Some new aspects on microstructural and morphological evolution of thermally aged duplex stainless steels

Surface Science 266 (1992} 4119-415 North-Holland

surface

science

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Some new aspects on microstructural and morphological evolution of thermally aged duplex stainless steels F. Danoix, B. Deconihout, A. Bostel a n d P. A u g e r Laboratoire de Microscopie lonique, URA CNRS 808, B.P. 118, 76 134 Mont Saint Aignan, Frame Received 5 August 1991; accepted for publication 2 September 1991

Improvements in the experimental procedure and refinements in the statistical analysis of atom-probe data allow a more accurate description of the kinetics of spinodal decomposition in duplex stainless steels aged between "t00 and 400°C. TI~,: lerrite decomposition process at these temperatures appears to be more complex than previously .x,nsidered. Whereas the initial stages follow what is expected for a spinodal decomposition, the a-a' bimodal Cr-concentration frecuency distribution tends towards a multimodal one as aging proceeds and temperature increases. On the t2r-rich t~' side of the h,stogram a Cr-60% peak develops, while the predominant Cr 40% peak seems to be quasi-metastable. Moreover, concentration profiles exhibit several wavelength families. The first wavelengths which appear are on the order of a few nanometers. Upon prolonged aging, larger correlation distances, one order or two orders of magnitude larger, emerge, leading to a structure with internal similarities.

1. Introduction An important part of investigations related to the embrittlement of duplex stainless steels used in primary coolant pipes of PWR nuclear plants deals with the study of the microstructurai evolution of the ferritic phase. In the temperature range 300-400°C, in addition to the precipitation of fine intcrmctallic G-phase particlcs, the i r o n chromium solid solution decomposes accerding to the model of isotropic spinodal decomposition [1-3]. Fine scale concentration fluctuations develop in the ferritic phase, leading to the formation of a (Fe-rich) and c~' (Cr-rich) domains. This unm!xing process is considered as mainly rcspons i n e for the material embrittlement at smwice temperatures, although G phase precipitation should not be neglected [4]. The study of relationships between embrittlement and a,
factor5,. Indeed, the diameter of the analyzed surface (&~) continuously increases during analysis. giving rise to nonstafionm5, convolution effects with the .ine soak microstructure. In order to keep con o[ution effects constant, the atomprobe has ~,een modified. A variable aperture iris-diaphra .m has bcm: positioned between ~,le ~ip and ~he detector. It is therefore possiNc ~o keep &a (and subscquci;t!y spatial con~olutions) constant by reducing the diaphragm diameter during analysis. The optimal &a value is a compromise between the best siqnal-to-noise ratio and a spatial resolution as dr st. :iminative as possible. In the particular case of :: otropic chromium concentration fluctuations , eh wavelength of approximately 5 nm it, ,he L rr,tic phase of duplex stainless steels, &~ value ha,~ m e n evaluated to be 1 nm [6]. With these parameters, a classical sampling block size of N = 50 detected ions corresponds to an analysed cy~ir~dcr i nm in diameter a~d ! 2 nm in depth.

2. Experimental The studied material is a molybdenum bearing duplex stainless steel, the composition of which is

~, ~.~)2 - Elsevier Science Publishers B.V. A!I rights reserved

F. Danoir et al. / Erolution of thermally aged duplex stainless steels

410

Table 1 Ferrite composition (at.%) o f t h e studied duplex stainless steel as m e a s u r e d by m e a n s uf an e n e r g y - c o m p e n s a t e d a t n m - p r o b e Elemcm

Si

Cr

Mn

Ni

Mo

C

Fe

r : , ,;i,, c. ,,,i,~,:qlion

2.v; + 0.4

24.() -+ 1.5

0.4 + (I.I

6.1 * ! !)

