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David L Mcdowell - One of the best experts on this subject based on the ideXlab platform.

  • atomistic modeling of dislocation cross Slip in nickel using free end nudged elastic band method
    2019
    Co-Authors: Dengke Chen, Luke L Costello, Clint B Geller, Ting Zhu, David L Mcdowell
    Abstract:

    Abstract Cross-Slip of screw dislocations plays an important role in the plastic deformation of face-centered cubic (FCC) metals and alloys. Here we use the free-end nudged elastic band (FENEB) method to determine the atomistic reaction pathways and energy barriers of cross-Slip in an FCC single crystal of Ni. We focus on the cross-Slip Process mediated by an array of pinning vacancy clusters in the form of stacking fault tetrahedra. We also study a competing Process of screw glide by direct cutting of those pinning obstacles on the original Slip plane. The activation energies of both cross-Slip and obstacle-cutting are determined for different stresses, obstacle spacings and sizes. Using FENEB-calculated energy barriers, we construct dislocation mechanism maps to reveal the effects of resolved shear stress, obstacle spacing and size on the rate-controlling dislocation Process for plastic deformation. We further evaluate the activation volumes of cross-Slip and obstacle-cutting. The latter result emphasizes the notion of finite strength of the atomically sized pinning obstacles to dislocation motion and also validates the Nabarro scaling law of the linear dependence of activation volume on obstacle spacing.

  • shear stress and line length dependent screw dislocation cross Slip in fcc ni
    2017
    Co-Authors: Liming Xiong, Youping Chen, David L Mcdowell
    Abstract:

    Abstract Screw dislocation cross-Slip is important in dynamic recovery of deformed metals. A mobile screw dislocation segment can cross Slip to annihilate an immobile screw dislocation segment with opposite Burgers vector, leaving excess dislocations of one kind in a crystal. Previous studies have found that the cross-Slip Process depends on both the local stress state and dislocation line length, yet a quantitative study of the combined effects of these two factors has not been conducted. In this work, we employ both dynamic concurrent atomistic-continuum (CAC) [L. Xiong, G. Tucker, D.L. McDowell, Y. Chen, J. Mech. Phys. Solids 59 (2011) 160–177] and molecular dynamics simulations to explore the shear stress- and line length-dependent screw dislocation cross-Slip in face-centered cubic Ni. It is demonstrated that the CAC approach can accurately describe the 3-D cross-Slip Process at a significantly reduced computational cost, as a complement to other numerical methods. In particular, we show that the Fleischer (FL) [R.L. Fleischer, Acta Metall. 7 (1959) 134–135] type cross-Slip, in which a stair-rod dislocation is involved, can be simulated in the coarse-grained domain. Our simulations show that as the applied shear stress increases, the cross-Slip mechanism changes from the Friedel-Escaig (FE) [B. Escaig, J. Phys. 29 (1968) 225–239] type to the FL type. In addition, the critical shear stress for both cross-Slip mechanisms depends on the dislocation line length. Moreover, the cross-Slip of a screw dislocation with a length of 6.47 nm analyzed using periodic boundary conditions occurs via only the FL mechanism, whereas a longer dislocation with length of 12.94 nm can cross-Slip via either the FE or FL Process in Ni subject to different shear stresses.

Chongyu Wang - One of the best experts on this subject based on the ideXlab platform.

  • cross Slip Process in model ni al solid solution an embedded atom method study
    2014
    Co-Authors: Chongyu Wang
    Abstract:

    Abstract The cross-Slip Process of a screw dislocation in model Ni(Al) random solid solution is studied using the climbing image nudged elastic band method with an embedded-atom method potential. The average stacking fault energy of model Ni(Al) solid solution decreases with increasing Al concentration from 0 to 10 at.%. However, the average activation energy under zero stress shows an initial increase and then plateaus with increasing concentration of Al. The excess activation energy is determined by comparing with results from linear-elastic continuum theory. The short-range repulsive solute atom pair and its interaction with the dislocation provide the excess activation energy in the cross-Slip Process of model Ni(Al) solid solution.

Tomohito Tsuru - One of the best experts on this subject based on the ideXlab platform.

  • atomistic study on the cross Slip Process of a screw dislocation in magnesium
    2015
    Co-Authors: Mitsuhiro Itakura, Hideo Kaburaki, Masatake Yamaguchi, Tomohito Tsuru
    Abstract:

