A COMPARISON OF CHARACTERISTIC DISTANCE MEASUREMENTS BY AP, TEM AND SANS

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A COMPARISON OF CHARACTERISTIC DISTANCE MEASUREMENTS BY AP, TEM AND SANS

M. Miller, L. Horton, Spooner

To cite this version:

M. Miller, L. Horton, Spooner. A COMPARISON OF CHARACTERISTIC DISTANCE MEASURE- MENTS BY AP, TEM AND SANS. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-409-C2-415.

�10.1051/jphyscol:1986263�. �jpa-00225697�

A COMPARISON OF CHARACTERISTIC DISTANCE MEASUREMENTS BY AP, TEM AND SANS

M.K. MILLER, L.L. HORTON and S. SPOONER'

Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.

'Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.

Abstract- C h a r a c t e r i s t i c d i s t a n c e measurements o f a s p i n o d a l l y decomposed i s o t r o p i c m i c r o s t r u c t u r e obtained from f i e l d - i o n and transmission e l e c t r o n micrographs, from a n a l y s i s of atom probe composition p r o f i l e s and from small angle neutron s c a t t e r i n g d a t a a r e compared. The r e s u l t s a r e i n general agreement although t h e r e a r e

s i g n i f i c a n t d i f f e r e n c e s i n t h e manner i n which t h e phase transformation and t h e aging time a f f e c t t h e measurements with t h e v a r i o u s instruments.

I INTRODUCTION

C h a r a c t e r i s t i c d i s t a n c e measurements may be used t o c h a r a c t e r i z e t h e nature and the k i n e t i c s o f phase transformation and coarsening i n an a l l o y . S e v e r a l techniques have been used t o o b t a i n t h e s e measurements - some d i r e c t l y from micrographs and o t h e r s from i n d i r e c t data. The p r e c i s e n a t u r e of t h e r e l a t i o n s h i p between t h e measurements and the m i c r d s t r u c t u r e i t s e l f is unclear. This paper r e p o r t s a comparison of c h a r a c t e r i s t i c d i s t a n c e s measured from F i e l d Ion Micrographs (FIM), Atom Probe (AP) composition p r o f i l e s , Transmission Electron Micrographs (TEM), and Small Angle Neutron S c a t t e r i n g (SANS) d a t a f o r an i s o t r o p i c s p i n o d a l l y decomposed a l l o y .

11 EXPERIMENTAL

The 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 a commercial Fe - 28.6 wt$ C r -

10.6 wt$ CO a l l o y . This m a t e r i a l was s e l e c t e d because i t e x h i b i t s good c o n t r a s t between t h e phases i n t h e FIM and TEM, h a s l a r g e neutron s c a t t e r i n g d i f f e r e n c e s between Fe, C r and CO, and can produce a m i c r o s t r u c t u r e with a s c a l e a c c e s s i b l e i n a l l t h r e e techniques. The f u l l chemical composition is given i n Table 1 . The a l l o y was s o l u t i o n t r e a t e d a t 925OC f o r 140 minutes, water quenched, isothermally aged a t 525OC f o r 2 , 8 , 24, 7 2 , 192 and 485 hours, and water quenched. The FIM wires, TEM d i s k s and SANS specimens were heat t r e a t e d simultaneously i n t h e same capsule t o ensure e q u i v a l e n t microstructures.

TABLE 1 . Alloy Composition

ElernentCr CO N i Mn S i Cu V Ti MO N P A 1 S C Fe

w t

28.5 10.6 .36 .12 .082 -034 .022 .021 .020 .016 .016 .013 .009 <.005 b a l The FIM micrographs used t o measure t h e l i n e a r i n t e r c e p t o f t h e two phases were taken a t i d e n t i c a l v o l t a g e and specimen t o screen d i s t a n c e

i . e . constant magnific- a t i o n

t o minimize e r r o r s . The atom probe composition p r o f i l e s were obtained i n t h e presence of a small t r a c e ( 7 x l 0 - ~ ~ a ) of neon image gas so t h a t t h e amount of m a t e r i a l removed could be v i s u a l l y monitored and c o r r e l a t e d with t h e atoms captured i n t h e mass spectrometer. The e f f e c t i v e s i z e of t h e probe a p e r t u r e was maintained a t l e s s t h a n 25% of t h e wavelength of t h e composition modulations.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986263

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TEM examinations were performed on P h i l i p s EMQOOT/FEG, P h i l i p s EM430T and JEM lOOC e l e c t r o n microscopes. The c h a r a c t e r i s t i c d i s t a n c e s of t h e m i c r o s t r u c t u r e s were measured from t h e micrographs using a l i n e a r i n t e r c e p t method.