_._ ' ~ + 0.3

0

balance

given table 1. Random an~,vses wine performed using an energy-compensate6 atom-probe with a tip temperature of 40 K and a puk~e to standing voltage ratio of 20%. The lateral res,~!utinn was kept constant at 1 nm. About 50000 a*oms (120~, nm) were collected for each aging treatment. Chromium concentration frequency distributions were obtained with a sampling block size of 50 atoms. Moving average was used to smood~ experimental histograms. The chromium concentration fluctuation amplitude values, AC, were deduced from frequency distributions and deconvoluted according to the model presented last year [6].

3. Amp[Rude of fluctuations

3. 1. Fit:s't stage of aging A~ 3()0°C, an'J aged up to 2500 h at 350°C, concentration profiles can roughly be described as ninus()idal conccntratkm waves (fig. la). On the basis of this assumpti,.m [7], simple analytical model: ~c~c dcxciopcd [2,8J to ~imulatc the ctJaresponding concentration frequency distributions. Simulated histograms are defined by a unique Table 2 A m p l i l u d c of c h r o m i u m c o n c e n t r a t i o n fluctuatk)ns ( a t , f ; ) . At 31|{F(', A ( ' is calculaled by m e a n s of sinus(tidal m o d e l (sin). At 35tF(', A(' can he calculated by means of sinusoidat m o d e l up 1o 2500 h, or by m e a n s o f LBM model for the whole kinetics, k ( ' values are d e c o n v o l u t e d according to [6] Aging lime

ll)l)ll 2"r'()ql l()(}(l() .qll}(ll)

parameter AC [8]. As shown fig. lc, simulated histograms fit well experimental ones. Table 2 gives AC deconvoluted values after 10000 and 30000 h at 300°C, and 1000 and 2500 h at 350°C.

3.2 ,c,,rond stage of aging Sinusoidal models fit well experimental distributions at 350°C at agings below 2500 h. Beyond this duration, statistical di¢~ance bf :aveen model and exp,,:; ...... :t^ ', d'~ r, atlor, s increases with aging time. Because concentration fluctuations loose their symmetrical nature, they can no longer be approximated to sine-like waves. A different approach has been initially developed by Hetherington et al. [9]. Based on the work of Langer et al. [10], this model, called LBM model, allows for the description of frequency distributions corresponding to nonsymmetrical

Cr

al ~,:

a

50

l O0

RIk)

b

C

,

--

nm SIM

O.5

Aging t e m p e r a t u r e (sin.)

(sin.)

_\("

A{ "

( '~,

("

('

10.()

25 24 24 23

24 I t) 18.5 15

26 2g 32.5 35.5

11.5

~).() q.5

.

~r

.3("

5.8

S.7

¢,.4

12.(~

7.2 6.1

15.{~ 17.4

j, --~ 5{} = 511 = 411 ----4q

It,

~,t

tlm

Fig. I. (a) Cr-cor~ccntration profile ~f the ferriti¢ phase of the studied duplex stainless steel aged 30,300 h at 308°C. (b) Cot l ~ p t m d i n g autocorrelogram. (c) C o r r e s p o n d i n g c o n c e n lrafion frequency distribution ( N = 50 a t . / b l o c k ) .

F. Danob~ et al. / Evo!ution of thermally aged duplex srainh'ss steels

concentration fluctuations. The basic hypothesis is that frequency distribution can be described as the sum of two Gaussians G~ and G_,. with the same width ~r and centred respectively on C and C,~,. Several improvements have been brought to the initial approach [9]: (a) In order to keep the model self-consistent, the calculated mean concentration has to be Co. This can be achieved by affecting each elemental distribution with relative weights A and B, in such a way that: A = ( C ~ , , - C o ) / ( C ., - C,,,) and

e = (Co- c.)/(c.,-

%

=

(I

-

[HV_ORETICAL IMATERIAL) L B.M DISTRIBUTION

\

/' .

.

C0)Co/N;

- the specific spread of concentration existing in the material [10], leading to a standard deviation called O'm~v Since these phenomena are independent:

.

.

