NEUTRON IRRADIATION OF Fe-Mn, Fe-Cr-Mn AND Fe-Cr-Ni ...

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The swelling and phase stability of neutron-irradiated. Fe-Cr-Mn and Fe-Cr-Ni alloys are compared in the range. 420-600 o C. While the behavior of the two alloy ...
Journal

294

of Nuclear

Materials 148 (1987) 294-301 North-Holland, Amsterdam

NEUTRON IRRADIATION OF Fe-Mn, Fe-Cr-Mn AND Fe-Cr-Ni ALLOYS AND AN EXPLANATION OF THEIR DIFFERENCES IN SWELLING BEHAVIOR F.A. GARNER,

H.R. BRAGER,

D.S. GELLES

*

and J.M. MCCARTHY

Hanford Engineering Development Laboratory, Richland, WA 99352, USA Received

17 October

1986; accepted

17 October

1986

The swelling and phase stability of neutron-irradiated Fe-Cr-Mn and Fe-Cr-Ni alloys are compared in the range 420-600 o C. While the behavior of the two alloy systems exhibits many similarities, that of the Fe-Cr-Mn system is more complex, involving a higher level of phase instabilities. However, the sensitivity of radiation-induced density changes to composition is less in the Fe-Cr-Mn system. In the Fe-Cr-Mn system there are three major components of density change, namely void growth, ferrite formation and lattice parameter changes of the retained austenite; whereas void swelling is the only major component of density change in the Fe-Cr-Ni system.

1. Introduction In an earlier paper [l] we described the first results of an irradiation program designed to develop Fe-Cr-Mn austenitic alloys for reduced activation service in fusion reactors. There are three groups of alloys in this program. The first group (table 1) includes simple Fe-Mn binaries and Fe-Cr-Mn ternary alloys in the annealed condition (1030° C/OS h/air cool). The second group (table 2) contains solute-modified temaries and the third contains five commercial Fe-Cr-Mn alloys (table 3). Both of these groups were prepared in a variety of thermal-mechanical starting conditions. These three groups of alloys are being irradiated as small microscopy disks in the Fast Flux Test Facility (FFTF) using the Materials Open Test Assembly (MOTA) in which the temperature is controlled within + 5 o C. Swelling data previously were available only at 520°C and 14 dpa for the simple binary Fe-Mn and ternary Fe-Cr-Mn alloys [l]. Data are now available at 420 o C and 9 dpa, 600 o C at 14 dpa, and 520 o C at 49.8 dpa for the simple alloys and for 14 and 49.8 dpa at 520° C for the commercial alloys. The swelling was measured as a change in density, using an immersion density technique accurate to +0.16%. These data are compared with comparable data on

* Presented at the Second International Reactor

Materials,

April 13-17.1986,

Conference on Fusion Chicago, Illinois, USA.

0022-3115/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

Fe-Cr-Ni ternary alloys irradiated in EBR-II [2-51. A summary of microscopy results is also presented for the simple Fe-Cr-Mn alloys in order to develop an understanding of the differences observed in swelling behavior.

2. Experimental results 2.1. Density change measurements of simple alloys in the 9-14 dpa range As shown in fig. 1 the swelling of eight simple alloys at 520° C and 14 dpa is remarkably insensitive to chromium content and only weakly dependent on the manganese level. The swelling of Fe-Cr-Ni alloys is much more sensitive to composition at this temperature [4]. One exception to the trend established by these eight alloys is shown in fig. 2 where the swelling levels of Fe-Xr-1SMn at 520 o C and 600° C are nearly identical but are much less than that of Fe-15Cr-15Mn. Table 1 Composition

R66 R67 R68 R69 R70

B.V.

of simple Fe-Cr-Mn

Fe

Cr

Mn

80 70 80 70 65

5 15 0 10 15

15 15 20 20 20

alloys (wtW)

R71 R72 R73 R74 R75

Fe

Cr

Mn

75 70 65 60 65

0 0 5 10 0

25 30 30 30 35

.

