JOURNAL OF ELECTRON MICROSCOPY TECHNIQUE 15:400-405 (1990)
A Replica Technique for Extracting Precipitates From Zirconium Alloys for Transmission Electron Microscopy Analysis J. NG-YELIM, O.T. WOO, AND G.J.C. CARPENTER Metals Technology Laboratories, CANMET, Energy, Mines and Resources Canada, Ottawa, Ontario K I A OGI, Canada (J.N.-Y., G.J.C.C.); Chalk Riucr Nuclear Laboratories, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1J0, Canada (0.T.W.I
KEY WORDS
Extraction replica, Zr alloys, Analytical electron microscopy
ABSTRACT A reliable two-stage carbon replica technique has been developed to extract precipitates from zirconium alloys. Using this technique, all precipitating phases can be extracted from Zircaloy-2, Zr-Cr-Fe, and Zr-Nb-Fe alloys. Precipitate identification using EDS X-ray analysis and convergent beam electron diffraction was greatly facilitated in comparison to thin foils. In addition, the sensitivity for the detection of trace elements in particles was increased using extraction replicas. The chemical compositions of the precipitates as determined from both replica and thin foils were in excellent agreement.
INTRODUCTION The rapid development of analytical electron microscopy has brought about a resurgence of interest in replica techniques for extracting small precipitates (less than approximately 100 nm diameter) onto suitable films, such as carbon. This permits a complete investigation of the chemical composition of the precipitates, without any interference from the matrix, using EDS X-ray analysis and electron energy loss spectroscopy (EELS); i t also facilitates crystal structure determination using convergent beam electron diffraction (CBED). Moreover, when the precipitates are small enough that absorption and fluorescence corrections can be neglected, X-ray microanalysis involves only a simple comparison of the intensities of the characteristic lines from the various elements (Cliff and Lorimer, 1975). Depending on the angle of the detector relative to the specimen, absorption corrections may be less important for extracted particles that are approximately equiaxed, because the X-ray path length to the detector may be reduced compared to that for a thin foil. For precipitates in thin foils, quantitative analysis is often more complex, involving 1)corrections for spurious X-rays produced from regions in the bulk of the specimen; 2) where overlap occurs, correction for the presence of the matrix, e.g., by using a n extrapolation procedure (Cliff et al., 1983); or 3) in certain cases, corrections for absorption andlor fluorescence (Zaluzec, 19791, which involve either foil thickness measurements or an extrapolation procedure (Van Cappellen, 1986; Horita et al., 1986; Carpenter et al., 1988). For some alloys, the extraction replica technique is fairly simple. The most straightforward procedure is to make a single-stage, direct carbon replica, where the etched surface is coated with a carbon film, which is subsequently stripped by further etching (Bradley, 1965). The main problem is to find a n etchant that attacks the matrix phase more rapidly than the precip-
GI 1990 WILEY-LISS, INC.
itates and that does not react so violently that the delicate carbon film breaks up. The greatest difficulties seem to occur when the matrix forms a coherent and stable oxide film, as in the case of Al, Zr, and Ti, where the precipitates are often attacked by a n etchant more rapidly than the matrix, making extraction impossible. In the course of a broader investigation of precipitation in Zr-2.5 wt% Nb-based alloys, it was found useful to develop a replica technique for extracting particles so that the chemical composition and crystal structure could be studied quickly and simply. Because of the protective oxide film, reactive etching agents such a s dilute hydrofluoric acid or bromine in methanol (Fong et al., 1982) must be used; however, these reagents tend to cause break up of the deposited carbon film with the one-stage technique. Micrographs of extraction replicas from the Zircaloys have been published (Fong et al., 1982; Chemelle et al., 1983; Versaci and Ipohorski, 1983; Kuwae et al., 19831, but full details of the technique were not always reported. In addition, there have been no reports of extraction of Zr-Ni-Fe precipitates, which tend to be attacked more rapidly than the matrix (Cox, 1970). This paper describes in detail a reliable two-stage extraction replica technique developed for Zirconium alloys. We illustrate the success of this technique with results from X-ray microanalysis and electron microdiffraction of the extracted particles. The results are also compared to those results with thin foils. EXPERIMENTAL TECHNIQUES The alloys used in this study were Zircaloy-2 (Zr1.5% Sn-0.15%1 Fe-0.1% Cr-0.05% Ni), Zr-1.25% Cr~
Received June 23, 1989, accepted in revised form October 3, 1989 Address reprint requests to J Ng-Yelim, Metals Technology Laboratories, CANMET, Energy, Mines and Resources Canada, 568 Booth Street, Ottawa, Ontario K1A OG1, Canada
EXTRACTING PRECIPITATES
SOlUt ion
Fresh
Carbon-Coated Replica with Extracted Precipitates
I
/
eve^ of Washing Solution \ I Copper Mesh
r
Driiining Old Solution Fig. 1. Apparatus for flow-washing extraction replicas to remove cellulose acetate tape.
