Artigo precipitação C Cr Fe

7/29/2019 Artigo precipitao C Cr Fe

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O veral l Rea ct ion Kinet ics and Morphologyof Au stenite Decom position betw een the Upp erNose and the Ms of a Hypoeutectoid Fe-C-Cr Al loyH . G O L D E N S T E I N a n d H . I. A A R O N S O NT h e o v e r a l l t r an s f o r m a t i o n k i n e t i cs a n d m i c r o s t r u c t u r es o f a n F e - 0 . 1 3 p c t C - 2 . 9 9 p c t C r a l l o yw e r e i n v e s t i g a t e d a b o v e a n d b e l o w t h e b a y t e m p e r a t u r e (To) w i t h o p t i c a l a n d e l e c t r o n m e t a l -l o g r a p h y . M o s t o f t h e k i n e t ic a n d m i c r o s t r u c t u r a l a s p e c t s o f t r a n s f o r m a t i o n a r e s i m i l a r t o th e i rc o u n t e r p a r ts i n F e - C - M o a l l o y s . T r a n s f o r m a t i o n s t a si s ( = i n c o m p l e t e t r a n s f o rm a t i o n ) w a s o b -s e r v e d a t a l l t e m p e r a t u r e s s t u d i e d b e l o w Tb . H o w e v e r , n o n m o n o t o n i c b e h a v i o r o f t h e p e r c en tt r a n s f o r m a t i o n a t s t as i s as a f u n c t i o n o f u n d e r c o o l i n g b e l o w Tb w a s f o u n d i n th e p r e s e n t a l l o ya n d a t t r i b u t e d t o a w i d e r t e m p e r a t u r e i n t e r v a l b e l o w Tb i n w h i c h t h e W i d m a n s t ~ i t t e n s t r u c t u r ei s la r g e l y s u p p r e ss e d . A l s o , u n l i k e F e - C - M o a l l o y s , fi b r o u s c a rb i d e s f o r m b e l o w a s w e l l a sa b o v e Tb; i n b o t h M o a n d C r a l l o y s , h o w e v e r , i n t e r p h a s e b o u n d a r y c a r b i d e s a p p e a r i n a s s o c ia t i o nw i t h p r o e u t e c t o i d f e r r it e b o t h a b o v e a n d b e l o w Tb. The s o l u te d r a g - l ik e e f f e c t , s o m e w h a t w e a k e ri n F e - C - C r t h a n i n F e - C - M o a l l o y s , p r o v i d e s a q u a l i t a t i v e e x p l a n a t i o n f o r m o s t o f t h e k i n e t i ca n d m i c r o s tr u c t u r a l o b s e r v a ti o n s m a d e .

I . I N T R O D U C T I O NTwo o f t h e p r e c e d i n g p a p e r s i n t h i s j o u r n a l r e p o r t t h er e s u l t s o f d e t a i l e d s t u d i e s o n t h e t r a n s f o r m a t i o n k i n e t i c sa n d m o r p h o l o g y o f a u s t e n i t e d e c o m p o s i t i o n p r o d u c t s a ta n d s o m e w h a t a b o v e t h e b a y t e m p e r a t u r e m a n d a t a n ds o m e w h a t b e l o w t h e b a y t e m p e r a t u r e [2] i n F e - C - M o a l -l o y s . T h e p r e s e n t i n v e s ti g a t i o n w a s u n d e r t a k e n t o d e t e r -m i n e t h e o v e r a l l re a c t i o n k i n e ti c s a n d t h e m o r p h o l o g y o ft r a n s f o r m a t i o n a t c o u n t e r p a r t t e m p e r a t u r e s b o t h a b o v ea n d b e l o w t h e b a y i n a n F e - C - C r a l l o y . T h e p r i m a r y p u r -p o s e o f t h i s w o r k w a s t o a s c e r t a in w h i c h o f t h e p r i n c ip a lr e s u lt s s e c u r e d o n F e - C - M o a p p e a r s t o b e g e n e r i c t o a u s -ten i t e decom pos i t ion in F e -C-X a l loys , where X i s a s t rongc a r b i d e - f o r m i n g s u b s t i t u t i o n a l a l l o y i n g e l e m e n t a n d i sp r e s e n t a t a c o n c e n t r a ti o n h i g h e n o u g h a t th e c a r b o n c o n -c e n t r a t i o n e m p l o y e d t o p r o d u c e a b a y i n t h e t i m e -t e m p e r a t u r e - t r a n s f o r m a t i o n ( T T T ) d i a g r a m a n d a l s o t h e" i n c o m p l e t e t r a n s f o r m a t i o n = t r a n s f o r m a t i o n s t a s i s" [31p h e n o m e n o n . T h e l a t t e r p r o c e s s h a s b e e n d e f i n e d a s t h ec e s s a t i o n o f f e r r i t e o r b a i n i t e f o r m a t i o n p r i o r t o t h ea p p e a r a n c e o f t h e L e v e r R u l e p r o p o r t i o n o f f e r r i t e . [4]A m o n g t h e f i n d i n g s r e p o r t e d i n F e - C - M o a l l o y s , i n -c o m p l e t e t r a n s f o r m a t i o n i s a b s e n t a b o v e t h e b a y t e m -pera tu re , Tb (de f ined a s the t em peraaLre in the in te rm edia tet r a n s f o r m a t io n t e m p e r a t u r e r e g i o n a t w h i c h t h e i s o th e r -m a l rea c t ion t im e requ i red to in i t i a te t rans form a t ion pas sest h r o u g h a m a x i m u m ) ; [1'3[ i n t h i s t e m p e r a t u r e r e g i o n , i s o -

H. GOLDENSTEIN, formerly Visiting Professor, Department ofMetallurgical Engineering and Materials Science, Carnegie MellonUniversity, is Assistant Professor with the Department of MetallurgicalEngineering, Escola Politecnica da Universidade de Sho Paulo, ShoPaulo, Brazil 05508. H.I. AARONSON, R. F. Meh l Professor, is withthe Department of Metallurgical Engineering and Materials Science,Carnegie Mellon University, Pittsburgh, PA 15213.This paper is based on a presentation made in the symposium"International Conference on Bainite" presented at the 198 8 WorldMaterials Congress in Chicago, IL, on September 26 and 27, 1988,under the auspices of the ASM INTERNATIONAL Phase Transfor-mations Committee and the TMS Ferrous Metallurgy Committee.

t h e r m a l r e a c t i o n k i n e t i c s a r e i n v a r i a b l y o f t h e s i m p l e s i g -m o i d a l t y p e . tn B e l o w t h e b a y , h o w e v e r , u p t o f o u rd i f f e r e n t p a t t e r n s o f o v e r a l l r e a c t i o n k i n e t i c s a r e o b -s e r v e d . I21 A t t h o s e t e m p e r a t u r e s w h e r e t r a n s f o r m a t i o ns t as i s o c c u r s , c a r b i d e p r e c i p i t a t i o n a t a : 3 ' b o u n d a r i e s i na s s o c i a t i o n w i t h t h e f e r r i t e i n i t i a l l y f o r m e d d o e s n o tc o m m e n c e u n t i l t h e e n d o f t h e s t a s is i n te r v al .t 2 ] F e r r i t em o r p h o l o g y i n F e - C - M o a l l o y s e x h i b i t in g s ta s is d i s p l a y sm a r k e d d i f f e r e n c e s [1,51 wi th re sp ec t to the us ua l pa t t e rnf o u n d i n t h e p r o e u t e c t o i d f e r r i t e a n d b a i n i t e r e g i o n s o fh y p o e u t e c t o i d F e - C , p l a i n c a r b o n s t e el s , a n d m a n y a l l o ys t e e l s . I n t h e l a t t e r s t e e l s , g r a i n b o u n d a r y f e r r i t e a l l o t -r i o m o r p h s p r e d o m i n a t e a t h i g h t e m p e r a t u r e s ; w i t h s u c -ces s ive reduc t ions in reac t ion t em pera tu re , Widm ans t~ t t ens i d e p l a t e s , a n d t h e n i n t r a g r a n u l a r p l a te s , d e v e l o p a n d b e -c o m e m a j o r f e a t u r e s o f t h e m i c r o s t r u c t u r e , t6 -9 [ A t s t il ll o w e r t e m p e r a t u r e s , i n d i v i d u a l f e r r i t e p l a t e s a r e r e p l a c e db y s h e a v e s o f p a r a l l e l p l a t e s [ 7- '~ ( w h i c h b e c o m e u p p e rb a i n i t e i n t h e p r e s e n c e o f a n o n l a m e l l a r d i s p e r s i o n o fc a r b i d e s ) a n d t h e n b y p a r a l l e l s i d e p l a t e s s y m p a t h e t i c a l l yn u c l e a t e d e d g e - t o - f a c e a t a r o u g h l y 5 5 d e g a n g l e I "[ w i t hrespec t to a " subs t ra te" fe r r i t e p la te [~2[ ( low er ba in i t e wh enn o n l a m e l l a r c a r b i d e s a r e p r e s e n t ) . I n t h e p r e s e n c e o f su f -f i c ie n t p r o p o r t i o n s o f M o a n d C , h o w e v e r , a b a y i s i n -t r o d u c e d i n t h e T T T c u r v e f o r i n it ia t io n o f t r a n s f o r m a t io n .A n " u p p e r n o s e " a n d a " l o w e r n o s e " t h u s a p p e a r int h is c u r v e . B e l o w t h e u p p e r n o s e , f o r m a t i o n o f f e r r i tep l a t e s i s i n c r e a s i n g l y s u p p r e s s e d , u n t i l a t Tb g r a i nb o u n d a r y a n d t w i n b o u n d a r y a l l o t ri o m o r p h s a r e th e o n l yf e r r i t e m o r p h o l o g i e s p r e s e n t , p a r t i c u l a r l y i n h i g h e r M oallo ys , t l,Sl Jus t be low Tb, t h e a l l o t r i o m o r p h s a r e s u p p l e -m e n t e d b y a n d t h e n w h o l l y r e p l a c e d b y a r e m a r k a b l yd e g e n e r a t e v e r s i o n o f t h e W i d m a n s t a t t e n s i d ep l a te a n di n t r a g r a n u l a r p l a t e m o r p h o l o g i e s . [5,~3[ T h i s d e g e n e r a c ys l o w l y d i m i n i s h e s w i t h d e c r e a s i n g r e a c t i o n t e m p e r a -t u r e [5[ a n d a l s o d o e s s o w i t h i n c r e a s i n g r e a c t i o n t i m e ,t h o u g h t h e l a t t e r e f f e c t a p p e a r s t o b e l a r g e l y a c o n s e -q u e n c e o f e x t e n s i v e i m p i n g e m e n t a m o n g a d j a c e n t f e r r it ep la te s. [~31 Transm is s ion e lec t ron m ic ro scop y (TE M ) s tud ies

