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Local Atomic Redistribution Under Irradiation in γ –NiFeCr Alloys
A. Menshikov, C. Dimitrov, A. Teplykh
To cite this version:
A. Menshikov, C. Dimitrov, A. Teplykh. Local Atomic Redistribution Under Irradiation inγ–NiFeCr Alloys. Journal de Physique III, EDP Sciences, 1997, 7 (10), pp.1899-1908. �10.1051/jp3:1997231�.
�jpa-00249689�
J. Phys. III IFance 7 (1997) 1899-1908 OCTOBER1997, PAGE 1899
Loca[ Atomic Redistribution Under Irradiation in
~i-Nifecr Alloys
A-Z- Menshikov (~), C. Dimitrov (~,*) and A-E- Teplykh (~)
(~) Magnetic Neutron Diffraction Laboratory, Institute of Metal Physics, S. Kovalevskaya str. 18, 620219 Ekaterinburg, GSP-170, Russia
(~) C-E-C-M-, C-N-R-S, IS rue G. Urbain, 94407 Vitry-sur-Seine Cedex, France
(Received 13 June 1996, revised 5 March 1997 and 23 June 1997, accepted 27 June 1997)
PACS.61.12 Ex Neutron scattering techniques (including small-angle scattering)
PACS.6166.Dk Alloys
PACS.61.80.Fe Electrons and positron radiation effects
Abstract. The atomic redistribution under irradiation in austenitic Nifecr alloys has been investigated by diffuse neutron scattering measurements m three alloys prepared with the ~~Ni isotope: N150Fe42 5Cr7
51 N148Fe3sCri4 and N148Fe34Cr20. The measurements were performed at
room temperature on polycristalhne powder, irradiated at 310 °C with 2.5 MeV electrons to a fluence of I X 10~~ e~ cm~~. The Warren-Cowley short-range order parameters were determined for three coordination spheres. The degree of order in the irradiated alloys was higher than the
one obtained by thermal treatments at higher temperatures. A model for atomic redistribu- tion under irradiation is proposed: it consists of a Ni(Cr,Fe) type short-range ordering with a
tendency to the formation of mainly Ni-Cr pairs.
Rdsum6. La redistribution atomique induite par irradiation a dtd dtudide par mesures de diffusion diffuse de neutrons dans des alliages Nifecr prdpards h partir de l'isotope ~~Ni
:
N150Fe42,5Cr7.5, Ni4sFe3sCrm et N146Fe34Cr20. Les mesures out dt6 effctudes h la tempdrature
ambiante sur des poudres cristallmes, irradides h 310 °C par des dlectrons de 2,5 MeV h
une
fluence de I X lo~~ e~ cm~~ Les paramAtres d'ordre h courte distance de Warren-Cowley out dtd d6termin6s sur trois sphAres de coordination. Le degr6 d'ordre dans les alliages irradids est plus dlevd que celui obtenu par des traitements thermiques effectuAs h plus haute temp6rature.
Le modAle propos6 pour la redistribution atomique est un ordre local du type Ni(Cr,Fe) avec
une tendance h la formation pr6f6rentielle de paires Ni-Cr.
1. Introduction
Austenitic stainless steels based on +f-Nifecr alloys are prospective materials for use in the first wall of nuclear fusion reactors, due to their high corrosion resistance and to their po-
tentially small radiation swelling. Minimum swelling of these alloys is achieved for a nickel content of about 50 at.%. Many studies [1-10] have been devoted to this effect, but no general agreement on its explanation has been reached. Among other possibilities, the redistribution (*) Author for correspondence (e-mail: dimitrovaflglvt-cnrs.fr)
@ Les (ditions de Physique 1997
of atomic species under irradation has been considered. In order to document the existence of such a redistribution, we have determined, by thermal neutron diffuse scattering measurements, the changes in short-range order induced by irradiation in +f-Nifecr alloys with a composi- tion corresponding to the minimum of swelling, and we have compared the degree of order to the one achieved by thermal treatments only. An important residual electrical resistivity increase, produced by 2 MeV electron irradiation in the temperature range 75-410 °C iii] or
by fast neutron irradiation at 128 °C [12], in alloys with 16 at.% Cr and 20 to 75 at.% Ni, has been interpreted in terms of short-range ordering resulting from radiation-enhanced diffusion.
