THE ATOM PROBE ANALYSIS OF A CAST DUPLEX STAINLESS STEEL

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THE ATOM PROBE ANALYSIS OF A CAST DUPLEX STAINLESS STEEL

T. Godfrey, G. Smith

To cite this version:

T. Godfrey, G. Smith. THE ATOM PROBE ANALYSIS OF A CAST DUPLEX STAINLESS STEEL.

Journal de Physique Colloques, 1986, 47 (C7), pp.C7-217-C7-222. �10.1051/jphyscol:1986738�. �jpa-

00225931�

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JOURNAL DE PHYSIQUE

C o l l o q u e C7, s u p p l 6 m e n t a u n o 11, Tome 47, N o v e r n b r e 1 9 8 6

THE ATOM PROBE ANALYSIS OF A CAST DUPLEX STAINLESS STEEL

T.J. GODFREY a n d G.D.W. SMITH

Department of Metallurgy and Science of Materials, University

of

Oxford, Parks Road, GB-Oxford OX1 3PH, ~ r e a t ~ ~ r i t a i n

A b s t r a c t

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Atom probe a n a l y s i s i s r e p o r t e d of a low Mo CF8 d u p l e x s t a i n l e s s s t e e l aged f o r 105,000h a t 280°C, 3,000h o r 70,000h a t 300°C, o r 3,000h a t 400°C. D e f i n i t e e v i d e n c e f o r a s p i n o d a l r e a c t i o n i n t h e a phase h a s been found a t a l l t h e t e m p e r a t u r e s s t u d i e d . This r e a c t i o n p r o c e s s i s most r e g u l a r and pronounced i n t h e m a t e r i a l aged a t 400°C b u t i s d e t e c t a b l e a f t e r t h e o t h e r h e a t t r e a t m e n t s . No evidence of G-phase p r e c i p i t a t i o n i s a p p a r e n t from t h e FIM micrographs, b u t s t a t i s t i c a l a n a l y s i s of t h e atom probe d a t a shows e v i d e n c e f o r t h e p r e s e n c e of s m a l l c l u s t e r s of S i and Mo atoms i n m a t e r i a l aged 70,000 h o u r s a t 300°C.

1

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INTRODUCTION

The a phase of d u p l e x s t a i n l e s s s t e e l s c a n undergo l o c a l c o m p o s i t i o n changes d u r i n g long t e r n a g e i n g i n t h e t e m p e r a t u r e range 300 t o 400°C [l-31. M i l l e r e t a l . have r e c e n t l y shown, u s i n g t h e FIM atom probe, t h a t i n a c a s t Mo-containing CF8M s t e e l t h e f i n e s c a l e p r e c i p i t a t i o n of a Mo, N i and S i r i c h G-phase o c c u r s , and t h a t s p i n o d a l decomposition of t h e a - m a t r i x a l s o t a k e s p l a c e (4-7). I n t h e p r e s e n t work, we have used t h e atom probe t e c h n i q u e t o s t u d y a s i m i l a r s t e e l of lower Ma c o n t e n t , CF8, comparing t h e r e a c t i o n s o c c u r r i n g a t 280, 300 and 400°C.

2

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EXPERIMENTAL

F i e l d i o n micrographs and atom p r o b e a n a l y s e s were o b t a i n e d u s i n g t h e Oxford FIMlOO atom probe (8).- Base p r e s s u r e s - w e r e t y p i c a l l y 5 x 10-I' mbar. Images were o b t a i n e d u s i n g neon a t -100K. Mass s p e c t r a were o b t a i n e d a t 65K o r loOK, u s i n g v o l t a g e p u l s e s , w i t h a pulse-to-standing v o l t a g e r a t i o of 15%. Some a n a l y s e s were c a r r i e d o u t i n vacuum, and o t h e r s i n t h e p r e s e n c e of t h e image g a s . No s i g n i f i c a n t d i f f e r e n c e s were found i n t h e a n a l y s e s o b t a i n e d under d i f f e r e n t e x p e r i m e n t a l c o n d i t i o n s . Four d i f f e r e n t m a t e r i a l s were s t u d i e d , and d e t a i l s of t h e i r c o m p o s i t i o n s and a g e i n g t r e a t m e n t s a r e summarised i n T a b l e s 1 and 2.