JL

L

Within the framework of LBM model, Crm.~ is the standard deviation of the distribution obtained after adding the elemental Gaussians G: and G,. it can therefore be wriuen as

o-,,',,, = ~r ~ - C ,) + C(,( C . + C,, / - C C . . This relationship links a measurable parameter (Gm,,t) with C,, C,, and o~. Among these last three parameters, only two are independent (Co is constant), and can be modified in order to

Cry' ~ t

_

.

_

lO0%Cr I

~

1 / N ,
)

--i

~

~q G(~ = (1-Co)

CO,' N

q

i

L~:'_

S

Co

..... ._

~

100 % Cr

"~.

A

C .... !I f

"1

_,

~

I

!P

Co~

SIMULATED OBSERVED DISTRIBUTION 2

2

[

2

' - ~ . s ~ Otot = o o + Omat

C°~' ~~'-h

100 % Cr

Fig. 2. N a t u r e of the diffcren, s l a n d a r d deviations. ( a ) ~&,.,~ I~ the s t a n d a r d deviatitm for a LBM Iheoretical distribution. ( ( . = 25 a t . ' ; , (',, = 15 al.<;, (',., = 4(l at.~;. ~r = f) a L ~ ; a n d f = 4lie,: ). (b) sampling convolution due to the finilc numb~-r ol a t o n e , / bh)ck. .\, (c) ~imukqcd LBM ' " ' " tion of (a) a n d (b).

r c t ~ c h ~]lC bc,~[ fit ~ ! c ~ c e i ~

C X p c I i H l C H ~ d ;IDd ~iIll-

ukll~d distrihulions. This model has been applied to the 350°C aging treatment. As shown by fig. 3d, the fit with experimental distributions is very good. According to Park [1% the effective AC value is given by the following relationship: AC=(C,,-C,;/2

O'm,,t + G(~.

\

Ca Co

c.).

According to the lever rule, A and B, respectively, account for ce and a ' transformed volume fractions (A + B = 1). (b) With respect to C, and o- values, G~ may extend on the side of concentration negative values. In order to avoid this difficulty, G t was replaced, when necessary, by a binomial distribution, B,. B, has the same parameters as G, (C. and G). (c) The standard deviation of the experimental distribution, o%t, accounts for two different phenomena (fig. 2): - sampling fluctuations due to the finite number of atoms per block (N = 51) atomsk The related standard deviation ~r,) is given by

-t l I

+.o.

TaNe 2 gives AC and the a ' transformed volume fraction i f , ) , for the 350°C decomposition kinctits. An important point to outline is that the tv,,~ models (sinusoida] and I.BM) gi~c similar A( ~a[ucs f,>i i]h~. ~:a:!i~.-, n~,ing tim,:, (i~iiiil ,~nd 25Ii{~ h). ,:urthcrmorc, for these agm': tm~c..,, the ',~-~ume fraction of a' domains, ( J , ) i s equal to 50c~. according to the hypothesis of symmetrical concentration fluctuations. Conversely, when aging proceeds, f,. decreases down to approximately

F. Dano& et al. / Evolution of thermally aged duplex stainless steels

412

at % Cr

///'•

R(k) t"~"J

LI

100 nm D <

,.j

•"0.5\ A ~ - ~

t/

8

r,~

a.a r LI 0 t J20/nr~

SIM

---

•

I EXP

t•

Fig. 3. (a) Cr-concentration profile of the studied steel aged 10000 h at 350°C, thin line: smoothing window, A , equal to 1 nm; thick line: smoothing window, ,4 x, equal to 30 nm. (b) Autocorrelogram corresponding to the thick line profile ( A = 30 nm). (c) Autocorrelogram corresponding to the thin line profile (,..l~ = 1 nm). (d) Corresponding concentration frequency distribution ( N = 50 at./block).

