A phenomenological dislocation mobility law for bcc metals

Abstract

a b s t r a c tDislocation motion in body centered cubic (bcc) metals displays a number of specific features that result in a strong temperature dependence of the flow stress, and in shear deformation asymmetries relative to the loading direction as well as crystal orientation. Here we develop a generalized dislocation mobility law in bcc metals, and demonstrate its use in discrete Dislocation Dynamics (DD) simulations of plastic flow in tungsten (W) micro pillars. We present the theoretical background for dislocation mobility as a motivating basis for the developed law. Analytical theory, molecular dynamics (MD) simulations, and experimental data are used to construct a general phenomenological description. The usefulness of the mobility law is demonstrated through its application to modeling the plastic deformation of W micro pillars. The model is consistent with experimental observations of temperature and orientation dependence of the flow stress and the corresponding dislocation microstructure.

Kink-limited Orowan strengthening explains the brittle to ductile transition of irradiated and unirradiated bcc metals

“…Using the Frenkel-Kontorova model [33] and kink-limited dislocation-obstacle simulations, we evidence a modified Orowan flow law [25,27,35]:…”

“…where ρ is the dislocation density. Employing dislocation mobility laws for unirradiated materials parametrized from atomistic simulations [25,27,36], we show that under temperatures and applied stresses appropriate for the brittle to ductile transition (BDT), the critical length L * falls to values of order 10 −1 μm, well within the range of typical obstacle spacings L . As illustrated in Fig.…”

“…In Sec. II we review dislocation mobility laws for unirradiated materials, parametrized from atomistic simulations [25,27,36] and produce quantitative estimates for the stress-and temperature-dependent critical length L * (σ,T ). In Sec.…”

“…The plasticity of bcc metals is thus controlled by an intrinsic lattice resistance even before the introduction of obstacles, complicating reconciliation with the phenomenology of line tension models, which assume that dislocation are free elastic lines before interaction with obstacles. In studies of bcc crystal plasticity [8,9,[25][26][27][28], the Friedel stress αμb/ L is typically subtracted from the resolved shear stress to yield an effective shear stress which acts against the intrinsic lattice resistance. Using thermally activated dislocation mobility laws [8,24,26,27,29] the flow stress σ F instead emerges as that required to maintain a realistic strain rate.…”

“…Our main result is the identification of a regime controlled by a well-known size effect in the kink nucleation rate [1,27,33,34] operative on longer-length scales, where the activation energy for kink-limited dislocation motion halves from 2F k to F k once the average dislocation segment length L becomes greater than a stress and temperature-dependent critical length:…”