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8/10/2019 Grain Refinement Through Thermal Treatment
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G r a in R e f in e m e n t T h r o u g h T h e rm a l
C y c l in g in a n F e N i T i C r y o g e n i c A l lo y
S . J I N , J . W . M O R R I S , J R . , A N D V . F . Z A C K A Y
A t h e r m a l c y c l i n g t e c h n i q u e w h i c h a l l o w s t h e g r a i n r e f i n e m e n t o f F e - 1 2 N i - 0 . 2 5 T i a l l o y
f r o m 4 0 ~ 6 0 / z m ( A S T M 5 ~ 6 ) to 0 .5 ~ 2 / z m ( A S T M 1 5 ~ 1 8 ) i n f o u r c y c l e s h a s b e e n d e -
v e l o p e d . T h e p r o c e s s c o n s i s t s o f a l t e r n a t e a n n e a l i n g i n 9' r a n g e a n d ( a + 7 ) r a n g e w i t h
i n t e r m e d i a t e a i r c o o l i n g . T h e t r a n s f o r m a t i o n b e h a v i o r , t h e c h a n g e of m i c r o s t r u c t u r e s
a n d c r y o g e n i c m e c h a n i c a l p r o p e r t i e s o n e a c h c y c l i n g s t e p a r e d e s c r i b e d .
D u e to th e u l t r a f i n e g r a i n s i z e , th e d u c t i l e - b r i t t l e t r a n s i t i o n t e m p e r a t u r e o f t h i s f e r r i t i c
a l l o y i n C h a r p y i m p a c t t e s t i n g w a s s u p p r e s s e d b e l o w 6 K . I n f r a c t u r e t o u g h n e s s t e s t i n g
a t 77 K , t h e m o d e o f f r a c t u r e w a s a l t e r e d f r o m b r i t t l e q u a s i - c l e a v a g e t o c o m p l e t e d u c t i l e
r u p t u r e t h r o u g h t h e g r a i n r e f i n i n g .
R E C E N T r e s e a r c h i n t h i s l a b o r a t o r y 1-4 h a s s h o w n
t h a t a l l o y s f r o m t h e F e - N i - T i s y s t e m e . g . , F e - 1 2 N i -
0 .2 5 T i t c an b e p r o c e s s e d t o h a v e a p r o m i s i n g c o m b i -
n a t i on o f s t r e n g t h a n d t o u g h n e s s a t c r y o g e n i c t e m p e r -
a t u r e . A s e x p e c t e d , t h e d u c t i l e - b r i t t l e t r a n s i t i o n t e m -
p e r a t u r e s o f t h e s e a l l o y s t e n d to d e c r e a s e i f t h e e f fe c -
t i v e g r a i n s iz e i s m a d e s m a l l . H e n c e a m a j o r f o c u s of
o u r r e s e a r c h h a s b e e n o n t h e d e s i g n of p r o c e s s e s w h i c h
a c c o m p l i s h s i g n i f i c a n t g r a i n r e f i n e m e n t .
P r e v i o u s w o r k 5 -r e s t a b l i s h e d t w o t e c h n i q u e s f o r r e -
f i n i n g t h e g r a i n s i z e o f F e - N i a l l o y s o f m o d e r a t e n i c k e l
c o n t e nt . F o l l o w i n g s t u d i e s b y G r a n g e , 8 P o r t e r a n d
D a b k o w s k i s d e m o n s t r a t e d g r a i n r e f i n e m e n t t h r o u g h a
t h e r m a l c y c l i ng p r o c e d u r e w h i ch i n c l ud e d r e p e a t e d
c y c l e s o f r a p i d a u s t e n i t i z i n g a nd c o o li n g . G r a i n s i z e
a s s i n a i 1 a s 3 t o 5 ~ m ( m e a n i n t e r c e p t l e n g t h : A S T M
1 3 - 1 4 ) w a s o b t a i n e d . S a u l e t a l 6 a l s o o b s e r v e d g r a i n
r e f i n e m e n t i n m a r a g i n g s t e e l s o n s im p l e r e p e a t e d a u -
s t e n i t i z i n g ( f r o m A S T M 2 t o 7 i n 4 c y c l e s ) . A n a l -
t e r n a t e t e c h n i q u e w a s u s e d b y M i l l e r , 7 w h o r e p o r t e d
u l t r a f i n e g r a i n s i z e ( 0 .3 t o 1. 1 ~ t m ; A S T M 1 7 to 1 9 )
i n a n F e - N i a l l o y w h i c h h a d b e e n s e v e r e l y c o l d - w o r k e d
a n d t h en a n n e a l e d i n t h e t w o - p h a s e ( a + 7 ) r a n g e .
