On the Nature of Similarity in Embrittlement of Metals by Hydrogen and Surfactants

Abstract

A mechanism for a similar effect of hydrogen and surface-active elements, surfactants, on the structure and properties of metals is discussed based on studies of austenitic steels and using the ab initio calculations and experimental measurements of the electron structure, mechanical spectroscopy and tension tests. The similarity is originated from the local enhancement of the metallic character of interatomic bonds. The obtained results are interpreted within the frame of the electron approach to the hydrogen-enhanced localized plasticity phenomenon, HELP, in hydrogen embrittlement. The surfactants increase the density of electron states at the Fermi level and, correspondingly, the concentration of free electrons. As a response, a local decrease of the shear modulus resulted in the initiation of dislocation sources, decrease of the line tension of dislocations and distance between their assembles is expected, which facilitates the opening and propagation of cracks and mechanical degradation. The comparison with the adsorption-induced dislocation emission hypothesis, AIDE, for hydrogen and liquid metal embrittlements is carried out. Keywords: On the Nature of Similarity in Embrittlement of Metals by Hydrogen and Surfactants 2/10Copyright: ©2017 Gavriljuk et al.Important for clarification of the operative mechanism was the observation that, including LME, embrittlement by surfactants is accompanied by a local increase of plasticity [22][23][24]. In other words, the locally enhanced plastic deformation precedes the macrobrittle fracture. This unusual behavior was firstly interpreted as a local disintegration of solids resulted in colloidal disperse systems [4,5]. Later, such approach was supported by the ideas about liquid channels [14,15] and corresponding grain boundary phase transitions [25,26].However, such interpretation of preceded plastic deformation is seriously discredited by a remarkable similarity between LME and hydrogen embrittlement, HE, where local plastic flow always precedes the fracture. Systematic comparative studies of LME and HE were carried out by Lynch [27]. Based on these studies, he has proposed the adsorption-induced localized-slip process to be responsible for both the LME and HE phenomena [28] and, correspondingly, developed the hypothesis of adsorption-induced dislocation emission, AIDE [29]. Its main idea amounts to the adsorption of hydrogen and surfactant atoms at the internal crack tips, which facilitates the nucleation and emission of dislocations. Once nucleated, the dislocations can readily move away from the crack tip under applied stress.Along with AIDE hypothesis, hydrogen embrittlement of metals is described by hypotheses of hydrogen-enhanced decohesion, HEDE, hydrogen-enhanced localized plasticity, HELP, and hydrogen-enhanced strain-induced vacancies, HESIV. Their critical analysis is presented, e.g., in the review articles [29,30]. The AIDE and HELP hypotheses seem to be the most acceptable for application to the both HE and LME processes.Probably first time, the ...