A study of γ → ε phase transformation in FeMn alloys induced by high pressure and plastic deformation

Scripta

METALLURGICA

A

Vol. 12, pp. 431-434, 1978 Printed in the United States

Pergamon

Press,

Inc.

STUDY OF 7 ÷ c PHASE TRANSFORMATION IN Fe-N~ ALLOYS INDUCED BY HIGH PRESSURE AND PLASTIC DEFORMATION F. Trichter Department o f P h y s i c s

A. Rabinkin, M. Ron and A. Sharfstein D e p a r t m e n t of Materials Engineering Technion, Israel Institute of Technology Haifa, Israel (Received January (Revised February I n Fe-Mn a l l o y s a r e t a i n e d

metastable

24, 1978) 19, 1978)

c - p h a s e may be o b t a i n e d e i t h e r

by r a p i d c o o l i n g

followed by plastic deformation, or byhigh pressure soaking (H.P.S.). The c-phase, which h a s t h e h i g h e s t d e n s i t y among t h e p h a s e s o f t h e Pe-Mn s y s t e m , i s s t a b i l i z e d u n d e r h i g h p r e s s u r e [1]. I t i s a g e n e r a l l y a c c e p t e d [ 2 ] f e a t u r e o f t h e mechanism o f r e c o n s t r u c t i n g F . C . C . - y t o H.C.P.-c-lattices t h a t n u c l e a t i o n and growth o f t h e c - p h a s e i s c o n t i n g e n t upon t h e e x i s t e n c e o f stacking faults in the y-matrix. Under t h e i n f l u e n c e o f t e m p e r a t u r e ( u n d e r - c o o l i n g ) o r p r e s s u r e , such s t a c k i n g f a u l t s b e g i n t o f u n c t i o n a s n u c l e a t i o n c e n t e r s . Upon p l a s t i c d e f o r m a t i o n o f t h e q u e n c h e d a l l o y s , new s t a c k i n g f a u l t s f o r m , w h i l e a t t h e same t i m e growth o f t h e m a r t e n s i t i ¢ c - p h a s e p l a t e s t a k e s p l a c e by s h e a r - a s s i s t e d s l i p mechanisms. P l a s t i c d e f o r m a t i o n can c o n v e r t q u e n c h e d F e - ~ a l l o y s from t h e y - s t a t e t o t h e c - m e t a s t a b l e s t a t e , b u t t h e m i c r o s t r u c t u r e o b t a i n e d a f t e r c o o l i n g h a s b e e n shown t o d i f f e r from t h a t c a u s e d by p l a s t i c d e f o r m a t i o n [ 3 ] ; t h e m a r t e n s i t i c p l a t e s o f t h e t h e r m a l l y i n d u c e d e - p h a s e b e i n g much s m a l l e r t h a n t h e ones p r o d u c e d by p l a s t i c d e f o r m a t i o n . According to a theoretical prediction [4], the y ÷ c transf o r m a t i o n i n F e - ( 2 2 - 5 6 ) a t % ~ a l l o y s s h o u l d be a c c o m p l i s h e d upon s o a k i n g a t p r e s s u r e s o f P > 40 k b a r . I t was t h e p u r p o s e o f t h e p r e s e n t work t o s t u d y some d e t a i l s o f t h e Y ÷ c t r a n s f o r m a t i o n i n d u c e d b o t h by h i g h p r e s s u r e s o a k i n g and by p l a s t i c d e f o r m a t i o n . I n t h e F e - 2 9 . 