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Structure and Mechanical Properties of Some Titanium

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Structure and Mechanical Properties of Some Titanium STRUCTURE AND MECHANICAL PROPERTIES OF SOME TITANIUM —HYDROGEN ALLOYS. A thesis submitted for the degree of Doctor of Philosophy in the University of London by STEWART MURRAY, BSc. Metallurgy Department, Imperial College, London, SW7 2BP. March, 1973. - 2 - ABSTRACT The microstructural effects of hydrogen on commercial purity (c.p.) IMI Till5 have been investigated with a more limited study of the a alloy Ti-5A1-21Sn and the a/ one Ti-6A1-4V. The main features studied in alloys of c.p. Ti with 0.8-7.0 at.% H2 have been (i) hydride platelet size as a function of cooling rate from the a (Ti) phase field and the changes produced by ageing quenched alloys. (ii) the morphologies of the three habit variants found; {lOIO} {1012} and {1252} and their formation as a function of hydrogen content. (iii) the relief markings (on pre- polished surfaces) associated with hydride formation. (iv) the influence of applied tensile stress during precipitation on the alignment of the hydrides. Techniques for determining habit planes and pseudomartensitic shape strains directly in the SEM were developed and these were found to require a correction procedure for SEM image distortion and a calibration of the electron-optical geometry and specimen movement facilities. Because of their general applicability these techniques are described in Appendix form. In association with the measurements of (i) above, the hardness and notch-impact toughness of c.p. titanium-hydrogen alloys have been investigated. For the Ti-6A1-4V alloy, the hydrogen partition between the a and $ phases has been determined as a function of hydrogen content and cooling rate. The notched stress-rupture properties of this alloy were investigated by means of elongation-time plots and fractographic techniques and the inferior resistance to hydrogen of coarse 8-matrix structures was associated with a/8 interfacial failure. - 3 - CONTENTS Page ABSTRACT 2 CONTENTS 3 ACKNOWLEDGEMENTS 8 CHAPTER 1. Introduction A Brief History of Research on Titanium-Hydrogen Alloys g CHAPTER 2. Literature Review 2.1. Introduction 12 2.1.1. The Saline Hydrides 13 2.1.2. The Covalent Metal Hydrides 14 2.2. Occlusion of Hydrogen by the Transition Metals 15 2.2.1. Endothermic Occlusion 16 2.2.1.1. Bonding in the Endothermic Occluders 18 2.2.2. Exothermic Occlusion 18 2.2.2.1. Structures and Phase Diagrams 20 2.2.2.2. Thermodynamics of Hydrogen Occlusion in the Exothermic Occluders 28 2.2.3. Statistical Mechanics of Metal Hydrogen Systems 30 2.2.4. Bonding in the Transition Metal Hydrides 31 2.2.4.1. The Protonic Theory 32 2.2.4.2. The Anionic Model 33 2.3. Constitution of Titanium-Hydrogen Alloys 34 2.3.1. Early Studies 34 2.3.2. The Equilibrium Diagram 38 2.3.3. The Phases of the System 43 2.3.3.1. Alpha 43 2.3.3.2. Beta 44 2.3.3.3. Hydride 46 2.3.4. The Effect of Ternary Additions to Ti-H Alloys 49 2.3.4.1. Interstitial Elements 49 2.3.4.2. Substitutional Elements 50 2.4. The State of Practical Titanium-Hydrogen Alloys 52 2.4.1. The Diffusion of Hydrogen in Titanium-Hydrogen 52 2.4.1.1. The Elements of Diffusion Theory 52 2.4.1.2. Diffusion in the a Phase 55 2.4.1.3. Diffusion in the a Phase 55 2.4.1.4. Diffusion in the y Phase 56 2.4.1.5. Thermal Gradient Diffusion 57 - 4- Page 2.4.2. The Kinetics of the Sorption of Hydrogen 58 2.4.2.1. Absorption from the Gaseous State 58 2.4.2.2. Desorption Studies 61 2.4.2.3. Absorption from Liquids 62 2.4.3. Microstructure of Titanium-Hydrogen Alloys 63 2.4.3.1. Morphology of Hydrides 63 2.4.3.2. Orientation Relationships 64 2.4.3.3. Nature of the Reaction 66 2.4.3.4. Kinetics and Stress Effects 67 2.5. Hydrogen Embrittlement of Titanium and its Alloys 69 2.5.1. Deformation Modes 70 2.5.1.1. Slip Modes 70 2.5.1.2. Twinning Modes 72 2.5.1.3. The Selection of Deformation Modes 73 2.5.1.4. Fracture Modes 74 2.5.2. Hydrogen Embrittlement of a Titanium 75 2.5.2.1. Iodide Titanium 75 2.5.2.2. Commercial Purity Titanium 76 2.5.2.3. a-Titanium Alloys 78 2.5.3. Hydrogen Embrittlement of a/f3 Alloys 79 2.5.4. A General Theory of Hydrogen Embrittlement 82 2.5.5. Localised Hydrogen Embrittlement of Titanium 84 CHAPTER 