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QUANTITATIVE STUDY OF SOLID STATE
REACTIONS BY ATOM PROBE MICROANALYSIS
G. Smith
To cite this version:
G. Smith. QUANTITATIVE STUDY OF SOLID STATE REACTIONS BY ATOM PROBE MICROANALYSIS. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-517-C6-522.
�10.1051/jphyscol:1987685�. �jpa-00226893�
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Colloque C6, suppl6ment au n O l l , Tome 48, novembre 1987
QUANTITATIVE STUDY OF SOLID STATE REACTIONS BY ATOM PROBE MICROANALYSIS
G.D.W. Smith
Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K.
A b s t r a c t
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A review i s p r e s e n t e d of some q u a n t i t a t i v e s t u d i e s of phase t r a n s f o r m a t i o n behaviour i n s t e e l s , c a r r i e d o u t u s i n g t h e techniques of f i e l d i o n microscopy and atom probe microanalysis. I t i s demonstrated t h a t c a r e f u l measurement of a l l o y element r e d i s t r i b u t i o n can g i v e d i r e c t i n s i g h t i n t o t h e mechanism of t h e p e a r l i t e r e a c t i o n . Also, t h e measurement of carbon d i s t r i b u t i o n on t h e atomic s c a l e provides v a l u a b l e evidence concerning t h e mechanism of t h e b a i n i t e r e a c t i o n , and r e v e a l s t h a t a s p i n o d a l mechanism i s o p e r a t i n g d u r i n g t h e low temperature ageing of high- carbon f e r r o u s m a r t e n s i t e .1
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INTRODUCTIONThe importance of s t e e l a s an e n g i n e e r i n g m a t e r i a l i s due i n p a r t t o i t s cheapness and ready a v a i l a b i l i t y , b u t a l s o t o i t s v e r s a t i l i t y . A wide range of d i f f e r e n t p r o p e r t i e s can be o b t a i n e d by only modest v a r i a t i o n s i n composition, processing and h e a t treatment. T h i s d i v e r s i t y of behaviour i s d i r e c t l y a t t r i b u t a b l e t o t h e complex phase t r a n s f o r m a t i o n s which e x i s t i n t h e iron-carbon system, and t o t h e s e n s i t i v i t y of t h e k i n e t i c s of t h e r e a c t i o n s t o t h e presence of s m a l l amounts Of s u b s t i t u t i o n a l s o l u t e s , such a s chromium, manganese, n i c k e l and molybdenum.
D e s p i t e t h e immense amount of r e s e a r c h c a r r i e d o u t on s t e e l s i n t h e l a s t s i x t y y e a r s , t h e r e remain many gaps i n our understanding of t h e s e r e a c t i o n s .
H i s t o r i c a l l y , t h e main experimental d i f f i c u l t i e s were t h e very f i n e s c a l e of t h e m i c r o s t r u c t u r e s of many s t e e l s , and t h e l a c k of microscopical and m i c r o a n a l y t i c a l techniques having s u f f i c i e n t s p a t i a l r e s o l u t i o n t o observe them. An a d d i t i o n a l problem was t h e absence of any s u i t a b l e experimental method t o measure l o c a l carbon c o n c e n t r a t i o n s a c c u r a t e l y . The development of t h e atom probe has gone a long way t o overcoming t h e s e d i f f i c u l t i e s . T h i s a r t i c l e w i l l review some of t h e advances i n understanding of t h e p e a r l i t e , b a i n i t e and m a r t e n s i t e tempering r e a c t i o n s which have taken p l a c e a s a r e s u l t of atom probe i n v e s t i g a t i o n s . R e s u l t s o b t a i n e d by o t h e r techniques w i l l a l s o be o u t l i n e d , where t h e i n f o r m a t i o n i s complementary t o t h a t o b t a i n e d from t h e atom probe.
