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JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 1600 1603

ispers ion beh aviour o f oxide part ic les in me chan ical ly a l loyed O S

steel

T. OKUDA, M. FUJIWAR A

Chofu- ldta P lant Ko be Steel Ltd. 2222-1 Ikeda On oe-cho Kakogaw a Hy og o 675 Japa n

Yt t r i um ox ide (Y203 ; y t t r i a ) i s one o f t he mos t

t h e r m o d y n a m i c a l l y s t a b le m a t e r i a l s , s o t h a t i t is

g e n e r a l l y u s e d a s a d i s p e r s o i d in m a n y k i n d s o f

d i s p e r s i o n s t r e n g t h e n e d a l l o y s . I n p r e v i o u s s t u d i e s

[ 1, 2 ] o f o x i d e d i s p e r s i o n s t r e n g t h e n e d ( O D S ) f e r r it i c

s t e e l s d e v e l o p e d f o r f u e l c l a d d i n g t u b e s i n f a s t

b r e e d e r r e a c t o rs , w e f o u n d a c o n f i g u r a t i o n c h a n g e o f

Y 2 03 p a r t i c l e s b y a a d d i t i o n o f a s m a l l a m o u n t o f

t i t a n i u m i n 1 2 C r b a s e f e r ri t ic s t e el s . T i t a n i u m

a d d i t i o n w a s v e r y b e n e f i c i a l i n p r o d u c i n g f i n e r o x i d e

p a r t ic l e s , a n d t h e r e f o r e c r e e p r u p t u r e s t r e n g t h o f T i -

c o n t a i n i n g O D S f e r r it i c st e e l w a s p a r t i c u l a r l y im -

p r o v e d . I n t hi s s t u dy , w e e x a m i n e d t h e m e c h a n i s m s

o f s i z e c h a n g e o f o x i d e p a r t i c l e s d u r i n g m e c h a n i c a l

a l l o y i n g ( M A ) a n d h e a t t r e a t i n g a f t e r M A .

T a b l e I s h o w s t h e c h e m i c a l c o m p o s i t i o n o f t h e

t e s t e d o x i d e d i s p e r s i o n s t r e n g t h e n e d s t e e l s . T h e a l l o y

i n t h i s s m d y c o n t a i n s a b o u t 1 0 t im e s t h e 5 1 20 3 a n d T i

a s a p r a c t i c a l o n e i n o r d e r t o i n c r e a s e t h e v o l u m e

f r a c t i o n o f o x i d e s a n d m a k e c l e a r th e s i z e c h a n g e o f

o x i d e p a r t ic l e s . M a s t e r a l l o y p o w d e r s w e r e a r g o n g a s

a t o m i z e d 1 3 C r b a s e d f e r ri t ic s t e e l w i t h a n a v e r a g es i z e o f 7 0 / x m ; o x i d e p o w d e r s w e r e p u r e Y 2 03 o x i d e

p o w d e r s w i t h m e a n d i a m e t e r 2 0 n m . T h e a l l o y a n d

o x i d e p o w d e r s w e r e m e c h a n i c a l l y a l l o y e d in a h i g h

e n e r g y a t t r it i o n b a l l m i l l . T h e m i l l c h a r g e c o n s i s t e d

o f a b o u t 1 k g w e i g h t o f a l l o y in g p o w d e r w i t h 1 5 k g

w e i g h t o f h a r d s t e e l b a ll s o f 9 . 5 m m d i a m e t e r . T h e

p o w d e r s w e r e a g i t a t e d f o r u p t o 4 8 h i n a n a r g o n g a s

a t m o s p h e r e a t a r o t a t io n a l s p e e d o f 2 9 0 rp m . T h e a s -

a t o m i z e d m e t a l p o w d e r s w i t h s p h e r i c a l e q u i a x e d

g r a in s b e c o m e m u l t i - la y e r e d f l a k y s h ap e b y k n e a d i n g

d u r i n g M A . P a r t o f t h e m e c h a n i c a l l y a ll o y e d p o w d e r s

w a s a n n e a l e d i n v a c u u m a n d t h e o t h e r p o w d e r s w e r e

p a c k e d i n m i l d s t e e l c a n s f o l l o w e d b y d e g a s s i n g a t

673 K in 1 .33 × 10 .2 Pa va cu um fo r 2 h , a nd ho t -

ex t ruded t o bar a t 1423 K and 1123 K.

