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Precipitate behavior in zirconium-based alloys in BWRs
MJgf
Journal of Nuclear Materials 205 (1993) 242-250 North-Holland
mab!riais
Precipitate behavior in zirconium-based alloys in BWRs R.M..Kruger
and R.B. Adamson
GeneralElectricCompany,Vallecitos NuclearCenter,Pleasanton,CA 94566, USA
The metallic elements Cr, Fe, Ni, Nb, and MO have low solubilities in zirconium, so that zirconium-based alloys made with these elements contain numerous small precipitates. Some of these undergo substantial changes in composition, crystalline structure, and size during in-reactor exposure. Data on these phenomenon have been obtained using the scanning transmission electron microscope (STEM) to study precipitates before and after reactor exposure. Results on the following alloys are reported here: Zircaloy-2 (Zr-1.SSn-O.15Fe-O.1OCr-O.O5Ni), Zry-02Nb (Zircaloy-2 with 0.2Nb), NoCr (ZrlSSn-O.l7Fe-O.l7Ni), NSF-2 (Zr-lNb-lSn-0.2Fe), and XLL (Zr-1.5Sn-0.3Nb-0.3Mo). The dissolution of Fe from various precipitates is apparent from direct compositional measurements. The dissolution of Cr can be inferred in Zry-0.2Nb from Cr : Nb gradients in amorphous Cr-Fe-Zr-Nb precipitates and from the formation of intergranular, Cr,Fe-rich precipitates during postirradiation annealing. Two types of phase change have been seen. First, crystalline phases can become amorphous, as with hexagonal Laves Zt(Fe,Cr), in Zircaloy-2 and cubic Fe(Zr,Nb), in NSF-2. In the second type, long-range order is eliminated, as with Laves (Mo,Nb)zZr precipitates in ?&L which become BCC. The precipitate size distributions can change in reactor. The greatest changes were measured in small, Cr,Fe-rich precipitates.
1. introduction
A number of metallic elements have been used in commercial alloys of zirconium for nuclear reactor applications, including Cr, Fe, Ni, Sn, Nb, and MO [l-5]. All of these elements can precipitate, subject to their concentrations, the simultaneous presence of other metallic elements, and the alloy processing. In standard Zircaloys, it has been shown that the precipitates themselves can then change in reactor, due to radiation damage processes coupled with moderately high temperatures, on the order of 570 K [6-g]. The changes to the precipitates include compositions, size distributions, and crystal structures. In this paper, it will be shown that in other Zr-base alloys such changes are also common, so that they should be expected by modelers and designers concerned with explaining and improving material performance [8,10-141. 2. Experimental 2.1. Materials
The materials referred to in this paper are listed in table 1, along with their compositions. All were fabricated with several cold-work/ anneal cycles, and with a 0022-3115/93/$06.00
final anneal that induced complete recrystallization. All materials were in a BWR for one cycle, and irradiated at 560 K to fluences of (0.85-1.5) X 10z n/m2 (E > 1 MeV). 2.2. Scanning transmksion electron microscope (STEM) All materials were characterized with a JEOL lOOCX-STEM equipped with a Be-window Si(Li) X-ray detector for energy dispersive X-ray spectroscopy (EDS). To prepare specimens, material was ground to 0.15 mm thickness, 3 mm diameter discs were punched from this material, and thin foils were made from the discs using the conventional twin-jet electropolishing method. The polishing solution was 10% perchloric acid in 90% ethanol, cooled to 233 K. The information on precipitates that was obtained included their sizes, compositions, which were quantified via EDS, and crystal lattices via selected area diffraction patterns (SADPs) or convergent beam diffraction patterns (CBDPs). All materials were characterized before and after irradiation. 2.3. Postirradiationannealing Some postirradiation annealing was performed under a vacuum of 5 X 10v3 Pa for 1 h at 848 K for the
0 1993 - Elsevier Science Publishers B.V. Ah rights reserved
R.M. Kruger, RB. Adamson / Precipitatebehavior in zirconium-basedalloysin BWRY
243
Table 1 Compositions and fluences of the alloys (E > 1 MeV) Alloy
Composition (wt%)
Fluence (n/m’)
Zircaloy-2 zry-0.2Nb NoCr XLL NSF-2
Zr-1.5Sn-O.1SFe-O.1OCr-O.O5Ni Zr-1.5Sn-0.18Fe-O.lOCCr-O.O5Ni-0.2Nb Zr-O.l7Fe-0.17Ni Zr-l.SSn-0.3Nb-0.3Mo-0.05Fe Zr-lNb-lSn-0.2Fe
0.85 x 10zs 1.5 x 1025 1.5 x lo= 1.5 x 1025 1.5 x 1025
alloys Zry-02Nb and NoCr and at 948 K for alloys XLL and NSF-2. The primary purpose was to determine if any new precipitates formed, which is an indirect indication that alloying elements dissolved during irradiation. The lower temperature anneal was chosen because it can cause precipitation of Cr, Fe, and Ni on grain boundaries in Zircaloy-2 [15], and the higher temperature anneal was chosen under the assumption that it would allow Nb and MO to diffuse fast enough for precipitation to occur.
3. Results
A summary of the precipitates and their changes due to irradiation is provided in table 2. 3.1. Zircaloy-2 Zircaloy-2 can be processed to have either large or small precipitates. In this instance, the precipitates in the unirradiated material were small, with a median
diameter of 0.036 urn (Fig. la). The precipitates were hexagonal Zr(Cr,Fe), (Laves MgZn, type, a = 0.51 nm and c = 0.82 nm) with xFc = 0.53 f 0.03 (xi+ = Fe/[Fe + Crl), and Zr,(Fe,Ni) (tetragonal &Al, type, with u,, = 0.66 nm, and cs = 0.55 nm> with y, = 0.54 f 0.03 ( yFe = Fe/[Fe + Nil). The scatter bands represent one standard deviation in the measurements. The precipitates in the irradiated material were altered. The Cr,Fe-rich precipitates all became completely amorphous, including precipitates as large as 0.08 pm in their smallest dimension (fig. lb). Their iron content decreased substantially, to xFc = 0.15 f 0.07. The range of xFe was 0.04 to 0.28. The Fe content of the Fe,Ni-rich precipitates was not significantly lower than in the unirradiated material, with These precipitates retained their YFe = 0.51 f 0.04. crystal structure. In other studies, it can be seen that Zr,(Fe,Ni) appears to dissolve [16], but no evidence of dissolution was recorded here. The dissolution of Fe has been implicated in the development of c-component dislocations at higher
Table 2 Summary of precipitates before and after exposure in BWR at 560 K to fluences of 0.85 x 10z n/m2 (E > 1 MeV) for Zircaloy-2 and 1.5 x loss n/m* (others) Alloy
Original precipitate
Final precipitate
Zircaloy-2
Hexagonal Zr(Cra.~,FeO,ss), Tetragonal Zr,(Fes,s,Ni,,,) Hexagonal (Zr,NbXCrO.~sFeO.,s), Tetragonal ZrZ(Fe,,s4NiaJ Orthorhombic Zrs(Fe,,,,Ni,,,,) Tetragonal Zr,(Fe,Ni) 0.27 5 Fe/(Fe + Ni) 5 0.51 Cubic MosZr, Nb (0.5 5 Nb : MO 5 0.67) and Fe (0.17 5 Fe : MO I 0.27) bee Nb, Nb/(Zr + Nb) 2 0.76 ZrsSi Cubic (Zr,Nb),Fe, Fe/(Cr + Fe + Nb) = 0.71 Hexagonal (Zr,Nb)Fes, Fe/(Cr + Fe + Nb) = 0.36
Amorphous, Fe/(Cr + Fe) = 0.15 Tetragonal Zr,(Fe,,s,Ni,,,) Amorphous, Fe/(Cr + Fe) = 0.26 Tetragonal Zr,(Fe,,,~Ni,,,) Orthorhombic Zr,(Fe,,s,Ni,,,) Tetragonal Zr,(Fe,Ni) Fe/(Fe + Ni) = 0.40 bee Zr-Nb-Mo, Nb (0.45 5 Nb : MO I 0.68) and Fe (0.00 I Fe : MO i; 0.03) bee Nb, Nb/(Zr + Nb) r 0.76 Zr,Si Amorphous
zry-0.2Nb NoCr
XLL NSF-2
Amorphous
244
RM. tiger,
R.B. Adamson
/ Precipitate behavior in zirconium-based alloys in BWRY
Fig. 1. Zircaloy-2 (a) before and (b) after irradiation. Amorphous precipitates in irradiated sample imaged with diffuse diffraction ring.
