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SHAPE MEMORY EFFECT AND MECHANICAL BEHAVIOUR OF AN Fe-30Mn-1Si ALLOY SINGLE
CRYSTAL
A. Sato, K. Soma, E. Chishima, T. Mori
To cite this version:
A. Sato, K. Soma, E. Chishima, T. Mori. SHAPE MEMORY EFFECT AND MECHANICAL BE-
HAVIOUR OF AN Fe-30Mn-1Si ALLOY SINGLE CRYSTAL. Journal de Physique Colloques, 1982,
43 (C4), pp.C4-797-C4-802. �10.1051/jphyscol:19824130�. �jpa-00222115�
JOURNAL DE PHYSIQUE
Colloque C4, suppl6ment au n o 12, Tome 43, dgcembre 1982 page C4-797
SHAPE MEMORY EFFECT AND MECHANICAL BEHAVIOUR OF AN F e - 3 0 M n - 1 S i ALLOY S I N G L E CRYSTAL
A. Sato, K. soma: E. Chishima and T. Mori
Department of Materials Science and Engineering, Tokyo I n s t i t u t e o f Technology, Nagatsuta, Midori-ku, Yokohama 227, Japan
(Accepted 9 August 1982)
Abstract.- The present paper deals with two main problems associated with the
~ Z E
transformation in an Fe-30th-1Si alloy. The first is the shape memory effect; it has been found that an Fe-30Mn-1Si alloy single crystal exhibits a nearly complete shape memory effect caused by entirely different origin from that adopted for the usual shape memory alloys such as TiNi and Cu based alloys. The second concerns the large hardening produced by the pre-injected E-martensites. This is in contrast to the relatively small hardeningobserved in an Fe-18Cr-14Ni alloy single crystal. Characteristic features of the crossing of an E plate or slip with a pre-injected E plate have been examined, confirming that the observed large hardening comes from the strong blocking by the pre-injected €-plate in absence of an additional EW transformation.Introduction.- With recognition that in an austenitic stainless steel plastic deformation occurs by stress induced martensitic transformation below the Md (y+~) temperature
[I]
and it obeys the Schmid law for the <112>{111} shear [ 2 ] , we have examined the plastic deformation properties associated with the stress induced~ Z E
martensitic transformation [3, 41. In one of these studies it has beendemonstrated that a tensile strain given by a uniaxial deformation below Md temperature
recovers (shrinks) by as much as 40% upon the E-ty reverse transformation. This shape memory effect is much larger than that found by Enami et a1 using a polycrys- talline Fe-18.5fifn alloy [5]. The difference can be essentially ascribed to the number of the €-martensite variants introduced by the applied stress. There is, however, an another factor complicating the interpretation of the experimental observation in Fe-18Cr-14Ni, i.e. formation of a martensites.
The same complication arises when we examine strengthening effect due to E-
martensites. An effect that martensitic transformation may help plasticdeformation has been suggested and called "Window Effect" by Suzuki et a1 [6]. They have concluded that a-martensites induced by plastic deformation (yw) are softening entities to the dislocation motion and act as a window for the gliding dislocation to penetrate through them, thus attributing the terminology (Window Effect) to explain positive temperature dependence appearing below the Md temperature. In opposition to this explanation, we have shown [2, 71 that the yield and flow of a single crystal below the Md temperature are controlled solely by the stress required to induce either the y e , EW or y-w martensitic transformation itself but not by letting dislocations to pass through the transformation products.
Main scheme of the present study is to examine these problems under a simpler experimental condition, by using a single crystal which undergoes the y+e martensi- tic transformation alone. Learning from the work by Gartstein and Rabinkin [8] and considering experimental convenience with respect to the transformation temperature, we have decided to use an Fe-30Mn-1Si alloy single crystal.
"
Presently at Technical Center, Bridgestone Tire Co., LTD., Kodaira-shi, Tokyo 187, JapanArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19824130
C4-798 JOURNAL DE PHYSIQUE
Ex erimental Procedures.- A base material of an Fe-30Mn alloy was prepared by a usEal melting and casting method with electrolytic iron under vacuum of loA2 Torr.
Single crystals of 20 mm in diameter and 150 nun in length were grown in Ar atmosphere by the usual Bridgman method. In the single crystal growing process, a small amount of Si was added by melting Fe3Si into the Fe-30Mn alloy.
