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 micronHe ...... 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

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 micronH e ...... 100

24. Mo-1 Re Alloy. 21°K. 1 micronH e ...... 102

25. Mo-1 Re Alloy. 21°K. 1 micron H e ...... 104

26. Mo-2 Re Alloy. 21°K. 1 micronH 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 faults.

To determine whether streaking in W-Re alloys is Indeed associated with intrinsic features such as stacking faults or whether such contrast effects are primarily artifacts is obviously an important problem.

B. Field-ion Microscopy of Alloys

The field-ion microscope has been developed to a point where almost routine studies of some phenomena in metals are possible. Considerable effort has been directed toward applying the technique to studies of phenomena occurring in alloys. For some types of alloy studies the field-ion microscope has proved largely unsuccessful so far, while in other areas the full potentialities of the field-ion microscope have been realized. The presence of a solute atom is expected to affect both the field ionization process and the field evaporation process. As a result, studies of solid solution alloys are very difficult in that the images are generally irregular and the Interpretation, which relies on easy identification of crystallographic features, is extremely difficult. Improvements in the understanding of the factors affecting the imaging process from alloys and a more detailed understanding of the field evaporation process will allow a t wider and wider range of studies to be envisaged. However, the possibility in this area of studying phenomena in concen­ trated alloys or in ternary alloy systems seems remote at present.

C. Present Work

A systematic study of a W-3 atomic percent Re alloy was carried out in this work to determine whether a definite correlation between streaking and some metallurgical condition of this alloy could be established. It was also hoped that the results of such a study might add further knowledge toward understanding the remarkable effect of Re on the ductility of 6 7 8 W and Mo. * * Comtflenrtentary field-ion observations of some

Mo-Re alloys were attempted. II. FIELD-ION MICROSCOPY

A. Historical Development

The principle of the field-ion microscope was first 9 10 described by Mueller in 1951. Mueller published his now classic review of the subject in I960. Since that time several reviews*1 j!5 ,16 technique have appeared. This microscope is the most powerful known today and is the only instrument that can directly show the atomic structure of a crystal lattice. Its development is an outgrowth of Mueller's work for over two decades with the field electron emission microscope and its application to the study of surfaces.

Oppenheimer1? in 1928 suggested the field ionization of free atoms by tunneling of an electron in the presence of an electric field. However, field ionization from the ground state was experimentally inaccessible because of the high field strengths required. Sufficiently high field strengths became a possibility with the introduction of the field 18 electron emission microscope by Mueller in 1936. In 19^1,

Mueller1^ found that barium and thorium, which had been adsorbed on the surface of a tungsten emitter in the field electron emission microscope, could be desorbed by reversing the polarity of the field. The realization that the resolution of the field electron emission microscope is determined by the tangential velocity of the emitted electrons, and to a lesser extent by their de Broglie wave­ length, coupled with further studies of the field ionization pn of gases, led Mueller in 1951 to the successful attempt of imaging an emitter surface with positive ions, rather than with electrons. In his original experiments Mueller did not cool the specimen; as a consequence both the resolution and the intensity of the image were relatively_poor. However, in the period 1955 to 1956 the significance of accommodating the imaging gas to the tip temperature was realized and led

Immediately to the operation of the microscope at cryogenic PI PP temperatures. » At the same time, during studies of the field desorption process, the important phenomena of "field 2 ^ evaporation" was discovered. The concept of the "hopping" 24 imaging gas atoms or molecules, also introduced by Mueller in 1956, led to such improvements in imaging procedures that true atomic resolution of large sections of the specimen surface was ultimately achieved. In 1957 (at the 4th Inter- 25 national Conference for Electron Microscopy) Mueller was able to present high resolution field-ion micrographs of a large number of refractory metals imaged at low temperatures.

By I960 most of the present day experimental techniques were developed such that high quality images of emitter surfaces could be obtained with magnifications exceeding one million diameters and with a resolution of less than 3 ft. 7

By 1961 most of the common defect structures had been imaged: stacking faults,^5 vacancies,^ grain boundaries 25 27 25 27 subboundaries * and dislocations, * and the first images 2 8 from alloys had been presented. The possibilities of the field-ion microscope as a tool for metallurgical research were thus clearly established. Since then detailed work has been initiated at a large number of laboratories all over the world.

Practically all the work mentioned so far had been carried out using helium as the Imaging gas, but recent attempts have been made to work with a variety of gases.

Great effort is now being expended with regard to the possi­ bility of using a variety of gases and gas mixtures including neon, argon and hydrogen. The practical use of hydrogen promotion for imaging certain non-refractory transition metals was fairly well established in 1965.2^ Other areas of consid­ erable interest and activity at present which may greatly increase the range of materials which can be studied and the amount of information that the technique can provide are: image intensification, cine-recording, and pulsed field evaporation.

B. Basic Principle and Operation of the Field-ion Microscope

The Field-ion microscope, like the field electron emission microscope, is a point projection microscope requiring no magnetic or electrostatic lenses to produce the image. A 8 high positive potential is applied to the specimen, which is a short, fine wire polished to a sharp point having a radius o of curvature between 100 and 1000 A. The specimen is attached to a wire loop and held between two electrodes which are generally cooled in liquid nitrogen, solid nitrogen or liquid hydrogen. Liquid neon and liquid helium have also been used. This assembly is mounted in a glass (or metal) chamber evacuated to less than 10-^ torr, into which the imaging gas is introduced at a pressure of about 10"3 torr.

(For various reasons, by far the best images are obtained with helium ions.) The field strength developed in the vicinity o of the tip lies typically in the 4 to 5 V/A range. Neutral gas atoms or molecules approach the tip, become polarized, and are pulled toward the tip with increasing velocity (see

Figure 1). As a result of the dipole interaction, about 5 to

6 times as many atoms or molecules strike the tip as calculated from kinetic theory for the same system without an applied field. When the field strength is sufficiently great, the polarized atoms or molecules may become ionized as they approach the tip. However, because of their high velocity, their transit time through the region of high field is short and the probability of their ionization is correspondingly small.

(An atom or molecule that ionizes before it reaches the tip does not contribute to the image of the tip. An' advantage of helium as the imaging gas appears here. Because of its high ionization energy, the field needed to ionize it is so high 9 that other kinds of atoms or molecules will generally ionize before they reach the tip. The tip will thus not become contaminated by impurities from the background gas.) As a result most of the Imaging gas atoms strike the tip and rebound.

However, their rebound velocity is considerably less than their approach velocity, because of the polarization forces which tend to pull the atoms back to the surface; the atoms therefore undergo a "hopping" motion on the surface, spending more time in regions of high field strength. After losing their polarization energy the imaging gas atoms bounce with essentially the kinetic energy of thermal equilibrium. At low tip temperatures this thermal energy enables them to make

"hops" of only a few Angstroms high on the surface. When the atoms become ionized above a region of locally enhanced field strength, generally above a protruding atom or lattice step, they are accelerated to the phosphor screen along the lines of force normal to the equipotentlals. The ion image produced on the screen is an image of the field strength distribution over the surface which is directly related to the local radius of curvature. Under suitable operating conditions the resolution of the instrument is sufficient to resolve the fluctuations in field strength caused by the individual surface atoms. C. Field Ionization and Image Formation

Field ionization, like field emission, occurs by tunneling,»^0»31 but ±n this case by the tunneling of electrons from the imaging gas atoms into the metal surface.

Let us consider the ionization event in some detail. Figure 2a shows the distortion of the potential well of the Is electron of a hydrogen atom produced by the high field near the surface.

