Temperature-induced screw dislocation core transformation and its effect on mobility in pure W (original) (raw)
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Journal of Physics: Condensed Matter, 2013
Screw dislocations in bcc metals display non-planar cores at zero temperature which result in high lattice friction and thermally activated strain rate behavior. In bcc W, electronic structure molecular statics calculations reveal a compact, non-degenerate core with an associated Peierls stress between 1.7 and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations can only be gained by using more efficient atomistic simulations based on semiempirical interatomic potentials. In this paper we assess the suitability of five different potentials in terms of static properties relevant to screw dislocations in pure W. As well, we perform molecular dynamics simulations of stress-assisted glide using all five potentials to study the dynamic behavior of screw dislocations under shear stress. Dislocations are seen to display thermally-activated motion in most of the applied stress range, with a gradual transition to a viscous damping regime at high stresses. We find that one potential predicts a core transformation from compact to dissociated at finite temperature that affects the energetics of kink-pair production and impacts the mechanism of motion. We conclude that a modified embedded-atom potential achieves the best compromise in terms of static and dynamic screw dislocation properties, although at an expense of about ten-fold compared to central potentials.
Physical Review B, 2015
The atomistic study of kink pairs on screw dislocations in body-centered cubic (bcc) metals is challenging because interatomic potentials in bcc metals still lack accuracy and kink pairs require too many atoms to be modeled by first principles. Here, we circumvent this difficulty using a one-dimensional line tension model whose parameters, namely the line tension and Peierls barrier, are reachable to density functional theory calculations. The model parameterized in V, Nb, Ta, Mo, W, and Fe, is used to study the kink-pair activation enthalpy and spatial extension. Interestingly, we find that the atomistic line tension is more than twice the usual elastic estimates. The calculations also show interesting group tendencies with the line tension and kink-pair width larger in group V than in group VI elements. Finally, the present kink-pair activation energies are shown to compare qualitatively with experimental data and potential origins of quantitative discrepancies are discussed.
Physical Review B, 2014
A density functional theory (DFT) study of the 1/2 111 screw dislocation was performed in the following body-centered cubic transition metals: V, Nb, Ta, Cr, Mo, W, and Fe. The energies of the easy, hard, and split core configurations, as well as the pathways between them, were investigated and used to generate the two-dimensional (2D) Peierls potential, i.e. the energy landscape seen by the dislocation as a function of its position in the (111) plane. In all investigated elements, the nondegenerate easy core is the minimum energy configuration, while the split core configuration, centered in the immediate vicinity of a 111 atomic column, has a high energy near or above that of the hard core. This unexpected result yields 2D Peierls potentials very different from the usually assumed landscapes. The 2D Peierls potential in Fe differs from the other transition metals, with a monkey saddle instead of a local maximum located at the hard core. An estimation of the Peierls stress from the shape of the Peierls barrier is presented in all investigated metals. A strong group dependence of the core energy is also evidenced, related to the position of the Fermi level with respect to the minimum of the pseudogap of the electronic density of states.
Theory and simulation of the diffusion of kinks on dislocations in bcc metals
Physical Review B, 2013
Isolated kinks on thermally fluctuating 1/2 111 screw, 100 edge, and 1/2 111 edge dislocations in bcc iron are simulated under zero stress conditions using molecular dynamics (MD). Kinks are seen to perform stochastic motion in a potential landscape that depends on the dislocation character and geometry, and their motion provides fresh insight into the coupling of dislocations to a heat bath. The kink formation energy, migration barrier, and friction parameter are deduced from the simulations. A discrete Frenkel-Kontorova-Langevin model is able to reproduce the coarse-grained data from MD at ∼10 −7 of the computational cost, without assuming an a priori temperature dependence beyond the fluctuation-dissipation theorem. Analytical results reveal that discreteness effects play an essential role in thermally activated dislocation glide, revealing the existence of a crucial intermediate length scale between molecular and dislocation dynamics. The model is used to investigate dislocation motion under the vanishingly small stress levels found in the evolution of dislocation microstructures in irradiated materials. PACS number(s): 61.72. Hh, 31.15.xv Dislocation motion is limited by two general processes: the formation and migration of kinks and pinning by impurities and other defects. 1 In this paper, we investigate the motion of kink-limited screw and edge dislocations in bcc Fe, where the kink formation energy is much larger than the thermal energy. To obtain dislocation motion on the time scales accessible to molecular dynamics (MD) simulations, some researchers have resorted to inducing kink formation by applying stresses some six orders of magnitude greater than those pertaining experimentally. 2,3 But, dislocation core structures and Peierls barriers are known to be highly stress dependent, 4 making it difficult to relate simulation to the vanishingly low stress conditions found in thermally activated evolution of dislocation microstructures.