doses 1171,and so a search for c-dislocations
was made, dislocations were found in the other materials studied below. The precipitates became smaller with irradiation (fig. 2). The median diameter decreased from 0.038 to 0.023 pm, and the smallest precipitates were 0.015 and 0.005 pm in the unit-radiated and irradiated materials, respectively. but none were found. Similarly, no c-component
Precipitate size distribution8 in Zircaloy-2 tubbIg
40
30
20
3.2. Zry-0.2Nb 10
Before irradiation, Zry-0.2Nb contained hexagonal (Zr,NbXCr,.,sFeo.4s)2 precipitates with Nb : Fe as high as 0.10 on an atom basis. It also contained tetragonal Zr2(Fe0_52NiOA8) precipitates that contained no Nb. Both types of precipitates were intragranular, with identical crystalTstru&res as in the Zkcaloy-2. The microstructure
of this material
appears
similar
of Zircaloy-2 having similar sizes of precipitates,
to that
which
” n
-I
4.015 0.02 0.M 0.06 0.08
0.10
0.12
0.14
0.16
diameter (r.lm) Fig. 2. Precipitate size distributions for Zircaloy-2; (a) before irradiation, (b) after irradiation.
RM. ICyer, RB. Adamson / Precipitate behavior in zirconium-based alloys in BETs
in this case ranged between 0.035 and 0.52 km, with a median of 0.11 pm. The irradiated Zry-0.2Nb contained both Cr,Fe-rich and Fe,Ni-rich precipitates. The Cr,Fe-rich precipitates that were electron transparent were completely amorphous, just like their counterparts in irradiated Zircaloy-2. In addition, the Fe content of these precipitates was substantially lower than in the unirradiated material. Representing the iron content by xi+ = Fe/(Cr + Fe), the composition at the centers of the . . precipitates was xre = 0.261 f 0.074, which is much lower than the value in the unirradiated material (xFe = 0.45). The diffusion of iron out of the precipitates was corroborated by the presence of gradients; xFc varied from 0.41 at the center to 0.30 at the edge of one precipitate, and from 0.28 to 0.28 in another. The Cr,Fe-rich precipitates still contained Nb. The Nb : Cr concentrations (atom basis) of 8 precipitates varied between 0.069 and 0.147, with a median of 0.088. This result is similar to the unirradiated material, for which 0.049 < Nb : Cr II 0.115, with a median of 0.091. Thus, Nb is retained by the Cr,Fe-rich precipitates in approximately its original ratio to Cr. The Nb:Cr ratio was higher at the edges than at the centers. In one case, the Nb: Cr ratio varied from 0.069 (center) to 0.328 (edge), and in another, Nb : Cr varied from 0.108 to 0.196. The Fe,Ni-rich precipitates in Zry-0.2Nb were crystalline, and the crystal structure, tetragonal Zr,(Fe,Ni), was the same as before irradiation. The iron content at the center of the precipitates, represented by yFe = Fe/(Fe + Ni), was y, = 0.475 f 0.03, or slightly lower than in the unirradiated material (yFe = 0.52 f 0.02). This indication of a slight loss of iron was supported by the measurement of gradients in yFe in two separate precipitates. In each case, y, was 0.50 at the center, and 0.42 at the edge. Postirradiation annealing of Zry-0.2Nb at 848 K for 1 h yielded Cr,Fe-rich precipitates at the grain boundaries (xFe = 0.59 f 0.06). No Ni-rich intergranular precipitates were found. The amorphous precipitates were not rectystallixed by the postirradiation anneal, but their iron content increased (from 0.26 before the anneal) to xFe = 0.39 f 0.05. The Fe,Ni-rich precipitates also gained Fe during the post-irradiation anneal, as the median value of y, increased (from 0.475 before the anneal) to 0.55 f 0.02. 3.3. NoCr Before irradiation, the NoCr contained two types of nearly spherical, intragranular precipitates. Some were
245
orthorhombic Zr,(Fe,s,Ni,,,) (a, = 0.332 run, 6, = 1.099 nm, c,, = 0.881 nm), which is not usually present in manufactured Zircaloy-2. The others were tetragonal Zr,(Fe,Ni), with 0.27 I, y, < 0.51. These tetragonal precipitates are identical to the Fe,Ni-rich precipitates typically found in Zircaloy-2, although in Zircaloy-2, the range of yFc is much narrower (as in section 3.1). The range of precipitate diameters was 0.09-0.72 pm, with a median of 0.31 pm. Visually, the precipitates were not changed by irradiation. Twelve precipitates were examined, and 11 of these were orthorhombic Zr,(Fe,Ni) with yi+. in the range of 0.56 < y, < 0.62, and a mean value of y, = 0.59 f 0.02. The lone precipitate with a low value of y,, equal to 0.399, was tetragonal Zr,(Fe,Ni). Thus, both types of precipitates were still present following irradiation. The ‘diameters of the precipitates ranged between 0.2 and 0.8 pm, with a median of 0.35 km. Postirradiation annealing of NoCr did not yield Fe,Ni-rich precipitates at the grain boundaries, as had been anticipated. Gradients in y, were found, the largest difference being y, = 0.39 at the center and 0.52 at the edge of a precipitate. 3.4. XLL Before irradiation, XLL contained spherical, intragranular precipitates that contained MO, Nb, Fe, and Zr. Their diffraction patterns were consistent with the crystal lattice of cubic Mo,Zr(MgCu, type Laves phase, with u,, = 0.759 nm), and they contained substantial concentrations of Nb (0.5 5 Nb: MO 5 0.67) and Fe (0.17 I Fe : MO < 0.27). For convenience, these precipitates will be referred to as (Nb,Mo),Zr, although there are no data concerning the sublattice occupied by Nb. In an analysis of 88 precipitates, the median diameter was 0.046 pm, and the range was 0.014 to 0.14 km. Irradiation caused two major changes in these precipitates. First, the iron content decreased measurably (0 < Fe : MO 5 0.03), indicating that iron diffised into the matrix. Second, the crystal structure became bee, with a, = 0.345 nm. This is slightly larger than for the bee metals MO (a, = 0.315 nm) and Nb (a, = 0.320 nm), but smaller than p-Zr (a, = 0.357 nm). The appearance of the precipitates was not dramatically altered by irradiation, and in particular, it was difficult to determine if any were dissolving. The precipitate diameter increased slightly due to irradiation, including a median diameter of 0.052 pm and a range of 0.020 to 0.38 km. The 13% increase in median diameter may be due to experimental scatter,
246
R.M. Kruger, R.B. Adamson / Precipitate behavior in zirconium-based alloys in BWRs
Fig. 3, NSF-2; 6) string of bee Nb precipitates before irradiation, ibl string of bee Nb precipitates amo~bous precipitate after irradiation.
