1

William Meere 2007

Development of Atom Probe Specimen

Preparation Technique for Ti6Al4V

Supervisors: Professor S. Ringer, Dr. B. Gault

ELECTRON MICROSCOPE UNIT

HEADQUARTERS OF THE AUSTRALIAN MICROSCOPY & MICROANALYSIS RESEARCH FACILITY AUSTRALIAN KEY

CENTRE FOR MICROSCOPY AND MICROANALYSIS

ARC CENTRE OF EXCELLENCE FOR DESIGN IN LIGHT METALS

WWW.EMU.USYD.EDU.AU

2

Table of Contents

I. Introduction ................................................................................... Error! Bookmark not defined.

II. Experimental techniques................................................................................................................5

The Atom Probe ..................................................................................................................................5

Specimen Preparation.........................................................................................................................7

Difficulties of Ti6Al4V specimen preparation ...................................................................................11

III. Mechanism of oxidation of Titanium surfaces.............................................................................12

Low Temperature Oxidation of Titanium .........................................................................................12

SEM analysis of Ti6Al4V specimens ..................................................................................................16

Suggested advanced oxidation mechanism......................................................................................19

Potential Solution .............................................................................................................................20

IV. Conclusion ....................................................................................................................................24

V. References ....................................................................................................................................26

Cover Page Image: APT map from a Ti-6Al-4V wt.% alloy heat treated 48 h at 900 ºC. Courtesy of T.

Honma

3

I. Introduction

Ti6Al4V remains the most heavily used titanium alloy, accounting for 50% of all titanium tonnage in the world (Boyer et al. 1994). Interest in Ti6Al4V remains high because of the prevalence of the material in the aerospace industry. The alloy’s light weight, high tensile strength, good high temperature mechanical properties, good creep resistance, and important corrosion resistance offers Ti6Al4V as an important and valuable material (Boyer et al. 1994), especially in the construction of aerospace vehicles and engines (Fig. 1).

Fig. 1. Incorporation of Ti6Al4V fan blades in the turbine of an aeroengine. Image courtesy of Hill, S. of Rolls

Royce.

Ti6Al4V is a titanium alloy that supports a mixture of alpha (α) and beta (β) phases at room temperature and is hence termed an alpha-beta alloy. The 6% (by weight) Aluminium component acts as a low temperature α-phase stabiliser, while the 4% (by weight) Vanadium component stabilises the small amount of high temperature β-phase. The resulting microstructures of the alloy can thus be described as grains consisting of pure alpha-phase surrounded by a mixture of alpha and a small amount of beta (Sittig et al., 1999). Importantly, the final microstructure of the Ti-alloy is dependent on the heat treatment it is subject to during formation and the alloying elements, as a result of changes to the stability of the α and β phases (Fig. 2). This investigation concerned Ti6Al4V alloys subject to two different heat treatments, each with a unique microstructure (Fig. 3). Microstructural analysis of Ti6Al4V remains important in better understanding the way in which physical properties and microstructure are related. The advent of the atom probe and its recent improvements that have seen the development of the Local Electrode Atom Probe (LEAP) provide one of the most interesting means by which to accurately analyse the microstructure of this material. Indeed, several recent studies, including those conducted by Larson and Miller (1999), and Lefebvre et al., (2002a,b) and have successfully utilized the atom probe to understand the phase structure and behaviour of varied titanium alloys.

This report aims to suggest improvements to current specimen preparation techniques applied to Ti6Al4V so as to able to easily and effectively analyse this important material in the atom probe. The suggestions put forward are phrased through an investigation of the Titanium oxidation mechanism, which is believed to be responsible for the difficulties of Ti6Al4V specimen preparation experienced up to this point in time. This paper begins with an introduction to the Atom Probe and current specimen preparation techniques, before explaining the difficulties of Ti6Al4V atom probe specimen preparation. An investigation into low temperature Titanium oxidation is then provided, and is followed by suggested amendments to current specimen preparation apparatus.

4

Fig. 2. Phase diagram of Ti6Al4V , showing four common heat treatments. Image Courtesy of Boyer et al.

(1994).

Fig. 3. Backscattered electron images of the microstructure developed in ultra fine grain Ti-6Al-4V during heat

treatment for a) 48h at 900oC and b) 4h at 955

oC. Images courtesy of Semiatin et al. (2004).

5

II. Experimental techniques

The Atom Probe

The development of the atom probe made the identification of selected individual atoms possible for the first time, allowing for the fundamental chemistry of a sample to be understood at the nano-meter scale (Miller et al., 1996). Essentially the atom probe draws on ideas utilised by the Field Ion Microscope combined with a mass spectrometer to allow for the position and identification of an atom to be assessed (Fig. 4). The microscope relies on a high electric field to ionise individual atoms from a specimen, then determines the original position of the atoms in the specimen by detecting their x and y positions on a position-sensitive single-atom detector. The z coordination of the atom may then be deduced from data including the evaporation sequence of atoms. The individual atoms are then identified on their mass-to-charge ratio, as figured by a time of flight mass spectrometer. The time of flight is recorded as the time between the high voltage pulse or laser pulse, used to ionise specimen atoms, and the arrival of the ion on the position sensitive detector.

Fig. 4. Schematic of Local Electrode Atom Probe (LEAP).

The process of field remoaval of an atom from its own lattice is called field evaporation. It requires an electric field strength in the range of 2 - 6 x 1010 V m-1 for common metals (Miller et al., 1996). Considering that the common potential applied to specimens in field ion microscopy is commonly 10kV, and that electric field production is a product of the applied potential (V0), the specimen apex radius (r0) and the field factor constant (kf - which is dependent on the taper angle of the specimen), such that

(9)

6

it is necessary to produce specimens for that have an end radius of between 50-100nm (Miller et al., 1996; Miller, 2000) (Fig. 5). Moreover, the importance of producing a fine hemispherical apex tip may also be understood.