Rectangular specimens of 15 x 2 x 0.5 and 10 x 7 x 0.5 mm were cut from the single crystal rods to have the tensiie axis along the [414] direction so that the Schmid factor for the primary [121](111) shear becomes 0.500. In these specimens since primary shear direction was taken nearly parallel to the specimen edge surface, mechanical restriction imposed by tensile axis rotation was sufficiently small. Choice of such an orientation was particularly important in examining hardening due to pre-injected E martensites by a double tensile deformation [9].
For a second step tensile deformation given along a [414] direction, shoulders were cut by a spark-cutting machine after introducing single variant of E-martensites
to have the final dimension of 2 x 0.5 x 0.5 nun inside the gage part. All the specimens were annealed in Ar atmosphere at 1273 K for lh, quenched into an oil bath at 373 K and immediately transfered and stored in an another bath at 473 K.
The Md and As temperatures for the y& transformation were 350 and 410 K, respectively.
Tensile tests were done on an Instron type testing machine at a strain rate of 2 x 10-~/s except those changed by a factor of 5 to measure the strain rate sensitivity. The accurate dimensional change associated with &+y reverse transfor- mation was measured directly by observing indentation markings put on the polished specimen surface with a precision tool microscope. For some specimens, length change was measured by a creep machine to examine a stress effect on the magnitude of the shape memory effect and on the temperature of reverse transformation. After a metallographic examination electron microscopic observation was made on an H-700 microscope equipped with a 60 degree tilting device at an acceleratingvoltage of 200 KV.
Experimental Results and Discussions.
1. Temperature and strain rate dependence of yield stress Figure 1 shows a critical resolved shear stress (CRSS) of Fe-30Mn-lSi, Fe-18Cr-14Ni and Fe-30Mn alloys for motion of a Shockley partial dislocation in the region of positive temperature dependence and of a perfect dislocation otherwise. It is clearly seen that addition of a small amount of Si induces a strong positive temperature dependence quite similar to that observed in an Fe-18Cr-l4Ni alloy. Drastic enhancement of formation of E-martensites and absence of a-martensites in the Fe-30Mn-1Sihavebeenconfirinedby an electron microscopic observation. Similarity of the two curves (Fe-Cr-Ni and Fe- Mn-Si) strongly suggests that the positive temperature dependence observed in an austenitic stainless steel is a direct consequence of stress induced y-x transforma-
Fig. 1 Temperature dependence of Fig. 2 Activation area determined a critical resolved shear stress by strain rate sensitivity tests
6 0 -
300
4 0
>
-
!- u a
110 FWERR
0 200 3 0 0 UOO 500 O 0 5 10 15
TEMPERRT'IRE ( K 1 FORCE [ IO.~NIH 1
t i o n . Judging from t h e l a r g e t e m p e r a t u r e dependence o f t h e y i e l d s t r e s s f o r t h e u s u a l s l i p (Fe-Mn c u r v e i n F i g . I ) , s t r a i n r a t e s e n s i t i v i t y of t h e flow s t r e s s i s e x p e c t e d t o b e a l s o l a r g e . A c t i v a t i o n a r e a determined from t h e s t r a i n r a t e s e n s i t i v i t y t e s t s i s p r e s e n t e d i n F i g . 2 , c l a r i f y i n g t h a t d i s l o c a t i o n motion i s b a s i c a l l y c o n t r o l l e d by overcoming of a s h o r t r a n g e o b s t a c l e . T h i s r e s u l t w i l l b e used l a t e r i n t h e q u a n t i t a t i v e a n a l y s i s o f a s h a p e memory e f f e c t .
2. Shape memory e f f e c t A f t e r t e n s i l e d e f o r m a t i o n below t h e Md t e m p e r a t u r e , l e n g t h
s u r f a c e was measured on t h e specimen s u r f a c e immersed i n a s i l i c o n e o i l b a t h d u r i n g warming up above As t e m p e r a t u r e . A t y p i c a l r e c o v e r y c u r v e n o r m a l i z e d w i t h r e s p e c t t o t h e i n i t i a l e l o n g a t i o n i s shown a s a f u n c t i o n of t e m p e r a t u r e i n F i g . 3. It i s i n t e r e s t i n g t o n o t e t h a t t h e t e n s i l e s t r a i n r e c o v e r s by a s much as 9 7 % i n c o n t r a s t t o t h e i n c o m p l e t e r e c o v e r y ( - 4 0 % a t maximum) found i n t h e similar t e s t of a n Fe-18Cr-14Ni a l l o y [ 3 ] . S i g n i f i c a n t d e v i a t i o n from t h e complete r e c o v e r y , however, occured by i n c r e a s i n g t h e i n i t i a l s t r a i n beyond 10%
due t o t h e o p e r a t i o n of t h e second s h e a r systems. T h i s i s o t h e r e v i d e n c e t o c o n s i d e r t h a t c o m p l i c a t i o n of t h e i n t e r n a l s t r u c t u r e , e i t h e r p r e s e n c e of a m a r t e n s i t e s o r o t h e r E - v a r i a n t s , s u p p r e s s e s t h e s h a p e memory e f f e c t c a r r i e d by t h e y ; ~ t r a n s f o r m a -
t i o n .