after i~adiation
and (c>
241
R M. Kruger, R B. Adatnson / Precipitate behavior in zirconium-based alloys in BU%s
but is relatively large. Postirrradiation annealing restored the precipitates to their original crystal lattice of Mo,Zr, restored the Fe, and reduced the median diameter from 0.052 to 0.049 urn. There was no evidence that Nb or MO dissolved into the matrix. The Nb : MO ratio was similar to its original value, with 0.45 < Nb : MO < 0.68, and it is unlikely that Nb and MO would diffuse away at identical rates. In addition, the minimum proportion of Zr in the spectra (for precipitates at the perforation of the foil) was equivalent to approximately 50 at%, similar to the unirradiated material. Postirradiation annealing at 948 K for 1 h did not yield any signs:of new precipitation, nor Nb : MO gradients within the ‘precipitates. 3.5. NSF-2 NSF-2 had the most varied microstructure of these alloys. Before irradiation, four types of precipitates were identified: (a) strings of bee, Nb-rich (at least 76 at% Nb), (b) Zr,Si, which is often found in Zircaloys, (c) large (0.4-1.2 p.m), fee (Zr,Nb),Fe with a, = 1.23 nm (equivalent to NiTi, type Zr,Fe for which a, = 1.21 nm) and a median value of xi+ = 0.71 f 0.04 (defined as xFe-- Fe/[Cr + Fe + Nb]), and (d) hexagonal (Zr, Nb)Fe, (Laves MgZn, type, a = 0.48 nm and c = 0.79 nm), with xFe = 0.36 f 0.02. An example of the precipitate strings, in which the median diameter was 0.046 urn, is shown in fig. 3a. The Fe,Nb-rich precipitates were much larger, with a median diameter of 0.40 pm. Although the hexagonal (Zr,Nb)Fe, was identified via diffraction, the measured atomic ratio of Fe to Nb was approximately 1: 2, but if Zr were present too, this ratio should be less than 2: 1. Thus, there remains uncertainty regarding the structure of these precipitates. In irradiated NSF-2, the large Nb, Fe bearing precipitates had a range of compositions. The proportion of iron was generally lower than in the unirradiated material, and varied between 0.17 I;X, I 0.41, and the proportion of Nb (defined as xm, = Nb/[Cr + Fe + Nb]) varied between 0.54 < xNb I 0.82. The proportion of Cr/[Cr + Fe + Nb] was always less than 0.05. The Fe content tended to increase with increasing precipitate diameter, which is consistent with diffusion control of iron dissolution (fig. 4). Diffusion of Fe from the precipitates can also be inferred from Fe gradients within them. For example, xre in fig. 3c was 0.41 at the center, 0.37 at the perforation of the foil, and 0.31 at the edge furthest from the perforation. The decrease in Fe between the center and edges indicates an iron gradient, and the diffusion of iron from the precipitate.
in Irrrdlti~ NSF-2
Iron contemtof pmeipbtee
q
0.4 : q
0
_
Fe
Fe+Nb+Cr
q
0.3q
1
0.14 0.0
I
I
1
0.1
0.2
0.3
1 0.4
.
preclpnatediametrr(pm) Fig. 4. Relationship between iron content of precipitates and precipitate diameter in irradiated NSF-2.
This particular precipitate was chosen for examination because it was thin enough for selected area diffraction, which showed it to be amorphous, with rings equivalent to spacings of 0.242 and 0.143 nm. No other Fe-bearing precipitates were examined that were both close enough to the foil’s perforation and thin enough for selected area diffraction. However, all Fe,Nb-rich precipitates were mostly or completely amorphous, given that their diffraction contrast was invariant with tilting. The precipitate in fig. 3c was 0.43 pm in its smallest dimension, demonstrating that amorphization can be completed by a fluence of 1.5 X 10E n/m* (E > 1 MeV) in relatively large precipitates. As in the unirradiated material, there were strings of precipitates whose diffraction patterns were consistent with bee Nb. They appeared to be smaller than before irradiation, with the median diameter decreasing from 0.046 to 0.036 p,m, and the range of diameters narrowed from 0.007-0.2 pm to 0.020-0.11 urn. (figs 3a and 3b). Based on a precipitate at the perforation of the foil, the atomic fraction of Nb is at least 0.76. Similar results were found in the unirradiated material, so the atomic fraction of Nb was essentially constant.