TABLE 1 BULK COMPOSITIONS OF CF8 TYPE STAINLESS STEEL (ATOMIC %) HEAT

KRB 280

Fe 66-46 66.46

Cr 23.01 22.60

N i 7.44 7.41

S i 2.27 2.65

Ma 0.1 0.14 Mn

0.31 0.50

N 0.15 0.11 C

0.28 0.13

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986738

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J O U R N A L D E PHYSIQUE

TABLE 2 FERRITE COMPOSITIONS OF THE FOUR MATERIALS STUDIED (ATOMIC %)

3

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RESULTS

a) Field Ion Images

In all materials studied, both ferrite and austenite FIM images appeared HEAT

KRB 280 280 280

relatively disordered, due to the large contents of alloy elements present.

Ferrite images were easily distinguishable from austenite images, because of greatly enhanced contrast between different crystallographic regions. Typical micrographs of each phase are shown in Fig.1. A striking difference between these ferrite micrographs and those obtained by Miller-et al. (4) from long term aged CF8M is the apparent absence of the brightly imaging G-phase. Faint traces of dark

'veining' can be seen in some ferrite micrographs, suggesting a fine scale

decomposition process is occurring, of the type reported by Brenner et al. and Zhu et al. in Fe-Cr and Fe-Co-Cr alloys (9-12).

AGEING 28ODC, -12 YEARS 300°C, 3,000 HRS 3OO0C, 70,000 HRS 40O0C, 3,000 HRS

Fe 65.3 66.5 64.7 64.2

b) Atom Probe Analysis

A typical mass spectrum of the ferrite phase is shown in Fig.2. In addition to the Fe, Cr, Ni, Si, Mn and Mo peaks small traces of Al, P and As are detectable.

Overall compositions of the ferrite regions of the four materials are summarised in Cr

26.2 24.3 25.6 27.4

Table 2.

Composition profiles through the ferrite regions of the four materials are shown in Fig.3. At least 30,000 ions were collected from each material.

Compositions are averaged over 100 ion blocks, and the vertical lines on the diagrams represent one standard deviation from the average analysis. All three long-term aged materials showed evidence of substantial local fluctuations in chromium content, indicating a spinodal decomposition reaction had occurred. The short-term aged material showed a less regular profile, with a small number of large composition fluctuations and a larger number of small fluctuations. (Note:

the depth scale of the composition profiles corresponds to -35 ions per (110) plane evaporated, for a typical D.C. tip voltage of IlkV).

Ni 4.8 4.7 5.4 4.7

Autocorrelation analyses for chromium are shown in Fig.4. All analyses are based on sample sizes of 100 ions, and extend over sequences of -30,000 ions. The material aged at 400°C showed the strongest periodicity. The shortest repeat distance observed was -708. A more prominent peak at -1408 appears to represent a multiple of this spacing. Specimens aged at 280°C and 300°C showed more limited periodicity. The autocorrelation curve for the material aged only 3000h at 300QC did not show any statistically significant long-range regularity, although from the corresponding composition profile it seemed that small regions of regular

fluctuations were present, suggesting that onset of decomposition had occurred.

