twin polishing for al alloys

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Preparation of high quality Al TEM specimens via a double-jet

electropolishing technique

Necip nl

Istanbul Technical University, Faculty of Chemistry-Metallurgy, Materials Science and Metallurgical Engineering Department, 34469 Maslak,

Istanbul, Turkey

A R T I C L E D A T A A B S T R A C T

Article history:

Received 2 February 2007

Received in revised form

28 March 2007

Accepted 6 April 2007

Obtaining clean, uniformly thin and high-quality TEM specimens entails a great dealof work

that has a number of parameters that have to be considered carefully depending on

specimen preparation technique(s). The parameters, such as voltage, current density,

temperature, time, electrolyte, and electrolyte flow rate, have the most significant

importance in a double-jet electropolishing technique. Useful hints to have the least

failures in preparing TEM specimens and optimum values of the above parameters for pure

Al are given and discussed.

2007 Elsevier Inc. All rights reserved.

Keywords:

Electropolishing

Aluminum

TEM

Specimen

1. Introduction

To understand and correlate the nature of the microstructure

of metals and their alloys directly with their physical,

chemical and mechanical properties, TEM analysis has been

a major tool. TEM analysis requires successfully prepared thin

foils about a few hundred nanometers or less thick from bulk

materials [1,2]; the success of the TEM analysis critically

depends on the quality of the thin foils prepared. Various

techniques, such as electropolishing using pointed cathodes

[3], jet machining [46], low voltage electropolishing using a

special cathode design [7], and jet electropolishing [4,810],

have been developed and used for the thin foil preparationsince 1949. Additionally, a number of experimental techniques

and theoretical knowledge on the preparation of thin films

have been summarized in specialized books, textbooks and

papers [1116]. In 1966, Schoone and Fischione [10] designed a

simple submerged double-jet technique [9] that enables the

polishing of metal disks simultaneously from both sides and

automatically stops the polishing operation when perforation

occurs. Basically, the preparation of the thin foils for TEM

analysis is comprised of three steps, (i) obtaining a sample

piece 12 mm thick, (ii) thinning the sample piece to about

0.2 mm, and (iii) electropolishing the sample to a thin foil

which enables sufficient electron beam penetration [17].

Electropolishing (EP) is a well-known method in an electric

potential passed through the chemical solution utilizing the

specimen as the anode [18,19]. Although this description

seems to be straightforward, to have reproducible optimum

conditions, the EP parameters (i.e., voltage, current density,

temperature, time, and flow rate) and characteristics of the

TEM specimen (thickness, conductivity, and its nature asbrittle or ductile) must be taken into consideration in the thin

foil preparation due to the fact that electropolishing rates and

ideal polishing conditions vary for most metals and alloys

[11,2023]. This paper describes the effects of both EP

parameters and the thickness of the TEM specimens with

regard to preparation of thin foils for TEM analysis.

M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 5 4 7 5 5 3

Tel.: +90 212 285 3382; fax: +90 212 285 2925.E-mail address: [email protected].

1044-5803/$ see front matter 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2007.04.003
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2. Experimental Details

The size, shape, and micro- and macrostructure of the hole

and time for hole formation were determined as a function of

the specimen thickness, electropolishing voltage and the

pump flow rate of the electropolisher. These characteristics

served as guides to determine the optimum electropolishingconditions. Pure Al sheet with 99.99999% purity used in this

study was obtained from Alfa Aesar, a Johnson Matthey

Company. Square 11 cm samples were gently cut from the

pure aluminum sheet by a precision diamond saw. Each

sample was mechanically ground on SiC abrasive papers (80,

120, 300, 600, 1000, and 1200) to five different thicknesses, 70,

100, 150, 200, and 250 m. During each grinding step, the

thickness was controlled using a Chicago Brand model digital

micrometer. 15 samples were prepared for each different

thickness. Then,3-mm diameter disks were punched from the

square foils by using a Gatan Disk punch. These samples were

electropolished using a solution consisting of 25 vol.% HNO3

and 75% methanol at different voltages in the range of 660 Vin a TenuPol-5 digitally controlled automatic electropolisher

with program storage capabilities. This instrument was

developed by Struers A/S [24] for twin-jet thinning samples

for transmission electron microscopy (TEM). The pump flow

rate of the electropolisher was adjusted as 15. The tempera-

ture of the electrolyte was held constantly at 202 C by

using Lauda Proline RP 870 model cooling system. The

polishing time was recorded from the digital screen of the

TenuPol-5 for each TEM foil. After polishing, the foil in the

holder was immediately rinsed three times in methanol. Then

the holder was opened gently and the foil was removed with

tweezers and rinsed in three small ethanol cups, gently

rinsing each sample 30 times in each cup. Each rinsing step

was done slowly so as not to damage the thinned area around

the hole. Some ethanol remaining on the tweezers with the

TEM sample was removed by blotting with a filter paper, and

the TEM sample was then put on another clean and dry filter

paper for 1 or 2 min to make it fully dry. These samples were

stored in labeled polyethylene vials to protect them from

mechanical damage and contamination. Each TEM samplewas investigated in detail under a Leica DM6000 M Model

optical microscope and Leica EC3 Model stereomicroscope;

images were recorded to describe the quality of the hole

formation.