4. Discussion 4.1. Trends in precipitate
behavior
Changes in composition, crystal structure, and size distribution occur in Zr-base alloys. Not all precipitates
248
R.M. Kruger, R.B. Adamson / Precipitate behavior in zirconium-based alloys in BWRs
change; no changes in Zr,Si have been reported, and ZrC is expected to coarsen very slowly [14]. However, most of the precipitates reported in this work were altered measurably. 4.2. Behavior of Fe and other interstitial metallic elements
It is common for Fe to diffuse out of precipitates during exposure of Zr-base alloys in BWRs. For example, Zr(Cr,Fe), loses a large fraction of its iron in Zircaloy-2 and Zircaloy-4 [6-9, 151, and in XLL, ZrMo, (which initially contains substantial Nb and Fe) loses virtually all of its iron. In both cases, the crystal structure changes, with Zr(Cr,Fe), gradually becoming amorphous, beginning at its interface with the matrix, and ZrMo, becoming a bee lattice similar to Nb or MO. Thus one might suspect that the dissolution of Fe somehow tied to the change in crystal structure. Most likely, fee (Zr,Nb),Fe precipitates release half of their Fe while becoming amorphous, changing from xre = 0.71 to Xt+ = 0.35 (the median value of the 7 largest precipitates in fig. 4), and these evolve into the amorphous precipitates with diameters greater than 0.2 urn. This is consistent with the data, in that the fee precipitates are the larger of the two types, with diameters of 0.39-1.17 km, versus 0.23 u.rn for (Zr,Nb)Fe,. Consistency with this interpretation requires that the (Zr,Nb)Fe, precipitates also lose approximately half of their original Fe, changing from xFe = 0.36 to xFe = 0.19 (the median value of the 4 smallest precipitates in fig. 4), and that they also decrease in size. While there may be a relationship between dissolution and crystal structure, dissolution of Fe can occur without a change in crystal structure. This is made clear by the data on Fe,Ni-rich precipitates in irradiated Zircaloy-2 and NoCr, in which the iron content yw decreased without any change in crystal structure. The Fe loss from Zr,(Fe,Ni) and Zr,(Fe,Ni) may be partially masked by the simultaneous dissolution of Ni. However, the data in this study do not support dissolution of Ni. The dissolution of Ni has been measured indirectly, by the formation of Fe,Ni-rich precipitates at grain boundaries during postirradiation annealing [15]. Similar annealing was performed in this work, but did not yield intergranular precipitation of Ni, possibly due to the lower neutron fluence. The observations of Etoh et al. [161 also support the dissolution of Ni. Chromium also dissolves during irradiation. This is shown indirectly by the Cr : Nb gradients in amorphous precipitates in Zry-0.2Nb, assuming that Cr diffuses
faster than Nb, and by the formation of Cr, Fe-rich precipitates at grain boundaries during the postirradiation annealing. The dissolution of Cr is evidently much slower than that of Fe. 4.3. Behavior of Nb and MO Thus far, there is no evidence for the dissolution of the substitutional elements Nb and MO in these alloys. Postirradiation annealing XLL and NSF-2 falls to show new precipitation, and there are no compositional gradients within the preexisting precipitates. Dissolution of Nb may have been inferred from changes in the diameters of the bee Nb precipitates, but was not supported by any obvious reprecipitation, as could have occurred in postirradiation annealing. Minor dissolution and reprecipitation of Nb has been inferred in Zr-2.5% Nb that was irradiated at 570 K and postirradiation annealed at 770 K for 170-1000 h [18], and so it is conceivable that this should also occur in NSF-2, which contains 1 wt% Nb.
4.4. Shifts in precipitate size distributions The median diameter can increase, decrease, or remain constant As shown in the examples here, irradiation induced changes in diameter may be accompanied by simultaneous dissolution, changes in crystal structure, and changes in morphology, so that explaining a shift in the median diameter may require more than a diffusion analysis. The hexagonal Zr(Cr,Fe), precipitates in Zircaloy-2 displayed a shift towards a smaller median diameter while simultaneously becoming amorphous, and smaller precipitates were measured after irradiation (fig. 2). Given the loss of Fe and Cr, precipitates should tend to become smaller, unless additional Zr or vacancies diffuse inwards; the data here do not address these two factors. The shift in the precipitate size distribution may reflect the dissolution of all precipitates, without ripening. For example, if we assume that all precipitates dissolve and that the rate of precipitate growth is independent of diameter, and subtract 0.015 km (the difference in the median values) from the precipitates measured to obtain the fig. 2a results, a size distribution is obtained that is almost identical to that of fig. 2b. This simple analysis supports the idea that the precipitates are dissolving, as opposed to ripening. As noted above, the amorphous Fe,Nb-rich precipitates in NSF-2 were generally smaller than the original ones and contained about half of the original Fe. This is consistent with the idea that they too are dissolving.
RM. Kruger, R.B. Aabnson / Precipitate behdvior in zirconium-based alloys in BWRs
The bee Nb precipitates in the material studied were smaller and in narrower bands than in the unirradiated material, which could be interpreted as a sign of Nb dissolution. However, much of the bee Nb was initially in a plate-like form (fig. 3a), and these plates may have simply become spherical, which would give the impression of smaller precipitates. The annealing data do not support the dissolution of significant amounts of Nb, although higher fluences or different anneals may yield different results. While the (Nb,Mo),Zr precipitates lost their Fe and became bee in structure, their median diameters changed slightly (+ 13%). Although this small increase may be due to experimental scatter, the volume per atom of a bee solid solution with a, = 0,345 nm is 13% greater than for Mo,Zr with a, = 0.759 nm. Thus, the change in crystal structure may result in larger precipitates. The 6% shrinkage of these precipitates due to postirradiation annealing, with the reabsorption of Fe, is consistent with a contraction due to the transformation of bee Zr-Nb-Mo back to (Nb,Mo),Zr. 5. Conclusions The following conclusions are for neutron irradiation in a BWR at 560 K to fluences of (0.85-1.5) X 10z n/m2 (E > 1 MeV). (1) The irradiation-induced dissolution of iron from precipitates in zirconium-based alloys is common. It is not known if iron can completely dissolve, or if some equilibrium is reached with the matrix. (2) Despite the substantial dissolution of Fe into the matrix, no c-component dislocations were found at these low fluences in any alloys. (3) The dissolution of Cr by a fluence of 1.5 x lO= n/m2 may be inferred in Zry-0.2Nb from Cr:Nb gradients in amorphous Cr-Fe-Zr-Nb precipitates and from the formation of intergranular, Cr,Fe-rich precipitates during postirradiation annealing. (4) The size distributions of some precipitates can change. For example, the median diameter of hexagonal ZxiCr,Fe), decreased 39% in Zircaloy-2 and of bee Nb decreased 22% in NSF-2. In XLL, (Nb,Mo),Zr transformed to bee Zr-Nb-Mo increased 13% in median diameter, but this small increase should be viewed with caution because of experimental scatter. (5) Most data concerning Nb do not show that it dissolves, with the. possible exception of the decrease in precipitate diameters of bee Nb in NSF-2. However, this diameter change may simply reflect a change in morphology.
249
(6) Amorphization of hexagonal (Zr,Nb) (C&Fe), in various alloys and of cubic (Zr,Nb),Fe in NSF-2 can be complete for fluences in the range of (0.851.5) x 10zs n/m2 (E > 1 MeV). (7) In XLL, (Nb,Mo),Zr, containing Fe, becomes bee Zr-Nb-Mo without Fe. There is no evidence for the loss of MO or Nb from these precipitates. Upon postirradiation annealing, bee Zr-Nb-Mo reverts to (Nb,Mo)zZr, and the Fe is reabsorbed.
Acknowledgements Much of this work was performed under a joint technical development program between General Electric Co., Hitachi Ltd., Toshiba Corp, Tokyo Electric Power Co., Tohoku Electric Power Co., Chubu Electric Power Co., Hokuriku Electric Power Co., Chugoku Electric Power Co., and Japan Atomic Power Co. The cooperation of Georgia Power Co. in the irradiation and retrieval of these materials is greatly appreciated. Assistance in specimen preparation was provided by Willie Mah. Postirradiation annealing was performed by Robert Blood.
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R.M. Kruger, R.B. Adamson / Precipitate behavior in zirconium-based alloys in BWRs
[ll] N. Sadaoka and M. Fuse, Proc. 8th Int. Symp. on Zirconium in the Nuclear Industry, ASTM-STP 1023 (ASTM, Philadelphia, 1989) p. 692. [12] MS. Anand, W. Mansei, G. Wallner and W. Week, J. Nucl. Mater. 126 (1984) 144. [13] A. Sarce, J. Nucl. Mater. 185 (1991) 214. [14] A. Sarce, J. Nucl. Mater. 184 (1991) 152. [15] R.M. Kruger, Precipitate Stability in Zircaloy-2, EPRI NP-6845-D (1990).
