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A Mössbauer study of single-crystal Zr-0.065 at% 57Fe
ELSEVIER
Journal of Nuclear Materials 218 (1995) 161-165
journalof nuclear materials
A M6ssbauer study of single-crystal Zr-0.065 at% 57Fe J.A. Sawicki, G.M. Hood, H. Zou, R.J. Schultz Reactor Materials Division, Atomic Energy of Canada Ltd., Chalk River Laboratories, Chalk River, Ontario, Canada KOJIJO Received 15 June 1994; accepted 6 September 1994
Abstract
Single crystals of Zr-0.065 at% Fe (95% 57Fe), from a common parent, have been studied by M6ssbauer spectroscopy, using both transmission and conversion-electron modes. In the "as grown" state the Fe is present as the meta-stable phase, t-ZrzFe. Vacuum annealing, in the range 795-973 K, results in gradual decomposition of t-Zr2Fe and the diffusion of Fe from the bulk to the surface, where it forms the stable Zr3Fe phase. The kinetics of Zr3Fe formation show fair accord with the known diffusion properties of Fe in a-Zr. Some results for other dilute Zr(Fe) single crystals are reported.
1. Introduction
The behaviour of Fe in Zr and in Zircaloy-2 and Zr-2.5% Nb is of particular interest. In the commercial alloys the state and distribution of the element (present at ~ 0.1 at%, unless specified otherwise, concentrations herein are in atomic units) are believed to have an influence on corrosion properties, hydrogen ingress, irradiation damage and possibly, mechanical properties, see references in Ref. [1]; in addition, there is now evidence of a correlation between the irradiation deformation of Zr-2.5Nb pressure tubes and the level of Fe in the alloy [2]. At a more elementary level it is apparent that trace Fe controls both self- and slow substitutional diffusion in a-Zr [3]. In terms of its physico/chemical behaviour in a-Zr, the small atomic radius of Fe leads to its low solubility and its ability to occupy sites in the interstitial sublattice of Zr: the latter property results in a very high diffusivity [3]. These behavioural aspects have made it difficult to charaeterise the equilibrium state of Fe in a-Zr [1,4]. In previous work [5] it has been shown that the disposition of Fe in binary Zr-0.1 Fe alloys is remarkably sensitive to annealing treatments. Single-crystal alloy specimens were found to exhibit rapid and massive transfer of Fe from internal precipitates to a surface-associated phase when the samples were annealed below the eutectoid temperature. Recent and Elsevier Science B.V. SSDI 0022-31 t5(94)00384-X
current work on dilute Zr(Fe) single crystals have shown that at modestly elevated temperatures, but below the eutectoid ( ~ 1070 K), an equilibrium is established between the bulk solid solution and an associated surface precipitate phase, Zr3Fe [1,4]. These latter studies have utilised secondary ion mass spectrometry, transmission electron microscopy, electron micro-probe analysis and micro-electron diffraction to determine properties of Fe in Zr. The present study utilises M6ssbauer spectrometry to characterise the nature of the internal and surface precipitate phases of Fe in single-crystal Zr-0.065 at% Fe (enriched to 95% in 57Fe) and to obtain information on the kinetics of the bulk-to-surface transfer of Fe. M6ssbauer spectrometry has the advantage (over microscopical methods) of yielding the averaged characteristics of the states of Fe over a relatively large sampled volume.
2. Experimental
2.1. Alloy fabrication /crystal growth /specimen preparation The starting alloy, Zr-0.1% 57Fe, was prepared from crystal-bar Zr and Fe enriched to 95% in 57Fe. Table 1 gives an analysis of the Zr. The alloy billet,
162
J.A. Sawicki et al. /Journal of Nuclear Materials 218 (1995) 161-165
made by arc-melting under Ar, was reduced, by cold extrusion, to 30 cm long, 6.0 mm diameter rods. Analyses of the rods by inductively-coupled plasma emission spectroscopy (ICPES) showed a homogeneous Fe concentration (0.1%). Single crystals were grown by an electron beam/float-zone-melting technique. The zone-melted rod contained some single crystal pieces, up to 3 cm long. Analyses (ICPES) of the rod in the as-zone-melted-state showed an Fe content, along the zone-melted length, of 0.065 _+0.008% Fe. Loss of Fe is attributed to evaporation during zone-melting. The surface preparation of the specimens, cut from the parent rod by a slow-speed diamond saw and generally 1-2 mm thick, was done by