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THE PEARLITE REACTIONIt has long been recognised t h a t t h e r a t e - c o n t r o l l i n g s t e p i n t h e p e a r l i t e r e a c t i o n i n a l l o y s t e e l s a t s m a l l undercoolings below t h e t r u e t e r n a r y e u t e c t o i d
temperatures must g e n e r a l l y be t h e d i f f u s i o n of s u b s t i t u t i o n a l s o l u t e atoms / I / . R e d i s t r i b u t i o n ( p a r t i t i o n i n g ) of each s o l u t e s p e c i e s t o t h e phase where i t i s thermodynamically most s t a b l e i s n e c e s s a r y i n o r d e r t o produce a p o s i t i v e d r i v i n g f o r c e f o r t h e r e a c t i o n . A t l a r g e r undercoolings, t h e s i t u a t i o n i s not s o c l e a r . Hultgren /2/ concluded t h a t t h e r e were two t y p e s of p e a r l i t e , which he c a l l e d
o r t h o p e a r l i t e and p a r a p e a r l i t e , depending on whether t h e a l l o y elements were, o r were n o t , p a r t i t i o n e d p r e f e r e n t i a l l y between t h e product phases. P u l s and Kirkaldy / 3 / proposed t h a t a l l o y elements which r a i s e d t h e e u t e c t o i d temperature, e s p e c i a l l y t h e s t r o n g c a r b i d e formers, would p a r t i t i o n over a very wide range of
t r a n s f o r m a t i o n temperatures, while t h o s e which lowered t h e e u t e c t o i d temperature might show much more l i m i t e d p a r t i t i o n i n g behaviour. E a r l y experimental s t u d i e s of a l l o y element p a r t i t i o n i n g were based on chemical a n a l y s i s of e x t r a c t e d c e m e n t i t e
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987685
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p l a t e s /2,3,4-71. Thin-foil a n a l y t i c a l e l e c t r o n microscopy (AEM) was subsequently a p p l i e d t o t h e problem by Ridley and coworkers. T h e i r s t u d i e s of manganese p a r t i t i o n i n g i n d i c a t e d a well-defined t e m p e r a t u r e of about 683OC below which no p a r t i t i o n i n g was observed 181. S i m i l a r measurements f o r chromium i n d i c a t e d t h a t no r e d i s t r i b u t i o n occurred below t h e thermodynamic n o - p a r t i t i o n temperature, 703O~.
It was d i f f i c u l t t o understand how t h e s e elements could e x e r t such a profound i n f l u e n c e on t h e k i n e t i c s of t h e p e a r l i t e r e a c t i o n over t h e whole t r a n s f o r m a t i o n temperature range i f t h e e x t e n t of t h e i r r e d i s t r i b u t i o n was so l i m i t e d . The f i r s t u s e of t h e atom probe i n t h i s f i e l d was by Turner
1111,
who r e p o r t e d enrichment of manganese i n t h e c e m e n t i t e phase of a p e a r l i t i c s t e e l , but d i d n o t g i v e any d e t a i l s of t r a n s f o r m a t i o n c o n d i t i o n s . More d e t a i l e d atom probe measurements by M i l l e r and Smith 112,131 d e m n s t r a t e d t h a t , c o n t r a r y t o e a r l i e r AEM r e s u l t s , s i g n i f i c a n t r e d i s t r i b u t i o n of manganese, s i l i c o n and chromium took p l a c e a t t r a n s f o r m a t i o n temperatures a s low as 560°c. Subsequently t h i s work was extended by Williams 114- 161, who demonstrated e x t e n s i v e chromium p a r t i t i o n i n g a t a temperature of 59T0c, more t h a n 80' below t h e previously-reported n o - p a r t i t i o n temperature. From measurements of p e a r l i t e spacing, growth r a t e and a l l o y element r e d i s t r i b u t i o n , Williams was a b l e t o show t h a t p a r t i t i o n i n g took p l a c e by means of a s h o r t - c i r c u i t d i f f u s i o n p a t h i n t h e t r a n s f o r m a t i o n f r o n t . The measured d i f f u s i o n f l u x was s l i g h t l y g r e a t e r (by a f a c t o r of -2.5) t h a n t h a t expected on t h e b a s i s of t h e g r a i n boundary d i f f u s i o n c o e f f i c i e n