Ab initiomodeling of the two-dimensional energy landscape of screw dislocations in bcc transition metals

Abstract

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.

First-principles prediction of kink-pair activation enthalpy on screw dislocations in bcc transition metals: V, Nb, Ta, Mo, W, and Fe

“…[40], it is too sparse for accurate calculations of bulk properties like the C 44 elastic constants in V and Nb due to the presence of a van Hove singularity near the Fermi level [41]. However, since this singularity is smeared out by the dislocation [21], this k-point density issue should not affect the present results. Residual forces after relaxation were smaller than 10 −3 eV/Å, and the convergence criterion for self-consistency on the total energy was 10 −7 eV.…”

“…These pseudopotentials have been tested and used in previous studies [21,[37][38][39]. 1 × 2 × 16 and 1 × 2 × 8 shifted k-point grids were used for the 1b and 2b cells respectively.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]), the main issue being to derive the dislocation mobility law from the atomic scale [18]. Among the different computational studies, calculations based on density-functional theory (DFT) have established some important features that were previously matter of debate in bcc transition metals, such as the nondegenerate dislocation core structure [6,10,17], the {110} glide plane [9], and the single-hump Peierls barrier [10,17,[19][20][21]. However, the main drawback of DFT-based methods applied to extended defects such as dislocations is the limited number of atoms that can be considered in the simulation cell, typically a few hundred.…”

Dislocation locking versus easy glide in titanium and zirconium

“…4a that the energy pathway, which corresponds to −2 ≤ ζ ≤ 0, has a high energy barrier (11.4 meV Å −1 ). Therefore, the glide of a dislocation in its pyramidal spreading plane is subjected to a high lattice friction, of the same order as in bcc metals 25,26 , where dislocation glide is thermally activated up to room temperature and requires the nucleation and propagation of kink pairs along the dislocation lines. Another potential glide mechanism is that the dislocation first cross-slips into a prismatic plane-that is, changes its spreading plane from pyramidal to prismatic-and then glides in the prismatic plane.…”

Decisive role of magnetism in the interaction of chromium and nickel solute atoms with 1/2〈111〉-screw dislocation core in body-centered cubic iron

“…1 (left to right accordingly). It should be mentioned that the full quadrupole of a screw dislocation with total Burgers vector equal to zero is a metastable configuration and is separate from the ground state, which is the perfect bcc lattice, by the barrier of~40 meV per Burgers vector [25,30].…”