Change in the electron structure caused by C, N and H atoms in iron and its effect on their interaction with dislocations

Based on the ab initio calculations of the electron structure, it is shown that nitrogen and hydrogen in the iron increase the state density at the Fermi level, whereas carbon decreases it, which is consistent with the available experimental data of conduction electron spin resonance. The results of calculations and measurements of the strain dependent internal friction and Snoek-Koster relaxation are used for the analysis of the interaction between dislocations and mobile or immobile interstitial atoms. It is shown that, if the moving dislocations in the iron-based austenite and ferrite are accompanied by the clouds of interstitial atoms, nitrogen and hydrogen increase the velocity of dislocations, whereas carbon decreases it. The obtained results are interpreted in terms of the local change in the density of free electrons within the dislocation atmospheres resulting in the corresponding change in the line tension of dislocations. Possible consequences for mechanical properties are discussed using the examples of hydrogen embrittlement and nitrogen-caused quasi-cleavage of austenitic steels. (c) 2005 Acta Materialia, Inc. Published by Elsevier Ltd. All rights reserved.