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Reversible shape memory effect in Fe-base alloys
Scripta METALLURGICA
Vol. 9, pp. 941-948, 1975 Printed in the United States
Pergamon Press,
Inc.
REVERSIBLE SHAPE MEMORY EFFECT IN Fe-BASE ALLOYS
K.Enami*, A.Nagasawa** and S.Nenno* *Department o f M e t a l l u r g y , F a c u l t y o f E n g i n e e r i n g , Osaka U n i v e r s i t y , Yamadakami, S u i t a , Osaka 565, J a p a n **Department o f P h y s i c s , Osaka C i t y U n i v e r s i t y , S u g i m o t o - c h o , Osaka 558, J a p a n (Received June 18, 1975)
Introduction As r e p o r t e d i n t h e p r e v i o u s p a p e r s ( 1 , 2 ) , we have f o u n d t h e r e v e r s i b l e shape memory (RSM) e f f e c t o c c u r r i n g i n t h e 8 - b r a s s t y p e a l l o y s such as N i T i , CuZn, NiA1 and Cu-Zn-Ga. The RSM e f f e c t i s a phenomenon i n which a s p e c i m e n o f each a l l o y d e f o r m e d below Ms, o r a t l e a s t below Md t e m p e r a t u r e , r e c a l l s r e v e r s i b l y and r e p e a t e d l y b o t h n o n d e f o r m e d and d e f o r m e d s h a p e s when h e a t e d above As (or more p r e c i s e l y above Af) and c o o l e d below Ms ( o r below Mf), r e s p e c t i v e l y . We have p r o p o s e d t h a t t h e n e c e s s a r y c o n d i t i o n f o r t h e RSM e f f e c t t o o c c u r i s t o d e f o r m s e v e r e l y t h e sample o v e r t h e r e c o v e r a b l e l i m i t o f t h e o r d i n a r y shape memory (OSM) e f f e c t . According to our p r o p o s a l , the r o l e of the severe d e f o r m a t i o n i s t o i n d u c e t h e s t r e s s f i e l d i n t o t h e s p e c i m e n , by which t h e g r o w t h d i r e c t i o n s o f t h e m a r t e n s i t e c r y s t a l s and h e n c e t h e mode o f t h e s p o n t a n e o u s deformation are controlled. These r e s u l t s s t r o n g l y s u g g e s t t h a t t h e RSM e f f e c t w i l l be f o u n d i n t h e m a r t e n s i t i c a l l o y s o t h e r t h a n t h e 8 - b r a s s t y p e o n e s , i f t h e above c o n d i t i o n
is satisfied.
In f a c t , t h e RSM e f f e c t o c c u r r i n g i n some F e - b a s e m a r t e n s i t i c a l l o y s has been f o u n d by our r e c e n t s t u d y . The p u r p o s e o f t h e p r e s e n t p a p e r i s t o r e p o r t t h e o b t a i n e d r e s u l t s on t h e RSM e f f e c t i n F e - b a s e a l l o y s d i s c u s s i o n on t h e g e n e r a l o u t l o o k o f t h e RSM e f f e c t .
and g i v e a b r i e f
Experimental The m a t e r i a l s
used in the p r e s e n t
work a r e an F e - 1 8 . 5 wt%Mn a l l o y
Fe-30 at%Ni a l l o y . The Fe-Mn a l l o y was a r c - m e l t e d ~ f o r g e d and hot-rolled, and f i n a l l y c o l d - r o l l e d t o a b o u t 0.1mm t h i c k . The c h e m i c a l 941
and an
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9, No. 9
composition in wt% is as follows: C 0.002, Si 0.002, Mn 18.5, P 0.002, S 0.011, Fe bal.. The Ms temperature of this alloy is estimated to be about 100--150°C, based on the data of Schumann (5). The As and Af temperatures were about 190 ° and 240°C respectively which were measured by a dilatometric method. The Fe-Ni alloy was prepared using Fe and Ni of 99.99% purity in a plasma-arc furnace under an argon atmosphere containing 5% hydrogen. After repeated melting for homogenization of the alloy, no measurable weight loss was found, therefore the composition of the alloy was estimated to be the nominal one. The Ms and Af temperatures of the alloy are estimated to be about -50 ° and 560°C respectively, based on the data of Honma (4). Specimens of the Fe-18.SMn and the Fe-50Ni alloys with the narrow ribbeRshape of about 0.1mm thick were annealed at 1050 ° and 1000°C respectively for i hr in vacuum-sealed quartz capsules followed by quenching into ice-water. Then they were deformed below their Ms temperatures, i.e. at room temperature for the Fe-18.SMn alloy and at liquid nitrogen temperature for the Fe-50Ni alloy. Results and Discussion Fig.l shows the RSM effect in the Fe-18.5Mn alloy. A specimen of the alloy with a straight form (l(a)) was severely bent at room temperature (below Ms) as shown in Fig.l(b). By heating to about 280°C (well above Af), the shape of the specimen was partially recovered (l(c)). This phenomenon is the OSM effect. When the specimen was cooled to room temperature without any mechanical treatment, however, its shape partially returned towards the low temperature, bent shape (l(d)); the amount of recovery is clearly seen in this double-exposed photograph. Although the spontaneous shape change is small, this phenomenon means that the RSM effect occurs in the Fe-18.SMn alloy as is the case in the B-brass type