F.A. Gurner et al. / Neutron irradiation

Table 2 Composition of solute-modified Fe-Cr-Mn Mn x-15 R76 RI7 RI8 RI9 R80 R81 R82 R83 R84 R85 R86 R81 R88 R89

30 30 30 30 30 30 20 15 15 15 15 15 15 15 15

Cr 2.0 2.0 2.0 5.0 10 10 15 5 5 5 15 15 15 15 15

alloys (wt%) N

C

0.15 0.05 0.15 0.15 0.10 0.10 0.10 0.10 0.05 0.10 0.15 0.35 0.10 0.30 0.10

0.10 0.60 0.40 0.05 0.05 0.50 0.05 0.40 0.60 0.10 0.05 0.10 0.10 0.30 0.50

Table 3 Composition of commercial Fe-Cr-Mn

295

of Fe - Mn, Fe - Cr - Mn and Fe - Cr - Ni

V

B

Al

W

Ni

Si

_

_

_

0.05 0.05 _ _

0.005 0.005 _ _

_ _ _ _

_ 0.05 _ _ _

0.005

1.0

1.0 _ _ 2.0

_ _ 1.0

_ _ _

0.05 0.05 -

0.005 0.005 _

_ 1.0

_ 2.0 2.0

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 1.0 0.4 0.4 0.4 1.0 0.4 0.4

P

_ -. 1.0 _ _ 2.0 _ 2.0 _ _ 2.0 2.0

0.005 _ _

austenitic alloys

Designation

Vendor

Composition (wt%)

Nitronic Alloy 32 18/18 Plus AMCR 0033 NMF3 Nomnag 30

Armco Cartech Creusot-Marrel Creusot-Marrel Kobe

18Cr-12Mn-1.5Ni-0.6Si-0.2Cu-0.2Mo-0.4N-O.lC-0.02P 18Cr-18Mn-0.5Ni-0.6Si-1.OCu-1.1Mo-0.4N-O.lC-O.O2P 10Cr-18Mn-0.1Ni-0.6Si-0.06N-0.2C 4Cr-19Mn-0.2Ni-0.7Si-0.09N-0.02P-0.6C 2Cr-14Mn-2.0Ni-0.3Si-0.02N-0.02P-0.6C

Fig.

3 shows that with somewhat more scatter the swelling of the other eight alloys at 600 o C is essentially equal to that at 520 o C, indicating a remarkable insensi-

4, SWELLING %

3 2 -

10

ALL OTHER’DATA AT 520°C AND 3.2 x l@ n/cm2 (E>O.l MeV) 15

20 wt.%

Fig. 1. Neutron-induced swelling of Fe-Mn and Fe-Cr-Mn alloys in FFTF at 52O“C and 14 dpa as determined by immersion density measurements [l].

25

30

35

40

45

MANGANESE

Fig. 2. Comparison of swelling of Fe-SCr-15Mn at 14 dpa with the trend of other Fe-Cr-Mn alloys at 520°C and 14 dpa.

Fe-lOCr-20Mn

^ Fe-15Cr-15Mn

Fe-lCCr-20Mn

Fe-lOCr-30Mn A n 0 SWELLING

00 5Cr 100

l 150

%

SWELLING o/o

2

42OT

0

9 dpa

wt.%

MANGANESE

-2

Fig. 3. Neutron-induced density changes of Fe-Mn and Fe-Cr-Mn alloys at (14 dpa. 600°C) and (9 dpa. 420°C). I