V a r i a n t s o f b o t h t h e s e p r o c e s s i n g t e c h n i q u e s w e r e
u s e d i n o u r r e s e a r c h 2 o n th e a l l o y F e - 1 2 N i - 0 . 2 5 T i ,
a n d b o t h l e d t o a l l o y s o f e x c e p t i o n a l s t r e n g t h a n d d u c -
t i l i t y a t c r y o g e n i c t e m p e r a t u r e . H o w e v e r , b o th p r o -
c e s s i n g t e c h n i q u e s ha d u n d e s i r a b l e f e a t u r e s . W h e n
t h e c y c l i c a u s t e n i t i z i n g p r o c e s s w a s u s e d , t h e g r a i n
s i z e s e e m e d t o s t a b i l i z e a t 5 t o 10 ~ tm . A f u r t h e r i m -
p r o v e m e n t i n l ow t e m p e r a t u r e d u c t i l i t y m i g h t b e o b -
t a i n a b l e w i t h f i n e r g r a i n s i z e . A m o r e r e f i n e d s t r u c -
t u r e c a n b e a c h i e v e d w i t h m e c h a n i c a l w o r k i n g , b u t
m e c h a n i c a l w o r k i n g is o f te n a n i m p r a c t i c a l o r u n d e -
s i r a b l e s t e p in f i n a l a l l o y p r o c e s s i n g . W e t h e r e f o r e
s o u g h t a n a l t e r n a t e t h e r m a l t r e a t m e n t w h i c h w o u l d
accomplish a grain refinement comparable to that ob-
tainable with mechanical work.
S. JIN, J . W. MOR RIS, Jr . , and V . F. ZACK AY are Assistant Re-
search Engineer, Associate Professor, and Professor, respectively,
Depa rtment of M aterials Science and Engineering, University o f
California and Center f or the D esign of All oys, Inorganic Materials
Research Division, Lawrence Berkeley Laboratory, Berkeley,
California 94720.
Manuscript submi tted January 16, 1974.
M E T A L L U R G I C A L T R A N S A C T I O N S A
A n i n i ti a l s o l u t io n to t h i s p r o b l e m w a s r e p o r t e d i n
R e f . 2 , w h e r e w e s h o w e d t h a t a n F e - 1 2 N i - 0 . 2 5 T i a l -
l o y c o u l d b e p r o c e s s e d t o a g r a i n s i z e n e a r 1 .0 ~zm b y
t r e a t i n g i t w i th a l t e r n a t e a n n e a l s i n t h e a u s t e n i t e ( 77
a n d t h e t w o - p h a s e ( ~ + 77 f i e l d s . T h e r e s u l t i n g a l l o y s
s h o w e d e x c e p t i o n a l d u c t i l i t y in f r a c t u r e t o u g h n e s s
t e s t s a t l i q u id n i t r o g e n t e m p e r a t u r e ( 77 K ) . T h e p r o -
c e s s i n g s e q u e n c e u s e d w a s , h o w e v e r , c o m p l e x , i n v o l v -
i n g n i n e s e p a r a t e a n n e a l i n g s t e p s . W e h a v e s i n c e f o un d
t h a t t h i s p r o c e s s i n g s e q u e n c e c a n b e s i m p l i f i e d c o n s i d -
e r a b l y w i t ho u t s a c r i f i c i n g t h e g r a i n r e f i n e m e n t o r t h e
l o w t e m p e r a t u r e d u c t i l i t y o f t h e r e s u l t i n g a l l o y s . T h e
g r a i n r e f i n e m e n t t e ch n i q u e s a n d a s s o c i a t e d c h a n g e s
i n l ow t e m p e r a t u r e m e c h a n i c a l p r o p e r t i e s a r e d e -
s c r i b e d i n t h e f o l l o w i ng s e c t i o n s .
I . TECHNICAL APPROACH
The technical approach leading to the grain refine-
ment processes used here and in Ref. 2 may be briefly
described as follows: The Fe-12 Ni-0.25 Ti alloy is ex-
pected to show roughly the same transformation be-
havior as the Fe-12 Ni binary, given the small amount
of Ti present and its partial consumption n scavenging
interstitials. Previous research on the transformation
behavior of Fe-Ni alloys in this composition range
summarized, for example, by Floreen9) indicates that
when the alloys are rapidly heated or cooled between
room temperature and the ~-field both the ~~ 7 and
T~ c~transformations occur primarily through a dif-
fusionless shear mechanism, while if the alloys are
annealed within the two phase c~+ 7) field the trans-
formation proceeds through a diffusional nucleation
and growth process leading to an equilibrium partition-
ing of nickel between the two phases. Either transfor-
mation mechanism may be used for grain refinement.
If c~is heated to the 7-field and then cooled to room
temperature, a decrease in apparent grain size or
martensite packet size) results, presumably to relieve
the internal strain built up during the shear transfor-
mation.6 If c~is annealed inside the two phase region
a very fine lath-like structure results, presumably
from preferential Y nucleation in the boundaries of the
martensite plates. 7
The fine lath-like structure obtained after two-phase
anneal is, however, not a desirable structure for high
VOLUME6A, JANUARY 975-141
8/10/2019 Grain Refinement Through Thermal Treatment
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Fe Ni PHASEDIAGRAM HEATTREATINGCYCLES
800 )
. . . . . . . . . . . . . . . . . . . . . . IA . . . 2A . . . . 73.0 C/ZHRS 7RANGE)
~600~ ~00 . . . . . . . . . . . . . . . . . . . . . ~ 18 _ 2B _650aC/2HRS r + RANGE)
I I I I - _ - R O O M T E M P E R A TU ~
e
20 40 60 80
Ni
ATOMICPERCENTIC;~JEL
XI~ 39 1884
F i g . l T h e F e N i e q u i l i b r i u m p h a s e d i a g r a m w i t h h e a t t r e a t
i n g c y c l e s .
t o u g h n e s s , a n d m a y e v e n c a u s e a d e c r e a s e i n t o u g h n e s s
d u e to e a s y c r a c k p r o p a g a t i o n p a r a l l e l t o t h e l a t h s . I n
M i l l e r s r e s e a r c h 7 t h e s t o r e d e n e r g y o f p r i o r c o l d w o r k
s e e m e d e f f e c t i v e in d e s t r o y i n g th e p r e f e r e n t i a l a l i g n -
m e n t o f t h o s e l a t h s . I t s e e m s p l a u s i b l e t h a t r e s i d u a l
s t r a i n f r o m a c y c l i c s h e a r t r a n s f o r m a t i o n m a y a c c o m -
p l i s h t he s a m e e f f e c t , p a r t i c u l a r l y i f t h e s t r a i n i s a l -
l o w e d t o a c c u m u l a t e t h r o u g h r e p e a t e d c y c l i n g .
