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AGEING OF Fe-Ni-C MARTENSITE
Li Chang, A. Cerezo, G. Smith, M. Miller, M. Burke, S. Brenner, K. Taylor, T. Abe, G. Olson
To cite this version:
Li Chang, A. Cerezo, G. Smith, M. Miller, M. Burke, et al.. AGEING OF Fe-Ni-C MARTENSITE.
Journal de Physique Colloques, 1984, 45 (C9), pp.C9-409-C9-416. �10.1051/jphyscol:1984968�. �jpa-
00224456�
JOURNAL DE PHYSIQUE
Colloque C 9 , supplement au n°12, Tome 45, d e c e m b r e 1984 page C9-409
AGEING OF Fe-Ni-C MARTENSITE
Li Chang, A. Cerezo, G.D.W. Smith, M.K. Miller , M.G. Burke , S.S. Brenner , K.A. Taylor**, T. Abe** and G.B. Olson**
Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, Oxford 0X1 ZPH, U.K.
+Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, U.S.A.
^Department of Metallurgy and Materials "Engineering, University of Pittsburgh, Pa 15261, U.S.A.
**Department of Materials Science and Engineering, M.I.T., Cambridge, Mass 02139, U.S.A.
Résumé - Nous décrivons une étude, entreprise en collaboration, portant sur les étapes initiales du vieillissement de la martensite Fe-Ni-C à haute teneur de carbone, et utilisant les techniques combinées de microscopie ionique de champ (FIM) et de microscopie ionique de champ à haute résolution
(TEM). Le matériau étudié est un alliage Fe-15%poids, Ni-l%poids, C, avec la température Ms inférieure à -35°C. L'alliage a été examiné à la fois avant et après vieillissement à 20°C pendant des durées atteignant 2000 hrs.
Les micrographies FIM ainsi que TEM révèlent une microstructure modulée à très fine échelle. Les microanalyses par sonde à atomes révèlent des bandes riches en carbone contenant jusqu'à 11% (%at) de C. Ce travail fournit une caractérisation positive des changements chimiques surviennent durant ce que l'on nomme l'étape "zéro" de la trempe ou du vieillissement, c'est-à-dire précédant la transition où se produit la formation de carbone.
Abstract We report a collaborative study of the early stages of ageing of high—carbon Fe—Ni—C martensite, using the combined techniques of atom probe field-ion microscopy (FIM) and high-resolution transmission electron microscopy (TEM). The material used in this investigation was an Fe- 15wt%Ni-lwt%C alloy, with Mg temperature below -35°C. The alloy was examined in both the unaged condition and after ageing at 20°6 for times up to 2000 hours. Both FIM and TEM micrographs revealed a very fine scale modulated microstructure. Atom probe microanalyses revealed carbon-rich bands containing up to llat%C. This work provides a positive
characterization of the chemical changes that occur duFing the so-called
"zeroth" stage of tempering or ageing - i.e. preceding transition carbide formation.
1 - INTRODUCTION
Ageing of high-carbon martensite has been of considerable interest in physical metallurgy for many years [l-12]. Due to the complicated microstructure of martensite, there are widely differing interpretations of results obtained from several techniques and a wide range of materials. The atom probe technique has been showrKto be very powerful for the determination of local carbon atom concentrations in martensites [8,10], although its ability to reveal information about crystallographic parameters is very limited. In order to clarify the structural evolution during ageing, therefore, a collaborative research programme has been initialed, using a range of different techniques, to study one material from the unagad s^ate to the stage before transition carbide formation. In this paper we report some preliminary results obtained by TEM and atom probe FIM.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984968
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EXPERIMENTALThe m a t e r i a l used i n t h i s i n v e s t i g a t i o n was an Fe-15wt%Ni-lwt%C a l l o y (Fe-14at%Ni -4.5at%C), w i t h an Ms t e m p e r a t u r e below -35'C. Heat t r e a t m e n t c o n s i s t e d of a u s t e n i z a t i o n f o r 1 hour a t 950°C, water quench and f u r t h e r c o o l i n g i n l i q u i d n i t r o g e n . Ageing t r e a t m e n t was done a t room t e m p e r a t u r e (20°C) f o r v a r i o u s p e r i o d s up t o 2000 hours.
TEM and FIM specimens were prepared from aged m a t e r i a l by s t a n d a r d
e l e c t r o p o l i s h i n g t e c h n i q u e s . Atom probe a n a l y s e s were done a t l i q u i d n i t r o g e n t e m p e r a t u r e , and i n random a r e a mode. The probe a p e r t u r e v a r i e d between approximately 1.5 and 3 nm, depending on t i p v o l t a g e .