t of chromium i n i r o n . P o s t - t r a n s f o r m a t i o n a n n e a l i n g t r e a t m e n t s were a l s o c a r r i e d o u t , and y i e l d e d a v a l u e f o r t h e bulk d i f f u s i v i t y of chromium i n f e r r i t e which was c o n s i s t e n t w i t h e x t r a p o l a t e d l i t e r a t u r e values.Mottishaw e t a 1 1171 extended t h i s work t o vanadium, and showed a complex p a t t e r n of r e d i s t r i b u t i o n , i n v o l v i n g p r e c i p i t a t i o n of secondary vanadium-chromium carbo- n i t r i d e s , a s w e l l a s p r e f e r e n t i a l p a r t i t i o n i n g of vanadium and chromium t o t h e c e m e n t i t e phase. Most r e c e n t l y , M a s t r a n t o n i s 1181 h a s s t u d i e d complex s t e e l s c o n t a i n i n g chromium, manganese, s i l i c o n and vanadium, and has demonstrated t h a t a l l of t h e elements concerned show a p p r e c i a b l e r e d i s t r i b u t i o n over a wide range of t r a n s f o r m a t i o n temperatures. Some of h i s r e s u l t s a r e i l l u s t r a t e d i n F i g s 1 and 2.
The a p p a r e n t d i s c r e p a n c y between t h e atom probe r e s u l t s and t h o s e o b t a i n e d p r e v i o u s l y by AEM have now been s a t i s f a c t o r i l y explained i n terms of t h e l a c k of s p a t i a l r e s o l u t i o n of e a r l y AEM instruments. Using a very high-resolution f i e l d e m i s s i o n STEM w i t h X-ray a n a l y s i s f a c i l i t i e s , Garratt-Reed e t a 1 /19,20/
demonstrated t h a t e x c e l l e n t agreement could be obtained with t h e atom probe r e s u l t s . Ridley and coworkers 121,221 a l s o c a r r i e d o u t f u r t h e r AEM work, u s i n g e x t r a c t i o n r e p l i c a s i n p l a c e of t h i n f o i l s , and a l s o o b t a i n e d good agreement with t h e atom probe. I n a s i g n i f i c a n t e x t e n s i o n t o t h i s work, Garratt-Reed e t a 1 1231 have a l s o used t h e f i e l d emission STEM t o s t u d y t h e chemistry of t h e a u s t e n i t e p e a r l i t e t r a n s f o r m a t i o n f r o n t , which i s not r e a d i l y a c c e s s i b l e t o atom probe a n a l y s i s . S i g n i f i c a n t enrichment of chromium and manganese was d e t e c t e d w i t h i n a few nanometres of t h e f e r r i t e growth i n t e r f a c e , i n specimens p a r t i a l l y transformed a t 630°C. T h i s enrichment i s s u f f i c i e n t t o e x p l a i n t h e enhanced r e d i s t r i b u t i o n of chromium (above t h e l e v e l s expected on t h e b a s i s of g r a i n boundary d i f f u s i o n c o e f f i c i e n t s ) , which was observed by Williams. More i m p o r t a n t l y , i t i n d i c a t e s t h a t a s o l u t e s e g r e g a t i o n e f f e c t , o r a p a r t i a l approach towards l o c a l e q u i l i b r i u m , i s t a k i n g p l a c e a t t h e growth i n t e r f a c e w e l l below t h e thermodynamic n o - p a r t i t i o n i n g temperature. A t p r e s e n t , t h e s p a t i a l r e s o l u t i o n of t h e AEM i s not s u f f i c i e n t t o d i s t i n g u i s h between t h e two p o s s i b l e mechanisms, but e i t h e r process c o u l d e x p l a i n t h e marked e f f e c t s of chromium and manganese on p e a r l i t e r e a c t i o n k i n e t i c s a t l a r g e undercoolings 1241. An o v e r a l l p i c t u r e i s t h u s emerging of a much more a c t i v e i n t e r f a c i a l chemistry t h a n was p r e v i o u s l y s u s p e c t e d , w i t h a l l o y elements
p a r t i t i o n i n g over a wide range of c o n d i t i o n s when k i n e t i c a l l y p o s s i b l e , r a t h e r t h a n o n l y when thermodynamically necessary. A s w e l l a s being of s c i e n t i f i c i n t e r e s t , t h e s e r e s u l t s have s t i m u l a t e d new work on t h e d e s i g n of microalloyed p e a r l i t i c s t e e l s f o r i n d u s t r i a l use, a development which i s a l r e a d y proving s u c c e s s f u l 1251.