Fig. 5. Tip suitable for atom probe analysis. Blurring of tip end indicates an appropriate tip radius, as tip

becomes too small to be viewed by visible light. Image courtesy of Choi et al., (2007).

Time-of-flight mass spectrometry uses known charge and distance to determine the mass of an unknown charged particle; m/z is based on the time it takes an ion to move from the specimen to the detector. If ions achieve their final velocities instantaneously then

(2)

Therefore, mass-to-charge ratio may ultimately be expressed as

(3)

where d is the know and constant distance in meters travelled by the ion from the specimen to the detector, e is the charge of an electron and t is the flight time of an ion over that distance in microseconds. Vo is in volts. (Miller et al., 1996).

The resolution of the time-of-flight mass spectrometer is dependent on equality of kinetic energy among ions of differing mass. Consequently a large kinetic energy spread among field evaporated ions would reduce the mass resolution of the atom probe (Kelly et al. 1996, Tsong, 1990). This is of importance for this investigation when considering that some elements of an alloy or compound may preferentially field evaporate from the specimen between the high energy pulses (Tsong, 1990, Kelly and Miller, 2007). In order to rectify this, the energy of the pulse must be raised to a high value (at least 15% of the total voltage). (Tsong, 1990). This produces a large kinetic energy spread of the field evaporated ions that consequently reduces mass resolution of the atom probe (Tsong, 1990).

The microscope used in this investigation is the local electrode atom probe, LEAP. This microscope incorporates a time-of-flight mass spectrometer and atom characterization and positioning techniques first used in the three dimensional atom probe (3DAP) (Kelly and Miller, 2007). While the 3DAP utilizes conventional time-of-flight mass spectrometry, it incorporates a position-sensitive single-atom detector in the mass spectrometer (Kelly and Miller, 2007). As a specimen atom is field evaporated the ion is project onto the position-

7

sensitive single-atom detector. The atom's hit position on the detector translates to its lateral position (i.e. its position in x and y). As the field evaporation for specimen atoms can be tightly regulated (as discussed above) an evaporation sequence can be established. The atoms position in z can thus be deduced from its position in the evaporation sequence (Kelly and Miller, 2007; Miller, 2000).

Significantly however, the local electrode atom probe positions the counter electrode of the microscope far closer to the specimen than in 3DAP and all other previous microscopes .The electrode is positioned around one aperture diameter distance away from the specimen tip (Kelly and Miller, 2007) (Fig. 6). This greatly enhances the electric field at the specimen tip, allowing the fields required for field evaporation to be obtained with a lower applied voltage (Kelly et al. 1996). A local electrode also allows for greater mass resolution in the time-of-flight mass spectrometer. The ions to reach the electrode very fast and thereafter are shielded from time-varying fields, consequently increasing mass resolution (Kelly and Miller, 2007).

As may be deduced, the preparation of a suitably fine needle shaped tip is of prime importance to effective specimen analysis. There are several techniques available that work to produce the required specimen tip.

Fig. 6. Ion milled specimen tip protruding from the milled V-shape in a TEM specimen disc. Cone shaped

electrode with aperture positioned directly opposite.

Specimen Preparation

The most common specimen preparation technique, and the one applied to titanium alloys, is electropolishing. This is the process by which a specimen is prepared by immersing the sample post in an electrolyte and applying a voltage, thereby crafting a tip via electrolysis of the sample post material.. It is often carried out as a two step process (Fig. 7). During the first stage, a thin layer (5-7mm) of electrolyte is floated over a dense, inert iquid such as carbon tetrachloride (Miller et al., 1996; Miller, 2000; Tsong, 1990). When a voltage is applied, material is rapidly and uniformly removed from the specimen to produce a necked region. The second stage follows, where the necked sample is moved to a vessel containing only electrolyte and material is removed at a much slower rate. This is allowed to continue until the weight of the bottom half of the sample is too great to be supported by the necked region, at which time the lower half disconnects from the upper and two specimens result (Miller et al., 1996; Miller 2000). While this may be seen as an effective means to produce

8

two specimens for atom probe analysis, Tsong (1990) contends that if carried out with too longer sample posts this procedure produces lattice defects in the sample. This may be of little use therefore to the scientist looking to understand the unaltered structure of the alloy. Moreover, this method may produce an uneven tip surface from where it fractured, producing a dropping off of electric field and consequently skewed images (Tsong, 1990).

A second, finer method for specimen preparation and/or specimen repair is micropolishing. This involves suspending a film of electrolyte in a small wire loop of an inert metal, commonly Platinum, with a diameter of approximately 3mm (Miller, 2000) (Fig. 8). Viewed under an optical microscope, the specimen is then inserted into the film of electrolyte. A voltage is applied between the specimen and the electrolyte and electropolishing takes place only on the parts of the specimen in the electrolyte film. Polishing takes place under the view of an optical microscope so as to be able to polish specific regions of the specimen. The specimen is mounted on a platform movable in the x and y dimensions, allowing for the specimen to be moved precisely. This movement allows for a tip to be crafted by differing the time specific points along the specimen needle that polishing is allowed to continue.