This distortion produces a finite potential barrier such that ionization can occur by tunneling through the barrier. This tunneling process can be thought of as a result of the

Heisenberg uncertainty principle: if the barrier is sufficiently small there is a finite probability of finding the electron outside the atom. If ionization occurs in free space, the potential barrier will be approximately triangular in shape with height, I, equal to the first ionization potential of the atom, and width, I/eF, where F is the applied field and e the electronic charge. For the one-dimensional case this barrier is formally Identical to that existing at the surface of a metal during field emission, with the work function of the metal, ^ , replaced by the ionization potential, I, and identical methods can be used for the tunneling probability, D. 11 The final result gives

D OZ exp -(0.68 I3/2/F) o where I is in eV and F in volts/A. 11

Ionization can only occur beyond a critical distance xc from the surface. When the atom Is too close to the surface the electronic energy level falls below that of the

Fermi level of the metal and tunneling Is prohibited since no accommodation can be made for the tunneling electron because there are no empty electronic states available in the metal

(see Figure 2b). The potential energy V(x) of the electron at a distance x from the emitter surface is given approximately by 2. — a 2. 2- V(x)= - A + r e x - £ + --

| X - * | I /W. in which the first term represents a Coulomb attraction to an ion at a distance xn from the emitter surface, and the last two terms are image potentials due to the electron and ion images respectively. Tunneling cannot occur approximately when

(neglecting the first and fourth terms) •t Fex - < | X | - | <|> |

where I is the ionization potential and ($> the work function.

The width of the forbidden region xc is for most metals 4 to o 5 A. Retarding potential experiments show that the ions actually do originate 5 to 100 \ above the surface depending on the field strength.

To reproduce the details of the surface topography, it is necessary that the ionization of the imaging gas occur 12 as near to the surface as possible. For best resolution, the average "hopping" height, h , should approximate the width of the forbidden ionization region, or 4 to 5 ft. Mueller^** has calculated that h is given approximately by

h -. -3 kTtr *■ 4 o< F where k is the Boltzmann constant, oC is the polarizability of the imaging gas, Tt is the absolute temperature of the tip, F is the average field strength at the tip, and r is the average radius of curvature of the tip. For He with °( « 2 X 10“^ cc, h « 4 to 5 ft at 20°K for r = 1000 ft. To achieve the same

"hopping" height at 77°K requires a tip with a radius of O curvature of 250 A. Since it is not practical to work with a tip radius of less than 200 ft, the operation of the field-ion 1 O microscope is limited to these low temperatures. Experi­ mentally, it is found that cooling to liquid or solid nitrogen is adequate for radii of less than 400 ft. For larger tips, the lower temperatures must be used. Also, going to lower temperatures can affect the topography of the specimen surface itself, for reasons not discussed yet, thus essentially affecting the actual applicability of the field-ion technique to a particular metal or alloy.

The formation of the field-ion image obviously depends intimately on the sensitivity of the ionization probability to local changes in the field strength over the emitter surface.

Mueller10 has estimated that a 155 field fluctuation may change the ionization probability by 30%. He assumes that field fluctuations are caused by protruding atoms or lattice steps on an almost hemispherical surface. Sites of atomic protru­ sions are sites of locally enhanced field strength and thus strong ionization centers. B r a n d o n ^ has explained image formation in terms of the extent to which the conduction electron cloud at the metal surface is drawn inwards by the applied field. This, he hypothesizes, depends on the screen­ ing of the field by the positive ion cores which are exposed in the process. As the ion-core density increases, the electron-cloud depression and the ripples of the equipotential lines becomes smaller. The visible atoms are those in ’’exposed" positions in the surface, such as exist in crystal faces of high index and at steps at the edges of crystal planes. The contrast in the ion image thus would depend on the coordination number of the atoms at each position. Brandon's hypothesis does explain the correlation between image contrast and coordination number of atoms at steps, and the lack of correla­ tion between step height and image contrast.

Other factors which can give rise to local differences in electric field other than local geometry are: (1) a variation in charge density associated with the effect of impurity atoms on the electronic structure of the solid solution and (2) a variation in ionic charge associated with impurities. Such changes in local field strength can alter the field ionization probability above the solute atom relative to the solvent. 14

This effect could result in a significant difference in con­ trast, depending on the extent and magnitude of the 4 perturbation.

The field-ion pattern produced at the fluorescent screen is thus a pattern of bright spots produced by the imaging gas ions from ionization centers on the emitter sur­ face. Since the resolution is about 3 ft, separate and visible images from each ionizing atom in the surface are obtained.

About 25/5 of the surface atoms are visible. The ion pattern shows a contour map of the tip, which can be considered to consist of intersecting stacks of lattice planes (see Figure 3).

The concentric rings in the pattern represent the edges of these planes, the height of the steps being equal to the interplanar spacing. Individual atoms can be resolved within the net planes only on the higher index planes because the modulation of the local field strength above the more closely packed planes is not large enough to give sufficient contrast.

Because the specimen tip is not a freely suspended sphere, the field lines are compressed and the angle of view is actually about 120°. The projection resembles most nearly a stero- graphic or orthographic projection. ^

D. Magnification and Resolution

If it is assumed that the atomic spacing at the sur­ face is equal to that in the bulk, the magnification of a field-ion pattern can be calculated directly from a knowledge 15 of the lattice parameter and crystal structure of the speci­ men. The magnification of a point projection microscope is essentially equal to the ratio of the screen distance to the tip radius. For a radius between 100 and 1000 X and a tip- to-screen distance of a few centimeters, a magnification of several million diameters is actually operationally obtained.

Neglecting scattering of the ions by the parent gas, the resolution of the field-ion microscope is determined mainly by three factors: (1) the extent to which the atomic structure of the emitter surface causes fluctuations in the local field strength through a surface xc above the emitter, where ionization predominantly occurs, (2) the extent to which this modulation of the field strength produces a variation of the ion current density leaving the emitter and (3) the extent to which this spatial variation of ion current density above the emitter is retained in the ion beam reaching the fluorescent screen. For helium ionization at 21°K^ at a field ° ° of m .5 V/A, it can be shown that a resolution of 1/2 A should be achieved for an emitter of 100 X radius, 1/6 X due to spreading of the ion beam associated with the transverse o velocity component of the ions and 1/3 A due to diffraction effects. The predicted resolution for an emitter of 1000 X o radius is about 1 A. Since the resolution is typically of o the order of 2 - 3 A for temperatures below liquid nitrogen

(77°K.) and tip radii of less than 1000 X it is clear that 16 the actual resolution is limited by the modulation of the electric field above the emitter surface.

Also, because of the finite size of the gas atom, there is also an uncertainty in the position of the tunneling electron to be considered which is of the order of the gas atom size. This size effect limits the resolution attainable with image gases other than hydrogen or helium, notably neon and argon.

E. The Field Stress

The applied electric field F generates tensile stresses at the surface of a field-ion microscope specimen given by

< T K = F/ 8 t r o y — — — 2.

o where F is in volts/A. Because the tip is attached to a coni­ cal shank, there will be an appreciable shear component of the

field s t r e s s . ^ Field stresses of the order of 500 to 2 2000 kg/mm are generated under the conditions of the field-ion microscope. Most b.c.c. metals deform plastically at shear stresses well below 100 kg/mm2 , even at low temperatures,^ so that it is at first sight surprising that any metal is capable of being handled in the field-ion microscope without simul­ taneous plastic deformation and eventual fracture. Apparently the whisker-like dimensions of the tip allow it to sustain the

unusual conditions imposed by this technique. However, in any 17 field-ion microscopy study the field stress or conditions of observation must always be considered for meaningful image interpretation.

F. Field Evaporation

The faceted surfaces depicted in a field-ion micro­ graph are not thermally equilibrated ones, but are produced by a process called "field evaporation."2^ Field evaporation occurs when the local field becomes sufficiently high to pull atoms out from their lattice positions and ionize them. This phenomena is of major consequence to the success of field-ion microscopy. Without it, the atomically smooth, faceted crystal surfaces could not be produced. Also, field evapor­ ation and the related phenomena of "field desorption" allow cleaning of specimen surfaces without heating; field evapor­ ation makes it possible to remove successive lattice planes from the specimen surface revealing the underlying structure of the bulk lattice. The term field evaporation is usually taken to refer to the evaporation of a metal from its own lattice and the term field desorption is used to describe' the analogous process for foreign atoms at the surface. However, the term field evaporation is used, in a more general sense, to include evaporation of impurities and solute atoms as well as evaporation of the solvent matrix.