111
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THE BAINITE REACTIONThe b a i n i t e r e a c t i o n i s t h e l e a s t w e l l understood of a l l t h e phase t r a n s f o r m a t i o n s i n iron-based systems. H u l t g r e n 121 f i r s t s t u d i e d t h e compositions of c a r b i d e s e x t r a c t e d from b a i n i t i c s t e e l s , and concluded t h a t t h e m i c r o s t r u c t u r e developed
without p a r t i t i o n i n g of a l l o y elements. T h i s was l a t e r e l e g a n t l y confirmed by Chance and R i d l e y 1221, who used AEM t e c h n i q u e s t o a n a l y s e c a r b i d e s e x t r a c t e d from p e a r l i t e and b a i n i t e c o l o n i e s of t h e same chromium steel specimen, transformed a t 550°C. Chromium was found t o be p r e f e r e n t i a l l y p a r t i t i o n e d t o t h e c e m e n t i t e phase i n t h e p e a r l i t i c r e g i o n s , but not i n b a i n i t e . Atom probe s t u d i e s of a chromium- manganese s t e e l by Barnard e t a 1 1261 produced r e s u l t s e q u i v a l e n t t o t h o s e of Chance and R i d l e y , w h i l e Bach e t a 1 1271 r e p o r t e d f i n d i n g some very small, l o c a l i s e d r e d i s t r i b u t i o n of a l l o y elements i n t h e r e g i o n of i n t e r f a c e s i n a b a i n i t i c chromium-molybdenum s t e e l transformed a t 423O~. Recently S t a r k h a s used t h e atom probe t o s t u d y b a i n i t i c silicon-manganese s t e e l s 1281 and molybdenum s t e e l s 129,301. Local r e d i s t r i b u t i o n of manganese was found o n l y a f t e r holding f o r 17 hours o r l o n g e r a t 3 5 8 ' ~ o r 326OC, and no r e d i s t r i b u t i o n of molybdenum was d e t e c t e d a f t e r t r a n s f o r m a t i o n f o r s h o r t times a t 470°C o r 430°C. Thus t h e d e t a i l e d experimental measurements l a r g e l y confirm t h e o r i g i n a l c o n c l u s i o n of H u l t g r e n 121 t h a t l o c a l e q u i l i b r i u m i s n o t a t t a i n e d i n t h e b a i n i t e r e a c t i o n , a t l e a s t a s f a r a s a l l o y elements a r e concerned.
The absence of a l l o y element r e d i s t r i b u t i o n d u r i n g t h e b a i n i t e r e a c t i o n provides a n o p p o r t u n i t y f o r t h e s t u d y of t h e bulk d i f f u s i o n of a l l o y i n g elements d u r i n g post- t r a n s f o r m a t i o n ageing a t e l e v a t e d temperatures. Barnard 1261 used t h e atom probe t o s t u d y t h e p r o g r e s s i v e r e d i s t r i b u t i o n of chromium from f e r r i t e t o c e m e n t i t e d u r i n g a n n e a l i n g of b a i n i t i c s t e e l s f o r p e r i o d s of up t o 40 days a t 486-C. The c o n c e n t r a t i o n g r a d i e n t s i n each phase were measured on t h e atomic s c a l e , and analysed u s i n g s t a n d a r d mathematical methods f o r two-phase d i f f u s i o n couples 1311.