Considering the need for a fine specimen tip, micropolishing may be the best specimen preparation technique. The micropolishing outlined above however is still somewhat rudimentary. This is because the continual application of voltage makes it difficult to remove very fine amounts of material when the tip begins to reach the suitable diameter (approximately 50-100 nm). That said, the application of a continued voltage during micropolishing is however suitable for producing a basic tip from which a finer tip may be developed. This basic tip is crafted in much the same manner that specimens are created in conventional electropolishing. The electrolyte attacks a region of the specimen, producing a “necked blank”. Soon, the weight of the far end of the specimen becomes too heavy to be supported by the necked region and the sample breaks. As the electrolyte continues to work this necked region is gradually eroded. At this point in time, the electrolyte concentration may be reduced so as to allow for finer polishing. As is often the case however, the horizontal orientation of the specimen during micropolishing means that when the necked region can no longer support the weight of the farther end of the specimen, this father end falls downward and is likely to bend the remaining tip. This may be rectified with a more dexterous method of specimen preparation that combines micropolishing with pulse polishing.

9

Fig. 7. Standard electrochemical polishing. Stage 1 is used to create a necked blank, while stage 2 completes

the preparation at a much slower rate, often producing two specimens. Image courtesy of Miller et al. (1996).

Pulse polishing itself is often used to produce a specimen with a desired structural feature at the specimen apex, such as a grain boundary. As described by Miller (2000) the specimen is first examined in a transmission electron microscope and the desired structural feature is identified at a particular point along the needle-shaped specimen. The specimen in then transferred to a conventional electropolishing cell (see Fig. 7(A)) where small amounts of material are removed by applying voltage is the form of short pulses. The short pulses allow for sufficiently small amounts of material to be removed so that the specific desired feature may consequently be in the analysable volume of the specimen (Miller et al., 1996). While this technique of examining the specimen and then pulse polishing it may be suitable for minor refinements, it is time consuming. Moreover, if specific structural features are not sought, the use of the transmission electron microscope in specimen preparation for the atom probe becomes somewhat redundant.

This idea of using short voltage pulses to remove small quantities of specimen material is however, easily applied to the micropolishing technique. In this way, a much finer tip than would be possible with continued voltage can be produced. An electrolyte of lowered concentration is set up as a film in the wire loop and the specimen is introduced through this film. Under approximately 30x magnification the blunt specimen tip is positioned so as to be viewed extruding from the farther surface for the electrolyte film. By means of a switch or by manually completing a broken circuit, DC voltage may be applied in short pulses. The short DC voltage pulse is applied the specimen tip is drawn backward, in the x dimension, into the electrolyte insofar as only the very tip of the specimen protrudes out the electrolyte surface film (Fig. 9.)

10

Fig 8. Micro-polishing. Specimen is moved in the x dimension to preferentially electropolish a point along the

specimen, thereby crafting a finer tip. Image courtesy of Miller et al. (1996)

Fig 9. Micro-polishing using intermitted DC voltage. Tip is withdrawn into electrolyte insofar as only the very

tip protrudes from the farthest electrolyte film surface. Image courtesy of Miller et al. (1996).

11

Difficulties of Ti6Al4V specimen preparation

While the analytical techniques of microscopy may be well advanced, the field of literature regarding titanium alloys examined by atom probe technology is relatively limited. There remains extensive literature on suitable specimen preparation techniques for the LEAP (see for example Miller et al., 1996; Miller, 2000; Tsong 1990) yet success in preparation of titanium alloys continues to be limited. It is suggested that this is primarily due to current techniques ignoring the difficulty of oxidation of the titanium alloy specimen during preparation. Indeed, great difficulties were encountered during attempts to prepare Ti6Al4V specimens using conventional techniques. Principally, the specimen could not be tapered to a fine enough point as to achieve the required radius of <100nm, and the specimen often fractured. Analysis under an optical microscope showed the distinct formation of an irregular white oxide, coating the Ti6Al4V alloy (Fig. 10). It was suggested this was the cause of the increased fracture rate and the difficulty sharpening the specimen. Consequently, the process of Ti6Al4V oxidation was investigated as a potential means to understand factors that may reduced its formation.

Fig 10. Ti6Al4V specimen viewed under optical microscope at 40X magnification. Specimen prepared by micro-

polishing with intermitted DC voltage applied using 2% perchloric acid, 37% buthanol, 61% ethanol electrolyte

and a voltage of 12V.

12

III. Mechanism of oxidation of Titanium surfaces

Low Temperature Oxidation of Titanium

The process of oxidation is fundamentally characteristic of this particular material. Titanium and its alloys are particularly useful because they are resistant to corrosion, due to a thin 4-6nm oxide film that forms spontaneously when the metal surface is exposed to air or water (Lausmaa et al., 1989), passivating the alloy surface. The predominant species in the oxide layer are titanium oxides. Delogu et al. (1994) investigating Al2O3/Ti6Al4V interfaces has shown that despite the presence of Al and V in the alloy and their appearance in their oxidised state in the oxide layer, the main constituent of the air oxidized alloy is titanium oxide, in the form TiO2. Indeed, Sittig et al (1999) studying Ti6Al4V and others under nitric acid treatment, have shown that the predominant oxidation state displayed by Ti is (II), i.e. TiO2 is the primary constituent of the oxide layer. This is in agreement with observations made by Vergara et al. (2002) studying high purity polycrystalline Ti, who show that at room temperature only TiO2 is present in the oxide layer produced. These results remain aligned with current literature on titanium surface films (see, for example, Moulder et al., 1992. Callen et al., 1995. Raikar et al., 1995, Lausmaa et al., 1989). It is therefore concluded that the principle reaction occurring during oxidation of Ti6Al4V is the formation of TiO2 from elemental Ti(II) and gaseous O2.

