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A STUDY OF SURFACE SEGREGATION OF A TYPE 304 STAINLESS STEEL BY ATOM-PROBE
K. Takahashi, Y. Ishikawa, T. Yoshimura, O. Nishikawa
To cite this version:
K. Takahashi, Y. Ishikawa, T. Yoshimura, O. Nishikawa. A STUDY OF SURFACE SEGREGATION
OF A TYPE 304 STAINLESS STEEL BY ATOM-PROBE. Journal de Physique Colloques, 1986, 47
(C7), pp.C7-233-C7-237. �10.1051/jphyscol:1986740�. �jpa-00225934�
JOURNAL DE PHYSIQUE
Colloque C7, suppl6ment au n o 11, Tome 47, Novembre 1986
A STUDY OF SURFACE SEGREGATION OF A TYPE 304 STAINLESS STEEL BY ATOM-PROBE
K. TAKAHASHI, Y. ISHIKAWA, T. YOSHIMURA and 0. NISHIKAWA"
Mechanical Engineering Research Laboratory, Hitachi Ltd., Kandatsu. Tsuchiura-shi, Japan 300
"Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama, Japan 227
Abstract - The uppermost layer of a type 304 stainless steel for use in an
-
& vacuum systen was characterized by an atom-probe. Specimns were prepared from a cold-worked wire heated at 100& for 5 min in vacuum. After heating at 45&, 600% and 80bC for 5 min in the atom-probe, surface
canpsitions were determined la er by layer. Tne surface Cr content was increased to 30 at .% in the 450g heating and to 50 at .% in the 600% and 800%
heating. The Cr segregated zone was limited to one or two atmic layers in each case. This was followed by a few atpic layers of a Cr depleted zone with a Cr content of nearly 0 at.% in the 450C heating. By raising the heating temperature to 600% and 800%, this Cr depleted zone was disappeared. Beneath the Cr
depleted layers, the Cr content was nearly the same value as in the bulk.
In the grain boundary, after heating at 600%, (Cr+Fe)23C6 and (Cr+Fe)f3 precipitates were observed.
I - INTRODUCTION
Austenitic stainless steels are widely used for building vacuum chambers and components because of their excellent corrosion resistance and outgassing characteristics./l/ In an ultrahigh vacuum system, baking procedure is routinely carried out for attaining the pressure lower than 10- Pa, because the outgassing rate can be greatly reduced by baking at elevated temperatures. In view of this procedure, the determination of the surface composition of stainless steel is particularly significant, because the surface composition must relate to the outgassing characteristics. Surface segregation of stainless steels has been extensively investigated by AES /2,3,4,5/, however, this does not provide the conposition of the uppermost layer, but the average compositions in the depth of several atomic layers. It is the uppermost layer that interacts with gas atoms, so the atom-probe is condsidered as a most powerful tool in providing the uppermost surface concentration profile.
With this is mind, we have undertaken an atom-probe study of surface segregation of a type 304 stainless steel in various heat treatments. During this investigation we have also observed grain boundary pricipatates in the stainless steel.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986740
C7-234 JOURNAL DE PHYSIQUE
I1 - EXPERIMENTAL METHODS
Our energy-focusing TOF atom-probe had been already described a t the l a s t IFES 161.
This atom-probe c o n s i s t s of three p a r t s , a storagelpreparation chamber, an FIM chamber and a TOF mass spectrometer i n the Poshenrieder configuration.
The specimens were prepared from a cold-worked type 304 s t a i n l e s s s t e e l wire of 0 . h d i m . heated a t 10007: for 5 min i n vacuum. A nominal c a n p s i t i o n of the wire was 19.7 a t . % Cr, 7.9 a t . % N i , 0.008 a t .% Mo and the r e s t Fe. The specimen t i p s were e l e c t r o l y t i c a l l y etched using a solution of concentrated HC1 a t 2-3 Vac. The t i p s could be heated r e s i s t i v e l y and t h e i r tenperatures were measured by a chromel- alumel thermocouple which was spot-welded t o the t i p Mo shank.
The as etched t i p s were f i e l d evaporated a t f i r s t t o r a v e the contaminated surface layers u n t i l t h e bulk composition of the steel was obtained. The t i p s were heated a t various temperatures for various durations of time. Finally each t i p was analysed layer by layer by atom-probe.
111 - RESULTS AND DISCUSSION
Fig.1 shows the atomic percentages of C r near the surface a s a function of the number of ions detected. Each d a t a p i n t corresponds t o the C r content per 30 ions detected and about 60 ions correspond t o one atomic layer throughout t h i s
investigation. Fig.2 shows the N i concentration p r o f i l e . These r e s u l t s were obtained from t h e same analysis for each heating temperature.
After heating a t 450k f o r 5 min, one atomic layer of Cr segregation was observed with the Cr content of about 30 at.%. This was followed by a few atomic layers of a C r depleted zone with a Cr content of nearly 0 at.%. Beneath the Cr depleted layers, t h e Cr content was nearly the same value as i n t h e bulk. On t h e other hand, N i was segregated in one atomic layer beneath the uppermst surface.