tivity to irradiation temperature. Fig. 3 also demonstrates that similar compositional trends occur at 420 o C and 9 dpa but that there is an increasing tendency to densify with increasing manganese content, reaching 2.2% densification at Fe-35Mn. Within the 420 and 520“ C data sets (figs. l-3) the widths of the range of swelling values are similar, suggesting that the difference in swelling arises from the different dpa levels, rather than different temperatures. Therefore, we can employ an insight gained from the Fe-Cr-Ni system [2,4] in which the swelling data in the Fe-Cr-Mn system are assumed to be independent of temperature below some critical temperature. Whereas this temperature is 540” C or less in the Fe-Cr-Ni system, the critical temperature for Fe-Cr-Mn appears to be above 600 o C. As shown in fig. 4 this temperature-insensitive approach implies that the steady-state swelling rate is - l%/dpa and that the minimum transient regime is on the order of 10 dpa, both of which agree with the behavior exhibited by Fe-Cr-Ni alloys [2-51. However, the agreement of the 420, 520 and 600 o C swelling data for Fe-Cr-Mn imply that the range of temperature-independent behavior is much larger than that of the Fe-Cr-Ni system.

5 10 15 20 25 DISPLACEMENTS PER ATOM Fig. 4. Plot of 420 and 52O’C data ignoring temperature, with the exception of Fe-SCr-15Mn at 52O’C. Note that the implied swelling rate is - lS/dpa, similar to that of the Fe-Cr-Ni system.

have already revealed the major reasons for the observed behavior. As shown in fig. 5 the matrix of the Fe-SCr-15Mn alloy has decomposed after irradiation at 600 o C into large, elongated ferrite precipitates (on the order of 2-10 pm in size) embedded in a matrix of retained austenite. The relative volume of the two phases is difficult to determine because the austenite is easily attacked and destroyed during electropolishing, leaving the ferrite particles protruding far from the edge and surfaces of the foil. Another interesting observation was that voids were found which appeared to be coated with a shell that also resisted electropolishing. Note in fig. 5 that some coated voids are suspended from precipitates hanging over the edge of the foil. At this point it is assumed that the void shells are ferritic in nature, but this assumption has not yet been tested for this specimen. 2.3. Examination

of specimens irradiated at 520°C

to 14

dpa 2.2. Microscopy examination of low fhence specimens Microscopy ternary alloys

examination of the simple binary and is in progress, but preliminary results

Fe-35Mn As shown decomposition

in fig. 6, once again we have observed a of the alloy into ferrite precipitates which

F.A. Gurner et al. / Neuiron irrudiution

of Fe - Mn, Fe - Cr - Mn and Fe - Cr - Ni

297

Fig. 5. Micrographs showing formation of large iron-rich ferrite precipitates protruding from the strongly etched austenite matrix in Fe-SCr-15Mn at 600 o C and 14 dpa. Voids are coated with ferrite which also resists electropolishing.

resist electropolishing and retained austenite which is more easily removed. The precipitates range from 0.2 to 1.5 pm in size. In this case the precipitates were analyzed by EDX and found to be essentially pure iron. Once again the voids appear to be coated and resistant to electropolishing. Within any one grain, the swelling and

Fig. 6. Micrograph

showing

decomposition

phase decomposition appears to be rather uniform, but adjacent grains often exhibit quite different levels of microstructural evolution. In some grains in which the precipitation was low, void swelling was present but at obviously lower levels.

of Fe-35Mn

at 520°C

and 14 dpa

Fe-IOCr-30Mn This specimen, as originally prepared, possessed a poorly polished surface and insufficient thin area. It was later flash-polished, yielding a thinner region for examination but also causing the ferrite precipitates to preferentially etch instead of the austenite matrix, as shown in fig. 7a. Once again it was obvious that ferrite formation had occurred. Since in this specimen the ferrite particles did not protrude from the foil edge and could not be easily analyzed for their composition, a broad beam EDX scan of a large area was performed. A drop in the overall iron concentration from 70 to 65% was observed, confirming that the lost ferrite precipitates were rich in iron. Fe-l OCr-20Mn As shown in fig. 7b, there is substantially less ferrite in this specimen, indicating that decreasing the manganese from 30 to 20% at 10% chromium affects the rate of formation of ferrite. The local swelling levels are in general larger than that of the Fe-lOCr-30Mn alloy. The dislocation evolution is easier to study in the absence of precipitation and appears to be typical of that observed in Fe-Cr-Ni alloys. The relative orientation

Fig. 7. (a) Preferential

attack

of ferrite particles during flash-polishing Fe-lOCr-2OMn after irradiation

of the ferrite precipitates was analyzed with respect to the matrix. They were found to exhibit the well-known Kurdjimov-Sachs relationship where (ill), 11(Oil), and --[iOf], II[1111,.