3. Results and Discussion

3.1. Affect of Electropolishing Voltage

A stereomicroscopic view of the pure Al specimens, 200 m

thick and 3 mm in diameter, prepared using different

electropolishing voltage values, i.e., 16, 30, 45 and 60 V withthe constant pump flow rate of 15 (arbitrary units as indicated

on the polishing unit), is presented in Fig. 1ad. Increasing the

electropolishing voltage above 12 V resulted in specimens

with smooth and clean surfaces, and holes were successfully

obtained near to the center of the polished circle area. The

peripheral alterations around the polished circle area due to

the increase in voltage from 16 V through 60 V can be seen in

Fig. 1ad. No hole formation occurred at the central areas of

the specimens exposed to excessive voltages such as 45 and

60 V. Fig. 2 shows optical micrographs of the holes produced

and the general effect of increasing the electropolishing

voltage; again the failure to form a hole at 45 V is seen in

Fig. 1 Stereomicroscope views of high purity Al disks, 200 m thick and 3 mm diameter in size, after electropolishing at

different voltages (a) 16 V, (b) 30 V, (c) 45 V, and (d) 60 V. The peripheral alterations around the polished circular areas areshown

with arrows.

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Fig. 2f. After electropolishing at different voltages, elliptical

holes were observed, as seen from Fig. 2ae. It is also evident

that the voltage above 12 V resulted in wider holes (Fig. 2de).

The variation of the time for hole formation and the

calculated hole area of the high purity Al disks as a function of

the electropolishing voltage are shown in Fig. 3. As can be

seen,it is clear that thetime for hole formation decreases with

increasing the applied electropolishing voltage. The time for

hole formation of thespecimens prepared at the voltage range

from 8 to 12 V decreases sharply from 380 s to 106 s. This value

decreased further to 15 and 12 s when the specimens were

prepared at 45 and 60 V, respectively, although for the latter

specimens the holes were not centrally located.

Determination of the suitable and correct electropolishing

parameters is not easy due to the fact that these parameters

have a wide range that makes repeatability difficult. The

voltage adjustment needs to be done properly to achieve the

ideal electropolishing. During this process, the formation of an

anodic viscous layer of electrolyte on the specimen surface is

responsible for thinning. The ideal thinning will create a

smooth surface and polished specimen by removing both the

macroscopic bumps and microscopic irregularities, respec-

tively [12,15]. Although it is not resolvable in Fig. 3, the cal-

culated hole area increases fourfold from 0.0035 mm2 at 8 V to

Fig. 2 Optical micrographs showing the resulting holes after electropolishing at different voltages (a) 8 V, (b) 10 V, (c) 12 V,

(d) 16 V, (e) 30 V, and (f) 45 V.

Fig. 3 The variation of the time for hole formation and the

calculated hole area of the high purity Al disks as a function

of electropolishing voltage.

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0.0141 mm2 at 16 V. Above 16 V, the hole area increases by

nearly a factor of five to 0.06500.0186 mm2 at 30 V. Even

though there is no circular or elliptical shaped hole to be

studied for the specimens prepared at 45 and 60 V, the

calculated hole areas are approximately 1.49 and 5.08 mm2,

respectively. The excessive voltage results in electrolysis of

the aqueous solution which creates bubbles on the specimen

surface; consequently these bubbles mask the surface locallyand cause pitting[25].

3.2. Affect of the Pump Flow Rate

Figs. 4 and 5 show the optical micrographs of the holes of the

pure Al specimens, prepared using five different pump flow

settings, i.e., 5, 15,25, 35,and 45,at a constant electropolishing

voltage, 8 V. Smooth, clean and well-polished surfaces were

observed on the specimens prepared in the range of pump

flow rates from 5 to 35 (Figs. 4a, c, e, and 5a). The effect of the

high flow rates, 35 and 45, on the specimen surface is obvious

in Fig. 5b and Fig. 5d, respectively. In the range of the pump

flow between 5 and 35, the locations of the holes were usually

near to the central area of the TEM disks. The location of the

hole for the specimen prepared with a 45 pump flow rate was

further removed from the central area (Fig. 5c and d); the

higher pump flow rates caused torn elliptical shaped holes

(Fig. 5b and d) and led to the surface distortion seen in Fig. 5c.