METALLURGICAL TRANSACTIONS A VOLUME 21A, JUNE 1990-- 1465


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h a v e s h o w n t h a t c a r b i d e m o r p h o l o g i e s a s s o c i a t e d w i t hf e r r it e i n F e - C - M o a l l o y s a re o f t h e i n t e r p h as e b o u n d a r yan d fib rou s car bid e typ es [~4'15] ab ov e the bay . [16'171 B el owt h e b a y , f i b r o u s c a r b i d e s p r o m p t l y d i s a p p e a r ; i n t e r p h a s eb o u n d a r y c a r b i d e s l a s t u n t i l s o m e w h a t l o w e r t e m p e r a -t u r e s , t13] L a r g e l a t h s o f M 0 2 C a p p e a r a t t e m p e r a t u r e s n o tt o o f a r b e l o w T o; a t l o w e r t e m p e r a t u r e s , t h e s e a r e r e -p l a c e d b y ( F e , M o ) 3 C l a t h s . [13] T h i s s e q u e n c e o f c a r b i d em o r p h o l o g i e s i s a l s o n o t c h a r a c t e r i s t i c o f e i t h e r p l a i nc a r b o n s t e e ls o r a l l o y s te e ls w h o s e T T T d i a g r a m s d o n o tc o n t a i n a b a y a t i n t e r m e d i a t e r e a c t i o n t e m p e r a t u r e s .In re spec t to aus ten i t e decom pos i t ion in reasonab ly pureF e - C - C r a l l o y s , r e s u l t s p r e v i o u s l y r e p o r t e d w h i c h a r er e l e v a n t , e i t h e r i m m e d i a t e l y o r a s f u n d a m e n t a l b a c k -g r o u n d , t o t h e p r e s e n t i n v e s ti g a t io n m a y b e s u m m a r i z e da s f o l l o w s . D a v e n p o r t a n d B a i n [~8] w e r e t h e f i r s t t od e m o n s t r a t e ( i n 1 9 3 0 ) a b a y i n t h e T T T c u r v e s f o r a nF e - C - C r a l l o y a t i n t e r m e d i a t e r e a c t io n t e m p e r a t u r e s . I n -c o m p l e t e t r an s f o r m a t i o n i n F e - C - C r a l l o ys w a s r e p o r t e db y W e v e r a n d J e l l i n g h a u s t19] i n 1 9 3 2 a n d b y R o s e a n dF i s c h e r [2~ i n 1 9 3 9 u s i n g d i l a t o m e t r i c a n d m a g n e t i c m e t h -ods . Rose and F i sche r [2~ a l so fou nd som e dev ia t ions f ro ms igm oida l behav ior in ove ra l l reac t ion k ine t i c s which theyd i d n o t d i s c u s s i n d e t a i l ; t h e s e d a t a w i l l b e c o n s i d e r e di n S e c t i o n I V . L y m a n a n d T r o i a n o [21] i n t e r p r e t e d d i l a -t o m e t r i c d a t a o n 3 p c t C r s t e e ls a s s h o w i n g t h a t t h e m a x -i m u m p r o p o r t i o n o f b a i n i te f o r m e d , p r i o r t o th e o n s e t o ft r a n s f o r m a t i o n s t a s i s , i n c r e a s e s s m o o t h l y f r o m z e r o t ou n i t y a s t h e r e a c t i o n t e m p e r a t u r e i s r e d u c e d b e l o w T b .B o s w e l l et a l . TM h a v e s h o w n t h a t s u f f i c i e n t p r o p o r t i o n so f C a n d C r c a u s e d e g e n e r a t i o n o f t h e W i d m a n s t~ i tt e nm o r p h o l o g i e s a t Tb i n t h e s a m e m a n n e r a s t h e y o b s e r v e di n F e - C - M o a l l o y s , t h o u g h t h e e f f e c t , p e r a t o m i cp e r c e n t X , i s l e s s s e v e r e i n F e - C - C r a l l o y s . R o s e a n dF i s c h e r 2 ~ r e p o r t e d t h a t d e c o m p o s i t i o n o f t h e a u s t e n i t er e m a i n i n g a f t e r c o m p l e t i o n o f s t a si s t a k e s p l a c e t h r o u g ht h e p e a r l i t e r e a c t i o n ; h o w e v e r , i t i s n o l o n g e r c e r t a i n t h a tt h i s i d e n t i f i c a t i o n w a s s e c u r e l y a c c o m p l i s h e d . K l i e r a n dLy m a n 12] and Ly m a n and Tro ian o , t2q us ing 3 pc t C r s t ee l si n s te a d o f t h e 0 . 2 4 t o 2 p c t C r a l l o y s e m p l o y e d b y R o s ea n d F i s c h e r , s t a t e d t h a t c o m p l e t i o n o f tr a n s f o r m a t i o n i sa c c o m p l i s h e d b y a d a r k - e t c h i n g , n o n p e a r l i t i c p r o d u c t .R e y n o l d s et a l . TMu s e d e l e c t r o n m i c r o s c o p y t o s h o w t h a tt h e b a i n i t e r e a c t i o n r e s u m e s a f t e r t r a n s f o r m a t i o n s t a s i sb e l o w T b i n F e - C - M o a l l o y s . S i m i l a r l y , " g r o w t h s t a s i s "( t h o u g h n o t t r a n s f o r m a t i o n s t a s is ) , o b s e r v e d a b o v e T b , i sa l so t e r m i n a t e d b y t h e r e n e w e d f o r m a t i o n o f b a i n it e i n -d i s t i n g u i s h a b l e f r o m t h a t a p p e a r i n g p r i o r t o g r o w t hs ta s i s. [u Man ner ko sk i , [14] Re land e r , t~5] and Ca m p be l l andH o n e y c o m b e 23] o b s e r v e d b o t h i n t er p h a s e b o u n d a r y a n df i b r o u s c a r b i d e s i n h i g h - C r F e - C - C r a l l o y s . W h i l e t h ea b s e n c e o f b u l k p a r t i t io n o f C r b e t w e e n a u s t e n it e a n df e r r i t e ( n o w k n o w n t o b e m i x t u r e s o f f e r r i t e a n d c a r -b ides [17'231) has bee n d em on s t ra t ed by m e ans o f e lec t ronp r o b e a n a l y s i s i n a 3 p c t C r a l l o y w i t h n e a r l y t h e s a m ec o m p o s i t i o n a s t h a t e m p l o y e d i n t h e p r e s e n t i n v e s t i g a -t i o n [24] a n d w a s l a t e r c o n f i r m e d i n t w o o t h e r 3 p c t C ra l loy s , tz5] bo t h (F e , Cr )3C and Cr7C3 a re fo rm ed in a s -socia t ion w ith ferr i te a t high rea ct ion temperatures .t26] On ly(Fe, C r)3C is o bse rve d at low er tem peratu res . 126] (Fe, Cr)7C3i s t h e e q u i l i b r i u m c a r b i d e i n t h e a l l o y u s e d i n th e p r e s e n tstu dy , t27]

II . E X P E R I M E N T A L P R O C E D U R E SThe a l loy used in th i s inves t iga t ion con ta ined ( in we igh tp e r c e n t ) 0 . 1 3 p c t C , 2 . 9 9 p c t C r , 0 . 0 0 2 p c t M n , 0 . 0 0 1p c t S i , 0 . 0 0 1 p e t P , a n d 0 . 0 0 6 p c t S . T h e a l l o y w a s p r e -p a r e d a t t h e S c i e n t i f i c L a b o r a t o r y o f F o r d M o t o r C o m -p a n y , D e a r b o r n , M I , b y v a c u u m m e l t i n g a n d c a s t i n gf o l l o w e d b y r e d u n d a n t h o t w o r k i n g . A f t e r i n v e st i g a t io no f C r d i s t r i b u t io n i n a u s t e n i t e w i t h e l e c t r o n p r o b e m i c r o -ana lys i s , the a l loy was h om oge nized fo r 3 days a t 1300 ~i n a q u a r t z c a p s u l e w i t h a p u r i f i e d A r a t m o s p h e r e . I n -

d i v i d u a l s p e c i m e n s 4 z 1 0 - 4 x 10 -3 X 1 0 - 3 m w e r ea u s t e n i t i z e d f o r 1 5 m i n u t e s a t 1 3 0 0 ~ i n a g r a p h i t e -d e o x i d i z e d , a r g o n - p r o t e c t e d B a C 1 2 b a t h , t28] y i e l d i n g a na u s te n i te g r a i n s iz e v a r y i n g f r o m A S T M N o . 2 t o c o a r s e rthan No. 1 . I so the rm a l reac t ion was conduc ted in s t i r red ,g r a p h i t e - p r o t e c t e d , g r a p h i t e - d e o x i d i z e d l e a d b a t h s a tt e m p e r a t u r e s f r o m 7 0 0 ~ t o 5 2 5 ~ a n d t h e n q u e n c h e di n i c e d b r in e . F o r o p t i c a l m e t a ll o g r a p h i c e x a m i n a t i o n , ab r o a d f a c e o f t h e s e s p e c i m e n s w a s m e c h a n i c a l l y p o l -i s h e d a n d t h e n e t c h e d w i t h L e P e r a ' s r e a g e n t J ~9,3~ T h ev o l u m e f r a c t io n o f a u s t e n it e t r a n s f o r m e d w a s d e t e r m i n e db y m e a n s o f p o i n t c o u n t i n g , t3~,32] F o r e x a m i n a t i o n w i t hr e p l i c a t i o n e l e c t r o n m i c r o s c o p y , s p e c i m e n s w e r e g i v e na f ina l po l i sh wi th a co l lo ida l s i l i ca s lu r ry , e t ched in n i t a l ,s h a d o w e d a t a n a n g l e o f a p p r o x i m a t e l y 1 5 d e g w i t h P t ,a n d t h e n c o a t e d w i t h a l a y e r o f c a r b o n a p p r o x i m a t e l y1 0 0 n m t h i c k . T h e s e r e p l ic a s w e r e c u t i n to 0 . 0 0 2 m s q u a r esegm ents and then e tch ed f ree o f the i r spec im ens in 10 pe tn i ta l . T h i n f o i l s p e c im e n s w e r e p r e p a r e d b y c h e m i c a lt h i n n in g t o a p p r o x i m a t e l y 1 0 0 / z a n d t h e n j e t p o l i sh i n gi n 2 0 p c t a n h y d r o u s s o d i u m c h r o m a t e i n g l a c ia l a c e t i ca c i d a t r o o m t e m p e r a t u r e a n d 4 0 t o 6 0 V . F o i l s w e r eo b s e r v e d i n a J E O L 1 2 0 C X m i c r o s c o p e o p e r a t e d a t1 2 0 k V .

I I I . R E S U L T SA. Overall Reaction Kinetics

F i g u r e 1 p r e s e n ts t h e T T T d i a g r a m f o r th e a l l o y i n -v e s t i g a t e d w i t h i n t h e t e m p e r a t u r e r a n g e s t u d i e d . T h e

oO I

700

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5 0 0 i I i I ~ I ~ I ~ I i0 1 2 3 4 5 6

L o g t i m eFig. 1-- TT T diagram at temperatures below the upper nose (ap-proxima tely 700 ~ and the Ms(approximately 500 ~ of an Fe-0.13pct C-2.99 pct Cr alloy.