40%. It confirms that fluctuations become asymmetric in relation to the m e a n concentration C,, As aging proceeds, LBM simulation slightly deviates from experimental distributions. This is mainly due to the cmcrgcncc of a third peak. centred on 69 at.q{ Cr (fig. 3d). Although present. this last contribution is ~-rcry~}'c~,lk ( ":: I'-'~ ~t the whole area). It therefore should not question the use of LBM model for 350°C heat treatments. The fact that it can only be detected with a lateral resolution of 1 nm indicates that the 60% Cr-rich domains are very small in extent ( = 1

I00 r m 2

a

20 nm

nnl

b ,,Rfk)

100 nm

A

).5

20 nm

05

t'-. . . .

C

I

V, 00'--,J .0eV'°m M_.. 0.5

d

2 nm

e

R(k) l

5 nm

~

0.25 nm

Fig. 4. (a) Cr-concentration profile of the steel aged 30000 h at 400°C, showing the different distance families. (b) Autocorrelogram obtained with A t = 40 rim. (c) Autocorrelogram obtained ~itil ,3~ = 8 nm. (d) Autocorrelogram obtained with A, = 0.8 nm. (e) Autocorrelogram obtained with ,.1, =11.05 rim.

Cr > 30 %

C

% all s

b

I

nq-I ).

1 "~ ':{ 60 %

3.3. Last stage of aging The characterization of the a'-oe' unmixed fertile aged at 400°C is much more complex than at 300 and 350°C (fig. 4a). Large differences in the shape of frequency distributions and conccnt,ation profiles are observed from one profile portion to another (fig. 5). This situation, which is very pronounced after 30000 k, is already observed after shorter aging times (250 h). Because

20

40.,_.~ 1 ~

•-

t.I ({- I1?

.0

40

60 M.~t ~q

Fig. 5 Cr-concentration fr, quency distributions for the fertile aged 37il)t!0 13 al 4Oil°(" (,'V : 50 at / b l o c k l , The original clmcentration protile (rig. 4a) had been cut up into 30 nm long elememaR profiles. (a) Corresponds to elemental profiles with a mean Cr-conccntration C m smaller than 20 at.¢~, (b) Corre• ....... ~' :c the ones with C m between 20 and 30 at.C4, (c) .,~.u,,~., Corresponds to the ones with C m greater than 3{) at.C}.

F. Dano& et aL / Evolution of thermally aged duplex stainless steers Table 3 Mea," distances (nm) between chromium peaks, ent aging times at 3(10 and 35(1°C Aging time (h)

tO00 2500 tO000 30000

d m

for differ-

4 i3

lengths. It is nevertheless difficult to determine these parameters for the longest wavelengths, because only few of them are observed.

Aging temperature 30WC

350°C

-

5 ± 1

4.0 ± 0.5 5.5 ±0.5

6± I 8 _+ l 9± t

of the experimental histograms diversity, it is quite impossible to define a unique AC value for samples aged at 400°C.

4. Evolution of wavelengths 4.1. Aging at 300°C

At 300°C, the concentration fluctuations can be described as sine shaped waves (fig. l a). It is therefore easy to d e t e r m i n e a single wavelength. The method which has been used [3] cons!sts of determining the m e a n distance between successive chromium peaks, dm. Table 3 sums-up d~ values fo; aging times of 10000 and 3(/000 h. It is also possible to use autocorretation profiles R(k } to determine correlation distances (fig. lb). Tl~c chara:,'teristic wavelengths, given by the mean distance between R ( k ) successive maxima, are very similar to d m values. 4.2. Aging at 350°C

Values of d m a r e given in table 3 for each aging time at 350°C. However, fluctuations with wavelengths one o r d e r of magnitude longer tend to develop. They can be shown on concentration profiles drawn with a large smoothing window (30 ~ ~,;,v I;. . . . . fig "z,~ ~ ~.. . . . . . . . . . ~ . . . . . . . . . relation profiKes (fig. 3b). As soon as these fluctuatkms are detected (~ _> 10000 h}, conccmrafio,,~ profiles must be described as the superimposition of two Feurier components. For this reason, at least 4 rmrameters are necessary to describe concentration profiles: two amplitudes and two wave-