Therefore, it was interesting to confirm this interpretation by direct structural determina- tions. Local ordering produced by thermal treatments in +f-Nifecr alloys was suggested by
reversible variations of the residual electrical resistivity for increasing and decreasing temper-
atures [13,14], and has been directly evidenced by neutron diffuse scattering measurements
[15,16] in thermally aged samples.
The short-range order parameters cannot be determined in these alloys by X-ray diffraction
or by classical thermal neutron scattering measurements~ due to the small difference between the scattering amplitudes of the natural isotopes of iron, chromiui and nickel. Nevertheless, local atomic order can be investigated by neutron scattering measurements in alloys prepared from appropriate isotopes. Cenedese et al. [15], on three single crystals of the same chemical
composition (N123Fe56Cr21) but of various isotopic compositions, have evaluated the Warren-
Cowley parameters of the three pairs Ni-Cr, Ni-Fe, Fe-Cr in thermally aged samples. They showed that ordering occurs mainly between Ni and Cr atoms. The same method of isotopic substitution was used by Teplykh and Menshikov [16] in three annealed +f-Nifecr alloys of different compositions, doped with the ~2Ni isotope. Their results were consistent with the data obtained by Cenedese et al. [15].
The irradiation of specimens for neutron diffuse scattering experiments may be performed
by neutrons or electrons. Neutron irradiation was ruled out since the experiments required
a substantial mass of material, leading to a high level of radioactivity m Nifecr alloys as a
result of nuclear reactions; the long decay of the radioelements did not allow to work in safe conditions. Homogeneous electron irradiation of bulk crystals could not be achieved because of the limited electron range in the materials. For these reasons, thin layers of +f-Nifecr powders were irradiated with 2.5 MeV electrons and combined to obtain a sufficient amount of irradiated material of each alloy.
In the present work, isotopic substitution was used, similarly to the investigation by Teplykh
and Menshikov [16] of thermally-induced short-range order in the same three Nifecr alloys doped with ~2Nii this isotope has a negative scattering length bNi = -0.87 X 10~~2
cm which allows to approach the "null matrix" condition b
= ~jbj cj
= 0 and to measure the incoherent
(diffuse) neutron scattering with a sufficient accuracyJ in the range of the (100) and (l10)
reflections of the fcc lattice due to the weakness of the coherent reflections. In a ternary alloy,
the diffuse intensity is related to three sets ofwarren-Cowley parameters. Such a determination is not possible in polycrystals; therefore, for the analysis ofthe diffuse scattering measurements of the investigated alloys, the materials were considered as pseudo-binary Ni(Fe,Cr) alloys.
For determining the specific redistribution of each type of atoms in these ternary alloys, other methods such as anomalous X-ray diffusion which allows to change the contrast of the different species (but requires a high flux density for the measurements) would have to be applied.
N°10 LOCAL ATOMIC ORDER IN IRRADIATED +f-Nifecr ALLOYS 1901
Table I. Atomic composition, average scattering lengths $, total Laue monotonic differen-
tial cross sections ~bA bB)~cAcB, total incoherent differential cross sections az~zcII ~, lattice
parameters a, and Curie temperatures Tc of the three investigated alloys. (These two last parameters were determined on the irradiated samples).
Alloy b (bA amc/4~ a TC
composition 10~~~ cm 10~~~ cm~ at. ~~_sr~~ 10~~~ cm~ at. ~~ sr~~ (K)
N150Fe42 5Cr7 5 0.0072 0.728 0.049 3.584 685
N14sFe38Cri4 0.0043 0.667 0.053 3.578 480
N146Fe34Cr20 0.0044 0.616 0.056 3.575 362
2. Experimental
Three model fcc alloys of compositions N150Fe42 5Cr7
5, N148Fe38Cr14 and N146Fe34Cr20 were prepared from electrolytic chromium (99.9%), Armco iron (99.9%) and isotopic ~~nickel. Ac-
cording to the supplier certification. the ~~Ni enriched nickel had
an average scattering length
$Ni = -0.846 X 10~~~
cm. Using respectively for pure iron and chromium bfe
= 0.95 X 10~~~
cm
and bcr
= 0.353 X 10~~~ cm, the average values obtained for the scattering length of the three
completely disordered alloys are given in Table I.
