Discrete stochastic model of point defect-dislocation interaction for simulating dislocation climb. (January 2021)

• Record Type: Journal Article
• Authors: McElfresh, Cameron; Cui, Yinan; Dudarev, Sergei L.; Po, Giacomo; Marian, Jaime
• Is Part Of: International journal of plasticity. Volume 136(2021)
• Publication Date: 2021-01
• Subjects: Dislocation climb -- Monte Carlo simulations -- Dislocation dynamics -- Vacancy diffusion -- Drift-diffusion process; Plasticity -- Periodicals; Plasticité -- Périodiques; Plasticity; Periodicals; 620.11233
• DOI: 10.1016/j.ijplas.2020.102848
• Languages: English
• ISSNs: 0749-6419

Abstract

Abstract: Dislocation climb is an important high temperature process in metals plasticity, responsible for the phenomena such as creep, swelling, or hardening. Climb is defined by the ability of dislocations to leave their original glide plane by interacting with point defects. As such, dislocation climb is controlled by point defect diffusion/absorption/emission, all of which involve thermal activation. The existing thermodynamically consistent models for climb are generally formulated in a continuum framework, through the definition of effective defect fluxes and climb propensities in response to thermodynamic driving forces. However, the point-wise discrete nature of vacancies (and/or self-interstitials) confers a highly discrete nature to the climb dynamics, which is also strongly affected by elastic forces. The combination of discreteness, thermal activation, and elasticity makes this process too challenging for direct atomistic methods such as molecular dynamics. Here we develop a kinetic Monte Carlo model that captures vacancy generation and transport kinetics acting in conjuction with the evolving elastic fields provided by discrete dislocation dynamics simulations. The two models are coupled via the applied stresses and stress gradients generated by dislocation structures at vacancy locations. Our simulations reveal two surprising results. First that climb is dominated by vacancy emission even when the background vacancy concentration is much higher than the equilibrium one. And, second, that climb velocities might be much faster than otherwise believed when one uses the classical theories of climb. These effects are due to the locality of vacancy-dislocation processes, which are not captured in classical treatments that assume smooth vacancy fluxes and homogeneous concentrations. We apply the method to study elementary climb processes in body-centered cubic iron and furnish climb mobility functions to be used in parametric dislocation dynamics and/or crystal plasticity simulations. We apply the technique to study non-conservative plastic bypass of spherical precipitates by edge dislocations and point out the differences between our discrete approach and existing continuum formulations. Highlights: We have developed a discrete stochastic model of vacancy evolution in the presence of arbitrary elastic fields furnished by discrete dislocation dynamics methods. Vacancy kinetics include emission, diffusion, and absorption, all rigorously coupled to the underlying dislocation fields. Dislocation-vacancy interactions change the dislocation microstructure with time, leading to updated elastic fields that are then inserted back into the vacancy kinetics simulator. The osmotic force has long been thought of as an 'unbiased' force (i.e. no mechanical driving force). We now show that the osmotic force indeed represents a bias, albeit a second-order one, via its connection to stress gradients. Contrary to classical theories, our work shows that the dislocation climb mobilities depend on the local nature of dislocation-vacancy interactions. As such, the vacancy concentration at the local level can suffer heavy fluctuations with respect to the global value. Climb mobilities derived from our discrete method are over two of orders of magnitude larger than classical ones. This could have important implications on the time scales of dislocation climb and high-temperature creep.