ICAMS / Interdisciplinary Centre for Advanced Materials Simulation

Light elements in iron and steel: Interfaces

The central goal of the project was to contribute to the understanding of hydrogen embrittlement in steels on the atomistic level by density-functional theory calculations. The addressed aspects include (a) the modification of elastic constants by interstitial hydrogen and external strain and (b) the interaction of H with solute atoms of other transition metals. Therefore, we studied the modification of the elastic properties of bcc-Fe by hydrostatic strain and by interstitial hydrogen. From our density-functional theory calculations, we observe a significant linear decrease in the elastic constants with increasing hydrostatic strain or with increasing concentration of interstitial hydrogen. We used the elastic constants from our single-crystal ab initio calculations to examine the dependence of the poly-crystalline elastic moduli B, E and G on the hydrogen concentration, and find good agreement with the few available measurements. Our results are in qualitative agreement with experimental observations of reduced shear modulus for a locally increased H concentration.

Shear Modulus (hardness):calculated for any plane sheared in any direction.

The extent of hydrogen embrittlement in steel depends strongly on the H distribution in the microstructure. Alloying elements might serve to detract hydrogen from regions prone to embrittlement and to distribute it within areas where it causes less damage. We investigated the interaction of interstitial hydrogen with substitutional transition-metal atoms in a-Fe. The TM solute atom causes an increase of the unit cell volume for nearly all TM elements, in line with the TM atomic volumes. For each TM element, the distance dependence of the TM–H interaction shows a range of about 5 Å with a monotonic decrease except for a notable deviation at separation distances of approximately 4 Å. This deviation is apparent for all TMs and corresponds to geometric arrangements close to elastic extremal axes. By comparing the TM–H interaction energy for the second and third nearest-neighbour separations across the TM series, we find an attractive interaction between H and the TMs with a nearly full or empty d-shell while the central d-band elements repel hydrogen.

Project Files:

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Left vertical axis scale: variation of stress–strain coefficients with H and volume-expected variation. Right vertical axis scale: difference between total and volume effect (dashed lines).
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Interaction of substitutional TM atom across the 4d series with an interstitial H atom in the second and third nearest-neighbour (NN) site. The solid (dashed) lines correspond to 3x3x3 and 4x4x4 supercell calculations.

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