Fig. 6 shows the variation of the time for hole formation

and the calculated hole area of the high purity Al disks as a

function of the pump flow setting. The values of the time for

hole formation significantly decrease from approximately610 s for a pump flow setting of 5 to approximately 240 s for

a pump flow setting of 25. When the pump flow was increased

through 45, the time for the hole formation slightly decreases

to about 210 s. The calculated hole area was determined as

0.0086 mm2 on average for the pump flow range from 5 to 25.

When the pump flow setting was increased from 25 to 45, the

calculated hole area significantly increased from 0.0120 mm2

to 0.1763 mm2. Fig. 6 also clearly shows that increasing the

pump flow rate from 5 to 25 resulted in a decrease in the time

for hole formation whereas no significant difference in the

values of the calculated hole areas was observed. An increas-

ing in the time for hole formation with a decrease of pump

flow rate from 25 to 5 indicates a delay in the polishing action

because of the presence of gas bubbles formed by anodic

Fig. 4Optical micrographs showing the holes after electropolishing at different pump settings: (a) and (b) 5, (c) and (d) 15, (e)

and (f) 25.



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dissolution of the metal, which remain in the polishing area

[26]. When the pump flow setting was increased from 25 to 45,

there was no significant change in the values of the time for

hole formation, but the values of the calculated hole areas

increased.

3.3. Affect of the Specimen Thickness

Fig. 7 shows optical micrographs of the holes produced in the

high purity Al disks starting with specimens thinned to five

different thicknesses, i.e., 70, 100, 150, 200 and 250 m, and

prepared using a constant 8 V electropolishing voltage and a

pump flow rate setting of 15. Elliptical-shaped holes were

produced in each, irrespective of starting thickness (Fig. 7ae).

In the present study, the thinnest specimen with a thickness

of 70 m (Fig. 7a) developed a larger hole and more etched

surface than thethicker specimens(Fig. 7be). The variation of

the time for hole formation and the calculated hole area of the

high purity Al disks as a function of the specimen thickness

are given in Fig. 8. The shortest time for hole formation, 84 s,

was observed with the 70 m thick specimens. This time

increased to about 130135 s, when the specimen thickness

increased to 100 and 150 m, respectively, and increased

further up to 440 s for specimen thicknesses from 150 m

through 250m. When the specimenthickness increased from

70 m to 100 m, the hole area decreased from approximately

0.009 mm2 to 0.003 mm2. However, for specimens thicknesses

greater than 100 m no further changes were observed.

4. Conclusion

The quality of the information from TEM analysis study is

directly related to the quality of the thin foils being examined.

Electropolishing is the most common and physically deforma-

tion-free specimen preparation technique available for exam-

ining electrically conductive materials. In the present study

the twin-jet electropolishing parameters for producing high-

quality disks of high purity Al were studied with the purpose

of both showing the effects of the polishing parameters,

achieving well-prepared reproducible TEM disks and adding

to the general knowledge basis for workers in this area of

scientific research. On the basis of the results reported in the

present investigations, the following conclusions can be drawn.

1. At a polishingvoltage range between8 and12 V, specimens

with smooth and clean surfaces and with holes near the

center of the polished area were successfully produced.

Fig. 5 Optical micrographs showing undesirable thin regions after electropolishing at different pump settings: (a) and (b) 35, (c)

and (d) 45.

Fig. 6 The variation of time for hole formation and the

calculated hole area of the high purity Al disks as a function

of the pump flow rate.



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2. It was clearly demonstrated that increasing the applied

electropolishing voltage resulted in decreasing the

required time for the hole formation. However, at higher

voltages, such as 45 and 60 V, the holes were unnecessarily

large and not centrally located in the disks.

3. Smooth, clean and well-polished surfaces were observed

on specimens prepared using the range of pump flow

settings from 5 to 35. Also, it was observed that the required

time for hole formation decreases with an increase of the

pump flow rate.

4. When the electropolishing voltage and the pump flow rateare maintained constant, there is a significant increase in

the specimen thickness; the variation in thesize of the hole

area is negligible due to the endpoint detection sensitivity

of the polishing unit.

5. An increase in the specimen thickness resulted in a signifi-

cant increase in the required time for the hole formation.

Acknowledgements

The author would like to thank Prof. Hseyin imenolu from

Istanbul Technical University for his help during the optical

and stereomicroscope investigations of this study. The author

Fig. 7 Optical micrographs showing the holes in the high purity Al disks thinned to five different thicknesses and prepared

using a constant 8 V electropolishing voltage and a pump setting of 15: (a) 70 m, (b) 100 m, (c) 150 m, (d) 200 m and (e)

250 m.

Fig. 8 The variation of time for hole formation and the hole

area of the high purity Al disks as a function of the sample

thickness.



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is grateful to Bernard J. Kestel for his supports on providing

the literature and sharing his own experience. In addition,

Santhana Eswaramoorthy and Eric Lass from University of

Virginia for their help on providing literature are gratefully

acknowledged. The author also would like to thank Dr. Brian

Gable for his helpful discussion.

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twin polishing for al alloys

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