0.1% Fe, and Zr-2.5% Nb containing either 0.1% or 0.5% Fe (all compositions given in wt%). The most important stage in making an extraction replica is to find an etching reagent that attacks the matrix material, leaving the precipitates intact. The standard chemical polishing solution, composed of 10%HF and 45% HNO, in water, was found to be satisfactory for some Zr-based alloys, e.g., for Zr-Cr-Fe alloys. However, in certain other cases, e.g., for the Zr-Nb based alloys, a solution containing 6-10% HF, 45% HNO,, and 45% lactic acid was found to be necessary. The replica technique used for extracting the precipitates is a refinement of the two-stage replica technique described by Bradley (1965). The specimen is first prepared by metallographic polishing and etched in the appropriate reagent. A piece of cellulose acetate tape, such as Bioden, is softened in a dish of acetone for a few seconds and placed onto the etched surface; the tape should make good contact with every part of the surface such that no air bubbles are trapped between
N" 200 0 d Y
150
tn 4
c 100 3
0 0 50
0 0
2
4
6
B
40 1
3
Energy (keV)
(b> Fig. 2. a: Zr(Fe,Cr), precipitate extracted from a Zr-1.25% Cr-O.l% Fe alloy. b X-ray spectrum from precipitate (the CuK peaks originate from the support grid). c: Convergent beam electron diffraction pattern from precipitate, 111261 zone axis.
J. NG-YELIM ET AL.
402
from breaking up, the level of the solvent is controlled so that it does not rise above the level of the copper grids. Only the precipitates remain on the surface of the carbon film, ready for examination and analysis in the transmission electron microscope. The two-stage replica technique has certain advantages over the single-stage method, namely 1) much greater control over the etching procedure, permitting a larger range of precipitate sizes to be examined; and 2) reduced tendency for disintegration of the carbon film, which can be a problem with single-stage replicas when strong reagents are used for stripping. The results presented here were obtained using a Philips EM 400T electron microscope operated at 120 keV, equipped with a n EDAX 9100160 EDS X-ray detection system. Calibration k factors for the EDAX analysis were obtained from various zirconium alloys and intermetallic compounds (Carpenter et al., 1988) using the extrapolation method.
RESULTS AND DISCUSSION
400,
I
320
0
d 240
-
u)
c,
c 1803
0
u 80
(b)
0
2
4
8
8
Energy (keV)
Fig. 3. a: Zr-Ni-Fe (A) and Zr-Fe-Cr (B) precipitates extracted from Zircaloy-2. b X-ray spectrum of Zr-Ni-Fe precipitate.
the two surfaces. The Bioden tape is allowed to harden, after which it is lifted off the surface with a pair of tweezers. A thin carbon film, 10-20 nm thick, is then deposited onto the side of the Bioden tape that contains the extracted particles. The Bioden replica is cut into strips and placed with the carbon-coated side facing upwards on 200 mesh electron microscope copper support grids. The grids are then flow-washed in solutions of acetone and water: starting with a 70% acetone solution, then progressing gradually to a 100% acetone wash (Fig. 1).Various washing techniques have been suggested in the literature (Bradley, 1965; Goodhew, 1972), but this technique was found to be particularly reliable and reproducible. To prevent the carbon film
Figure 2a is a micrograph of a thin faulted precipitate extracted from a Zr-Cr-Fe alloy. The particles were clean and uncontaminated; for example, details of the faults could be observed with excellent resolution. From the energy dispersive spectrum (EDS) (Fig. 2b) it was deduced that the particles had the composition Zr(Fe,Cr),. The CuK peaks originated from the grids used to support the carbon replica. CBED patterns of the type shown in Figure 2c confirmed the crystal structure to be hexagonal, with a = 0.508 nm and c = 0.828 nm, a s expected. A major advantage of working with extracted precipitates is the greater ease with which i t is possible to obtain high symmetry zone axis patterns without interference from the matrix phase. Similar Zr-Fe-Cr precipitates have been extracted from Zircaloy-2 specimens using a n etching solution of nitric acid, hydrofluoric acid, and water. However, the Zr-Ni-Fe particles were not extracted using this acid etch. There is evidence that the acid solution preferentially dissolves the Zr-Ni-Fe particles; indeed, scanning electron micrographs of intergranular cracks in Zircaloy-2 after stress corrosion cracking in methanol solution have shown that most Zr-Ni-Fe particles were lost from the cracks, leaving pits instead (Cox, 1970). An attempt to extract these particles from Zircaloy-2 was therefore made using a n alternative etchant containing nitric acid, hydrofluoric acid, and lactic acid. This reagent was found to be most effective when the sample was swabbed lightly. A replica showing Zr-NiFe precipitates successfully extracted in this way is shown in Figure 3. This is the first report of Zr-Ni-Fe precipitates being extracted from Z r alloys. The Zr-FeCr precipitates were also extracted using this etchant. The chemical composition of the Zr-Ni-Fe precipitates was determined by EDS X-ray analysis to be approximately Zr2Feo,5Nio,4, in agreement with earlier results (Chemelle et al., 1983; Arias et al., 1987). Traces of Cr (0.1-0.396) and Sn (1.0-1.596) were also detected in some precipitates (Fig. 3b). CBED patterns confirmed the crystal structure to be body-centered tetragonal, with a = 0.68 nm and c = 0.56 nm, in agreement with previous work (Chemelle et al., 1983).