W e h a v e , t h e r e f o r e , i n v e s t i g a t e d t h e r m a l c y c l i n g
t r e a t m e n t s o f t h e t y p e i l l u s t r a t e d s c h e m a t i c a l l y in
F i g . 1 ( th e F e - N i e q u i l i b r i u m p h a s e d i a g r a m w a s o b -
t a i n e d f r o m H a n s e n l~ i n w h i c h a u s t e n i t e r e v e r s i o n s
a r e a l t e r n a t e d w i th t w o - p h a s e d e c o m p o s i t i o n s . T h e
a n n e a li n g t e m p e r a t u r e s w e r e c h o s e n f r o m t h e r e s u l t s
o f d i l a t o m e t r i c s t u d i e s o f th e k i n e t i c s o f t h e p h a s e
t r a n s f o r m a t i o n s , w i t h t h e g u id i n g p r i n c i p a l t h a t t h e
a u s t e n i t i z i n g t e m p e r a t u r e s h o u l d b e l ow , t o m i n i m i z e
g r a i n g r o w t h , w h i le t h e t w o - p h a s e d e c o m p o s i t i o n t e m -
p e r a t u r e s h o u ld b e h i g h, to m a x i m i z e t h e r a t e o f d e -
c o m p o s i t i o n . T h e a n n e a l in g t i m e s a n d t o t a l n u m b e r o f
c y c l e s w e r e c h o s e n f r o m m e t a l l o g r a p h i c s t u d i es o f
g r a i n r e f i n e m e n t a n d l o w - t e m p e r a t u r e m e c h a n i c a l
t e s t s o f t h e r e s u l t i n g a l l o y s .
I I. M A T E R I A L S A N D PR O C E S S O F
G R A I N R E F I N E M E N T
a . M a t e r i a l s P r e p a r a t i o n
A lo w c a r b o n a l l o y of n o m i n a l c o m p o s i t i o n F e - 1 2 N i -
0 .2 5 T i w a s o b t a i n e d fr o m p u r e s t a r t i n g m a t e r i a l s ( 99 .9
p c t p u r it y ) b y i n d uc t i o n m e l t i n g i n a n i n e r t g a s a t m o -
s p h e r e . T w o t w e n t y p o u n d i n g o t s of 2 .7 5 i n . ( 7 .0 c m )
d i a m e t e r w e r e p r e p a r e d b y s lo w c a s t i n g i n a r o t a t i n g
c o p p e r c h i l l m o l d . I n go t c o m p o s i t i o n w a s d e t e r m i n e d
t o b e ( i n w t p o t ) : 1 2 . 0 7 N i , 0 . 2 6 T i , 0 . 0 0 1 C , 0 . 0 1 4 N ,
0 . 00 3 P , 0 .0 0 4 S , w i t h th e b a l a n c e F e . T h e i n g o t s w e r e
h o m o g e n i z e d u n d e r v a c u u m a t 10 50 ~ f o r 1 20 h , c r o s s -
f o r g e d a t ~ l l 0 0 ~ t o t h i c k p l a t e s 4 i n . (1 0 .2 c m ) w i d e
b y 0 .7 5 i n. ( 1 .9 c m ) t h i c k , t h en a i r - c o o l e d t o r o o m t e m -
p e r a t u r e . T h e p l a t e s w e r e t h en a n n e a l e d a t 9 00 ~ f o r
t w o h o u r s t o r e m o v e m o s t o f t h e p r i o r d e f o r m a t i o n
s t r a i n a n d a i r c o o l e d t o r o o m t e m p e r a t u r e . T h i s f i n a l
a n n e a l w a s i n c l u d e d t o e s t a b l i s h a s t a n d a r d i n i t i a l s t a t e
f o r r e s e a r c h p u r p o s e s ; o u r p r i o r w o r k h as s h o w n t h a t
t h e p r o c e s s e d a l l o y s h a v e s o m e w h a t b e t t e r c r y o g e n i c
m e c h a n i c a l p r o p e r t i e s w h en g r a i n r e f i n e d d i r e c t l y f r o m
t h e a s - f o r g e d c o n d i t i o n .