Repulsion leads to coupled dislocation motion and extended work hardening in bcc metals
Nature Communications
Work hardening in bcc single crystals at low homologous temperature shows a strong orientation-dependent hardening for high symmetry loading, which is not captured by classical dislocation density based models. We demonstrate here that the high activation barrier for screw dislocation glide motion in tungsten results in repulsive interactions between screw dislocations, and triggers dislocation motion at applied loading conditions where it is not expected. In situ transmission electron microscopy and atomistically informed discrete dislocation dynamics simulations confirm coupled dislocation motion and vanishing obstacle strength for repulsive screw dislocations, compatible with the kink pair mechanism of dislocation motion in the thermally activated (low temperature) regime. We implement this additional contribution to plastic strain in a modified crystal plasticity framework and show that it can explain the extended work hardening regime observed for [100] oriented tungsten single...
International Journal of Plasticity, 2015
Thermally-activated 1 /2 111 screw dislocation motion is the controlling plastic mechanism at low temperatures in body-centered cubic (bcc) crystals. Motion proceeds by nucleation and propagation of atomic-sized kink pairs susceptible of being studied using molecular dynamics (MD). However, MD's natural inability to properly sample thermally-activated processes as well as to capture {110} screw dislocation glide calls for the development of other methods capable of overcoming these limitations. Here we develop a kinetic Monte Carlo (kMC) approach to study single screw dislocation dynamics from room temperature to 0.5T m and at stresses 0 < σ < 0.9σ P , where T m and σ P are the melting point and the Peierls stress. The method is entirely parameterized with atomistic simulations using an embedded atom potential for tungsten. To increase the physical fidelity of our simulations, we calculate the deviations from Schmid's law prescribed by the interatomic potential used and we study single dislocation kinetics using both projections. We calculate dislocation velocities as a function of stress, temperature, and dislocation line length. We find that considering non-Schmid effects has a strong influence on both the magnitude of the velocities and the trajectories followed by the dislocation. We finish by condensing all the calculated data into effective stress and temperature dependent mobilities to be used in more homogenized numerical methods.
Atomistic simulations of kinks in 1 / 2 a 〈 111 〉 screw dislocations in bcc tantalum
Physical Review B, 2003
Two types of equilibrium core structures ͑denoted symmetric and asymmetric͒ for 1/2a͗111͘ screw dislocations in bcc metals have been found in atomistic simulations. In asymmetric ͑or polarized͒ cores, the central three atoms simultaneously translate along the Burgers vector direction. This collective displacement of core atoms is called polarization. In contrast, symmetric ͑nonpolarized͒ cores have zero core polarization. To examine the possible role of dislocation core in kink-pair formation process, we studied the multiplicity, structural features, and formation energies of 1/3a͗112͘ kinks in 1/2a͗111͘ screw dislocations with different core structures. To do this we used a family of embedded atom model potentials for tantalum ͑Ta͒ all of which reproduce bulk properties ͑density, cohesive energy, and elastic constants͒ from quantum mechanics calculations but differ in the resulting polarization of 1/2a͗111͘ screw dislocations. For dislocations with asymmetric core, there are two energy equivalent core configurations ͓with positive ͑P͒ and negative ͑N͒ polarization͔, leading to 2 types of ͑polarization͒ flips, 8 kinds of isolated kinks, and 16 combinations of kink pairs. We find there are only two elementary kinks, while the others are composites of elementary kinks and flips. In contrast, for screw dislocations with symmetric core, there are only two types of isolated kinks and one kind of kink pair. We find that the equilibrium dislocation core structure of 1/2a͗111͘ screw dislocations is an important factor in determining the kink-pair formation energy.