In order to investigate the early stages of the decomposition process in more detail, frequency distribution curves were plotted for the chromium content of successive 100-ion data blocks from material aged for either 3000 hours or 70,000 hours at 300°C. The results are shown in Figure 5, the dotted lines indicating the values expected from a random binomial distribution function- The overall

distribution curve after 3000h did not differ significantly from the binomial, but Si

3.2 4.0 3.7 3.2

Mn 0.3 0.3 0.4 0.3

Mo 0.14 0.16 0.09 0.09

C'

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some s l i g h t broadening appeared a t t h e e x t r e m i t i e s of t h e c u r v e , g i v i n g t h e f i r s t i n d i c a t i o n t h a t t h e s p i n o d a l r e a c t i o n had begun. The d i s t r i b u t i o n o b t a i n e d a f t e r 70,000 h o u r s showed a much more e x t e n s i v e d e p a r t u r e from randomness, i n d i c a t i n g t h a t s u b s t a n t i a l decomposition had o c c u r r e d .

Contingency t a b l e s were c o n s t r u c t e d t o t e s t f o r t h e p o s s i b l e c l u s t e r i n g of S i and Mo w i t h i n t h e f e r r i t e , which would i n d i c a t e t h e p o s s i b l e ons t of f o r m a t i o n

7

of G-phase. One such s e t of t a b l e s i s shown i n F i g u r e 6. For t h e c a s e of m a t e r i a l aged f o r 70,000 h o u r s a t 30O0C, and t a k i n g a d a t a b l o c k s i z e of 34 i o n s ( i . e . one atom l a y e r ) , t h e r e i s e v i d e n c e f o r t h e c l u s t e r i n g of one o r more S i atoms w i t h one Mo atom. T h i s r e s u l t i s s i g n i f i c a n t a t t h e 0.3% l e v e l , and probably i n d i c a t e s t h e i n c i p i e n t f o r m a t i o n of G-phase. T h i s a c c o r d s w i t h t h e TEM r e s u l t s of M i l l e r and B e n t l e y [ 7 ]

-

4

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CONCLUSION

The f e r r i t e phase of t h e low-Mo d u p l e x s t a i n l e s s s t e e l CF8 undergoes a s p i n o d a l r e a c t i o n d u r i n g l o n g term a g e i n g a t e i t h e r 300 o r 400°C. The r e a c t i o n i s w e l l d e f i n e d a f t e r 70,000 h o u r s a t 300°C o r 3000 h o u r s a t 400°C. It a p p e a r s t h a t t h e p r o c e s s can j u s t be d e t e c t e d a f t e r 3000 hours a t 300°C. The presence of s m a l l c l u s t e r s of S i and Mo atoms i s d e t e c t a b l e a f t e r 70,000 h o u r s a t 300°C.

Acknowledgements

F i n a n c i a l s u p p o r t f o r t h i s work was provided by t h e C e n t r a l E l e c t r i c i t y Research L a b o r a t o r y and by t h e Science and Engineering Research Council. We t h a n k Dr.P.H.Pumphrey and Dr.K.N.Akhurst (CERC) f o r t h e i r a s s i s t a n c e , and a l s o

acknowledge u s e f u l d i s c u s s i o n s w i t h D r . M . K . M i l l e r (Oak Ridge N a t i o n a l L a b o r a t o r i e s , U.S.A.) and Dr.O.K.Chopra (Argonne N a t i o n a l L a b o r a t o r y , U.S.A.).

R e f e r e n c e s

[ l ] Chopra ^O.K. and A y r a u l t G., Nucl. E n g i n e e r i n g and Design

86

(1985) 69.

[ 2 ] Chung H.M. and Chopra O.K., Proc. 2nd I n t . Symposium on Environmental

Degradation of M a t e r i a l s i n Nuclear Power Systems

-

Water R e a c t o r s , Monterey, e d s . J.T.Roberts, J.R.Weeks and G.J.Theus, American Nuclear S o c i e t y (1985).

p.289

[ 3 ] Chopra O.K. and Chung H.M., Proc. 1 3 t h Water Reactor S a f e t y Research I n f o r m a t i o n Meeting, N a t i o n a l Bureau of S t a n d a r d s , G a i t h e r s b u r g , (1985).