grinding through metallographic papers down to 600 grit, followed by 6, then 3, ~m diamond grinding on plate glass. The final steps were chemical polishing in a solution of H 2 0 : H N O 3 : H F , in the ratio 50:45:5, by volume, followed by electropolishing in 5% perchloric acid in ethanol at - 40°C. Thin (100-250 Ixm), parallel-sided specimens for transmission M6ssbauer spectrometry studies were cut from the rod. Two such specimens, 250 Ixm thick, were made especially for kinetic studies: one had basal (0001) and the other prism (1010) surfaces. Specimen annealing was done under vacuum at residual pressures of 10 6 Pa. The specimens were placed on polished tungsten in Ta containers. 2.2. M6ssbauer spectrometry
M6ssbauer spectroscopy has been used extensively in studies of Z r - F e intermetallics, see, for example, Refs. [6,7]. On the Zr-rich side of the phase diagram three crystalline phases occur: orthorhombic Zr3Fe, tetragonal body-centered t-Zr2Fe and cubic diamond c-Zr2Fe [6]. The M6ssbauer spectra of these phases are well known and can be clearly distinguished by their characteristically different quadrupole splittings (QS). These compounds display only one Fe lattice site, with practically axial symmetry, asymmetry parameter r / = 0 in c-Zr2Fe and t-ZrzFe, and r / = 0.6 in Zr3Fe [6]. However, in Zr3Fe and c-Zr2Fe the Fe atom has a positive QS whereas QS is negative in t-Zr2Fe. The present measurements were done with a 30 mCi source of 57Co in a 6 tzm thick metallic rhodium matrix (Amersham, Code No. CTD.56), having a narrow line width ( F S= 0.115 mm/s). A Halder Electron-
ics M6ssbauer spectrometer, combined with a Canberra S100 PC-based spectra acquisition system, was used. A sine-waveform drive was used and the data folded to provide a constant background. A K r - C O 2 proportional counter was used as the detector of 14.4 keV v-rays in transmission measurements (TMS). Conversion-electron M6ssbauer spectroscopy (CEMS) measurements, sensitive to a depth of about 0.3 ~m, were performed in backscatter geometry with a He-flow proportional counter at atmospheric pressure. Typical count rates were 5000 c / s in TMS measurements and 200 c / s in CEMS measurements. The spectra were evaluated by least-squares fitting the quadrupole doublets with the Lorentzian-shape lines. Two quadrupole doublets, both with an isomer shift IS = - 0 . 3 2 m m / s , and with quadrupole splitting QS = 0.72 m m / s and 0.90 m m / s , have been ascribed to t-Zr2Fe and Zr3Fe, respectively, in agreement with the reference data for the intermetallics [6,7]. Henceforth, t-Zr2Fe will simply be referred to as Zr2Fe.
3. R e s u l t s a n d d i s c u s s i o n
3.1. As-grown alloy
Examples of the M6ssbauer spectra are shown in Figs. l a and lb. The TMS spectrum (a) shows a slightly asymmetric quadrupole doublet of narrow absorption lines ( W = 0.26 _+ 0.01 mm/s), with large negative isomer shift (IS = - 0 . 3 2 _+ 0.01 m m / s ) and quadrupole splitting (QS = 0.72 _+ 0.01 mm/s), in accord with the reference data for the Zr2Fe phase [6,7]. Since the CEMS spectrum (b) of the same specimen represents a 0.3 ~m-thick surface region, and a correspondingly small amount of Fe, it yields a small resonant effect (0.3%). The remarkable asymmetry of the CEMS spectrum, with the line on the right being about 1.8 times stronger than on the left, is ascribed to preferential alignment of the ZrzFe crystallites in the surface plane of the specimen, in agreement with microscopic observations [1]. The values of IS and QS of this spectrum also match well with the values characteristic of Zr2Fe. In addition, TMS and CEMS spectra of other as-grown specimens, both 57Fe enriched basal-cut and prism-cut, as well as non-enriched ones, indicate only the presence of Zr2Fe (c.f. Table 2). The lack in all spectra of any measurable spectral component attributable to Fe in solid solution, in both
Table 1 Chemical analysis (wppm) of crystal-bar Zr (from Teledyne Wah Chang, Albany, Oregon) used for production of the Zr-0.065 57Fe alloy O C N Fe AI Ti Hf Cr Cu Ni W U H AI B Cd Co Mn Si 55
150
6
86
< 35
40
70
< 50
< 10
< 35
< 25
< 1.0
5
35
0.2
0.2
10
< 25
< 40
J.A. Sawicki et al. / Journal of Nuclear Materials 218 (1995) 161-165 as-grown a n d a n n e a l e d specimens, is consistent with the very low solid solubility of Fe in a - Z r ( ~ 180 p p m at 1070 K, falling exponentially to ~ 1 p p m at 770 K [4]).