4.3. Aging at 400°C

At this temperature, the results are more complex. Several wavelength domains develop with aging time as shown fig. 4a. At first sight, three wavelength ranges can be detected. The first one corresponds to characteristic distances larger than 100 nm, the second one in the range 15-20 nm and the last one corresponds to distances in the range 2 - 4 nm. A m o r e detailed study can be undertaken using c h r o m i u m autocorrelation profiles. The smoothing length was chosen in order to best show each family. At least 4 wavelength domains can be detected: - The first autoeorrelogram displayed in fig. 4b is obtained with a smoothing length of 4(} nm. It clearly shows evidence for fluctuations with wavelengths (A 1) larger than 100 nm, and amplitudes of 15 at.C~. - The second ene (fig. 4c. 8 nm smoothing length) shows evidence for the second family. with wavelength ( A , ) dose to 20 nm. I~ is noticeable tha~ R l k ) t h r e e successive maxima decrease wifla incrcaqng k. This i~ duc to Lhc first !emil5 fluctuations ( > 100 nm). - T h e third one (fig. 4d) is obtained with a smoothing length of 0.8 nm. A concentration signal with wavelength (A 3) nearly 2 nm, is observed, superimposed onto 20 to 30 nm fluctuations. - At last, with the smatlest smoothing window (0.05 nm), superimposed on a 20 nm fluctuation, 0.20 nm long perturbations are detected (fig. 4el. The existence of so short fluctuations l-~ d,,~: } i:; vl,,,ll~ UlI~,I~,.,LLJKL gr~ ~A~Jng~,L]li

~,[4J++.~.t d

ill~LO, lllUl~'lk.~.n

~TkYLIOL

of view. it i~ ran< ![kc]y d~.~c to prcfcrcnli:,d retention of Fe in the field evaporation s c e u e n c c Indeed, as suggested by FIM images (Cr-rich or' domains appear with a dark contrast), the activation energy for chromium field evaporation is lower than for iron. For a given evaporated atomic

414

F. Danoix et al. / Ecolution of thermally aged duplex stainless steels

plane, chromium atoms will tend to leave the specimen surface before iron atoms, giving rise to concentration modulations with a period of op.e atomic plane. lit is noteworthy that for these heat treatmems (350-400°C), the previously published results only deal with A 2 type fluctuations [13,14]. The question which arises is: why was the existence of the other families not reported? Concerning A 3 type, the reason is that the very high spatial resolution of the iris-diaphragm equipped atom-probe is able to detect wavelengths down to 2 nm. It is thus likely that during most of the classical atom-probe analyses (O,, = 2 nm), A 3 type fluctuations are averaged and cannot be distinguished from A, ones. The detection of A~ type fluctuations is again due to the new analysis method used. The reason is not directly due to the high lateral resolution but is a consequence of it: the possibility to perform very long analyses. Reducing the analyzed surface reduces the number of atoms detected per length unit. The 30000 atoms which can be detected within 8 h represent, for a classical analysis, approximately, between 100 and 200 nm; with an analysis diameter equal to 1 nm, they correspond to a~ analyzed depth of 800 nm. The detection of ~. lype fluctuations becomes possib!e. Nevertheless, only a few periods have been observed and a precise determination of their characteristics (A. _~(')is difficult. Furthermore, such fluctuations cannot bc detected l rum Small Angle Neutron Scattering (SANS) experiments, because the corresponding distances in reciprocal space are situated inside the central beam. The maximal lengths to be observed in SANS experiments are typically a few tens of nanometers.

