Hydrogen embrittlement, revisited by in situ electrochemical nanoindentation Dissertation Zur Erlangung des Grades des Doktors der Ingenieurwissenschaften (Dr.-Ing.) der Naturwissenschaftlich-Technischen Fakultät III Chemie, Pharmazie, Bio- und Werkstoffwissenschaften der Universität des Saarlandes Von Dipl.-Ing. Afrooz Barnoush Saarbrücken, 2007 Eingereicht am: 16.10.2007 Tag der Kolloquiums: 14.03.2008 Dekan: Prof. Dr. Uli Müller Vorsitzender: Prof. Dr. Hempelmann Berichterstatter: Prof. Dr. H. Vehoff Prof. Dr. W. Arnold Prof. Dr. R. Johnsen Akad. Mitarbeiter: Dr. Isabella Gallino CONTENTS LIST OF FIGURES................................. vi LIST OF TABLES.................................. xvi ACKNOWLEDGMENT............................... xvii ABSTRACT...................................... xix ZUSAMMENFASSUNG............................... xx ACRONYMS..................................... xxiii SYMBOLS....................................... xxv 1. Introduction....................................1 2. Hydrogen Embrittlement............................5 2.1 Phenomenology of hydrogen embrittlement..............5 2.2 Entry of hydrogen into metals......................8 2.2.1 Gas phase..............................8 2.2.2 Liquid phase............................9 2.2.2.1 Mechanism of the cathodic evolution of hydrogen from aqueous electrolytes............... 10 2.2.2.2 Entry of electrolytic hydrogen into metals..... 13 2.2.2.3 Promoter of hydrogen entry into metals...... 14 2.3 Hydrogen interaction with defects in metal.............. 14 2.3.1 Point defects............................ 15 2.3.2 Solutes and solute-defect complexes.............. 16 2.3.3 Dislocations............................. 17 2.3.4 Internal boundaries........................ 19 2.4 Experimental methodologies of HE study............... 20 2.4.1 Conventional Methods...................... 22 2.4.2 Environmental transmission electron microscopy...... 25 ii 2.5 HE mechanisms.............................. 28 2.5.1 Hydride-induced embrittlement................. 28 2.5.2 Hydrogen enhanced decohesion................. 28 2.5.3 Hydrogen enhanced localized plasticity............ 31 2.6 A new approach to HE study....................... 32 3. NI-AFM...................................... 35 3.1 Nanoindentation.............................. 35 3.2 Contact Mechanics............................. 36 3.3 Depth sensing nanoindentation..................... 48 3.4 Instrumentation.............................. 55 3.4.1 Hysitron Triboscope........................ 55 3.4.2 Nanoindentation tips....................... 59 3.5 NI-AFM in liquid.............................. 63 3.5.1 Complexities of NI in liquid................... 64 3.5.1.1 Meniscus force...................... 65 3.5.1.2 Buoyant force...................... 66 3.5.2 Controlling the forces acting on the tip............ 67 3.6 Indentation phenomena.......................... 68 3.6.1 Geometry-based phenomena................... 69 3.6.1.1 Surface roughness................... 69 3.6.1.2 Inhomogeneities..................... 71 3.6.1.3 The indentation size effect.............. 72 3.6.2 Material-based phenomena................... 74 3.6.2.1 Pile-up and Sink-in................... 74 3.6.2.2 Phase transformation................. 77 3.6.2.3 Pop-in........................... 77 3.6.2.4 Creep........................... 84 3.6.2.5 Fracture......................... 85 4. Experimental................................... 87 4.1 General aspects of sample preparation................. 87 4.1.1 Electropolishing procedure.................... 90 4.2 Mechanical property measurements.................. 92 4.2.1 Microindentation.......................... 92 4.2.2 Nanoindentation.......................... 93 4.3 In situ electrochemical NI-AFM..................... 93 iii 5. Results....................................... 99 5.1 Why not ex situ tests?........................... 100 5.1.1 Ex situ electrochemical hydrogen charging.......... 100 5.1.1.1 Nanoindentation measurements on ex situ charged nickel........................... 100 5.1.1.2 Microhardness measurements on ex situ charged nickel........................... 103 5.1.2 Ex situ hydrogen hydrogen charging in autoclave...... 106 5.2 In situ ECNI-AFM tests on copper................... 107 5.3 In situ ECNI-AFM tests on aluminum................. 111 5.3.1 ECNI-AFM of aluminum in pH 6, sulfate solution...... 118 5.3.2 ECNI-AFM of aluminum in pH 8.9, borate buffer...... 118 5.3.3 pH effect on pop-in load in aluminum............. 122 5.4 In situ ECNI-AFM tests on Fe-3wt.%Si................ 126 5.5 In situ ECNI-AFM tests on a FeAl intermetallic alloy........ 133 5.6 In situ ECNI-AFM tests on Nickel................... 140 5.6.1 Time delay experiments..................... 