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d r a m a t i c i n c r e a s e i n t h e t i m e d i f f e r e n c e s b e t w e e n t h e 5 0a n d 9 9 p c t t r a n s f o r m a t i o n c u r v e s a t t e m p e r a t u r e s b e l o wt h a t o f th e b a y ( To i s a p p r o x i m a t e l y 6 0 0 ~ f a m i l i a rf r o m p r e v i o u s i n v es ti g at io n s ,i ~ ~ p r o v i d e s a n i n i ti a l i n -d i c a t i o n ( t h o u g h n o t p r o o f ) o f t h e p r e s e n c e o f i n c o m p l e t et r a n s f o r m a t i o n . F i g u r e s 2 ( a ) t h r o u g h ( j ) a r e is o t h e r m a l

r e a c t io n c u r v e s a t e a c h o f t h e r e a c t io n t e m p e r a t u r e s u t i -l i zed . I n d i scus s i ng t hese cur ves , t he c l a s s i f i ca t i on schemed e s c r i b e d i n F i g u r e 2 ( k ) t2,33] w i l l b e e m p l o y e d . N o t e t h a to n l y t h e m i d d l e s t a g e o f T y p e I V k i n e t i c b e h a v i o r c o r -r e s p o n d s t o t r a n s f o r m a t i o n s t as i s. F i g u r e s 2 (a ) t h r o u g h( f ) , r e p r e s e n t in g r e a c t i o n t e m p e r a t u r e s f r o m 7 0 0 ~ d o w n

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( j ) ( k )Fig. 2--Pe rce nta ge o f austenite isothermally transformed as a function of the isotherm al reaction tim e at (a) 700 ~ (b) 675 ~ (c) 650 ~(d) 625 ~ (e) 610 ~ (f ) 600 ~ (g) 585 ~ (h) 570 ~ (i) 555 ~ and (j ) 525 ~ (k) The types of isothermal reaction curves found belowt he ba y t e mpe r a t u r e . t 2z 3 ]

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to 600 ~ are al l properly described by the Type I , i . e . ,s igmoidal or "Joh nson -M ehl ," [34] form of ov eral l t rans-formation kinetics. Figures 2(g) through (j), on the otherhand, presenting the overall kinetics from 585 ~ through525 ~ al l d isplay Type IV behavior. Except in one par-t icular case, these resul ts are those class ical ly acceptedfor baini te described by the overal l react ion kinet ics def-ini t ion, e . g . , Referen ce 35. T his except ion is that thepercent t ransformat ion at which s tas is occurs does notincrease smooth ly wi th unde rcoo l ing be low Tb. Ins tead ,this quantity is 25 pct at 585 ~ 12 pct at 570 ~ 42 pctat 555 ~ and 90 pct at 525 ~ The percent t ransfor-mat ion required to develop s tas is at 585 ~ is thus anom-alously high. An explanat ion for this anom aly wil l becomeapparent when the microstructures associated with theset rans fo rmat ion t empera tures a re examined .B. Optical Microstruc tures

Figure 3 i l lus trates a typical W idmanst~it ten ferri temicrostructure develo ped at 700 ~ the upper nose inthe TTT curve fo r in i t i a t ion o f t rans fo rmat ion . Thesep la tes exh ib i t the Type A degeneracy observed amongsideplates in a plain carbon s teel (0 .29 pct C, 0 .76 pctMn, 0 .25 pct Si) , part icularly as i l lus t rated inFigures 47(a) and ( f ) and 48(a) o f Reference 36 . Th i si s one o f th ree c rys ta l lograph ica l ly based fo rms o f fe r r i tes ideplate degeneracy ident i f ied in this s teel . These de-generacies also occur in al loy s teels , and i t i s importantto dis t inguish them fro m the below-Tb degenera cies char-ac ter i s t i c o f Fe-C-Mo and Fe-C-Cr a l loys when su f f i -cient proportions of C and X are present. With decreasingreact ion temperature, the Widmanst~i t ten s t ructure di-minishes in importance. As i l lus t rated in Figure 4 , how-ever, i t remains a n important feature of the microstructure.The W idmanst~it ten morpho logy now undergoes a some-what subt le change in i ts detai led shape. As shown inFigure 4 , the de generacy changes f rom the p ro -nouncedly crystal lographic s tyle of Figure 3 to the morei r regu lar , quas i - random mode o f sub-bay degeneracy .Comparison with Figures 11 and 12 of the paper byBoswel l et al. ]51 on Fe-C -M o al loys dem onstrates that Cr

Fig. 4--Degenerate Widmanst~itten structure of a more irregular typedeveloped at the bay temperature. Reacted 4020 s at 600 ~

i nduces the same type o f degeneracy among fer r it e p la tesfo rmed a t the bay (Figure 4 ) and jus t be low the bay(Figure 5), as does Mo. Figure 5 , taken of a specimenreacted at 585 ~ during the stas is interval, show s thesame type o f degeneracy as Figure 4 reac ted a t 600 ~(Tb) and also demonstrates that Cr has now further sup-pressed the Widmanst~i tten mor pholog y. Figure 6 is ah igher magni f i ca t ion v iew of s idep la te degeneracy a t585 ~ Althou gh the i rregularit ies displayed wou ld al -most certainly have a more crystal lographic appearancewhen v iewed a t the much h igher reso lu t ions permi t t edby TEM , the seeming ly random natu re o f the degeneracyand the blurred out l ine of individual plates are again ap-parent in this higher magnificat ion i l lus t rat ion.Figure 7 dem onstrates that whi le ferri te plates are s t i l ldegenera te at 570 ~ the crystal lographic appearancecharacteris t ic of this morphology has now returned andparal lel planari t ies of a: y boundaries again s t rongly in-fluence the appearance of the microstructure. Again,

Fig. 3--Cr ysta llogr aphic ally degenerate ferrite plates with unre-solved eutectoid structure beginning to form in the austenite trappedbetween them. Reacted 200 s at 700 ~

Fig. 5--Only a small amount of Widmanstatten structure is presentafter reaction 15 ~ below the bay temperature in a micros tructu re ofwhich grain boundary and twin boundary allotriomorphs are now amajor component. Reacted 6300 s at 585 ~

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F i g . 6 - - H i g h e r m a g n i f i c a ti o n op t ic a l m i c r o g r a p h o f a s u b - b a y d e-genera te fe rr i te . Note g ray-e teh ing , unres o lved eu tec to id s t ruc ture abovet h e a u s t e n i t e g ra i n b o u n d a ry p a s s i n g l a t e ra l l y a c ro s s t h e mi c ro g ra p h .Reacted 2820 s a t 585 ~

F i g . 8 - -F u r t h e r r e d u c t i o n o f Wi d ma n s t ~ i t te n s t ru c t u re d e g e n e ra c y a ta s t i l l lower tempera ture . Reacted 400 s a t 555 ~

Boswel l et al . t s l made equivalent observat ions on Fe-C-Mo al loys (compa re their Figures 12 (630 ~ and 13(600 ~ t aken o f an Fe-0 .11 pc t C-1 .95 pc t Mo a l loyw h o s e Tb = 650 ~ with our Figures 6 and 7). That fur-ther undercool ing of the present al loy causes addi t ionald iminu t ion in "sub-bay" degeneracy , now a t a s lowerra te , i s shown by compar i son o f Figure 8 , t aken af t e rreact ion in the s tas is region at 555 ~ with Figure 7 .Althou gh the eutectoid s t ructures developed within thetemperature range s tudied in this al loy formed on toofine a scale to permit resolut ion of individual carbideswi th op t ica l microscopy , a b r i e f overv iew o f the ex terna lmorphology, dis t r ibut ion, and proport ion of these s t ruc-tures is , nonetheless , an important part of the present"s tory." While al l of the ferri te s t ructures so far de-scribed sooner or later contained carbides , their externalmorpho logy was l a rge ly de termined by the g rowth k i -net ics and mechanisms of the ferri te phase. At later re-act ion t imes, however, eutectoid s t ructures appeared in

Fi g . 7 - - Pa r t i a l r e c o v e ry o f l i n e a r - s i d e d Wi d ma n s t ~ i tt e n s t ru c t u re at al o we r r e a c t i o n t e mp e ra t u re . R e a c t e d 2 6 0 s a t 5 7 0 ~

wh ich, as rec ently desc ribed , 133'371 the exter nal mor -pho logy w as jo in t ly de termined by crys ta l s o f the fe r r i teand carbide phases . Pea rl i te was observed at 700 ~ inthe present al loy but not at 675 ~ or at any lower tem-perature. F igure 9 sho ws the external shape of the eu-tectoid structures developed at reaction temperatures from675 ~ to 570 ~ Figure 9(a) displays clearly the nod-ular external shape of these s t ructures at 675 ~ Evenwhen f i ll ing in the aus ten i te l e f t un t rans fo rmed be tweencomplexes o f fe r r i t e c rys ta l s (which have been c lose lyenveloped themselves by unreso lved eu tec to id s t ruc-tures) , as in Figure 9(b) (650 ~ the nodular morpho l-ogy o f the eu tec to id s t ruc tu re which t rans fo rmed theremaining austeni te is clearly evident . Figure 9(c) showstha t a t 610 ~ jus t above the bay t empera tu re , mos t o fthe austenite matrix is transform ed to bainite nodules ratherthan to ferrite-dominated structures. Figures 9(d) through(f) i l lus t rate the role of the nodular eutectoid s t ructurein decompos ing p rogress ive ly d imin i sh ing vo lumes o fausteni te not previously t ransform ed to ferri te-domina tedstructures as the react ion tempe rature is successively de-creased to 600 ~ 585 ~ and 570 ~ The eutectoidmicrostructures at late s tages of t ransform at ion at 555 ~and 525 ~ are not easily distinguished from that at 570 ~and, hence, are not i l lus t rated here.C . E l e c t r o n M i c r o s t r u c t u r e s

Both rep l ica t ion and t ransmiss ion e lec tron m icroscopywere used to resolve and elucidate the nature of the var-ious ferri te + carbide s t ructures developed. Figure 10 isa good example o f the carb ide s t ruc tu re wi th in nodu larba in it e . Th i s m orpho logy i s bas ica l ly the same as tha t o fnodular baini te, as observed in an Fe-0.24 pct C-0.93pct Mo al loy (Figure l (b) o f Reference 38), a hypo-eutectoid Ti-Cr al loy (Figures 9(a) and (b) o fRefere nce 39), and a hyp ereutectoid Ti-Cr al loy(Figures 5(a) and (b) of Reference 40).Interphase boundary carbides and fibrous carbides as-sociated with proeutectoid ferrite crystals, usually thoughnot a lways in the a l lo t r iomorph ic morpho logy , a re nowa wel l -accepted feature of the electron microstructure of

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Fig. 9- -O pt ic al micrographs of eutectoid s t ructures developed at success ively decreasing react ion temperatures: (a) 675 ~ s ;(b) 650 ~ s ; (c) 610 ~ s ; (d) 600 ~ s ; (e) 585 ~ s ; and ( f ) 570 ~ s .

hypoeutec to id (and approximate ly eu tec to id) s tee ls con-ta in ing apprec iable propor t ions of one or more s t rongcarbide - forming a l loying e lements , such as Cr orMo . [16,17,26] Fig ure 1 1 is a TE M mic rog rap h of inte rpha seboundary ca rb ides , he re , in a pa r t ia l ly curv i l inea rmo de , 123,41-431 as a co ns equ enc e of gre ater com ple xit ies

in the mo dus operan di o f the ledg e m echani sms, t44]F igure 12 is a r ep l ica t ion mic rograph of an unco mm onlyregula r f ibrous s t ruc ture . As a cont r ibut ion towardunderstanding the origin of this structure, Figure 13 showsthe init ia l stages in the evolution of sideneedles f rom ap-proximately equiaxed carbide crysta ls nucleated a t an a : y

