SNOEK LIKE RELAXATION IN Fe-Ni-C VIRGIN MARTENSITE

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SNOEK LIKE RELAXATION IN Fe-Ni-C VIRGIN

MARTENSITE

C. Prioul

To cite this version:

JOURNAL DE PHYSIQUE

C o l l o q u e C10, s u p p l k m e n t a u n 0 1 2 , Tome 46, d k c e m b r e 1985 p a g e C10-665

SNOEK LIKE RELAXATION I N Fe-Ni-C VIRGIN MARTENSITE C. PRIOUL

Laboratoire Matkriaux, Ecole Centrale des Arts et Manufactures, 92290 Chdtenay-Malabry, France

R6sum6

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Nous proposons d 1 i n t e r p r 6 t e r par l a t h 6 o r i e de SCHOECK e t SEEGER l e s p e c t r e de f r o t t e m e n t i n t g r i e u r , relev6 dans l a m a r t e n s i t e rgcemment tremp6e d ' a l l i a g e s Fe-Ni-C. Le m6canisme de r e l a x a t i o n envisag6 c o n s i s t e e n d e s s a u t s r 6 v e r s i b l e s d e s atomes d e carbone, d ' u n s i t e o c t a i d r i q u e un a u t r e , dans l e champ de c o n t r a i n t e d e s d i s l o c a t i o n s v i s . Pour l e s temp6ratures s u p 6 r i e u r e s

1

150K, c e processus pr6sente un c a r a c t i r e i r r 6 v e r s i b l e de p l u s e n p l u s marqu6, conduisant a i n s i

ti

l ' a n c r a g e d e s d i s l o c a t i o n s p a r l e carbone.

Abstract

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The i n t e r n a l f r i c t i o n p l o t r e p o r t e d i n v i r g i n Fe-Ni-C m a r t e n s i t e i s i n t e r p r e t e d i n terms of SCHOECK-SEEGER theory. The r e l a x a t i o n pheno- menon considered h e r e r e s u l t s from r e v e r s i b l e carbon r e d i s t r i b u t i o n i n t h e screw d i s l o c a t i o n s t r e s s f i e l d . For temperatures h i g h e r than 150K t h i s pro- c e s s becomes more and more i r r e v e r s i b l e . , t h u s inducing d i s l o c a t i o n pinning by carbon.

I

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INTRODUCTION

I n a r e c e n t overview on tempering of v i r g i n m a r t e n s i t e [l] t h r e e s t a g e s were considered, a s proposed e a r l i e r by WINCHELL e t a l . [2]. The f i r s t one, named " r e l a x a t i o n s t a g e " , occurs a t low temperatures ( ( 2 2 0 ~ ) . The second one c a l l e d "aging" t a k e s place i n t h e ( 2 2 0 - 3 0 0 ) ~ temperature range. F i n a l l y , "tempering"

refers t o c l u s t e r i n g and p r e c i p i t a t i o n phenomena occuring above room temperature. We have demonstrated

[3]

t h a t a n isothermal m a r t e n s i t i c transformation e x h i b i t i n g

C curve behavior develops i n t h e temperature range of t h e " r e l a x a t i o n " s t a g e . Furthermore, we have shown [4] t h a t , except f o r t h e isothermal m a r t e n s i t i c component, t h e i n t e r n a l f r i c t i o n p l o t i s n e a r l y r e v e r s i b l e i n t h e ( 7 7 - 1 9 0 ) ~ temperaturf, range, whereas i r r e v e r s i b l e behavior i s observed f o r h i g h e r temperatures, where aging occurs.

The purpose of t h i s paper i s t o p r e s e n t a new approach of a n e l a s t i c behavior i n t h e so-called " r e l a x a t i o n " and "aging" s t a g e s . Our r e s u l t s a r e shown t o be c o n s i s t e n t with SCHOECK-SEEGER'S theory [5].