Although this preparation is essential to the field- ion technique, unfortunately, it sets a severe limit to the applicability of the technique. In order to obtain a stable ion image, the specimen surface should not evaporate at the field necessary to ionize the imaging gas. Also, to form atomically smooth, faceted surfaces it is necessary that the specimen material not yield mechanically under the field stress during the field evaporation conditioning. In many cases, however, one must be willing to accept the occurrence of some limited plastic deformation, and the question of whether an observed detail is intrinsic or an artifact becomes a serious problem.

Considerable progress has been made recently in ex­ tending the applicability of the field-ion microscope to the less refractory metals and to the technically important transition metals such as Pe, Ni, and Co. The promotion of field evaporation by the addition of hydrogen has made it possible to perform field evaporation at reduced field strengths. The mechanism of this effect is not yet under­ stood, but the reduction of the field stress can reach ^0%.

Also, significant improvement of thermal accommodation of the impinging helium atoms can be achieved by apparently allowing hydrogen to act as an "intermediate collision partner" at the emitter surface. This effect allows a reduction of the imaging field of helium and practical observation of surfaces of marginal stability formed by hydrogen promoted field 29 evaporation can be realized. Observation of these materials is greatly facilitated by the use of photoelectronic image intensification. 19

1. Theory of Field Evaporation

Field evaporation is a thermally activated process and has an activation energy dependent on the applied field strength. At a sufficiently high field strength the activa­ tion energy can be reduced to an arbitrarily low value, so that field evaporation can be made to occur at any temperature.

Field enhancement over protruding surface atoms reduces the activation energy for evaporation in these regions, so that atomic protrusions at the surface of a field-ion microscope tip are evaporated preferentially. Field evaporation is therefore used, as mentioned previously, to clean and smooth the tip before examination and thus represents the final stage of specimen preparation. With careful control of the evaporation process a specimen can be taken apart atom by atom and in principle the bulk structure can be analyzed on an atomic scale. The field evaporation process is therefore an integral part of the technique of field-ion microscopy.

Field evaporation is generally treated in terms of the deformation of one-dimensional potential curves by applied fields. As suggested by Mueller-^ and later by Gomer and

Swanson,field evaporation can be treated as a special case of field desorption. The activation barrier essentially con­ sists of two terms: (1) an intrinsic term described by a thermionic cycle wherein the surface atom is vaporized, ionized, and the electrons removed are returned to the metal 20 surface and (2) a term accounting for the effect of the applied field on the Ionic potential curve.

For most metals the Image potential model provides a reasonable first approximation for calculation of the acti­ vation energy and evaporation field. The potential energy of an atom, V^(x), as a function of distance x from the surface is shown in Figure 4a. The minimum corresponds to the sub­ limation energy of the atom,-A-. a similar curve for an ion in the gas phase is shown in Figure 4b. The energy required to ionize an atom in the gas phase is ^ I where n is the charge n on the ion and In is the n-th ionization potential. If the electrons are returned to the metal an energy n

A minimum in the potential curve will appear. On applying an electric field to the specimen the potential energy of the ion will be reduced by a factor nFex as shown in Figure 4c.

At distances which are sufficiently large to ignore the repul­ sion potential, the potential energy of the ion is given by

2 2 — VT (x) » nFex + - e - . 1 4x 21

Setting yields c^- ^

A F o"L €- and

- v x c > w ) = C e 3 F )*-

The energy at this maximum is referred to as the"Schottky hump.” Superposition of the Ionic and atomic potential curves produces three cases as shown In Figures 5a, 5b and 5c. The case where the atomic state is stable but the ionic and atomic curves cross over to the left of the "Schottky hump" leading to evaporation in the ionic state, as shown in Figure 5b, seems to correspond to the situation of field evaporation of most metals. The activation energy for the field evaporation process is then given by t

where

The time't' in which an atom will be evaporated at a tempera- ture T if it attacks the activation barrier with a frequency

is given by

t = 22 where is essentially the vibrational frequency of the bound particle and k is the Boltzmann constant. The field necessary to evaporate an atom within the time ~"C from a metal surface is then given by10

= ~C 3 e .'3 j_A_ + ~ ] i

Since the term is small at low temperature*;s com- pared to the sum of the parameters-/'— , ^ 1 , and

Mueller1® has calculated the evaporation fields for metals at 0° K. for singly charged ions. These results are listed in Table 1. Comparing these values to an image field of 4.5 v/X for helium it appears that a cut-off in image stability occurs at about the evaporation field of iron. This is in general agreement with experimental results. Brandon has considered evaporation of multiply charged ions and has included polarization corrections. That is, Q = + ^

Figure 5c corresponds to the case where the ionization potential of the atomic species is so high that the image hump lies above the atomic potential curve. The energy maxi­ mum then lies below and to the right of this hump, at the point of Intersection of the atomic and ionic curves, so that Q becomes a linear function of F for removal of the ion. Non- metallic impurities on a metal surface may evaporate by this 39 lin ill mechanism. * The former case is referred to as the

"image potential" model and this latter case is referred to as the "intersection" model.

2. The Structure of Field Evaporated Surfaces— Pure Metals

The stable endform of a pure metal is reached when the variations in the local radius of curvature compensate for:

(1) the field enhancement at an atomic site related to the lattice step height and local geometry and (2) the variation of the structure of the electrical double layer (surface polarization) with orientation. Regions of relatively large lattice steps and high work function will be flattened. Since these parameters show a systematic variation with crystallo- -aq graphic orientation, faceting results. J

The most successful model for the evaporated surface 42 is the computer model developed by Moore. The basic assump­ tion in this treatment is that the field evaporated surface includes all atoms whose sites lie within a smooth section taken through an infinitely extended crystal lattice and no atoms whose sites lie outside this section. Moore also assumes that the atoms appearing in the image lie within a fixed distance of the smooth section and that this section is approximately hemispherical. Correlation between the computed field-ion image and the observed image is quite good.

Lattice defects can often give rise to preferential evaporation and corresponding surface irregularities making image interpretation difficult. Polarization effects can often stabilize atoms in protuberant positions giving rise to atoms occupying metastable sites at the surface which have no existence in the bulk lattice. "Zone lines" commonly observed 4 3 in many metals may be caused by such an effect.

3. The Field Evaporation Behavior of Alloys; Field Evaporation of Impurities

In an alloy the binding energy of an atom depends not only on its position in the lattice, but also on its atomic species and the atomic species of its neighbors. Thus, the addition of a second component introduces variables which do 25 not systematically depend on crystal orientation, the excep­ tion being the case of an ordered alloy.39,4l ^he regular structure of field-ion patterns generally deteriorates progressively with increasing solute content. Mueller has postulated that the lack of faceting is associated with the random distribution of solute, which also randomizes the work function. In support of this hypothesis, he has shown that only in the ordered state is faceting in concentrated Pt-Co alloys observed.

Preferential evaporation or retention of a given species can occur in alloys leading to a distortion of the field-ion pattern. From consideration of the basic parameters involved in the field evaporation process it can be readily seen that there will be genuine differences in the field required to evaporate atoms of different atomic species, in the same geometric position, and, that there may be superposed on these differences an effect for atoms of the same species of different atomic environment. A solute atom can also cause a difference in local charge density leading to dif­ ferences in the effective local field. If the solute is preferentially evaporated and the solute concentration is greater than 10 - 15% the regularity of the surface can be effectively destroyed. If preferential retention of the solute atoms occurs a skeleton of solute atoms at the surface will result giving rise to a highly irregular field-ion ■so in image. ^ * Thus, the presence of solute atoms generally leads to distortion and irregularities in the field evaporation endform of alloys, especially in alloys of high solute con­ centration, often making image interpretation difficult.

