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SOFFA, William Anthony, 1939- a FIELD-ION MICROSCOPY STUDY of SOME TUNGSTEN-RHENIUM and MOLYBDENUM- RHENIUM ALLOYS

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SOFFA, William Anthony, 1939- a FIELD-ION MICROSCOPY STUDY of SOME TUNGSTEN-RHENIUM and MOLYBDENUM- RHENIUM ALLOYS This dissertation has been microfilmed exactly as received 67-10,926 SOFFA, William Anthony, 1939- A FIELD-ION MICROSCOPY STUDY OF SOME TUNGSTEN-RHENIUM AND MOLYBDENUM- RHENIUM ALLOYS. The Ohio State University, Ph.D., 1967 Engineering, metallurgy University Microfilms, Inc., Ann Arbor, Michigan All Rights Reserved A FIELD-ION MICROSCOPY STUDY OF SOME TUNGSTEN-RHENIUM AND MOLYBDENUM-RHENIUM ALLOYS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Decree Doctor of Philosophy in the Graduate School of The Ohio State University By William Anthony Soffa, 13.S. , M.S. * # # # The Ohio State University 1967 Approved by / ..Adviser/] Department of Metallurgical Engineering To my Wife and Daughter ACKNOWLEDGMENTS The author is grateful for the continued encouragement and guidance of Professor K. L. Moazed during the course of this work. The author also gratefully acknowledges the many faceted contribution and inspiration of Professor J. P. Hirth throughout his undergraduate and graduate studies. ii VITA June 1, 1939 Born - Pittsburgh, Pennsylvania 1961 .... B.S., Carnegie Institute of Technology, Pittsburgh, Pennsylvania 1961-1963 . Graduate Assistant, Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, New York 1963 . M.S., Rensselaer Polytechnic Institute, Troy, New York 1963-1967 . Research Fellow, The Department of Metallurgical Engineering, The Ohio State University, Columbus, Ohio FIELDS OF STUDY Major Field: Physical Metallurgy Studies in Physical Metallurgy. Professors K. L. Moazed, Gordon W. Powell and J. W. Spretnak Studies in Mechanical Metallurgy. Professor J. W. Spretnak Studies in Dislocation Theory. Professor J. P. Hirth Studies in Thermodynamics and Kinetics. Professors R. A. Rapp and R. Speiser Studies in Corrosion and Oxidation. Professors M. G. Fontana and R. A. Rapp iii CONTENTS Page I. INTRODUCTION ........................... 1 Streak Contrast and Stacking Faults in B.C.C. Metals and Alloys .................... 1 Field-Ion Microscopy of Alloys ................ 3 Present W o r k .................................... 4 II. FIELD-ION MICROSCOPY ............................. 5 Historical Development ......................... 5 Basic Principle and Operation of the Field-Ion Microscope ......................... 7 Field Ionization and Image Formation ......... 10 Magnification and Resolution .................. 14 The Field S t r e s s ............................... 16 Field E v a p o r a t i o n ............................. 17 Theory of Field Evaporation .................. 19 The Structure of Field Evaporated Surfaces— Pure M e t a l s .................................. 2 3 The Field Evaporation Behavior of Alloys; Field Evaporation of Impurities ........... 24 III. EXPERIMENTAL...................................... 27 Microscopy: General Microscope Design, Operation and Vacuum Performance ........... 27 Microscope Screen Preparation ................ 29 Image Recording; Photographic Equipment and P r o c e d u r e ............................... 31 iv CONTENTS (Contd.) Page Liquid Hydrogen Cooling; Liquid Hydrogen Transfer Technique ............. 32 M a t e r i a l s ...................................... 33 Specimen Preparation ........................... 3^ IV. RESULTS AND DISCUSSION........................... 36 Streak Contrast in Tungsten-Rhenium Alloys . 36 Field-Ion Microscopy Study of Some Dilute Molybdenum-Rhenium Alloys; Anomalous Field Evaporation E f f e c t s .................. *J3 V. CONCLUSIONS ...................................... 50 APPENDIX .................................................... 112 BIBLIOGRAPHY .............................................. 115 v ILLUSTRATIONS Figure ' Page 1. Ionization of Helium Ions In the Field-Ion M i c r o s c o p e ...................................... 52 2. Effect of Applied Field on Electron Energy . 54 3. Field-Ion Image of a Tungsten T i p ................ 56 4. Atomic and Ionic Potential Curves . .............. 58 5. Potential Energy Curves ............................ 60 6. (a) Field-Ion M i c r o s c o p e ......................... 62 (b) Schematic Diagram of the Field-Ion M i c r o s c o p e .................................. 64 7. Schematic Diagram of the Vacuum System .......... 66 8. Schematic Diagram of Liquid Hydrogen Transfer Equipment........................................ 68 9. Tungsten: 77°K. 1 micron He .................... 70 10. Tungsten: 77°K. 1 micron H e ...................... 72 11. Tungsten: 77°K. 1 micron H e ...................... 74 12. Field-Ion Micrograph of W-3 Re Alloy Exhibiting Approximately <f230 Axis. 77°K. 1 micron H e ...................................... 