I1

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EXPERIMENTAL PROCEDURE

The i n t e r n a l f r i c t i o n experiments were performed on a n automatic i n v e r t e d t o r s i o n pendulum [6]. Unless otherwise s t a t e d t h e t e s t frequency was 1.5 Hz. Three Fe-Ni-C a l l o y s having s i m i l a r Ms temperature (?r 2253) were t e s t e d : A (Fe-30 Ni-0.02 c ) , B ( F e - 2 7 ~ i - 0 . 1 8 ~ ) and C ( F e - l g ~ i - 0 , 5 1 ~ ) . A f t e r i n s i t u quenching a t 77K (1.5K/min.) and a 100

mitt?

holding a t 77K, measurements where r e a l i z e d d u r i n g r e h e a t i n g t o room temperature.

JOURNAL

DE

PHYSIQUE

Fig. 1

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I n t e r n a l f r i c t i o n e v o l u t i o n s i n Fig. 2

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Influence of t h e h e a t i n g r a t e a s quenched Fe-Ni-C a l l o y s . on t h e i n t e r n a l f r i c t i o n (1,

2.

. .

) and r e l a t i v e frequency ( 1 '

,

2',

. .

) e v o l u t i o n s .

111

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EXPERIMENTAL RESULTS

We have reported i n Figure 1 t h e i n t e r n a l f r i c t i o n , e v o l u t i o n s

,

f o r a l l o y s A, B

and C , d u r i n g r e h e a t i n g (1.5K/min) v i r g i n m a r t e n s i t 6

iron

up t o room temperature. Three maxima l a b e l l e d r e s p e c t i v e l y

@

,

M and B a r e observed. The f i r s t one (

@

), a s s o c i a t e d t o t h e i s o t h e r m a l r n a r t e n s i t i c transformation [j]

i s d i s c u s s e d i n a n o t h e r c o n t i b u t i o n

[7].

Although i n c r e a s i n g carbon c o n t e n t i n c r e a s e s t h e maxima

@

and

6 ,

t h e temperature of maximum

@

remains constant ( 2 1 8 ~ ) .

The i n f l u e n c e of t h e h e a t i n g r a t e on

@

and

@

( ~ i ~ . 2 ) h a s been s t u d i e d f o r a l l o y B, a f t e r having suppressed anomaly

@

by thermal c y c l i n g t h e sample i n t h e ( 7 7 - 1 7 0 ) ~ temperature range ( s e e i n s e r t i n ~ i g . 2 ) . We can observe t h a t i n c r e a s i n g h e a t i n g r a t e s r a i s e maximum

@

without any change i n its temperature, whereas maximum

@

is s h i f t e d towards h i g h e r temperatures and tends t o become only a shoulder of t h e main peak

@

(Fig.2

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curve 4). Furthermore, r e l a t i v e frequency e v o l u t i o n s (curves 1'

,

2', 3 ' , 4 ' ) i n d i c a t e t h a t t h e i n c r e a s e of t h e modulus i s g r e a t e r t h e lower t h e h e a t i n g r a t e . No i n f l u e n c e of t h e h e a t i n g r a t e i s observed below 170K.

Figure 3, which p r e s e n t s t h e i n f l u e n c e of t h e t e s t frequency on t h e i n t e r n a l f r i c t i o n p l o t f o r a l l o y B, shows t h a t frequency does n o t a f f e c t t h e maximum

@

Fig.

3

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Influence of t h e t e s t frequency Fig. 4

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Schematic r e p r e s e n t a t i o n of on t h e i n t e r n a l f r i c t i o n (1,2. .) t h e model f o r t h e i n t e r n a l and r e l a t i v e frequency ( 1 ' , 2 ' . .) f r i c t i o n e v o l u t i o n s . e v o l u t i o n s ( a l l o y B).

of

@

.

R e l a t i v e frequency e v o l u t i o n s (curves 1'

,

2 ' . 3 ' , 4 ' ) i n d i c a t e t h a t i n c r e a s i n g t h e t e s t frequency tends t o i n c r e a s e t h e modulus i n t h e ( 1 5 0 - 3 0 0 ) ~ temperature range. Using r e s u l t s reported i n Fig.3 we c a l c u l a t e d t h e a c t i v a t i o n energy

E

= (0.785.05) eV. a s s o c i a t e d t o t h e peak

@

.