The field evaporation of dilute alloys has been treated quantitatively by Brandon^ in terms of the heats of solution or adsorption of the solute species and the appropri­ ate ionization potentials. Evaporation fields have been

calculated for various solute species and can be used to predict whether the solute is preferentially retained or evaporated in the lattice. It is shown that non-metallic

impurities, because of their high ionization potentials, tend

to be preferentially retained or may possibly evaporate as molecular ions. III. EXPERIMENTAL

A. Microscopy: General Miscroscope Design, Operation and Vacuum Performance

The field-ion microscope used in this work as shown

in Figures 6a and 6b was basically a Mueller design except

that a Varian ultra-high vacuum Conflat flange (2 1/2" I.D.) was used in preference to a demountable grease joint. A

simple pyrex-glass vacuum system as shown in Figure 7 was

employed. The pumping station was comprised of a mechanical

fore pump (Welch Duo-Seal) and an air-cooled, three-stage,

glass oil diffusion pump. The vacuum was measured using a

Bayard-Alpert ionization gauge. A background contamination

Q pressure of less than 5 X 10"° torr could be obtained in the

microscope chamber in less than 12 hours after a bake-out at

250°C and a vacuum of better than 1 X 10“® torr could be

obtained after a 12 hour bake-out at 400°C. Helium was used

as the imaging gas. The helium was admitted by diffusion

through a commercially available diffuser employing a heated

vycor tube. A ground-glass seat valve (totally enclosed and

containing a steel rod for manual operation with an external

ring magnet) was employed between the pumps and microscope

chamber, making it possible to build up the imaging gas

pressure to operational levels. A zeolite sorption pump

27 2 8 consisting of a small quantity of zeolite in a conventional cold trap design cooled to liquid nitrogen temperature was used to help maintain the low background pressure during operation. The microscope design allowed specimens to be examined at liquid nitrogen (78°K.) and liquid hydrogen

(21°K.) temperatures. The horizontal screen of the microscope was viewed using a front surface mirror at forty-five degrees; images were also photographed in this manner.

Specimens were replaced by removing the screen portion of the flange coupling. Immediately after resealing the micro­ scope chamber, the mechanical pump and diffusion pump were turned on simultaneously at full power. The mechanical pump reduced the pressure to the micron range in a few minutes; however, the diffusion pump required about 15 minutes to reach operating temperature. When the diffusion pump reached operating temperature, the zeolite sorption pump and zeolite trap were then heated with Veeco hot-air blowers to about

100-150°C for about 1 hour and then allowed to cool. A vacuum

of less than 1 X 10"^ torr could readily be achieved in less than 8 hours following this procedure. After a ^ - 6 hour

bake-out at 250°C the background pressure fell to about O 5 X 10 torr. Cooling the zeolite sorption pump to liquid nitrogen temperature further Improved the vacuum to about

1 X 10”° torr. As mentioned above, longer bake-outs at higher

temperatures made it possible to obtain vacuums of less than

1 X 10“ 8 torr, but to increase the number of specimens which 29 could be examined, the procedure described here was established as an optimum. The helium pressure in the microscope chamber could readily be established at approximately 1 X 10"^ torr within 15 minutes after closing the seat valve by heating the vycor tube to about 300°C. with a He pressure of slightly greater than 1 atm. maintained in the diffuser Jacket. The helium pressure could be measured by the ion gauge using a conversion factor or directly using a thermocouple gauge.

B. Microscope Screen Preparation

The fluorescent screen of the microscope was made demountable because ion bombardment damage in the phosphor eventually decreased the intensity of the image and resulted in prohibitively long exposure times for photography. A zinc orthosilicate phosphor was used as the screen material.

The conductive coating of the screen was prepared according to the following procedure. All parts of the pyrex tube section to be left non-conducting were coated with

Aquadag suspension and allowed to dry. (It was import suit to mask about 1/2" of the neck of the tube so that it later could be Joined readily to the flsmge adapter section.) The tube was then placed in sui oven and heated slowly to 450 - 475°C.

The flat screen portion of the tube was mounted horizontally, parallel to the oven floor with the neck down.Stannous chloride was then blown up into the tube and allowed to exhaust through the small side-arm provided for attaching the ground lead to the screen. (The side-arm section was also 30 masked with Aquadag.) The vapor which impinged on the hot walls of the tube remained on these surfaces as stannous oxide.

Careful control of the amount of stannous chloride introduced into the tube made it possible to obtain uniform transparent coatings with sufficient electrical conductivity (much less than a kilohm). After this operation, the tube was cooled slowly and the Aquadag was thoroughly removed by washing.

The fluorescent screen was prepared using the follow­ ing procedure suggested by S. S. Brenner (private communica­ tion). A suspension of finely ground phosphor in distilled water was prepared and allowed to settle for several hours.

The remaining suspension of fines was sprayed onto the flat screen portion of the tube using a commercially available air brush while the screen was maintained at about 100-150°C. by

Veeco hot-air blowers. By carefully controlling the air brush pressure (by controlling the gas pressure of the oxygen or nitrogen used to operate the brush) this simple method produced quite uniform screens. After applying the phosphor coating to the screen the tube was then baked at about 250°C. for several hours. An electrical connection was then made to the conduc­ tive film through the small side-arm using a tungsten wire lead and standard seal. Spring contact of the tungsten wire lead provided sufficient conductivity. Finally, the prepared tube was attached to the flange adapter section. 31

C. Image Recording; Photographic Equipment and Procedure

The light Intensity of the image produced on the field-ion microscope screen is extremely low making mandatory the use of ultra-high speed photographic techniques. A Nikon

P 35mm fully-automatic reflex camera with an fl.2 lens was employed in this work. Film speed was enhanced by using a high-sensitivity film, Kodak Spectroscopic Film 103 a-G.

Kodak 2475 Recording Film was also found to be of comparable speed. The latter has the advantage of not requiring refrig­ eration during storage whereas the Kodak 103 a-G Film must be maintained below 32°F. Both films have their greatest sensitivity to wavelengths in the blue-green region as pro­ duced by the screen phosphor. Exposure times were typically of the order of 5 to 7 minutes at an image voltage of 10 kv at liquid nitrogen temperature and about $0% shorter when liquid hydrogen cooling was used.

The Kodak Spectroscopic Film 103 a-G was processed with highly active Ethol developer for 5 minutes at 68°F. The development of the Kodak 2475 Recording Film was carried out in Kodak D-19 solution at 75°F. for 4 3/4 minutes with approx­ imately a 7 second increase for every degree below this base.

Standard Kodak Acid Fixing Solution sufficed for fixing. 32

D. Liquid Hydrogen Cooling; Liquid Hydrogen Transfer Technique

The microscope design shown in Figure 6 was capable of accommodating liquid hydrogen as a coolant when suitable auxiliary equipment was employed as shown in Figure 8. A rather simple transfer technique was employed to introduce the cryogenic fluid into the cold finger of the microscope. A

5 liter dewar of liquid hydrogen was mounted securely behind the microscope and an all-glass, vacuum jacketed transfer tube connected the dewar and cold finger chamber. All connections of this coupling were made leak tight simply by using commer­ cial adhesive tape wrappings. The system was checked carefully for leaks using a commercial hydrogen gas detector. Before transferring hydrogen from the dewar, the cold finger chamber was purged thoroughly with helium gas. The transfer of hydro­ gen was then initiated by pinching-off the dewar exhaust. The pressure build-up above the liquid reservoir was sufficient to cause a steady flow of liquid hydrogen within about one minute.

It Is important that this transfer be carried out slowly, especially during the initial cool down to avoid rapid pres­ sure build-up in the cold finger chamber due to rapid evolu­ tion of hydrogen gas. Hydrogen gas from the dewar and cold finger exhausts was vented through an explosion proof hood mounted on the ceiling above the microscope. This hood pro­ vided excellent ventilation of the working area and ran continuously during any experiment. The use of a glass 33 transfer tube made it possible to transfer during operation without having to turn-off the high voltage.

Liquid hydrogen is a dangerous fluid and explosive conditions can arise with concentrations of hydrogen gas in air as low as 4 percent by volume. The upper explosion limit is about 75 percent by volume. Maximum safety precautions should be employed especially when complete segregation of electrical apparatus is not possible. Efficient ventilation of the working area is mandatory to prevent possible danger­ ous accumulation. Before using liquid hydrogen as a coolant in the field-ion microscope the experimenter should consult the plethora of literature available on the handling of this cryogenic fluid.

E. Materials

The tungsten specimens examined in the microscope and the tungsten loops used for mounting the specimens were pre­ pared from General Electric 218 W wire. The W-3 Re alloy specimens were prepared from General Electric 3D218W tungsten- rhenium alloy wire (see appendix for chemical analysis).

Specimens of commercially pure molybdenum were pre­ pared from a Sylvania powder metallurgical wire product.

