Hydrogen-Vacancy Interactions in Fe-C Alloys

Abstract

Energetics and concentrations of hydrogen-containing point defect clusters (PDCs) in Fe-C alloys are calculated and cast into a PDC dominance diagram. Because of the strong binding effects of iron vacancies on the stability of the clusters, hydrogen accumulation requires the total hydrogen and vacancy concentrations to be comparable. As a result of the interplay between repulsive and attractive binding processes, PDC populations in Fe-C-H effectively decouple into the binary systems Fe-C and Fe-H. This results in significant vacancy-hydrogen PDC populations even for low total hydrogen concentrations. DOI: 10.1103/PhysRevLett.103.085501 PACS numbers: 61.72.JÀ, 61.72.SÀ, 71.15.Mb, 71.15.Nc In the past few decades, a variety of fundamentally new phenomena has been observed in metals and their alloys under hydrogen-rich conditions, such as large volume contractions in body-centered cubic (bcc) -Fe [1,2], and the enhancement of diffusion at metal-metal junctions [3]. Furthermore, the well-known problem of hydrogeninduced degradation of the mechanical properties of metals has had significant impact in Fe-rich alloys such as hardened steels that suffer severe embrittlement [4,5]. Experimental and theoretical evidence [1][2][3][4][5][6][7] suggests that one unifying theme behind these hydrogen-mediated effects is the strong interaction between hydrogen impurities and other point defects in the material, and the subsequent microstructural changes that occur as a result of that interaction. Although a rich experimental literature detailing macroscopic effects exists, fundamental understanding of the interactions of hydrogen with point defect clusters (PDCs) in metals is limited. In particular, a solid theoretical description of PDC stability, accumulation, and the changes in the PDC concentrations is not yet available.In this Letter, we study the formation, binding, and concentrations of PDCs in bcc Fe-C alloys containing hydrogen. We use density functional theory (DFT) calculations for the energetics, coupled with a free-energy functional to determine the PDC concentrations as a function of total hydrogen and vacancy concentrations. The processes by which hydrogen binds to PDCs in Fe-C alloys are found to be dominated by the interaction of hydrogen (H) and carbon (C) with bcc iron vacancies (Va). By graphically presenting our results on the PDC concentrations in a dominance diagram, the important role of vacancies in determining the PDC populations is readily appreciated. The PDC dominance representation directly indicates the essential changes in the point defect population as a function of point defect composition in the alloy bulk, analogous in spirit to a phase diagram. An effective decoupling between the Va x H z and Va x C y populations is found, simplifying the full Fe-C-H system into a superposition of the PDC populations of the Fe-C and Fe-H binary systems. The strong Va-H interaction implies that the accumulation of hydrogen into PDCs comprising molecular hydrogen (H 2 ) is favored over PDCs containing atom...

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