76 13. Field-Ion Micrograph of W-3 Re Alloy Exhibiting Approximately <^231^ Axis. 77°K. 1 micron H e ...................................... 78 14. Cluster Maintained in (110) Net During Field Evaporation of Tungsten. 77°K. 1 micron He . 80 15. Streaking in Deformed W-3 Re Alloy. 77°K. 1 micron He . ......................... 82 16. Streaking in Deformed W-3 Re Alloy. vi ILLUSTRATIONS (Contd.) Figure Page 17. Streaking in Deformed W-3Re Alloy. 77°K. 1 micronH e .............................. 86 18. Streaking in Annealed W-3 Re Alloy. 77°K. 1 micronH e .............................. 88 19. Streaking in Annealed W-3 Re Alloy. 77°K. 1 micronH e .............................. 90 20. (a) Helium Field-Ion Image of Commercially Pure Molybdenum. 21°K. 1 micron He ................. 92 (b) Commercially Pure Molybdenum. 21°K. 1 micron H e ...................................... 94 21. Mo-. 5 Re Alloy. 21°K. 1 micron H e .............. 96 22. Mo-1 Re Alloy. 21°K. 1 micron H e .............. 98 23. Mo-1 Re Alloy. 21°K. 1 micron H e ................. 100 24. Mo-1 Re Alloy. 21°K. 1 micron H e ................. 102 25. Mo-1 Re Alloy. 21°K. 1 micron H e ................. 104 26. Mo-2 Re Alloy. 21°K. 1 micron H e ................. 106 vii TABLES Table Page 1. Field Evaporation of Ions at 0 ° K ................... 107 2. Calculated Evaporation Fields .................. 109 3. Energy to Field-Evaporate Non-Metallic Impurities......................................... Ill viii I. INTRODUCTION The field-ion microscope has a resolution of 2-3 X and is capable of magnifications exceeding one million diameters. Hence, despite some serious disadvantages, this microscope is potentially the most powerful tool available today for studying the structure of metals and alloys. However, in order to utilize the field-ion technique as a metallographic tool it is extremely important to establish precisely the relation between various contrast effects observed in field-ion micrographs and intrinsic metallurgical structure. One of the more important and more controversial of these effects is that of streak contrast. A. Streak Contrast and“Stacking Faults in B.C.C. Metals and Alloys Bright streaks are frequently observed in field-ion images of specimens which have been subjected to certain critical treatments. Ranganathan et al.1 have attempted to catalogue and explain the sources and origin of streak contrast effects. Streaking arising from image superposition and possibly stacking faults are discussed. The more general type of streaking, found in images from specimens with an elliptical cross section, which has eluded explanation is discussed in terms of likely causes such as dislocations, 2 2 slip bands, etc. Brandon, however, has suggested that the primary cause of streak contrast is tip asymmetry which may 3 result from faulty electropolishing. Ralph and Bowkett have disputed the general applicability of such an explanation but agree that the model may account for some specific streaks. Despite such attempts to explain streak contrast effects, considerable discussion continues and no general agreement as to their origin has yet been reached. Ralph and Brandon** encountered the first reproducible observations of image streaking in their study of the W-Re system. They reported that small streaks were found in deformed W-5 atomic percent Re alloys which they attributed to steps produced by stacking faults intersecting the crystal surface. These streaks were found to be always parallel to the line of intersection of \ 112 } planes with the surface and were only one atom in width. They showed that such a step could be produced by an % / l l l ^ partial dislocation on a plane. Preferential evaporation of atoms situated on the step was thought to be unlikely since the atoms essentially have the same coordination number and binding energy (though with a different disposition of neighbors) as the other atoms giving rise to image points. However, atoms on the step should give bright images because of field enhancement and focusing of field contours. 5 Ryan and Suiter have observed an apparently planar defect structure in tungsten, the so-called "cross-over structure." The (Oil) net planes are observed to be drawn Inwards locally. This fault was found to disappear during the removal of a hundred layers and then to reappear again in c; the same place. Ryan and SuiterJ interpret the cross-over structure in terms of an extended dislocation on the (111) plane. Stacking faults produce characteristic interference fringes in transmission electron micrographs. However, if the separation between partlals is of the order of 100 \ or less, identification becomes ambiguous. Thus, in the case of stacking faults of small widths and high energies in b.c.c. metals and alloys, the field-ion microscope has potentially an important role to play in the observation and study of these
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