After melting, the alloys were homogenized at 1000 °C for 100 hours. Then the ingots were
ground to a powder of 0.I mm particle size on an abrasive disc. These alloys are ferromagnetic
at room temperature, their Curie temperature (Tab. I) was determined by low field magne- tization measurements, and the separation between metallic and abrasive disc powders was performed with a permanent magnet The powder of each alloy was divided into two equal parts: the first one was considered as a reference of the completely disordered state of the
deformed material, the second one was used to determine the evolution of the structural state after the following successive treatments: I) quenching into water after a thermal treatment at 1050 °C under vacuum in a sealed silica tube, it) successive annealings for one hour at 400, 500 and 600 °C, iii) electron irradiation
The samples were irradiated under helium gas at 310 °C with 2.5 MeV electrons to a fluence of i X 10~~ e~ cm~~, on the Van de Graaf accelerator of the "Laboratoire des Solides Irrad16s"
(#cole Polytechnique, Palaiseau, France). The irradiation holder was a copper box (19 mm in diameter, 1.5 mm in depth) closed by a is pm stainless steel foil and containing 2.2 g of powder.
Several runs were performed for irradiating 6.7 to 7.9 g of powder of each alloy. The irradiation temperature was set to 310 ~ 0.2 °C and selected on the basis of previous determinations of a high degree of short-range order in electron-irradiated Ni-rich Nifecr alloys iii]. A chromel-
alumel thermocouple was located at the bottom of the box. Heat, resulting from the energy absorbed by the powder during irradiation, was evacuated to a water cooled coil, ma a stainless steel tube. The temperature was controlled by an additional heating resistor, wound on the
stainless steel tube and monitored by a temperature controller.
Neutron diffuse scattering mea8urements were performed at room temperature on a diffrac- tometer, 8et up in the IVV-2M reactor of the In8titute of Metal Phy8ics (Ekaterinburg, Russia). A neutron beam of wave-length 1
= 0.181 nm was produced by a monochroma-
tor consisting of two single crystals respectively of plastically deformed germanium and of
pyrographite. The incoherent and coherent neutron scattering intensities were measured in the range q = (4~ sine) Ii
= 0.8-4 5 i~~ which includes the (100) and 1110) superstructure
reflections and the (III) and (200) fundamental reflections. Previously to each neutron scatter-
ing measurement on the quenched, thermally treated or irradiated samples, measurements on the completely disordered reference powder, which does not exhibit any local order and has a random texture (Fig. 2a, curvel), were performed in the same conditions. The experimental in- tensities were converted into differential scattering cross-section units from incoherent neutron
scattering measurements on a vanadium sample. The validity of the comparison between the neutron patterns obtained in the different experiments is supported by observing the coherent scattering intensity of the (006) A1203 reflection which originates from a residual amount of
abrasive material in the alloy samples. It turns out that this intensity does not change for the different treatments of the alloys
3. Experimental Results
Figure I compares the neutron diffuse scattering patterns, observed in the 1.2-3.8 i~~
range of the transferred momentum q = 4~ sin H/~, in the irradiated N150Fe42 5Cr7
5 alloy, to the ones previously obtained in similar experimental conditions, after thermal treatments or quenching.
In the range of the (100) and (l10) reflections, an increase of the neutron diffuse scattering intensity was observed after electron irradiation of the alloy, as compared to the data of the
annealed and quenched samples The same behaviour was found in the two other alloys, indi- cating that irradiation induces a higher degree of atomic order than the one achieved by thermal
treatments only. The Warren-Cowley short-range order parameters of the alloys were evaluated from the experimental diffuse neutron scattering data. In a ternary alloy, the diffuse intensities
are related to three sets ofparameters corresponding to the three different atomic pairs. These
cannot be determined in polycrystals; consequently, the investigated alloys were considered as
pseudo-binary Ni(Fe,Cr) alloys. This assumption was based on the results obtained on ther-
mally aged +f-Nifecr alloys by Cenedese et al. [15] who found a strong interaction between chromium and nickel and no tendency to short-range ordering between iron and chromium. It
was also suggested by the existence of stable Ni~fei-x and Ni~cri-x intermetallics, whereas binary Fe~cri-~ compounds have not been evidenced at low temperatures. Therefore, the
determination of the short-range order parameters was limited to the interaction between Ni
atoms and average (Fe,Cr) atoms, considering a scattering length IA
" $Ni for the nickel (A)
atoms and an average scattering length bB
" cfebfe +ccrbcr for the iron+chromium (B) atoms
in the pseudo-binary AB alloys of composition Ni~(Fecr)i-x.