145 5.7 In situ ECNI-AFM tests on stainless steels.............. 151 5.7.1 Austenitic Stainless steel..................... 151 5.7.2 Super Duplex Stainless steel.................. 160 5.7.3 Hydrogen effect on stainless steels............... 173 6. Discussion..................................... 183 6.1 Indentation induced homogeneous dislocation nucleation..... 183 6.2 Hydrogen effect on dislocation nucleation............... 193 6.2.1 Shear modulus........................... 204 6.2.2 Stacking fault energy....................... 206 6.2.3 Dislocation core radius...................... 208 6.3 Time delay experiments.......................... 210 7. Conclusion and outlooks............................. 213 7.1 Conclusion.................................. 213 7.2 Outlooks................................... 214 7.2.1 Micro compression tests..................... 215 7.2.2 Low temperature ECNI-AFM.................. 215 APPENDIX iv A. Pop-in finder program.............................. 218 B. In situ ECNI-AFM operation.......................... 220 B.1 Overview................................... 220 B.2 Procedure.................................. 221 B.2.1 Starting the software....................... 221 B.2.2 Install the sample in the electrochemical cell......... 221 B.2.3 Install the electrochemical cell on the microscope stage.. 222 B.2.4 Install the nanoindentation head................ 223 B.2.5 Align the nanoindentation head on the microscope..... 224 B.2.6 Put the microscope inside the chamber............ 225 B.2.7 Engage the tip in air....................... 225 B.2.8 Engage the tip in electrolyte................... 226 PUBLICATIONS................................... 227 Peer-reviewed publications........................... 227 Conference papers................................ 228 REFERENCES.................................... 229 INDEX......................................... 256 v LIST OF FIGURES 1.1 Global description of HE interaction aspects...............2 2.1 Damage parameter for different single-crystalline and polycrystalline super-alloys..................................6 2.2 Schematic of critical variables affecting the threshold values (KTH) and the crack growth rate da/dt.......................7 2.3 Schematic diagram of the metal/electrolyte interface, showing fully and partially solvated ions.......................... 11 2.4 Schematic presentation of defects in metal and accumulation of hy- drogen atoms in the low-concentration range............... 15 2.5 Embrittlement index from 465 tests on 34 different steel grades as a function of yield stress........................... 22 2.6 The effect of hydrogen charging condition and temperature on σUTS(H ydrogen) versus σUTS(Air) ................................. 23 2.7 The effect of in situ hydrogen charging on the flow stress of high purity iron at various temperatures.................... 24 2.8 The effect of hydrogen on the mobility of dislocations in ®-Ti..... 26 2.9 Reduction of the separation distance between dislocations in a pileup in 310s stainless steel due to solute hydrogen.............. 27 2.10 The dependence of in situ measured crack tip opening angle, ®, on hydrogen pressure for Fe-3wt%Si..................... 29 2.11 Crack tip opening angles obtained in Fe-3wt%Si single crystals after straining..................................... 30 3.1 Schematic of the interaction between a rigid spherical indenter and a flat surface.................................. 37 3.2 Graphical representation of the σr in Huber stress tensor...... 41 vi 3.3 Graphical representation of the σθ in Huber stress tensor...... 42 3.4 Graphical representation of the σz in Huber stress tensor...... 43 3.5 Graphical representation of the ¿rz in Huber stress tensor...... 44 3.6 Graphical representation of the σ1 principle stress........... 45 3.7 Graphical representation of the σ3 principle stress........... 46 3.8 Graphical representation of the ¿13 principle shear stress....... 47 3.9 Loading profile and the resulted load displacement curve....... 49 3.10 Representative load-displacement data demonstrating differences in elasticity.................................... 50 3.11 Schematic of an indenter at maximum load Pmax with an associated total depth of hmax............................... 53 3.12 Schematic of the NI-AFM system..................... 56 3.13 Schematic circuit diagram of the Hysitron TriboScope transducer assembly..................................... 57 3.14 Schematic of a three plate capacitor force-displacement transducer of the Hysitron Triboscope.......................... 57 p 3.15 Graph of 1/S versus 1/ Pmax for a series of indents performed in fused quartz.................................. 59 3.16 Schematic of nanoindentation probe tips................. 60 3.17 Schematics of a Berkovich