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8/14/2019 Dispersion Behaviour of Oxide Particles in Mechanically Alloyed ODS
http://slidepdf.com/reader/full/dispersion-behaviour-of-oxide-particles-in-mechanically-alloyed-ods 1/4
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
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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
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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
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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