Originally low temperature oxidation of metals was assumed to follow the mechanism outlined by Cabrera and Mott (1949), who considered the extended process to be essentially dependent on cation migration to the oxide-gas interface across a quickly developed oxide monolayer. This cation movement became possible with the establishment of a potential across the developing oxide layer after electron tunneling from metal to oxygen. The potential resulted from the dissociation of oxygen molecules on the surface of the oxide to produce atoms or energy “traps” with energy levels below that of the Fermi level of the metal. The number of such dissociation events and resulting energy traps allowed for a constant potential V to be developed across the oxide layer. This produced an electric field through the flim Ffilmsuch that

x

VFfilm = (1)

where x is the thickness of the oxide film. As a result of this field, the activation energy W to move a cation to the surface of the oxide becomes

filmqaFW 21− (2)

where q is the charge of the ion and a is the distance between the ions original and final position after movement; i.e. the jump distance. This translates to a rate of growth described by

−−≈

Tkx

qaVW

Nadt

dx

B

2exp4ν (3)

13

where N is the number of mobile ions per unit volume in the oxide, ν a phonon frequency and Bk the Boltzmann constant. This therefore leads to the inverse logarithmic law.

( )tBAx

ln**1 −= (4)

Where oxide thickness, x, is a function of time, t, and A* and B* are constants.

Considering the importance of the activation energy required to move the cation to this notion of oxide growth, it can be seen that oxidation becomes appreciable when the value of F increases enough to significantly lower the value of W. Essentially therefore, the larger the voltage acting on the oxide the greater the growth rate.

When considering the fact that titanium oxidation occurs via oxygen anion diffusion (Boyer et al. 1994) however, and not titanium cation diffusion, this theory of oxidation kinetics becomes problematic. Indeed, if the Cabrera-Mott mechanism was effective, the anions produced on the surface of the oxide would move away from the surface before the full voltage had been established. Consequently the notion of a constant voltage that aids the movement of ions must be understood to be replaced by a constant field (Fehlner and Mott, 1970).

The work of Cabrera and Mott has therefore been elaborated upon, first by Fehlner and Mott (1970), and then by Eley and Wilkinson (1970) in consideration of the possibility for anion and well as cation movement. As outlined by Fehlner and Mott (1970), it is now understood that a period of fast, linear oxidation occurs through the process of place exchange between cation and anion in a continuation of the chemisorption process. This period is followed by a slower logarithmic oxide growth where an electric field lowers the activation energy of ion migration. However, the structural nature of the oxide is also said to affect the oxidation mechanism. In the case of Ti, and other metals such as W, Mo and Nb, it is the formation of a glass like covalent network within the oxide that allows for anion migration and thus continued oxide formation. In all cases of low temperature oxidation, the rate of the oxide growth is limited by the displacement of the ions through the barrier.

The extensions of the Cabrera-Mott theory made by Eley and Wilkinson are focused on a different understanding of how the activation energy for ion movement changes in time. While their understanding of the mechanism considers place exchange rather than any direct movement of ion, Wa the activation energy of this “movement” is expressed by

xWWa µ+= 0 (5)

where W0 is the activation energy of ion movement before the effects of oxide film structure are considered, and µx is a constant dependent on oxide film structure. The growth law is then described by

+−≈Tk

xWC

dt

dx

B

µ0exp' (6)

14

This therefore leads to the logarithmic growth law

( )BtAx += 1ln (7)

with A and B as constants and oxide thickness, x, a function of time, t. The logarithmic nature of oxide growth is explained by Eley and Wilkinson to essentially result from the continued increase in activation energy for ion transport. This increase is due to structural changes in within the oxide, including the consolidation of the amphorous structure and a change to the open glass-like network of the oxide. This change may be facilitated by the presence of the electric field or the presence of water in the case of anodic oxide films (Fehlner and Mott, 1970).

Importantly, Eley and Wilkinson's notion of activation energy helps to explain how field across the oxide remains constant. Felhner and Mott (1970) extend this, offering that the field remains constant owing to the fact that the rate at which anions move into the oxide is balanced by the rate at which oxygen ions are partially incorporated into the oxide surface and hence contribute to the potential. Significantly, it is noted that the activation energy required to move the anions into the oxide layer through which they may easily traverse is lowered by the field. Felhner and Mott therefore propose a cohesion of the effects of field and oxide structure on the activation energy of anion movement, expressed as being proportional to

−+

−Tk

qaFxW

BB

film

2exp0

0

µ (8)

Consequently it may be understood that the value of ½qaF decreases the value of W and therefore also the activation energy of anion movement. Due to the rapid formation of ions, as a result of electron tunnelling, Felhner and Mott (1970) consider ion movement to be rate determining. If this is the case, then considering that ion movement is dependent on the extent to which the activation energy for movement in lowered, and that rate is dependent on the number of moving anions, the equation that describes activation energy changes also defines the rate of reaction. Consequently, Eq. (8) may be understood as the rate expression for the titanium oxidation reaction.

This understanding of field effects has been adopted by many studying the outcomes of ranging sample biases and thus electric fields on the rate of oxidation of different metals. Notably, Gwo et al. (1999) reports that the strong electric fields exerted on the titanium nitride films enhances the injection of ions into the oxide film. Dubios and Fontaine (1997) also rely directly on the Cabrera-Mott theory to explain similar results. Work by Nowak et al. (2006) on tungsten nanowires, who's observations are suggested to be largely material independent, shows that the natural oxidative process is extended by the application of an external electric field.

15

Fig. 11. a) AFM image of two vertically aligned grids of oxidized lines on TiN. b) Cross-sectional line profiles

along two grid directions. In this plot, the oxide line height and the oxide linewidth (FWHM) along the x and y

directions are 3.5 , 60 nm and 4.5, 30 nm, respectively. The distance between lines is 150 nm. Local oxidation

conditions: tip scanning speed 0.25 mm/s, sample bias 18 V, relative humidity 65%. Images courtesy of Gwo et

al. (1999).