A phenomenological dislocation mobility law for bcc metals
Acta Materialia, 2016
Dislocation motion in body centered cubic (bcc) metals displays a number of specific features that result in a strong temperature dependence of the flow stress, and in shear deformation asymmetries relative to the loading direction as well as crystal orientation. Here we develop a generalized dislocation mobility law in bcc metals, and demonstrate its use in discrete Dislocation Dynamics (DD) simulations of plastic flow in tungsten (W) micro pillars. We present the theoretical background for dislocation mobility as a motivating basis for the developed law. Analytical theory, molecular dynamics (MD) simulations, and experimental data are used to construct a general phenomenological description. The usefulness of the mobility law is demonstrated through its application to modeling the plastic deformation of W micro pillars. The model is consistent with experimental observations of temperature and orientation dependence of the flow stress and the corresponding dislocation microstructure.
npj Computational Materials, 2020
In traditional body-centered cubic (bcc) metals, the core properties of screw dislocations play a critical role in plastic deformation at low temperatures. Recently, much attention has been focused on refractory high-entropy alloys (RHEAs), which also possess bcc crystal structures. However, unlike face-centered cubic high-entropy alloys (HEAs), there have been far fewer investigations into bcc HEAs, specifically on the possible effects of chemical short-range order (SRO) in these multiple principal element alloys on dislocation mobility. Here, using density functional theory, we investigate the distribution of dislocation core properties in MoNbTaW RHEAs alloys, and how they are influenced by SRO. The average values of the core energies in the RHEA are found to be larger than those in the corresponding pure constituent bcc metals, and are relatively insensitive to the degree of SRO. However, the presence of SRO is shown to have a large effect on narrowing the distribution of dislocation core energies and decreasing the spatial heterogeneity of dislocation core energies in the RHEA. It is argued that the consequences of the mechanical behavior of HEAs is a change in the energy landscape of the dislocations, which would likely heterogeneously inhibit their motion. npj Computational Materials (2020) 6:110 ; https://doi. INTRODUCTION Previous investigation of the fundamentals of deformation in body-centered cubic (bcc) transition metals have revealed that the core properties of the ½〈111〉 screw dislocations play an essential role in their plasticity 1 , especially at low temperatures where the deformation is thermally activated through the kink-pair nuclea-tion mechanism 2 , and is expected to be strongly temperature dependent. The high lattice friction associated with such screw dislocation motion is a result of nonplanar core structure 1,3 and is related to the height of the Peierls potential 4. Due to the importance for plastic deformation, extensive atomistic simulation studies have been devoted to computing core structures and corresponding mobilities of screw dislocations in bcc transition metals 3,5-8. In these studies, one of the significant challenges has been the variation in properties derived from different models for the interatomic potentials. For example, early studies based on classical potential models often predicted a metastable split core structure 9-11 , which leads to a camel-hump shape in the Peierls potential. Later density functional theory (DFT) calculations produced symmetric and compact dislocation cores in Mo, Ta, and Fe 12-16 ; similar compact cores have been found in other bcc transition metals, such as W, Nb, and V 17,18. In DFT studies of the energy landscape of screw dislocations in bcc transition metals 18-20 , it was found that nondegenerate cores lead to a single humped curve in the Peierls potential, implying that the split core structure might not be metastable. Alloying effects on the Peierls potential of W have also been explored 21. Recently developed machine learning-based potentials 22-24 and new embedded atom method potentials that consider quantum effects on lattice vibrations 25 and extra constraints 26 all lead to predictions of a single humped curve in the Peierls potential. Due to the dependence of the results for screw dislocations in bcc transition metals on the model for interatomic bonding, DFT-based approaches are of interest to provide benchmarks for subsequent modeling at higher scales.
Physical Review Letters, 2000
We report the first ab initio density-functional study of 111 screw dislocations cores in the bcc transition metals Mo and Ta. Our results suggest a new picture of bcc plasticity with symmetric and compact dislocation cores, contrary to the presently accepted picture based on continuum and interatomic potentials. Core energy scales in this new picture are in much better agreement with the Peierls energy barriers to dislocation motion suggested by experiments.