Fe- 1 Xr-20Mn The ferrite density in this specimen seems to have decreased somewhat relative to that of Fe-lOCr-20Mn but the swelling is comparable. The precipitates were found to have an average iron concentration of 75-808. 2.4. Examination

of specimens irradiated at 420°C

to 9

dpa Two specimens have been examined thus far, Fe-35Mn and Fe-30Mn. Both exhibited a high density of contrast features typical of low temperature irradiation. These were found to be small (< 500 A) dislocation loops, largely unfaulted, and small voids ( < 100 A). No precipitates were observed. Streaks observed in the diffraction patterns were found to be associated with a mixed oxide-hydroxide of iron and manganese on the surface of the foil. This oxidation reflects the lack of chromium in these alloys.

of Fe-lOCr-30Mn (52O”C, to the same conditions.

14 dpa);

(b) microstructure

of

blA. Garner et al. / Neutron irradiation of Fe - Mn, Fe - Cr - Mn and Fe - Cr - Nl I

I

I

I

0-

Fe-1OCdOMn IFe-lOCr-MMn , Fe-15Cr-16MnI I

SWELLING ” %

x-

FedCr-15Mn

AMCR 003

299

Fig. 8 confirms that the swelling rate at 520” C declines in the range of 14 to 49.8 dpa. The resulting tendency toward saturation of swelling appears to increase for a given manganese level as the chromium level decreases. Fig. 8 also shows that reductions in swelling can be accomplished via solute modification and/or thermal-mechanical treatment, using the commercial alloy AMCR 0033 as an example. Swelling data for other commercial alloys are given in table 4. The data for solute-modified alloys are presented elsewhere but show that solute modification and thermal-mechanical treatment can be used to reduce swelling [6].

3. Discussion 20 40 DISPLACEMENTS

0

80 60 PER ATOM

Fig. 8. Swelling of simple Fe-Cr-Mn alloys at 520°C in FFTF-MOTA. The swelling of commercial alloy AMCR 0033 is also shown.

2.5. Density change

measurements

for 49.8 dpa specimens

at 520°C

The tendency toward ferrite formation may increase with fluence beyond 14 dpa but this assumption has not yet been confirmed by microscopy. One consequence of such a behavior would be a reduction in the austenite fraction of the matrix, leading to a reduction in bulkaveraged swelling rate as the lower-swelling ferrite phase increases in volume.

Fig. 9 shows that the sensitivity of swelling of ironbased alloys to manganese content is less than that exhibited with nickel content at comparable irradiation conditions. Swelling in Fe-Cr-Ni alloys is also sensitive to chromium content, unlike the behavior shown in figs. 1 and 3. There are a number of reasons for these differences. First, the displacement rates in FFTF are 2-4 times those of EBR-II in the irradiation capsules employed. For Fe-Cr-Ni alloys this would tend to extend the temperature and composition-insensitive regime of swelling and we would anticipate a similar effect to occur in Fe-Cr-Mn alloys. Second, the sharp decrease in swelling with increasing nickel above a critical tem-

Table 4 Swelling of commercial alloys at 520 o C Alloy

Nitronic 32 18/18 Plus AMCRO033 AMCR 0033 AMCR 0033 NMF3 Nonmag 30 Nonmag 30 Nonmag 30

Condition

Swelling 14 dpa

49.8 dpa

cw cw CW CWA SAA cw cw

-0.02 - 0.28 0.62 -0.02 0.61 - 0.23 0.62

3.5 2.2 1.9 2.5 4.4 0.1 1.0

CWA SAA

- 0.02 0.17

0.42 2.87 10

CW = 1030 o C/O.5 h/air cool + 20% cold-work. CWA = cold-worked condition + 650 o C/l h/air cool.