Importantly, much of the aforementioned work also suggests that such a mechanism is at work even over short time periods, and validates this investigations claims that oxidation occurs significantly even during micro-polishing with DC voltage applied intermittedly. Work by Nowak et al. (2006) shows that the thickness of the oxide layer formed under an external electric field becomes virtually stationary after a reaction time of just 10 seconds, and most of the reaction was found to take place within the first second. Indeed, work by Gwo et al. (1999) has shown distinct oxide formation at relatively low voltage (8V) and over short time periods (Fig. 11). Such is also displayed by the work of Dubios and Fontaine (1997). Additionally, Nowak et al. (2006) has demonstrated that oxide growth becomes limited after relatively short periods of time. Oxidation reactions with the specimen under biases between just 0.3V and 0.6V essentially ceased after just 10 seconds, with most of the reaction occurring in the first second. While during micro-polishing the conditions are somewhat variable, by virtue of the fact that an elevated macroscopic electric field is applied for many short period intervals (half a second), growth is still suggested to be significant.

What must be noted is that due to the small radius of curvature of the tip, the field acting upon the specimen during electrochemical polishing will be significantly increased. Nowak et al. suggest that an electric field in the order of 10 x 106 V m-1 may be generated if a moderate voltage is applied. The reason for the large extend of oxidation on the specimen is therefore relatively simple to comprehend when the affect of field on ion movement activation energy W is considered. As briefly summarized above, the relationship shown in Eq. (8) is such that anion movement activation energy decreases with an increase in the value of ½ qaF. Therefore if the field is dramatically increased, the value of ½ qaF will rise significantly and ion movement will be more favoured as activation energy is reduced. Principally, the work by Gwo et al. (Fig. 12), and Dubios and Fontaine on titanium alloys provides experimental verification of this theoretical assumption. Such is also provided by Nowak et al. through his work on tungsten nanowires (Fig. 13)

16

Fig. 12. Four oxidized lines formed on the titanium nitride film by the AFM conductive probe induced local

oxidation process at different sample biases (110, 19, 18, 17 V, respectively). The tip scanning speed during

line writing was fixed at 0.1 mm/s and relative humidity was 65%. Image courtesy of Gwo et al. (1999).

Fig. 13. Series of TEM images of a nanoscaled tip after voltages of (a) 0.3 V, (b) 0.36 V, (c) 0.42 V, (d) 0.48 V,

(e) 0.54 V, and (f) 0.6 V were applied for 10 s, respectively. Image courtesy of Nowak et al (2006).

SEM analysis of Ti6Al4V specimens

Coupled with the aggressive nature of the active electrolyte constituent (perchloric acid (HClO4), the relatively large voltages (10V) used during micro-polishing suggests that oxidation will occur at a greatly increased rate during the short polishing time period. We performed qualitative analysis of a tip (Fig 14.). Results aligned with those presented above

17

and indicates that oxidation occurs significantly during conventional micro-polishing with DC voltage applied over short repeating intervals .

Fig. 14. Ti6Al4V specimen tip viewed by SEM at 25 kV and 1.31 x 103X magnification. Specimen prepared by

micro-polishing with intermitted DC voltage applied using 2% perchloric acid, 37% buthanol, 61% ethanol

electrolyte and a voltage of 12V

Additionally, Fig. 14. shows the extremely rough nature of the oxide formed during micro-polishing. Qualitative analysis (Fig. 15. and Fig. 16) of different sections of the same specimen shown in Fig 15. exhibits the difference in surface roughness of the oxide among areas of the tip that experienced different amounts of oxidation. During micro -polishing there is a decrease in oxidation time the further up the tip one goes. While a difference in oxidation time may understandably translate to a difference in oxide thickness, the link between increased oxidation and increased roughness may seem difficult to explain. Conversely, if anything it could be assumed that roughness would decrease as the extension of oxidation time results in the crystallisation and ordering of the amphorous oxide structure (Hart, 1956. Dell'Oca and Young, 1969. Fehlner and Mott, 1970). Alternatively, it is suggested that at least under the conditions of micro-polishing with DC voltage applied intermittedly, surface roughness leads to further surface roughness with each subsequent oxidation event.

18

Fig. 15. Middle of Ti6Al4V specimen tip viewed by SEM at 25 kV and 1.31 x 103X magnification. Specimen

prepared by micro -polishing technique using 2% perchloric acid, 37% buthanol, 61% ethanol electrolyte and a

voltage of 12V.

Fig. 16. Upper Ti6Al4V specimen tip viewed by SEM at 25 kV and 1.31 x 103X magnification. Specimen prepared

by micro -polishing technique using 2% perchloric acid, 37% buthanol, 61% ethanol electrolyte and a voltage of

12V.

19

Suggested advanced oxidation mechanism

This suggestion is premised on the idea that surface irregularities lead to local increases in field strength, which enhances oxide formation. This is made in light of the fact that the strength of a field emanating from a curved body may be expressed as proportional to V/R

R

VF ∝ (9)

where R is the local radius of curvature . While this is often considered during the development of a macroscopic field using the whole specimen, especially during sample analysis in the LEAP, it may be considered significant at a much more finite level. It is suggested that the surface irregularities of the oxide locally manipulate the strength of the electric field applied to the specimen during micro-polishing. Just as the tip-like nature of the specimen macroscopically increases the field strength such that an applied voltage of moderate strength may translate to a high electric field, protrusions of the oxide along the tip surface produce a local increase in field strength. These points with once enormous radii of curvature become almost tip like in shape (see, for example, bottom left of tip in Fig 14.) or otherwise, and thus have a vastly decreased radii of curvature. Field strength consequently increases. The local increase in field strength allows for greater anion movement and translates therefore into a greater rate of oxide formation. Expectantly, this produces further pronounced surface irregularities that consequently allow for further enhancement of the oxidation process at these points during the next voltage application during micro -polishing . Indeed, Nowak et al. (2006) illustrate this idea, showing oxide formation occurring to a far greater extent near the tip apex, where the radius of curvature is smaller and electric field is consequently higher (Fig. 13).