b . P h a s e T r a n s f o r m a t i o n S t u d ie s
D i l a t o m e t r i c s t u d i e s w e r e c o n d u c t e d t o d e t e r m i n e
p h a s e t r a n s f o r m a t i o n k i n e t i c s i n t h i s a ll o y . T u b e -
2
IC
,. I, 8
_z
4
~ 2
C
- 2
- ~
I I
As=675=C
MS= 75 *C
4 0 0 8 1 0
TEMPERATURE,C
a)
2 /
IO
8
~G
g2
5C
-2
i 1
iSOTHERMALDECOMPOSITK~N
AT 650 C
2
~ 20hrs _
M
i I
25 400 800
TEMPERATURE,
b)
F i g . 2 - - D i l a t o m e t r i c a n a l y s i s o f t h e p h a s e t r a n s f o r m a t i o n s i n
F e - 1 2 N i - 0 . 2 5 T i a l l o y , ( a ) on c o n t i n u o u s h e a t i n g a n d c o o l i n g ,
( b) o n i s o t h e r m a l d e c o m p o s i t i o n w i t h t h e ( ~ + 7 ) ra n g e f o r d i f -
f e r e n t p e r i o d s .
s h a p e d d i l a t o m e t r y s p e c i m e n s 1 .5 i n . ( 3 . 8 c m ) l o n g
a n d 0 . 2 5 i n . ( 0 .6 4 c m ) i n d i a m w i t h 0 . 0 4 i n . ( 0. 1 c m )
w a l l t h i c k n e s s w e r e m a c h i n e d f r o m t h e a n n e a l e d s t a r t -
i ng m a t e r i a l . T e m p e r a t u r e s w e r e m o n i t o r e d w i th a
c h r o m e l - a l u m e l t h e r m o c o u p l e , s p o t w e l d e d a t th e m i d -
p o i nt o f t h e s p e c i m e n l e n g t h . T h e e x p e r i m e n t s w e r e
c a r r i e d o u t w i th h e a t i n g a nd c o o l i n g r a t e s o f a p p r o x i -
m a t e l y 1 5 ~ r o u g h l y d u p l i c a t i n g t h e r a t e s u s e d
i n h e a t t r e a t m e n t o f t h e a l l o y .
S a m p l e d i l a t o m e t r i c c u r v e s a r e g i v e n i n F ig . 2 . F i g .
2 (a ) i l l u s t r a t e s t r a n s f o r m a t i o n b e h a v i o r w h e n an
( m a r t e n s i t e ) i s c o n t i n u o u s l y h e a t e d i n t o t h e ~ r e g i o n ,
t h e n c o o l e d t o r o o m t e m p e r a t u r e . R e v e r s i o n t o a u s t e n -
i re b e g i n s a t a t e m p e r a t u r e s l i g h t l y b e l o w th e m a x i m u m
i n t h e d i l a t i o n c u r v e a t 67 3 ~ ( w h i c h w e h a v e t a k e n a s
a m e a s u r e o f t h e a u s t e n i t e s t a r t t e m p e r a t u r e ,
A s ) a n d
i s c o m p l e t e d a t a t e m p e r a t u r e s l i g h t l y a b o v e t h e m i n i -
m u m i n t h e c u r v e a t 7 15 ~ ( w h i c h m e a s u r e s t h e a u s t e n -
i t e f i n i s h t e m p e r a t u r e ,
A f ) . O n
c o o l i n g, th e m a r t e n s i t e
t r a n s f o r m a t i o n b e g i n s n e a r 4 7 3~ M s ) a n d i s l a r g e l y
c o m p l e t e b y 4 12 ~ M r) . On c y c l i n g , t h e s a m p l e u n d e r -
g o e s a s m a l l p e r m a n e n t s e t , m e a s u r e d b y t h e o f f s e t i n
t h e c o o l i n g c u r v e a t l o w - t e m p e r a t u r e . T h i s o f fs e t r e -
s u l t s f r o m t r a n s f o r m a t i o n s t r a i n o f t h e s a m p l e a n d d o e s
n o t s ig n i f y t h e p r e s e n c e o f r e t a i n e d a u s t e n i t e ; n o a u s t e n -
i t e is d e t e c t e d i n X - r a y s t u d i e s o f s a m p l e s c y c l e d b a c k
t o r o o m t e m p e r a t u r e . F i g . 2 (a ) s u g g e s t s a n a u s t e n i t i z -
142-VOLUME 6A, JANUARY 1975 METALLURGICAL TRANSACTIONS A
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~~ o
T I M E P ~ U R 5
Fig. 3--Estimated isothermal transformation rate at 650~
I I
673~C
I ~ 5 ~ 6 2
,r
I
5
4
8 0 0
T E M P E R A T U R E ~
Fig. 4--(a) Dilato metry for an initia lly homogeneous spec i-
men (same as Fig. 2(a)), (b) dilatome try for a specimen ini -
tially decomposed at 650~ for 2 h. After the transfor matio n
of the duplex structure to ? is finished, the specimen was
held at 730~ for two hours for par tial homogenization and
then cooled to room temperature, (c) same as (b) except that
the homogenization was minimiz ed by holding at 730~ for
jus t a few seconds. (d) The homogenizati on cycle (b) was fo l-
lowed by anoth er con tinuous heat ing to the 7 rang e to see the
change in A s Af temperatures from those of (a) and (b).
ing tem per atu re of about 730~ to insu re that the t ra ns-
format ion i s completed , and a maximum two-phase de-
composi t i on tem per atu re of about 650~ to insu re that
shear reve rs ion has no t become s ign i f i can t.