    The cross-Slip Process of a screw dislocation from the basal to the prismatic plane in magnesium was studied using the density functional theory and the molecular dynamics calculations. An atomistic method for calculating the total Peierls energy map has been devised to track the transition path of a dissociated and/or constricted screw dislocation in the cross-Slip Process. The barrier of a screw dislocation from the basal to the prismatic plane is estimated by the density functional theory for the first time to be meV per Burgers vector length. The activation enthalpy for the cross Slip is calculated using a line tension model based on the density functional theory to be 1.4–1.7 eV, which is in reasonable agreement with experiments. On the basis of the results, the effect of temperature on the cross-Slip Process of the dissociated screw dislocation on the basal plane is studied in detail using the molecular dynamics method with the embedded-atom-method (EAM) interatomic potential, in which the critical resolved shear stress for the cross Slip is evaluated. It is confirmed that the bowed-out dislocation line on the prismatic plane consists of slightly dissociated rectilinear segments with connecting jogs at low temperatures and, as the temperature rises, the curved dislocation line becomes smooth with many segments. The motion of an dislocation on the prismatic plane is jerky in the low temperature region, while it is retarded by the formation of the largely dissociated plateau segment above the room temperature. A large reduction of the critical shear stress for the cross Slip is obtained when the screw dislocation interacts with a hard-sphere particle placed on the basal plane in the low temperature region.

Dan Mordehai - One of the best experts on this subject based on the ideXlab platform.

  • cross Slip in face centred cubic metals a general full stress field dependent activation energy line tension model
    2019
    Co-Authors: Alon Malkamarkovitz, Dan Mordehai
    Abstract:

    ABSTRACTCross-Slip is a thermally activated Process by which a screw dislocation changes its Slip plane. Understanding and modelling the activation barrier of the cross-Slip Process as a free-energy barrier that depends on the stress conditions at the vicinity of the dislocation is crucial. In this work, we employ the line-tension model for the cross-Slip of screw dislocations in face-centred cubic (FCC) metals in order to calculate the energy barrier when both Escaig stresses are applied on the primary and cross-Slip planes and Schmid stress is applied on the cross-Slip plane. We propose a closed-form expression for the activation energy for cross-Slip in a large range of stresses, without any fitting parameters. The results of the proposed model are in good agreement with previous numerical results and atomistic simulations. We also show that, when Schmid stress is applied on the cross-Slip plane, the energy barrier is decreased, and in particular, cross-Slip can occur even when the Escaig stress in the...

  • Cross-Slip in face centred cubic metals: a general full stress-field dependent activation energy line-tension model
    2019
    Co-Authors: Alon Malka-markovitz, Dan Mordehai
    Abstract:

    Cross-Slip is a thermally activated Process by which a screw dislocation changes its Slip plane. Understanding and modelling the activation barrier of the cross-Slip Process as a free-energy barrier that depends on the stress conditions at the vicinity of the dislocation is crucial. In this work, we employ the line-tension model for the cross-Slip of screw dislocations in face-centred cubic (FCC) metals in order to calculate the energy barrier when both Escaig stresses are applied on the primary and cross-Slip planes and Schmid stress is applied on the cross-Slip plane. We propose a closed-form expression for the activation energy for cross-Slip in a large range of stresses, without any fitting parameters. The results of the proposed model are in good agreement with previous numerical results and atomistic simulations. We also show that, when Schmid stress is applied on the cross-Slip plane, the energy barrier is decreased, and in particular, cross-Slip can occur even when the Escaig stress in the primary plane is smaller than that on the cross-Slip plane. The proposed closed-form expression for the activation energy can be easily implemented in dislocation dynamics simulations, owing to its simplicity and universality. This will allow cross-Slip to be more accurately related to macroscopic plasticity.

W A Curtin - One of the best experts on this subject based on the ideXlab platform.

  • analysis of double cross Slip of pyramidal i c a screw dislocations and implications for ductility in mg alloys
    2020
    Co-Authors: Rasool Ahmad, W A Curtin
    Abstract:

    Abstract Solute accelerated cross-Slip of pyramidal 〈 c + a 〉 screw dislocations has recently been recognized as a crucial mechanism in enhancing the ductility of solid-solution Mg alloys. In pure Mg, cross-Slip is ineffective owing to the energy difference between the high energy pyramidal I and low energy pyramidal II 〈 c + a 〉 screw dislocations. A small addition of solutes, especially rare earth (RE) elements, can reduce this energy difference and accelerate cross-Slip, thus enabling enhanced ductility. With increasing solute concentrations, the pyramidal I dislocation can become energetically favorable, which switches the primary 〈 c + a 〉 Slip plane and alters the cross-Slip Process. Here, the transition path and energetics for double cross-Slip of pyramidal I 〈 c + a 〉 dislocations are analysed in the regime where the pyramidal I dislocation is energetically more favorable than the pyramidal II. This is achieved using nudged elastic band simulations on a proxy MEAM potential for Mg designed to favor the pyramidal I over pyramidal II. The minimum energy transition path for pyramidal I double cross-Slip is found to initiate with cross-Slip onto a pyramidal II plane followed by cross-Slip onto a pyramidal I plane parallel to the original pyramidal I plane. A previous mechanistic model for ductility is then extended to higher solute concentrations where pyramidal I is favorable. The model predicts an upper limit of solute concentrations beyond which ductility again becomes poor in Mg alloys. The model predictions are consistent with limited experiments of Mg-RE alloys at high concentrations and motivate further experimental studies in the high concentration regime.