Impact of magnetism on screw dislocations in body-centered cubic chromium

Abstract

The influence of magnetism on the properties of screw dislocations in body-centered cubic chromium is investigated by means of ab initio calculations. Screw dislocations having Burgers vectors 1/2 111 and 100 are considered, following experimental observations showing activity for both slip systems. At low temperature, chromium has a magnetic order close to antiferromagnetism along 100 directions, for which 1/2 111 is not a periodicity vector. Hence, dislocations with Burgers vectors 1/2 111 generate magnetic faults when shearing the crystal, which constrain them to coexist and move pairwise, leading to dissociated 111 super-dislocations. On the other side, 100 is a periodicity vector of the magnetic order of chromium, and no such magnetic fault are generated when 100 dislocations glide. Dislocation properties are computed in the magnetically ordered and non magnetic phases of chromium for comparison purposes. We report a marginal impact of magnetism on the structural properties and energies of dislocations for both slip systems. The Peierls energy barrier opposing dislocation glide in {110} planes is comparable for both 1/2 111 {110} and 100 {110} slip systems, with lower Peierls stresses in the magnetically ordered phase of chromium.

Screw dislocations in BCC transition metals: from ab initio modeling to yield criterion

“…On the other hand, magnetic order in an antiferromagnetic phase, like BCC Cr at low temperature, 5 is not compatible with a 1/2〈111〉 Burgers vector: dislocations with such a Burgers vector necessarily introduce a magnetic fault in the crystal. It has been proposed that these dislocations coexist pairwise to bound the magnetic fault, thus leading to super-dislocations with a 〈111〉 Burgers vector [96].…”

Ab initio modeling of slip activity in body-centered cubic chromium

“…Existence of these 100 dislocations was later confirmed by Hale and Henderson Brown [4] who determined Burgers vectors through extinction experiments in TEM ( g. b contrast). Although these 100 dislocations have a larger Burgers vector than 1/2 111 dislocations, both have close core and elastic energy [7,9]. The latter is a consequence of the strong elastic anisotropy of bcc Cr: with an anisotropy ratio A = C 44 /(C 11 − C12) smaller than 1 (A 0.68), 100 screw dislocations have actually a lower elastic energy than 1/2 111 ones in chromium [9].…”

“…These dislocations glide in the {110} planes [2,3,6] and have a Burgers vector equal to the smallest periodicity vectors of the bcc crystal lattice, 1/2 111 . As 1/2 111 is not a periodicity vector of the AF magnetic order, such dislocations should trail magnetic faults while gliding [7,8]. But, except this magnetic fault, ab initio calculations have shown that magnetism has a limited impact on the core properties of this screw dislocation [7].…”

“…As 1/2 111 is not a periodicity vector of the AF magnetic order, such dislocations should trail magnetic faults while gliding [7,8]. But, except this magnetic fault, ab initio calculations have shown that magnetism has a limited impact on the core properties of this screw dislocation [7].…”

Crystal Structure, Microstructure and Electronic Properties of a Newly Discovered Ternary Phase in the Al-Cr-Sc System

“…We took the non-magnetic Cr crystal as the reference to calculate the formation energies. Obviously, this is not the realistic ground state for bcc chromium [31,32]. Indeed, neutron diffraction and coherent X-ray diffraction experiments showed the magnetic ground state of bcc Cr to be a spin-density wave (SDW) with an incommensurate period regarding the crystal lattice below its Néel temperature of 311 K. The SDW corresponded to a quasi-sinusoidal modulation of the magnitude of the magnetic moments along the propagation of the wave, keeping a locally antiferromagnetic order.…”