Molybdenum-rhenium alloy specimens were prepared from a powder metallurgical wire product supplied by the Rhenium Division of

Chase Brass, Waterbury, Connecticut. The wire was in the as- drawn condition (see appendix for chemical analysis). 34

P. Specimen Preparation

Specimens of the materials examined in this study were generally prepared from 5 to 10 mil diameter wire using essen­ tially the same electropolishing technique. A short piece of wire was polished in a suitable electrolyte to a point, about

2-5 mil, on one end. This reduced section was then spot welded to a 10 mil tungsten wire loop. The loop arrangement was suspended above the electrolyte bath so that the meniscus was just below the loop with the wire immersed. The wire was electropolished until the lower section fell away leaving a short, fine tip mounted on the tungsten loop. After the bottom portion had dropped the electropolishing was continued for a short time to allow back-polishing and thus removal of any deformed region which may have been produced in the vicinity of the separation. Very sharp tips could be prepared readily using this technique.

Tungsten and tungsten-rhenium alloy specimens were prepared using either 1 N sodium hydroxide or potassium hydroxide as an electrolyte and platinum as the other electrode.

Polishing was carried out at 0 to 8 volts A.C. Potassium hydroxide was found to be somewhat better for preparation of alloy specimens.

Very fine tips of molybdenum and molybdenum-rhenium alloys could be produced using a thin layer of 20 percent potassium cyanide solution floating on a bath of carbon tetrachloride. Polishing was carried out at 0 to 10 volts

A.C. with platinum as the other electrode.

No problem of preferential etching at grain boundaries or formation of etch pits occurred in the preparation of these materials using these techniques. This is important in that such effects lead to frequent tip failure in the microscope. IV. RESULTS AND DISCUSSION

A. Streak Contrast in Tungsten-Rhenium Alloys

Specimens of the W-3 Re alloy were examined in essen­ tially four different metallurgical conditions: (1) as-received or as drawn, (2) annealed, (3) as-received material deformed by cyclic bending at room temperature, (4) annealed followed by cyclic bending at room temperature. Annealing was carried out at 1100°C. for 1 hour in dry hydrogen. This treatment provided a recovery anneal. The mode of deformation employed was quite severe. It was hoped that these different conditions could be correlated with the occurrence of streaking in field-ion micrographs of these alloys. More precisely, it was hoped that such a correlation combined with analysis of the nature of the streaking might establish whether the streaks oould be associated with stacking faults or some other intrinsic condition of the lattice.

Figures 9, 10, and 11 show field-ion micrographs of tungsten specimens examined during the course of this work.

Streaking was never observed in micrographs of tungsten in any of the above conditions. Field-ion patterns of W-3 Re exhibiting approximately a 0 3 1 axis instead of a usual

< 1 1 0 > are shown in Figures 12 and 13. These micrographs

36 are typical of the patterns of the W-3 Re alloy specimens examined during this study in the majority of cases in all conditions. The W-3 Re alloy micrographs are not noticeably less regular than those obtained from pure tungsten but do appear somewhat "spotty" in comparison. This was much more evident while examining the pattern on the microscope screen by eye. Variations in intensity may be associated with the rhenium solute because of a difference in ionization probabil­ ity above the solute species. However, unambiguous identifi­ cation of atomic species allowing an analysis of the image to find the distribution of rhenium is not possible at present.

The field evaporation behavior of the alloy was observed to be different from pure tungsten in that atoms or small clusters quite often were observed to persist in protuberant positions on the surface and were not evaporated until the underlying net was evaporated or nearly collapsed. These effects are sometimes observed in tungsten and are generally associated with impurities (see Figure 14). The'field evaporated sur­ faces of the alloy were generally more atomically rough because of irregularities introduced into the field evaporation process. However, this effect was not marked because of the dilute nature of the alloy. As might be expected, the presence of rhenium apparently affected both the field ionization and field evaporation processes.

Streak contrast effects were observed in field-ion patterns of about 20 percent of all specimens examined in conditions (2), (3), and (4). Streaking was never observed in as-received or as-drawn material. This latter result is not considered significant but rather a statistical effect within the limitations of this experiment. Because of the essentially random observation of streaks no correlation with the metallurgical condition of the specimens could categori­ cally be established.

Figure 15 shows streaking in a specimen deformed by cyclic bending at room temperature (condition (3)). Field evaporation of the surface indicated that the streaking had a planar nature and appeared to be on a type plane.

(Note the dislocation structure suggested in the ■[ Ollj rings

In the lower right of the micrograph.) Figure 16 shows the same specimen after subsequent field evaporation. Some splitting of the streaking seems to have occurred. Although the streaking effect did generally appear to behave as a planar feature during field evaporation, its movement was often discontinuous, occurring in bursts. Streaking in a deformed specimen (condition (*0) exhibiting an elliptical cross- section is shown in Figure 17. Again the streaking behaved as if it were a planar discontinuity on a ■^1 1 2 }' type plane.

Figures 18 and 19 show streaking in annealed specimens

(condition (2)) and the behavior of these contrast effects was generally identical to those already mentioned. Both micrographs exhibit elliptical cross-sections and Figure 19 shows a large streak as well as a series of small streaks. 39

The streaking observed in this study of a W-3 Re alloy appeared to have a planar nature and always appeared to be parallel to the line of intersection of •^112} planes with the surface. Streaking was only observed in relatively irregular, distorted, or asymmetric patterns and was never observed superposed on patterns such as shown in Figures 12 and 13. Also, streaking was never '^uncovered" by field evapor­ ation but always appeared during the initial stages of specimen conditioning.

Basically, streaks appear to be a planar type discon­ tinuity or fault and appear as a gross structural feature but are never clearly resolved. Unalloyed tungsten can show images with a considerable amount of streaking1 and carburi- zation of tungsten can lead to streaked images which have been tentatively associated with carbon segregation to stacking

faults. Streaks have also been observed in deformed W-5 Re alloys and have been interpreted as the result of stacking

faults intersecting the surface.1* The streaking observed in this work differed somewhat from the streak contrast observed by Ralph and Brandon in their study of the W-5 Re alloy.

They generally observed rather small streaks; whereas, the

streaks observed in the micrographs of the W-3 Re alloy were more of a gross feature. However, their basic nature appears to be the same.

Streaking can sometimes be unambiguously identified

such as that associated with obvious image superposition or that associated with interfaces such as grain boundaries.1

Asymmetric tips generally show the most drastic cases of streaking. Streaking seems to be found quite often in images 2 from specimens with an elliptical cross-section. Brandon has suggested that streaks are a structural feature introduced during the process of specimen preparation. The streaking is considered to be a result of field focusing over a tip which is-not cylindrically symmetrical. The asymmetry may not be removed by field evaporation, which merely adjusts the princi­ pal radii of curvature at each point on the surface to give a uniform overall field strength. The ion beams generated over such a tip are not radially projected unto the screen, but, in extreme cases may actually cross-over. Similar effects on a smaller scale can occur. These streaking effects are then best regarded as resulting from large differences in local magnification in the field-ion images parallel and perpendicu­ lar to the streaks. Although Brandon's argument is plausible it does not generally explain the specific details of streak contrast effects.

Stacking faults intersecting the surface should pro­ duce a net displacement, corresponding to the resolved com­ ponent of the fault vector at the plane of the surface. Very few clear examples, if any, of such faults have been observed.

The association of streaking with stacking faults in deformed 2| W-5 Re alloys rests mainly on the following criteria: (1) the streaks appeared to be associated with a planar fault and 41 are parallel to the Intersection of ■£ll2j' type planes with the surface which are slip planes in the b.c.c. lattice,(2)

100 percent of deformed specimens showed image streakingwhile only about 10 percent of annealed specimens exhibited this effect, (3) a partial dislocation of the type ^ / ^ l l l ^ could produce a step at the surface giving rise to field enhancement and focusing of field contours; preferential evaporation of atoms situated on such a step would not be likely.

The fact that 100 percent of the deformed specimens exhibited streak contrast in itself suggests that such effects are artifacts rather than a manifestation of Intrinsic faults such as dislocations or stacking faults. Since the field of view of the field-ion image is of the order 10"11 to 10”^2 2 cm the probability of finding a dislocation is usually small even for uniform dislocation densities of 10^ to l O ^ c m ”2 .

