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Alexander Hartmaier - One of the best experts on this subject based on the ideXlab platform.
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3d discrete Dislocation dynamics study of creep behavior in ni base single crystal superalloys by a combined Dislocation climb and vacancy diffusion model
Journal of The Mechanics and Physics of Solids, 2017Co-Authors: Marc Fivel, Alexander HartmaierAbstract:Abstract A three-dimensional (3D) discrete Dislocation dynamics (DDD) creep model is developed to investigate creep behavior under uniaxial tensile stress along the crystallographic [001] direction in Ni-base single crystal superalloys, which takes explicitly account of Dislocation Glide, climb and vacancy diffusion, but neglects phase transformation like rafting of γ ′ precipitates. The vacancy diffusion model takes internal stresses by Dislocations and mismatch strains into account and it is coupled to the Dislocation dynamics model in a numerically efficient way. This model is helpful for understanding the fundamental creep mechanisms in superalloys and clarifying the effects of Dislocation Glide and climb on creep deformation. In cases where the precipitate cutting rarely occurs, e.g. due to the high anti-phase boundary energy and the lack of superDislocations, the Dislocation Glide in the γ matrix and the Dislocation climb along the γ / γ ′ interface dominate plastic deformation. The simulation results show that a high temperature or a high stress both promote Dislocation motion and multiplication, so as to cause a large creep strain. Dislocation climb accelerated by high temperature only produces a small plastic strain, but relaxes the hardening caused by the filling γ channels and lets Dislocations further Glide and multiply. The strongest variation of vacancy concentration occurs in the horizontal channels, where more mixed Dislocations exit and tend to climb. The increasing internal stresses due to the increasing Dislocation density are easily overcome by Dislocations under a high external stress that leads to a long-term Dislocation Glide accompanied by multiplication.
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Influence of misfit stresses on Dislocation Glide in single crystal superalloys: A three-dimensional discrete Dislocation dynamics study
Journal of the Mechanics and Physics of Solids, 2015Co-Authors: Siwen Gao, Marc Fivel, Alexander HartmaierAbstract:In the characteristic gamma/gamma' microstructure of single crystal superalloys, misfit stresses occur due to a significant lattice mismatch of those two phases. The magnitude of this lattice mismatch depends on the chemical composition of both phases as well as on temperature. Furthermore, the lattice mismatch of gamma and gamma' phases can be either positive or negative in sign. The internal stresses caused by such lattice mismatch play a decisive role for the micromechanical processes that lead to the observed macroscopic athermal deformation behavior of these high-temperature alloys. Three-dimensional discrete Dislocation dynamics (DDD) simulations are applied to investigate Dislocation Glide in gamma matrix channels and shearing of gamma' precipitates by superDislocations under externally applied uniaxial stresses, by fully taking into account internal misfit stresses. Misfit stress fields are calculated by the fast Fourier transformation (FFI) method and hybridized with DDD simulations. For external loading along the crystallographic [001] direction of the single crystal, it was found that the different internal stress states for negative and positive lattice mismatch result in non-uniform Dislocation movement and different Dislocation patterns in horizontal and vertical gamma matrix channels. Furthermore, positive lattice mismatch produces a lower deformation rate than negative lattice mismatch under the same tensile loading, but for an increasing magnitude of lattice mismatch, the deformation resistance always diminishes. Hence, the best deformation performance is expected to result from alloys with either small positive, or even better, vanishing lattice mismatch between gamma and gamma' phase.
Ulrich Messerschmidt - One of the best experts on this subject based on the ideXlab platform.
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Dislocation Glide and grain boundary decohesion in polycrystalline molybdenum disilicide during plastic deformation
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002Co-Authors: L Junker, Martin Bartsch, Ulrich MesserschmidtAbstract:Abstract Molybdenum disilicide polycrystals were deformed in constant strain rate tests in compression followed by analyses of the deformed specimens by means of optical microscopy and scanning and transmission electron microscopy. The strain rate sensitivity was measured by stress relaxation tests. Using a low strain rate of 2.5×10 −7 s −1 , it was possible for the first time to achieve plastic flow at low-temperatures down to 495 °C and thus in the temperature range of the flow stress anomalies of different slip systems in single crystals. In addition, in situ straining experiments in a high-voltage electron microscope were performed to observe the deformation processes directly. The deformation is controlled by Dislocation Glide on the {0 1 1}〈1 0 0〉 and {1 1 0}〈1 1 1〉 slip systems below 1000 °C, however, by visco-elastic grain boundary Glide and decohesion above this temperature.