SAA = 1030 o C/l h/air cool + 760 o C/2 h/air cool.

15

20

25 % NICKEL

a

35

40

OR MANGfaNESE

Fig. 9. Comparison of swelling of Fe-Cr-Ni and Fe-Cr-Mn alloys at comparable irradiation conditions [l].

45

perature is thought to arise from the effect of nickel in increasing the effective vacancy diffusion coefficient and thereby decreasing the void nucleation rate [7]. This role is surprising in that nickel is known to be the slowest diffusing element compared to iron and chromium and yet it enhances the diffusivity of all three species. Chromium is the fastest diffusing element but tends to decrease the overall diffusivity. In the Fe-Cr-Mn system, however, the diffusivities of chromium and manganese are nearly identical and are larger than that of iron [S]. Therefore, the decrease in swelling with manganese content cannot be ascribed simply to its effect on vacancy diffusion. The Fe-Cr-Ni system at comparable nickel levels does not exhibit a tendency toward ferrite formation during irradiation. The data presented in this paper show that there is a relationship in Fe-Cr-Mn alloys between the onset of precipitation and that of swelling and that there is also an influence of composition on precipitation. As shown in fig. 10 the two-phase ((Y + y) boundary lies within the lower manganese portion of our test matrix [9]. Therefore it is not surprising that the Fe-SCr-15Mn alloy transformed to ferrite, probably very early in the irradiation experiment. This would have substantially reduced the volume of austenite and probably accounts for the atypical swelling behavior shown in fig. 2. Chromium additions change the phase boundarier somewhat, with the observed consequence that ferrite

forms less quickly at higher chromium levels. There is not much impact of chromium level on swelling in the portion of the test matrix which lies outside the twophase regime, however. The most surprising result of this study is that the alloys at higher manganese content also form ferrite. Based on the formation of ferrite on void surfaces it is inferred that the phase decomposition is initiated via the inverse Kirkendall mechanism, whereby the slowerdiffusing iron segregates by default at the base of vacancy gradients developed at various microstructural sinks. This conclusion has recently been confirmed in electron irradiation studies by Takahashi and coworkers [lOI. The formation of less-dense ferrite precipitates will also lead to a contribution to the overall density change induced by radiation. However, ferrite and void formation will not be the only contributions. Fig. 11 shows that the density of the austenite phase exhibits a steep dependence on manganese content. Enrichment of the austenite in manganese content during ferrite formation will also cause the density to decrease. However, at 400° C both Fe-30Mn and Fe-35Mn

8.0

I

I

I

I

I

30

I 35

7.9

ORIGINAL DENSITY OF ANNEALED ALLOY

1400

7.6

gm/cm3

TEMPERATURE DC 1000

Y-Fe. Y-Mn

I’ ’ I’ : I : I I

8-Ml-l 7.7

7.6

I

/

1

15

26

25

wt 96 MANGANESE wt%Mn

Fig. 10. Phase diagram for Fe-Mn system [lo], showing range of manganese levels and irradiation temperatures explored in this experiment.

Fig. 11. Density of unirradiated Fe-Mn and measured in this study using an immersion Also shown are the densifications exhibited Fe-35Mn after irradiation to 9 dpa

Fe-Cr-Mn alloys density technique. by Fe-30Mn and at 420°C

F.A. Garner et al. / Neutron irradiation of Fe - Mn, Fe - Cr - Mn and Fe - Cr - Ni

were observed to increase in density, although very little swelling was observed and no precipitation occurred. This implies that the lattice parameter exhibits a maximum above 35% Mn and that the austenite matrix is decomposing into small regions with higher manganese levels, separated by regions of lower manganese level. A similar effect has been observed in Fe-Ni and Fe-Cr-Ni alloys at comparable high levels of nickel [11,12]. In a soon to be published paper Mekhrabov and coworkers have used the Mossbauer technique after 4 and 8 MeV electron irradiation at temperatures in the 200-400°C range to study the decomposition of Fe-xMn alloys, where x = 34, 40, 45, 50 and 55% manganese [13]. These studies showed that concentration inhomogeneities of the type discussed above indeed develop during low temperature irradiation.