[16] Y. Etoh, K. Kikuchi, T. Yasuda, S. Koizumi and M. Oishi, Fuel for the 90s Proc. Int Topical Meeting on LWR Fuel Performance, Avignon, France, April 1991 (ANSI p. 691. [17] M. Griffiths and R.W. Gilbert, J. Nucl. Mater. 150 (1987) 169. [18] C.E. Coleman, R.W. Gilbert, G.J.C. Carpenter and G.C. Weatherly, Phase Stability During Irradiation (TMSAIME, Warrendale, Pennsylvania, 1981) p. 587.
Journal of Nuclear Materials 205 (1993) 242-250 North-Holland
mab!riais
Precipitate behavior in zirconium-based alloys in BWRs R.M..Kruger
and R.B. Adamson
GeneralElectricCompany,Vallecitos NuclearCenter,Pleasanton,CA 94566, USA
The metallic elements Cr, Fe, Ni, Nb, and MO have low solubilities in zirconium, so that zirconium-based alloys made with these elements contain numerous small precipitates. Some of these undergo substantial changes in composition, crystalline structure, and size during in-reactor exposure. Data on these phenomenon have been obtained using the scanning transmission electron microscope (STEM) to study precipitates before and after reactor exposure. Results on the following alloys are reported here: Zircaloy-2 (Zr-1.SSn-O.15Fe-O.1OCr-O.O5Ni), Zry-02Nb (Zircaloy-2 with 0.2Nb), NoCr (ZrlSSn-O.l7Fe-O.l7Ni), NSF-2 (Zr-lNb-lSn-0.2Fe), and XLL (Zr-1.5Sn-0.3Nb-0.3Mo). The dissolution of Fe from various precipitates is apparent from direct compositional measurements. The dissolution of Cr can be inferred in Zry-0.2Nb from Cr : Nb gradients in amorphous Cr-Fe-Zr-Nb precipitates and from the formation of intergranular, Cr,Fe-rich precipitates during postirradiation annealing. Two types of phase change have been seen. First, crystalline phases can become amorphous, as with hexagonal Laves Zt(Fe,Cr), in Zircaloy-2 and cubic Fe(Zr,Nb), in NSF-2. In the second type, long-range order is eliminated, as with Laves (Mo,Nb)zZr precipitates in ?&L which become BCC. The precipitate size distributions can change in reactor. The greatest changes were measured in small, Cr,Fe-rich precipitates.
1. introduction
A number of metallic elements have been used in commercial alloys of zirconium for nuclear reactor applications, including Cr, Fe, Ni, Sn, Nb, and MO [l-5]. All of these elements can precipitate, subject to their concentrations, the simultaneous presence of other metallic elements, and the alloy processing. In standard Zircaloys, it has been shown that the precipitates themselves can then change in reactor, due to radiation damage processes coupled with moderately high temperatures, on the order of 570 K [6-g]. The changes to the precipitates include compositions, size distributions, and crystal structures. In this paper, it will be shown that in other Zr-base alloys such changes are also common, so that they should be expected by modelers and designers concerned with explaining and improving material performance [8,10-141. 2. Experimental 2.1. Materials
The materials referred to in this paper are listed in table 1, along with their compositions. All were fabricated with several cold-work/ anneal cycles, and with a 0022-3115/93/$06.00
final anneal that induced complete recrystallization. All materials were in a BWR for one cycle, and irradiated at 560 K to fluences of (0.85-1.5) X 10z n/m2 (E > 1 MeV). 2.2. Scanning transmksion electron microscope (STEM) All materials were characterized with a JEOL lOOCX-STEM equipped with a Be-window Si(Li) X-ray detector for energy dispersive X-ray spectroscopy (EDS). To prepare specimens, material was ground to 0.15 mm thickness, 3 mm diameter discs were punched from this material, and thin foils were made from the discs using the conventional twin-jet electropolishing method. The polishing solution was 10% perchloric acid in 90% ethanol, cooled to 233 K. The information on precipitates that was obtained included their sizes, compositions, which were quantified via EDS, and crystal lattices via selected area diffraction patterns (SADPs) or convergent beam diffraction patterns (CBDPs). All materials were characterized before and after irradiation. 2.3. Postirradiationannealing Some postirradiation annealing was performed under a vacuum of 5 X 10v3 Pa for 1 h at 848 K for the
0 1993 - Elsevier Science Publishers B.V. Ah rights reserved
R.M. Kruger, RB. Adamson / Precipitatebehavior in zirconium-basedalloysin BWRY
243
Table 1 Compositions and fluences of the alloys (E > 1 MeV) Alloy
Composition (wt%)
Fluence (n/m’)
Zircaloy-2 zry-0.2Nb NoCr XLL NSF-2
Zr-1.5Sn-O.1SFe-O.1OCr-O.O5Ni Zr-1.5Sn-0.18Fe-O.lOCCr-O.O5Ni-0.2Nb Zr-O.l7Fe-0.17Ni Zr-l.SSn-0.3Nb-0.3Mo-0.05Fe Zr-lNb-lSn-0.2Fe
0.85 x 10zs 1.5 x 1025 1.5 x lo= 1.5 x 1025 1.5 x 1025
alloys Zry-02Nb and NoCr and at 948 K for alloys XLL and NSF-2. The primary purpose was to determine if any new precipitates formed, which is an indirect indication that alloying elements dissolved during irradiation. The lower temperature anneal was chosen because it can cause precipitation of Cr, Fe, and Ni on grain boundaries in Zircaloy-2 [15], and the higher temperature anneal was chosen under the assumption that it would allow Nb and MO to diffuse fast enough for precipitation to occur.