A s - M A a n d a n n e a l e d p o w d e r s w e r e p r e ss e d i n to

d i s c s a n d e x a m i n e d b y X - r a y d i f f r a c t i o n , s m a l l a n g l e

X - r a y s c a t t ë r in g ( S A X S ) a n d t r a n s m i s s i o n e l e c t ro n

ù m i c r o s c o p y ( T E M ) . T h i n f o i l s f o r T E M s p e c i m e n s

w e r e e l e c t r o p o l i s h e d d i r e c t ly f r o m t h e d i sc . A

c o n v e n t i o n a l X - r a y d i f fr a c t io n m e t h o d w a s u t il i z e d

w i t h C u K « r a d i a t i o n o p e ra t in g a t 2 0 0 m A , 4 0 k V .

T AB L E I Chemical om positionof ODS ferriticsteel samples wt )

Alloy Fe Cr Ti Y203 O

3 Ti-3 Y~O3 Bal. 13.5 6 2.9 1 2.85 0.18Practical Bal. 13 0.3 0.3

1600

S A X S [ 3] u s i n g M o K « w a s o b t a i n e d f r o m a ro t a t i n g

a n o d e g e n e r a t o r o p e r a t i n g a t 2 4 0 m A , 5 0 k V . T h e

d i s p e r s o i d s i z e d i s t r i b u t i o n s i n M A p o w d e r s w e r e

d e t e r m i n e d b y a c a l c u l a t i o n o f l o g - n o r m a l d i s t r i b u -

t i o n p a r a m e t e r s f r o m t h e S A X S s p e c tr a .

F i g . 1 s h o w s X - r a y d i f f r a c t i o n p a t t e rn s o f M A

p o w d e r s m i l l e d f o r 4 8 h a n d p o w d e r s a f te r M A

a m l e a l e d a t te m p e r a t u r e s f r o m 1 17 3 K t o 1 57 3 K f o r

1 h . E v e r y d i f fr a c t io n p e a k o f a s - M A p o w d e r s w a s

i d e n t i f ie d w i t h t h e p e a k s o f b . c . c, i r on . P e a k s o f Y 2 0 3

o r a n y o t h e r k i n d o f o x i d e w e r e n o t d e t e c te d . T h i s

m e a n s Y 20 3 w o u l d b e d e c o m p o s e d t o y t tr i u m a n d

o x y g e n a t o m s d u r i n g t h e M A p r o c e s s a n d d i s s o l v e d

in t he 13 Cr f e r r it i c s t ee l . I t i s we l l kn ow n tha t

c a r b i d e , n i t r i d e o r i n t e r m e t a l l i c c o m p o u n d c a n

d i s s o l v e i n m e t a l d u r i n g M A b y f o r c e . I t i s k n o w n

t h a t o x i d e s s u c h a s Y 2 0 3 , w h i c h d o n o t h a v e a n y

s o l u b i l i t y l i m i t , c a n d i s s o l v e i n m e t a l d u r i n g a M A

process .

T h e p a t t e r n o f t h e a n n e a l e d p o w d e r s a t 1 17 3 K

a f t e r M A w a s t h e s a m e a s t h a t o f a s - M A p o w d e r s .

