Preparation of TEM Samples

Selective Aluminium Matrix Dissolution from Al-TM Solidified Alloys

Oct. 26, 2011

Fig. 1: Structure of rapidly solidified Al-5.5 wt. % Cr-3 wt. % Fe-1.5wt. %Ti ribbon (1 – quasicrystalline phase Al80Cr20, 2 – Al13Cr2) (TEM)

Fig. 1: Structure of rapidly solidified Al-5.5 wt. % Cr-3 wt. % Fe-1.5wt. %Ti ribbon (1 – ... more

$Fig. 2: XRD diffraction patterns of rapidly solidified Al-5.5 wt. % Cr-3 wt. % Fe-1.5wt. %Ti ribbon ...$

A special method of Transmission Electron Microscopy (TEM) samples preparation is described in this article. Rapidly solidified alloy have very fine structure formed by matrix and intermetallic phases. Common TEM sample of bulk material allows observe the distribution of intermetallic particles but not their detail characteristic because of their small size. To describe the intermetallic phases is necessary to extract them from matrix. It is possible to do that by selective matrix dissolution in the solution of tartaric acid and iodine in methanol. Extracted phases can be observed by TEM and described in detail by X-Ray Diffraction (XRD).

Introduction

Rapid solidification is one of few possible methods for preparing thermally stable aluminium alloys. The structure of such alloys is very fine (with average grain size about 1 µm and even smaller intermetallic particles with size of few hundreds nanometers [1]). To characterize the material especially two methods are used - TEM for structure observing and XRD for phase analysis. When whole rapidly solidified ribbons are analyzed, the main obtained information come from fcc-Al matrix. To study formed intermetallic phases it is necessary to extract them from the matrix. The tradition method - extraction replica - is not suitable in this case, because for TEM observing it is also useful to spread the intermetallic particles on the copper grid and so prevent their overlapping. The easy and effective method in this case is matrix dissolution.

Experiment

An alloy with composition of Al-5.5 wt. % Cr-3 wt.% Fe-1.5 wt.% Ti was prepared by melting appropriate amounts of master alloys and pure metals (Al-11wt.% Cr, Al-4wt.% Ti, Al and Fe) in induction furnace. Consequently, rapidly solidified ribbons were prepared by melt spinning at cooling wheel circumferential speed of 28 m/s.

Thin foils, the TEM samples, were prepared by grinding and etching in solution containing C₂H₅OH:HNO₃ equal to 3:1 under following conditions: 10 V and -20°C. Intermetallic phases were extracted from rapidly solidified ribbons by selective dissolving of fcc-Al in solution of 250 g methanol, 25 g tartaric acid and 10 g iodine [2].

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TEM observing was performed on Jeol 3010 at accelerating voltage 300 kV. Phase composition of all materials was determined by X-Ray diffraction (XRD) on PAN analytical X'Pert PRO + High Score Plus.

Results, Discussion and Conclusion

The rapidly solidified ribbon is formed by fcc-Al matrix, quacrystalline phase Al₈₀Cr₂₀ (marked by 1) and crystalline phase Al₁₃Cr₂ (marked by 2) as shown in figure 1. Chemical composition of selected intermetallic particles is given in table 1. A part of Cr atoms is substituted by Fe atoms, which is possible due to their almost similar atomic ratio.

XRD diffraction patterns of rapidly solidified ribbon and extracted phases are given in figure 2. The phase composition obtained by XRD is in good agreement with TEM observations. The XRD patterns of RS alloys were measured on the wheel side of the ribbons that are covered by very thin layers of Al₂O₃, which amount is under a detection limit of XRD. The oxide layer on the free side of RS ribbon is slightly more massive. During selective Al-matrix dissolution, increases the relative content of Al₂O₃, so the peaks of this compound are observed in XRD pattern.

The extracted phases are shown in figure 3. Spherical particles marked by 1 belong to quasicrystalline phase Al₈₀Cr₂₀ and irregular shape particles are formed of Al₁₃Cr₂ crystalline phase. These results show that the fcc-Al matrix can be successfully dissolved while intermetallic particles stay untouched.

Conclusion

Extraction of intermetallic particles from matrix can be applied for detail study of intermetallic phases formed in rapidly solidified Al-Cr-Fe-Ti alloy. XRD diffraction pattern of extracted phases does not include peaks from fcc-Al covering the less intensive peaks from phases. Extracted phases can be directly observed in TEM.

Acknowledgement
This research was financially supported by Ministry of Education, Youth and Sports of the Czech Republic (project no. MSM6046137302).

References
[1] Vojtˇe ch D. et al.: J. of Alloys Compounds 475, 151 (2009)
[2] Bartova B. et al.: J. of Alloys Compounds 387, 193 (2005)
[3] Michalcová A. et al.: Metal 2009, Conference proceedings,
401- 404 (2009)

Authors
Ing. Alena Michalcová
Institute of Chemical Technology
Department of Chemical Technology of Monuments Conservation
Prague, Czech Republic
http://www.vscht.cz

Dr. Ing. Dalibor Vojtěch
Ing. Pavel Novák, PhD
Institute of Chemical Technology
Department of Metals and Corrosion Engineering
Prague, Czech Republic

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Selective Aluminium Matrix Dissolution from Al-TM Solidified Alloys

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