Journal of Microscopy, Vol. 237, Pt 3 2010, pp. 431–434
doi: 10.1111/j.1365-2818.2009.03279.x
Received 26 November 2008; accepted 23 April 2009
Electron microscopy investigations of the microstructure of a CrRe alloy E. FORTUNA-ZALESNA, W. ZIELINSKI & K.J. KURZYDLOWSKI Faculty of Materials Science and Engineering, Warsaw University of Technology, Warszawa, Poland
Key words. CrRe alloy, EDS, microstructure, phase identification, SEM, TEM.
Summary Transmission and scanning electron microscopic investigations were carried out to characterize Cr-35Re alloy. A continuous film of precipitates at the grain boundaries and precipitates inside the grains were observed. Identification of the phases was based on the analysis of the chemical composition and the diffraction patterns. It has been found that the matrix is a solid solution of Re in Cr and sigma phase CrRe2 was one of the phases present at the grain boundaries. In the majority of the precipitates chromium, rhenium and nitrogen were detected, which indicates the existence of a ternary compound. Introduction Chromium and chromium-based alloys have the potential for high temperature applications because of their high creep strength and good oxidation resistance. Unfortunately, like other VIA group bcc metals (Cr, Mo, W), a limitation is room temperature brittleness. This phenomenon is caused by the low solubility of interstitial impurities, which results in the precipitation of carbides, nitrides and oxides. It has been known since the 1950s that this brittleness could be reduced by the addition of Re (Geach and Hughes, 1956; Jaffe et al., 1959; Agnew and Leonhardt, 2003; Gimeno-Fabre, 2006). The addition of Re at concentrations close to the solubility limit significantly improves the mechanical properties of W and Mo-based alloys, mainly due to its effect on the lowering of the DBTT (Ductile-to-Brittle Transition Temperature), and by increasing the strength (the so-called ‘rhenium effect’). It is also known that the addition of Re dramatically increases the solubility of interstitial impurities. Recently, the improvement of ductility of alloys with Re addition has been explained by Correspondence to: Elzbieta Fortuna-Zalesna, Faculty of Materials Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warszawa, Poland. Tel: +48 22 234 87 48; fax: +48 22 234 87 50; e-mail:
[email protected]
C 2010 The Authors C 2010 The Royal Microscopical Society Journal compilation
their higher propensity to mechanical twinning compared to pure metals (Agnew and Leonhardt, 2003; Gimeno-Fabre, 2006). The W-Re and Mo-Re alloys are produced on industrial scale for specific high temperature applications (www. plansee.com). W-Re alloys are used for propulsion nozzles in the aerospace industry and for thermo-elements in high temperature furnaces, operating at up to 2000◦ C. MoRe alloys are used as rocket propulsion components, high temperature thermo-elements and in heat exchangers for liquid metals and inert gases. Theoretical predictions (Medvedeva et al., 2002) and experimental results (Ro Yoshikazu et al., 2002; GimenoFabre, 2006) suggest that Cr alloyed with an addition of Re close to the solid solubility limit produces a significant improvement of the mechanical properties. As part of the FP6 IP ExtreMat [www.extremat.org] Cr35Re alloy, as a possible material for space applications, has been investigated. The paper reports on the examinations of the microstructure of the Cr35Re alloy fabricated by an Extremat Partner, the EADS Corporate Research Center in Germany.
Material and experimental procedure The material was Cr-35 at.%Re alloy in the recrystallized state. The content of impurities measured by a Str¨ohlein ON-mat N and O analyser was 347 ppm of oxygen and 664 ppm of nitrogen. Characterization of the microstructure was carried out on an ultra high resolution Hitachi S-5500 scanning electron microscope equipped with a scanning transmission electron imaging detector and an energy dispersive spectrometer (EDS) made by the Thermo Electron Corporation. A JEOL 1200 EX transmission electron microscope (TEM) was employed to obtain electron diffraction patterns of the precipitates. TEM thin foils were prepared by ion milling using a precision ion polishing system, PIPS, manufactured by Gatan.