3
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RESULTSTEM micrographs of t h i n f o i l s prepared a f t e r 3h and 1150h a g e i n g t r e a t m e n t s , F i g s . l a and l b , show a tweed morphology c o n s i s t i n g of very f i n e s t r i a t i o n s roughly a l o n g t h e t r a c e s of (012) and (012) p l a n e s . The c o a r s e , d a r k f e a t u r e s a r e
c o n t r a s t from screw d i s l o c a t i o n s l y i n g a l o n g 111 d i r e c t i o n s . The TEM micrograph of a FIM t i p prepared a f t e r 1440h a g e i n g i s shown i n Fig.2a. The FIM image i n Fig.2b a l s o r e v e a l e d t h e f i n e s t r i a t i o n s . The d a r k bands c o n t a i n i n g high-carbon r e g i o n s were observed i n a sequence of FIM, carbon and i r o n images u s i n g t h e imaging atom probe t e c h n i q u e , Fig. 3a. Another example of high-carbon bands w i t h -2nm width and i n t e r - b a n d d i s t a n c e of -6nm i s shown i n Fig.3b. The composition p r o f i l e , Fig.4, o b t a i n e d by p r o b i n g down through t h e (011) p o l e of a specimen aged f o r 1580 hours r e v e a l e d q u i t e r e g u l a r v a r i a t i o n s of carbon c o n c e n t r a t i o n s w i t h an average maximum amplitude of about l l a t % and wavelength of roughly 30 atomic l a y e r s (50 i o n s =l l a y e r ) . Fig.5 shows t h e composition p r o f i l e a f t e r 1870h a g e i n g , t o g e t h e r w i t h a diagram of a u t o c o r r e l a t i o n a n a l y s i s i n which t h e s i z e of high-carbon r e g i o n of 2000 i o n s and t h e wavelength of 4000 i o n s a r e c l e a r l y demonstrated. To show t h e v a r i a t i o n of carbon c o n c e n t r a t i o n , evePy p o i n t i n Fig.5 was averaged over 200 i o n s . Averaging over s m a l l e r numbers of i o n s only i n c r e a s e s t h e n o i s e on t h e background. The p e r i o d i c i t y of t h e composition p r o f i l e s shows one of t h e c h a r a c t e r i s t i c s of a modulated s t r u c t u r e . The frequency d i s t r i b u t i o n which can r e v e a l t h e p r o g r e s s of t h e decomposition d u r i n g a g e i n g [14] i s shown i n Fig.6. The sample s i z e i n each c a s e i s 100 i o n s . The d i s t r i b u t i o n i n t h e unaged c o n d i t i o n i s s i m i l a r t o t h e c a l c u l a t e d binomial d i s t r i b u t i o n of a homogeneous s o l i d s o l u t i o n . The f r e q u e n c i e s a t 0 and 1%C a r e i n c r e a s e d w i t h a g e i n g time, and a l s o h i g h carbon c o n c e n t r a t i o n r e g i o n s become p r o g r e s s i v e l y more abundant. T h i s p r o v i d e s evidence t h a t t h e a l l o y i s undergoing continuous phase decomposition.
S t r i k i n g l y , t h e r e i s a peak a t l l a t % C on t h e frequency d i s t r i b u t i o n curve a f t e r 1580h a g e i n g , which may r e p r e s e n t t h e carbon c o n t e n t of t h e high-carbon region.
There i s no evidence of Fe4C composition (2Oat%C) d u r i n g t h e p e r i o d of a g e i n g up t o 2000h examined i n t h e p r e s e n t s t u d y .