O n t h e o t h e r h a n d n e w p e a k s w e r e o b s e r v e d i n t h ep a t t e r n s o f p o w d e r s a n n e a l e d a t 1 2 7 3 K . T h e s e n e w

p e a k s w e r e c o n s i s t e n t w i t h y t t r i u m a n d t i t a n i u m b i -

o x i d e s a n d t i t a n i u m o x i d e s . T h e b i - o x i d e s w e r e

iden t i f i ed wi th Y 2TiO5 and Y2Ti207 . In an ann ea l i ng

p r o c e s s a b o v e 1 27 3 K , d is s o l v e d o x y g e n i n t h e M A

p r o c e s s w o u l d b e b o n d e d w i t h Y a n d T i t o m a k e

m o r e s t a b l e b i - o x i d e t h a n s i m p l e Y 2 0 3 m o n o - o x i d e .

Y - T l b i - ox i d e s h a v e tw o t y p e s o f c o m b i n a t io n o f

Y 2 03 + T i O 2 , i .e . Y 2 T i O 5 a n d Y 2 03 + 2 T i O » i .e .

Y 2 T i2 07 . B o t h b i - o x i d e s a r e t h e r m o d y n a m i c a l l y

c

10 20 40 602e degrees)

80

Figure 1 X-raydiffraction atternof M A powders: (a) as-MA;and heattreated (b) 1173 K/1 la; (c) 1273 K/1 h; (d) 1373 K/1 h; (e) 1473 K/1 la;(f) 1573 K/1 h.

0261-8028 © 1995 Chapman Hall



stable at elevated temperature, even a bove 2273 K

[4]. Residual titanium oxides, other than those

consumed for Y-Ti bi-oxides, were not ruti le type

TiO2, but non-stoichiometric TiOl.».

Fig. 2 shows transmission electron micrographs of

thin foils of MA powders. Even when the dislocation

density was extremely high, no precipitates or oxides

were obs erved in the matrix. Also, no carbides M23-

C6, TiC) or nitrides TiN) were observed. Solubility

limits of the interstitial atoms C, N) in the ferritic

steel are enlarged by M A [5] , We found that oxide

like Y203, which had no solubility limit originally

even at high temperatures near the melting point of

the alloy, could have some solubili ty by MA.

Figure TEM of th in fo i ls o f MA powder

16 1



Fig. 3 shows the size distribution of dispersoids,

which is analysed by SAXS, in as-MA and the

annealed pow ders. Althou gh the average size of Y203

before MA was 20 nm, most of the dispersoids

became smaller than 1 nm after MA. Because the

lattice parameter o f Y203 is 1.03 nm [5], the size of

dispersoids in MA powder, which is equivalent to

atomic state, practically corresponds to solid solution

in the ferritic steel. Although Y203 originally has

little solubility limit in solid steel, part of it can

dissolve in liquid steel. A metastable condition such

as l iquid or amorphous, which can dissolve any

oxide, would be produced not only by latt ice defects

generated by MA, but also by chemical reaction; for

example, reduction of oxides and rapid diffusion of

substitutional atoms.

The size of dispersoids after annealing ove r

1273K is larger than in as-MA. This means

precipitation of new dispersoids from the solid

solution. The size distributions of annealed powders

annealed at 1273 K and 1473 K are almost the same.

These kinds of Y-T i bi-oxides may be too stable togrow slowly even at high temperatures af ter nuclea-

tion of oxides. The size of dispersoids in titanium-

free 13Cr-3 7203 steel also reduces during the MA

process, but the size is not so small as in titanium-

added steel . Titanium may promote dissolving

reactions and may be able to reduce Y203 to yttrium

and oxygen atoms under low oxygen environments

as inside the ferritic steel.

In the hot-extruded bar, almost the same results as

for the annealed powder were obtained. The X-ray

diffraction pattern o f the bar extruded at 1123 K

showed only peaks of the b.c.c, structure of fenit ic

steel, but the pattern o f the bar extrude d at 1423 K

had Y-Ti bi-oxide and steel peaks. Fig. 4 shows thin

foils of heat-treated bar extruded at 1423 K an d cold

rolled (60 reduction) and annealed at 1473 K for

1 h. A uniform dispersion of very f ine oxides about

5 nm is observed. This microstructure is necessary to

obtain high creep strength alloy for long-time service.

