Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.740349
Title: Modelling early stages of hydrogen embrittlement and surface oxidation of iron using density functional theory
Author: Chohan, Urslaan
ISNI:       0000 0004 7225 7032
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2018
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Abstract:
In this project, I have modelled the adsorption and diffusion of hydrogen and oxygen on and through the three low-index planes of two phases of iron, namely the body-centred cubic ferromagnetic alpha iron, alpha-Fe, and face-centered cubic gamma iron phase, gamma-Fe. This was done using spin-polarised Density Functional Theory, and the minimum energy path for the diffusion calculation was derived from potential energy surfaces created from a tight 3D mesh through the crystal. It was found that oxygen and hydrogen atoms strongly chemisorb on the (110) phase. Oxygen strongly chemisorbs on alpha-Fe(110) at the quasi-threefold site, with a surface stretch ~500 cm-1 for higher coverage. The structural changes at the highest coverage (>0.5 ML) indicated the incipient formation of FeO(111) from the O-Fe(110) overlay. Studying the electronic properties of the formation of FeO(111) yields an understanding of the earliest stage of oxide formation. Hydrogen was found to strongly chemisorb on the (110) surface of alpha-Fe. The hydrogen adsorbs at the quasi-threefold site with an adsorption energy of ~3 eV/H atom and surface stretches at ~1100 cm-1 for higher coverages. The (111) surface of gamma-Fe has been found to have the highest barrier for bulk-like diffusion. The bulk-diffusion barrier for hydrogen through gamma-Fe is ~0.7 eV for the (111) surface, which is ~0.2 eV higher than the (110) surface. The presence of magnetism in the (001) surface of gamma-Fe resulted in a lowering in the bulk-like diffusion barrier, with an ~0.2 eV barrier in the ferromagnetic surface as opposed to the ~0.6 eV in the non-magnetic surface. The high barrier for the (111) surface of gamma-Fe demonstrates that producing textured austenitic steel components with this surface exposed to the hydrogen source may work to lower the hydrogen damage in these samples. The strong effect of magnetism in lowering the barrier for diffusion demonstrates the importance of avoiding ferromagnetic austenitic steel alloys in environments where hydrogen is in abundance. These results may be applied in the process of development of Gen IV fission and fusion reactors. Ferritic and austenitic steels are ideal candidates for a number of components in these reactors, such as the first wall/breeding blanket. There is an abundance of presence of hydrogen in nuclear reactors. Hydrogen may enter the metallic matrix through diffusion processes, leading to the embrittlement of these components. Additionally, oxygen is readily present in the environment, which may oxidise components. In this project, I have modelled the adsorption and diffusion of hydrogen and oxygen on and through the three low-index planes of two phases of iron, namely the body-centred cubic ferromagnetic alpha iron, alpha-Fe, and face-centered cubic gamma iron phase, gamma-Fe. This was done using spin-polarised Density Functional Theory, and the minimum energy path for the diffusion calculation was derived from potential energy surfaces created from a tight 3D mesh through the crystal. It was found that oxygen and hydrogen atoms strongly chemisorb on the (110) phase. Oxygen strongly chemisorbs on alpha-Fe(110) at the quasi-threefold site, with a surface stretch ~500 cm-1 for higher coverage. The structural changes at the highest coverage (>0.5 ML) indicated the incipient formation of FeO(111) from the O-Fe(110) overlay. Studying the electronic properties of the formation of FeO(111) yields an understanding of the earliest stage of oxide formation. Hydrogen was found to strongly chemisorb on the (110) surface of alpha-Fe. The hydrogen adsorbs at the quasi-threefold site with an adsorption energy of ~3 eV/H atom and surface stretches at ~1100 cm-1 for higher coverages. The (111) surface of gamma-Fe has been found to have the highest barrier for bulk-like diffusion. The bulk-diffusion barrier for hydrogen through gamma-Fe is ~0.7 eV for the (111) surface, which is ~0.2 eV higher than the (110) surface. The presence of magnetism in the (001) surface of gamma-Fe resulted in a lowering in the bulk-like diffusion barrier, with an ~0.2 eV barrier in the ferromagnetic surface as opposed to the ~0.6 eV in the non-magnetic surface. The high barrier for the (111) surface of gamma-Fe demonstrates that producing textured austenitic steel components with this surface exposed to the hydrogen source may work to lower the hydrogen damage in these samples. The strong effect of magnetism in lowering the barrier for diffusion demonstrates the importance of avoiding ferromagnetic austenitic steel alloys in environments where hydrogen is in abundance. These results may be applied in the process of development of Gen IV fission and fusion reactors. Ferritic and austenitic steels are ideal candidates for a number of components in these reactors, such as the first wall/breeding blanket. There is an abundance of presence of hydrogen in nuclear reactors. Hydrogen may enter the metallic matrix through diffusion processes, leading to the embrittlement of these components. Additionally, oxygen is readily present in the environment, which may oxidise components.
Supervisor: Jimenez-Melero, Enrique ; Koehler, Sven Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.740349  DOI: Not available
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