3.2. Annealing effects In Figs. l c a n d l d the T M S s p e c t r u m (c) a n d C E M S s p e c t r u m (d), t a k e n after p r o l o n g e d t~-phase annealing, show a q u a d r u p o l e d o u b l e t with t h e p a r a m e t e r s (IS = - 0 . 3 2 + 0.01 m m / s a n d QS = 0.90 + 0.01 m m / s ) of the Z r 3 F e p h a s e [6,7]. E q u a l a m p l i t u d e s of b o t h lines indicate n o p r e f e r r e d o r i e n t a t i o n of the crystallites, consistent with e l e c t r o n microscopy observations [1]. T h e m u c h larger total a r e a of s p e c t r u m (d) c o m p a r e d to s p e c t r u m (b), t a k e n prior to annealing, indicates p r o m i n e n t , h e a t - i n d u c e d surface segregation. T h e results of an e x p e r i m e n t to d e t e r m i n e the surf a c e / b u l k n a t u r e of the Z r a F e / Z r 2 F e intermetallics
163
are given in T a b l e 2. In this e x p e r i m e n t a 2.5 m m thick s p e c i m e n of unspecified o r i e n t a t i o n was v a c u u m ann e a l e d for 8.5 h at 923 K a n d t h e n sectioned into t h r e e s e p a r a t e slices, preserving the original s p e c i m e n surfaces o n t h e two o u t e r sections. O n e of the o u t e r specimens, S1, a n d the i n n e r one, $2, were polished o n the freshly-cut surfaces to give s p e c i m e n s ( ~ 100 ~ m thick) suitable for TMS. R e f e r e n c e to T a b l e 2 shows t h a t for S1, Fe on t h e original surface is mainly in the form of Z r 3 F e ( C E M S spectrum), with a m i n o r Z r z F e c o m p o n e n t , with t h e reverse characteristics for the bulk Fe (TMS spectrum): t h e T M S data for $2 (100% Z r z F e ) indicate t h a t the m i n o r Z r 3 F e e l e m e n t of the T M S s p e c t r u m of S1 is a surface c o m p o n e n t . E n e r g y dispersive X-ray analyses ( c o m b i n e d with s c a n n i n g e l e c t r o n microscopy) of the surface precipitates of S1 were consistent with Zr3Fe. T h e kinetics of iron r e d i s t r i b u t i o n were studied as a function of a n n e a l i n g time a n d s p e c i m e n o r i e n t a t i o n at
Table 2 Parameters of the M6ssbauer spectra of Zr-0.065 at% Fe crystals: IS is the isomer shift with respect to 57Fe in metallic iron a-Fe, QS is the electric quadrupole splitting, W is the line width, A 2 is the relative area of the ZraFe component and A is the total spectral area. Last digit(s) errors are given in parentheses. Values given without errors were fixed during the fit Treatment (See also
Zr z Fe particles phase 1
text)
IS1 (mm/s)
Zr 3Fe particles phase 2
W (mm/s)
A2 (%)
A (arb.u.)
QS1 (ram/s)
IS 2 (mm/s)
QSz (mm/s)
0.72(1) 0.73(1) 0.71(1) 0.71 0.71(1) -
- 0.32 -0.32(1) -0.32(1) -0.33(1) -0.32(9)
0.90 0.90 0.90(1) 0.89(1) 0.90(1)
0.26(1) 0.30(3) 0.27(1) 0.24(1) 0.27 0.25(1) 0.29(2) 0.29(1)
0.71(1) 0.71(1) 0.71(1) 0.71(1) 0.71(1)
- 0.32 -0.32 -0.32 -0.32 - 0.32
0.90 0.90 0.90 0.90 0.90
0.27(1) 0.25(1) 0.27(1) 0.25(1) 0.26(1)
0.8(4.9) 2.4(4.8) 2.0(5.0) 15.9(3.6) 32.8(3.9)
1.3 1.9 2.1 1.5 1.2
0.72(1) 0.70(1) 0.69(1) 0.69(1) 0.70(1)
-0.32 -0.32 - 0.32 -0.32 -0.32
0.90 0.90 0.90 0.90 0.90
0.26(1) 0.26(1) 0.25(1) 0.28(1) 0.26(1)
0.6(6.1) 6.4(4.3) 15.6(4.6) 24.1(4.2) 26.9(4.1)
1.8 2.0 1.9 2.1 1.1
0.71
- 0.31
0.90
0.28(3)
56.4(8.6)
0.9
Crystals cut in arbitrary orientation as-grown as-grown a S1 S1 a $2 973 K/24 h 923 K/84 h " 873 K/144 h a,b
-0.32(1) -0.31(1) - 0.31(1) -0.29(1) -0.31(1) _
-
-
16.9(4,0) 85.2(2.6) 100 100 100
1.4 0.1 0.8 1.5 1.9 3.8 0.2 1.7
Crystal cut in basal plane as-grown 795 K/1 h /5h /21 h /85 h
- 0.31(1) -0.31(1) -0.32(1) -0.32(1) - 0.32(1)
Crystal cut in prism plane as-grown 795 K / 1 h /5 h /21 h /85 h
-0.32(1) -0.32(1) - 0.32(1) -0.32(1) -0.32(1)
Surface of the single-crystal rod as-grown a
- 0.32(2)
a CEMS spectra. b Crystal has been grown using Fe with natural isotopic composition.
164
J.A. Sawicki et al. /Journal of Nuclear Materials 218 (1995) 161-165
795 K - see Fig. 2 and Table 2. Fig. 2 shows only M6ssbauer spectra obtained for the basal-cut crystal, but a similar set of spectra was obtained for the prismcut crystal. All spectra could be consistently fitted by assuming the superposition of two quadrupole doublets corresponding to Zr2Fe and Zr3Fe particles. In the final fits, represented in Table 2, the magnitudes of IS and QS of the Zr2Fe doublet and the relative areas of both doublets were free parameters, the magnitudes of IS and QS of the Zr3Fe doublet were fixed at their best average values; the absorption widths of both doublets were equal to each other. The results of the kinetic measurements (Fig. 3) show reasonable accord with a t °s dependence, suggesting a diffusion-limited process. A standard segrega-
o ~
S'f / AS-fiROWN
2
V
0
t,
o
2
o 5h
2
~-i
-2 0.2
b
i
I
i
0
1
2
VELOCITY (ram/s}
0.1
o
i
-1
Fig. 2. M6ssbauer transmission spectra of basal plane surface discs of Zr-0.065 at% -STFe following successive annealing periods at 795 K. The solid lines represent the best fits for a superposition of Zr2Fe and Zr3Fe components.