3. Results
A summary of the precipitates and their changes due to irradiation is provided in table 2. 3.1. Zircaloy-2 Zircaloy-2 can be processed to have either large or small precipitates. In this instance, the precipitates in the unirradiated material were small, with a median
diameter of 0.036 urn (Fig. la). The precipitates were hexagonal Zr(Cr,Fe), (Laves MgZn, type, a = 0.51 nm and c = 0.82 nm) with xFc = 0.53 f 0.03 (xi+ = Fe/[Fe + Crl), and Zr,(Fe,Ni) (tetragonal &Al, type, with u,, = 0.66 nm, and cs = 0.55 nm> with y, = 0.54 f 0.03 ( yFe = Fe/[Fe + Nil). The scatter bands represent one standard deviation in the measurements. The precipitates in the irradiated material were altered. The Cr,Fe-rich precipitates all became completely amorphous, including precipitates as large as 0.08 pm in their smallest dimension (fig. lb). Their iron content decreased substantially, to xFc = 0.15 f 0.07. The range of xFe was 0.04 to 0.28. The Fe content of the Fe,Ni-rich precipitates was not significantly lower than in the unirradiated material, with These precipitates retained their YFe = 0.51 f 0.04. crystal structure. In other studies, it can be seen that Zr,(Fe,Ni) appears to dissolve [16], but no evidence of dissolution was recorded here. The dissolution of Fe has been implicated in the development of c-component dislocations at higher
Table 2 Summary of precipitates before and after exposure in BWR at 560 K to fluences of 0.85 x 10z n/m2 (E > 1 MeV) for Zircaloy-2 and 1.5 x loss n/m* (others) Alloy
Original precipitate
Final precipitate
Zircaloy-2
Hexagonal Zr(Cra.~,FeO,ss), Tetragonal Zr,(Fes,s,Ni,,,) Hexagonal (Zr,NbXCrO.~sFeO.,s), Tetragonal ZrZ(Fe,,s4NiaJ Orthorhombic Zrs(Fe,,,,Ni,,,,) Tetragonal Zr,(Fe,Ni) 0.27 5 Fe/(Fe + Ni) 5 0.51 Cubic MosZr, Nb (0.5 5 Nb : MO 5 0.67) and Fe (0.17 5 Fe : MO I 0.27) bee Nb, Nb/(Zr + Nb) 2 0.76 ZrsSi Cubic (Zr,Nb),Fe, Fe/(Cr + Fe + Nb) = 0.71 Hexagonal (Zr,Nb)Fes, Fe/(Cr + Fe + Nb) = 0.36
Amorphous, Fe/(Cr + Fe) = 0.15 Tetragonal Zr,(Fe,,s,Ni,,,) Amorphous, Fe/(Cr + Fe) = 0.26 Tetragonal Zr,(Fe,,,~Ni,,,) Orthorhombic Zr,(Fe,,s,Ni,,,) Tetragonal Zr,(Fe,Ni) Fe/(Fe + Ni) = 0.40 bee Zr-Nb-Mo, Nb (0.45 5 Nb : MO I 0.68) and Fe (0.00 I Fe : MO i; 0.03) bee Nb, Nb/(Zr + Nb) r 0.76 Zr,Si Amorphous
zry-0.2Nb NoCr
XLL NSF-2
Amorphous
244
RM. tiger,
R.B. Adamson
/ Precipitate behavior in zirconium-based alloys in BWRY
Fig. 1. Zircaloy-2 (a) before and (b) after irradiation. Amorphous precipitates in irradiated sample imaged with diffuse diffraction ring.
doses 1171,and so a search for c-dislocations
was made, dislocations were found in the other materials studied below. The precipitates became smaller with irradiation (fig. 2). The median diameter decreased from 0.038 to 0.023 pm, and the smallest precipitates were 0.015 and 0.005 pm in the unit-radiated and irradiated materials, respectively. but none were found. Similarly, no c-component
Precipitate size distribution8 in Zircaloy-2 tubbIg
40
30
20
3.2. Zry-0.2Nb 10
Before irradiation, Zry-0.2Nb contained hexagonal (Zr,NbXCr,.,sFeo.4s)2 precipitates with Nb : Fe as high as 0.10 on an atom basis. It also contained tetragonal Zr2(Fe0_52NiOA8) precipitates that contained no Nb. Both types of precipitates were intragranular, with identical crystalTstru&res as in the Zkcaloy-2. The microstructure
of this material
appears
similar
of Zircaloy-2 having similar sizes of precipitates,
to that
which
” n
-I
4.015 0.02 0.M 0.06 0.08
0.10
0.12
0.14
0.16
diameter (r.lm) Fig. 2. Precipitate size distributions for Zircaloy-2; (a) before irradiation, (b) after irradiation.
RM. ICyer, RB. Adamson / Precipitate behavior in zirconium-based alloys in BETs
in this case ranged between 0.035 and 0.52 km, with a median of 0.11 pm. The irradiated Zry-0.2Nb contained both Cr,Fe-rich and Fe,Ni-rich precipitates. The Cr,Fe-rich precipitates that were electron transparent were completely amorphous, just like their counterparts in irradiated Zircaloy-2. In addition, the Fe content of these precipitates was substantially lower than in the unirradiated material. Representing the iron content by xi+ = Fe/(Cr + Fe), the composition at the centers of the . . precipitates was xre = 0.261 f 0.074, which is much lower than the value in the unirradiated material (xFe = 0.45). The diffusion of iron out of the precipitates was corroborated by the presence of gradients; xFc varied from 0.41 at the center to 0.30 at the edge of one precipitate, and from 0.28 to 0.28 in another. The Cr,Fe-rich precipitates still contained Nb. The Nb : Cr concentrations (atom basis) of 8 precipitates varied between 0.069 and 0.147, with a median of 0.088. This result is similar to the unirradiated material, for which 0.049 < Nb : Cr II 0.115, with a median of 0.091. Thus, Nb is retained by the Cr,Fe-rich precipitates in approximately its original ratio to Cr. The Nb:Cr ratio was higher at the edges than at the centers. In one case, the Nb: Cr ratio varied from 0.069 (center) to 0.328 (edge), and in another, Nb : Cr varied from 0.108 to 0.196. The Fe,Ni-rich precipitates in Zry-0.2Nb were crystalline, and the crystal structure, tetragonal Zr,(Fe,Ni), was the same as before irradiation. The iron content at the center of the precipitates, represented by yFe = Fe/(Fe + Ni), was y, = 0.475 f 0.03, or slightly lower than in the unirradiated material (yFe = 0.52 f 0.02). This indication of a slight loss of iron was supported by the measurement of gradients in yFe in two separate precipitates. In each case, y, was 0.50 at the center, and 0.42 at the edge. Postirradiation annealing of Zry-0.2Nb at 848 K for 1 h yielded Cr,Fe-rich precipitates at the grain boundaries (xFe = 0.59 f 0.06). No Ni-rich intergranular precipitates were found. The amorphous precipitates were not rectystallixed by the postirradiation anneal, but their iron content increased (from 0.26 before the anneal) to xFe = 0.39 f 0.05. The Fe,Ni-rich precipitates also gained Fe during the post-irradiation anneal, as the median value of y, increased (from 0.475 before the anneal) to 0.55 f 0.02. 3.3. NoCr Before irradiation, the NoCr contained two types of nearly spherical, intragranular precipitates. Some were
245
orthorhombic Zr,(Fe,s,Ni,,,) (a, = 0.332 run, 6, = 1.099 nm, c,, = 0.881 nm), which is not usually present in manufactured Zircaloy-2. The others were tetragonal Zr,(Fe,Ni), with 0.27 I, y, < 0.51. These tetragonal precipitates are identical to the Fe,Ni-rich precipitates typically found in Zircaloy-2, although in Zircaloy-2, the range of yFc is much narrower (as in section 3.1). The range of precipitate diameters was 0.09-0.72 pm, with a median of 0.31 pm. Visually, the precipitates were not changed by irradiation. Twelve precipitates were examined, and 11 of these were orthorhombic Zr,(Fe,Ni) with yi+. in the range of 0.56 < y, < 0.62, and a mean value of y, = 0.59 f 0.02. The lone precipitate with a low value of y,, equal to 0.399, was tetragonal Zr,(Fe,Ni). Thus, both types of precipitates were still present following irradiation. The ‘diameters of the precipitates ranged between 0.2 and 0.8 pm, with a median of 0.35 km. Postirradiation annealing of NoCr did not yield Fe,Ni-rich precipitates at the grain boundaries, as had been anticipated. Gradients in y, were found, the largest difference being y, = 0.39 at the center and 0.52 at the edge of a precipitate. 3.4. XLL Before irradiation, XLL contained spherical, intragranular precipitates that contained MO, Nb, Fe, and Zr. Their diffraction patterns were consistent with the crystal lattice of cubic Mo,Zr(MgCu, type Laves phase, with u,, = 0.759 nm), and they contained substantial concentrations of Nb (0.5 5 Nb: MO 5 0.67) and Fe (0.17 I Fe : MO < 0.27). For convenience, these precipitates will be referred to as (Nb,Mo),Zr, although there are no data concerning the sublattice occupied by Nb. In an analysis of 88 precipitates, the median diameter was 0.046 pm, and the range was 0.014 to 0.14 km. Irradiation caused two major changes in these precipitates. First, the iron content decreased measurably (0 < Fe : MO 5 0.03), indicating that iron diffised into the matrix. Second, the crystal structure became bee, with a, = 0.345 nm. This is slightly larger than for the bee metals MO (a, = 0.315 nm) and Nb (a, = 0.320 nm), but smaller than p-Zr (a, = 0.357 nm). The appearance of the precipitates was not dramatically altered by irradiation, and in particular, it was difficult to determine if any were dissolving. The precipitate diameter increased slightly due to irradiation, including a median diameter of 0.052 pm and a range of 0.020 to 0.38 km. The 13% increase in median diameter may be due to experimental scatter,
246
R.M. Kruger, R.B. Adamson / Precipitate behavior in zirconium-based alloys in BWRs
Fig. 3, NSF-2; 6) string of bee Nb precipitates before irradiation, ibl string of bee Nb precipitates amo~bous precipitate after irradiation.