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Fig. 10--TEM micrograph showing internal structure of nodularbainite. Reacted 20,040 s at 570 ~

boundary . The spac ing be tween ad jacent s ideneedles inFigure 13 is near ly the same as that between adjacentf ibers in Figure 12.A le ss f ami l ia r ca rb ide morphology i s shown inF igure 14 in the form of r ibbons . Branching of these r ib-bons , presumably by sympa the t ic nuc lea t ion , i s seen tobe a mechanism through which the i r numbers can bemultiplied. An equivalent carbide microstructure has beenreported b y B ee and Edm ond s taSj in Fe-0.21 pct C-3 .13pc t Mo and Fe-0 .5 pc t C-10 .4 pc t Cr a l loys . They havea lso been obse rved , more f requent ly than in the presentalloy, in an F e-0.2 2 pct C -10.6 pct Cr steel , t26~One fur the r , somewha t more subt le , f e r r i te + ca rb idemic ros t ruc ture was obse rved . This cons is ts of sma l l f e r -r i te s idepla te s , obvious ly deve loped a t a la te s tage oft r ansformat ion , wi th in which ca rb ides a re embedded. Amicros t ruc ture of th is type i s shown in F igure 15 , wi tha r rows poin t ing out some examples of ca rb ides .Hultg ren t46~ obs erved this structure in an Fe-0 .54 pct C-O. 82 pct M o alloy reacted a t i ts bay tem peratu re , as did

Fig. l 1-- Dar k-f iel d TE M mic rograph of interphase boundary car-bide structure imaged with a carbide reflection. Reacted 10,020 s at585 ~

Fig. 12--Replication micrograph of an array of fibrous carbides.Reacted 15,900 s at 675 ~

Habraken and Econ omo poulo s I471 in a comp lex com-mercial steel . Hultgren descr ibed the sideplates as"shoots . " F igure 16 shows shoots deve loping a t somelarge ferr i te crysta ls nucleated a t austenite grain bound-a r ie s . The shoots have form ed a t the a : y boundar ie s oc -cupying approximate ly the fo rmer loca t ions of the gra inboundaries. While transformation stasis was st i l l inprogress a t the t ime i so the rmal r eac t ion of th is spec imenwas te rmina ted , the obse rva t ions of th is inves t iga t ion in-d ica te tha t when ca rb ides have formed a t an apprec iab lepropor t ion of the a : 7 bou ndar ie s present , t r ansformat ionresumes wi th de tec tab le k ine t ics . T hese ca rb ides can de -ve lop as a compon ent of shoot s t ruc tures but do so moref requent ly a s pa r t o f e i the r f ibrous ca rb ide or in te rphaseboundary ca rb ide a r rays . The da rk-e tch ing , unreso lvedfr inge in Figure 6, similar ly replacing an austenite grainboundary, is the optical micrographic equivalent of e i therF igure 16 or an equiva len t s t ruc ture o f f ibrous o rin te rphase boundary ca rb ides .F igures 17 and 18 a re montages of br ight- and da rk-f ie ld TEM mic rographs , r e spec t ive ly , taken of a eu tec -to id s t ruc ture deve lop ed f rom a poss ib ly a l lo t r iomorphica r ray of f e r f ite c rys ta ls . T here a re some indica t ions tha tthe ca rb ides a re of the in te rphase boundary type , thoughthe t i l t angle of the thin foil with respect to the e lectronbeam was unfortunately not optimized for visualiz ing thistype of mic ros t ruc ture . By u t i l iz ing both of these mon-tages, one can establish fa ir ly well that the ferr i t ic com-ponent of the in te rphase boundary ca rb ide s t ruc ture d idnot form by the cont inued growth of the proeutec to idfe r r i te subs t ra te c rys ta ls . Ins tead , we l l -de f ined bound-a r ie s sepa ra te the "eutec to id fe r r i te " f rom the "pro-eutec to id fe r r i te . " Upon c lose r examina t ion , theseboundar ie s , in some ins tances , can be seen to conta inc lose ly spaced d is loca t ions , ind ica t ing tha t they a reprobably of the smal l -angle type . F rom the obse rva t ionso f R e y n o ld s e t a l . i381 on Fe-C -M o alloys an d o f Me nonand Aa ron so n E481 on a Ti -C r al loy , th is in dicate s that th eeutec to id fe r r i te was a lmost ce r ta in ly formed by sym-pathetic nu cleation tl~ a t aust enit e :pro eute ctoid ferr i teboundar ie s . As wi l l be noted in Sec t ion IV, th is obse r -va t ion provides fur the r ev idence for an impor tan t a spec tof the mechanism of the ba in i te r eac t ion .

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Fi g . 1 3 - - R e p l i c a t i o n mi c ro g r a p h o f f i b e r s d e v e l o p i n g f ro m c a rb i d e s n u c l e a t e d a t a n a : y b o u n d a ry . R e a c t e d 2 0 0 s a t 7 0 0 ~

F i g . 1 4 - - R e p l i c a t i o n m i c r o g r a p h o f r i b bo n - t yp e c a r b i d e s w i t hb ra n c h i n g . C a rb i d e s a re i n a f e r r it e ma t r i x b u t a re i n c o n t a c t wi t hma r t e n s i t e ma t r i x ( l o we r l e f t -h a n d c o me r o f mi c ro g ra p h ) . R e a c t e d20 ,040 s a t 585 ~

I V . D I S C U S S I O NA. Solute Drag-Like Effect Explanation fo r Effects ofCr on Ferrite and Carbide Formation

As in Fe -C -M o al lo ys , I1'2'5'13'491 he s olu te dr ag -li ke e f-fec t (SDLE) appears to be the bes t explana t ion present lyava i lab le for the d i f fe rences in overa l l t r ansformat ion k i -ne t ic s , f e r r i te morphology, and ca rb ide morphology anddis t r ibut ion be tween those repor ted he re and previous lyin Fe-C-Cr a lloys and those observed in Fe-C alloys, pla inca rbon s tee ls , and a l loy s tee ls which exhib i t ne i the r abay in the i r TTT diagram nor t r ansformat ion s ta s is . Ini t s present form, a s desc r ibed in anothe r paper in th isissue, t21 the S DL E is based upon the gradu al swee pingup of a nonequ i l ibr ium concent ra t ion of an a l loying e le -ment w hich markedly dec reases the ac t iv i ty of ca rbon inaus teni te a t the mobi le a reas of advanc ing a :y bound-ar ies. (These areas necessar i ly have a disordered struc-ture; tS~ on an a tomic scale , their eff ective a rea nowappears to be usua l ly no more than tha t of k inks on ther isers of growth ledg es on a :y boundaries.t51~) Th e Xconcent ra t ion thus adsorbed d iminishes the ca rbon ac t iv-i ty gradient in aus teni te , dr iv ing the growth of f e r r i te

Fi g . 1 5 - -S h o o t s o f f e rr i t e e me r g i n g f ro m t h e si d e s o f f er r i te p l a te sw i t h c a r b i d e s e m b e d d e d a m o n g t h e m a l o n g f o r m e r a : y b o u n d a r i e s .Reacted 10 ,200 s a t 555 ~

F i g . 1 6 - -F r i n g e o f " sh o o t " s t ru c t u re ro u n d fe r ri t e c ry s t a l s n u c l e a t e dat austen i te g ra in boundaries . Reacted 100 s a t 570 ~

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Fig. 17 --M on tag e of br ight- f ield TEM micrographs showing probab le smal l -angle ferr i te: ferr ite boundar ies between proeutectoid ferri te crystalsand interphase Nmndary carbide s t ructure. Arrows indicate some locat ions of these boundar ies . Reacted 20,040 s at 570 ~

Fig. 1 8- -S am e as Fig. 17 but in a dark f ield.

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un der para equ ilib rium t52~ conditions.t53~ W he n this gra-dient is reduced to zero, growth stasis occurs.[53] If allferrite crystals in a specimen are simultaneously in growthstasis, despite the spectrum of t imes over which they havenucleate d, th en transfo rmatio n stasis occurs, f ll Wh en theSDL E is suf f ic ien t ly s t rong , anothe r f ac tor en te r s whicheventually expedites the development of both growth andtransformation stasis , even though i t works init ia l ly inthe oppos i te sense . The reduc t ion in the late ra l migra t ionra te of growth ledges (and l ike ly a lso in the rate of ledgegenera t ion) provides enhanced oppor tuni t ie s for the sym-pathetic nucleation of new ferr i te crysta ls a t the terracesof growth ledges. I21 The newly formed ferr i te crysta lscan grow rapid ly , a lbe i t a t a cont inuous ly d iminish ingrate , t~,21 unti l they have accum ulated a suff ic ient con-cent ra t ion of X a t the mobi le a reas o f the i r a : 3, bound-a r ie s to en te r growth s ta s is , whereupon the processrepeats, tl,Z~ Ho we ver, wh en the interiors of austenite grainsconta in a suf f ic ien t num ber o f sympa the t ica l ly nuc lea tedferr i te crysta ls for a t ime long enough so that the min-imum ca rbon concent ra t ion in the remain ing aus ten i ter i ses apprec iab ly above tha t of the bulk ca rbon concen-t ra t ion in the a l loy , the d r iv ing force for sympa the t ic nu-cleation is soon diminished suff ic iently to preven t fur thernuc lea t ion . In th is manner , the i so the rmal r eac t ion t imeand pe rhaps even the propor t ion of f e r r i te r equi red toinduce t r ansformat ion s ta s is should be s igni f icant ly re -duced , inasmuch as the ave rage d i f fus ion d is tance re -qui red to produce over lap of the ca rbon d i f fus ion f ie ldsof nea rby fe r r i te c rys ta ls wi l l be ma te r ia l ly d iminishedwhen a l lo t r iomorphs growing uni formly f rom the gra inboundaries toward the inter iors of austenite grains arereplaced by the "ske le ta l" degenera te Widmanst / i t tenmorp holo gy tSl produced by repea ted sympa the t ic nuc le -a t ion and by both the more nea r ly i so t ropic growth re -suiting from additional X adsorption at the more disorderedareas o f a : y boun daries tS~ and by the more exten sive fo r-mat ion of h igh super ledges a t a:3, boundaries.t21 Thisinc rease in the ca rbon concent ra t ion throughout the re -main ing u nt ransformed aus ten i te a lso fur the r d iminishesthe ca rbon ac t iv i ty gradien t dr iv ing growth . Hence , theoccurrence of both grow th and transformation stasis takesplace sooner than in the s i tua t ion where fe r r i te growthis la rge ly conf ined to the th ickening of gra in boundarya l lo t r iomorphs .

