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Title:A new approach to discovering the fundamental mechanisms of hydrogen failure
Author(s):Martin, May
Director of Research:Robertson, Ian M.
Doctoral Committee Chair(s):Robertson, Ian M.
Doctoral Committee Member(s):Sofronis, Petros; Bellon, Pascal; Dillon, Shen J.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Hydrogen Embrittlement
Failure
Liquid Metal Embrittlement
Microstructure
Transmission electron microscopy
Focused ion beam
Fractography
Abstract:The recent interest in developing a hydrogen-based energy economy has put the need for hydrogen compatible materials to the fore-front. In particular, metals that are resistant to the effects of hydrogen embrittlement are needed. Despite the long history of study into the embrittlement of metals by hydrogen, there is little consensus as to the mechanism by which hydrogen degrades the mechanical response. Contributing to this situation is a lack of experimental data at the microstructural and atomistic scales. To begin to address this problem, this present work examines the microstructure immediately beneath fracture surfaces and relates it to the fracture surface in an effort to understand the microstructural deformation processes leading to failure. This was done through a variety of techniques, including scanning electron microscopy and atomic force microscopy studies of the fracture surface, and the adaptation of the lift-out technique on the focused ion beam microscope to extract thin foils from fracture surface features for examination in the transmission electron microscope. A variety of structural materials under different loading conditions were studied to get a broad view of the underlying effects of hydrogen. The materials and loading conditions were chosen to represent materials of interest for a Hydrogen Economy and loading conditions likely to be encountered. Pipeline steels, considered for hydrogen transport, were loaded in compact tension configuration in high pressure hydrogen gas. Nickel, charged with a high concentration of hydrogen, was tested in uniaxial tension. Two stainless steel alloys, used in manifolds, fueling hoses, and valves, were fatigue loaded. To contrast the effect of hydrogen with that of liquid metals, which are considered by some to operate by similar mechanisms, a martensitic steel, candidate material for nuclear reactors, and a commercial purity iron were loaded in center crack in tension mode in contact with liquid metal. Each of these samples displayed fracture modes typical of hydrogen embrittlement. The microstructures beneath these typical features are complex and showed striking commonalities between the different materials and loading conditions. The complexity and extent of the plasticity observed beneath “brittle” fracture features suggests that extensive plasticity occurs prior to cracking. The evidence suggests that hydrogen accelerates this plasticity. From this, it can be concluded that inferences about the underlying microstructure based upon fracture surface morphologies tend to be erroneous, due to the overly simple models of deformation that are assumed to operate. From these observations, a new mechanism for hydrogen embrittlement is proposed based upon previously published mechanisms. Hydrogen accelerates the plasticity, resulting in work-hardening of the material and redistribution of the solute hydrogen, leading to locally high concentrations. This leads to the weakening of local features by the combination of the strain and hydrogen concentration. The weakest microstructural features fail, and fracture follows the weakest path through the material.
Issue Date:2013-05-24
URI:http://hdl.handle.net/2142/44358
Rights Information:Copyright 2013 May Martin
Date Available in IDEALS:2013-05-24
Date Deposited:2013-05


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