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Dislocation Glide and grain boundary decohesion in polycrystalline molybdenum disilicide during plastic deformation
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2002Co-Authors: L Junker, Martin Bartsch, Ulrich MesserschmidtAbstract:Molybdenum disilicide polycrystals were deformed in constant strain rate tests in compression followed by analyses of the deformed specimens by means of optical microscopy and scanning and transmission electron microscopy. The strain rate sensitivity was measured by stress relaxation tests. Using a low strain rate of 2.5 × 10 − 7 s − 1 , it was possible for the first time to achieve plastic flow at low-temperatures down to 495 °C and thus in the temperature range of the flow stress anomalies of different slip systems in single crystals. In addition, in situ straining experiments in a high-voltage electron microscope were performed to observe the deformation processes directly. The deformation is controlled by Dislocation Glide on the {0 1 1} 100 and {1 10 } 111 slip systems below 1000 °C, however, by visco-elastic grain boundary Glide and decohesion above this temperature. © 2002 Elsevier Science B.V. All rights reserved.
Jian Wang - One of the best experts on this subject based on the ideXlab platform.
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quantifying the resistance to Dislocation Glide in single phase fecral alloy
International Journal of Plasticity, 2020Co-Authors: Dongyue Xie, Guisen Liu, Kaisheng Ming, Jian WangAbstract:Abstract The resistance to Dislocation Glide associated with slip systems {110} and {112} in single phase FeCrAl alloy is measured via micromechanical testing in a scanning electronic microscopy at room temperature. Two important factors, the shape and orientation of a pillar, are discussed with respect to the Glide resistance and stress-strain response. Maximizing Schmid factor of one specific slip system while minimizing others is recommended in order to diminish obvious Dislocations-induced hardening during in-situ testing. Apparent Schmid factor analysis is conducted to select grains with preferred orientations. Two types of pillars with conventional cylindrical shape or dog-bone shape are tested under compression to estimate the resistance to Dislocation Glide and evaluate the effect of pillar shape on the compression stress-strain response. One dog-bone pillar is tested under tension to check the tension-compression isotropy of Dislocation slip. We find that the shape of a pillar to a smaller extent affects the measured resistance but strongly influences the stress strain response. Cylindrical pillars exhibit apparent hardening associated with early yielding due to stress or strain concentration at contact region, while dog-bone pillars show an obvious yielding and continuous shearing without hardening. The resistance is 220 MPa for slip system {110} and 230 MPa for slip system {112} . Finite element analysis is performed to account for the influence of pillar shape and contact condition on mechanical response.
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quantifying the resistance to Dislocation Glide in single phase fecral alloy
Social Science Research Network, 2019Co-Authors: Dongyue Xie, Guisen Liu, Kaisheng Ming, Jian WangAbstract:The resistance to Dislocation Glide associated with slip systems {110} and {112} in single phase FeCrAl alloy is measured via micromechanical testing in a scanning electronic microscopy at room temperature. Two important factors, the shape and orientation of a pillar, are discussed with respect to the Glide resistance and stress-strain response. Maximizing Schmid factor of one specific slip system while minimizing others is recommended in order to diminish obvious Dislocations-induced hardening during in situ testing. Apparent Schmid factor analysis is conducted to select grains with preferred orientations. Two types of pillars with conventional cylindrical shape or dog-bone shape are tested under compression to estimate the resistance to Dislocation Glide and evaluate the effect of pillar shape on the compression stress-strain response. One dog-bone pillar is tested under tension to check the tension-compression isotropy. We find that the shape of a pillar to a smaller extent affects the measured resistance but strongly influences the stress strain response. Cylindrical pillars exhibit apparent hardening associated with early yielding due to stress or strain concentration at contact region, while dog-bone pillars show an obvious yielding and continuous shearing without hardening. The resistance is 220 MPa for slip system {110} and 230 MPa for slip system {112} . Finite element analysis is performed to account for the influence of pillar shape and contact condition on mechanical response. Furthermore, slip transmission across two high-angle grain boundaries is experimentally investigated and discussed with respect to the misorientation and the continuity of slips across GBs.
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influence of loading directions on Dislocation slip mechanism of nanotwinned ni with void defect at the twin boundary
Computational Materials Science, 2018Co-Authors: Jun Ding, Jian Wang, Haonan Zhao, Lusheng Wang, Xia Huang, Kun Song, Xiangguo ZengAbstract:Abstract In this study, a molecular dynamics model is used to simulate the compressive loading process along different crystal orientations of nano-twinned Ni with void defect at the twin boundaries. The loading angle is defined as the angle between the loading direction and the twin boundary, loading angles of 0, 15, 30, 45, 60, 75, and 90° were investigated in this study. The effects of different loading directions on the mechanical properties and the Dislocation Glide mechanisms were investigated. The Dislocation Glide process during the initial stage of plastic deformation for different loading directions was also studied. The results show that the Dislocation Glide mainly occurs along the {1 1 1} plane that is inclined to the twin boundaries when the loading direction is 0°. The Dislocation Glide process is constrained by the twin boundaries and, therefore, slips along the twin layers. As the loading angle increases from 0° to 45°, the Dislocation gradually shifts and slips along the (1 1 1) slip plane that is parallel to the twin boundaries and twin migration and twinning occur. As the loading angle continues to increase to 90°, the Dislocation slips along the {1 1 1} plane again during the loading process. In addition, the Dislocation slips toward the adjacent twin layers because they are strongly hindered by the twin boundaries.