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Fig. 1. Images of the precipitates located at a grain boundary: (a) SE and (b) TE mode.
Preliminary observations revealed that the material contained a number of non-metallic inclusions, which were identified as chromium oxides. Results The electron microscopic studies revealed a complex microstructure of the alloy with precipitates present at the grain boundaries and in the matrix. Two phases were identified at the grain boundaries (Fig. 1) based on their contrast in the secondary electron mode. In addition, eutectic-like areas, shown in Fig. 1(a), were observed. The images in Figs 1(a) and (b) show the same area, in secondary and transmitted electrons (SE, TE) in SEM. It can be noted that the phase contrasts in these two images are reverse. Also, radiation damages induced by ion milling can be observed in the matrix, with the precipitates being intact. This was confirmed by tilting the specimen with goniometer. The chemical composition of the precipitates was investigated by EDS measurements carried out on the specimen used for TEM studies. The results shown in Table 1 indicate presence of nitrogen in the dark contrast phase (SE mode) and eutectic-like structural element. In the majority Table 1. Results of EDS analysis (at.%). N 1∗ 2 2 3 4 4 ∗1
21.9 21.3 38.2 35.7
Cr
Re
68.8 62.3 55.2 53.6 58.4 61.0
31.2 15.8 23.5 46.4 3.4 3.3
– matrix, 2 – eutectic element, 3 – white phase, 4 – dark phase.
of the precipitates, three elements were detected: chromium, rhenium and nitrogen, indicating the presence of a ternary compound. In the bright phase, nitrogen was not detected. In order to identify the precipitates, a series of diffraction patterns was recorded. Because of the size of investigated precipitates, the convergent electron beam diffraction patterns has been recorded with a beam diameter of about 10 nm. The images of the precipitates located at the grain boundaries and those in matrix, together with some diffraction patterns used to determine the interplanar distances, are presented in Fig. 2. The interplanar distances calculated for the matrix and the data from the ASTM card (06-0694) for chromium are presented in Table 2. A good agreement between the two indicates that the matrix is a solid solution of rhenium in chromium. The interplanar distances calculated for the TE imaged dark phase and the data from ASTM cards for CrRe2 (20–313) are presented in Table 3. The agreement between the listed values leads to the conclusion that the investigated phase is CrRe2 – sigma phase. The interplanar distances for the TE imaged bright phase found at the grain boundaries (‘A’), the precipitates observed inside grains (‘B’) and the data from ASTM cards for Cr2 N (35-0803) and CrN (11-0065), are presented in Table 4. The interplanar distances for the phases designated as A and B are comparable. These results suggest that A and B constitute precipitates of the same phase. The results indicate that the interplanar distances for these phases correspond to chromium nitride, either Cr2 N or CrN.
Discussion and conclusions Transmission and scanning electron microscopic investigations of the microstructure of the Cr-Re alloy were undertaken on specimens prepared by ion milling. C 2010 The Authors C 2010 The Royal Microscopical Society, Journal of Microscopy, 237, 431–434 Journal compilation
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Precipitates located at the grain boundaries and in the matrix were observed. Identification of the phases, based on analyses of the chemical composition and electron diffraction patterns, helped to identify the matrix as a solid solution of Re in Cr and one of the phases at the boundaries as CrRe2 (sigma phase). The other phase detected at the grain boundaries and in the matrix is very likely to be Cr2 N. However, one has to consider also another possibility based on the work of Medvedeva et al. (2002). The authors, using the full potential linear muffin-tin orbital (LMTO) method, studied the possibility of the formation of an A15 Cr3 Re phase in the Cr-Re system. It has been demonstrated that although the A15 Cr3 Re is unstable as a bulk compound, carbon impurities lead to stabilization of the phase. A similar effect could be expected for other interstitial elements such as nitrogen or oxygen. It should be noted that the A15 metastable phase has been observed in W-Re and Mo-Re films (Tournier et al., 1998) and is also present in a large number of Cr-Me (Pt, Ir, Rh, Ru, Os) systems. A tetragonal phase (Cr0.82 Re0.18 )2 C was found experimentally in the Cr-Re-C system (Grytsiv et al., 1997). In summary, the results presented in this paper indicate that as-recrystallized Cr-35Re alloy contains numerous precipitates, which may have a deleterious influence of its properties.