F i g . 3 Shrinkage due t o &+y r e v e r s e t r a n s f o r m a t i o n
A s e r i e s of photographs p r e s e n t e d i n F i g . 4 d e m o n s t r a t e t h e s h a p e memory e f f e c t
o b s e r v e d i n r e ~ e a t e d bendinn and t e m p e r a t u r e
( E l
-
c y c l i n g t e s t s . F o r e a c h y%+y c y c l e , we
d e f i n e a f r a c t i o n of r e c o v e r y f a s F i g . 4 Shape memory e f f e c t examined by bending t e s t s
measured a s shown i n F i g . 5 and f i n d f = 0 . 8 0 , 0 . 6 3 and 0 . 6 1 f o r t h e f i r s t , second and t h i r d c y c l e s , r e s p e c t i v e l y . Thus, d e g r e e of s h a p e memory e f f e c r becomes less prominent i n bending t e s t and d e c r e a s e s w i t h r e p e t i t i o n of t h e t r a n s f o r m a t i o n c y c l e . There a r e two p o s s i b l e r e a s o n s t o a c c o u n t f o r t h i s i n c o m p l e t e n e s s : F i r s t l y , s e v e r e d e f o r m a t i o n t a k e s p l a c e s a t l a r g e d i s t a n c e f r o m t h e c e n t e r zone, s u p p r e s s i n g o p e r a t i o n o f a s i n g l e s h e a r s y s t e m upon r e v e r s e t r a n s f o r m a t i o n . Secondly, d e f o r m a t i o n i s complex i n t h e compressed zone where two e q u i v a l e n t s h e a r s y s t e m s a r e
o p e r a t i v e . I n c a s e o f a n Fe-18Cr-14Ni a l l o y , ~ i5 ~~ ~. f i of ~ 0 . i tand i ~0 ~
t h e s h a p e memory e f f e c t i s u s u a l l y i n v i s i b l e 1 f
i n a bending t e s t . T h i s i s e x p l a i n e d by t h e f a c t t h a t f o r m a t i o n of a - m a r t e n s i t e s i s
a c c e l e r a t e d by c o l l i s i o n of two € - v a r i a n t s [ 4 , 91.
C4-800 JOURNAL DE PHYSIQUE
I I
0 5 10
RESOLVED SHERR S T R E S S [ M N / M 2 1
Fig. 6 Temperature of reverse transformation at its maximum rate
0.01 I
5 10 15
RESOLVED SHEAR STRESS ( UN/U 21
Fig. 7 Effect of tensile stress on the amount of shrinkage Temperature of the reverse transformation as well as magnitude of the shrinkage is affected substantially by a tensile stress applied during the reverse transformation as shown in Figs. 6 and 7. These stress dependences are important in analyzing the shape memory effect due to the
GE
transformation. By expressing the free energy required to move a dislocation overcoming a short range obstacle under the influence of chemical driving force and applied stress asS*
AF* =
i
lf(s*)ds*-
(2dAg-
~ ~ b ) s *1 0 EYY 1 (la)
we may write the shrinkage rate in terms of the motion of Shockley partial dislocations:
AFT AF$
ds
= A(1-
E / E ~ ) [plexp(- - p2(- -i;~- )I
(2)On the other hand, rate of the reverse transformation may be given by
AFT AF$
= A(1
-
E / E ~ ) [plexp(- + P~~XP(--
kT )I
( 3 )Shrinkage-fraction measured as a function of tensile stress (Fig. 7) is now approxi- mated by cs/i leading to
with
AFf
-
AFqa = exp(-
kT )
The free energy difference AF* - AF;, describing the magnitude of the shrinkage under applied stress is given by 1
OFT
-
AF? = is*f(s*)ds*sf -
f s* 1 1+
f2s$ (6)2 with
The function of f(s*) for the reverse transformation is represented by that deter- mined for the
y+&
transformation (Fig. 2). Substituting relevant values [lo] weo b t a i n p /p2