~-~
I.-
O' t i o n / d i f f u s i o n analysis has been applied, see, for example, Ref. [8]. The equation is ( C s - C b ) / C b = ( 2 / d ) ( D t / a r ) °5,
I -2
1 -I
i 0
~ I
I 2
VELOCITY (mm/s)
Fig. I. (a) M6ssbauer transmission spectrum and (b) conversion electron M6ssbauer spectrum of 57Fe in a Zr-0.065 at% S7Fe, as-grown, single crystal: (c) and (d), as for (a) and (b), respectively, except after prolonged high temperature vacuum annealing. The spectra of the as-grown and fully annealed specimen are associated with Zr2Fe and Zr3Fe, respectively, see also text.
(1)
where C s and C b are the surface and bulk concentrations of Fe, respectively, d is the thickness of the surface layer, D is the appropriate diffusion coefficient and t, the annealing time. The analysis is appropriate to thin surface films and an initially homogeneous composition. It has been assumed that the C u is equal to the terminal solid solubility at 795 K, 2.0 × 10 6 at.fn. [4], and that it is independent of time; values of C S were obtained from Fig. 3 and a value of one micron was used for d; S E M / E D X examinations of Zr3Fe precipitates indicate that their thicknesses are generally of the order of, or less than, 1 ~m. Despite the non-ideality of the experimental conditions, inhomogeneous bulk and surface Fe distributions, the Eq. (1) analysis gives D values that are in good overall agreement with experimental results for
J.A. Sawicki et al. /Journal of Nuclear Materials 218 (1995) 161-165 I
I
I
I
I
sion from the bulk are influencing the results, for example, surface diffusion, nucleation and growth.
1,0
4. C o n c l u d i n g
30
'°
0
I
I
I
I
3o ~
remarks
It has been shown, in accordance with the equilibrium phase diagram, that the Zr2Fe phase particles in as-grown, low-iron zirconium single crystals are metastable. Annealing in a relatively high temperature range, but below the eutectoid, causes decomposition of Zr2Fe and subsequent formation, at the specimen surfaces, of the stable Zr3Fe phase. The rate of formation of Zr3Fe shows fair accord with a bulk diffusion limited process. The results of this work indicate a strong barrier to the nucleation of Zr3Fe in the a-Zr matrix.
CRYSTAL
20
165
Acknowledgements
20
100I
CRYSTAL
I
1
I
I
I
I
0
20
40
60
80
100
120
ANNEALING TIME AT 795K (hours)
Fig. 3. Comparison of the fraction of Fe in the form of Zr3Fe in Zr-0.065 at% 57Fe on both basal and prism planes. The data are from M6ssbauer transmission spectra as a function of annealing time at 795 K.
59Fe tracer diffusion in dilute, polycrystalline Zr(Fe) alloys [9]. For example, after 20 h, where the kinetic curve is better defined, see Fig. 3, the D values from Eq. (1) are about 3 × 10 -14 and 8 × 10 -14 m2/s, for the basal and prism plane specimens, respectively; the corresponding value (795 K) from Ref. [9] is 2.5 × 10-14 m2/s. It is noted, however, that the observation of initially faster ZraFe formation on the prism plane specimen (c.f. Fig. 3) is not in accord with the anisotropy of diffusion of Fe in nominally pure single crystal ct-Zr [9]: anisotropy data are not available for Zr(Fe) alloys. This might suggest that factors other than Fe out-diffu-
The authors acknowledge valuable discussions with Drs. M. Griffiths and O.T. Woo. Thanks are accorded Mr. J.F. Watters, who assisted with the growth of the single crystals and who did the S E M / E D X analyses, and Mr. W. Edwards and Mr. V. Corriveau for performing the neutron activation analyses. This work was supported by the CANDU Owners Group, COG working party 32.
References
[1] H. Zou, G.M. Hood, J.A. Roy and R.J. Schultz, Metall. Mater. Trans. 25A (1994) 1359. [2] R.G. Fleck, J.E. Elder, A.R. Causey and R.A. Holt, Proc. 9th Int. Symp. on Zirconium in the Nuclear industry, Baltimore MD, June 1993, in press. [3] G.M. Hood, Def. Diffusion Forum 95-98 (1993) 755. [4] H. Zou, G.M. Hood, J.A. Roy, R.J. Schultz and J,A. Jackman, J. Nucl. Mater. 210 (1994) 239. [5] G.M. Hood and R.J. Schultz, Def. Diffusion Forum 66-69 (1989) 371. [6] F. Aubertin, U. Gonser, S.J. Campbell and H.G. Wagner, Z. Metallk. 76 (1985) 237. [7] B.M. Mauer, J.M. Friedt and J.P. Sanchez, J. Phys. F15 (1985) 1449. [8] S. Hofmann and J. Erlewein, Scripta Metall. 10 (1976) 857. [9] H. Nakajima, G.M. Hood and R.J. Schultz, Philos. Mag. B58 (1988) 319.