EXTRACTING PRECIPITATES
100
403
40
Ir L JbL
r\
"U
Fe
h
32 0 4 v
24
cn C
c, C 16 3
40
3 0
0 20
-
0 0
8
Cr
0 0
4
8
12
16
,
E n e r g y ( ke V )
Energy (keV)
(c> Fig. 4. a: Extracted Zr-Nb-Fe-Cr precipitates from a cold-rolled Zr-2.5% Nb-0.5% Fe alloy. b: X-ray spectrum of extracted precipitate. c: X-ray spectrum of similar precipitate in thin foil.
Figure 4a is a micrograph of a n extraction replica from a cold-rolled Zr-2.5% Nb-0.5% Fe alloy, lightly etched in the lactic acid reagent. A typical X-ray spectrum of the extracted particles (Fig. 4b) showed that they contained Zr, Nb, Fe, and Cr. The X-ray spectra from similar precipitates in a metal foil (e.g., a s shown in Fig. 4c) did not contain the CuK peaks but had contributions from 1) the zirconium surrounding the particle and 2) spurious X-rays produced by the entire specimen. The latter signal is caused by the interaction of unwanted high-energy X-rays and stray electrons
with the bulk of the specimen (Williams et al., 1986). The normal procedure in compensating for this effect is to subtract a L L h o l e - ~ ~correction ~nt7' obtained by passing the electron beam through the hole in the middle of the specimen. In the case of particles extracted onto a suitable film, a hole-count correction is rarely necessary. When comparing the two spectra, it was evident that the Cr peak was generally less distinguishable in the metal foil than in the replica. Extraction replicas from specimens of the cold-rolled Zr-2.5% Nb-0.1% Fe alloy, etched heavily with the lac-
J. NG-YELIM ET AL.
404
Fig. 5. Precipitates and P-Zr and P-Nb phases in a cold-rolled Zr-2.5% Nb-0.1% Fe alloy. P, Zr. Nb-Fe-Cr precipitate; Zr, P-Zr phase; Nb, P-Nb phase.
TABLE 1. Chemical compositions (in w~%G) of intermetallic particles in Zr-2.5 wt% Nb-Fe alloys Zr
Alloy Thin foil 0.5 wt% Fe alloy 0.1 wt% Fe alloy Zr, ,Nb, ,Fez Extraction replica 0.5 wt% Fe alloy3 0.1 wt% Fe alloy4
Nb
Fe
Cr
40.8 (22.4) 43.5 (22.5) 15.6 (21.0) 0.1 ( k O . 1 ) 40.1 ( 5 2 . 2 ) 44.0 (13.0) 14.9 (20.9) 1.0 (20.1)' 41.2 42.0 16.8 41.0 (23.6) 44.4 (23.0) 14.3 (22.0) 0.3 (t0.1) 39.0 (20.6) 45.5 (20.9) 14.4 (20.9) 1.1( i 0 . 4 )
'Approximate value. 'Calculated. 'Average of nine spectra. 4Average of four spectra.
