2024 journal article
An atomistic-to-microscale characterization of the kink-controlled dislocation dynamics in bcc metals through finite-temperature coarse-grained atomistic simulations
ACTA MATERIALIA, 262.
Adopting bcc tungsten (W) as a model material, we characterize the temperature and stress dependence of kink dynamics on a dislocation line with length L ranging from 60 nm to 1 μm using finite-temperature coarse-grained (FT-CG) atomistic simulations. The main novelty of this work is to accommodate major salient aspects, namely the motion of μm-long dislocation lines, the atomic-scale kink dynamics, and the full spectrum of phonon dynamics, all in one single FT-CG model. At a fraction of the cost of molecular dynamics (MD) calculations, the FT-CG simulation predicts: (a) a dislocation-induced degeneration of the phonon density of states (PDoS) of W; (b) the kink-induced dislocation core structure transition from a “soft” (non-planar, compact) to a “hard” configuration (planar, split); and (c) the crossover from the line tension (LT) regime to the elastic interaction (EI) regime in the temperature dependence of the flow stress. Several findings arise from the simulations: (1) the kink activation stress, σf, not only depends on the temperature, T, but also exhibits a sensitivity to the dislocation line length, L. For μm-long dislocations, it approaches experimental results but σf for kink activation on nm-long dislocations does not; (2) upon an increase of T, the σf reduction for the sample containing μm-long dislocations is significantly larger than that for the one with nm-long dislocations; (3) based on data extracted from FT-CG simulations of “temperature jump tests”, the L- dependence of the kink activation enthalpy, ΔH, is characterized. It can be as high as ∼3 eV for a dislocation with a length of tens of nm but reduces to an experimentally comparable level of ∼1.5 eV when L is 0.3 μm or longer. This suggests an easier kink activation on a longer dislocation. Such a dislocation line length dependence of ΔH can be further amplified at an even lower applied stress; (4) the entropic kink activation barrier, ΔHT, is linearly proportional to T. The slope of the ΔHT – T relation, however, will be largely underestimated in nanoscale MD simulations, but can be comparable with that from experiments when L is ∼ 0.3 μm or longer. These findings highlight the limitations of nanoscale MD models in simulating kink-controlled dislocation dynamics. The knowledge gained here can support the development of mobility laws that incorporate the stress-, temperature-, and line length-dependence all into one formulation for understanding plasticity in bcc metals and other high-Peierls-stress alloys.