after i~adiation
and (c>
241
R M. Kruger, R B. Adatnson / Precipitate behavior in zirconium-based alloys in BU%s
but is relatively large. Postirrradiation annealing restored the precipitates to their original crystal lattice of Mo,Zr, restored the Fe, and reduced the median diameter from 0.052 to 0.049 urn. There was no evidence that Nb or MO dissolved into the matrix. The Nb : MO ratio was similar to its original value, with 0.45 < Nb : MO < 0.68, and it is unlikely that Nb and MO would diffuse away at identical rates. In addition, the minimum proportion of Zr in the spectra (for precipitates at the perforation of the foil) was equivalent to approximately 50 at%, similar to the unirradiated material. Postirradiation annealing at 948 K for 1 h did not yield any signs:of new precipitation, nor Nb : MO gradients within the ‘precipitates. 3.5. NSF-2 NSF-2 had the most varied microstructure of these alloys. Before irradiation, four types of precipitates were identified: (a) strings of bee, Nb-rich (at least 76 at% Nb), (b) Zr,Si, which is often found in Zircaloys, (c) large (0.4-1.2 p.m), fee (Zr,Nb),Fe with a, = 1.23 nm (equivalent to NiTi, type Zr,Fe for which a, = 1.21 nm) and a median value of xi+ = 0.71 f 0.04 (defined as xFe-- Fe/[Cr + Fe + Nb]), and (d) hexagonal (Zr, Nb)Fe, (Laves MgZn, type, a = 0.48 nm and c = 0.79 nm), with xFe = 0.36 f 0.02. An example of the precipitate strings, in which the median diameter was 0.046 urn, is shown in fig. 3a. The Fe,Nb-rich precipitates were much larger, with a median diameter of 0.40 pm. Although the hexagonal (Zr,Nb)Fe, was identified via diffraction, the measured atomic ratio of Fe to Nb was approximately 1: 2, but if Zr were present too, this ratio should be less than 2: 1. Thus, there remains uncertainty regarding the structure of these precipitates. In irradiated NSF-2, the large Nb, Fe bearing precipitates had a range of compositions. The proportion of iron was generally lower than in the unirradiated material, and varied between 0.17 I;X, I 0.41, and the proportion of Nb (defined as xm, = Nb/[Cr + Fe + Nb]) varied between 0.54 < xNb I 0.82. The proportion of Cr/[Cr + Fe + Nb] was always less than 0.05. The Fe content tended to increase with increasing precipitate diameter, which is consistent with diffusion control of iron dissolution (fig. 4). Diffusion of Fe from the precipitates can also be inferred from Fe gradients within them. For example, xre in fig. 3c was 0.41 at the center, 0.37 at the perforation of the foil, and 0.31 at the edge furthest from the perforation. The decrease in Fe between the center and edges indicates an iron gradient, and the diffusion of iron from the precipitate.
in Irrrdlti~ NSF-2
Iron contemtof pmeipbtee
q
0.4 : q
0
_
Fe
Fe+Nb+Cr
q
0.3q
1
0.14 0.0
I
I
1
0.1
0.2
0.3
1 0.4
.
preclpnatediametrr(pm) Fig. 4. Relationship between iron content of precipitates and precipitate diameter in irradiated NSF-2.
This particular precipitate was chosen for examination because it was thin enough for selected area diffraction, which showed it to be amorphous, with rings equivalent to spacings of 0.242 and 0.143 nm. No other Fe-bearing precipitates were examined that were both close enough to the foil’s perforation and thin enough for selected area diffraction. However, all Fe,Nb-rich precipitates were mostly or completely amorphous, given that their diffraction contrast was invariant with tilting. The precipitate in fig. 3c was 0.43 pm in its smallest dimension, demonstrating that amorphization can be completed by a fluence of 1.5 X 10E n/m* (E > 1 MeV) in relatively large precipitates. As in the unirradiated material, there were strings of precipitates whose diffraction patterns were consistent with bee Nb. They appeared to be smaller than before irradiation, with the median diameter decreasing from 0.046 to 0.036 p,m, and the range of diameters narrowed from 0.007-0.2 pm to 0.020-0.11 urn. (figs 3a and 3b). Based on a precipitate at the perforation of the foil, the atomic fraction of Nb is at least 0.76. Similar results were found in the unirradiated material, so the atomic fraction of Nb was essentially constant.