tic acid reagent, revealed 6-Zr and P-Nb particles as well as Zr-Nb-Fe-Cr precipitates. These are shown in Figure 5. The p-Zr phase contained approximately 25.3% Nb and 0.7% Fe, and the P-Nb phase contained approximately 14.3% Zr, 0.3% Fe, and 0.3% Cr. The extraction of p-Zr and P-Nb phases from zirconium alloys has not been reported previously. The chemical composition of the Zr-Nb-Fe-Cr precipitates, as determined from replicas and from thin foils, using the extrapolation technique (Woo and Carpenter, in preparation) are shown in Table 1. It is clear that the results from both methods are in good agreement, ex-
cept for the small amount of Cr, -0.3%, in the 0.5% Fe alloy, which was detected more readily in the replica spectra (0.2-0.4% in the replica vs. 0-0.2% in the thin foil). Because the Cr concentration in the 0.1 wt% Fe alloy was higher than that in the 0.5 wt% Fe alloy, X-ray spectra from precipitates in both the thin foils and the replica detected the 1.0% Cr present. We thus conclude that, for the lower Cr concentration, the Cr peak was likely obscured by the background in a thin foil analysis, whereas, in a n extraction replica, the bremsstrahlung level was sufficiently low for the peak to be distinguishable. CBED patterns from the Zr-NbFe-Cr precipitates in extraction replicas and in thin foils showed them to be a new phase having a hexagonal crystal structure, with a = 0.54 nm, and c = 0.88 nm, belonging to the space group P6,immc (Woo and Carpenter, in preparation).
CONCLUSIONS A simple two-stage replica technique has been developed for extracting second-phase particles from zirconium alloys. By using a n appropriate etchant, i t was found for the first time to be possible to extract all the precipitating phases in Zircaloy-2, Zr-Cr-Fe, and ZrNb-Fe alloys. In particular, Zr-Ni-Fe precipitates have been successfully extracted from Zircaloy-2. The ad-
EXTRACTING PRECIPITATES
vantages of using extraction replicas are that EDS Xray analysis, EELS, and electron diffraction are facilitated in comparison with thin foil specimens. Another advantage of the use of an extraction replica is the greater sensitivity for the detection of trace elements.
ACKNOWLEDGMENTS We thank Andrew S. Danis (University of Waterloo) for his help with preliminary work on these techniques and M.C. Jacklin (CRNL) for experimental assistance. We are also grateful to B. Cox (CRNL) for valuable discussions and for suggesting the use of a lactic acid etchant to extract Zr-Ni-Fe precipitates. This work is partially funded by Canadian Utilities under COG/ CANDEV agreement WP 3 111. REFERENCES Arias, D., Palacios, T., and Turrillo, C. (1987) J. Nuclear Mater. 148: 227. Bradley, D.E. (1965) In: Techniques for Electron Microscopy. D.H. Kay, ed. Blackwell Scientific Publications, Oxford, p. 96.
405
Carpenter, G.J.C., Charest, M., and Woo, O.T. (1988) Electron Microsc. SOC. Am. Bull., 18:57. Chemelle, P., Knorr, D.B., van der Sande, J.B., and Pelloux, R.M. (1983) J. Nuclear Mater. 11358. Cliff, G., and Lorimer, G.W. (1975) J . Microsc. 103:203. Cliff, G., Powell, D.J., Pilkington, R., Champness, P.E., and Lorimer, G.W. (1983) Inst. Physics Conf. Series No. 68, Proc. EMAG, Guildford, p. 63. Cox, B. (1970) Atomic Energy of Canada Ltd. Report, AECL-3551. Fong, W.L., and Northwood, D.O. (1982) Metallography 15:27. Goodhew, P.J. (1972) In: Practical Methods in Electron Microscopy. A. Glauert, ed. Vol. 1, American Elsevier Publishing Company, Inc., New York, p. 137. Horita, Z., Sano, T., and Nemoto, M. (1986) J . Microsc. 143:215. Kuwae, R., Sato, K., Higashinakagawa, E., Kawashima, J., and Nakamura, S. (1983) J. Nuclear Mater. 119:229. Van Cappellen, E. (1986) Proc. 11th Int. Congress on X-ray Optics and Microanalysis. J.D. Brown and R.H. Packwood, eds. Ontario. The University of Western Ontario Press, p. 409. Versaci, R.A., and Ipohorski, M. (1983) J. Nuclear Mater. 116:321. Williams, D.B., Goldstein, J.I., and Fiori, C.E. (1986) In: Principles of Analytical Electron Microscopy. D.C. Joy, A.D. Romig, and J.I. Goldstein, eds. Plenum Press, New York, p. 123. Zaluzec, N.J. (1979) In: Introduction to Analytical Electron Microscopy. J.J. Hren, J.I. Goldstein, and D.C. Joy, eds. Plenum Press, New York, p. 121.