Patrick Cordier - One of the best experts on this subject based on the ideXlab platform.
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on Dislocation Glide in mgsio3 bridgmanite at high pressure and high temperature
Earth and Planetary Science Letters, 2016Co-Authors: Antoine Kraych, Philippe Carrez, Patrick CordierAbstract:Abstract Dislocation Glide in MgSiO3 bridgmanite with Pbnm perovskite structure is modeled at 30 and 60 GPa for the [100](010) and [010](100) slip systems. The velocity of screw Dislocations is calculated in the thermally activated regime based on the kink-pair mechanism. We show that the Dislocation velocity determination can rely on the atomic scale calculations of a limited amount of parameters: the Peierls stress τ p , and the formation enthalpy of a single kink H k . From the Dislocation velocities, the evolution of stress as a function of temperature can be derived from the Orowan equation at any strain rate. Calculations performed at laboratory strain-rates of 10 − 5 s − 1 reproduce well the high stress levels found experimentally. This demonstrates the influence of lattice friction in the mechanical properties of bridgmanite. The same calculations are performed at mantle strain-rate ( 10 − 16 s − 1 ). They demonstrate that in the lower mantle, bridgmanite would always be in the thermally activated regime and that stresses close to 1 GPa are still necessary to move Dislocations in bridgmanite. In the uppermost lower mantle, Dislocation Glide is inhibited and other deformation mechanisms, involving diffusion, are needed.
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modeling Dislocation Glide and lattice friction in mg2sio4 wadsleyite in conditions of the earth s transition zone
American Mineralogist, 2016Co-Authors: Sebastian Ritterbex, Philippe Carrez, Patrick CordierAbstract:Thermally activated Dislocation Glide in Mg 2 SiO 4 wadsleyite at 15 GPa has been modeled to investigate its potential contribution to plastic deformation of wadsleyite in the Earth’s transition zone. Modeling is based on a multiphysics approach that allows calculating the constitutive equations associated with single slip for a wide range of temperatures and strain rates typical for the laboratory and the Earth’s mantle. The model is based on the core structures of the rate limiting ½ {101} and [100](010) dissociated screw Dislocations. After quantifying their lattice friction, Glide is modeled through an elastic interaction model that allows calculating the critical configurations that trigger elementary displacements of dissociated Dislocations. The constitutive relations corresponding to Glide are then deduced with Orowan’s equation to describe the average intracrystalline plasticity. The high stresses predicted by the model are found to be in good agreement with experimental data on plastic deformation of wadsleyite at high-pressure conditions. Moreover, it is found that even at appropriate mantle strain rates, Glide of Dislocations remain difficult with critical resolved shear stress ( CRSS ) values typically larger than 100 MPa. This implies the inefficiency of Dislocation Glide to the overall plastic deformation of Mg 2 SiO 4 wadsleyite under transition zone conditions.
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modeling Dislocation Glide in mg2sio4 ringwoodite towards rheology under transition zone conditions
Physics of the Earth and Planetary Interiors, 2015Co-Authors: Sebastian Ritterbex, Karine Gouriet, Philippe Carrez, Patrick CordierAbstract:Abstract Deformation resulting from thermally activated plastic slip is modeled in Mg2SiO4 ringwoodite at 20 GPa for a wide range of temperatures. The model relies on the structures of the rate controlling 1 / 2 〈 1 1 0 〉 screw Dislocations which have been modeled using the Peierls–Nabarro–Galerkin method. These calculations are parametrized by density functional theory calculations of γ -surfaces of the { 0 0 1 } , { 1 1 0 } and { 1 1 1 } planes. At finite temperatures, Dislocation mobility is controlled by kink-pair nucleation on the thermally activated 1 / 4 〈 1 1 0 〉 partial screw Dislocations as they occur in ringwoodite. Single slip critical resolved shear stresses (CRSS) corresponding to this mechanism are deduced from Orowan’s equation. The results are found to be in reasonably good agreement with experimental data at 20 GPa which show high effective flow stresses under laboratory conditions. Finally, the CRSS’s are calculated for typical mantle strain rates of ∊ = 10 - 16 s−1 at appropriate temperatures expected in the lower transition zone. Results show that Dislocation Glide remains difficult and that lattice friction is not yet negligible in ringwoodite under natural conditions.