4. Discussion 4.1. Trends in precipitate
behavior
Changes in composition, crystal structure, and size distribution occur in Zr-base alloys. Not all precipitates
248
R.M. Kruger, R.B. Adamson / Precipitate behavior in zirconium-based alloys in BWRs
change; no changes in Zr,Si have been reported, and ZrC is expected to coarsen very slowly [14]. However, most of the precipitates reported in this work were altered measurably. 4.2. Behavior of Fe and other interstitial metallic elements
It is common for Fe to diffuse out of precipitates during exposure of Zr-base alloys in BWRs. For example, Zr(Cr,Fe), loses a large fraction of its iron in Zircaloy-2 and Zircaloy-4 [6-9, 151, and in XLL, ZrMo, (which initially contains substantial Nb and Fe) loses virtually all of its iron. In both cases, the crystal structure changes, with Zr(Cr,Fe), gradually becoming amorphous, beginning at its interface with the matrix, and ZrMo, becoming a bee lattice similar to Nb or MO. Thus one might suspect that the dissolution of Fe somehow tied to the change in crystal structure. Most likely, fee (Zr,Nb),Fe precipitates release half of their Fe while becoming amorphous, changing from xre = 0.71 to Xt+ = 0.35 (the median value of the 7 largest precipitates in fig. 4), and these evolve into the amorphous precipitates with diameters greater than 0.2 urn. This is consistent with the data, in that the fee precipitates are the larger of the two types, with diameters of 0.39-1.17 km, versus 0.23 u.rn for (Zr,Nb)Fe,. Consistency with this interpretation requires that the (Zr,Nb)Fe, precipitates also lose approximately half of their original Fe, changing from xFe = 0.36 to xFe = 0.19 (the median value of the 4 smallest precipitates in fig. 4), and that they also decrease in size. While there may be a relationship between dissolution and crystal structure, dissolution of Fe can occur without a change in crystal structure. This is made clear by the data on Fe,Ni-rich precipitates in irradiated Zircaloy-2 and NoCr, in which the iron content yw decreased without any change in crystal structure. The Fe loss from Zr,(Fe,Ni) and Zr,(Fe,Ni) may be partially masked by the simultaneous dissolution of Ni. However, the data in this study do not support dissolution of Ni. The dissolution of Ni has been measured indirectly, by the formation of Fe,Ni-rich precipitates at grain boundaries during postirradiation annealing [15]. Similar annealing was performed in this work, but did not yield intergranular precipitation of Ni, possibly due to the lower neutron fluence. The observations of Etoh et al. [161 also support the dissolution of Ni. Chromium also dissolves during irradiation. This is shown indirectly by the Cr : Nb gradients in amorphous precipitates in Zry-0.2Nb, assuming that Cr diffuses
faster than Nb, and by the formation of Cr, Fe-rich precipitates at grain boundaries during the postirradiation annealing. The dissolution of Cr is evidently much slower than that of Fe. 4.3. Behavior of Nb and MO Thus far, there is no evidence for the dissolution of the substitutional elements Nb and MO in these alloys. Postirradiation annealing XLL and NSF-2 falls to show new precipitation, and there are no compositional gradients within the preexisting precipitates. Dissolution of Nb may have been inferred from changes in the diameters of the bee Nb precipitates, but was not supported by any obvious reprecipitation, as could have occurred in postirradiation annealing. Minor dissolution and reprecipitation of Nb has been inferred in Zr-2.5% Nb that was irradiated at 570 K and postirradiation annealed at 770 K for 170-1000 h [18], and so it is conceivable that this should also occur in NSF-2, which contains 1 wt% Nb.
4.4. Shifts in precipitate size distributions The median diameter can increase, decrease, or remain constant As shown in the examples here, irradiation induced changes in diameter may be accompanied by simultaneous dissolution, changes in crystal structure, and changes in morphology, so that explaining a shift in the median diameter may require more than a diffusion analysis. The hexagonal Zr(Cr,Fe), precipitates in Zircaloy-2 displayed a shift towards a smaller median diameter while simultaneously becoming amorphous, and smaller precipitates were measured after irradiation (fig. 2). Given the loss of Fe and Cr, precipitates should tend to become smaller, unless additional Zr or vacancies diffuse inwards; the data here do not address these two factors. The shift in the precipitate size distribution may reflect the dissolution of all precipitates, without ripening. For example, if we assume that all precipitates dissolve and that the rate of precipitate growth is independent of diameter, and subtract 0.015 km (the difference in the median values) from the precipitates measured to obtain the fig. 2a results, a size distribution is obtained that is almost identical to that of fig. 2b. This simple analysis supports the idea that the precipitates are dissolving, as opposed to ripening. As noted above, the amorphous Fe,Nb-rich precipitates in NSF-2 were generally smaller than the original ones and contained about half of the original Fe. This is consistent with the idea that they too are dissolving.
RM. Kruger, R.B. Aabnson / Precipitate behdvior in zirconium-based alloys in BWRs
The bee Nb precipitates in the material studied were smaller and in narrower bands than in the unirradiated material, which could be interpreted as a sign of Nb dissolution. However, much of the bee Nb was initially in a plate-like form (fig. 3a), and these plates may have simply become spherical, which would give the impression of smaller precipitates. The annealing data do not support the dissolution of significant amounts of Nb, although higher fluences or different anneals may yield different results. While the (Nb,Mo),Zr precipitates lost their Fe and became bee in structure, their median diameters changed slightly (+ 13%). Although this small increase may be due to experimental scatter, the volume per atom of a bee solid solution with a, = 0,345 nm is 13% greater than for Mo,Zr with a, = 0.759 nm. Thus, the change in crystal structure may result in larger precipitates. The 6% shrinkage of these precipitates due to postirradiation annealing, with the reabsorption of Fe, is consistent with a contraction due to the transformation of bee Zr-Nb-Mo back to (Nb,Mo),Zr. 5. Conclusions The following conclusions are for neutron irradiation in a BWR at 560 K to fluences of (0.85-1.5) X 10z n/m2 (E > 1 MeV). (1) The irradiation-induced dissolution of iron from precipitates in zirconium-based alloys is common. It is not known if iron can completely dissolve, or if some equilibrium is reached with the matrix. (2) Despite the substantial dissolution of Fe into the matrix, no c-component dislocations were found at these low fluences in any alloys. (3) The dissolution of Cr by a fluence of 1.5 x lO= n/m2 may be inferred in Zry-0.2Nb from Cr:Nb gradients in amorphous Cr-Fe-Zr-Nb precipitates and from the formation of intergranular, Cr,Fe-rich precipitates during postirradiation annealing. (4) The size distributions of some precipitates can change. For example, the median diameter of hexagonal ZxiCr,Fe), decreased 39% in Zircaloy-2 and of bee Nb decreased 22% in NSF-2. In XLL, (Nb,Mo),Zr transformed to bee Zr-Nb-Mo increased 13% in median diameter, but this small increase should be viewed with caution because of experimental scatter. (5) Most data concerning Nb do not show that it dissolves, with the. possible exception of the decrease in precipitate diameters of bee Nb in NSF-2. However, this diameter change may simply reflect a change in morphology.
249
(6) Amorphization of hexagonal (Zr,Nb) (C&Fe), in various alloys and of cubic (Zr,Nb),Fe in NSF-2 can be complete for fluences in the range of (0.851.5) x 10zs n/m2 (E > 1 MeV). (7) In XLL, (Nb,Mo),Zr, containing Fe, becomes bee Zr-Nb-Mo without Fe. There is no evidence for the loss of MO or Nb from these precipitates. Upon postirradiation annealing, bee Zr-Nb-Mo reverts to (Nb,Mo)zZr, and the Fe is reabsorbed.
Acknowledgements Much of this work was performed under a joint technical development program between General Electric Co., Hitachi Ltd., Toshiba Corp, Tokyo Electric Power Co., Tohoku Electric Power Co., Chubu Electric Power Co., Hokuriku Electric Power Co., Chugoku Electric Power Co., and Japan Atomic Power Co. The cooperation of Georgia Power Co. in the irradiation and retrieval of these materials is greatly appreciated. Assistance in specimen preparation was provided by Willie Mah. Postirradiation annealing was performed by Robert Blood.
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