These surface irregularities pose serious concern to both specimen preparation and analysis. Principally, as has been suggested, they allow for the oxide to grow at a greater rate than would otherwise be possible and prevent the extended electrochemical narrowing of the specimen needed to produce the small tip radii (50-100nm). This essentially prevents the development of the desired electric field during analysis in the LEAP, which is required for field evaporation of the specimen. Additionally, the presence of an irregular oxide coating contributes significantly to a rise in brittleness of the specimen tip and often results in tip breakage. Moreover, Miller et al. (1996) and Reissig et al. (2005) have proposed that an increase in the roughness of the tip surface increases the chance for tip breakage when a high electric field is applied to a specimen. Breakage results in the blunting or disfiguration of the tip to the extent that it cannot develop the required field strength in the LEAP. Additionally, the irregular surface translates to an irregular electric field when a potential is applied during specimen analysis in the LEAP. This is not conducive to accurate analysis, as the difference in field strength may cause differential field evaporation across the specimen. Also, removal of the oxide often requires the increase of field strength. It therefore follows that the removal of areas of thicker oxide requires further increase in field strength. This can have a two-fold problematic effect. Firstly, the removal of the oxide commonly also removes some of the alloy beneath it. Indeed, Mulson and Müller (1963) note that the field evaporation of atoms absorbed onto the tip sometimes results in the removal of the metal atoms making up the tip. Secondly, the increased field may mean that there is unwanted evaporation of the alloy at the standing DC voltage, between the high voltage pulses. This will not only significantly contribute to the level of background noise, but will mean these atoms are lost to analysis, as their evaporation occurs during the unrecorded period.Additionally, the specimen remains subject to a high degree of electrostatic pressure during analysis, which contributes to an increased embrittlement of the tip. This pressure results from the electric field, as electrostatic pressure is proportional to F2. Consequently,

20

further increases in the electric field, as a result of surface irregularities, may lead to greater electrostatic pressure. This will increase embrittlement of the specimen, translating to a greater risk of specimen rupture during analysis.

Potential Solution

Low Temperature Micropolishing

Minimising the original formation of surface irregularities would therefore be beneficial to the entire specimen preparation and analysis process. This is understood to be very hard, not least because of the speed of formation of an oxide monolayer on the alloy, upon which further oxidation takes place. The topography of the monolayer may establish the pattern of occurrence of surface protrusions. Considering that the oxide monolayer is just a few molecules diameter thick (Fehlner and Mott, 1970) and evenly coats the substrate alloy, the topography of the alloy surface may thus heavily influence initial surface irregularity of the oxide. Indeed, while Ressig et al. (2005) contend that the surface irregularities of a tip after electrochemical polishing is the result of differences in phase and grain etching characteristics of the alloy, leading to preferential etching in some regions (Fig. 17) it is suggested that this surface irregularity is of the oxide that comes to cover these differently etched regions. Retardation of the oxide growth rate, in a bid to minimise any oxide formation, including at the points of early and small surface irregularities, is therefore perhaps the only means by which irregularity of the final specimen surface structure can be minimised.

Fig. 17. TEM-image of a rough tip, polished with 2% perchloric acid, 37% buthanol, 61% ethanol, perhaps due

to two phase structure of Ti6Al4V. Image courtesy of Reissig et al (2005).

The importance of temperature must not be excluded from this examination of the oxidation process. Indeed, Fehlner and Mott tentatively suggest that in addition to the effects of field, an anion requires thermal activation to traverse the oxide film. The importance of temperature however comes to the fore when the rate determining expression (Eq. (8)) is considered. The rate of anion movement decreases with a decrease in temperature. As the aim is to decrease the rate of oxidation, this is of paramount importance. Keeping in mind that a certain potential must always be applied in order to achieve electrochemical polishing, field strength takes on the value of an unchangeable constant and therefore essentially the only way to manipulate oxidation rate is by temperature. This idea is fundamentally supported by Miller et al. (1996) when considering titanium specimen preparation. Here the ideal conditions for specimen preparation include temperatures of between -60oC and -50oC. The same conditions are used for electrochemical preparation of titanium specimens for the TEM, for understandably the same reasons.

21

Changes to specimen preparation apparatus

Reducing the temperature at which the oxidation reaction takes place requires a development of current specimen preparation techniques. Importantly, temperature must be lowered without contamination of the electrolyte or interruption of the voltage supplied, and within the confines imposed by working under an optical microscope. Moreover, temperature must be lowered in such a way that the physical process of changing the position of the specimen in the electrolyte film is not hindered. Considering these factors, it is suggested that sufficient lowering of the temperature may be achieved through indirect cooling of the electrolyte. This requires the simple modification of pre-existing apparatus (Fig. 18) and the inclusion of liquid nitrogen as a coolant (Fig. 19). It is suggested that to lower the temperature of the electrolyte, the platinum wire loop through which the electrolyte film is suspended during micro-polishing (see Fig. 8) should be cooled. As this platinum loop is ultimately connected to a conducting, cylindrical metal platform where the application of DC voltage is gated, cooling of the platinum wire loop may be achieved by cooling this cylindrical metal platform. This may be realised by creating a well in the cylindrical metal platform and filling this with liquid nitrogen. In order to maintain sufficiently low temperatures, the coolant would be periodically replaced. Additionally, in order to sufficiently “transfer” the low temperature, the platinum wire loop should be replaced with a thicker, sturdier length of conducting metal. The highly malleable nature and good conduction properties of gold make it an attractive option for this part of the apparatus.