--lo4,
implying t h a t o r i g i n of t h e n e a r l y complete shape memory e f f e c t i s a t t r i b u t a b l e t o t h e p r e f e r e n t i a l m u l t i p l i - c a t i o n of a s i n g l e t y p e of Shockleyp a r t i a l d i s l o c a t i o n s . z
3. Hardening due t o € - p l a t e s F i g u r e 8
shows r e p r e s e n t a t i v e s t r e s s - s t r a i n c u r v e s
:
e x h i b i t i n g a remarkably l a r g e h a r d e n i n g due
t o t h e p r e - i n j e c t e d & - p l a t e s . S i n c e t h e
2
200 _____--- -- t e n s i l e a x e s a r e o r i e n t e d c r y s t a l l o g r a p h i - + S T E P Ic a l l y i d e n t i c a l l y f o r a l l t h e f i r s t and
________---
second s t e p t e s t s i t i s e v i d e n t t h a t e i t h e r
,,
t h e s t r e s s r e q u i r e d t o move a p a r t i a l
I
TRUE G T R R I N 3 2d i s l o c a t i o n (A) o r a p e r f e c t d i s l o c a t i o n (B)
becomes e x t r e m e l y l a r g e , e x c e e d i n g 600MN/m2, F i g . 8 S t r e s s s t r a i n c u r v e s i n t h e i n t h e p r e s e n c e of € - p l a t e s . On t h e o t h e r s u c c e s s i v e two s t e p t e s t s . S t e p 1 hand, i n a n i d e n t i c a l t e s t u s i n g t h e Fe- and s t e p 2-A a t 330 K , and s t e p 2-B 18Cr-14Ni h a r d e n i n g due t o s t e p 1 deforma- a t 380 K. Note t h a t Md and As t i o n i s q u i t e s m a l l a s demonstrated i n t h e t e m p e r a t u r e s a r e 350 and 420 K , f i g u r e by broken l i n e s . Thus, comparison r e s p e c t i v e l y .
between t h e Fe-Mn-Si and Fe-Cr-Ni c e r t a i n l y shows t h a t f o r m a t i o n of a i n t h e b l o c k i n g
& p l a t e a i d s t h e p e n e t r a t i o n o f t h e c r o s s i n g & o r s l i p . I n t h i s s e n s e , t h e cx
f o r m a t i o n i s understood t o produce a "Window E f f e c t " . However, i t s h o u l d b e remembered t h a t t h e a c c u m u l a t i o n of
a
produces a l a r g e h a r d e n i n g [ 2 ] .Concerning c o l l i s i o n of € - p l a t e s , some i n t e r e s t i n g o b s e r v a t i o n s made by an i n t e r f e r e n c e and e l e c t r o n microscope a r e p r e s e n t e d i n F i g s . 9 and 10. Although t h e s t e p c o n t r a s t i s n o t s h a r p a t t h e E-~y boundary b e c a u s e of a n e l e c t r o l y t i c p o l i s h i n g g i v e n a f t e r d e f o r m a t i o n , one can imagine how t h e v e r t i c a l € - p l a t e s have p a s s e d through t h e h o r i z o n t a l b l o c k i n g &. Some o f t h e growing p l a t e s a r e c o m p l e t e l y s t o p p e d a t t h e t h i c k p l a t e s , l e a v i n g a s t r o n g l y s t r e s s e d r e g i o n a t t h e i n t e r s e c t i o n . D e t a i l e d m i c r o s t r u c t u r e a t s u c h an i n t e r s e c t i o n c a n b e s e e n i n a m a g n i f i e d v i e w of F i g . 10. Here, b o t h p l a t e s A (formed by t h e s t e p 1 d e f o r m a t i o n ) and B ( s t e p 2) a r e a l i g n e d p a r a l e l l t o t h e e l e c t r o n beam [ O l l l y . It i s n o t e d t h a t p l a t e B t e r m i n a t i n g i n s i d e p l a t e A i s b e n t a t t h e end where i t IS c o m p l e t e l y bound by t h e & - p l a t e A.
