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  • 2014 Edward DeMille Campbell Memorial LectureASM Internatinal

    Hydrogen Embrittlement Understood

    IAN M. ROBERTSON, P. SOFRONIS, A. NAGAO, M.L. MARTIN, S. WANG,D.W. GROSS, and K.E. NYGREN

    The connection between hydrogen-enhanced plasticity and the hydrogen-induced fracture me-chanism and pathway is established through examination of the evolved microstructural stateimmediately beneath fracture surfaces including voids, quasi-cleavage, and intergranularsurfaces. This leads to a new understanding of hydrogen embrittlement in which hydrogen-enhanced plasticity processes accelerate the evolution of the microstructure, which establishesnot only local high concentrations of hydrogen but also a local stress state. Together, thesefactors establish the fracture mechanism and pathway.

    DOI: 10.1007/s11663-015-0325-y The Minerals, Metals & Materials Society and ASM International 2015

    I. INTRODUCTION

    JOHNSON, in 1875, launched the field of study ofhydrogen embrittlement of metals with the publication

    of his observations of the influence of immersing apiece of iron in different acids on the mechanicalproperties.[1] He summarized the macroscale effect as:

    IAN M. ROBERTSON, Dean of the College of Engineering and aProfessor, is with Department of Materials Science and Engineering andDepartment of Engineering Physics, University of Wisconsin Madison,Madison, WI 53706, and also with International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744Motooka, Nishi-ku, Fukuoka, Fukuoka 819-0395, Japan. Contact e-mail:[email protected] P. SOFRONIS, James W. Bayne Professor, iswith theDepartment ofMechanical Science andEngineering,University ofIllinois at Urbana-Champaign, Urbana, IL 61801, and also withInternational Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University. A. NAGAO, Senior Researcher, is withInternational Institute for Carbon-Neutral Energy Research (WPI-I2CNER), KyushuUniversity, and also withMaterial Surface & InterfaceScience Research Department, Steel Research Laboratory, JFE SteelCorporation, 1-1 Minamiwatarida-cho, Kawasaki-ku, Kawasaki, Kana-gawa210-0855, Japan.M.L.MARTIN,HumboldtPostdoctoralFellow, iswith Institut fur Materialphysik, Georg-August-Universitat Gottingen,Friedrich-Hund-Platz 1, 37077 Gottingen, Germany. S. WANG, Post-doctoral Fellow, is with International Institute for Carbon-Neutral EnergyResearch (WPI-I2CNER), Kyushu University, and also with Division ofMaterials Science and Engineering, Graduate School of Engineering,Hokkaido University, N-13, W-8, Sapporo, Hokkaido 060-8628, Japan.D.W. GROSS, and K.E. NYGREN, Graduate Students, are with the

    Department ofMaterials Science and Engineering, University of Illinois atUrbana-Champaign, Urbana, IL 61801.

    Ian M. Robertson is the Dean of the College of Engineering and aprofessor in Materials Science and Engineering as well as EngineeringPhysics at theUniversityofWisconsin-Madison.His research focuseson theuse of the electron microscope as an experimental laboratory in whichdynamic experiments can be conducted to reveal the atomistic processesresponsible for the macroscopic response of a material. He has applied thistechnique to enhance our understanding of the reaction pathways andkinetics that occur during deformation, phase transformation, irradiationand hydrogen embrittlement of metallic materials. His insight to themechanisms responsible for hydrogen embrittlement of metals was recog-nized by the Department of Energy in 1984 when he, along with HowardBirnbaum, received the DOE prize for Outstanding Scientific Accomplish-ment in Metallurgy and Ceramics. In 2011, he received the DOE EE FuelCell Program award for contributions to our understanding ofmechanismsof hydrogen embrittlement.Hewas selected recently as the 2014 recipient ofthe ASM Edward DeMille Campbell Memorial Lectureship. He is theEditor-in-Chief of the review journal Current Opinion in Solid State andMaterials Science.

    Manuscript submitted January 17, 2015.Article published online March 28, 2015.