(Admittedly, the probability is higher for split dislocations.)

Unless the field stress is producing some unusual behavior of dislocations at or near the surface it seems that the high frequency of streaking is really strong evidence against this effect being associated with stacking faults. Also, through field evaporation never revealed an associated partial; e.g. ^ < 1 1 2 > .

The results obtained with the W-3 Re alloy described in this text also suggest that streaks are artifacts. No correlation between the metallurgical condition of the specimens 42 and the frequency of streaking could be established in this study. Streaking only occurred in irregular and asymmetric specimens and was never revealed by field evaporation but always appeared during the Initial conditioning of the sur­ face. The crystallographic regularity of the streaking, specifically, the fact that the streaks appear to be parallel to slip planes, may be misleading. It is concluded here, 2 essentially in accord with Brandon, that the primary cause of streaking in fleld-ion images is tip asymmetry. This asymmetry is related to the ’’polished endform” or "dissolution endform" produced during specimen preparation. Deformation and alloying could affect this geometry. It is very possible that asymmetries in the prepared specimen surface might exhibit some crystallographic regularity related to the dissolution process occurring during specimen preparation. It is also concluded here that although streaking appears to be an arti­ fact related to faulty tip preparation a general explanation of these effects is still lacking.

The results of this work suggest that the streaking observed by Ralph and Brandon was also the result of faulty tip preparation and not associated with stacking faults since there is little evidence to suspect any gross difference in the nature of dislocations in the W-5 Re alloy as compared to the W-3 Re alloy based on mechanical properties. 43

B. Field-Ion Microscopy Study of Molybdenum and Some Dilute Molybdenum-Rhenium Alloys; Anomalous Field Evaporation Effects

While attempting to make a complimentary study of

Mo-Re alloys anomalous field evaporation effects were observed.

The field evaporation behavior of commercially pure molybdenum,

Mo - .5 atomic percent Re , Mo - 1 atomic percent and Mo - 2 atomic percent Re specimens was studied. The field evapor­ ation behavior of these materials varied markedly from speci­ men to specimen of the same composition under apparently the same experimental conditions. Sudden increases in evaporation rate at a given voltage and sudden image instability at an apparent BIV were quite often observed. Also, in most cases a large number of random spots not occupying regular lattice positions were observed during field evaporation and in the field-evaporated endform. This disorder could not be removed by field evaporation at higher temperatures or by flashing the specimen at high temperatures. It was apparent that the random spots markedly affected the evaporation of the central

£ lioj planes and the development of higher index planes.

Figures 20a and 20b show a field-ion image of commer­ cially pure molybdenum typically observed during this study.

The usual regularity of the pattern is disturbed by the pres­ ence of a large number of random spots. Figure 21 shows a similar pattern of a Mo - .5 Re alloy. Field-ion micrographs of a relatively well-developed image of a Mo - 1 Re specimen are shown in Figures 22 and 23. This specimen did not exhibit any of the above mentioned irregularities during field evaporation or imaging. Molybdenum or dilute molybdenum alloys never show an image of uniform intensity but always exhibit a bright triangular region around the lll^ poles.

These contrast features are related to variations in surface energy, work function, polarizability, and energy of the Fermi surface as well as to the radius of curvature of the tip.

The blurred <^00l')> region is the result of ionization of He in space at relatively large distances from the surface. The local field enhancement is associated with the small radius of curvature which is maintained in this low work function area.

Also characteristic of Mo patterns is the appreciable field induced deformation of the 111/* areas. The amorphous nature of these regions can be attributed to dislocations at the surface or can be the result of strain energy release by dislocations emerging during field evaporation. Figures 24 and

25 show endforms of Mo-1 Re alloy specimens which did show anomalous field evaporation effects. Again the presence of a large number of random spots is quite evident. Figure 26 shows an extreme condition observed in a Mo-2 Re alloy. Ho Brandon also has recently reported marked irregular­ ities in the field evaporation behavior of Mo. He attributes these irregularities to strongly bound non-metallic impurities and to plastic deformation accompanying the field evaporation 46 process. Nakamura and Mueller have observed similar endforms of tantalum containing oxygen but were able to restore the 45 surface by field evaporation. In addition, Mueller has reported very amorphous field-evaporated endforms in high percentage 47 alloys of Mo - Re. 4l Brandon has considered the field evaporation of dilute alloys in terms of appropriate thermodynamic quantities and ionization potentials based oncne* dimensional potential energy curves. He shows that non-metallic impurities can be prefer­ entially retained during field evaporation of the matrix and has established criteria for the possible evaporation of molecular ions. This treatment will be outlined in the fol­ lowing since it is directly applicable to the results quoted above. The discussion is confined to field evaporation at low temperatures of alloys at infinite dilution. Thus each of the second components is assumed to be unaffected by clustering or ordering and is surrounded by atoms of the solvent matrix.

Polarization effects will be neglected.

The average energy required to remove an ion of charge n from the metal lattice has been shown to be given by37,38 Q = -A. +■ — -n cf> O ''VX. where — is the average energy required to remove the uncharged atom or molecule (the sublimation energy in a pure metal), is the sum of the ionization potentials, is the averaged t work function. The image force theory gives Q 0 = where Fe (n) is the field required to evaporate the ion at 0°K. and the charge of the ion can be predicted from the condition 46 that Fe (n) is an m i n i m u m . 39

For a monatomlc ion of a solute-A. is given to a first approximation by one of the following expressions:

_A_ Ci -A-0 " A -A. oi + H s

- A _ a E d +

Here -A. 0 is the heat of sublimation of the pure solute, -A H$ is the relative partial molar enthalpy of the solute in

Infinite dilution; Hs is the heat of solution per gramatom of solute; Ed is the dissociation energy per gramatom when the solute is a diatomic gas in the standard state, and Ha is the heat of adsorption per gramatom of the gas in its standard state. For some diatomic gases, values of Hg as well as Ha have been determined and it has been observed that Hg < Ha ; indeed in general Ha > Hs , where - A is the heat of formation of the appropriate compound per gramatom of the gas.

For field evaporation of the second component, Ha is the quantity required, but the difference must be shared between neighboring solvent atoms at the surface and subtracted from the value of Q0 for these atoms. In other words, if the second component has a lower free energy at the surface than in the bulk, then the difference in free energy must occur as a reduction in the average bonding energy of nearest neighbor atoms when these reach the surface. Therefore, as field 47 evaporation proceeds, solvent atoms will evaporate preferen­ tially if by doing so they uncover underlying impurity atoms.

The energies to evaporate oxygen, nitrogen, and carbon in a variety of solvents based on available data for H& , Hg , o r - ^ H f are listed in Table 3. These calculated values of and Q 1 are compared with the corresponding Q' for the solvent 2 n matrix. (Q^ is the activation energy for the evaporation of an ion of charge n.) The application of the image force theory or image potential model to non-metallic impurities thus predicts evaporation fields more than twice those of the solvent metals. The use of the intersection model would not effect these high values of predicted for these non-metallic impurities.

An alternative to the simple image force model worthy of consideration is the possibility of evaporation of a molecular ion. This situation involves simultaneous removal of a solvent atoms and impurity species as a molecular ion complex. The condition that Q0 for the molecular ion should be less than Q0 for removal of a single impurity species is given by

-A-.v\ “ C + ^OL )' ^ ^ nrv ^ no­ "^“d where A is the sublimation energy of the molecule from the metal lattice, is the sum of the ionization potentials of the non-metallic impurity, is the sum of the ionization potentials for the molecular species. Molecular ions are 48 expected for large values of Ha as long as the molecular species Is reasonably volatile. Oxygen on molybdenum may 4l evaporate as a molecular Ion complex.

Two Important results stem from the above discussion.