The neutron s c a t t e r i n g measurements were performed with t h e 30 m SANS instrument a t the National Center f o r Small Angle Research a t Oak Ridge National Laboratory.

Monochromatic neutrons with an i n c i d e n t wavelength, A , of 0.475 nm were used.

Experimental data from the p o s i t i o n s e n s i t i v e p r o p o r t i o n a l counter were c o r r e c t e d f o r background and d e t e c t o r s e n s i t i v i t y and reduced t o r a d i a l l y averaged i n t e n s i t y , I , t o give I ( K ) curves where

is t h e magnitude of t h e s c a t t e r i n g v e c t o r [ l ] .

RESULTS

The isothermal aging treatment a t 525OC w i t h i n t h e m i s c i b i l i t y gap produced a f i n e s c a l e f u l l y interconnected m i c r o s t r u c t u r e of a b.c.c. iron-rich phase and a b.c.c.

chromi.um-enriched phase [2,3]. Examples of t h e m i c r o s t r u c t u r e observed i n t h e FIM and TEM a r e shown i n f i g u r e s l and 2 r e s p e c t i v e l y . A TEM c h a r a c t e r i z a t i o n of t h e m a t e r i a l revealed t h e presence of 5-10% a u s t e n i t e and small chromium n i t r i d e p r e c i p i t a t e s a t t h e g r a i n boundaries. The s c a l e of t h e s p i n o d a l l y decomposed m i c r o s t r u c t u r e was uniform throughout t h e matrix and i n t h e v i c i n i t y of g r a i n boundaries but was c o a r s e r next t o t h e a u s t e n i t e regions. Thin f o i l s were found t o be s u s c e p t i b l e t o s u r f a c e o x i d a t i o n , e s p e c i a l l y m a t e r i a l aged f o r s h o r t times. The oxide produced a s i m i l a r image c o n t r a s t but with a d i f f e r e n t s c a l e from t h e spinodal phase s e p a r a t i o n .

An AP distance-monitored composition p r o f i l e f o r a specimen aged 24 h is shown i n f i g u r e 3. Estimates of t h e average t h i c k n e s s of t h e chromium-enriched phase, T, and t h e mean i n t e r c e p t or spacing of t h e s e r e g i o n s , L , were made from t h e s e composition p r o f i l e s . The compositions of t h e chromium-enriched and i r o n - r i c h phases a t t a i n e d t h e i r Steady s t a t e v a l u e s a f t e r aging f o r 24 t o 72 h. Estimates of t h e p e r i o d i c i t y of t h e m i c r o s t r u c t u r e were a l s o obtained from a u t o c o r r e l a t i o n a n a l y s i s of t h e composition p r o f i l e s C21 from (k2-kl)d where kl and k2 a r e t h e l a g s corresponding t o the f i r s t and second maxima and, d, is t h e l a t t i c e spacing of t h e monitored plane.

An example is shown i n f i g u r e 4.

Figure 1 . Neon F i e l d Ion micrograph of a Figure 2. TEM micrograph of

specimen aged 8 h a t 525OC. specimen aged 72 h a t 525OC.

PO

Figure 3. Distance-monitored Atom Probe Composition P r o f i l e o f specimen aged 2Q h a t 525OC.

Om4L-OlLs 85 8373,

AUTOCORRELATION

l 9 2 h al 525%

Figure 4. Autocorrelation a n a l y s i s of Atom Probe Composition P r o f i l e of specimen aged 192 h a t 525OC.

The c h a r a c t e r i s t i c wavenumber, K*, may be determined from t h e SANS s c a t t e r i n g curves which e x h i b i t a broad low angle peak a s shown i n f i g u r e 5. The curves may be scaled upon a master curve, a s shown i n f i g u r e 6 , which i n d i c a t e s t h e expected dynamic s c a l i n g i n t h i s a l l o y system. The i n t e r p r e t a t i o n of K* i n terms of a c h a r a c t e r i s t i c wavelength,

with K*

2

, is expected t o have only q u a l i t a t i v e s i g n i f i c a n c e

i n terms of t h e s p e c i f i c s t r u c t u r e but showed t h e time dependent dynamic s c a l i n g .

