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

Abstract

We show here how density functional theory calculations can be used to predict the temperatureand orientation-dependence of the yield stress of body-centered cubic (BCC) metals in the thermallyactivated regime where plasticity is governed by the glide of screw dislocations with a 1/2〈111〉 Burgers vector. Our numerical model incorporates non-Schmid effects, both the twinning/antitwinning asymmetry and non-glide effects, characterized through ab initio calculations on straight dislocations. The model uses the stress-dependence of the kink-pair nucleation enthalpy predicted by a line tension model also fully parameterized on ab initio calculations. The methodology is illustrated here on BCC tungsten but is applicable to all BCC metals. Comparison with experimental data allows to highlight both the successes and remaining limitations of our modeling approach.

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

“…In addition to the Volterra elastic field, dislocations also induce a short-range dilatation field in the vicinity of their cores which can be modeled as an Eshelby inclusion defined by a relaxation volume tensor [16]. The dislocation positions and relaxation volume tensor can be evaluated from ab initio calculations taking advantage of the stress variation ∆σ induced by moving dislocations in a simulation cell with fixed periodicity vectors [16,17]. These variations are caused by changes in the distance between the two dislocations of the dipole, and by the variations of their relaxation volume.…”

“…Such departures from the Schmid law have been shown to result from the trajectory of the gliding dislocation which locally deviates from the {110} plane [15] and from the variations along this trajectory of the dislocation relaxation volume [16]. With all corresponding quantities extracted from ab initio calculations, one can build a generalized yield criterion going beyond the Schmid law to take full account of the specificities of the bcc crystal [17].…”

Mobility of screw dislocation in BCC tungsten at high temperature in presence of carbon

“…bτ, with a slight stress dependence which can be neglected at low stress. This contrasts with the Peierls regime at low temperature existing in pure BCC metals, in particular in tungsten, where kinks can freely glide along the dislocation, leading to an activation enthalpy for dislocation velocity which simply equals the kink-pair formation enthalpy [3,4,37]. Because of the covalent bonding between C and W atoms inside the reconstructed core, similar to the one existing in WC tungsten carbide [9], kinks on screw dislocation in presence of carbon are highly localised and are difficult to move.…”

Secondary slip of screw dislocations in zirconium

“…29 in Ref. [34]) using parameters obtained in the low stress regime for the Kocks law and assuming that parameters T m and ν are the same as the ones given by MD in the high stress regime. For reasonable values of the dislocation density (10 10 -10 16 m −2 ) and of the strain rate (10 −3 -10 −1 s −1 ), the obtained athermal temperature is between 450 and 570 K, thus further confirming that the lattice friction opposing glide should have a negligible effect in the high-temperatures regime explored by Akhtar [5].…”