Consequently it has been shown that Y203 particle

size becomes smaller with MA processing t ime, and

after 48 h MA , all the Y203 particles eventua lly

disappeared. In MA powders annealed above 1273 K,

precipitation o f Y2Ti207 and Y2TiO5 com plex fine

oxide particles with an average diam eter of about5 nm were observed. These results s how a solid

Latt ice Param eter of Y203

1.03 nm)100

~ 8 o i i i i ~ i ,--+ -.] - . ~........ .~-i-~44.

s ii ii \ i iiii

= 40 ~ i i / i i l i i

_~ 20

rr 0

0.1 1 10 100Diam eter of dispersoid nm)

Figure Size distribution of dispersoids: 13Cr-3Ti-3Y203 [] as-M A

(4811); O M A + 1273 K/1 h; A M A + 1473 K/1 h. 13 Cr- 3Y203 •

as -M A (48 11). • Y203 pow der.

1 6 0 2

Figure 4 TEM of thin foils of heat-treated bar.

solution of 7 2 0 3 oxide in the ferritic base matrix

during the MA proeess, and in the following

annealing process, a thermochemical reaction be-

tween Y203 and Ti occurred. This non-equilibrium



s o l i d s o l u t i o n s t a t e o f t h e o x i d e i n a f e r r it i c b a s e

m a t r i x w a s m a i n t a i n e d i n a h o t e x t m s i o n p r o c e s s a t

1 2 7 3 K . T h i s p r o c e s s i s c o n s i d e r e d t o b e e f f e c t i v e f o r

c o n t r o l l i n g m i c r o s t r u c t u r e s [ 6 ] r e c r y s t a l l i z a t i o n [7 ] )

a n d f o r i m p r o v i n g t h e m e c h a n i c a l p r o p e r t i e s o f O D S

s t e e l s a t e l e v a t e d t e m p e r a t u r e s .

R e f e r e n c e s

1. T . O K U D A , S . N O M U R A , S . S I L L K A K U R A , K .

A S A B E , S . T A N O U F a nd M . F U J I W A R A , i n Proceedings

of the International Con ference o n S o l i d s t at e p o w d e r

processing I n d i a n a p o l i s , U S A , e d i t e d b y A . H , C l a u e r a n d

J . J. d e B a r b a d i l l o , T h e M i n e r a l s , M e t a l s a n d M a t e r i a l s S o c i e t y

T M S , U S A , 1 9 9 0 ) p p . 1 9 5 - 2 0 2 .

2 . S . N O M U R A a n d T . O K U D A , ibid. T M S , U S A , 1 9 9 0 ) p p .

2 0 3 ~ 2 1 1 .

3 . J . J . S T E P H E N S a n d S . S P O O N E R , A e t a M e t a l l . 3 4 1 9 8 6 )

3 0 3 - 3 1 2 .

4 . R . S . R O T H , A . E . M c H A L E a n d R . S. R O T H , J . A m .

C e r a m . S o c . 6 9 1 9 8 6 ) 8 2 7 .

5 . M . R U H L E a n d T h . S T E F F E N S , Z M e t a l l k d 8 3 1 9 9 2 ) 4 3 6 -

4 4 0 .

6. T h . K E H A G I A S , L . C O H E U R a n d P . D E L A V I G N E T T E ,

J . Mater . Sc i . Le t t . 1 2 1 9 9 3 ) 1 0 5 9 - 1 0 6 1 .

7 . A . A L A M O , H . R E G L E , G . P O N S a n d J . L . B E C H A D E ,M a t e r . S c i . F o r u m 8 8 - 9 0 1 9 9 2 ) 1 8 3 - 1 9 0 .

R e c e i v e d 2 A p r i l

and accepted 5 June 995

16 3