Another measure of t h e m i c r o s t r u c t u r e may be obtained from t h e Guinier p l o t of t h e

logarithm of i n t e n s i t y versus t h e square of t h e s c a t t e r i n g v e c t o r , f i g u r e 7. A t

angles g r e a t e r than t h a t corresponding t o K*, a s t r a i g h t l i n e may be f i t t e d t o t h e

curve. The Guinier r a d i u s , RC, which is given by t h e g r a d i e n t of t h i s s t r a i g h t l i n e

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m u l t i p l i e d by J3, is u s u a l l y assumed t o measure t h e a v e r a g e p r e c i p i t a t e s i z e . I n a n i n t e r c o n n e c t e d m i c r o s t r u c t u r e s u c h a n i n t e r p r e t a t i o n is q u e s t i o n a b l e .

ORNL-DWG 85-43021

7000 I I I I I I I I

0

0.060 0.420 0.i8

SCATTERING VECTOR

ORNL-DWG 85-43022

70001 I

, ^{, ,} , ,

0

0.06 0.12 0.48

SCATTERING VECTOR ( P ' )

F i g u r e 5. SANS S c a t t e r i n g I n t e n s i t y , I , F i g u r e 6. S c a t t e r i n g c u r v e s o f f i g u r e 5 p l o t t e d as a f u n c t i o n o f t h e S c a t t e r i n g have been c o l l a p s e d o n t o a m a s t e r c u r v e V e c t o r , K, d e f i n e d a s 4 a s i n @ / A . by l i n e a r s c a l i n g o f t h e X a n d Y a x e s . A t h i r d e s t i m a t e may b e made i n t e r m s o f a l i n e a r c o r r e l a t i o n l e n g t h , lc, which is e x p r e s s e d i n t e r m s o f a s c a t t e r i n g d e n s i t y c o r r e l a t i o n f u n c t i o n a n d may b e o b t a i n e d from t h e s c a t t e r i n g c u r v e

T h i s e s t i m a t e d e p e n d s less on t h e s p e c i f i c s h a p e o f t h e s c a t t e r i n g c u r v e and is a p p l i c a b l e t o t h e i n t e r c o n n e c t e d m i c r o s t r u c t u r e a s w e l l a s d i s t i n c t p r e c i p i t a t e s .

ORNL-DWG

85-

F i g u r e 7. SANS G u i n i e r p l o t where t h e s t r a i g h t l i n e p o r t i o n d e f i n e s ^RG a n d t h e

maximum o f t h e c u r v e d e f i n e s

Specimen aged 24 h a t

t o a ta type power law. The time exponent, a , was obtained from t h e g r a d i e n t of t h e log-log p l o t of t h e c h a r a c t e r i s t i c d i s t a n c e versus aging time using a l i n e a r

r e g r e s s i o n a n a l y s i s , Table 3. For d a t a t h a t e x h i b i t e d a d i s t i n c t d i s c o n t i n u i t y i n g r a d i e n t t h e time exponent was c a l c u l a t e d f o r both s e c t i o n s ( a l and

and t h e o v e r a l l curve, ao. For t h e o t h e r d a t a s e t s t h e quoted

and

values a r e f o r t h e s e c t i o n s of t h e p l o t from 2 t o 24 and from 24 t o 485 h r e s p e c t i v e l y . The

c o e f f i c i e n t s o f determination, R ~ , a r e included i n Table 3 t o show t h e goodness of f i t o f t h e d a t a t o a ta power law behavior.

TABLE 2. Comparison of C h a r a c t e r i s t i c Distance Measurements (nm).

TABLE 3. Comparison of Time Exponents.

Aging Time a t

h 2 8 24 72 192 485

I V DISCUSSION FIM

5.3 6.4 7.9 9.5 12.6 15.3

Time Exponent

ao f i t +

The r e s u l t s f a l l i n t o two main c a t e g o r i e s : those t h a t a r e a s s o c i a t e d with t h e mean s i z e of t h e chromium-enriched phase and those a s s o c i a t e d with t h e mean d i s t a n c e between regions o r p e r i o d i c i t y of t h e chromium-enriched phase. However, t h e p r e c i s e r e l a t i o n s h i p of t h e s e experimentally determined parameters with the i s o t r o p i c m i c r o s t r u c t u r e where both phases a r e f u l l y interconnected is d i f f i c u l t t o a s c e r t a i n .