Di la tomet r i c curves moni tor ing i so therm al decompo-
siti on at 650~ are shown in Fig. 2(b). The
M s
t e m -
pera tur e i s essen t i a l ly independent o f t rans f ormat io n
tim e (t___ 30 min.) indi cat ing that the n icke l co ntent of
the 7 formed is e ssen t ial ly independent of the 7 volume
fract ion . Thi s 7 apparen t ly t ran s for ms complete ly to
a on cool ing; no reta ined au steni te is detected in X-
ray diffract ion analyses of samples cooled to room
tem per atu re. (We do, however, f ind reta ined austeni te
after decomposi t ion at te mpe rat ure s < 6000C).
Given the cur ves in Fig. 2(b) we can phras e a rough
est i mate of the rate of decompo si t ion which is useful
in select in g an ann eal ing t ime. If we assume that the
d i l a t ion i s a l ine ar funct ion of the f rac t ion t ra ns fo rmed
and es t imate the d i l a t ion on complete t rans forma t ion
fro m the total offset in the cur ves of Fig. 2(a), we ob-
ta in the es t imated i so thermal t ran s form at ion curve
shown in Fig. 3. This curve has an asymptote indica t-
ing equi l i briu m with s l ight ly more than 80 pct 7, which
agre es roughly with an est imate based on the Fe-N i
bin ary phase diagram . We infer from Fig. 3 that an
ann eal ing time of 2 h at 650~ will yield a product
which is roughly 60 pct 7, providing an overal l grain
ref ine ment near the op t imum. This es t imate i s sup-
por ted by the meta l lographic re su l t s g iven below.
If the 730~176 cycle is repea ted after decom -
posi t ion at 650~ the kinet ic s of t ra nsf orm ati on are
complica ted by the inhomogeneou s dis t r ibut ion of
n ickel th rough the micros t r uctu re . A sample annealed
for two hour s at 650~ will co nsi st of roug hly 60 pct 7
of composi t ion ~ 13 pct Ni (est imated from the Fe-Ni
b inar y phase d iagram) together wi th ~40 po t res idual
a of mean nickel concen trat ion nea r 10 pct (wel l above
the equi l ibr ium at ~ 4 pct Ni) which may be heter oge-
neously dis t r ibuted. If the mater ial is ai r cooled to
room temper ature and then reheated to 730~ reg ions
of d if feren t Ni concent ra t ion wil l t rans f orm at d i f feren t
t emp erat ures (and perhaps by d i f feren t mechanisms)
lead ing to the more complex t rans for mat io n behavior
i l lu stra ted by the di lato meter t r ace shown in Fig. 4.
I t nonetheless appears tha t the p ronounced shear r e-
vers io n i s essen t i a l ly complete a t 730~
Tran sforma t ion behavior on subsequent coo l ing de-
pends on the extent of homo geni zat ion at 730~ and
hence on the t ime of anneal , as i l lus t ra ted by curves
(b) and (c) of Fig. 4. An an ne al of 2 h at 730~ is not
sufficient to homogenize the al loy. The cont inued pr es -
ence of regions of relat ive ly high Ni concent rat i on is
indicated by the su ppre ssio n of the A s t em p era t u re o n
reheat ing (curve (d) of Fig. 4). However, after the sec-
ond 2 h ann eal at 730~ the A s t emper a ture in the nex t
heat ing l ies above 650~ indicat ing that shea r rev er -
s ion to 7 does not beco me pron ounced unt i l the temp er -
ature is raised above 650~
c. Micros t r uctura l Changes on Cycl ing
On the bas i s o f d i l a tomet r i c and micros t ructura l
s tud ies we se lec ted a g ra in ref in ing process cons i s t ing
of alt er nat e two- hour ann eal s at 730~ and 650~ A
four-cycl e p rocess i s shown schemat ic a l ly in Fig. i ,
where the succ essi ve s teps are label led 1A (730~
IB (650~ 2A (730~ and 2B (650~ The evo lut ion
of the micr os t r uctu re dur ing th i s cycl ing i s i l lus t ra ted
by the opt ical micrographs given in Fig. 5 and by the
scanning e lec t ron micro graphs in Fig. 6. Af ter f ina l
cycle 2B the bulk of the mic ros tru ct ure consi s ts of a
fine mixture of platelet grains approximately 1 to 4
microns long and a fract ion of a micron in the short
d imens ion . The preferen t i a l o r i en ta t ion of these gra ins
has been l argely e l iminated .
A p rec i s e ch a rac t e r i za t i o n of th e m i c ro s t ru c t u re s
developed during this thermal cycl ing depends on the
re s u l t s o f t r an s m i s s i o n e l ec t ro n m i c ro s co p y s t u di e s
now in pro gre ss . The general features of the grain
ref ine ment p rocess are , however , apparen t in Fig . 5
and Fig. 6. Fig. 5(a) shows the mic ros tr uct ure of the
annealed s tar t ing mater ia l . The apparen t g ra in s i ze
is 40 to 60~m (ASTM 5 and 6). After cycle lAt he
appar ent gra in s ize (Fig. 5(b)) has redu ced to ~15 ~m
(ASTM 9). These gr ains cons is t of blocky laths of
d i s located mar te ns i t e . Dur ing cycle 1B the mar tens i t e
i s i so ther mal ly decomposed to g ive the micro s t ru c-
ture shown in Fig. 5(c) and in Fig. 6(a). T he mi cro -
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a) c)
b) d)
Fig. 5--Change of optical mic ros tru ctu res on therma l cyclings. a) star ting micr ostr uctu re 900~ 2 h annealed), b) after cycle
1A, c) afte r cycle 1B, d) afte r cycle 2A, e, page 145) afte r cycle 2B final mic ros tru ctu re) .
s t ructure after cycle 1B is not a desirable one for low
tempe ratur e toughness . The s t rong prefe ren t i a l o r i en-
ta t ion of the micros t ructura l fea tures p rov ides po ten-
t i a l pa ths for eas y crack propagat ion . The preferen t i a l
o r ien ta t ion i s l a rge ly removed dur ing the second 730~
650~ cycl ic t rea tmen t .