Journal of Nuclear Materials 218 (1995) 161-165
journalof nuclear materials
A M6ssbauer study of single-crystal Zr-0.065 at% 57Fe J.A. Sawicki, G.M. Hood, H. Zou, R.J. Schultz Reactor Materials Division, Atomic Energy of Canada Ltd., Chalk River Laboratories, Chalk River, Ontario, Canada KOJIJO Received 15 June 1994; accepted 6 September 1994
Abstract
Single crystals of Zr-0.065 at% Fe (95% 57Fe), from a common parent, have been studied by M6ssbauer spectroscopy, using both transmission and conversion-electron modes. In the "as grown" state the Fe is present as the meta-stable phase, t-ZrzFe. Vacuum annealing, in the range 795-973 K, results in gradual decomposition of t-Zr2Fe and the diffusion of Fe from the bulk to the surface, where it forms the stable Zr3Fe phase. The kinetics of Zr3Fe formation show fair accord with the known diffusion properties of Fe in a-Zr. Some results for other dilute Zr(Fe) single crystals are reported.
1. Introduction
The behaviour of Fe in Zr and in Zircaloy-2 and Zr-2.5% Nb is of particular interest. In the commercial alloys the state and distribution of the element (present at ~ 0.1 at%, unless specified otherwise, concentrations herein are in atomic units) are believed to have an influence on corrosion properties, hydrogen ingress, irradiation damage and possibly, mechanical properties, see references in Ref. [1]; in addition, there is now evidence of a correlation between the irradiation deformation of Zr-2.5Nb pressure tubes and the level of Fe in the alloy [2]. At a more elementary level it is apparent that trace Fe controls both self- and slow substitutional diffusion in a-Zr [3]. In terms of its physico/chemical behaviour in a-Zr, the small atomic radius of Fe leads to its low solubility and its ability to occupy sites in the interstitial sublattice of Zr: the latter property results in a very high diffusivity [3]. These behavioural aspects have made it difficult to charaeterise the equilibrium state of Fe in a-Zr [1,4]. In previous work [5] it has been shown that the disposition of Fe in binary Zr-0.1 Fe alloys is remarkably sensitive to annealing treatments. Single-crystal alloy specimens were found to exhibit rapid and massive transfer of Fe from internal precipitates to a surface-associated phase when the samples were annealed below the eutectoid temperature. Recent and Elsevier Science B.V. SSDI 0022-31 t5(94)00384-X
current work on dilute Zr(Fe) single crystals have shown that at modestly elevated temperatures, but below the eutectoid ( ~ 1070 K), an equilibrium is established between the bulk solid solution and an associated surface precipitate phase, Zr3Fe [1,4]. These latter studies have utilised secondary ion mass spectrometry, transmission electron microscopy, electron micro-probe analysis and micro-electron diffraction to determine properties of Fe in Zr. The present study utilises M6ssbauer spectrometry to characterise the nature of the internal and surface precipitate phases of Fe in single-crystal Zr-0.065 at% Fe (enriched to 95% in 57Fe) and to obtain information on the kinetics of the bulk-to-surface transfer of Fe. M6ssbauer spectrometry has the advantage (over microscopical methods) of yielding the averaged characteristics of the states of Fe over a relatively large sampled volume.
2. Experimental
2.1. Alloy fabrication /crystal growth /specimen preparation The starting alloy, Zr-0.1% 57Fe, was prepared from crystal-bar Zr and Fe enriched to 95% in 57Fe. Table 1 gives an analysis of the Zr. The alloy billet,
162
J.A. Sawicki et al. /Journal of Nuclear Materials 218 (1995) 161-165
made by arc-melting under Ar, was reduced, by cold extrusion, to 30 cm long, 6.0 mm diameter rods. Analyses of the rods by inductively-coupled plasma emission spectroscopy (ICPES) showed a homogeneous Fe concentration (0.1%). Single crystals were grown by an electron beam/float-zone-melting technique. The zone-melted rod contained some single crystal pieces, up to 3 cm long. Analyses (ICPES) of the rod in the as-zone-melted-state showed an Fe content, along the zone-melted length, of 0.065 _+0.008% Fe. Loss of Fe is attributed to evaporation during zone-melting. The surface preparation of the specimens, cut from the parent rod by a slow-speed diamond saw and generally 1-2 mm thick, was done by grinding through metallographic papers down to 600 grit, followed by 6, then 3, ~m diamond grinding on plate glass. The final steps were chemical polishing in a solution of H 2 0 : H N O 3 : H F , in the ratio 50:45:5, by volume, followed by electropolishing in 5% perchloric acid in ethanol at - 40°C. Thin (100-250 Ixm), parallel-sided specimens for transmission M6ssbauer spectrometry studies were cut from the rod. Two such specimens, 250 Ixm thick, were made especially for kinetic studies: one had basal (0001) and the other prism (1010) surfaces. Specimen annealing was done under vacuum at residual pressures of 10 6 Pa. The specimens were placed on polished tungsten in Ta containers. 