The an omalous ly h igh pe rcent tr ansformat ion a t whichtransform ation stasis occurs a t 585 ~ pct insteadof roughly 6 pc t ( s ince Tb = 600 ~ and stasis occurs a t12 pc t t r ansformat ion dur ing reac t ion a t 570 ~vides a useful test of the sym pathetic nucleation/degeneratefe r r i te component of the SDLE explana t ion of growthand transform ation stasis . Fig ure 5 demonstra t es thatdur ing s ta s is of 585 ~ only a smal l p ropor t ion of de -generate Widmanst/Rten structure is present. Hence, stasismust be accom pl ished main ly by reduc t ion of the ca rbonac t iv i ty in aus ten i te in contac t wi th a :3 , boundar ie s tothat of the austenite in the bulk a lloy. Howe ver , Figure 19shows (he re somewha t a f te r comple t ion of s ta s is ) tha tthe degenera te W idmanst / i tten s t ruc ture deve lops rap id lythroughou t the in te r ior s of the aus ten i te gra ins a t 570 ~thereby permitt ing (as just descr ibed) increases in the ac-t iv i ty of ca rbon in the reg ions o f aus ten ite fur thes t awayf rom the advanc ing a :3 , boundar ie s to cont r ibute to r e -

Fi g . 1 9 - -R e t u ru o f t h e Wi d ma n s t~ i t t en s tru c t u re in l a rg e q u a n t i t ie su p o n fu r t h e r u n d e rc o o l i n g b e l o w t h e b a y t e mp e ra t u re . R e a c t e d 2 6 0 sa t 570 ~

duc t ion in the dr iv ing force for the growth of f e r r ite . Th einc reased dr iv ing force ava i lab le for sympa the t ic nuc le -ation as the reaction temperature was reduced from 585 ~to 570 ~ i s probably pr imar i ly r e spons ib le for the morerapid and ex tens ive format ion o f degenera te s t ruc ture a tthe lower tempera ture , s ince sympa the t ic nuc lea t ion hasnow been recog nize d ~2j as a m ajor contr ib utor to the de-velopment of this special var iant of the Widmanst~it tenpla te morphology.Diminution of the f requency with which Widmans~ttenmorphologies appear be tween the upper nose in the TTTdiagram and Tb, degenera t ion of the Widmanst i i t ten mor -phologies be low Tb, a n d d e v e lo p m e n t o f i n c o m p le t et ransformat ion be low the bay (wi th the except ion d is -cussed in the preceding paragraph) are a ll quali ta t ivelythe same as in Fe-C-Mo alloys. [1 '2 '5 '131 As Boswell et al. tS~noted and the present s tudy conf i rms in more de ta i l , Cris le ss e f fec t ive , pe r a tomic pe rcent X, than Mo in pro-duc ing these e f fec ts . S ince the inf luence of Cr upon theactivity of carbon in austenite is near ly the same as thatof Mo,t54~ one m ay suggest instead that the smaller ato mics ize d i f fe rence be tween Cr and Fe than be tween Mo andFe resu l t s in a le sse r tendency for Cr to be adsorbed a ta give n type of a : 3, boun dary, t551 Conti nuation of f ib rouscarbide formation to temperatures appreciably below thato f Tb in Cr but no t in Mo a l loys l ike ly re su l t s f rom thelesse r degeneracy of the fe r r i te formed in Fe -C-Cr a l -loys . The seve re na ture of th is degeneracy in Fe -C-Moalloys, i t has been sugges ted, ~131 inhibits unif orm devel-opm ent o f paralle l f ibers, s ince this requires that the a : 3,boundar ie s f rom which they deve lop re ta in an approxi -mate ly cons tan t or ien ta t ion over an apprec iab le a rea i f a"colo ny" of f ibrous ca rb ides i s to deve lop , a s i l lus tra tedin Figure 13.The "sho ot" [46] microst ructure (Figure 15) is a lso ex-p l icable on the basis of the SDL E. Carb ide prec ip i ta t iona t a : 3 ' boundar ie s dra ins Cr f rom them through d i f fus iona long these boundar ie s . Lo ca l d iminut ion of the SDL Efol lows. Th e an iso t ropy of a : 3, bound ary mobi l i ty w hichobta ins a t these tempera tures in the absence of the SDL E( i .e . , in Fe-C alloys and plain carbo n steels I6j) can thusdeve lop loca l ly , lead ing to the format ion of s idepla te s .

1474--V OLU ME 21A, JUNE 1990 METALLURGICAL TRANSACTIONS A


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Th e obse rva t i on by Hu l t g ren t461 o f t h i s m i cros t ru c t u re i na n F e - C - M o a l l o y , p r e v i o u s l y n o t e d , i s in d i c a t i v e o f t h eg e n e r a l i t y o f t h i s m o r p h o l o g i c a l d e v e l o p m e n t a n d p r o -v i d e s s o m e f u r t h e r s u p p o r t f o r t h e S D L E - b a s e d e x p l a -n a t i o n o f f e r e d .B . O v e r a l l R e a c t i o n K i n e t ic s B e h a v i o r s

L y m a n a n d T r o ia n o m l d e t e r m i n e d t h e u p p e r a n d l o w e rl i m i t i n g t e m p e r a t u r e s o f i n c o m p l e t e t r a n s f o r m a t i o n a s af u n c t i o n o f c a r b o n c o n c e n t r a t i o n i n 3 p c t C r s t e e l s . O nt he ex t r apo l a t i on t ha t To i n t he p resen t a l l oy i s t ha t a tw h i c h s t a s i s b e g a n a t 0 p c t t r a n s f o r m a t i o n , t h i s t e m p e r -a t u r e i s i n g o o d a g r e e m e n t w i t h t h a t i n t e r p o l a t e d f r o mt h e i r p l o t o f To vs p e r c e n t C . H o w e v e r , a t c a r b o n c o n -c e n t r a t i o n s r a n g i n g f r o m 0 . 0 8 t o 1 . 0 2 p c t , t h e y f o u n dt h a t t h e t o t a l d i l a t o m e t r i c e x p a n s i o n a t t h e t i m e t r a n s -f o r m a t i o n s t a s i s b e g a n i n c r e a s e d s m o o t h l y a s t h e r e a c -t i o n t e m p e r a t u r e w a s d e c r e a s e d b e l o w To . T h e a n o m a l yf o u n d d u r i n g t h e p r e s e n t s t u d y i n t h e p e r c e n t t r a n s f o r -m a t i o n a t s t a s i s a s a f u n c t i o n o f r e a c t i o n t e m p e r a t u r e i sn o t c o n s i s t e n t w i t h t h e i r re s u l t s. I n v i e w o f t h e s m a l li n t e r v a l s o f r e a c t i o n t e m p e r a t u r e w h i c h L y m a n a n dT r o i a n o w e r e a b l e t o e m p l o y a n d t h e w i d e r a n g e o f t o ta le x p a n s i o n s t h e y f o u n d a t e a c h t e m p e r a t u r e , i t is s u r p r is -i n g t h a t t h is a n o m a l y w a s n o t d e t e c t e d w i t h d i l a t o m e t r y .L y m a n a n d T r o i a n o t2~l d i d n o t p r e s e n t a n y p l o ts o f d i -l a t o m e t r i c e x p a n s i o n vs i s o t h e r m a l r e a c t i o n t i m e . R o s ea n d F i s c h e r , tz ~ h o w e v e r , s h o w e d a s e r i e s o f i s o t h e r m a lt r a n s f o r m a t i o n c u r v e s , d e t e r m i n e d b y a m a g n e t i c t e c h -n i q u e , i n b o t h t h e p e a r l i t e a n d b a i n i t e r a n g e s o f a0 . 6 4 p c t C , 1 . 3 2 p c t C r s t e e l ( al s o c o n t a i n i n g 0 . 1 7 p c tM n a n d 0 . 2 6 p c t S i ). I n t h i s a l l o y , t h e p e a r l it e a n d b a i n -i te r e a c t io n s d i d o v e r l a p a p p r e c i a b l y . F u r t h e r , i s o t h e r m a lr eac t i on t i mes u t i li zed d i d no t ex t end beyond 400 seconds .N o n e o f t h e i s o t h e r m a l r e a c t i o n c u r v e s p r e s e n t e d e x h i b -i t ed T y p e I V b e h a v i o r ( F i g u r e 2 ( k )) d e m o n s t r a t i n gt r a n s f o r m a t i o n s ta s i s . O n l y T y p e I a n d I I b e h a v i o r s w e r es h o w n . I t is p o s s i b l e t h a t T y p e IV k i n e t i c s , i n c l u d i n gt r a n s f o r m a t i o n s t a s i s , w o u l d h a v e d e v e l o p e d a t h i g h e rt e m p e r a t u r e s i n t h e a b s e n c e o f th e p e a r l i t e re a c t i o n . O nt h e f i n d i n g s o f R e y n o l d s et al. t21 o n F e - C - M o a l l o y s ,howe ver , i t is a l so poss i b l e t ha t t r ans fo rmat i on s tas is m ayn o t o c c u r a t a n y t e m p e r a t u r e i n t h i s a l l o y .T h e a b s e n c e o f T y p e I I a n d I I I b e h a v i o r s i n t h e p r e s-e n t a l lo y m a y h a v e r e s u lt e d f r o m t h e c i r c u m s t a n c e t h a tt h e c o m b i n a t i o n o f C a n d C r c o n c e n t r a t i o n s w a s s u f f i -c i e n t t o p la c e t h is a l l o y f a r e n o u g h i n t o t h e c o m p o s i t i o nr e g i o n i n w h i c h s t a s i s o c c u r s s o t h a t T y p e I V b e h a v i o rc o u l d p r e v a i l a t a l l t e m p e r a t u r e s s t u d i e d . I n F e - C - M oa l l o y s , i t w a s f o u n d t h a t T y p e I I I b e h a v i o r , i n p a r t ic u l a r ,d e v e l o p e d i n a l l o y s l o c a t e d c l o s e r t o t h e C - M o c o n c e n -t r a t ion r eg i on i n w h i ch t r ans fo rmat i on s t asi s d i d no t occ u ra t a n y t e m p e r a t u r e b e l o w T b .C . D e v e l o p m e n t o f B a i n i t e N o d u l e s