The micros t ruc ture af t er cycle 2A is shown in Fig.
5 d) and Fig. 6 b). The evident micr os tru ct ura l
changes are two. Fir s t , a react i on which we ass ume
to be an homogenizat io n rea ct io n ini t iate s along lath
packet boundar ies in the micro s t ru ctur e 1B, g iv ing
rise to the network of the white regions in Fig. 5 d).
Second, there is some decompos i t ion of the al igned
laths of the 1B structu re.
The oriented substructure of the al loy is broken up
during the next two-phase decomposi t ion, c ycle 2B.
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o s t a t . S u b s i z e s p e c i m e n s o f 0 . 5 i n . (1 . 27 c m ) g a u g e
l e n g t h a n d 0 .1 2 5 i n . ( 0 .3 2 c m ) d i a m w e r e t e s t e d a t a
c r o s s h e a d s p e e d o f 0 .0 2 i n . / m i n . ( 0. 05 c m / m i n . ) . T w o
s p e c i m e n s w e r e t e s t e d a t e a c h s t a g e of t h e c y c l i n g
t r e a t m e n t . T h e d e v i a t i o n in y i e l d s t r e n g t h b e t w e e n
t e s t s w a s ~ 5 k s i ( 35 n e w t o n / m 2 ) .
(e)
T h e r e s u l t i n g m i c r o s t r u c t u r e i s s ho w n i n F i g . 5 ( e) a n d
a n d i n F i g . 6 (c ) . T h e r e s u l t i n g g r a i n s i z e ( 0. 5 t o
2 .0 m ; A S T M 1 5 t o 1 8 ) i s c o n s i d e r a b l y b e l o w t h e
b e s t w e h a v e b e e n a b l e t o o b t a i n w it h s i m p l e c y c l i c
a u s t e n i t i zi n g t r e a t m e n t s a n d is c o m p a r a b l e t o t h at
M i l l e r 7 o b t a i n e d a f t e r s e v e r e c o l d w o r k i n g .
T h e g r a i n - r e f i n e d a l l o y i s a p p a r e n t l y c o m p l e t e l y
f e r r i t i c . N o r e t a i n e d a u s t e n i t e w a s d e t e c t e d u s i n g c o n -
v e n t i o n a l X - r a y t e c h n i q u e s . R . L . M i l l e r n c o n f i r m e d
t h i s r e s u l t u s i n g a m o d i f i c a t i o n of h i s r e p o r t e d t e c h -
n i q u e s 12 '~ s w h i c h p e r m i t s d e t e c t i o n o f a s l i t t l e a s 0 . 1
p c t r e t a i n e d a u s t e n i t e .
W e h a v e e x p l o r e d t h e e f f e c t o f a d d i n g a d d i t i o n a l
7 3 0 ~1 7 6 c y c l e s . T h e s e d o g i v e a n a p p a r e n t a d d i -
t i o n a l r e f i n e m e n t o f t h e l a r g e r g r a i n s r e m a i n i n g i n t h e
m i c r o s t r u c t u r e , b u t t h e e f fe c t i s s m a l l a n d h a s n o o b v i -
o u s i n f lu e n c e o n c r y o g e n i c m e c h a n i c a l p r o p e r t i e s .
III. MECHAN ICAL PROPER TIES AT
CRYOGENIC TEMPERATURES
The mechan ical test specim ens were taken from the
material prepar ed as descri bed in section II a). The
ann eal ed 0.75 in. 1.9 cm) thick plates were cut into
piece s 2.75 in. 7.0 cm) long. Th ese piece s were then
given selected therma l pr ocessing. After processing,
one fract ure toughness specimen and two Charpy im-
pact specimens were machined from each piece along
the longitudinal direc tion of forging, ensuring the same
heat treatmen t for both tests. Tensile specimens
tra nsv ers e directio n) were obtained from the far end
of the broken frac tur e toughness specimen s.
( a) C r y o g e n i c T e n s i l e P r o p e r t i e s
T e n s i l e t e s t s w e r e c o n d u c t e d a t l i q u i d n i t r o g e n t e m -
p e r a t u r e u s i n g an I n s t r o n m a c h i n e e q u i p p e d w it h a c r y -
(a)
b)
Fig . 6 - -Scanning e lec tro n micro s truc ture s . ( a) spec imen 1B,
(0) specim en 2A, (c) spec ime ns 2B, on following page.
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Fire 6 c}
T h e r e s u l t s o f t h e t e n s i l e t e s t s a r e s h o w n i n T a b l e I .
