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



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



~~ 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 -

METALLURGICAL TRANSACTIONS A VOLUME 6A, JANUARY 1975-143



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.

144-VOLUME 6A, JANUARY 1975 METALLURGICAL TRANSACTIONS A



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.

METALLURGICAL TRANSACTIONS A VOLUME 6A,JANU ARY 1975 145



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.

146-VO LUME 6A, JANUAR Y 1975 METALLU RGICAL TRANSACTIONS A



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



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

148-VOLUME 6A JANUARY 1975 METALLURGICAL TRANSACTIONS A



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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Grain Refinement Through Thermal Treatment