T h e p r o p o s a l h a s b e e n r e c e n t l y m a d e t h a t t h e a p p r o x -i m a t e l y e q u ia x e d n o d u l e i s th e f u n d a m e n t a l m o r p h o l o g yo f m i c r o s t r u c t u r a l l y d e f i n e d b a i n i t e . I371 T h e m u c h m o r ef a m i l i a r p l a t e m o r p h o l o g i e s , e . g . , u p p e r a n d l o w e r b a i n -i te i n s t e e l a s w e l l a s a l l o t r io m o r p h i c a n d i n v e r s e b a i n i t e ,w e r e s u g g e s t e d t o b e d e r i v a t i v e m o r p h o l o g i e s r e f l e c t i n g

p r i m a r i l y t h e m o r p h o l o g y o f t h e s u b s t r a t e p r o e u t e c t o i dp h a s e c r y s t a l ( o r g r o u p i n g o f c r y s t a l s ) a t w h i c h f o r m a -t i o n o f a p a r t i c u l a r b a i n i t i c a g g r e g a t e m o r p h o l o g y w a si n i t i a t ed , t33,37j Co nd i t i on s wh i ch m us t be fu l f i l l ed fo r t hed e v e l o p m e n t o f n o d u l a r b a i n it e w e r e d e d u c e d 1371 a n d ap -p l i e d t o e x p e r i m e n t a l o b s e r v a t i o n s i n b o t h T i - C r a l -lo y s 37,4~ an d s t e e l s . [ 37 '3 8 '5 7 ] These cond i t i ons , here s t a t edi n t h e t e r m i n o l o g y o f a u s t e n i t e d e c o m p o s i t i o n , i n c l u d et h e f o l lo w i n g : ( 1 ) s u b s t a n t i a l c o n t i g u o u s v o l u m e s o f u n -t r a n s f o r m e d a u s t e n i t e ; ( 2 ) m u c h h i g h e r n u c l e a t i o n r a t e so f c a r b i d e a t e u t e c t o i d f e r r i t e : a u s t e n i t e t h a n a t p r o -eu t ec t o i d f e r r it e : aus t en i te boundar i es ; and (3) m uch morer a p i d f o r m a t i o n o f g r o w t h l e d g e s a t t h e f o r m e r t h a n a tt he l a tt e r boundar i es . Cond i t i ons (2 ) and (3 ) l ead t o h i ghe ra n d m o r e n e a r l y i s o t r o p i c g r o w t h k i n e t i c s o f e u t e c t o i dt h a n o f p r o e u t e c t o i d f e r r i t e , e s p e c i a l l y a t l a t e s t a g e s i nt h e p r o e u t e c t o i d f e rr i te r e a c t i o n . T h e t w o p r o d u c t p h a s e so f e u t e c t o i d d e c o m p o s i t i o n , f e r r i te a n d c a r b i d e , n o w b e -c o m e m o r e n e a r l y e q u a l p a r t n e r s i n g o v e r n i n g t h e k i -ne t i cs and m orpho l ogy o f g rowt h . U n t i l t he c l o s i ng s t ageso f a u s t e n it e d e c o m p o s i t i o n , w h e n t h e n o d u l a r m o r p h o l -o g y m u s t a s s u m e t h e s h a p e o f th e v o l u m e s o f a u s te n i ter e m a i n i n g u n t r a n s f o r m e d , t h e e x t e r n a l m o r p h o l o g y o fn o d u l a r b a i n i t e i s a p p r o x i m a t e l y e q u i a x e d , i . e . , a b o u ts p h e r i c a l i n t h e i n t e r io r s o f a u s t e n i t e g r a i n s a n d r o u g h l yhem ispher ical at grain boundar ies . [4~ Par t icular ly w he nt h e i n t e r n a l s t r u c t u r e o f n o d u l a r b a i n i t e i s n o t f u l l y r e -s o l v e d o r w h e n t h e c a r b i d e s i n b a i n i t e n o d u l e s a r e e l o n -g a t e d i n th e d i r e c t i o n o f g r o w t h , t h e r e i s a p r o n o u n c e dten de nc y to m ista ke no du lar b aini te for pea r l i te . 157,581

B y d i s c o u r a g i n g t h e d e v e l o p m e n t o f b o t h s i d e p l a t e sa n d i n t r a g r a n u l a r p l a t e s , t h e S D L E p r o v i d e s t h e l a r g ec o n t i g u o u s v o l u m e s o f u n o c c u p i e d a u s t e n i t e c o n s t i t u t i n gc o n d i t i o n ( 1 ) f o r n o d u l a r b a i n i t e f o r m a t i o n . T h i s i s p a r -t i c u l a r ly w e l l i l l u s t ra t e d i n F i g u r e 9 ( a ) . E v e n a t l o w e rr e a c t i o n t e m p e r a t u r e s , w h e n a p p r e c i a b l e n u m b e r s o f d e -g e n e r a te W i d m a n s ~ t t e n s tr u c tu r e s h a v e d e v e l o p e d w i th i na u s t e n i t e g ra i n s , t h e o t h e r c o m p o n e n t s o f F i g u r e 9 d e m -o n s t r a t e t h a t r e l a t i v e l y " b u l k y " r e g i o n s o f a u s t e n i t e r e -m a i n u n t r a n s f o r m e d a n d a r e t h u s a v a i l a b l e f o r t h ed e v e l o p m e n t o f n o d u l a r b a i n i te .D . C o n t i n u e d G r o w t h ( H u h g r e n ) v s S y m p a t h e t i cN u c l e a t i o n M e c h a n i s m s o f E u t e c t o id F e r ri t e F o r m a t i o n

T h e l a s t is s u e t o b e c o n s i d e r e d i s th a t o f t h e H u l t g r e n SJm e c h a n i s m f o r t h e f o r m a t i o n o f t h e f e r r i t i c c o m p o n e n to f ( u p p e r ) b a i n i t e a n d , m o r e g e n e r a l l y , f o r t h e d e v e l -o p m e n t o f t h e m a j o r i t y p h a s e o f a b a in i t ic m i c r o s t r u c t u r ew h e n t h i s p h a s e i s t h e s a m e a s t h e p r o e u t e c t o i d p h a s ef o r m e d b e f o r e e u t e c t o i d d e c o m p o s i t i o n b e g a n . H u l tg r e nm a d e t h e l o g i c a l p r o p o s a l t h a t c a r b i d e p r e c i p i t a t i o n a ta : y b o u n d a r i e s s h o u l d e x p e d i t e c o n t i n u e d g r o w t h o f t h ef e r r i t e a r o u n d t h e c a r b i d e s b e c a u s e o f t h e l o c a l i n c r e a s ei n th e d r i v i n g f o r c e f o r f e r r it e g r o w t h . H o w e v e r , b y t a k -i n g a d v a n t a g e o f t h e s l o w e r t r a n s f o r m a t i o n r a t e s o f t h e( h c p ) p r o e u t e c t o i d a r e a c t i o n i n a h y p o e u t e c t o i d T i - C oa l l o y a n d o f r e t e n t i o n o f t h e u n t r a n s f o r m e d ( b c c ) / 3 m a -t r ix p h a s e d u r i n g q u e n c h i n g t o r o o m t e m p e r a t u r e f o l l o w -i n g i s o t h e r m a l t r a n s f o r m a t i o n , i t w a s s h o w n b y m e a n so f T E M t h a t " e u t e c to i d a " f o r m e d b e y o n d a l i n e o f Ti 2C oc r y s t a l s p r e c ip i t a te d a t p r o e u t e c t o i d a : / 3 b o u n d a r i e s w a ssepara t ed f rom i t s p roeu t ec t o i d a subs tr a t e by sm al l - ang leMETALLU RGICAL TRANSACTIONS A VOLUME 21A, JUNE 1990--- 1475


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a : a boundar ie s. [39] S uc h bound ar ie s hav e been sho wn tos e p a r a t e s y m p a t h e t i c a l l y n u c l e a t e d p r o e u t e c t o i d a c r y s -t a l s f r o m t h e p r o e u t e c t o i d a c r y s t a l s a g a i n s t w h i c h t h e yw e r e n u c l e a t e d . [48] T h e s e b o u n d a r i e s a r e a l s o p r e s e n t b e -t w e e n t h e i r r e g u l a r f e r r i t e c r y s t a l s w h i c h , a l i g n e d i n r a g -g e d r o w s t h r o u g h s y m p a t h e t i c n u c l e a t io n , c o n s t i t u t e th ed e g e n e r a t e W i d m a n s t ~ i t t e n s t r u c t u r e c h a r a c t e r i s t i c o f i n -c o m p l e t e t r a n s f o r m a t i o n i n F e - C - M o , [2] F e - C - M n , [59[ a n dF e - C - C r 2~J ( t h e p r e s e n t i n v e s t i g a t i o n ) a l l o y s . F i g u r e s 1 7a n d 1 8 d e m o n s t r a t e t h e s a m e p h e n o m e n o n o n a l a r g e rs c a l e . I n s t e a d o f j u s t o n e r o w o r s h e e t o f c a r b i d e s o rT i 2 C o c r y s t a l s , m a n y s u c h r o w s , e a c h s e p a r a t e d b y e u -t e c t o i d ( i . e . , b a i n i t i c ) f e r r i t e , a r e p r e s e n t w i t h i n t h e b a i n -i te n o d u l e e n c o m p a s s e d b y t h e m o n t a g e s o f F i g u r e s 1 7a n d 1 8 . N o t e t h a t a d d i t i o n a l s m a l l - a n g l e b o u n d a r i e s c a nb e d i s c e r n e d w i t h i n t h i s s t r u c t u r e . T h i s s u g g e s t s t h a t r e -p e a t e d s y m p a t h e t i c n u c l e a t i o n o f e u t e c t o i d f e r r i t e o c -c u r r e d d u r i n g t h e f o r m a t i o n o f t h i s s t r u c t u r e .O n e i s t e m p t e d t o p r o p o s e , b y a n a l o g y t o g r ai n g r o w t hp h e n o m e n a i n p u r e m e t a l s a n d s o l i d s o l u t i o n s , t h a t c a r -b i d e p r e c i p i t a t i o n a t a : y b o u n d a r i e s h a s t e m p o r a r i l yp i n n e d t h e s e b o u n d a r i e s , t h u s r e q u i r i n g s y m p a t h e t i c n u -c l e a t io n i n o r d e r t o r e i n i ti a t e f e r r it e f o r m a t i o n . H o w e v e r ,e x p e r i m e n t a l e v i d e n c e a n d t h e o r y a r e a c c u m u l a t i n g t oi n d ic a t e th a t c a r b i de s n u c l e a t e o n l y r a r e l y o n m o v i n g a : yb o u n d a r i e s . I n s t e a d , c a r b i d e n u c l e a t i o n m u s t t a k e p l a c ea t t h e i m m o b i l e t e r r a c e s o f l e d g e s o r a t o t h e r i n t e r f a c e si m m o b i l i z e d b y t h e S D LE .I 2,6 ~ A m o r e s a t i s f a c t o r y a l -t e r n a t i v e m e c h a n i s m i s t h a t c a r b i d e p r e c i p i t a t i o n t a k e sp l a c e w h e n e i t h e r th e g r o w t h l e d g e d e n s i t y o n a : yb o u n d a r i e s i s l o w o r t h e d r i v i n g f o r c e f o r g r o w t h h a sb e e n s e r i o u s l y d e p l e t e d . W h e n a d j a c e n t p a r a l l e l s i d e -p la te s have th ickened nea r ly in to con tac t wi th each o the r ,t o th e p o i n t w h e r e a l m o s t t h e L e v e r R u l e p r o p o r t i o n o ffe r r i t e has loca l ly p rec ip i ta ted , the d r iv ing fo rce fo r g row thw i l l a p p r o a c h z e r o . U n d e r t h i s c i r c u m s t a n c e , t h e r e s u l t so f A i k i n a n d P l i c h t a [63[ o n t h e c o a r s e n i n g k i n e t i c s o f 1 9 'A 1 -C u p l a t es s u g g e s t t h e n u c l e a t i o n r a t e o f n e w g r o w t hl e d g e s i s a ls o s h a r p l y r e d u c e d . E v e n i n t h e a b s e n c e o f ar e d u c e d d r i v i n g f o r c e , h o w e v e r , i t h as o f t e n b e e n f o u n dtha t a t l a t e r s t ages o f g rowth , the k ine t i c s o f g rowth l edgef o r m a t i o n , b y w h a t e v e r m e c h a n i s m , t e n d t o b e d i m i n -i s h e d , t o a c o n s i d e r a b l e e x t e n t , i n s e v e r a l a l l o y s y s -tem s. [~1,65[ Th is c ircu msta nce is fav orab le to the occ urr enc eo f s y m p a t h e t i c n u c l e a t io n . H e n c e , i t w o u l d n o w a p p e a rt h a t t h e f o l l o w i n g s e q u e n c e m a y o b t a i n d u r i n g t h e i n i t i a lp r e c i p i t a t i o n o f c a r b i d e s a n d s u b s e q u e n t s t a g e s o f t h eba in i t e reac t ion (a s de f ined m i c r o s t r u c t u r a l l y [33,53,641 ra the rt h a n o n t h e b a s i s o f o v e r a l l r e a c t i o n k i n e t i c s ) :( 1 ) g r o w t h l e d g e d e n s i ty a t a n d n e a r a p a r t i c u la r b o u n d -a r y o r i e n t a t io n i s m a r k e d l y d i m i n i s h e d ;( 2 ) c a r b i d e n u c l e a t i o n o c c u r s a t t e r r a c e s b e t w e e n w i d e l ys e p a r a t e d g r o w t h l e d g e s a t s u c h b o u n d a r i e s ;( 3 ) c a r b i d e g r o w t h i n c r e a s e s l o c a l l y t h e s u p e r s a t u r a t i o nf o r t h e s y m p a t h e t i c n u c l e a t i o n o f n e w f e r r i t e c r y s t a ls a n dt h u s t h e k i n e t i c s o f s u c h n u c l e a t i o n ; a n d( 4 ) g r o w t h o f t h e s y m p a t h e t i c a l l y n u c l e at e d c r y st a l s o c -c u r s u n t i l g r o w t h l e d g e d e n s i t y a t t h e s a m e i n t e r p h a s eb o u n d a r y o r i e n t a ti o n a s i n ( 1 ) a g a in b e c o m e s l o w , t h e r e b ye n c o u r a g i n g r e p e t i t i o n o f t h i s c y c l e .A s F i g u r e s 1 7 a n d 1 8 s u g g e s t , t h i s s e q u e n c e d o e s n o tf o r e c l o s e c o n t i n u e d g r o w t h o f f e r r i te a r o u n d b a i n it i c c a r -