T h i s work permitted t h e f i r s t d i r e c t measurement of d i f f u s i o n c o n s t a n t s a s low a s m2 8-l, and showed t h a t t h e d i f f u s i v i t y of chromium i n f e r r i t e was l e s s (by a f a c t o r of -10) t h a n p r e d i c t e d on t h e b a s i s of e x t r a p o l a t i o n of high temperature d a t a . T h i s discrepancy i s b e l i e v e d t o be due t o anomalous changes i n d i f f u s i o n behaviour i n i r o n a t t h e C u r i e temperature. The r e s u l t s may be of u s e i n a s s e s s i n g t h e s e r v i c e l i f e t i m e s of power p l a n t components, which o p e r a t e f o r long p e r i o d s a t high temperatures.
The r e d i s t r i b u t i o n of carbon d u r i n g t h e b a i n i t e r e a c t i o n h a s a l s o been t h e s u b j e c t of atom probe i n v e s t i g a t i o n s . The experiments a r e d i f f i c u l t t o c a r r y o u t and i n t e r p r e t a c c u r a t e l y , a s i t i s not g e n e r a l l y p o s s i b l e t o i d e n t i f y which p a r t of t h e o v e r a l l t r a n s f o r m a t i o n i n t e r f a c e i s being observed i n t h e FIM. There i s a n
a d d i t i o n a l r i s k of some carbon d i f f u s i o n t a k i n g p l a c e during quenching from t h e r e a c t i o n temperature. D e s p i t e t h e s e d i f f i c u l t i e s u s e f u l r e s u l t s have been
obtained. Bach e t a 1 1271 f i r s t showed t h e enhancement of carbon c o n c e n t r a t i o n i n r e t a i n e d a u s t e n i t e r e g i o n s of chromium-molybdenum b a i n i t i c s t e e l s , w i t h l e v e l s of up t o 8 a t % being reported. F u r t h e r d e t a i l e d i n v e s t i g a t i o n s were c a r r i e d o u t by Bhadeshia and Waugh 1321 and S t a r k 1291, and involved t h e s t u d y of b a i n i t i c silicon-manganese s t e e l s , i n which c a r b i d e p r e c i p i t a t i o n was suppressed d u r i n g transformation. The r e s u l t s show two p a r t i c u l a r l y s i g n i f i c a n t f e a t u r e s : t h e f e r r i t e phase i s found t o grow with a p a r t i a l s u p e r s a t u r a t i o n of carbon, and t h e d e g r e e of carbon enrichment i n t h e p a r e n t a u s t e n i t e i s l e s s t h a n t h a t p r e d i c t e d on t h e b a s i s of e q u i l i b r i u m thermodynamics. Bhadeshia and Waugh i n t e r p r e t t h e s e r e s u l t s i n terms of a m a r t e n s i t i c t y p e growth mechanism f o r b a i n i t e sub-units.
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THE TEMPERING OF MARTENSITEThe s t u d y of m a r t e n s i t i c s t e e l s h a s proved t o be one of t h e most f m i t f u l a r e a s of atom probe research. V i r t u a l l y a l l a s p e c t s of m a r t e n s i t e tempering have now been s t u d i e d , ranging from t h e i n i t i a l r e d i s t r i b u t i o n of carbon atoms during low-- temperature ageing, t o t h e p r e c i p i t a t i o n of c e m e n t i t e and a l l o y c a r b i d e s . Only a b r i e f o u t l i n e of t h i s work can be a t t e m p t e d here.
