ICAMS / Interdisciplinary Centre for Advanced Materials Simulation


Atomistic modelling of diffusion processes of light elements in metals

Date: 17.06.2014
Place: CIMTEC 2014: 6th Forum on New Materials, Montecatini Terme, Italy

Jutta Rogal

The mobility of light elements such as hydrogen, carbon or boron, and their segregation behaviour towards point and extended defects play an important role in understanding the mechanical properties of metals. In iron and steels structural defects such as vacancies, grain boundaries, and dislocations can, e.g., trap hydrogen, and a local accumulation of hydrogen at these defects can lead to a degradation of the materials properties. This so-called hydrogen embrittlement is a complex problem that is still not fully understood. An important aspect in obtaining insight into hydrogen embrittlement on the atomistic level is to understand the diffusion of hydrogen in these materials.

Modelling the diffusion of hydrogen requires atomistic simulations over extended time scales that go far beyond what can be reached with regular molecular dynamics simulations. Here, we apply a kinetic Monte Carlo (KMC) approach combined with highly accurate ab initio calculations to model hydrogen diffusion in BCC iron including the effect of point and extended defects. All input data to the KMC model, such as available sites, solution energies, and diffusion barriers are obtained using density functional theory. In particular we investigate the effect of different microstructures on the distribution and mobility of hydrogen. We find that hydrogen mainly diffuses within interface regions with an overall diffusivity that is lower than in pure BCC bulk iron. Based on our numerical results we can furthermore derive an analytic expression to describe the macroscopic diffusion behaviour as a function of hydrogen concentration and temperature.

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