[ 4 ] M i l l e r M.K., Bentley J., Brenner S.S. and S p i t z n a g e l J.A. J o u r n a l de Physique, Colloque C9, Supplement t o No.12

45

(1984) C9-385.

[ 5 ] M i l l e r M.K., B e n t l e y J., Brenner S.S. and S p i t z n a g e l J.A., Proc. 43rd Annual Meeting of EMSA, 1985, ed. G.W.Bailey, Publ. San F r a n c i s c o P r e s s , U.S.A.

p.326.

[ 6 ] Bentley J , M i l l e r M.K, Brenner S.S. and S p i t z n a g e l J.A., Proc. 43rd Annual Meeting of EMSA, 1985, ed. G.W.Bailey, Publ. San F r a n c i s c o P r e s s USA, p.328.

[ 7 ] M i l l e r M.K. and B e n t l e y J, t h i s volume.

[ 8 ] Cerezo A, Smith G.D.W. and Waugh A.R., J o u r n a l de Physique, Colloque C9, Supplement t o No.12

65

(1984) C9-329.

[ 9 ] Brenner S.S., M i l l e r M.K. and S o f f a W.A., S c r i p t a Met.

16

(1982) 831.

[ l o ]

Brenner S.S., Camus P., M i l l e r M.K. and S o f f a W.A., Acta Met.

32

(1984) 1217.

[ll] Zhu F , Wendt H and Haasen P., S c r i p t a Met.

2

(1982) 1175.

[ 1 2 ] Zhu F, Haasen P. and Wagner R., Acta Met.

34

(1986) 457.

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C7-220 JOURNAL

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(a) (b)

Figure 1 Neon FIM images of (a) austenite and (b) ferrite phases of CF8 steel.

Figure 2 High resolution atom probe mass spectrum of ferrite phase of C F 8 .

100 -

LOG - -

As ⁿ ^o

-

O 34 5 4

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I 00-

2 ,.

a

---i : : : : I ---&--+e----t--c-i--.-i

0 NUPlBER OF I O N S 5250 0 NUI'IBER OF IONS 5000

I

^--+--^4-- ^f^-^--1-- ^-^+-

---,--+

^-^--4

L

^: ^: ^{+ :} ^: ^: ^: ^: ^I

0 I ~ J I - ~ U ~ K IJF I UI-IS 50M0 0 NUPIBER OF IUNS 5000

Figure 3 Composition p r o f i l e s for the four ageing treatments studied.

(averaged over 100 ion blocks; v e r t i c a l bar = 1 standard deviation).

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JOURNAL

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Ruto-~orr=lation: Cr 3OO0C 3000 hr

HEAT 280

&to-correlation: Cr 300°C 70,000 hr

HEAT 280

shl-squared- 19 69

300°C 3000hr H e a t 280

Cr IONS IN SRnPLE

Ruto-correlation: Cr

300°C 70000hr H e a t 280

18 Cr IONS I N SRnPLE 5 8

+l

R(k)

Figure 5 Frequency distribution curves for Cr in material aged at 300°C.

(Sample size 100 ions).

OBSERVED EXPECTED

I

70 72

4 21 0 2 3

5 10 0 11

> = 6 1 0 0

Total 772 29 0 801

29DaC %I2 yrs KRB PUMP COVER

/\2

' V "-4 --"--%/AL,/ ^A-J- A*Lr-'.

,I

Si\Mo

166.7 6.3 0.0 173 69.4 2.6

22.2 0.8 0.0 23

10.6 0.4 0.0 11

> = 6 1.0 0.0 0.0

Total 772 29 0 801

-1

Figure 6 Contingency table for Si

+

Mo clustering in material aged for 70,000 hours at 300°C. (Sample size 34 ions).

-1/

I

B NUllBER Or 3RnPLES (k) llB 0 NUqBER OF SRnPLES (kt 180

Figure 4 Autocorrelation analyses corresponding to Fig.3. (Sample size 100 ions).