Fig. 18. Current apparatus for micro-polishing with intermitted DC voltages. a) External DC voltage source, b)

specimen placed through the electrolyte film, c) platinum wire with loop, d) cylindrical metal platform. Note

the broken circuit, which is completed by the tapping of the electrical wire on the cylindrical metal platform.

a)

b)

c)

d)

22

Fig. 19. Changes to part of the micro-polishing apparatus. a) The drilling of a hole into the cylindrical metal

platform to allow for cooling with liquid nitrogen. b) Substitution of the platinum wore loop for a thin length of

gold sheet, with appropriate hole for electrolyte film suspension.

Lowering the temperature of the electrolyte may have benefits in addition to reducing oxidation rate. These are associated with the increase in electrolyte viscosity that accompanies the temperature decrease. It is suggested that this increase in viscosity may lead to greater ease in specimen preparation. This idea is supported by the work of Gerstl et al. (2004) and Reissig et al (2005), who replaced the ethanol component of the standard electrolyte used for titanium specimen preparation (2% perchloric acid, 37% buthanol, 61% ethanol), with butoxyethanol. Viscosity is a significant difference between these two solvents, and suggests that the higher viscosity of butoxyethanol facilitates finer tip preparation. Gerstl et al. (2004), who used this new electrolyte during the latter stages of preparation, were able to produce a tip with a radius of 50nm, while Reissig et al (2005), who used the new solution throughout, produced specimens with tip radii of 10-20nm (Fig. 20). It is suggested that higher viscosity allows for greater specificity during micropolishing, as the electrolyte resists spreading outward from the electrolyte film and along the tip. Moreover, higher viscosity may be especially significant in tip preparation of Ti6Al4V as the firmer electrolyte film resists oxygen diffusion. This may reduce the amount of oxygen available at the titanium-electrolyte interface and reduce the subsequent formation of anions on the oxide surface

Fig. 20. a) Ti6Al4V tip prepared with an electrolyte solution of 2% perchloric acid, 37% buthanol, 61% ethanol.

b) Ti6Al4V tip prepared with an electrolyte solution of 2% perchloric acid, 37% buthanol, 61% butoxyethanol.

Image courtesy of Reissig et al (2005).

a)

b)

23

Further Preparation Techniques

While this investigation has considered the process of specimen preparation by electrochemical polishing and has offered amendments accordingly, other non chemical preparation techniques exist. For these techniques, oxide formation may be minimal and thus not pose a problem; allowing for an appropriate tip to be developed. The technique principally referred to here is that of ion milling. This process involves the directed bombardment of ions onto a sample surface. This causes erosion of the sample surface and may allow for the development of a tip appropriate for analysis (Fig. 21). Importantly however, the use of the Focused Ion Beam (FIB) method remains a draw out process. Additionally, it is difficult to view the specimen during tip production. In a further pragmatic sense, the need for high quality equipment means the opportunities for tip preparation using the FIB are often rare. Regardless, this specimen preparation technique offers exciting prospects for the future, and needs further investigation and consideration when dealing with the problem of Ti6Al4V atom probe specimen preparation.

Fig 21. The development of a Ti6Al4V tip using the FIB technique.

24

IV. Conclusion

While the theoretical solution to reduce oxidation of Ti6Al4V during electrochemical polishing may be relatively simple, its implementation remains somewhat more difficult. Through an investigation of the mechanism of low temperature oxidation of Titanium it has been suggested that the rate of reaction is dependent on the lowering of the activation energy for anion movement. Such lowering is affected by both temperature and the strength of the electric field acting upon the specimen. Considering that during electrochemical polishing, the electric field strength remains constant, as a result of a required set voltage, temperature becomes the only means by which to reduce anion movement. It is put forward that a lowering of temperature significantly minimises the reduction of activation energy that results from the presence of the high strength electric field.

To date there have been changes made to the electrochemical specimen preparation apparatus, in a bid to accommodate for the suggestions made, however they have involved changing the polishing technique used during the rough stage of preparation prior to fine micro-polishing. These changes have involved using a technique similar to standard electrochemical polishing (see Fig. 7) and cooling the electrolyte by immersing it in liquid nitrogen. Other attempts have seen the electrolyte cooled by situating it atop the liquid nitrogen coolant. Additionally, attempts have been made to limit oxidation by reducing oxygen concentration around the specimen during preparation. This was achieved by directing a flow of Argon directly onto the specimen, thereby flooding the electrochemical polishing region with inert gas in a bid to evacuate the region of oxygen. The success of this procedure remains unevaluated. Furthermore, the extent of oxidation of the tip during polishing could not be continually monitored, due to the apparatus being unable to be situated beneath an optical microscope. While conventional rough electropolishing is not usually carried out under an optical microscope, it may have well been useful in this instance to see if oxide formation was significantly reduced throughout the process. Regardless, this process remains an important first step in the process of creating a suitable Ti6Al4V tip by electrochemical means. It is hoped that the specimen produced by this rough electrochemical polishing may be used with the amended micro-polishing apparatus.

It is suggested that with future attempts to produce Ti6Al4V specimen tips with the methods that include cooling of the electrolyte, greater success than is otherwise currently experienced may be achieved. It is hoped that setting up a micro-polishing apparatus similar to that suggested within this report may allow for greater ease during specimen preparation. Increased use of this new apparatus will not only lead to greater familiarisation and competency, but also allow for successful elements of the procedure to be further examined and developed. This developed technique may also be useful for preparing specimens of Magnesium alloys, and certain Aluminium alloys, as well as other metallic materials that are easily oxidised by the electrolyte.