T h e y i e l d s t r e n g t h a t 7 7 K i s i n t h e r a n g e 1 4 0 t o 1 5 0 k s i
9 . 6 6 t o 1 0 . 3 5 x 1 08 n e w t o n / m 2) a n d i s r a t h e r i n s e n s i t i v e
t o m i c r o s t r u c t u r e . T h e s a m p l e s t r e a t e d to s t a g e s 1 B
a n d 2B h a v e s l i g h t l y h i g h e r y i e l d s t r e n g t h a n d l o w e r
t e n s i l e d u c t i l i t y th a n t h o s e a t s t a t e s 1 A a n d 2 A. T h e s e
m i n o r e f f e c t s a p p e a r t o b e d u e t o t h e p r e c i p i t a t i o n o f
N i z T i i n t h e a p h a s e a t 6 5 0 ~ E l e c t r o n m i c r o s c o p i c
s t u d i e s i n d i c a t e t h a t t h e e x t e n t o f t h i s p r e c i p i t a t i o n i s
s m a l l .
b ) C r y o g e n i c I m p a c t P r o p e r t i e s
C h a r p y V - n o t c h i m p a c t t e s t s w e r e c o n d u c t e d a t l i q u id
n i t r o g e n t e m p e r a t u r e 7 7 K ) u s i n g A S T M st a n d a r d t e c h -
n i q u e s , 14 a n d n e a r l i q u i d h e l i u m t e m p e r a t u r e 5 t o 6 K ),
u s i n g a m e t h o d r e c e n t l y d e v e l o p e d i n t h i s l a b o r a t o r y , t s
Table I Results of the Tensile Tests at 77 K
Yield Strength Tensile Stre ngth Elo ng ,
Specimen ksi newton/m2) ksi newto n/m2) pet R.A., pet
1A 134 9.2 5X 10a) 142 9.8 0X 10a) 31.1 73.8
IB 145 10.09 X 10a) 151 10.42 X l0 s) 24.8 70.5
2A 141 9.7 3 X I0*) 150 10.35 • 108) 29.3 72.9
2B 149 10.28• 1oa) 154 10.63• lO ) 26.8 72.1
Fig. 7 Phot ogra phs of the brok en Char py bars after testing at 77 K and 6 K.
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T w o s p e c i m e n s w e r e t e s t e d a t e a c h co n d i t i o n . T h e r e -
s u l t s o f t h e s e t e s t s a r e p r e s e n t e d i n T a b l e I I. T h e
b r o k e n s p e c i m e n s a r e s h o w n i n F i g . 7 .
A t 77 K a l l f o u r s t r u c t u r e s s h o w e d h i g h t o u g h n e s s.
H o w e v e r , a t 6 K s p e c i m e n s 1 A a n d 1 B w e r e b r i t t l e .
F r a c t o g r a p h s t a k e n at th e c e n t e r o f t h e f r a c t u r e s u r -
f a c e i n t h e s e s p e c i m e n s r e v e a l e d a q u a s i - c l e a v a g e
f r a c t u r e m o d e . S p e c i m e n s 2 A a n d 2 B r e m a i n e d t o ug h
a t 6 K ; t h e t r a n s i t i o n t e m p e r a t u r e s i n C h a r p y t e s t i n g
f o r t h e b c c s t r u c t u r e s 2 A a n d 2B a r e b e l o w t h i s t e m -
p e r a t u r e .
FRACTURE TOUGHNESS TESTING AT 77 K
27,00 (07 thick Compocf Yenshm Specimen)
~4,00
88, 48
z z~c_ _
o 15,00 IA ~ ZA ~
HEATTRF ~,~ CYCLES
S 9c x
6 3 3O
3OOO
L _ l I I I
Ol O2 03
IN CH 0~4 05 06
DISPLACEMENT,
XBL
7 3 9 - 1 8 5 4
Fig. 8 Load crack opening displacement curves in fracture
t o u g h n e s s t e s t i n g a t 7 7 K .
c ) C r y o g e n i c F r a c t u r e T o u g h n e s s
F r a c t u r e t o u g h n e s s t e s t s w e r e c o n d u c t e d a t 77 K o n
a n M T S m a c h i n e e q u i p p e d w i th a l i q u i d n i t r o g e n c r y o -
s t a t . C o m p a c t t e n s i o n W O L ) s p e c i m e n s o f 0 .7 0 in .
1 .7 8 c m ) t h i c k n e s s w e r e p r e p a r e d a n d f a t i g u e p r e -
c r a c k e d a c c o r d i n g t o A S T M s p e c i f i c a t i o n s . T T w o s p e c -
i m e n s w e r e t e s t e d a t e a c h s t a g e o f t h e t h e r m a l c y c l i n g
t r e a t m e n t .
T h e r e s u l t s o f t h e s e t e s t s a r e s h o w n in F i g . 8 . W i t h
t h e p o s s i b l e e x c e p t i o n o f s a m p l e 1 B , t h e s e s p e c i m e n s
w e r e w e l l a w a y f r o m p l a n e s t r a i n c o n d i t i o n s ; a v a l u e
of
Q ~
1 40 K S I . ~ w a s c o m p u t e d f r o m t h e l o a d -
c r a c k o p e n i n g d i s p l a c e m e n t C O D ) c u r v e s . 16 A t s t a g e s
1 A, 1 B , a n d 2 A t h e a l l o y e x h i b i t e d u n s t a b l e c r a c k p r o p -
a g a t i o n a s m a r k e d b y t h e d o t t e d l i n e s i n t h e l o a d - C O D
c u r v e s . H o w e v e r th e f i n e - g r a i n e d a l l o y 2 B s e e m e d
Table II Charpy Imp act Energy at 77 and 6 K
Impact Energy at 77 K, Impact Energy at 6 K
Specimen ft-lb newton-meter) ft-lb newton-meter)
1A 154 209) 55 75)
1B 116 158) 43 59)
2A 131 178) 116 158)
2B 115 156) 99 134)
F i g. 9 - - P o s t - t e s t f r a c t u r e t o u g h n e s s s p e c i m e n s .