Acknowledgements
Fig. 2. TEM images of: (a) precipitates located in the matrix, (b) dark contrast phase and (c) bright contrast phase together with the corresponding micro-diffraction patterns.
This work has been performed within the framework of the Integrated European Project ‘ExtreMat’ (contract NMPCT-2004-500253) with financial support by the European Community. It only reflects the view of the authors and the European Community is not liable for any use of the information contained therein. The authors acknowledge the support provided by the Polish Ministry of Science and Higher Education through Grant No. 134/E-365/6 PR/DIE 317/2005–2008. Finally, the authors would like to thank their colleague from the ExtreMat consortium, Achim Schoberth of EADS, Corporate Research Center, Germany for providing the material for the investigation.
References Table 2. Calculated interplanar distances for matrix and ASTM data for chromium, in [Å]. Calculated ASTM hkl
2.04 2.04 110
1.47 1.44 200
1.23 1.18 211
Agnew, S.R. & Leonhardt, T. (2003) The low-temperature mechanical behavior of molybdenum-rhenium. JOM October, 25–29. Geach, G.A. & Hughes, J.E. (1956) The alloys of rhenium with molybdenum or with tungsten and having good high temperature properties, 2nd Plansee Seminar (ed. by F. Benesovsky) Pergamon, London. Gimeno-Fabre, L. (2006) Design, manufacture and properties of CrRe alloys for application in satellite thrusters. PhD Thesis, Technical University of Catalonia.
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Table 3. Calculated interplanar distances for TEM dark contrast phase and ASTM data for CrRe2 , in [Å]. Calculated ASTM hkl
4.60 4.65 200
4.29 101
4.08 4.16 210
3.67 3.89 111
3.29 220
3.06 3.15 211
2.83 2.94 310
2.72 221
2.62 2.61 301
2.58 320
Table 4. Calculated interplanar distances for TEM bright contrast phase (A), precipitates observed inside grains (B), ASTM data for β Cr2 N and for CrN, in [Å]. A B β Cr2 N CrN β Cr2 N hkl CrN hkl
3.05 101 -
2.53 -
2.45 2.41 2.39 110 111
2.30 2.30 -
2.23 2.23 2.24 002 -
Grytsiv, A.V., Bondar, A.A. & Varlikanova, T.Ya. (1997) Rhenium sold solution in the Cr-Re-C ternary system at solidus temperature. J. Alloys Compd. 262–263, 402–405. Jaffe, R.I., Sims, C.T. & Harwood, J.J. (1959) The effect of rhenium on the fabricability and ductility of molybdenum and tungsten, 3rd Plansee Seminar (ed. by F. Benesovsky) (Springer-Verlag, Vienna, Austria. Medvedeva, N.I., Gornostyrev, Yu.N. & Freeman, A.J. (2002) Carbon stabilized A15 Cr3 Re precipitates and ductility enhancement of Crbased alloys. Acta Mater. 50, 2471–2476.
2.16 2.16 2.12 111 -
2.10 2.04 2.08 2.07 200 200
1.84 1.89 201 -
1.67 1.63 1.64 112 -
1.49 1.46 211 220
1.36 1.33 1.39 300 -
Ro, Y., Koizumi, Y., Nakazawa, S., Kobayashi, T., Bannai, E. & Harada, H. (2002) Development of Cr-base alloys and their compressive properties. Scr. Mater. 46, 331–335. Tournier, S., Vinet, B., Pasturel, A., Ansara, I. & Desre, P.J. (1998) Undercooling-induced metastable A15 phase in the ReW system from drop-tube processing. Phys. Rev. B57(6), 3340– 3344. www.extremat.org. www.plansee.com.
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