The same p l a t e B i n t h e r e g i o n , w h e r e y-matrix i s l e f t a t l e a s t p a r t i a l l y w i t h o u t y+E t r a n s f o r m a t i o n , l i e s almost e x a c t l y on t h e second s h e a r p l a n e (111). From t h e g e o m e t r i c a l c o n s i d e r a t i o n i t i s p o s t u l a t e d t h a t t h i s bending i s caused by t h e [ l i 2 ]
(111) s h e a r which d i d n o t o p e r a t e i n t h e f i r s t s t e p d e f o r m a t i o n . The e x p e c t e d b e n t a n g l e i s - 1 0 " f o r t h e complete Y+E t r a n s f o r m a t i o n , which i s somewhat s m a l l e r t h a n
F i g . 9 S u r f a c e r e l i e f s o b s e r v e d F i g . 1 0 M i c r o s t r u c t u r e a t t h e by a i n t e r f e r e n c e microscope. i n t e r s e c t i o n of two & - p l a t e s viewed
a l o n g [ O l l I y .
C4-8 02 JOURNAL DE PHYSIQUE
t h e o b s e r v e d v a l u e of 15' i n F i g . 10. T h i s d i f f e r e n c e can, however, be a t t r i b u t a b l e t o t h e motion o f a n a d d i t i o n a l p a r t i a l d i s l o c a t i o n . A r e a c t i o n s u c h a s
may o c c u r t o i n c r e a s e t h e b e n t a n g l e , w i t h o u t l e a v i n g t o o many s t a c k i n g f a u l t s i n a c c o r d a n c e w i t h t h e e x p e r i m e n t a l o b s e r v a c i o n . They were s p a c e d 2 0 0 ~ 1 0 0 0 (111) p l a n e s a p a r t .
The t h i c k e n i n g o f p l a t e A observed a s above i s a n o t h e r f a c t o r , i n a d d i t i o n t o t h e a b s e n c e of ~ - t a t r a n s f o r m a t i o n , t o c a u s e t h e l a r g e h a r d e n i n g s i n c e a t h i n E-
p l a t e i s r e l a t i v e l y e a s i l y t r a n s m i t t e d t h r o u g h by t h e growing E p l a t e s o r s l i p s w h i l e a t h i c k e r one i s n o t . W e c o n s i d e r t h a t i n p r i n c i p l e t h e l a r g e h a r d e n i n g produced by t h e p r e - i n j e c t e d & - p l a t e s comes from a l a r g e P e i e r l s s t r e s s f o r a non- b a s a l t y p e o f d i s l o c a t i o n motion i n a n hcp s t r u c t u r e . But d e t a i l e d e x a m i n a t i o n of t h e non-basal s l i p s i n t h e & - p l a t e s remains t o b e f u r t h e r s t u d i e d , i n o r d e r t o d e s c r i b e t h e h a r d e n i n g q u a n t i t a t i v e l y .
Acknowledgement.
The a u t h o r s would l i k e t o e x p r e s s t h e i r t h a n k s t o D r . T. Matsuo a t Tokyo I n s t i t u t e of Technology f o r h i s c o o p e r a t i o n i n Fe-Mn a l l o y p r e p a r a t i o n . They a r e
i n d e b t e d t o M i s s Atomi Yoshizawa f o r t h e e d i t o r i a l a s s i s t a n c e . R e f e r e n c e s .
[ l ] J . F . B r e e d i s and W.D. Robertson, Acta M e t a l l .
11
(1963) 547.121
A. S a t o , Y. Sunaga and T. Mori, Acta M e t a l l .5
(1977) 627.[ 3 ] A. S a t o , H. Kasuga and T. Mori, Proc. ICOMAT 79, p.183 Cambridge, MA, U.S.A.
[ 4 ] A. S a t o , H. Kasuga and T. Mori, Acta M e t a l l .
8
(1980) 1223.[ 5 ] K. Enami, A. Nagasawa and S. Nenno, S c r i p t a M e t a l l .
2
(1975) 941: Proc. ICOMAT 76, p.139, Kobe, Japan.[61 T. S u z u k i , H. Kojima, K. S u z u k i , T. Hashimoto and M. I c h i h a r a , Acta M e t a l l . 25 (1979) 367.
[ 7 ]
A,
S a t o , M. Kato, Y. Sunaga, T. Miyazaki and T. Mori, Acta M e t a l l . (1979) 367.[ a ] E. G a r t s t e i n and A. Robinkin, Acta M e t a l l .
27
(1979) 1053.[ 9 ] J . A . Venables, P h i l . Mag. 7 (1962) 35.
1101 A. S a t o , K. Soma and T. ~ o f i , Acta M e t a l l . t o a p p e a r (1982).