2.2. M6ssbauer spectrometry
M6ssbauer spectroscopy has been used extensively in studies of Z r - F e intermetallics, see, for example, Refs. [6,7]. On the Zr-rich side of the phase diagram three crystalline phases occur: orthorhombic Zr3Fe, tetragonal body-centered t-Zr2Fe and cubic diamond c-Zr2Fe [6]. The M6ssbauer spectra of these phases are well known and can be clearly distinguished by their characteristically different quadrupole splittings (QS). These compounds display only one Fe lattice site, with practically axial symmetry, asymmetry parameter r / = 0 in c-Zr2Fe and t-ZrzFe, and r / = 0.6 in Zr3Fe [6]. However, in Zr3Fe and c-Zr2Fe the Fe atom has a positive QS whereas QS is negative in t-Zr2Fe. The present measurements were done with a 30 mCi source of 57Co in a 6 tzm thick metallic rhodium matrix (Amersham, Code No. CTD.56), having a narrow line width ( F S= 0.115 mm/s). A Halder Electron-
ics M6ssbauer spectrometer, combined with a Canberra S100 PC-based spectra acquisition system, was used. A sine-waveform drive was used and the data folded to provide a constant background. A K r - C O 2 proportional counter was used as the detector of 14.4 keV v-rays in transmission measurements (TMS). Conversion-electron M6ssbauer spectroscopy (CEMS) measurements, sensitive to a depth of about 0.3 ~m, were performed in backscatter geometry with a He-flow proportional counter at atmospheric pressure. Typical count rates were 5000 c / s in TMS measurements and 200 c / s in CEMS measurements. The spectra were evaluated by least-squares fitting the quadrupole doublets with the Lorentzian-shape lines. Two quadrupole doublets, both with an isomer shift IS = - 0 . 3 2 m m / s , and with quadrupole splitting QS = 0.72 m m / s and 0.90 m m / s , have been ascribed to t-Zr2Fe and Zr3Fe, respectively, in agreement with the reference data for the intermetallics [6,7]. Henceforth, t-Zr2Fe will simply be referred to as Zr2Fe.
3. R e s u l t s a n d d i s c u s s i o n
3.1. As-grown alloy
Examples of the M6ssbauer spectra are shown in Figs. l a and lb. The TMS spectrum (a) shows a slightly asymmetric quadrupole doublet of narrow absorption lines ( W = 0.26 _+ 0.01 mm/s), with large negative isomer shift (IS = - 0 . 3 2 _+ 0.01 m m / s ) and quadrupole splitting (QS = 0.72 _+ 0.01 mm/s), in accord with the reference data for the Zr2Fe phase [6,7]. Since the CEMS spectrum (b) of the same specimen represents a 0.3 ~m-thick surface region, and a correspondingly small amount of Fe, it yields a small resonant effect (0.3%). The remarkable asymmetry of the CEMS spectrum, with the line on the right being about 1.8 times stronger than on the left, is ascribed to preferential alignment of the ZrzFe crystallites in the surface plane of the specimen, in agreement with microscopic observations [1]. The values of IS and QS of this spectrum also match well with the values characteristic of Zr2Fe. In addition, TMS and CEMS spectra of other as-grown specimens, both 57Fe enriched basal-cut and prism-cut, as well as non-enriched ones, indicate only the presence of Zr2Fe (c.f. Table 2). The lack in all spectra of any measurable spectral component attributable to Fe in solid solution, in both
Table 1 Chemical analysis (wppm) of crystal-bar Zr (from Teledyne Wah Chang, Albany, Oregon) used for production of the Zr-0.065 57Fe alloy O C N Fe AI Ti Hf Cr Cu Ni W U H AI B Cd Co Mn Si 55
150
6
86
< 35
40
70
< 50
< 10
< 35
< 25
< 1.0
5
35
0.2
0.2
10
< 25
< 40
J.A. Sawicki et al. / Journal of Nuclear Materials 218 (1995) 161-165 as-grown a n d a n n e a l e d specimens, is consistent with the very low solid solubility of Fe in a - Z r ( ~ 180 p p m at 1070 K, falling exponentially to ~ 1 p p m at 770 K [4]).