After heating a t 600k f o r 5 min, t h e surface Cr content was increased t o 50 at.%, while the Cr segregation was limited t o one atomic layer. And i n the Cr depleted zone, t h e C r content was about 10 at.%. N i behaved s i m i l a r l y as i n the 450C heating.
After heating a t 800'C f o r 5 m$, t h e concentration p r o f i l e of Cr was almost the same a s t h e one obtained by the 600C heating, except for a s l i g h t increase of the Cr segregated layers and t h e disappearance of the Cr depleted zone. N i behaved s i m i l a r l y a s i n the other beatings.
On the other hand, i n AES a n a l y s i s the reported surface C r contents were not more than 30 at.%, because AES r e s u l t s indicate t h e average value of more than two or t h r e e atomic layers. The average surface Cr contents over two or t h r e e atomic layers by the atan-probe analysis agree with the AES r e s u l t s . The impurities S and P segregations were observed up t o several percent i n the AES s t u d i e s /&/,while they were not observed i n our atom-probe analysis. The reason is not c l e a r a t present.
During t h e analysis, Cr-C p r e c i p i t a t e s were detected in the g r a i n boundary. Fig.3 shows the C r and C concentration p r o f i l e from t h e beginning of the analysis t o the detection of the second Cr-C p r e c i p i t a t e s . After t h a t , t h e analysis was continued u n t i l 6350 ions were detected and the t h i r d p r e c i p i t a t e y a s observed. During t h i s analysis, several heat treatments were undertaken a t 600C for the purpose of promoting t h e p r e c i p i t a t i o n . Each heating duration is indicated with the arrow i n the figure.
After the heating f o r a t o t a l of 50 min, the Cr content was abruptly increased t o 55
a t .% a s w e l l a s the C content.
I
II
II
1I
II
I0 50 100 150 200 250 300 350 400 450 CUMULATIVE NUMBER OF IONS DETECTED Fig. 1 The concentration profile of Cr as a function
of cumulative number of ions detected. One atomic layer corresponds to about 60 ions detected. The heating duration is 5 min at each temperature.
CUMULATIVE NUMBER OF IONS DETECTED Fig. 2 The concentration profile of Ni as a function
of cumulative number of ions detected. The
results were obtained from the same experiments
as in Fig. 1.
JOURNAL DE PHYSIQUE
Heating Temp. 600"
60 40 20 0
CUMULATIVE NUMBER OF IONS DETECTED
Fig.3 A composite atom-probe profile of a Cr segregated zone in a grain boundary. T h e heating temperature is fixed a t 600 C and t h e duration is dictated in the figure with a n arrow.
Table 1- Compositions of the precipitates.
Precipitate
the first
the second
the third
a total heating time a t 600°C
50 m i n
55 m i n
55 rnin
number of ions detected Fe Cr C rest 415 442 181 44 2 0 1 452 1 4 6 1 3 159 475 224 13
99 242 88 17
c o m p o s i t i o n
(11.9CrSll.lFe)23C6 (15~.9Cr+7.1Fe)~~C~
(5.2Cr+1.8Fe),C3
(16.3Cr+6.7Fe)23C6
Fig.4 shows the concentration of C and Cr+Fe for these p r e c i p t a t e s as a lzdder diagram. The upper figure shows the f i r s t p r e c i p t a t e and the lower the second one which was observed during detection of 2200 t o 4000 ions. The f i r s t p r e c i p i t a t e showed the c a n p s i t i o n of (Cr+Fe)23C6 and t h e second (Cr+Fe) C and
(Cr+Fe) C3. The t h i r d was similar t o the f i r s t one. ~ r o m d ~ ? h g s e p r e c i p i t a t e s , the Cr Jepleted zone was observed with the Cr content of about 10 at.% / 7 / . The concentration r a t i o between Cr and Fe i n these p r e c i p i t a t e s a r e l i s t e d i n Table 1. The Fe content decreased a s the heating repeated.
IV - CONCLUSION
The surface segregation of Cr of a type 304 s t a i n l e s s s t e e l was investigated.
After the heat treatment, t h e Cr content c l e a r l y increased i n the uppermst layer.
These Cr segregation were observed i n one or two atomic layers. And in the g r a i n boundary, t h e (Cr+Fe)23~6 and t h e (Cr+Fe) 7C3 p r e c i p i t a t e s were
detected.
REFERENCES
/1/ C Geyari, Vacuum 26,7 (1976) 287.
/2/ A.Boschi and C.FeEo, J.Vac.Sci. Technol, 16,4 (1979) 1037.
/3/ C.L. Briant and R.A.Mulford, Metall. TransTA, (1982) 745.
/4/ K.Yoshihara, M.Tosa and K . N i i , J.Vac. Sci. Technol. A,2,4 (1985) 1804.
/5/ R.O.Adams, J.Vac. Sci. Technol. A,1,1(1983) 12.
/6/ Y ,Ishikawa, K.Takahashi and T . ~ o s h h u r a , The 32nd IFES. (1985) t o be published.
/7/ A.Henjered, H.I\Jorden, T.Thorvaldsson anc! Fi-0. l . d r ~ n , S r r i n t a Met., (1983) 1275.
THE FIRST
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400 -
- THE SECOND PRECIPITATE 200 -
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