3. Experimental Techniques 3.1. Hydrogen Charging 90 3.1.1. Preliminary Methods 90 3.1.2. Horizontal Charging Furnace 94 3.1.2.1. Hydrogen Charging Procedure 100 3.1.2.2. Calculation of the Hydrogen Pressure for Hydrogenation 101 3.1.2.3. Reliability of the Furnace System 103 3.2. Metallographic Preparation 105 3.2.1. Mechanical Preparation 105 3.2.2. Chemical Preparation 106 3.2.3. Thinning of Electron Microscope Foils 109 3.2.4. Preparation of Two-Surface Specimens 111 3.3. Heat Treatment 115 3.3.1. End Quenching Experiments 117 3.4. Metallographic Techniques 118 3.4.1. Optical Microscopy 118 - 5 - Page 3.4.2. Scanning Electron Microscopy 120 3.4.2.1. SEM Hot Stage 125 3.5. Mechanical Test Methods 128 3.5.1. Hardness Testing 128 3.5.2. Notched Stress-Rupture Testing 128 3.5.2.1. Apparatus 128 3.5.2.2. Specimen Preparation 128 3.5.3. Cooling Under Tensile Stress 129 3.5.4. Notched Impact Tests 129 3.6. X-Ray Diffractometry 131 3.6.1. Apparatus 131 3.6.2. Specimen Preparation 134 3.6.3. Determination of Lattice Parameters 135 3.6.3.1. Errors in Diffractometry 135 3.6.3.2. Numerical Solution 137 CHAPTER 4. Results 4.1. Hydrogen Contents 138 4.1.1. Starting Materials 138 4.1.2. Hydrogenation 138 4.2. The Structure of Titanium-Hydrogen Alloys from 10-50 at.% Hydrogen 149 4.3. Hydride Precipitation from Commercial Purity Titanium 152 4.3.1. Morphology of Hydrides after Cooling from the a Phase Field 154 4.3.1.1. End-Quenched Specimens 166 4.3.1.2. Cold-Worked Ti-H Alloys 167 4.3.2. The Effect of Ageing on Quenched Ti-H Alloys 168 4.3.3. Surface Relief Observations 173 4.3.3.1. Optical Microscope Hot Stage 173 4.3.3.2. SEM Hot Stage 179 4.3.3.3. Nature of the Surface Relief 185 4.3.4. Habit Plane and the Shape Strain of Hydride Precipitation 189 4.3.4.1. Testing of the SEM Two-Surface Analysis Method 189 4.3.4.2. Grain Growth of Titanium 191 4.3.4.3. Habit Planes 194 4.3.5. The Effects of Stress during Cooling 202 4.3.6. Notch-Impact Specimens 204 4.3.7. Hydride Precipitation in Ti-5A1-21-Sn 204 (IMI Ti317) Page 4.4. The Examination of Ti6%A14W Containing Hydrogen 206 4.4.1. Hydrogen Partition between the a and 8 Phases 206 4.4.2. Notched Stress-Rupture Tests 213 CHAPTER 5. Discussion of Results 5.1. Quantitative Analysis of'Lamellar Features using the Scanning Electron Microscope 218 5.2. Hydride Precipitation in Titanium 221 5.2.1. Nucleation of Titanium Hydride in the a Phase 221 5.2.1.1. Nucleation and Supersaturation 221 5.2.1.2. Heterogeneity of Hydride Formation 224 5.2.1.3. The Effects of Stress on Hydride Nucleation 226 5.2.2. Hydride Growth 227 5.2.2.1. Diffusion Control 227 5.2.2.2. Energy Control 230 5.2.2.3. The Mechanism of Hydride Growth 236 5.2.3. The Coarsening of Hydride Dispersions in a Titanium- 238 5.3. Effect of Hydrides on the Mechanical Properties of Titanium 240 5.3.1. Hardness 240 5.3.2. Notch Impact Tests 242 5.3.3. Relevance to Stress-Corrosion of Titanium 244 5.4. Hydrogen Distribution in Ti-6%A1-4%V 245 5.5. Hydrogen Embrittlement of Ti-6%A1-4%V 247 CHAPTER 6. Conclusions and Suggestions for Future Work 249 APPENDIX A The Correction of Distortion in Stereoscan Micrographs A.1 Origins and Effects of SEM Distortions Al A.1.1. Scanned Raster in the Electron Optical Column A3 A.1.2. Display Tube Scanning A5 A.1.3. External Errors A8 A2 Correction of Errors Produced by SEM Distortion A8 A.2.1. Internal Standardisation A8 A.2.2. Electronic Compensation A10 A.2.3. Mathematical Correction A10 A.2.4. Measurement of Distortion All A.2.5. Mathematical Technique A14 A.2.6. Results of Distortion Measurement A25 - 7 - Page A.2.7. Correction of Micrographs A29 A.2.8. Computer Program Listings A31 APPENDIX B. Stereoscan Calibration Bl X, Y and Z Translation Controls B2 B2 The Specimen Holder B5 B.2.1. Calibration of Tilt Control B5 B.2.2. The Effect of Rotation on Tilt B9 B.2.3. The Effect of X, Y and Translations on Tilt B9 B3 Establishment of Working Distance and Scan Rotation B11 APPENDIX C. Determination of Precipitate Habit Planes by Two Surface Analysis Cl Introduction C2 C2 Geometry and Analysis C2 _ C.2.1. Computer Program Listing Cll APPENDIX D. The Calculation of the Shape Strain of Shear-Type Regions using the SEM Dl Geometry D2 D2 Mathematics D3 D.2.1. Computer Program Listing D7 APPENDIX E.
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