A Replica Technique for Extracting Precipitates From Zirconium Alloys for Transmission Electron Microscopy Analysis J. NG-YELIM, O.T. WOO, AND G.J.C. CARPENTER Metals Technology Laboratories, CANMET, Energy, Mines and Resources Canada, Ottawa, Ontario K I A OGI, Canada (J.N.-Y., G.J.C.C.); Chalk Riucr Nuclear Laboratories, Atomic Energy of Canada Limited Research Company, Chalk River, Ontario KOJ 1J0, Canada (0.T.W.I
KEY WORDS
Extraction replica, Zr alloys, Analytical electron microscopy
ABSTRACT A reliable two-stage carbon replica technique has been developed to extract precipitates from zirconium alloys. Using this technique, all precipitating phases can be extracted from Zircaloy-2, Zr-Cr-Fe, and Zr-Nb-Fe alloys. Precipitate identification using EDS X-ray analysis and convergent beam electron diffraction was greatly facilitated in comparison to thin foils. In addition, the sensitivity for the detection of trace elements in particles was increased using extraction replicas. The chemical compositions of the precipitates as determined from both replica and thin foils were in excellent agreement.
INTRODUCTION The rapid development of analytical electron microscopy has brought about a resurgence of interest in replica techniques for extracting small precipitates (less than approximately 100 nm diameter) onto suitable films, such as carbon. This permits a complete investigation of the chemical composition of the precipitates, without any interference from the matrix, using EDS X-ray analysis and electron energy loss spectroscopy (EELS); i t also facilitates crystal structure determination using convergent beam electron diffraction (CBED). Moreover, when the precipitates are small enough that absorption and fluorescence corrections can be neglected, X-ray microanalysis involves only a simple comparison of the intensities of the characteristic lines from the various elements (Cliff and Lorimer, 1975). Depending on the angle of the detector relative to the specimen, absorption corrections may be less important for extracted particles that are approximately equiaxed, because the X-ray path length to the detector may be reduced compared to that for a thin foil. For precipitates in thin foils, quantitative analysis is often more complex, involving 1)corrections for spurious X-rays produced from regions in the bulk of the specimen; 2) where overlap occurs, correction for the presence of the matrix, e.g., by using a n extrapolation procedure (Cliff et al., 1983); or 3) in certain cases, corrections for absorption andlor fluorescence (Zaluzec, 19791, which involve either foil thickness measurements or an extrapolation procedure (Van Cappellen, 1986; Horita et al., 1986; Carpenter et al., 1988). For some alloys, the extraction replica technique is fairly simple. The most straightforward procedure is to make a single-stage, direct carbon replica, where the etched surface is coated with a carbon film, which is subsequently stripped by further etching (Bradley, 1965). The main problem is to find a n etchant that attacks the matrix phase more rapidly than the precip-
GI 1990 WILEY-LISS, INC.
itates and that does not react so violently that the delicate carbon film breaks up. The greatest difficulties seem to occur when the matrix forms a coherent and stable oxide film, as in the case of Al, Zr, and Ti, where the precipitates are often attacked by a n etchant more rapidly than the matrix, making extraction impossible. In the course of a broader investigation of precipitation in Zr-2.5 wt% Nb-based alloys, it was found useful to develop a replica technique for extracting particles so that the chemical composition and crystal structure could be studied quickly and simply. Because of the protective oxide film, reactive etching agents such a s dilute hydrofluoric acid or bromine in methanol (Fong et al., 1982) must be used; however, these reagents tend to cause break up of the deposited carbon film with the one-stage technique. Micrographs of extraction replicas from the Zircaloys have been published (Fong et al., 1982; Chemelle et al., 1983; Versaci and Ipohorski, 1983; Kuwae et al., 19831, but full details of the technique were not always reported. In addition, there have been no reports of extraction of Zr-Ni-Fe precipitates, which tend to be attacked more rapidly than the matrix (Cox, 1970). This paper describes in detail a reliable two-stage extraction replica technique developed for Zirconium alloys. We illustrate the success of this technique with results from X-ray microanalysis and electron microdiffraction of the extracted particles. The results are also compared to those results with thin foils. EXPERIMENTAL TECHNIQUES The alloys used in this study were Zircaloy-2 (Zr1.5% Sn-0.15%1 Fe-0.1% Cr-0.05% Ni), Zr-1.25% Cr~
Received June 23, 1989, accepted in revised form October 3, 1989 Address reprint requests to J Ng-Yelim, Metals Technology Laboratories, CANMET, Energy, Mines and Resources Canada, 568 Booth Street, Ottawa, Ontario K1A OG1, Canada
EXTRACTING PRECIPITATES
SOlUt ion
Fresh
Carbon-Coated Replica with Extracted Precipitates
I
/
eve^ of Washing Solution \ I Copper Mesh
r
Driiining Old Solution Fig. 1. Apparatus for flow-washing extraction replicas to remove cellulose acetate tape.