8 ~h a l l o y u n d e r s t u d y h e r e o n l y t h e y and e - p h a s e s can e x i s t [ 5 ] . o The s p e c i m e n s were c u t and h e l d a t 850°C f o r 2 h o u r s . They were t h e n q u e n c h e d i n o i l t o 20 C u n d e r vacuum. The s p e c i m e n s so p r e p a r e d were c o l d worked t o v a r i o u s d e g r e e s o f r e d u c t i o n , ~ = h o - h / h o , where h e o and h a r e , r e s p e c t i v e l y , t h e i n i t i a l t h i c k n e s s and t h e t h i c k ness after a step of rolling. Some s p e c i m e n s were s u b s e q u e n t l y c o o l e d t o 77 and 4.2K. The h i g h p r e s s u r e s o a k i n g up t o 30 k b a r f o r 1 . 0 h o u r was a p p l i e d i n a h y d r o s t a t i c l i q u i d - m e d i u m c e l l , w h i l e f o r s o a k i n g a t p r e s s u r e s o f up t o 100 k b a r s o l i d - m e d i u m c e l l s h a v i n g o n l y s m a l l l d e v i a t i o n s o f p r e s s u r e from u n i f o r m i t y (1-2%) were u s e d . The p h a s e t r a n s i t i o n s were t r a c e d w i t h t h e a i d o f X - r a y d i f f r a c t i o n and M~ssbauer s p e c t r o s c o p y . The ~ 6 s s b a u e r s p e c t r o m e t e r h a s b e e n d e s c r i b e d e l s e w h e r e [ 6 ] . The s p e c t r a were a n a l y z e d by t h e l e a s t - s q u a r e s b e s t - f i t method w i t h t h e a i d o f an I.B.M. 380/158 Computer a s a s u p e r position of Lorentzian lines. All isomer shifts are given with respect to u-Fe. The ~ 6 s s b a u e r s p e c t r a were a n a l y z e d as a s u p e r p o s i t i o n o f one o r two d o u b l e t s and one s i n g l e l i n e a t t r i b u t e d t o t h e Y1, Y2 and c - p h a s e s r e s p e c t i v e l y . The r e l a t i v e amounts o f t h e e, Y1 and Y2 p h a s e s were t a k e n t o b e p r o p o r t i o n a l t o t h e r e l a t i v e a r e a a t t r i b u t e d t o a p a r t i c u l a r p h a s e : A ~ A , Ay1/A , Ay2/A r e s p e c t i v e l y . The 4 0 - ~ - t h i c k a b s o r b e r had an e f f e c t i v e t h i c k n e s s of t a ~ 5 for a single line. Spectra taken with specially prepared specimens having t a < 1 were found t o c o n s i s t o f e x a c t l y t h e same components as t h e t h i c k e r o n e s . A l l l i n e w i d t h s , F, were e x t r a p o l a t e d t o z e r o a b s o r b e r t h i c k n e s s t 7 ] . R o o m - t e m p e r a t u r e MSssbauer s p e c t r a a r e shown i n F i g . l a , b , c. The M~ssbauer s p e c t r a o f t h e quenched s p e c i m e n s a r e s e e n a t t h e t o p o f F i g . l b and l c . A s p e c t r u m o f a specimen cooled to 4.2 K after quenching is seen in Fig. la. The f e a t u r e s o f t h i s s p e c t r u m a r e e s s e n t i a l l y t h e same as t h o s e o f t h e quenched s a m p l e s . The r e l a t i v e a r e a A~/A a t t r i b u t e d t o t h e e - p h a s e i s s e e n t o i n c r e a s e t o 1 as a r e s u l t o f p l a s t i c d e f o r m a t i o n by r o l l i n g ( F i g . l b ) . relative

The e f f e c t o f h y d r o s t a t i c p r e s s u r e on t h e M3ssbauer s p e c t r a i s s e e n i n F i g . l ( . The a r e a a t t r i b u t e d t o t h e r e t a i n e d e - p h a s e , A J A , h a s i n c r e a s e d , as compared w i t h t ~ e

431

432

PHASE

TRANSFORMATION

as-quenched state, to a value of 0.7. t h e E - p h a s e o n l y f o r ~ > 5%.

IN Fe-Mn A L L O Y S

X-ray diffraction

Vol.

