Screw dislocation-carbon interaction in BCC tungsten: an ab initio study

Abstract

The interaction between carbon and screw dislocations in tungsten is investigated using ab initio calculations. The presence of carbon atoms in the vicinity of the dislocation induces a reconstruction, with the dislocation relaxing to a configuration, the hard core structure, which is unstable in pure tungsten. The reconstruction corresponds to a strong binding of carbon in the prismatic sites created by the dislocation which is perfect for high concentrations of carbon segregated on the dislocation line. However, the reconstruction is only partial for lower atomic fractions, with the dislocation tending to fall back in its easy core ground state. This pinning by carbon atoms of the dislocation in an unstable position is well described by a simple line tension model. A strong carbon-dislocation attraction is also evidenced at larger separation distances, when the solute is in the fourth nearest neighbour octahedral sites of the reconstructed core. The equilibrium concentrations of carbon in these different segregation sites are modelled with an Ising model and using a mean-field approximation. This thermodynamic model evidences that screw dislocations remain fully saturated by carbon atoms and pinned in their hard core configuration up to about 2500 K.

Synergistic coupling in ab initio-machine learning simulations of dislocations

“…The dislocation goes back to 'easy' core configuration at distance of 10 |b| from the impurity. Similar relaxation lengths were obtained with fully periodic cells for carbon stabilised hard core in tungsten [25].…”

“…The resulting DFT cluster contained 396 atoms with 24 atoms in QM region (blue atoms) and 373 atoms in Buffer (orange atoms). The left part of the figure demonstrates the extracted dislocation core position by fitting theoretical displacement field to the displacement extracted from the relaxed atomic positions with |b|/3 discretisation step [23,25]. It can be seen that the in contrast to 'disk' cell results He stabilises the 'split' core locally.…”

“…The requirement of periodicity along the line direction makes studies of impurity segregation similarly expensive, even with amenable dislocations, due to long ranged core reconstructions as we show below. Recent studies have simulated well over a thousand DFT atoms to infer the converged structure [25].…”

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

“…In addition, some substitutional solute atoms have been shown to induce a change of the core structure of the screw dislocation through the variation of the electronic density [52,[101][102][103], with the dislocation going from a symmetric compact to a degenerate polarized core. Ab initio calculations have also evidenced core reconstructions of the screw dislocation induced by interstitial solutes, with H stabilizing the split configuration [104] and bigger interstitial solute atoms like carbon stabilizing the hard core [105][106][107][108][109][110]. Integrating the elementary interaction mechanisms revealed by these ab initio calculations in higher scale models, allows to tackle more complex phenomena, like the Portevin-Le Chatelier effect [111] or the reappearance of a Peierls regime and dynamic strain ageing at temperatures where solute diffusion is activated [112,113].…”

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

“…Ab initio calculations have shown the same strong attraction between C atoms and screw dislocations in tungsten [9][10][11] and thermodynamics predicts that screw dislocations remain fully saturated by C atoms up to 2500 K [11]. A reappearance of the Peierls mechanism is therefore expected also in tungsten, but at a higher temperature than in iron because of the higher activation energy for C diffusion in tungsten than in iron, respectively 2.32 and 0.83 eV [12].…”