0.1% Fe, and Zr-2.5% Nb containing either 0.1% or 0.5% Fe (all compositions given in wt%). The most important stage in making an extraction replica is to find an etching reagent that attacks the matrix material, leaving the precipitates intact. The standard chemical polishing solution, composed of 10%HF and 45% HNO, in water, was found to be satisfactory for some Zr-based alloys, e.g., for Zr-Cr-Fe alloys. However, in certain other cases, e.g., for the Zr-Nb based alloys, a solution containing 6-10% HF, 45% HNO,, and 45% lactic acid was found to be necessary. The replica technique used for extracting the precipitates is a refinement of the two-stage replica technique described by Bradley (1965). The specimen is first prepared by metallographic polishing and etched in the appropriate reagent. A piece of cellulose acetate tape, such as Bioden, is softened in a dish of acetone for a few seconds and placed onto the etched surface; the tape should make good contact with every part of the surface such that no air bubbles are trapped between
N" 200 0 d Y
150
tn 4
c 100 3
0 0 50
0 0
2
4
6
B
40 1
3
Energy (keV)
(b> Fig. 2. a: Zr(Fe,Cr), precipitate extracted from a Zr-1.25% Cr-O.l% Fe alloy. b X-ray spectrum from precipitate (the CuK peaks originate from the support grid). c: Convergent beam electron diffraction pattern from precipitate, 111261 zone axis.
J. NG-YELIM ET AL.
402
from breaking up, the level of the solvent is controlled so that it does not rise above the level of the copper grids. Only the precipitates remain on the surface of the carbon film, ready for examination and analysis in the transmission electron microscope. The two-stage replica technique has certain advantages over the single-stage method, namely 1) much greater control over the etching procedure, permitting a larger range of precipitate sizes to be examined; and 2) reduced tendency for disintegration of the carbon film, which can be a problem with single-stage replicas when strong reagents are used for stripping. The results presented here were obtained using a Philips EM 400T electron microscope operated at 120 keV, equipped with a n EDAX 9100160 EDS X-ray detection system. Calibration k factors for the EDAX analysis were obtained from various zirconium alloys and intermetallic compounds (Carpenter et al., 1988) using the extrapolation method.
RESULTS AND DISCUSSION
400,
I
320
0
d 240
-
u)
c,
c 1803
0
u 80
(b)
0
2
4
8
8
Energy (keV)
Fig. 3. a: Zr-Ni-Fe (A) and Zr-Fe-Cr (B) precipitates extracted from Zircaloy-2. b X-ray spectrum of Zr-Ni-Fe precipitate.
the two surfaces. The Bioden tape is allowed to harden, after which it is lifted off the surface with a pair of tweezers. A thin carbon film, 10-20 nm thick, is then deposited onto the side of the Bioden tape that contains the extracted particles. The Bioden replica is cut into strips and placed with the carbon-coated side facing upwards on 200 mesh electron microscope copper support grids. The grids are then flow-washed in solutions of acetone and water: starting with a 70% acetone solution, then progressing gradually to a 100% acetone wash (Fig. 1).Various washing techniques have been suggested in the literature (Bradley, 1965; Goodhew, 1972), but this technique was found to be particularly reliable and reproducible. To prevent the carbon film
Figure 2a is a micrograph of a thin faulted precipitate extracted from a Zr-Cr-Fe alloy. The particles were clean and uncontaminated; for example, details of the faults could be observed with excellent resolution. From the energy dispersive spectrum (EDS) (Fig. 2b) it was deduced that the particles had the composition Zr(Fe,Cr),. The CuK peaks originated from the grids used to support the carbon replica. CBED patterns of the type shown in Figure 2c confirmed the crystal structure to be hexagonal, with a = 0.508 nm and c = 0.828 nm, a s expected. A major advantage of working with extracted precipitates is the greater ease with which i t is possible to obtain high symmetry zone axis patterns without interference from the matrix phase. Similar Zr-Fe-Cr precipitates have been extracted from Zircaloy-2 specimens using a n etching solution of nitric acid, hydrofluoric acid, and water. However, the Zr-Ni-Fe particles were not extracted using this acid etch. There is evidence that the acid solution preferentially dissolves the Zr-Ni-Fe particles; indeed, scanning electron micrographs of intergranular cracks in Zircaloy-2 after stress corrosion cracking in methanol solution have shown that most Zr-Ni-Fe particles were lost from the cracks, leaving pits instead (Cox, 1970). An attempt to extract these particles from Zircaloy-2 was therefore made using a n alternative etchant containing nitric acid, hydrofluoric acid, and lactic acid. This reagent was found to be most effective when the sample was swabbed lightly. A replica showing Zr-NiFe precipitates successfully extracted in this way is shown in Figure 3. This is the first report of Zr-Ni-Fe precipitates being extracted from Z r alloys. The Zr-FeCr precipitates were also extracted using this etchant. The chemical composition of the Zr-Ni-Fe precipitates was determined by EDS X-ray analysis to be approximately Zr2Feo,5Nio,4, in agreement with earlier results (Chemelle et al., 1983; Arias et al., 1987). Traces of Cr (0.1-0.396) and Sn (1.0-1.596) were also detected in some precipitates (Fig. 3b). CBED patterns confirmed the crystal structure to be body-centered tetragonal, with a = 0.68 nm and c = 0.56 nm, in agreement with previous work (Chemelle et al., 1983).