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5

measurements discern the presence of

The change i n t h e M~ssbauer p a r a m e t e r s o f t h e E - p h a s e as a f u n c t i o n o f c o l d work, ~, a r e shown i n Fig. 2a, b. For s m a l l d e g r e e s o f d e f o r m a t i o n t h e i s o m e r i c s h i f t o f t h e s - p h a s e , I . S . s , becomes more n e g a t i v e b u t r e a c h e s a c o n s t a n t v a l u e o f - 0 . 1 3 mm/sec f o r ~ ~ 10%. The line width extrapolated to zero absorber thickness is seen to decrease steeply to a constant v a l u e o f 0 . 2 5 mm/sec a t ~ ~40%. The i n c r e a s e o f t h e r e l a t i v e a r e a , As/A, w i t h ~ i s shown i n F i g . 2a. The y ~ s t r a n s f o r m a t i o n i s s e e n t o be a c c o m p l i s h e d a t ~ ~ 60%. The c h a n g e s i n t h e M~ssbauer p a r a m e t e r s o b s e r v e d upon i n c r e a s e o f p r e s s u r e a r e s i m i l a r t o t h o s e due t o i n c r e a s i n g degrees of plastic deformation. The y + s t r a n s f o r m a t i o n was n o t c o m p l e t e d even a f t e r s o a k i n g f o r 5 h o u r s a t 90 k b a r . I n t h e a s - q u e n c h e d s t a t e a s u p e r p o s i t i o n o f a s i n g l e l i n e and two d o u b l e t s p r o v i d e d t h e b e s t f i t f o r t h e e x p e r i m e n t a l s p e c t r a , f o r which t h e M~ssbauer p a r a m e t e r s were f o u n d t o be as follows: Q.S.Y1 = 0 . 5 9 ± 0.015 mm/s, Q.S.Y2 = 0 . 9 9 ± 0.15 mm/s, t h e I . S . y 1 = I . S . y 2 = I . S . s . The r e l a t i v e a r e a were AY1/A ~ 0 . 3 0 and AY2/A ~ 0 . 4 4 . Here Q.S. d e n o t e s q u a d r u p o l e s p l i t t i n g . The e x i s t e n c e o£ two y - p h a s e s i n Fe-Mn a l l o y s h a s b e e n s u g g e s t e d [ 8 ] . They were t h o u g h t t o h a v e d i f f e r e n t s p e c i f i c v o l u m e s , t h e one b e i n g p o s t u l a t e d t o be p a r a m a g n e t i c , t h e o t h e r antiferromagnetic. T h i s h y p o t h e s i s does n o t , however, seem t o be j u s t i f i e d , because different