While the success of this amended technique remains unevaluated, there has been recent success in preparing Ti6Al4V specimens via the FIB method. Specimens suitable for analysis were crafted and provided limited, but none the less valid data (Fig. 22, Fig. 23). This reaffirms the aforementioned suggestion that the FIB offers exciting prospects for preparation of Titanium alloys, and other easily oxidised materials. Nevertheless, advances

25

in electrochemical polishing techniques remain important, as they may offer opportunities to produce tips with greater ease and efficiency.

Fig. 22. Mass spectrum of Ti6Al4V heat treated at 900oC for 48h.

Fig. 23. A 2D map of Ti6Al4V heat treated at 900oC for 48h, showing a homogeneous distribution of

constituent elements.

26

V. References

Boyer, R., G. Welsch, and E. W. Collings. (eds). 1994. Materials Properties Handbook: Titanium Alloys. ASM International. Ohio

Cabrera, N., and N. F. Mott. 1949. Theory of the oxidation of metals. Report on the Progress of Physics. 12: 163-184

Callen, B. W., B. F. Lowenberg, S. Lugowski, R. N. S. Sodhi, and J. E. Davis. 1995. Journal of Biomedical Materials. Research. 29: 279

Chio, P-P., Kwon, Y-S., Kim, J-S., and Al-Kassab, T. 2007. Transmission electron microscopy and atom probe specimen preparation from mechanically alloyed powder using the focused ion-beam lift-out technique. Journal of Electron Microscopy 56: 43–49

Dell'Oca, C. J., and L. Young. 1969. Mechanism of Ionic Conduction at High Fields. Applied Physics Letters 13: 228

Delogu, P., T. Dikonimos-Makris, R. Giorgi, J. Lascovich, L. Caneve, and S. Scaglione. 1994. XPS and AES Study of A120,/Ti6A14V Interface: Influence of Oxidation and Nitruration of the Metal Surface. Surface and Interface Analysis 22: 236-241

Dubios, E., and P. A. Fontaine. Nanometer Scale Lithography of Silicon and Titanium using Scanning Probe Microscopy. Extended Abstracts in Solid State Devices and Materials Conference (Hamamatsu, Japan, 1997) p. 474

Fehlner, F. P., and N. F. Mott. 1970. Low Temperature Oxidation. Oxidation of Metal 2: 59-99

Gerstl, S. S. A., Y. W. Kim, and D. N. Seidman. 2004. Atomic Scale Chemistry of α 2/γ Interfaces in a Multi-Component TiAl Alloy. Interface Science. 12: 300-310

Gwo, S. C.-L. Yeh, P.-F. Chen, Y.-C. Chou, T.-S. Chao, S.-F. Hu, and T.-Y. Huang. Local electric-field-induced oxidation of titianium nitride films. Applied Physics Letters 74: 1090-1092

Hart, R. K. 1956. The Oxidation of Aluminium in Dry and Humid Oxygen Atmospheres. Proceeding of the Royal Society (London) Ser. A 236: 68- 88

Larson, D. J., and M. K. Miller. 1999. Solute Partitioning and Interfacial Segregation in TiAl-Based Alloys. Proc. High Temperature Ordered Intermetallic Alloys VIII, E. P.

Lausmaa, J., M. Ask, U. Rolander, and B. Kasemo. 1989. Materials Research Society. Synopsis Proceedings. 110: 647

Lefebvre, W., A. Loiseau, and A. Menand. 2002a. Field evaporation behaviour in the gamma phase in Ti-Al during analysis in the tomographic atom probe. Ultramicroscopy. 92: 77-87

Lefebvre, W., A. Menand, A. Loiseau, and D. Blavette. 2002b. Atom probe study of phase transformations in a Ti-48 at.% Al alloy. Materials Science and Engineering. A327: 40-46

Miller, M. K., A. Cerezo, M. G. Hetherington, and G. D. W. Smith. 1996. Atom Probe Field Ion Microscopy. Oxford Science Publications. Oxford

27

Moulder, J. F., W. F. Stickle, P. E. Sobol, K. D. Bomben, and J. Chastain. 1992. Handbook of X-ray photoelectron spectroscopy” (Perkin-Elmer Corporation, Physical Electronics Division, MN)

Mulson, J. F., and E. W. Müller. 1963. Corrosion of Tungsten and Iridium by Filed Desorption of Nitrogen and Carbon Monoxide. The Journal of Chemical Physics 38: 2615-2619

Raikar, G. N., J. C. Gregory, J. L. Ong, L.C. Lucas, J. E. Lemons, D. Kawahara, and M. Nakamura. J. Vac. Sci. Technol. A13: 2633

Reissig, L., U. Czubayko, N Wanderka, R Völkl, and U. Glatzel. 2005. Comparison of oxide measurement techniques applied to Ti6Al4V. Materials Characterization 55: 153-159

Semiatin, S. L., Kirby, B. C., and Salishchev, G. A. 2004. Coarsening Behavior of an Alpha-Beta Titanium Alloy. Metallurgical and Materials Transactions A 35A: 2809-2819

Sittig, C., M. Textor, and N. D. Spencer. 1999. Surface characterization if implant materials c.p. Ti, Ti-6Al-7Nb and Ti-6Al-4V with different pretreatments. Journal of Materials Science: Materials in Medicine 10: 35-46

Vergara, L. I., M. C. G. Passeggi Jr., and J. Ferran. 2002. The role of passivation in titanium oxidation: thin films and temperature effects. Applied Surface Science 187: 199-206