EXTRACTING PRECIPITATES
100
403
40
Ir L JbL
r\
"U
Fe
h
32 0 4 v
24
cn C
c, C 16 3
40
3 0
0 20
-
0 0
8
Cr
0 0
4
8
12
16
,
E n e r g y ( ke V )
Energy (keV)
(c> Fig. 4. a: Extracted Zr-Nb-Fe-Cr precipitates from a cold-rolled Zr-2.5% Nb-0.5% Fe alloy. b: X-ray spectrum of extracted precipitate. c: X-ray spectrum of similar precipitate in thin foil.
Figure 4a is a micrograph of a n extraction replica from a cold-rolled Zr-2.5% Nb-0.5% Fe alloy, lightly etched in the lactic acid reagent. A typical X-ray spectrum of the extracted particles (Fig. 4b) showed that they contained Zr, Nb, Fe, and Cr. The X-ray spectra from similar precipitates in a metal foil (e.g., a s shown in Fig. 4c) did not contain the CuK peaks but had contributions from 1) the zirconium surrounding the particle and 2) spurious X-rays produced by the entire specimen. The latter signal is caused by the interaction of unwanted high-energy X-rays and stray electrons
with the bulk of the specimen (Williams et al., 1986). The normal procedure in compensating for this effect is to subtract a L L h o l e - ~ ~correction ~nt7' obtained by passing the electron beam through the hole in the middle of the specimen. In the case of particles extracted onto a suitable film, a hole-count correction is rarely necessary. When comparing the two spectra, it was evident that the Cr peak was generally less distinguishable in the metal foil than in the replica. Extraction replicas from specimens of the cold-rolled Zr-2.5% Nb-0.1% Fe alloy, etched heavily with the lac-
J. NG-YELIM ET AL.
404
Fig. 5. Precipitates and P-Zr and P-Nb phases in a cold-rolled Zr-2.5% Nb-0.1% Fe alloy. P, Zr. Nb-Fe-Cr precipitate; Zr, P-Zr phase; Nb, P-Nb phase.
TABLE 1. Chemical compositions (in w~%G) of intermetallic particles in Zr-2.5 wt% Nb-Fe alloys Zr
Alloy Thin foil 0.5 wt% Fe alloy 0.1 wt% Fe alloy Zr, ,Nb, ,Fez Extraction replica 0.5 wt% Fe alloy3 0.1 wt% Fe alloy4
Nb
Fe
Cr
40.8 (22.4) 43.5 (22.5) 15.6 (21.0) 0.1 ( k O . 1 ) 40.1 ( 5 2 . 2 ) 44.0 (13.0) 14.9 (20.9) 1.0 (20.1)' 41.2 42.0 16.8 41.0 (23.6) 44.4 (23.0) 14.3 (22.0) 0.3 (t0.1) 39.0 (20.6) 45.5 (20.9) 14.4 (20.9) 1.1( i 0 . 4 )
'Approximate value. 'Calculated. 'Average of nine spectra. 4Average of four spectra.