The d a t a s e t s from t h e FIM and TEM micrographs a r e i n good agreement. This

agreement i n d i c a t e s t h a t t h e FIM a n a l y s i s taken from a s l i g h t l y curved 2-dimensional plane does not s i g n i f i c a n t l y d i f f e r from t h e TEM a n a l y s i s obtained from t h e

i n t e r a c t i o n s of the e l e c t r o n beam with a 3 dimensional s e c t i o n of m a t e r i a l . The mean i n t e r c e p t , L, measured from t h e d i s t a n c e monitored composition p r o f i l e s and t h e k2-kl values from a u t o c o r r e l a t i o n a n a l y s i s of t h e p r o f i l e s a r e a l s o i n good

agreement with these values. The AP and FIM d a t a a r e a l s o c o n s i s t e n t with r e s u l t s r e p o r t e d i n an Fe-Cr binary a l l o y [41. The SANS 1, d a t a a l s o provides a measure of t h e s t r u c t u r a l s c a l e t h a t is c l o s e l y comparable t o t h e FIM, TEM and AP d i r e c t measurements.

TEM

---

5.8 7.4 10.2 11.0 14.6

FIM

.20 .g93

TEM

.22 .g90

SANS

2

lc R G

4.4 5.7 1.6 5.0 6.6 1.8 5.8 7.5 2.1 7.8 9.7 2 . 7 10.1 13.5 3.6 12.1 16.3 4.1 Atom Probe

P r o f i l e

L T

4.9 2.6 6.2 3.0 7.7 2.9 9.2 3.4 12.8 4.4 17.1 4.2

Autocorrelation

k2-kl

d

4.6 6.1 8.3 11.0 14.0 18.0

SANS

2

R G

19 .20 -18

.g66 ,959 .g64 Atom Probe

P r o f i l e

L T

.22 . l 0 -977 .a72

Autocorrelation (kz-kl )d

25

.998

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The FIM, TEM, k2-kl a u t o c o r r e l a t i o n , and AP L d a t a s e t s a l l i n d i c a t e a continuous i n c r e a s e i n t h e s c a l e of t h e m i c r o s t r u c t u r e with aging time. The SANS d a t a , however, a l l c o n t a i n a d i s t i n c t s t a t i s t i c a l l y s i g n i f i c a n t d i s c o n t i n u i t y a f t e r aging f o r 24 h. The mean s i z e of t h e chromium-enriched phase, T, measured from the AP composition p r o f i l e a l s o h a s t h i s d i s c o n t i n u i t y a f t e r approximately t h e same time although t h e e r r o r a s s o c i a t e d with these measurements is l a r g e r . This

d i s c o n t i n u i t y i n s i z e has been previously observed by APFIM i n t h i s a l l o y a t higher aging temperatures 121. The time a t which t h e d i s c o n t i n u i t y occurs corresponds t o t h e time a t which t h e phases a t t a i n a steady s t a t e condition with r e s p e c t t o composition and volume f r a c t i o n . For t h e very e a r l y aging times t h e width of t h e i n t e r f a c e region between t h e phases is s l i g h t l y more extended than f o r t h e longer aging times. The RG, and AP t h i c k n e s s measurements a r e i n general agreement f o r t h e l l s i z e n o f t h e chromium-enriched regions. The SANS 2 r / ~ * measurements which give a measure of t h e p e r i o d i c i t y of t h e microstructure f a l l between t h e s i z e and

p e r i o d i c i t y c a t e g o r i e s .

The time exponent which is f r e q u e n t l y used t o compare experimental d a t a t o

t h e o r e t i c a l models shows some v a r i a t i o n but is s i g n i f i c a n t l y l e s s i n a l l c a s e s than t h e c l a s s i c a l Lifshitz-Slyozov-Wagner value of 0.33. This experimental v a r i a t i o n w i l l make comparison with o t h e r t h e o r e t i c a l models d i f f i c u l t .

V CONCLUSIONS

While t h e r e s u l t s a r e i n general agreement, t h i s study has shown t h a t s i g n i f i c a n t d i f f e r e n c e s a r e measured by t h e t h r e e instruments i n an evolving m i c r o s t r u c t u r e where t h e r e a r e s e v e r a l changes occurring simultaneously. The r e s u l t s a l s o

h i g h l i g h t . t h e advantages of using a v a r i e t y of techniques t o f u l l y c h a r a c t e r i z e t h e s c a l e and n a t u r e of a microstructure.

Acknowledgments

The a u t h o r s would l i k e t o thank D r . S.S. Brenner o f t h e University of P i t t s b u r g h f o r use of t h e Atom Probe. Research sponsored by the Division of M a t e r i a l s Sciences, U.S. Department of Energy, under c o n t r a c t DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.

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29th I n t . F i e l d Emission Symposium;Gothenburg. (1982), ed. Andren, H-O., and

Norden, H., Pub. Almqvist and Wiksell, Stockholm. (1982) 489-496