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slip systems and plastic shear anisotropy in mg 2 sio 4 ringwoodite insights from numerical modelling
European Journal of Mineralogy, 2006Co-Authors: Philippe Carrez, Patrick Cordier, David Mainprice, Andréa TommasiAbstract:Knowledge on the deformation mechanisms of Mg 2 SiO 4 ringwoodite is important for the understanding of flow and seismic anisotropy in the Earth9s mantle transition zone. We report here the first numerical modelling of Dislocation structures in ringwoodite. The Dislocation properties are calculated through the Peierls-Nabarro model using the generalized stacking fault (GSF) results as a starting model. The GSF are determined from first-principle calculations using the code VASP. They enable us to determine the relative ease of slip for Dislocation Glide systems in ringwoodite. The Dislocation properties such as core spreading and Peierls stresses were determined for the easy Dislocation Glide systems. Our results show that 1/2 {110} and 1/2 {111} are the easiest slip systems in ringwoodite at 20 GPa and 0 K. These results are used as input of a viscoplastic model to predict the deformation of a ringwoodite rich aggregate. Calculated crystal preferred orientation (CPO) accounts satisfactorily for experimental data available from either diamond anvil cell or D-DIA experiments.
David J. Srolovitz - One of the best experts on this subject based on the ideXlab platform.
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the effect of randomness on the strength of high entropy alloys
Acta Materialia, 2019Co-Authors: David J. Srolovitz, Luchan Zhang, Yang Xiang, Jian HanAbstract:Abstract High-entropy alloys (HEAs), i.e., single-phase, (nearly) equiatomic multicomponent, metallic materials, are associated with novel mechanical properties, such as high strength, fracture resistance etc. In this paper, a stochastic Peierls-Nabarro (PN) model is proposed to understand how random site occupancy affects intrinsic strength. The stochastic PN model accounts for the randomness in the composition, characterized by both the standard deviation of the perturbation in the interplanar potential and the correlation length within the spatial compositional distribution. The model presented includes the effects of non-uniform compositional distribution both in the direction of Dislocation Glide and along a Dislocation line to predict overall Dislocation Glide resistance. The model predicts the intrinsic strength of HEAs as a function of the standard deviation and the correlation length of the randomness. We find that, in most of the parameter space, the compositional randomness in an HEA gives rise to an intrinsic strength that far exceeds that of any of the pure metals from which the HEA is composed. This approach provides a fundamental explanation to the origin of the high strength of HEAs.
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Solute effects on Dislocation Glide in metals
Acta Materialia, 2003Co-Authors: Jeffrey M. Rickman, Richard Alan Lesar, David J. SrolovitzAbstract:Abstract We examine theoretically the impact of dissolved solute atoms on the motion of Dislocations in metals and, hence, on their plastic response. Two regimes of behavior are distinguished; a low velocity regime, in which the Dislocation drags a near equilibrium solute cloud, and a high velocity regime, in which the Dislocation is alternately trapped and free running. In both regimes, the overdamped motion of a Dislocation in the alloy is exactly as it would be in the pure system but with regime-dependent Dislocation mobilities. In the low velocity regime, the forces between Dislocations are replaced by forces between Dislocation-solute cloud “quasiparticles.” This paper provides analytical estimates for the mobilities in the two regimes, as well as expressions for calculating quasiparticle interactions. Finally, a prescription is provided for carrying out Dislocation dynamics simulations without explicitly incorporating solute degrees of freedom.
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first passage time markov chain analysis of rare events for kinetic monte carlo double kink nucleation during Dislocation Glide
Modelling and Simulation in Materials Science and Engineering, 2002Co-Authors: Chaitanya Deo, David J. SrolovitzAbstract:We describe a first passage time Markov chain analysis of rare events in kinetic Monte Carlo (kMC) simulations and demonstrate how this analysis may be used to enhance kMC simulations of Dislocation Glide. Dislocation Glide is described by the kink mechanism, which involves double kink nucleation, kink migration and kink–kink annihilation. Double kinks that nucleate on straight Dislocations are unstable at small kink separations and tend to recombine immediately following nucleation. A very small fraction (<0.001) of nucleating double kinks survive to grow to a stable kink separation. The present approach replaces all of the events that lead up to the formation of a stable kink with a simple numerical calculation of the time required for stable kink formation. In this paper, we treat the double kink nucleation process as a temporally homogeneous birth–death Markov process and present a first passage time analysis of the Markov process in order to calculate the nucleation rate of a double kink with a stable kink separation. We discuss two methods to calculate the first passage time; one computes the distribution and the average of the first passage time, while the other uses a recursive relation to calculate the average first passage time. The average first passage times calculated by both approaches are shown to be in excellent agreement with direct Monte Carlo simulations for four idealized cases of double kink nucleation. Finally, we apply this approach to double kink nucleation on a screw Dislocation in molybdenum and obtain the rates for formation of stable double kinks as a function of applied stress and temperature. Equivalent kMC simulations are too inefficient to be performed using commonly available computational resources.