b i d e s w h e n t h e a r e a s o f l o w g r o w t h l e d g e d e n s i t y a r es u f f i c i e n t l y s m a l l s o t h a t t h e y c a n b e e a s i l y s u r r o u n d e da n d o v e r g r o w n b y t h e a d j a c e n t a r e as o f t h e a : ~ b o u n d -a r y . H o w e v e r , w h e n a s u b s t an t ia l a : 3 ' b o u n d a r y a r e a w i t ha l o w d e n s i t y o f g r o w t h l e d g e s r e a p p e a rs , t h e s y m p a -t h e ti c n u c l e a ti o n m e c h a n i s m w i l l a g a i n b e c o m e o p e r a t i v e .V . S U M M A R Y

O p t i c a l m i c r o s c o p y , q u a n t i t a t i v e o p t i c a l m e t a l l o g r a -p h y , a n d b o t h r e p l i c a t i o n a n d t r a n s m i ss i o n e l e c t r o n m i -c r o s c o p y h a v e b e e n u t i l i z e d t o i n v e s t ig a t e t h e o v e r a l lr e a c t io n k i n e t ic s a n d t h e m o r p h o l o g y o f t r a n s f o r m a t i o na t r e a ct i o n t e m p e r a tu r e s b e t w e e n t h e u p p e r n o s e ( 7 0 0 ~i n th e T T T c u r v e f o r i n it i at i on o f t r a n s f o r m a t i o n a n d at e m p e r a t u r e ( 5 2 5 ~ a p p r e c i a b l y b e l o w t h a t o f t h e b a y( 6 0 0 ~ i n a n F e - 0 . 1 3 p c t C - 2 . 9 9 p c t C r a ll o y . C o m -p a r i s o n o f t h e t r a n s fo r m a t i o n k i n e t i c s a n d m i c r o -s t r u c t u r e s w i t h t h o s e p r e v i o u s l y r e p o r t e d i n F e - C - M oa l loys [L2,51 was one o f the p r inc ipa l ob je c t ive s o f th i si n v e s t i g a t i o n .A t t e m p e r a t u r e s a b o v e t h a t o f t h e b a y , T b , o v e r a l lt r a n s f o r m a t io n k i n e t ic s w e r e a l w a y s o f t h e s i g m o i d a l o rJ o h n s o n - M e h l ]341 t y p e . A t a l l t e m p e r a t u r e s s t u d i e d b e l o wTb, t h r e e - s t a g e k i n e t i c s d e v e l o p e d . I n t h e f i r s t s t a g e , n oc a r b i d e p r e c i p i t a t i o n w a s o b s e r v e d , a s i n F e - C - M o a l -l o y s ; [21 d u r i n g t h e m i d d l e s t a g e , t r a n s f o r m a t i o n c e a s e de n t i r e l y , i . e . , t r a n s f o r m a t i o n s t a s i s o b t a i n e d , a n d i n t h et h i r d s t a g e , t h e b a i n i t e r e a c t i o n ( n o t t h e p e a r l i t e r e a c t i o n )c o m p l e t e d d e c o m p o s i t i o n o f t h e a u s te n i te m a t r i x . H o w -e v e r , i n s t e a d o f s ta s i s a p p e a r i n g a t c o n t i n u o u s l y h i g h e rp e r c e n t a g e s o f t h e a u s t e n it e m a t r i x t r a n s f o r m e d w i t h i n -c r e a s in g u n d e r c o o l i n g b e l o w Tb, a s r e p o r t e d i n 3 p c t C rs t e e l s w i t h v a r i o u s c a r b o n c o n t e n t s b y L y m a n a n dT r o i a n o 2~l o n t h e b a s i s o f d i l a t o m e t r i c s t u d i e s , s t a s is o c -cur red a t 25 pc t t rans form a t ion a t 585 ~ 12 pc t a t 570 ~4 2 p c t a t 5 5 5 ~ a n d 9 0 p c t a t 5 2 5 ~ E x c e p t f o r t h ea n o m a l o u s l y h i g h p e r c e n t t r a n s f o r m a t i o n a t s ta s is f o u n da t 5 8 5 ~ a n d th e o c c u r r e n c e o f t h e s a m e t y p e o f tr a n s-f o r m a t i o n k in e t ic s ( T y p e I V o n t h e s c h e m e o f F i g u r e 2 ( j )a n d R e f e r e n c e 3 3 ) a t a l l t e m p e r a t u r e s b e l o w T b , t h e k i -n e t i c b e h a v i o r i s s i m i l a r t o t h a t f o u n d i n F e - C - M oa ll o y s. [1,2,51

Mic ros t ruc tura l ly , pea r l i t e appea red a t 700 ~ ( the upp ern o s e i n t h e T T T d i a g r a m ) b u t n o t a t a n y l o w e r t e m p e r -a t u r e . A s i m i l a r l a c k o f i n t e r f e r e n c e b y t h e p e a r l i t e r e -a c t io n w i t h t r a n s f o r m a t i o n p h e n o m e n a i n t h e v i c in i t y o fthe ba y r eg ion ob tains in F e- C- M o al lo ys , t1,1,5,~61 Th eW i d m a o s t a t t e n s t r u c t u r e d i m i n i s h e s i n i m p o r t a n c e a tt e m p e r a t u r e s b e l o w t h a t o f th e u p p e r n o s e , a s i n F e - C -M o a l l o y s , TM b u t r e m a i n s p r e s e n t e v e n b e l o w Tb. H o w -ever , a som ewh a t l a rge r t em pera tu re in te rva l ob ta ins be lowTb t h a n i n F e - C - M o a l l o y s b e f o r e t h e W i d m a n s t / i t t e ns t r uc t u r e ag a i n b e c o m e s p r o n o u n c e d , d u r i n g w h i c h g r a i na n d t w i n b o u n d a r y a l l o t r i o m o r p h s p r e d o m i n a t e . A g a i n ,h o w e v e r , w h e n t h e W i d m a n s t ~ i t t e n s t r u c t u r e r e s u m e s i t sp r e p o n d e r a n c e , i t i s h ig h l y d e g e n e r a t e i n s e e m i n g l y n o n -c r y s t a l l o g r a p h i c f a s h i o n , a l s o a s i n F e - C - M o a l l o y s . TMA n d a g a i n , t h e n o r m a l , p r e d o m i n a n t l y p l a n e - s i d e dW i d m a n s t a tt e n s t ru c t u r e s l o w l y r e a p p e a r s w i t h f u r t h e rd e c r e a s e s i n r e a c t i o n t e m p e r a t u r e .

C a r b i d e s f o r m i n a s s o c i a t i o n w i t h t h e f o r e g o i n g p r o -e u t e c t o i d f e r r i t e s t r u c t u r e s a n d a l s o a s a c o m p o n e n t o f

1 4 7 6 - - V O L U M E 2 1 A , J U N E 1 9 90 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


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n o d u l a r b a i n i t e . B o t h i n t e r p h a s e b o u n d a r y a n d f i b r o u sc a r b i d e s a p p e a r a b o v e a n d b e l o w Tb , w h e r e a s i n a nF e - C - M o a l l o y , [~31 t h e f i b r o u s c a r b i d e s d i s a p p e a r i m -m e d i a t e l y b e l o w To . A d d i t i o n a l l y , s m a l l a m o u n t s o fr ibbon- shaped ca r b ides and f e r r i t e shoots , a s soc ia ted wi thc a r b i d e s, w e r e o b s e r v e d ; b o t h h a v e t h e i r c o u n t e rp a r t s i nF e - C- M o a l loys . [45,47] F ur the r e v id enc e wa s s ecu r ed w i thT E M f o r t h e f o r m a t i o n o f t h e f e rr i ti c c o m p o n e n t o fm i c r o s t r u c tu r a l l y d e f i n e d b a i n i te b y s y m p a t h e t i c n u c l e -a t i o n a t p r o e u t e c t o i d f e r r i t e i n t e rf a c e s r a t h e r t h a n b y c o n -t i n u e d g r o w t h o f f e r ri t e a r o u n d c a r b i d e s n u c l e a t e d a t a : 3 'b o u n d a r i e s , a s o r i g i n a l l y p r o p o s e d b y H u l t g r e n . [ 8 ~ T h es y m p a t h e t i c n u c l e a t i o n - b a s e d m e c h a n i s m w a s p r e v i -o u s l y o b s e r v e d w i th T E M i n a h y p o e u t e c t o i d T i - C oal lo y. [391

A s i n F e - C - M o a l l o y s , t h e S D L E w a s f o u n d t o b e c a -p a b l e o f p r o v i d i n g a q u a l i ta t i v e e x p l a n a t i o n f o r b o t h t h er e a c t i o n k i n e t i c s a n d m i c r o s t r u c t u r a l p h e n o m e n a o b -s e r v e d . T h e a n o m a l o u s l y h i g h p e r c e n t t r a n s f o rm a t i o n a ts t a si s d u r i n g r e a c t i o n a t 5 8 5 ~ c o r r e s p o n d s d i r e c t l y w i t ht h e s p a r s i t y o f t h e W i d m a n s t ~ i t t e n s t r u c t u r e a t t h i s t e m -p e r a t u r e a n d t h u s w i t h t h e l a r g e r a m o u n t o f t r a n s f o r -m a t i o n w h i c h m u s t o c c u r b e f o r e t h e c a r b o n a c t i v i t y i na u s t e n i t e a t l o c a t i o n s r e m o t e f r o m a : 3 ' b o u n d a r i e s b e -c o m e s e q u a l t o th a t a t t h e s e b o u n d a r i e s . T h e l a t e r c a r b o na c t i v i t y i s a p p r e c i a b l y r e d u c e d b y t h e S D L E b u t e v i -d e n t l y n o t s u f f i c i e n t l y t o h a l t t r a n s f o r m a t i o n w i t h o u t t h ea s s i s t a n c e o f c a r b o n e n r i c h m e n t o f t h e r e m a i n i n g a u s -t e n it e . T h e m a i n d i f f e r e n c e b e t w e e n F e - C - M o a n d F e -C - C r a l l o y s i n r e s p e c t t o t h e i r t r a n s f o r m a t i o n b e h a v i o ri n t h e v i c i n i t y o f To i s t h a t t h e S D L E a p p e a r s t o b e l e s se f f e c t i v e a t a g i v e n a t o m i c p e r c e n t X i n t h e l a tt e r a l lo y s .T h e s m a l l e r si z e d i f f e r e n c e b e t w e e n C r a n d F e t h a n b e -t w e e n M o a n d F e m a y b e r e s p o n s i b le f o r th e l e s se r S D L Ei n t h e F e - C - C r s y s t e m .