tic acid reagent, revealed 6-Zr and P-Nb particles as well as Zr-Nb-Fe-Cr precipitates. These are shown in Figure 5. The p-Zr phase contained approximately 25.3% Nb and 0.7% Fe, and the P-Nb phase contained approximately 14.3% Zr, 0.3% Fe, and 0.3% Cr. The extraction of p-Zr and P-Nb phases from zirconium alloys has not been reported previously. The chemical composition of the Zr-Nb-Fe-Cr precipitates, as determined from replicas and from thin foils, using the extrapolation technique (Woo and Carpenter, in preparation) are shown in Table 1. It is clear that the results from both methods are in good agreement, ex-
cept for the small amount of Cr, -0.3%, in the 0.5% Fe alloy, which was detected more readily in the replica spectra (0.2-0.4% in the replica vs. 0-0.2% in the thin foil). Because the Cr concentration in the 0.1 wt% Fe alloy was higher than that in the 0.5 wt% Fe alloy, X-ray spectra from precipitates in both the thin foils and the replica detected the 1.0% Cr present. We thus conclude that, for the lower Cr concentration, the Cr peak was likely obscured by the background in a thin foil analysis, whereas, in a n extraction replica, the bremsstrahlung level was sufficiently low for the peak to be distinguishable. CBED patterns from the Zr-NbFe-Cr precipitates in extraction replicas and in thin foils showed them to be a new phase having a hexagonal crystal structure, with a = 0.54 nm, and c = 0.88 nm, belonging to the space group P6,immc (Woo and Carpenter, in preparation).
CONCLUSIONS A simple two-stage replica technique has been developed for extracting second-phase particles from zirconium alloys. By using a n appropriate etchant, i t was found for the first time to be possible to extract all the precipitating phases in Zircaloy-2, Zr-Cr-Fe, and ZrNb-Fe alloys. In particular, Zr-Ni-Fe precipitates have been successfully extracted from Zircaloy-2. The ad-
EXTRACTING PRECIPITATES
vantages of using extraction replicas are that EDS Xray analysis, EELS, and electron diffraction are facilitated in comparison with thin foil specimens. Another advantage of the use of an extraction replica is the greater sensitivity for the detection of trace elements.
ACKNOWLEDGMENTS We thank Andrew S. Danis (University of Waterloo) for his help with preliminary work on these techniques and M.C. Jacklin (CRNL) for experimental assistance. We are also grateful to B. Cox (CRNL) for valuable discussions and for suggesting the use of a lactic acid etchant to extract Zr-Ni-Fe precipitates. This work is partially funded by Canadian Utilities under COG/ CANDEV agreement WP 3 111. REFERENCES Arias, D., Palacios, T., and Turrillo, C. (1987) J. Nuclear Mater. 148: 227. Bradley, D.E. (1965) In: Techniques for Electron Microscopy. D.H. Kay, ed. Blackwell Scientific Publications, Oxford, p. 96.
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Carpenter, G.J.C., Charest, M., and Woo, O.T. (1988) Electron Microsc. SOC. Am. Bull., 18:57. Chemelle, P., Knorr, D.B., van der Sande, J.B., and Pelloux, R.M. (1983) J. Nuclear Mater. 11358. Cliff, G., and Lorimer, G.W. (1975) J . Microsc. 103:203. Cliff, G., Powell, D.J., Pilkington, R., Champness, P.E., and Lorimer, G.W. (1983) Inst. Physics Conf. Series No. 68, Proc. EMAG, Guildford, p. 63. Cox, B. (1970) Atomic Energy of Canada Ltd. Report, AECL-3551. Fong, W.L., and Northwood, D.O. (1982) Metallography 15:27. Goodhew, P.J. (1972) In: Practical Methods in Electron Microscopy. A. Glauert, ed. Vol. 1, American Elsevier Publishing Company, Inc., New York, p. 137. Horita, Z., Sano, T., and Nemoto, M. (1986) J . Microsc. 143:215. Kuwae, R., Sato, K., Higashinakagawa, E., Kawashima, J., and Nakamura, S. (1983) J. Nuclear Mater. 119:229. Van Cappellen, E. (1986) Proc. 11th Int. Congress on X-ray Optics and Microanalysis. J.D. Brown and R.H. Packwood, eds. Ontario. The University of Western Ontario Press, p. 409. Versaci, R.A., and Ipohorski, M. (1983) J. Nuclear Mater. 116:321. Williams, D.B., Goldstein, J.I., and Fiori, C.E. (1986) In: Principles of Analytical Electron Microscopy. D.C. Joy, A.D. Romig, and J.I. Goldstein, eds. Plenum Press, New York, p. 123. Zaluzec, N.J. (1979) In: Introduction to Analytical Electron Microscopy. J.J. Hren, J.I. Goldstein, and D.C. Joy, eds. Plenum Press, New York, p. 121.
