Dispersion Behaviour of Oxide Particles in Mechanically Alloyed ODS
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JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 1600 1603 ispersion behaviour of oxide particles in mechanically alloyed O S steel T. OKUDA, M. FUJIWARA Chofu-ldta Plant Kobe Steel Ltd. 2222-1 Ikeda Onoe-cho Kakogawa Hyogo 675 Japan Yttrium oxide (Y203; yttria) is one of the most therm odynam ically stable materials, so that it is generally used as a dispersoid in many kinds of dispersion strengthened alloys. In previous studies [1, 2] of oxide dispersion strengthened (ODS) ferritic steels developed for fuel cladding tubes in fast breeder reactors, we found a configuration change of Y203 particles by a addition of a small amount of titanium in 12 Cr base ferritic steels. Titanium addition was very beneficial in producing finer oxide particles, and therefore creep rupture strength of Ti- containing ODS ferritic steel was particularly im- proved. In this study, we examined the mechanisms of size change of oxide particles during mechanical alloying (MA) and heat treating after MA. Table I shows the chemical composition of the tested oxide dispersion strengthened steels. The alloy in this smdy contains about 10 times the 51203 and Ti as a practical one in order to increase the volume fraction of oxides and make clear the size change of oxide particles. Master alloy powders were argon gas atomized 13 Cr based ferritic steel with an average size of 70/xm; oxide powders were pure Y203 oxide powders with mean diameter 20 nm. The alloy and oxide powders were mechanically alloyed in a high energy attrition ball mill. The mill charge consisted of about 1 kg weight of alloying powder with 15 kg powders were agitated for up to 48 h in an argon gas atmosphere at a rotational speed of 290 rpm. The as- atomized metal powders with spherical equiaxed grains become multi-layered flaky shape by kneading during MA. Part of the mechanically alloyed powders was annealed in vacuum and the other powders were packed in mild steel cans followed by degassing at 673 K in 1.33 × 10 .2 Pa vacuum for 2 h, and hot- extruded to bar at 1423 K and 1123 K. As-MA and annealed powders were pressed into discs and examined by X-ray diffraction, small angle X-ray scattëring (SAXS) and transmission electron ùmicroscopy (TEM). Thin foils for TEM specimens were electropolished directly from the disc. A conventional X-ray diffraction method was utilized with CuK« radiation operating at 200 mA, 40 kV. T A B L E I Chemical omposition of ODS ferritic steel samples wt ) Alloy Fe Cr Ti Y203 O 3 Ti- 3 Y~O3 Bal. 13.5 6 2.91 2.85 0.18 Practical Bal. 13 0.3 0.3 1600 SAXS [3] using MoK« was obtained from a rotating anode generator operating at 240 mA , 50 kV. The dispersoid size distributions in MA powders were determined by a calculation of log-normal distribu- tion parameters from the SAXS spectra. Fig. 1 shows X-ray diffraction patterns of MA powders milled for 48 h and powders after MA amlealed at temperatures from 1173 K to 1573 K for 1 h. Every diffraction peak of as-MA powders was identified with the peaks of b.c.c, iron. Peaks of Y203 or any other kind of oxide were not detected. This means Y203 would be decomposed to yttrium and oxygen atoms during the MA process and dissolved in the 13 Cr ferritic steel. It is well known that carbide, nitride or intermetallic compound can dissolve in metal during MA by force. It is known that oxides such as Y203, which do not have any solubility limit, can dissolve in metal during a MA process. The pattern of the annealed powders at 1173 K after MA was the same as that of as-MA powders. On the other hand new peaks were observed in the patterns of powders annealed at 1273 K. These new peaks were consistent with yttrium and titanium bi- oxides and titanium oxides. The bi-oxides were identified with Y2TiO5 and Y2Ti207. In an annealing process above 1273 K, dissolved oxygen in the MA more stable bi-oxide than simple Y203 mono-oxide. Y-Tl bi-oxides have two types of combination of Y203 +TiO2, i.e. Y2TiO5 and Y203 +2TiO» i.e. Y2Ti207 . Both bi-oxides are thermodynamically c 10 20 40 60 2e degrees) 80 Figure 1 X-ray diffraction attern of M A powders: (a) as-MA; and heat treated (b) 1173 K/1 l a; ( 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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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
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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