3.2. Annealing effects In Figs. l c a n d l d the T M S s p e c t r u m (c) a n d C E M S s p e c t r u m (d), t a k e n after p r o l o n g e d t~-phase annealing, show a q u a d r u p o l e d o u b l e t with t h e p a r a m e t e r s (IS = - 0 . 3 2 + 0.01 m m / s a n d QS = 0.90 + 0.01 m m / s ) of the Z r 3 F e p h a s e [6,7]. E q u a l a m p l i t u d e s of b o t h lines indicate n o p r e f e r r e d o r i e n t a t i o n of the crystallites, consistent with e l e c t r o n microscopy observations [1]. T h e m u c h larger total a r e a of s p e c t r u m (d) c o m p a r e d to s p e c t r u m (b), t a k e n prior to annealing, indicates p r o m i n e n t , h e a t - i n d u c e d surface segregation. T h e results of an e x p e r i m e n t to d e t e r m i n e the surf a c e / b u l k n a t u r e of the Z r a F e / Z r 2 F e intermetallics
163
are given in T a b l e 2. In this e x p e r i m e n t a 2.5 m m thick s p e c i m e n of unspecified o r i e n t a t i o n was v a c u u m ann e a l e d for 8.5 h at 923 K a n d t h e n sectioned into t h r e e s e p a r a t e slices, preserving the original s p e c i m e n surfaces o n t h e two o u t e r sections. O n e of the o u t e r specimens, S1, a n d the i n n e r one, $2, were polished o n the freshly-cut surfaces to give s p e c i m e n s ( ~ 100 ~ m thick) suitable for TMS. R e f e r e n c e to T a b l e 2 shows t h a t for S1, Fe on t h e original surface is mainly in the form of Z r 3 F e ( C E M S spectrum), with a m i n o r Z r z F e c o m p o n e n t , with t h e reverse characteristics for the bulk Fe (TMS spectrum): t h e T M S data for $2 (100% Z r z F e ) indicate t h a t the m i n o r Z r 3 F e e l e m e n t of the T M S s p e c t r u m of S1 is a surface c o m p o n e n t . E n e r g y dispersive X-ray analyses ( c o m b i n e d with s c a n n i n g e l e c t r o n microscopy) of the surface precipitates of S1 were consistent with Zr3Fe. T h e kinetics of iron r e d i s t r i b u t i o n were studied as a function of a n n e a l i n g time a n d s p e c i m e n o r i e n t a t i o n at
Table 2 Parameters of the M6ssbauer spectra of Zr-0.065 at% Fe crystals: IS is the isomer shift with respect to 57Fe in metallic iron a-Fe, QS is the electric quadrupole splitting, W is the line width, A 2 is the relative area of the ZraFe component and A is the total spectral area. Last digit(s) errors are given in parentheses. Values given without errors were fixed during the fit Treatment (See also
Zr z Fe particles phase 1
text)
IS1 (mm/s)
Zr 3Fe particles phase 2
W (mm/s)
A2 (%)
A (arb.u.)
QS1 (ram/s)
IS 2 (mm/s)
QSz (mm/s)
0.72(1) 0.73(1) 0.71(1) 0.71 0.71(1) -
- 0.32 -0.32(1) -0.32(1) -0.33(1) -0.32(9)
0.90 0.90 0.90(1) 0.89(1) 0.90(1)
0.26(1) 0.30(3) 0.27(1) 0.24(1) 0.27 0.25(1) 0.29(2) 0.29(1)
0.71(1) 0.71(1) 0.71(1) 0.71(1) 0.71(1)
- 0.32 -0.32 -0.32 -0.32 - 0.32
0.90 0.90 0.90 0.90 0.90
0.27(1) 0.25(1) 0.27(1) 0.25(1) 0.26(1)
0.8(4.9) 2.4(4.8) 2.0(5.0) 15.9(3.6) 32.8(3.9)
1.3 1.9 2.1 1.5 1.2
0.72(1) 0.70(1) 0.69(1) 0.69(1) 0.70(1)
-0.32 -0.32 - 0.32 -0.32 -0.32
0.90 0.90 0.90 0.90 0.90
0.26(1) 0.26(1) 0.25(1) 0.28(1) 0.26(1)
0.6(6.1) 6.4(4.3) 15.6(4.6) 24.1(4.2) 26.9(4.1)
1.8 2.0 1.9 2.1 1.1
0.71
- 0.31
0.90
0.28(3)
56.4(8.6)
0.9
Crystals cut in arbitrary orientation as-grown as-grown a S1 S1 a $2 973 K/24 h 923 K/84 h " 873 K/144 h a,b
-0.32(1) -0.31(1) - 0.31(1) -0.29(1) -0.31(1) _
-
-
16.9(4,0) 85.2(2.6) 100 100 100
1.4 0.1 0.8 1.5 1.9 3.8 0.2 1.7
Crystal cut in basal plane as-grown 795 K/1 h /5h /21 h /85 h
- 0.31(1) -0.31(1) -0.32(1) -0.32(1) - 0.32(1)
Crystal cut in prism plane as-grown 795 K / 1 h /5 h /21 h /85 h
-0.32(1) -0.32(1) - 0.32(1) -0.32(1) -0.32(1)
Surface of the single-crystal rod as-grown a
- 0.32(2)
a CEMS spectra. b Crystal has been grown using Fe with natural isotopic composition.
164
J.A. Sawicki et al. /Journal of Nuclear Materials 218 (1995) 161-165
795 K - see Fig. 2 and Table 2. Fig. 2 shows only M6ssbauer spectra obtained for the basal-cut crystal, but a similar set of spectra was obtained for the prismcut crystal. All spectra could be consistently fitted by assuming the superposition of two quadrupole doublets corresponding to Zr2Fe and Zr3Fe particles. In the final fits, represented in Table 2, the magnitudes of IS and QS of the Zr2Fe doublet and the relative areas of both doublets were free parameters, the magnitudes of IS and QS of the Zr3Fe doublet were fixed at their best average values; the absorption widths of both doublets were equal to each other. The results of the kinetic measurements (Fig. 3) show reasonable accord with a t °s dependence, suggesting a diffusion-limited process. A standard segrega-
o ~
S'f / AS-fiROWN
2
V
0
t,
o
2
o 5h
2
~-i
-2 0.2
b
i
I
i
0
1
2
VELOCITY (ram/s}
0.1
o
i
-1
Fig. 2. M6ssbauer transmission spectra of basal plane surface discs of Zr-0.065 at% -STFe following successive annealing periods at 795 K. The solid lines represent the best fits for a superposition of Zr2Fe and Zr3Fe components.