Firstly, non-metallic impurities should be preferentially retained during field evaporation especially when the solvent matrix has a relatively low evaporation field such as molybdenum. Such effects should not be observed in tungsten because of the high evaporation field of tungsten. Secondly, there is a strong possibility that Mo-0 complexes can affect the field evaporation process of Mo when oxygen is present as an impurity at the surface. It is suggested here that the random spots observed in the field-ion images of Mo and dilute

Mo-Re alloys are associated with strongly bound non-metallic impurities or possibly associated complexes. Oxygen could originate as a bulk impurity. Carbon could originate as a bulk impurity as well as from the vacuum ambient because of the employment of oil diffusion pumps in the vacuum system.

Nitrogen adsorption from the vacuum background as well as from the bulk must also be considered. The apparent high concentra­ tion of impurities at the surface would result from accumula­ tion during field evaporation. The presence of Re in solid solution can affect the association and interaction of oxygen 4 n with the surface. As suggested by Brandon, plastic deforma­ tion during imaging or field evaporation can explain the other irregularities noted. Thus, field-ion microscopy of molybdenum and molybdenum base alloys appears to be extremely dependent on the precise metallurgical condition of the material employed and on variations in vacuum background. V. CONCLUSIONS

1. Streak contrast effects observed In field-ion micrographs are artifacts; no relation between metallurgical condition and streaking could be established for the case of a W-3 Re alloy. The primary cause of streaking is associated with tip asymmetry produced during specimen preparation.

2. Strongly bound non-metallic impurities can lead to marked Irregularities in the field-ion images and field evaporation behavior of metals of relatively low evaporation fields such as Mo. Field induced plastic deformation can lead to image instabilities and irregularities in the field-evapor­ ated endforms of Mo and dilute Mo-Re alloys. Field-ion microscopy of Mo and Mo-base alloys is apparently dependent on the precise metallurgical condition of the specimen material and on variations in vacuum background.

50

\ FIGURE 1

IONIZATION OF HELIUM IONS IN FIELD ION MICROSCOPE Specimen +3 to 30 KV Tip

Ionisation zones over atom ic------protrusions Gas atoms attracted by polarisation forces

Radial ion trajectories

Phosphor Screen FIGURE 2

EFFECT OF APPLIED FIELD ON ELECTRON ENERGY / S e a

lb) 55

FIGURE 3

FIELD-ION IMAGE OF A TUNGSTEN TIP

The plane at the center of the image is the ( 110) . Tip temperature is 77°K. Helium pressure is 1 micron. 4. i'- vv

* • V • ' 0 . * t / ••. - v - /. % ' * « « . • oo* 1 *» 0 # » « * N k * V V’ * ' * <% 0 « •

AV fe: m , « • > ' * 0

■ T . - v -

'. *•*. : •*. .-.• i-O. . X m ' - • 400 .t *'dL / _ \ •' *> ‘*^ 2IC«

. ' ■ . . ■ v ^ - r ; FIGURE H

ATOMIC AND IONIC POTENTIAL CURVES VA(x) Iae Potential Image I

VtU) max X nFex - c) (c \ 00 ji FIGURE 5

POTENTIAL ENERGY CURVES FOR FIELD EVAPORATION 0 V(x) (o) Ionic Curve tmc Curve Atomic x e F n - > ) b ( Curve Atomic Curve ) c ( oi^s lonic^ v-nFex CurveIoAtomic ic's n o o\ Curve FIGURE 6 a) FIELD-ION MICROSCOPE

FIGURE 6

(b) SCHEMATIC DIAGRAM OF THE FIELD-ION MICROSCOPE 64

© ©

To Vacuum

Standard 2 1/2" Varian OFHC Copper Gasket Conflat Flanges

111*—-" ~j Liquid Nitrogen or Liquid Hydrogen

Aluminum Cathode Cylinder Specimen Tip

Conductive Screen Coating FIGURE 7

SCHEMATIC DIAGRAM OF THE VACUUM SYSTEM Metal Bellows To Microscope Chamber Magnetic Valve Ionization Gauge

Cold Trap Zeolite with Zeolite Trap

Helium Diffuser Oil Vacuum Diffusion Pump Stopcock

Fore Pump 67

FIGURE 8

SCHEMATIC DIAGRAM OF LIQUID HYDROGEN TRANSFER EQUIPMENT 41

Glass Transfer Tube

To Hood Exhaust

To Hood Exhaust

Field — Ion Microscope

5 Liter Dewar

Liquid Hydrogen FIGURE 9

TUNGSTEN: 77°K. 1 micron He 70

♦ 4

• * * ► m * . * ' 4 : * ' i

%% • % % % * V

• • , <* V * r *

S' ■'•# « •«

4 . * • %•*** V * * I V *• £ • • % % # FIGURE 10

TUNGSTEN: 77°K. 1 micron He M

4

w * - *

% » *

•

f. * fA' % . t *\%L

* I FIGURE 11

TUNGSTEN: 77°K. 1 micron He

FIGURE 12

FIELD-ION MICROGRAPH OF W-3 Re ALLOY EXHIBITING

APPROXIMATELY <231> AXIS

77°K. 1 micron He 76

♦ 77

ff.l

FIGURE 13

FIELD-ION MICROGRAPH OF W-3 Re ALLOY EXHIBITING

APPROXIMATELY <'231> AXIS

77°K. 1 micron He /:

0 0

• . \ , > . , ¥ * " V ! •

" , * $ m i ■* > -■ • * <•• > ‘‘W ♦ V • t * , '• 4

/ 5 r v # * : s ... • •*

f* : #

.,*-»/*/ * * V * *. - *-4 ‘ • • s • * * I . ‘ • i ' * M ••*,. .. 7 / V •. .. • • V& '* . 1 \ >*• \> '« 4 » sAivi' » - * V # • .. • .. | f v * • '

. •* «t*/ ■;, •*« % . I; fV*V ••",* • I.*;-:, - ;

. \ •%. *\v •■. FIGURE li\

CLUSTER MAINTAINED IN (110) NET DURING FIELD

EVAPORATION OF TUNGSTEN

77°K. 1 micron He 80

+ «•

I*

< * ’>: ", * s * «

* A • , • >.. . * . *•: / , ' ♦ f V%

" # * ♦* - L * / : i I V • * .v. % . * f FIGURE 15

STREAKING IN DEFORMED W-3 Re ALLOY

77°K. 1 micron He 82 FIGURE 16

STREAKING IN DEFORMED W-3 Re ALLOY

77°K. 1 micron He

FIGURE 17

STREAKING IN DEFORMED W-3 Re ALLOY

77°K. 1 micron He

FIGURE 18

STREAKING IN ANNEALED W-3 Re ALLOY

77°K. 1 micron He V ., V" ... ..V . T i

* «

# * t I % « i 4 *■ « ft v v * i

i * |l *•# ♦ I FIGURE 19

STREAKING IN ANNEALED W-3 Re ALLOY

77°K. 1 micron He

91

FIGURE 20

(a) HELIUM FIELD-ION IMAGE OF COMMERCIALLY PURE MOLYBDENUM

21°K. 1 micron He 92 FIGURE 20

(b) COMMERCIALLY PURE MOLYBDENUM

21°K. 1 micron He 94 95

FIGURE 21

Mo-.5 Re ALLOY

21°K. 1 micron He - % » * • ' ______

. j I t

*- > -x . S i

% . . Bk. k b.. * V,. » k ^ * ' * • ‘ V i * ♦ , . * ; - ' J> k ll v V

•*.*r -. 4 • > i v **•* ' .•« jp « * • - \ .»** m ' > t v* •:•>*. (

.. * . • % •• > < *...

■ . ?*<"-* . *. t wa .•>• s« 4 * * . •» •* ' # « * •< ' • *• ».*« ♦ « # «. «. * i . '*•••.•♦ ,*< IU ,«• ,* * *# t . v , . *' »r ‘ *t 4 *'»'•■

> ., ** * • V.^ * * I-,' . H . . ^ V 4 . 7 t* 9 .... *" ..* . • . •%*'.. ► ■ * * * ^ ..i % m * V* ?:•' * •• Fk * ► » • j • .-v: ■• • v „ • •V* IV - - * * - FIGURE 22

Mo-1 Re ALLOY

21°K. 1 micron He

FIGURE 23

Mo-1 Re ALLOY

21°K. 1 micron He 100 101

FIGURE 24

Mo-1 Re ALLOY

21°K. 1 micron He 102

t . •:#.***»"’•»• v»% • .. • '2* -r • ► • *" • * stA?*-' • •

"• > *•' * * 'x % ' ** * • «*•> * **-* ► *• 1 T ' i* -t •••?; :* * ^ I .V _V .il; • * ? >-■'' ■ / > * « > * ' ' V . v ‘ * * * ' % «. • ~•

» I ' ’> • % * / . 4 * * * M \ : ; \ •: • x *: • r %' * ; i % y * 4 vvv ' • r A t r*.