    IAN M. ROBERTSON

    METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 46B, JUNE 20151085

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  • After a few minutes immersion (half a minute willsometimes suffice) in strong hydrochloric or dilutesulfuric acida piece breaking after being bent onceon itself, while before immersion it would bear bendingon itself and back again two or three times beforebreaking. He further noted that evident to any oneby the extraordinary decrease in toughness and break-ing-strain of the iron so treated, and is all the moreremarkable as it is not permanent, but only temporaryin character, for with lapse of time the metal slowlyregains its original toughness and strength. AlthoughJohnsons description remains appropriate, it now hasbeen shown to occur in most metals within a narrowtemperature and strain rate range and is reversible if thehydrogen is removed. These signature dependencies ofhydrogen embrittlement are important to remember asany approach to discern the physical mechanism mustdemonstrate them. Furthermore, the mechanism mustbe universal as it would be extremely unfortunate if itwas not broadly applicable across a particular class ofhydrogen-induced failures. Of course, it is recognizedthat there is more than one class and more than onemechanism may exist. For example, the hydride forma-tion and cleavage mechanism certainly operates inGroup Vb pure metals and their alloys, as well as intitanium and zirconium.[2,3] However, even in thesesystems, other mechanisms may operate, but not neces-sarily dominate the response.[4] Despite the extensivedatabase of hydrogen-induced failures of metallic struc-tures, the fundamental mechanisms remain in debate;for details on hydrogen effects on mechanical properties;for details the reader is referred to recent conferenceproceedings, book chapters, and references therein.[5,6]

    Discovery of the underlying fundamental mechanismsof hydrogen embrittlement is a complex multi-facetedchallenge that requires knowledge of the hydrogensource, and how hydrogen molecules interact with theoxide on the metal surface, then how hydrogen entersthe base metal, diffuses through the lattice or alonggrain boundaries, interacts with and is trapped bymicrostructural features, modifies the properties ofmetals, moves with mobile dislocations, and results inductile, quasi-cleavage, and intergranular fracturemodes. The emphasis of this manuscript is on themodifications to properties of dislocations and defor-mation mechanisms by hydrogen, hydrogen-defect in-teractions, and how these link to the hydrogen-inducedfracture mode and path. Specifically, the emphasis ofthis manuscript is captured in two assertive statementsmade by Beachem in 1972,[7] namely, that the hydro-gen-steel interaction appears to be an easing of disloca-tion motion or generation, or both, and fracturemodes operate depending upon the microstructure.Evidence supporting the former has been providedprimarily by the deformation experiments performedin an electron microscope equipped with an environ-mental cell;[8,9] key results of this work are described indetail later. Experimental evidence supporting a corre-lation between the microstructure and the fracture modehas been lacking. However, progress in establishing thislinkage has been enabled through recent advances inexperimental tools and methods. For example, focused-

    ion beam machining affords the opportunity to extract avolume of material from a site-specific location, includ-ing ones on complex fracture surfaces for subsequentexamination in a transmission electron microscope.[10]

    This approach has been used to determine themicrostruc-ture that has evolved beneath different types of hydrogen-induced fracture surfaces including flat features onpipeline steels,[11] quasi-cleavage fracture surfaces onpipeline steels,[12] microvoids,[13] quasi-cleavage andintergranular surfaces in lath martensite steels,[13,14] andintergranular facets in Ni[15] and Fe.[16] As detection ofhydrogen at specific locations remains one of the out-standing issues in hydrogen-induced degradation ofmetals, it is inferred from observed responses. Theremainder of thismanuscript begins with a brief summaryof themechanisms that have been proposed to account forhydrogen embrittlement; a description of key experimen-tal findings supporting hydrogen-enhanced dislocationmotion; examination of evolved microstructure and itscorrelation to the fracture mechanism and pathway; andends with discussion and conclusions.

    II. PROPOSED MECHANISMS

    Numerousmechanisms have been proposed to accountfor hydrogen embrittlement and these have been reviewedextensively.[17-19] The proposed mechanisms include:

    High hydrogen pressure bubble or void;[20-22] Hydrogen-induced reduction in surface energy;[23,24] Hydrogen-enhanced dislocation ejection from the

    surface or near surface region;[17]

    Hydrogen-induced reduction in cohesive strength;[6,18,25-28] Hydrogen-enhanced localized plasticity;[7,9,29-31] Hydrogen- and deformation-assisted vacancy pro-

    duction;[32-34]

    Hydrogen-triggered ductile to brittle transition;[35,36] Hydride formation and cleavage;[2,3] Hydrogen- and strain-induced phase transforma-

    tions;[37,38] and Reactants and hydrogen.[39]

    Of these possibilities, the operative ones depend onthe material, the hydrogen charging conditions, and theloading. For example, the high pressure bubble mechan-ism is related to the precipitation of H2 and it occursunder high fugacity charging conditions. The hydrideformation and cleavage mechanism operates in GroupVb pure metals and their alloys as well as in titaniumand zirconium systems.[2,3] I