~-~
I.-
O' t i o n / d i f f u s i o n analysis has been applied, see, for example, Ref. [8]. The equation is ( C s - C b ) / C b = ( 2 / d ) ( D t / a r ) °5,
I -2
1 -I
i 0
~ I
I 2
VELOCITY (mm/s)
Fig. I. (a) M6ssbauer transmission spectrum and (b) conversion electron M6ssbauer spectrum of 57Fe in a Zr-0.065 at% S7Fe, as-grown, single crystal: (c) and (d), as for (a) and (b), respectively, except after prolonged high temperature vacuum annealing. The spectra of the as-grown and fully annealed specimen are associated with Zr2Fe and Zr3Fe, respectively, see also text.
(1)
where C s and C b are the surface and bulk concentrations of Fe, respectively, d is the thickness of the surface layer, D is the appropriate diffusion coefficient and t, the annealing time. The analysis is appropriate to thin surface films and an initially homogeneous composition. It has been assumed that the C u is equal to the terminal solid solubility at 795 K, 2.0 × 10 6 at.fn. [4], and that it is independent of time; values of C S were obtained from Fig. 3 and a value of one micron was used for d; S E M / E D X examinations of Zr3Fe precipitates indicate that their thicknesses are generally of the order of, or less than, 1 ~m. Despite the non-ideality of the experimental conditions, inhomogeneous bulk and surface Fe distributions, the Eq. (1) analysis gives D values that are in good overall agreement with experimental results for
J.A. Sawicki et al. /Journal of Nuclear Materials 218 (1995) 161-165 I
I
I
I
I
sion from the bulk are influencing the results, for example, surface diffusion, nucleation and growth.
1,0
4. C o n c l u d i n g
30
'°
0
I
I
I
I
3o ~
remarks
It has been shown, in accordance with the equilibrium phase diagram, that the Zr2Fe phase particles in as-grown, low-iron zirconium single crystals are metastable. Annealing in a relatively high temperature range, but below the eutectoid, causes decomposition of Zr2Fe and subsequent formation, at the specimen surfaces, of the stable Zr3Fe phase. The rate of formation of Zr3Fe shows fair accord with a bulk diffusion limited process. The results of this work indicate a strong barrier to the nucleation of Zr3Fe in the a-Zr matrix.
CRYSTAL
20
165
Acknowledgements
20
100I
CRYSTAL
I
1
I
I
I
I
0
20
40
60
80
100
120
ANNEALING TIME AT 795K (hours)
Fig. 3. Comparison of the fraction of Fe in the form of Zr3Fe in Zr-0.065 at% 57Fe on both basal and prism planes. The data are from M6ssbauer transmission spectra as a function of annealing time at 795 K.
59Fe tracer diffusion in dilute, polycrystalline Zr(Fe) alloys [9]. For example, after 20 h, where the kinetic curve is better defined, see Fig. 3, the D values from Eq. (1) are about 3 × 10 -14 and 8 × 10 -14 m2/s, for the basal and prism plane specimens, respectively; the corresponding value (795 K) from Ref. [9] is 2.5 × 10-14 m2/s. It is noted, however, that the observation of initially faster ZraFe formation on the prism plane specimen (c.f. Fig. 3) is not in accord with the anisotropy of diffusion of Fe in nominally pure single crystal ct-Zr [9]: anisotropy data are not available for Zr(Fe) alloys. This might suggest that factors other than Fe out-diffu-
The authors acknowledge valuable discussions with Drs. M. Griffiths and O.T. Woo. Thanks are accorded Mr. J.F. Watters, who assisted with the growth of the single crystals and who did the S E M / E D X analyses, and Mr. W. Edwards and Mr. V. Corriveau for performing the neutron activation analyses. This work was supported by the CANDU Owners Group, COG working party 32.
References
[1] H. Zou, G.M. Hood, J.A. Roy and R.J. Schultz, Metall. Mater. Trans. 25A (1994) 1359. [2] R.G. Fleck, J.E. Elder, A.R. Causey and R.A. Holt, Proc. 9th Int. Symp. on Zirconium in the Nuclear industry, Baltimore MD, June 1993, in press. [3] G.M. Hood, Def. Diffusion Forum 95-98 (1993) 755. [4] H. Zou, G.M. Hood, J.A. Roy, R.J. Schultz and J,A. Jackman, J. Nucl. Mater. 210 (1994) 239. [5] G.M. Hood and R.J. Schultz, Def. Diffusion Forum 66-69 (1989) 371. [6] F. Aubertin, U. Gonser, S.J. Campbell and H.G. Wagner, Z. Metallk. 76 (1985) 237. [7] B.M. Mauer, J.M. Friedt and J.P. Sanchez, J. Phys. F15 (1985) 1449. [8] S. Hofmann and J. Erlewein, Scripta Metall. 10 (1976) 857. [9] H. Nakajima, G.M. Hood and R.J. Schultz, Philos. Mag. B58 (1988) 319.