\V ? v * ' •“ - •- FIGURE 25

-1 Re ALLOY

K. 1 micron He 104 FIGURE 26

Mo-2 Re ALLOY

°K. 1 micron

107

TABLE 1

FIELD EVAPORATION OF IONS AT 0°K

-A- I Fe Element (ev) (ev) (ev) (v A )

C 7.40 11.26 4.34 14.28

W 8.67 7.98 4.35 7.86

Ta 8.10 7.70 4.20 9.30

Re 8.10 7.87 5.1 8.19

Ir 6.50 9.2 5.0 7.92

Nb 6.87 6.88 4.01 6.59

Mo 6.82 7.13 4.30 6.48

Pt 5.84 8.96 5.32 6.26

Zr 6.33 6.84 4.12 5-69

Be 3.38 9.32 3.92 5.36

« oo

Rh 5.77 7.46 • 4.95

Ru 5.52 7-36 4.52 4.85

Si 4.90 8.15 4.80 4.73

Au 3.68 9.22 4.82 4.54

Fe ,4.30 7.90 4.17 4.48

Co 4.40 7.86 4.40 4.29

U 5.09 6.0 3.27 4.24

V 5.32 4.74 4.4 4.08

4.92 6.83 4.17 3.99

4.08 8.33 4.99 3.83

Ni 4.38 7.63 5.01 3.51 108

TABLE 1 (continued)

... _ _/V X Element (ev) (ev) (ev) (V/S)

Ge 3.99 7.88 4.80 3.47

Cu 3.52 7.72 4.55 3.12

La 4.33 6.51 3.3 3.07

Hg 0.64 10.39 4.52 2.94

Zn 1.36 9.39 4.31 2.88

Adapted from Mueller.*0 109

TABLE 2

CALCULATED EVAPORATION FIELDS

Atomic Number Element (V/ft) Ion Expected

4 Be 4.6 Be2+

5 B 6.5 B+

6 C 8.2 C2+

12 Mg 2.4 Mg2+

13 A1 1.8 Al2+

14 Si 3.4 Si2+

20 Ca 1.6 Ca2+

22 Ti 2.3 T12+

23 V 2.6 V2+

24 Cr 3.1 Cr2+

25 Mn 2.8 Mn2+

26 Fe 3.6 Fe2+

27 Co 3.7 Co2+

28 Ni 3.5 Ni2+

29 Cu 3.1 Cu+ „ 2+ 30 Zn 3.5 Zn

31 Ga 1.6 Ga+

32 Ge 2.9 Ge2+

40 Zr 3.1 Zr2+

41 Nb 3.4 Nb2+

42 Mo 3.8 Mo 2 +

44 Ru 3.8 Ru 2+ 110

TABLE 2 (continued)

Atomic Number Element (V/X) Ion Expected

45 Rh 3.9 Rh2+

46 Pd 3.8 Pd2+

47 Ag 2.3 Ag+

48 Cd 3.1 Cd2+

50 Sn 2.3 Sn2+

57 La 2.5 La2+

73 Ta 4.6 Ta2+

74 W 5.7 w2+

75 Re 4.3 Re2+

76 Os 4.6 Os2+

77 Ir 4.4 Ir2+

78 Pt 4.1 Pt2+

79 Au 4.3 Au 2+

80 Hg 2.9 Hg+

81 T1 1.1 Ti+

82 Pb 2.3 Pb2 +

83 Bi 2.6 Bi2 +

Adapted from Brandon.^ TABLE 3

ENERGY TO PIELD-EVAPORATE NON-METALLIC IMPURITIES

) 2 1 ) Q ^ v ) ( *Vl ^ Metal 0 NC (eV) 1 2 Metal 0 N c Metal 0 N C

Fe 4.2 4.2 3.1 6.6 4.17 7.9 24.4 8.0 13.6 13-4 13.6 7.9 15.7 13-7 11.9

N1 4.4 3.0 3.7 7.0 5.01 7.63 25.8 7.08 11.5 13.2 13.2 7.15 14.7 13.3 11.5

Mo 6.8 5.4 6.0 7.3 4.30 7.13 23.3 8.98 14.6 16.2 14.2 7.4 16.0 14.6 12 t 7

W 8.7 5-5 4.5 6.7 4.52 7.98 25.7 12.10 14.6 14.5 13.4 9.0 15.9 14.0 11.8

Pt 5.3 3.0 -- 5.32 8.96 27.5 9.26 11.2 - - 7.7 14.5 - -

X i „ i 13.55 14.48 11.22 II =sum of ionization. potentials of non-metallic impurity. 'n. O (eV)2 48.48 43-95 35.49 II n=sum of ionization, potentials of metallic species. n

Adapted from Brandon.^ APPENDIX

Typical Impurity Analysis for Grade 218W Tungsten and

3D218W Tungsten-Rhenlum Alloys (General Electric)

a) Interstitial Impurities (ppm)

C 307

0 52

N 42

b) Substitutional Impurities (ppm)

A1 102 Ca 46

Fe 50 Si 66

Cr 35 Mn 33

Ni 31 Mg 76

Cu 29 Sn 15.5

Mo 115 Co 31

Ti 38

Ag 17

Pb 9

Zr 20

112 Typical Impurity Analysis for Commercial Grade Molybdenum

(Sylvania)

a) Interstitial Impurities (ppm)

C 50-80

0 50-60

N 1-10

H 1-4

b) Substitutional Impurities (ppm)

Si 150

Mn 3

Mg 6

Pb 8

Sn 9

Cr 9

Ni 158

Fe 111

Cu 6

A1 21

Ca 23 114

Typical Impurity Analysis for Molybdenum-Rhenium Alloys

(Chase Brass) a) Interstitial Impurities (wt.Jf)

C 0.002

0 0.003

N 0.001

H 0.0001 b) Substitutional Impurities (ppm)

A1 30

Ca 5

Cr 3

Cu 5

Fe 52

K 1

Mg 5

Mn 1

Fe 66

Na 1

Ni 12

Si 15 BIBLIOGRAPHY

1. Ranganathan, S., Bowkett, K. M., Hren, J., and Ralph, B., Phil. Mag. 12, 841 (1965).

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3. Ralph B . , and Bowkett, K. M . , Phil. Mag. 13, 1283 (1966).

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9. Mueller, E. W. , Z. Physik 131 , 136 (1951).

10. Mueller, E. W., Adv. Electronics Electron Physics 13, 83 (I960).

11. Gomer, R., Field Emission and Field Ionization, Harvard University Press (1961).

12. Brenner, S. S., Metal Surfaces A.S.M., p. 305 (1962).

13- Cottrell, A. H., J. Inst. Metals £0, 449 (1962).

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15. Brandon, D. G., Modern Techniques of Metallography, Van Nostrand, Princeton, New Jersey, p. 197 (1966).

16. Brandon, D. G., Hlgh-Temperature and Hlgh-Resolution Metallography, edited by Ansell, G. S. and Aaronson, H,, A.I.M.e T (1966).

115 116

17. Oppenheimer, J. R., Phys. Rev. 31, 67 (1928).

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30. Inghram, M. G., and Gomer, R., Z. Naturforsch 10a, 863 (1955).

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33. Brandon, D. G., Phil. Mag. 7, 1003 (1962).

34. Sonthon, M. J. , Field-Ion Microscopy Short Course, University of Florida (1966).

35. Mueller, E. W., Acta Met. 6, 620 (1958).

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37. Mueller, E. W., Phys. Rev. 102, 618 (1956). 117

38. Gomer, R. , and Swanson, L. W. , J. Chem. Phys. 38, 1613 (1963).

39. Brandon, D. G., Surface Science 3, 1 (1965).

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