ICAMS / Interdisciplinary Centre for Advanced Materials Simulation
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Modification of mechanical properties of alpha-iron by hydrogen

Date: 30.11.2010
Place: Materials Research Society (MRS) Fall Meeting 2010, Boston, USA

Demetra Psiachos
Thomas Hammerschmidt
Ralf Drautz

The design of modern steels with tailored mechanical properties requires an understanding of the fabrication process and a reliable prediction of failure during operation. The complex interplay of effects on different length and time scales hampers a full understanding of the contribution of the microscopic processes causing hydrogen embrittlement in steels. Different mechanisms have been proposed in the literature to explain how the presence of hydrogen triggers mechanical failure in many steels. Our aim is to use the results obtained from atomistic simulations of the materials properties of relatively simple systems, in models on the mesoscopic length scale of realistic steels undergoing mechanical loading. To that end, we have parameterised the results of our atomistic simulations for subsequent incorporation into higher length-scale schemes. We will describe the results from our calculations of the modification of the elastic properties of alpha-iron as a function of hydrogen concentration. Upon determining the dependence of the stress-strain coefficients on H concentration using first-principles electronic structure theory, we then used these results in continuum elasticity theory in order to examine the effect of H on the anisotropic elastic moduli. We find that increasing the hydrogen concentration reduces the elastic moduli. We analyze the various contributions behind the effect of hydrogen on the material properties of iron. We find that while hydrogen leads to overall softening, most of this behaviour is caused by hydrogen increasing the local volume rather than by electronic effects. Using these results for the effect of H on the elastic properties of alpha-iron, we have further derived expressions for the dependence of the solubility of H, and by extension, the local concentration of H, on strain fields typical of those found near dislocations. Our analytical expressions for the modification of H-solubility are in excellent agreement with our first-principles results. We will describe some of the applications to the mesoscale which will hopefully aid in achieving greater understanding of the underlying mechanisms of H embrittlement.

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