I n t h e c a s e of low-carbon l a t h m a r t e n s i t e s , i t was proposed many y e a r s ago t h a t t h e absence of t e t r a g o n a l i t y i n t h e as-quenched s t a t e may be due t o t h e s e g r e g a t i o n of carbon t o l a t t i c e d e f e c t s 1331. The f i r s t d i r e c t c o n f i r m a t i o n of t h i s came from t h e work of Chang 1341, who used imaging atom probe techniques t o demonstrate t h e s e g r e g a t i o n of carbon atoms t o d i s l o c a t i o n l i n e s i n a Fe-0.2 w t %C martensite, F i g u r e 3. The carbon atoms a p p e a r t o form " C o t t r e l l atmospheres" w i t h i n -3nm of
C6-520 J O U R N A L DE PHYSIQUE
t h e d i s l o c a t i o n c o r e s , l e a v i n g a v e r y low level of c a r b o n i n t h e f e r r i t e m a t r i x . The carbon atoms a r e d i s t r i b u t e d somewhat i r r e g u l a r l y a l o n g t h e l e n g t h of t h e d i s l o c a t i o n l i n e s . The d i s t r i b u t i o n a p p e a r s u n a f f e c t e d by a low t e m p e r a t u r e h e a t t r e a t m e n t i n t h e r a n g e 100°c.
The composition of t h e t h i n f i l m s of r e t a i n e d a u s t e n i t e found i n i n t e r l a t h r e g i o n s of t h e l a t h m a r t e n s i t e s h a s a l s o been measured u s i n g t h e atom probe 135-371. A s u b s t a n t i a l enrichment i n carbon w a s found, by a f a c t o r o f 2-3 over t h e nominal b u l k l e v e l . The h i g h carbon l e v e l i s a t l e a s t p a r t i a l l y r e s p o n s i b l e f o r t h e enhanced thermal s t a b i l i t y of r e t a i n e d a u s t e n i t e i n t h e s e s t e e l s , a l t h o u g h mechanical s t a b i l i s a t i o n a l s o seems t o be i n v o l v e d 1361. Tempering a t 2 0 0 ' ~ produces very s t r o n g enrichment of carbon i n t h e f e r r i t e - a u s t e n i t e i n t e r f a c e s , t o l e v e l s of -20 a t % 1351. T h i s presumably r e p r e s e n t s t h e o n s e t of i n t e r l a t h c a r b i d e p r e c i p i t a t i o n , which i s b e l i e v e d t o be r e s p o n s i b l e f o r t h e phenomenon of tempered m a r t e n s i t e e m b r i t t l e m e n t 1361.
I n t h e c a s e of e u t e c t o i d plain-carbon m a r t e n s i t e s , w a t e r quenched and aged a t room t e m p e r a t u r e f o r -24 h o u r s , M i l l e r e t a 1 1371 have shown e x t e n s i v e s e g r e g a t i o n of carbon t o c o h e r e n t twin i n t e r f a c e s , t o g e t h e r w i t h t h e f o r m a t i o n of carbon-rich c l u s t e r s i n t h e m a r t e n s i t e m a t r i x . F u r t h e r s e g r e g a t i o n t o t w i n i n t e r f a c e s , and t h e o n s e t of i n t e r f a c i a l c a r b i d e p r e c i p i t a t i o n , were observed a f t e r tempering a t 160°C.
Because of t h e somewhat h i g h M, t e m p e r a t u r e s of p l a i n carbon s t e e l s , i t i s h a r d t o d i s t i n g u i s h between e f f e c t s caused by a u t o t e m p e r i n g d u r i n g quenching, and t h o s e due t o subsequent low-temperature ageing. I n o r d e r t o overcome t h i s problem, e x t e n s i v e s t u d i e s have been c a r r i e d o u t of t h e a g e i n g of iron-nickel-carbon m a r t e n s i t e s h a v i n g Ms t e m p e r a t u r e s below O°C 138-401. Carbon composition f l u c t u a t i o n s of i n c r e a s i n g l y r e g u l a r p e r i o d i c i t y and a m p l i t u d e develop as room-temperature a g e i n g p r o g r e s s e s . The p r o c e s s a p p e a r s t o have a l l t h e c h a r a c t e r i s t i c s of a s p i n o d a l r e a c t i o n , t h e f i n a l composition of t h e c o h e r e n t carbon-rich c l u s t e r s being about 11 at%C. c o r r e s p o n d i n g t o a composition FeaC ( o r Fe16C2). Ageing a t h i g h e r t e m p e r a t u r e s produces, s u c c e s s i v e l y , p r e c i p i t a t i o n of more s t a b l e t r a n s i t i o n c a r b i d e s and c e m e n t i t e p l a t e l e t s 1391.