Under this assumption, we have determined the short-range order parameters for three coordination spheres. The diffuse scattering intensity in a partially ordered polycrystalhne binary AB alloy where cA and cB are the concentrations of atoms A and B is expressed as:
z=n
Iq = IL[q) l
+ ~j Z~az(rz)(sin qrz)/qrz (I)
I i=1
IL(q)
= (bA bB)~cAcB is the Laue scattering intensity; Z~ and r~ are respectively the coor- dination number and the radius of the I-th atomic sphere, q = (4~sinG)/~ is the transferred
momentum
The calculation of the Warren-Cowley parameters from relation ii) requires to know the difference between the modulated intensity Iq and the Laue intensiiy IL in absolute values.
We have determined this difference from two experimental measureljients. The first
one was
performed on the deformed material, which corresponds to a fully diso/dered state,
as evidenced
by the monotonous intensity variation in the range where the (100) and (l10) reflections should appear on the neutron diffraction patterns (Fig. 2a, curvel). From this, we deduce
N°10 LOCAL ATOMIC ORDER IN IRRADIATED +f-Nifecr ALLOYS 1903
2.0
j
~
1.6 .(
(loo) (I10) )(Ill) (200)
.(l13)
~
A12°3
'# '~ ~~~~~~~~~~
006)
T < AI~C~ if
G jj
] 0.8
u
-
(
c~t 0.4 1.2
c~ annealed
~
> ji 0.8
.2 °.~
l
< quenched
0. 8
~i
~'~
l.5 2.0 2.5 3.0 3.5
q=4T sin8/~ (i~~)
Fig. I. Neutron diffraction patterns of the N150Fe42 5Cr7
5 alloy, after quenching from 1050 °C (o), annealing at 400, 500, 600 °C for I hour at each temperature (A), irradiation at 310 °C by 2 5 MeV electrons (.).
the intensity ID which was considered as the sum of the following contributions: the Laue scattering intensity (IL), the nuclear incoherent scattering intensity (INIC), the spin nuclear
incoherent scattering intensity (IsNic), the magnetic incoherent scattering intensity (IM), the
multiple neutron scattering (IMS), the incoherent scattering intensity (IDis) due to the (static
and dynamic) displacement of atoms and the background intensity (IBG)1
ID ~ IL + iNIC + iSNIC + iM + iMS + iDIS + iBG. (~)
From the second measurenients performed on the quenched, or annealed or irradiated samples, assuming that these treatments do not change the different contributions considered above for the deformed alloys, we get the experimental diffuse intensity (Iexp):
Iexp = ID + IL(q) z=n~j Zzai(r~)(sinqr~)/qr~ (3)
z=1
a)
fl 2A e
) 2.0
z 1.6
H
l.2 2
I
# 0.8
0A b)
,
12 Ii
o
-0.4
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
q=4n sine/~ (A~l)
Fig. 2. a) Comparison of the diffraction patterns obtained for the N150Fe42 5Cr7
5 alloy -I) in the reference deformed sample (fully disordered) -2) in the irradiated sample (partially locally ordered).
b) Difference between the diffuse intensities obtained in the two order states.
Then, the modulation of the diffuse scattering was derived from equations (2) and (3):
IL(q) z=n~ ZzcYz(rz)(Sillqrz)/qrz
# Iexp ID " hi(q). (4)
z=1
Figure 2a compares the experimental diffuse scattering patterns determined respectively in the
completely disordered and in the irradiated N150Fe42 5Cr7
5 alloy; the modulation of the diffuse scattering intensity obtained by difference of the two patterns is given in Figure 2b. Figure 3
shows, for the three irradiated alloys, satisfactory fits of the experimental diffuse scattering data by the calculated lines. The observed small shifts might originate from displacement effects which have not been taken account in the analysis.