Assuming t h a t WE =1 f o r t h e maximum of t h e r e l a x a t i o n phenomenon, we can deduce t h e value of t h e pre- exponential f a c t o r :

co

= 5.10-l7s.

I V

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DISCUSSION

A s observed i n Fig.1, t h e maxima

@

and@ a r e r e l a t e d t o t h e presence of carbon i n t h e m a r t e n s i t i c s t r u c t u r e . Since we have shown previously

14J

t h a t i n c r e a s i n g d i s l o c a t i o n d e n s i t y i n v i r g i n m a r t e n s i t e i n c r e a s e s t h e i n t e n s i t y of M and B

,

t h e i n t e r p r e t a t i o n of t h e s e maxima must account f o r t h e i n f l u e n c e of t h e s e two parameters.

C10-668 JOURNAL DE PHYSIQUE

According t o these experimental observations the i n t e r n a l f r i c t i o n p l o t would r e s u l t from two contributions :

1 ) a r e l a x a t i o n phenomenon developing i n the (100-300)~ temperature range, with a maximum of t h e r e l a x a t i o n s t r e n g t h a t 253K ( f o r 1.5Hz experiments).

2) a n i r r e v e r s i b l e process which should be s i g n i f i c a n t i n the (150-300)K tempe- r a t u r e range, thus i n t e r f e r i n g with the r e l a x a t i o n phenomenon by increasing the modulus and lowering the i n t e r n a l f r i c t i o n .

We h a v e * presented i n Fig.4 a schematic p l o t of this phenomenological i n t e r p r e t a t i o n of the i n t e r n a l f r i c t i o n evolution, assuming a pure r e l a x a t i o n peak. The difference between the t h e o r e t i c a l behavior and the r e a l p l o t reveals the negative contribution of the i r r e v e r s i b l e component ( l a b e l l e d "pinning"). This model i s c o n s i s t e n t with the influence of frequency and heating r a t e .

We believe t h a t , among e x i s t i n g models of r e l a x a t i o n phenomenon involving carbon and d i s l o c a t i o n s , SCHOECK and SEEGER theory L5J can give a convenient d e s c r i p t i o n of our experiments. They pointed out t h a t carbon can respond t o a screw d i s l o c a t i o n s t r e s s f i e l d , j u s t a s

i t

can respond t o an applied s t r e s s , by d i f f u s i n g among its octaedral l a t t i c e s i t e s . This process i s expected t o give r i s e t o a SNOEK-like r e l a x a t i o n peak, never mentioned before because r e v e r s i b l e behavior i s only observed f o r ,very low frequency experiments performed i n the low temperature range. For increasing temperatures (T>150K) the carbon atoms can be progressively trapped on d e f e c t s ( d i s l o c a t i o n s , twins, i n t e r f a c e s ) , thus inducing an i r r e v e r s i b l e pinning of the d i s l o c a t i o n s , which i s revealed by t h e increase of the modulus and the decrease of the i n t e r n a l f r i c t i o n . The k i n e t i c s of t h i s s h o r t range d i s t a n c e rearrangement i s extremely high. This pinning process decreases the i n t e r n a l f r i c t i o n (thus inducing the maximum

@

)and lowers the maximum@so t h a t f o r high frequency experiments ( > 1 0 ~ z ) t h i s peak (which should appear a t 280K) i s not observed.

Such an i r r e v e r s i b l e SNOEK pinning has been proposed t o i n t e r p r e t the f i r s t s t a g e of s t r a i n ageing i n i r o n

18-11]

and i n i r o n n i c k e l carbon a l l o y s L12,13J.

Our i n t e r p r e t a t i o n i s supported by the a c t i v a t i o n energy ( ~ ~ 0 . 7 8 eV.) calculated f o r the r e l a x a t i o n process. The very low pre-expdnential f a c t o r could be a t t r i b u t e d t o t h e extremely high i n t e r n a l s t r e s s e s e x i s t i n g i n the m a r t e n s i t i c p l a t e s .

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OLSON G.B., COHEN M., Met. Trans. 14A. (1983) 1057.

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1111

HUMPHREYS J.H., PLUMTREE A., BRATINA W.J., Acta Met., 17 (1969) 775.

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