nearest neighbour configurations of Fe atoms may also cause different Q.S. and different magnetic ordering characteristics. From t h e p r e s e n t r e s u l t s , t h e f o l l o w i n g c a n be deduced: The d e c r e a s e i n I . S . s may be t a k e n t o be an i n d i c a t i o n o f a d e c r e a s e i n t h e s p e c i f i c volume o f t h e s - p h a s e , w h i l e t h e n a r r o w i n g o f t h e r E l i n e c a n be c o n s i d e r e d as r e f l e c t i n g a l a t t i c e s t r a i n r e c o v e r y . In the as-quenched state the coherent E-phase is subjected to high tensile stresses, since its density is higher than that of the y-phase. S u b s e q u e n t growth o f t h e E - p h a s e upon t h e a p p l i c a t i o n o f e i t h e r H . P . S . o r p l a s t i c d e f o r m a t i o n l e a d t o t h e r e m o v a l o f c o h e r e n c y s t r e s s e s , which i n t u r n result in the line narrowing. According to Ershova et al. [4], the application of pressure extends to the region,of t h e m e t a s t a b l e s - p h a s e i n t h e Fe-MO m e t a s t a b l e p h a s e d i a g r a m . With i n c r e a s i n g p r e s s u r e a single phase region of stable s-phase appears in the T-C-P-phase diagram. Subsequent release o f p r e s s u r e s h o u l d n o t be f o l l o w e d by t h e r e v e r s e s ÷ y p h a s e t r a n s i t i o n . Thus, a s i n g l e E-phase structure should be observed after the removal of pressure. Our e x p e r i m e n t a l r e s u l t s , however, show t h a t t h e y ÷ s p h a s e t r a n s f o r m a t i o n does n o t end upon s o a k i n g a t u n i f o r m p r e s s u r e s , e v e n as h i g h as 90 k b a r , c o n t r a r y t o t h e above m e n t i o n e d t h e o r e t i c a l p r e d i c t i o n [ 4 ] and c e r t a i n e x p e r i m e n t a l r e s u l t s [ 9 ] . T h i s a p p a r e n t d i s c r e p a n c y c a n be a t t r i b u t e d t o t h e e f f e c t o f t h e 10% p l a s t i c d e f o r m a t i o n a p p l i e d i n t h e e x p e r i m e n t t o g e t h e r w i t h 50 k b a r p r e s s u r e i n [ 9 ] . On t h e o t h e r hand t h e t r a n s f o r m a t i o n i s a c c o m p l i s h e d by 60% c o l d work. I t i s i n t e r e s t i n g t o n o t e t h a t p l a s t i c d e f o r m a t i o n , which does n o t c o n v e r t t h e E - p h a s e t o t h e s t a b l e s t a t e b u t k i n e t i c a l l y p r o m o t e s i t s n u c l e a t i o n and g r o w t h , becomes a more e f f e c t i v e f a c t o r f o r developing the y ÷ s transformation than either pressure or temperature. This is in accordance w i t h t h e t h e o r y r e c e n t l y p u t f o r w a r d by Olson and Cohen [ 2 ] on t h e r o l e p l a y e d by t h e s t a c k i n g defects in the y ÷ s transformation. References

i. T.P. Ershova, E.G. Poniatovski, Izv. Akad. Nauk USSR, Metally, ~, 156 (1967). 2. G.B. Olson, Morris Cohen, Met. Trans., 7A, 1897 (1976). 3. H . Schumann, Pract. Metallographie, 12, 511 (1975). 4. T.P. Ershova, E.G. Poniatovski, I.L. Aptekar, Jorn. Phys. Chimii, XLII, NS, 748 (1968). 5. M. Hansen, Constitution of Binary alloys, McGraw-Hill (1958). 6. A. Biran, A. Shoshani, P.A. Moutano, Nucl. Instrum. Meth., 89, 21 (1970). 7. F. Hornstein, M. Ron, Acta. Met., 22, 1557 (1974). 8. G.N. Belozerskiy, V. Gittsoyich, Y. Kramar, O. Sokoloy, Yu, ~imich, The Phys. of Metals and Metallography; 35__, N3, 20 C19731. 9. E.G. Poniatovskl, T.P. HTshova, O,P, ~aksimova, ~.N, Rosenberg, Izv. Akad, Nauk USSR, Metally, 4_, NS, 225 [1967].

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PHASE TRANSFORMATION

Fe-29.8

J

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FZ o

ot.% Mn

IN Fe-Mn ALLOYS

I

433

Fe- 29.8 ot % Mn

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324 1

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Fig,l:

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Mossbauer s p e c t r a f o r Fe-29,8 at,% alloy.

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b~

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c)

Quenched and soaked a t h i g h p r e s s u r e ,

-I 0 VELOCITY RELATIVE TO ~-Fe

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434

PHASE TRANSFORF~TION IN Fe-Mn ALLOYS

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Vol.

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Fig.2. Changes in the isomer s h i f t , l i n e width [ e x t r a p o l a t e d t o zero absorber t h i c k n e s s ) and relative spectral area for the c-phase in the Fe-29,8 at,% ~ alloy vs, , a} reduction of thickness, ~; b} soaking pressure, P,

5