The values of the Warren-Cowley short-range order parameters, determined from the fits of the experimental difference curves for three coordination spheres, are given in Table II for the
three alloys treated in different conditions: I) quenched from 1050 °C, it) then annealed up to 600 °C, iii) finally electron irradiated at 310 °C. Short-range order is characterized by negative
N°10 LOCAL ATOMIC ORDER IN IRRADIATED +f-Nifecr ALLOYS 1905
~ ~ ~~ N150Fe42 5Cr75
0 2 '
o-o
-0 2 ~ °
] -o.4 m
G
m[ b) N148Fe38Cr14
Q~
$
~
~ -02
" -04
~~ N146Fe34Cr20
02 °
oo
-02
>
-0 4
0 1.5 2.0 2.5 3.0 3.5 4.0 4 5
q=4T sin8/~ (i'~)
Fig. 3. Modulation of the diffuse scattering intensity obtained by di?erence of the two patterns determined in the partially ordered state (after irradiation) and m the fully disordered state (reference)
in the three investigated alloys, a) N150Fe42 5Cr7
5, b) N148Fe38Cri4, c) N146Fe36Cr20. The solid lines, calculated for three coordination spheres, were fitted to the experimental data points (open dots).
values of the Warren-Cowley parameters in the first coordination sphere. From these values,
we can conclude that:
I) the highest degree of order is obtained in the irradiated alloys, for which the highest absolute value of al is found. Considering the relationship between the short-range order parameter al and the probability (pAA) for an atome A to be the nearest neighbour of an atom A:
FAA # CA + al CB, (5)
the average number of nickel atoms on the first coordination sphere around a nickel atom
(nAA = Z pAA) decreases under irradiation, since ai varies from 0 in the disordered state to a large negative value in the irradiated alloys. By contrast, the number of Ni-(Fecr) pairs
increases.
ii) the degree of order in the irradiated alloys becomes higher when the chromium concentration varies from 7.5 to 20 at.%. The height of the diffuse maxima observed in Figure 3 and the
absolute values of al (Tab. II) increase with increasing chromium content.
Table II. Short-range order parameters az for three coordination spheres of +f-Nifecr alloys: i) quenched from 1050 ° C, n) annealed successively at $00, 500 and 600 ° C for 1 hour, ni) irradiated at 310 °C iuith 2.5 MeV electrons. (uncertainty on az parameters: ~0.005 ).
Alloy treatment al a2 a3
composition N150Fe42 5Cr7
5 quenched 0.049 0 015 -0.017
annealed -0.093 0.134 0.001
irradiated -0,121 0.148 0.002
N148Fe38Cri4 quenched -0.048 0.018 -0 015
annealed -0.091 0.082 0.005
irradiated -0.126 0.089 -0.004
N146Fe34Cr20 quenched -0.042 0.017 -0.016
annealed -0.088 0.086 0.003
irradiated -0.152 0.098 0.008
4. Discussion
The observed formation of short-range order supports the interpretation proposed for the increase of the residual electrical resistivity under irradiation, in a temperature range where atomic diffusion results from interstitial and vacancy mobility [11,12].
To determine the type of local ordering in the inve8tigated +f-Nifecr alloys, we have con- sidered the possibility of atomic arrangements in the nearest coordination sphere, similar to the ones existing in the long-range ordered Nife, N12Cr and N13Fe compounds The maximum
values ay~ of the short-range order parameters, in the nearest coordination sphere of fully long-range ordered Ni(Fecr) alloys, were estimated according to the above three structures.
This was performed by introducing, in the expression: al
~ (FAA cA)/cB, derived from (5),
the following relations 11?]:
pAA = cA (nf~ni)/3cA for a Ni(Fe,Cr) type ordering
pAA = cA (nf~ni)/2cA for a N12(Fe,Cr) type ordering
pAA = cA (nf~il()/cA for a N13(Fe,Cr) type ordering,
where +f
= cA(I u) Iv for cA < u and
+f = cB for cA > u; u is the fraction of lattice sites
belonging to the A sublattice. q~ is a long-range order parameter defined so as to vary from 0 in the disordered state to I for maximum order, both in stoichiometric and in off-stoichiometric
compounds [18]:
~l* " (P~ CA)/~
where p( is the probability for a site of the A sublattice to be occupied by an atom A. Assuming
J~~ = I, we obtain:
of~~ = -+f~/3cAcB for a Ni(Fe,Cr) type ordering a/~~ = -+f~/2cAcB for a N12(Fe,Cr) type ordering af~~ = -+f~/cAcB for a N13(Fe, Cr) type ordering.
The calculated values ay~~ of the maximum short-range order paraldeters are given in Table III for the compositions of the three investigated alloys. In the irradiited alloys which have the
