Non-glide effects and dislocation core fields in BCC metals

Abstract

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=2h111i screw dislocations during glide. We predict quantitatively nonglide 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 nonglide effects.

Hybrid quantum/classical study of hydrogen-decorated screw dislocations in tungsten: Ultrafast pipe diffusion, core reconstruction, and effects on glide mechanism

“…1(e) and shows a curved path between two easy-core configurations (left and right blues boxes). The saddle point is located close to the "split" core configuration in agreement with previous DFT calculations using quadrupole configurations [37,42,44,45].…”

“…[42] used for the MM part in QM/MM scheme. Dashed lines with hollow circles give previously reported DFT results with periodic quadrupole configurations containing 135 atoms [42,44,45]. Our results demonstrate that treating only 23 atoms around the dislocation core at the DFT level allows us to obtain an MEP and absolute barrier height in much better agreement with reference DFT results, within the range of the reported DFT data (≈10 meV).…”

“…No elastic correction for the resulting energy barrier was applied, though previous studies have shown this correction is minimal [41,44,45], as we confirm below.…”

“…The minimum energy path (MEP) of the dislocation core passes close to a third type of core configuration-the "split" configuration when the dislocation core is centered close to an atomic column [red cross in Fig. 1(e)] resulting in a single hump-shaped glide MEP profile [37,[41][42][43][44][45] presented with dashed lines in Fig. 2.…”

Repulsion leads to coupled dislocation motion and extended work hardening in bcc metals

“…This core structure is believed to make screw dislocations glide by the successive nucleation and motion of pairs of kinks on an otherwise straight screw dislocation line and lead to the thermally activated deformation behaviour of bcc metals at low temperatures 18 . Atomistic simulations and first principle calculations [19][20][21][22][23][24][25] have been used to investigate the properties of screw dislocations and of kink-pair formation for bcc materials. The core structure is found to be non degenerate and compact.…”