4
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DISCUSSIONBoth t h e TEM and FIM atom probe r e s u l t s d e s c r i b e d above s u p p o r t t h e d e d u c t i o n from p r e v i o u s TEM s t u d i e s [7,11,15
]
t h a t a modulated s t r u c t u r e i s formed d u r i n g t h e e a r l y s t a g e s of ageing. A FIM atom probe s t u d y by M i l l e r e t a l . on a Fe-23wt%Ni- 0.42wt%C m a r t e n s i t e w i t h Ms t e m p e r a t u r e -50°C[ l o ]
a l s o demonstrated t h a t a t t h e e a r l y s t a g e s of a g e i n g t h e maximum carbon c o n c e n t r a t i o n s of high-carbon r e g i o n s were of t h e o r d e r of 10at%C, c o n s i s t e n t w i t h t h e p r e s e n t r e s u l t s . To e x p l a i n t h e o c c u r r e n c e of t h e modulated s t r u c t u r e t h e most l i k e l y - m o d e l G t h a t o f - s p i n o d a l decomposition [16]. The h y p o t h e t i c a l s p i n o d a l r e g i o n on t h e phase diagram i s shown s c h e m a t i c a l l y i n Fig.7. The s o l i d l i n e s a r e t h e phase boundaries and t h e dashed l i n e s r e p r e s e n t t h e coherent s p i n o d a l . The composition f i g u r e s a r e based on t h e atom probe r e s u l t s from t h e 0.42wt%C a l l o y [ l o ] , and t h e l w t % C a l l o y used i n t h e p r e s e n t s t u d y , t o g e t h e r w i t h evidence from Mossbauer s t u d i e s by Choo and Kaplow [ 4 ] which i n d i c a t e t h a t Fe-1.86wt%C a l l o y s a l s o undergo decomposition. The s l o p e s of t h e l i n e s a r e u n c e r t a i n a t p r e s e n t . When v i r g i n m a r t e n s i t e decomposes i n s p i n o d a l mode, t h e carbon c o n c e n t r a t i o n s i n high-carbon r e g i o n s i n c r e a s e w i t h ageing time a s t h e frequency d i s t r i b u t i o n i m p l i e s . A f t e r r e a c h i n g a m e t a s t a b l e(a)
.1
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E l e c t r o n micrographs of m a r t e n s i t e aged a t room temperature ( a ) f o r 3h; and (b) f o r 1150h.Both a r e b r i g h t f i e l d images taken with t h e i n c i d e n t e l e c t r o n beam n e a r l y along
[loo],
but with t h e f o i l t i l t e d s l i g h t l y about [010]Fig.2
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( a ) E l e c t r o n micrograph of a FIM specimen a f t e r 1440h ageing.( b ) FIM image of t h e same specimen taken a t 10 kV.
C9-412 JOURNAL
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( a ) Sequence of ( i ) FIM image, ( i i ) gated carbon and ( i i i ) gated iron images a f t e r 1680h ageing, taken a t 11.5 kV. ( b ) Another gated carbon image taken a t 10.9 kV.0 1
1pole
number of ions x 1000
Fig.4
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Composition p r o f i l e probing through (011) pole with each date point averaged over 100 i o n s . The specimen was aged f o r 1580h.Autocorrelation function for carbon in RT aged Fe-Ni-C.
Carbon composition profile for RT aged Fe-Ni-C.
+I
1
(MYIE WE m OM,201 LUIE. m m s ,
Total number of ions collected (~1000).
F i g . 5
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Composition p r o f i l e and diagram of autocorrelation a n a l y s i s a f t e r 1870h ageing with o r i g i n a l data points averaged over 200 i o n s .JOURNAL DE PHYSIQUE
FREQUENCY. %
CARBON CONCENTRATION. %
FREQUENCY. %
I
1 5 8 0 hrs
CARBON CONCENTRATION. %
Fig.6
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Frequency d i s t r i b u t i o n of carbon content i n 100 ions blocks. Total number of ions c o l l e c t e d i n each ageing condition was over 10000.m- z
15-
10-
x : R e f . 4
0: Ref.10 A : This paper
x
13 hrs m-
15-
10-
C A R B O N ATOMIC PERCENT
5 -
Fig.7
-
Schematic phase diagram of spinodal region of martensite. The s o l i d l i n e s are phase boundaries and the dashed l i n e s represent the coherent spinodal.5-
. I "
5 10 15
equilibrium stage of decomposition at -2at% and llat%C it would appear to proceed into the conventional first stage of tempering by a process of nucleation and growth.
The occurrence of an average maximum carbon concentration of llat%C can tentatively be interpreted in terms of the development of a coherent phase of composition Fe16C?. This composition corresponds to that of the first
intermediate precipitate formed during the low-temperature ageing of iron-nitrogen martensites, Fe16N2 (b.c.t. structure) [17]. There appears to be no other
evidence in the literature suggesting the existence of an iron carbon phase at this composition. X-ray and electron diffraction and Mossbauer data have conventionally been interpreted in terms of the Fe4C phase (fcc structure), although it has been recognised that this may be below stoichiometry in carbon content [18]. A close relationship exists between the two unit cells, since the removal of 50% of the interstitial atoms from the Fe4C structure in an ordered manner produces the Fe16N2 structure [17
1.
The absence of any separatediffraction evidence for a Fe16C2 phase suggests that the carbon atoms may not be fully ordered in this structure. If only partial ordering of the interstitial atoms occurred, then the main diffraction observed would be that from the Fe4C structure. However, weaker features due to the higher order structure might be observable with care. It would be of interest to investigate this point further.
Further work over a wider range of ageing times and temperatures is currently being undertaken, with the intention of fully establishing the limits of carbon concentration reached in the coherent phase, before nucleation of transition carbides occurs.
Acknowledgements
Financial support for this work was kindly provided by the Science and Engineering * Research Council (UK), and the National Science Foundatioin (U.S.A). Li Chang wishes to thank the Ministry of Education Taiwan, Republic of China, for the provision of a graduate scholarship. Oak Ridge National Laboratory is operated by the Division of Materials Science, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
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