Ab Initio Study of Screw Dislocations in Mo and Ta: A New Picture of Plasticity in bcc Transition Metals (original) (raw)
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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.
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.
Philosophical Magazine, 2009
To cite this Article Chiesa, S., Gilbert, M. R., Dudarev, S. L., Derlet, P. M. and Van Swygenhoven, H.(2009) 'The nondegenerate core structure of a ½〈111〉 screw dislocation in bcc transition metals modelled using Finnis-Sinclair potentials: The necessary and sufficient conditions', Philosophical Magazine, 89: 34, 3235 -3243 To link to this Article:
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.
Ab initio investigation of the Peierls potential of screw dislocations in bcc Fe and W
Acta Materialia, 2013
The easy, hard and split core configurations of the h1 1 1i screw dislocation and the energy pathways between them are studied in body-centered cubic (bcc) Fe and W using different density functional theory (DFT) approaches. All approaches indicate that in Fe, the hard core has a low relative energy, close to or even below that of the saddle configuration for a straight path between two easy cores. This surprising result is not a direct consequence of magnetism in bcc Fe. Moreover, the path followed by the dislocation core in the (1 1 1) plane between easy cores, identified here using two different methods to locate the dislocation position, is almost straight, while the energy landscape between the hard core position and the saddle configuration for a straight path is found to be very flat. These results in Fe are in contrast with predictions from empirical potentials as well as DFT calculations in W, where the hard core has an energy about twice that of the maximum energy along the Peierls barrier, and where the dislocation trajectory between easy cores is curved. Also, the split core configuration is found to be unstable in DFT and of high energy in both Fe and W, in contrast with predictions from most empirical potentials.
Plastic anisotropy and dislocation trajectory in BCC metals
Nature communications, 2016
Plasticity in body-centred cubic (BCC) metals at low temperatures is atypical, marked in particular by an anisotropic elastic limit in clear violation of the famous Schmid law applicable to most other metals. This effect is known to originate from the behaviour of the screw dislocations; however, the underlying physics has so far remained insufficiently understood to predict plastic anisotropy without adjustable parameters. Here we show that deviations from the Schmid law can be quantified from the deviations of the screw dislocation trajectory away from a straight path between equilibrium configurations, a consequence of the asymmetrical and metal-dependent potential energy landscape of the dislocation. We propose a modified parameter-free Schmid law, based on a projection of the applied stress on the curved trajectory, which compares well with experimental variations and first-principles calculations of the dislocation Peierls stress as a function of crystal orientation.
Dislocations with edge components in nanocrystalline bcc Mo
We report high-resolution transmission electron microscopy (HRTEM) observation of a high density of dislocations with edge components (;10 16 m À2 ) in nanocrystalline (NC) body-centered cubic (bcc) Mo prepared by high-pressure torsion. We also observed for the first time of the ½,111. and ,001. pure edge dislocations in NC Mo. Crystallographic analysis and image simulations reveal that the best way using HRTEM to study dislocations with edge components in bcc systems is to take images along ,110. zone axis, from which it is possible to identify ½,111. pure edge dislocations, and edge components of ½,111. and ,001. mixed dislocations. The ,001. pure edge dislocations can only be identified from ,100. zone axis. The high density of dislocations with edge components is believed to play a major role in the reduction of strain rate sensitivity in NC bcc metals and alloys.
Non-glide effects and dislocation core fields in BCC metals
npj Computational Materials
A hallmark of low-temperature plasticity in body-centered cubic (BCC) metals is its departure from Schmid’s law. One aspect is that non-glide stresses, which do not produce any driving force on the dislocations, may affect the yield stress. We show here that this effect is due to a variation of the relaxation volume of the 1/2\langle 111\rangle$$1∕2⟨111⟩ screw dislocations during glide. We predict quantitatively non-glide effects by modeling the dislocation core as an Eshelby inclusion, which couples elastically to the applied stress. This model explains the physical origin of the generalized yield criterion classically used to include non-Schmid effects in constitutive models of BCC plasticity. We use first-principles calculations to properly account for dislocation cores and use tungsten as a reference BCC metal. However, the methodology developed here applies to other BCC metals, other energy models and other solids showing non-glide effects.
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.
Dislocation nucleation in bcc Ta single crystals studied by nanoindentation
Physical Review B, 2007
The study of dislocation nucleation in close-packed metals by nanoindentation has recently attracted much interest. Here, we address the peculiarities of the incipient plasticity in body centered cubic ͑bcc͒ metals using low index Ta single crystals as a model system. The combination of nanoindentation with high-resolution atomic force microscopy provides us with experimental atomic-scale information on the process of dislocation nucleation and multiplication. Our results reveal a unique deformation behavior of bcc Ta at the onset of plasticity, which is distinctly different from that of close-packed metals. Most noticeably, we observe only one rather than a sequence of discontinuities in the load-displacement curves. This and other differences are discussed in the context of the characteristic plastic deformation behavior of bcc metals.