5. Discussion The dctectitm~f I~ lypc tluCttlali~m,,isncx~. ~t can be shown that they are not due to an inhomogcnc[ty of the material, but are a characteristic ef the unmixing process. These fluctuations arc nei-

ther detected in the as received material, nor in the samples aged up to 30000 h at 300°C (fig. la). They appear and develop as aging proceeds at 350 and 400°C (fig. 3a and 4a). The concentration profile obtained after 30 000 h at 400°C (fig. 4a) has been cut up into 30 nm long parts (---A1/3) in order to separate Cr-enriched from Cr-impoverished domains. The different parts have been shared out amongst three categories, according to their mean chromium concentration, Cm: the first one for Cm ~<20%, the second one for 20% ~i 30%. The corresponding frequency distributions (displayed in, respectively, figs 5a, b and c) are very different. Their only common point is the presence of a peak at approximately 10-12%. This value, the same as encountered after 30000 h at 350°C [6], appears to be the chromium solubility limit in a domains at these temperatures. The general shape of experimental histograms is yet very different. For C m ~< 20%, the analyzed regions are weakly decomposed. The driving force is very low. On the other hand, for C m > 30%, the decomposition is very well pronounced, and can partly be characterized by the presence of 60% Cr-enriched domains. This is due to the fact that alloys with these composition are deeply inside the miscibility gap, where tile driving force is much higher than close to the spin',~da! [inc. The intermediate concentration category (2(IC~ <~ ('m ~ 3/ICi) cxhibils a less advanced decomposition, characterized by 40% Crenriched domains. The presence of several wavelength families coexisting in the aged material evokes a fractal structure. This idea has already been discussed by Camus et al. [15] and Hetherington et al. [16]. Such a description leads us to the idea that, although not observed, fluctuations with wavelengths one or two orders of magnitude larger than ~1~ cougd exist. The upper limit ibr such fluctuations would bc the ferrite grains size (a few b-m). it is aiso possible to imagine that the fractal nature could extend ~o ~hc who~e fertile and ;mstenitc micn~structurc. Bonnet [171 showed that the ferrite phase forms a sponge like continuous network which is highly interconnected w'[th the au.~;tcnitic phase. This microstructure is very

F. DanoLr et al. / Evohction of Uc'rmally aged &¢pl~:rseamless ste,4~

similar to the one observed in the fine scale c~-a" decomposition [ ! 8].

415

This research was financially supported by E.D.F., DtSpartemeni des Mat6riaux under conIracl E.D.F./C.N.R.S. no. ~1 ~'~'~

6. Conclusion "_( •

The new experimental methods developed allow for a more refined description of the spinodally decomposed ferrite. The unmixing process appears to be continuous, with an enhancing complexity as time and temperature increase. First stages may be described by means of analytical models, complexity of which increases with the decomposition progress: - One wavelength and one amplitude parameter, AC, are sufficient to characterize the solid solution at 300°C, and up to 2500 h at 350°C, - two wavelengths and three parameters (C,,, C,~, and o') for longer times at 350°C. For latest stages of unmixing, the existence of long-range order fluctuations ( > 100 nm) modifies the local thermodynamic equilibrium. The decomposed solid solution cannot be described any longer in simple analytical terms. The occurrence of several wavelength families with self-similarities evokes fractal structures. This could bc an al~.ernative, to the classical models, unable to describe these last stages.

Acknowledgements The authors would like to thank Professor D. Blavette, Dr. M. Guttmann (I.R.S.I.D.) and Dr. J.P. Massoud (EDF) for fruitful discussions, and Dr. S. Chambreland for ketpful assistance.

%'e[lc~s

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all0 ]t.'c'l~ll**l

i~

t~ (i,J,Jl!b

115} P P t'm~m',. \ V . . \ <';,~li;l. % 5 l-L,,_'r~,:z m..! k~ X \~ !},' Phys. (Paris) 45 (1984t C9-265. [16] M.G. Heiherington and M.K. Miller, J, Phys. (Paris) 50 ( 1989} C8-535. [17] S. Bonnet, J. Bourgoin, J. Champredonde, D. Gullmann and M. Guttmann. Mater. Sci. and Technol. 6 (1990) 221. [18] A. Cerezo and M.G. Hetherington, J. Phys. (Paris) 50 {I989} C8-523.