4. Conclusions Unstabilized Fe-Cr-Mn alloys exhibit a more complex phase evolution than do comparable Fe-Cr-Ni alloys, involving ferrite formation under some conditions and micro-segregation without precipitation at other conditions. While ferrite forms naturally at some compositions, radiation-induced point defect gradients combined with the inverse Kirkendall effect also cause ferrite formation to occur in alloys that normally would be totally austenitic. The post-transient swelling behavior of Fe-Cr-Mn alloys is similar to that of Fe-Cr-Ni alloys with the exception that continued ferrite formation tends to decrease the swelling rate as the irradiation proceeds. Whereas the density changes observed in Fe-Cr-Ni alloys are largely due to void growth only, those in Fe-Cr-Mn alloys arise from a combination of void growth, ferrite formation and lattice parameter changes in the retained austenite. Preliminary results of irradiation of solute-modified and commercial Fe-Cr-Mn alloys show that swelling in this system can

be reduced by compositional mechanical treatment.

301

changes and/or thermo-

References [l] H.R. Brager, F.A. Gamer, D.S. Gelles and M.L. Hamilton, J. Nucl. Mater. 133 t 134 (1985) 907. (21 F.A. Garner, J. Nucl. Mater. 122 & 123 (1984) 459. [3] F.A. Gamer, J. Nucl. Mater. 133 & 134 (1985) 113. [4] F.A. Garner and H.R. Brager, in: Optimizing Materials for Nuclear Applications, Eds. F.A. Garner, D.S. Gelles and F.W. Wiffen (TMS-AIME, Warrendale, PA, 1985) pp. 87-109. [S] F.A. Garner and H.R. Brager, in: Twelfth International Symposium, ASTM SIP 870, Eds. F.A. Garner and J.S. Penin (American Society for Testing and Materials, Philadelphia, 1985 pp. 187-201. [6] F.A. Garner and H.R. Brager, Neutron induced swelling of Fe-Mn and Fe-Cr-Mn austenitic alloys, in: Proc. Thirteenth ASTM Intern. Symp. on Effects of Radiation on Materials, Seattle, WA, 1986. [7] F.A. Garner and W.G. Wolfer, J. Nucl. Mater. 122 & 123 (1984) 201. [8] H. Oikawa, Technology Reports, Tohoku University 47 (1982) 215; 48 (1983) 7. [9] Constitution of Binary Alloys, Ed. M. Hansen (McGraw-Hill, New York, 1958). [lo] H. Takahashi, F.A. Garner, H. Itoh, B. Hu and S. Ohnuki, in: Proc. Thirteenth ASTM Intern. Symp. on Effects of Radiation on Materials, Seattle, WA, 1986. [11] F.A. Gamer, H.R. Brager, R.A. Dodd and T. Lauritzen, Nucl. Instr. and Meth. B16 (1986) 244. [12] H.R. Brager and F.A. Garner, in: Optimizing Materials for Nuclear Applications, Eds. F.A. Garner, D.S. Gelles and F.W. Wiffen (TMS-AIME, Warrendale, PA, 1985) pp. 141-166. 1131 A.O. Mekhrabov, K.G. Bimratov, Amdulla 0. Mekhrabov and T.A. Shukyurov, The influence of electron irradiation on the structural state of y-Fe-Mn alloys, in: Proc. Thirteenth Intern. Symp. on Effects of Radiation on Materials, Seattle, WA, 1986.