Low-temperature a g e i n g s t u d i e s of i r o n - n i t r o g e n m a r t e n s i t e s have a l s o been
i n i t i a t e d r e c e n t l y by S t a r k ( u n p u b l i s h e d r e s e a r c h , Oxford U n i v e r s i t y ) . Ageing of a 1.6wt % n i t r o g e n m a r t e n s i t e a t room t e m p e r a t u r e produces remarkably l i t t l e change i n n i t r o g e n c o m p o s i t i o n p r o f i l e s , a l t h o u g h e l e c t r o n microscopy i n d i c a t e s t h a t some f i n e s c a l e s t r u c t u r a l changes have occurred. I n c o n t r a s t , ageing f o r 24 hours a t 1 2 0 ' ~ produces a r e g u l a r d i s p e r s i o n of a " (Fe16N2) p l a t e l e t s . I t seems p o s s i b l e t h a t i n t h i s system t h e low t e m p e r a t u r e a g e i n g p r o c e s s may i n v o l v e o r d e r i n g r a t h e r t h a n s p i n o d a l decomposition. C l e a r l y , t h e r e a r e s t i l l many q u e s t i o n s t o be
r e s o l v e d i n t h i s a r e a , and t h e atom probe i s l i k e l y t o c o n t i n u e t o p r o v i d e a w e a l t h of new and i m p o r t a n t i n f o r m a t i o n .
Acknowledgements
I a m i n d e b t e d t o many s t u d e n t s and c o l l e a g u e s who have c o n t r i b u t e d t o t h e work i n t h i s f i e l d o v e r t h e p a s t t e n y e a r s , i n c l u d i n g M. K. M i l l e r , P. A. Beaven, P. R.
Williams, A. J. G a r r a t t Reed, S. J. Barnard, T. D. Mottishaw, L i Chang and I. S t a r k . The atom probe r e s e a r c h was s u p p o r t e d by t h e S c i e n c e and E n g i n e e r i n g Research C o u n c i l ( U . K . ) . F i e l d e m i s s i o n STEM X-ray i n v e s t i g a t i o n s were s u p p o r t e d by t h e O f f i c e of Naval Research and t h e N a t i o n a l S c i e n c e Foundation (U.S.A.).
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C6-522 JOURNAL
DE
PHYSIQUEF i g u r e 1 (above) Neon FIM image of c e m e n t i t e p l a t e ( d a r k ) i n a l l o y p e a r l i t i c s t e e l . ( c o u r t e s y N. M a s t r a n t o n i s ) .
- Wn F i g u r e 2 ( r i g h t ) composition p r o f i l e s a c r o s s p e a r l i t i c
c e m e n t i t e p l a t e f o r V , C r , C , 100
S i , , M n and Fe. Alloy at%
transformed 900 s e c s a t 610°C Fe H o r i z o n t a l s c a l e : 1000 i o n s ~ 5 n m
( c o u r t e s y N. M a s t r a n t o n i s )
0 WERW IONS 6000
F i g u r e 3. S e g r e g a t i o n of carbon atoms t o a
d i s l o c a t i o n l i n e i n a Fe-0.2wt% carbon a l l o y . Top l e f t : d i s l o c a t i o n
s p i r a l v i s i b l e i n c e n t r a l (110) pole.
Top r i g h t : g a t e d d e s o r p t i o n image f o r C* s p e c i e s . Bottom l e f t : s u p e r p o s i t i o n
of FIM and g a t e d d e s o r p t i o n image.
Bottom r i g h t : s k e t c h showing l o c a t i o n of carbon atoms
I+ surrounding t h e d i s l o c a t i o n core.
\ ( c o u r t e s y L i Chang)