A C K N O W L E D G M E N T SThe contr ibution of HG was supported by theConse lho Nac iona l De se nvo lv ime nto Cie nt i f i c o eTe c hn olog i c o - -CN P q-B r az i l through G r ant No . EM 20-01 28 /87 . Addi t i ona l support f r om the US X Cor por ationand the Air Force Off ice of Sc ient i f ic Research throughG r ant No . AF O SR 84-0303 i s gr atefu ll y ac know le dge d.

R E F E R E N C E S1. G.J. Shiflet and H.I. Aaronson: Metal l . Trans. A, 1990, vol. 21A,pp. 1413-32.2. W.T. Reynolds, Jr., F.Z. Li, C.K. Shui, and H.I. Aaronson:Me t a l l . Tra n s . A , 1990, vol. 21A, pp. 1433-63.3. F. Wever and H. Lange: Mi t t . K a i se r -Wi l h e l m - In s t . E i se n f o rsc h . ,1932, vol. 14, p. 71.4. R.F. Hehemann, K.R. Kinsman, and H.I. Aaronson: Me t a l l .T r a n s . , 1972, vol. 3, pp. 1077-94.5. P.G. Boswell, K.R. Kinsman, G.J. Shiflet, and H.I. Aaronson:M e c h a n i c a l P r o p e r t i e s a n d P h a s e T r a n s f o r m a ti o n s i n E n g i n e e r -i n g Ma t e r i a l s , TMS-AIME, Warrendale, PA, 1986, p. 445.6. C.A. Dube, H.I. Aaronso n, and R.F. Mehl: R e v . M e t a l l . , 1958,vol. 55, p. 201.7. S. Modin: J e r n k o n t o re t s A n n . , 1958, vol. 142, p. 37.8. A. Hultgren: T r a n s . A S M , 1947, vol. 39, p. 915.9. W,C. Hagel and M. Ruoff: T r a n s . A S M , 1957, vol. 50, p. 184.10. H.I. Aaronson and C. Wells: T r a n s . A I M E , 1956, vol. 206,p. 1216.

11. ASTM Subcommittee XI: P r o c . A m . S o c . T e s t i n g M a t e r . , 1950,vol. 50, p. 444.12. G. Spanos, H.S. Fang, and H.I Aaronson: Me t a l l . Tra n s . A , 1990,vol. 21A, pp. 1381-90.13. H. Tsubakino and H.I. Aaronson: Me t a l l . Tra n s . A , 1987,vol. 18A, pp. 2047-60.14. M. Mannerkoski: A c t a P o l y t e c h . S c a n d . , 1964, ch. 26, p. 7,15. K. Relander: A c t a P o l y t e c h . S c a n d . , 1964, ch. 34, p. 7.16. F.G. Berry and R.W.K. Honeycombe: Me t a l l . Tra n s . , 1970,vol. 1, pp. 3279-86.17. R.W .K. Honeycombe: Me t a l l . Tra n s . A , 1976, vol. 7A,pp. 915-36.18. E.S. Davenport and E.C. Bain: T r a n s . A I M E , 1930, vol. 90,p. ' l17.19. F. W ever and W. Jellinghaus: Mi t t . K a i se r -Wi l h e l m - In s t .E i se n f o rsc h . , 1932, vol. 14, p. 105.20. A. Rose and W. Fischer: Mit t . Kaiser-Wi lhelm-Inst . Eisen forsch . ,1939, vol. 21, p. 133.21. T. Lyman and A.R. Troiano: T r a n s . A S M , 1946, vol. 37, p. 402.22. E.P. Klier and T. Lyman: T r a n s . A I M E , 1944, vol. 158, p. 394.23. K. Campbell and R.W .K. Honeycombe: M e t . S c i . , 1974, vol. 8,p. 197.24. H.I. Aaronson: T r a n s . T M S - A I M E , 1962, vol. 224, p. 870.25. H.I. Aaronson and H.A. Domain: T r a n s . T M S - A I M E , 1966,vol. 236, p. 781.26. J.V. Bee, P.R. Howell, and R.W.K. Honeycombe: Me t a l l .Tra n s . A , 1979, vol. 10A, pp. 1207-12.27. Kehsin Kuo: J . I ro n S t e e l In s t . , 1953, vol. 173, p. 363.28. J. R. Bradley, T. Abe, and H.I. Aaronson: Re v . S c i . l n s t ru m . ,1982, vol. 53, p. 98.29. F.S. Le Pera: M e t a l l o g r a p h y , 1979, vol. 12, p. 263.30. C.K. Shui, W.T. Rey nolds, Jr., G.J. Shiflet, and H.I. Aaronson:M e t a l l o g r a p h y , 1988, vol. 21, p. 91.31. T. Gladman and J.H. Woodhead: J . I ro n S t e e l In s t . , 1960,vol. 194, p. 189.32. J.E. Hilliard and J.W. Cahn: T r a n s . T M S - A I M E , 1961, vol. 221,p. 344.33. H.I. Aaronso n, W.T. R eynolds, Jr., G.J. Shiflet, and G. Spanos:Me t a l l . Tra n s . A , 1990, vol. 21A, pp. 1343-80.34. W.A. Johnson and R.F. Mehl: T r a n s . A 1 M E , 1939, vol. 135,p. 416.35. R.F. Hehemann and A.R. Troiano: Me t . Pro g . , 1956, vol. 70 (2),p. 97.36. H.I. Aaronson: D e c o m p o s i t i o n o f A u s t e n it e b y D i f f u s io n a l P r o -c e sse s , Interscience, New York, NY, 1962, p. 387.

37. H.J. Lee, G. Spanos, G.J. Shiflet, and H.I. Aaronson: A c t aM e t a l l . , 1988, vol. 36, p. 1129.38. W.T. Reynolds, Jr., H.I. Aaronson, F.Z. Li, and C.K. Shui:P h a s e T r a n s f o r m a t i o n s ' 8 7 , Institute of Metals, London, 1988,p. 330.39. H.J. l ee and H.I. Aaronson: J . Ma t e r . S c i . , 1988, vol. 23, p. 150.40. H.J. Lee and H.I. Aaronson: Ac t a M e [a l l ., 1988, vol. 36, p. 1141.41. F.G. Berry, A.T. Davenport, and R.W.K. Honeycombe: T h eMe c h a n i sm o f Ph a se Tra n s f o rm a ti o n s i n C ry s t a l l in e S o l i d s , Instituteof Metals, London, 1969, p. 288.42. R.A. Ricks, P.R. Howell, and R.W.K. Honeycombe: Me [a l l .Tra n s . A , 1979, vol. 10A, pp. 1049-58.43. R.W. K. Honeycombe: M e t . S c i . , 1982, vol. 14, p. 201.44. T. Furuhara and H.I. Aaronson: S c r i p t a Me [a l l . , 1988, vol. 22,p. 1635.45. J.V. Bee and D.V. Edmonds: Me t a l l o g ra p h y , 1979, vol. 12, p. 3.46. A. Hultgren: K g l . S v e n sk a Ve t e n sk ap sa k a d . H a n d l . , 1953, Set. 4,vol. 4.47. L.J. Habraken and M. Economopoulos: Tra n s f o rm a t i o n a n dH a rd e n a b i l i t y i n S t e e l s , Climax Molybdenum Co., Ann Arbor,MI, 1967, p. 69.48. E.S.K . Menon and H.I. Aaronson: A c t a M e t a l l ., 1987, vol. 35,p. 549.49. K.R. Kinsman and H.I. Aaronson: Tra n s f o rm a t i o n a n dH a rd e n a b i l i t y i n S t e e l s , Climax Molybdenum Co., Ann Arbor,MI, 1967, p. 39.50. H.I. Aaronson, C. Laird, and K.R. Kinsman: P h a s e T r a n s f o r -m a t i o n s , ASM, Metals Park, OH, 1970, p. 313.51. H.I. Aaronson: Tra n s . In d i a n In s t . Me t . , 1979, vol. 32, p. 1.52. J.B. Gilmour, G .R. Purdy, and J.S. Kirkaldy: Me t a l l . Tra n s . ,1972, vol. 3, pp. 1455-64.

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53. H.I. Aaronson: T h e M e c h a n i s m o f P h a s e T r a n s f o r m a t io n s i nC ry s t a l l i n e S o l i d s , Institute of Metals, London, 1969, p. 270.54. J.C. Greenbank: J . I ro n S t e e l In s t . , 1971, vol. 209, p. 986.55. J.R. Bradley and H.I. Aaronson: Me t a l l . Tra n s . A , 1981, vol. 12A,pp. 1729-41.56. H.J. Lee and H.I. Aaronson: Ac t a Me t a l l . , 1988, vol. 36, p. 1155.57. G. Spanos, H.S. Fang, D.S. Sarma, and H.I. Aaronson: Me t a l l .T r a n s . A , 1990, vol. 21A, pp. 1391-411.58. N.F. Kennon and N.A. Kaye: Me t a l l . Tra n s . A , 1982, vol. 13A,pp. 975-78.59. W .T. R eynolds, Jr., S.K. Liu, F.Z. Li, S. Hartfield, and H.I.Aaronson: Me t a l l . Tra n s . A , 1990, vol. 21A, pp. 1479-91.

60. A.T. Davenport and R.W. K. Honeycombe: Pro c . R . S o c k , 1971,vol. 322, p. 191.61. V.K. Heikkinnen: A c t a M e t a l l . , 1973, vol. 21, p. 709.62. H.I. Aaronson, T. Furuhara, J.M. Rigsbee, W.T. Reynolds, Jr.,and J.M. Howe: Me t a l l . Tra n s . A , in press.63. R.M. Aikin and M.R. Plichta: Ac t a Me t a l l . , 1990, vol. 38, p. 77.64. H.I. Aaronson and W.T. Reynolds, Jr.: P h a s e T r a n s f o r m a t io n s"87, Institute of Metals, London, 1988, p. 301.65. K.R. Kinsman, E. Eichen , and H.I. Aaronson: M e t a l l . T r a n s . A ,1975, vol. 6A, pp. 303-17.

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