding).
Sebastian S Schmidt - One of the best experts on this subject based on the ideXlab platform.
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Stacking Fault reduction during annealing in cu poor cuinse2 thin film solar cell absorbers analyzed by in situ xrd and grain growth modeling
2019Co-Authors: Helena Stange, Stephan Brunken, D Greiner, Marc Daniel Heinemann, Daniel Antonio Barragan Yani, Leonard A Wagele, Chen Li, Ekin Simsek Sanli, Max Kahnt, Sebastian S SchmidtAbstract:Buried wurtzite structures composed by Stacking Faults of the {111} planes in zinc-blende and {112} planes in chalcopyrite structures can result in barriers for charge carrier transport. A precise understanding of Stacking Fault annihilation mechanisms is therefore crucial for the development of effective deposition processes. During co-evaporation of Cu(In,Ga)Se2—a photovoltaic absorber material showing record efficiencies of up to 22.9% for thin film solar cells—a reduction of Stacking Faults occurs at the transition from a Cu-poor to a Cu-rich film composition, parallel to grain growth, which is suggesting that the two phenomena are coupled. Here, we show by in situ synchrotron X-ray diffraction during annealing of Cu-poor CuInSe2 thin films that Stacking Faults can be strongly reduced through annealing, without passing through a Cu-rich film composition. We simulate the evolution of the X-ray diffraction Stacking Fault signal with a simple numerical model of grain growth driven by Stacking Fault energy and grain boundary curvature. The results support the hypothesis that the Stacking Fault reduction can be explained by grain growth. The model is used to make predictions on annealing times and temperatures required for Stacking Fault reduction and could be adapted for polycrystalline thin films with similar morphology.Buried wurtzite structures composed by Stacking Faults of the {111} planes in zinc-blende and {112} planes in chalcopyrite structures can result in barriers for charge carrier transport. A precise understanding of Stacking Fault annihilation mechanisms is therefore crucial for the development of effective deposition processes. During co-evaporation of Cu(In,Ga)Se2—a photovoltaic absorber material showing record efficiencies of up to 22.9% for thin film solar cells—a reduction of Stacking Faults occurs at the transition from a Cu-poor to a Cu-rich film composition, parallel to grain growth, which is suggesting that the two phenomena are coupled. Here, we show by in situ synchrotron X-ray diffraction during annealing of Cu-poor CuInSe2 thin films that Stacking Faults can be strongly reduced through annealing, without passing through a Cu-rich film composition. We simulate the evolution of the X-ray diffraction Stacking Fault signal with a simple numerical model of grain growth driven by Stacking Fault energ...
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Stacking Fault reduction during annealing in cu poor cuinse2 thin film solar cell absorbers analyzed by in situ xrd and grain growth modeling
2019Co-Authors: Helena Stange, Stephan Brunken, D Greiner, Marc Daniel Heinemann, Daniel Antonio Barragan Yani, Leonard A Wagele, Ekin Simsek Sanli, Max Kahnt, Sebastian S Schmidt, Janpeter BackerAbstract:Buried wurtzite structures composed by Stacking Faults of the {111} planes in zinc-blende and {112} planes in chalcopyrite structures can result in barriers for charge carrier transport. A precise understanding of Stacking Fault annihilation mechanisms is therefore crucial for the development of effective deposition processes. During co-evaporation of Cu(In,Ga)Se2—a photovoltaic absorber material showing record efficiencies of up to 22.9% for thin film solar cells—a reduction of Stacking Faults occurs at the transition from a Cu-poor to a Cu-rich film composition, parallel to grain growth, which is suggesting that the two phenomena are coupled. Here, we show by in situ synchrotron X-ray diffraction during annealing of Cu-poor CuInSe2 thin films that Stacking Faults can be strongly reduced through annealing, without passing through a Cu-rich film composition. We simulate the evolution of the X-ray diffraction Stacking Fault signal with a simple numerical model of grain growth driven by Stacking Fault energy and grain boundary curvature. The results support the hypothesis that the Stacking Fault reduction can be explained by grain growth. The model is used to make predictions on annealing times and temperatures required for Stacking Fault reduction and could be adapted for polycrystalline thin films with similar morphology.
Alessandro Mottura - One of the best experts on this subject based on the ideXlab platform.
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first principles modeling of superlattice intrinsic Stacking Fault energies in ni3al based alloys
2018Co-Authors: A Breidi, Joshua Allen, Alessandro MotturaAbstract:Abstract High-throughput quantum mechanics based simulations have been carried out to establish the change in lattice parameter and superlattice intrinsic Stacking Fault (SISF) formation energies in Ni3Al-based alloys using the axial Ising model. We had direct access to the variation in SISF energies due to finite compositional change of the added ternary transition metal (TM) element through constructing large supercells, which was equally necessary to account for chemical disorder. We find that most added TM ternaries induce an important quasi-linear increase in the SISF energy as a function of alloying composition x. The most pronounced increase corresponds to Fe addition, while Co addition decreases the SISF energy monotonically. Our results shed light on the role played by TM elements on strengthening L12 Ni3Al precipitates against Stacking Fault shear. The data are of high importance for designing new Ni-based superalloys based on computational approaches.
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a first principles study of the effect of ta on the superlattice intrinsic Stacking Fault energy of l12 co3 al w
2012Co-Authors: Alessandro Mottura, Anderson Janotti, Tresa M PollockAbstract:Abstract New Co-based alloys containing a L12 reinforcement phase display exceptional high-temperature properties. Early research has shown that the quaternary alloy Co-8.8Al-9.8W-2Ta (at.%) has a high-temperature strength comparable to single-crystal Ni-based superalloys above 1200 K. Associated with high strength is an unusual high density of intrinsic Stacking Faults within the γ′ precipitates. In this work, Density Functional Theory, the Axial Next Nearest Neighbor Ising model and Special Quasi-random Structures have been used to calculate the Stacking Fault energy of L12-Co3(Al,W) and the effect of small Ta additions on the Stacking Fault energy. The model predicts a superlattice intrinsic Stacking Fault energy of 90–93 mJ/m2, which increases up to 30% when one Ta atom is substituted on the Al/W sub-lattice. This effect can be explained by considering d-band effects resulting from the addition of Ta.