M E T A L L U R G I C A L T R A N S A C T I O N S A
V O L U M E 6 A , J A N U A R Y 1 9 7 5 - 1 47
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Fig. 10--Scanning elect ron microscope fractograp hs of the specimens shown in Fig. 9.
immune to uns tab le crack propagat ion . The specimen
was ful ly plast ic and the pre- indu ced crack grew
slowly in a s table mann er unt i l the test was s topped.
The pos t - t es t f rac ture toughness specimens are com-
pared to one another in Fig. 9. The bri t t l ene ss of spec -
imens 1A and 1B the repeated crack a r re s t in speci -
men 2A and the duct i l i ty of spec imen 2B are visu al ly
apparen t .
The f rac tur e surf aces were examined by scanning
elec t ron microscopy . Fig . 10 shows scanning e lec t ron
fract ograph s taken s l ight ly ahead of the pre-ind uce d
fat igue crack along the cent er l ine of the sample. Spec-
imens proc esse d to s tages 1A and 1B propagated fra c-
ture in a quas i-cl eava ge mode. At s tage 2A the fra c-
ture mode was a mixture of quas i-clea vag e and ductile
rupture. Fully grain refined specimens 2B) showed
ductile dimple rupture over the whole fra ctu re sur face.
CONC LUS ONS
I) By alterna te phase tran sfor matio n in y and or + y)
range, the g rain si ze of an Fe-12 Ni-0.25 Ti al loy was
refined from 40~60 /ira to 0.5 ~2/i m in 4 cycles.
2) With his ultrafine grain size, the ductile brit tle
transition temperatu re of this ferr iti c alloy in Charpy
impact testin g was supp ress ed below liquid helium
temperature.
3) The ducti le-b rittle ra nsition tempe rature in a
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f r ac tu re toughness tes t ing wi th a sharp c r ack was sup -
p ressed be low l iqu id n i t roge n tempera tu re and the f r ac -
t u r e m o d e w a s c h a n g e d f r o m b r i t t l e q u a s i - c l e a v a g e t o
c o m p l e t e l y d u c t i l e d i m p l e r u p t u r e .
AC KNOWLEDGMENTS
The au tho r s w ish to thank Dr . R . L . Mi l le r o f the
U . S . S t e e l C o r p o r a t i o n f o r a s s i s t a n c e i n r e t a i n e d a u -
s t e n i t e a n a l y s i s . T h e y a r e a l s o g r a t e f u l to P r o f e s s o r
R . M. Fu l r a th fo r as s i s ta nce wi th the d i la tomet r i c and
X- ray ana l y ses and to P ro fesso r G . Thomas fo r he lp -
fu l d i scuss ions .
The work r epo r te d here was suppo r te d by the Of f ice
o f Nava l R esearc h under con t r ac t N00014-69 -A-1062
NR 031-762 and by the Atomic Energy C om miss io n
th rough the Ino rgan ic Mater ia l s R e search Div is ion o f
t h e L a w r e n c e B e r k e l e y L a b o r a t o r y .
R E F E R E N E S
1. W. A. Horwood: M.S. Thesis Universityof California Berkeley 1972.
2. S. Jin J. W. Morris Jr. and V. F. Zackay:
Aduan. Cryog. Eng.
vol. 19 1974
p. 379.
3. G. Sasaki: Ph.D Thesis Universityof California Berkeley 1973.
4. V. F. Zackay:Proceedings of the Thud International Conference on the
Strength o f Metals and Alloys vol. 1 August 1973 p. 591.
5. L. F. Porter and D. S. Dabkowski: Ultrafine Grain Metals p. 113 Syracuse
UniversityPress 1972.
6. G. Saul J. A. Robertson and A. M. Adair:Met. Trans. 1970 vol. 1 p. 383.
7. R. L. Miller:
Met. Trans.
1972 vol. 2 p. 905.
8. R. A. Grange:Trans. ASM 1966 vol. 59 p. 26.
9. S. Floreen:
Met. Rev.
1968 p. 115.
10. M. Hansen:Constitution of Binary Alloys 2nd Ed. p. 677 McGraw-Hill
New York 1958.
11. R.L. Miller: ResearchLab. of U.S. Steel Corp Monroevine Pennsylvania
Private Communication.
12. R. L. Miller: Trans. ASM 1964 vok 57 p. 892.
13. R. L. Miller:
Trans. ASM
I968 vol. 61 p. 592.
14. 1973 Book of ASTMStandards part 31 E 23-72 p. 277.
15 S. Jin W. A. Horwood J. W. Morris Jr. and V. F. Zackay:
Advan. Cryog.
Eng.
vol. 19 1974 p. 373.
16.
1973Book of ASTMStandards
part 31 E 399-72 p. 960.
METALLURGICAL TRANSACTIONS A VOLUME 6A JANUARY 197 5-149
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