alloys (1,2). Even after 15 cycles of heating and cooling, the RSM effect of the alloy shows no hysteresis as shown in Fig.l(e). The RSM effect in the Fe-50Ni alloy is demonstrated in Fig.2. A specimen of the alloy with a straight form (2(a)) was severely bent in a liquid nitrogen bath (2(b)). When the bent specimen was heated to about 400°C (above Af), a small shape recovery occurred (2(c)). By subsequent immersion into liquid nitrogen, the Specimen showed a spontaneous bending (2(d)): this is also the RSM effect. Even after 12 cycles of heating and cooling, the RSM effect was still found as shown in Fig.2(e). All the photographs in Fig.2 were taken at room temperature after the martensitic and the reverse transformations had occurred. As clearly shown in Figs. I and 2, the present study proves evidently that the RSM effect occurs also in the Fe-base alloys. The RSM effect in the present case is characterized by a small amount of the spontaneous shape change compared to the cases of the B-brass type alloys (1,2). However, it must be emphasized here that the essential natures of the RSM effect, i.e. spontaneous, reversible and repeatable shape changes corresponding to the repeated cooling
Vol. 9, No. 9
REVERSIBLE SHAPE MEMORY
943
and heating cycles, are also observed in the Fe-base martensitic alloys, as is the case in the 8-brass type alloys. As was proposed in our previous papers
(1,2), some sort of residual strain
or stress field should remain in the specimen even above Af in order for the RSM effect to occur. be responsible,
In the case of Fe-base alloys, dislocations are thought to
at least in part, for residual strain or stress field.
When the
specimen is deformed below Md (or Mr), dislocations due to slip deformation are introduced in the retained austenite or in the martensite.
When the specimen is
heated above Af and then cooled again below Ms, these dislocations not only act as the nucleation sites of the martensitic transformation the direction of growth of the martensite crystals.
(5) but also control
In these Fe-base alloys,
the RSM effect does not occur when deformation is light (6).
Thus, severe
deformation is also the necessary condition for Fe-base alloys to introduce excess amount of dislocations.(or other lattice defects) which act as the origin of the RSM effect.
Therefore,
the necessary condition for the RSM effect
proposed in (i) is confirmed again by the present study. The next step to discuss is the difference in the amount of shape recovery associated with the RSM effect between the Fe-base and the 8-brass type alloys. This difference may be explained in connection with the deformation mechanism. Firstly,
in the Fe-base martensitic alloys, deformation below the Md temperature
proceeds with both transformation to martensite
(SIM) and slip.
of deformation is responsible for the irreversible shape change. Mf temperature,
The latter part Even below the
deformation will proceed with slip in addition to twinning or
reorientation of variants of martensite crystals.
Secondly, on the other hand,
in the 8-brass type alloys, irreversible part of deformation due to slip is very small even when the deformation is performed above Ms (below Md), because stress-induced martensitic transformation can easily occur without slip when the specimen &s stressed and deformation-induced or possibly stacking faults are mobile.
lattice defects such as twins (7)
The differences in these deformation
mechanisms are mainly responsible for the difference in the RSM manner in the two types of alloys. considered,
Other factors for this difference, of course, can be
such as crystal structure, the number of the crystals
(variants) involved in the martensite phase or grain size of the parent phase. The above arguments can also be applied to explaining the difference in the OSM manner in the two types of alloys (8).
However, dislocations or other lattice
defects which act as the barrier to reduce the OSM effect become the origin of the RSM effect, and the existence of enough dislocations or other lattice defects and therefore enough stress from these defects is needed to control the growth direction of the martensite crystals in the RSM effect. We now consider the outline of the relation between deformation process, lattice defects and the RSM effect of the martensitic alloy in g e n e r a l .
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Fig.
1
Reversible shape memory effect in the Pe-18.S%Mn alloy. (a) Initial form of the specimen. (b) Deformed at from temperature. (c) Heating to 280vC, OSM occurred. Low temperature (l.t,) and high temperature (h.t.) forms of the specimen were double-exposed by use of an electronic flash. High temperature form~ were photographed at about 270-290 C. (d) Cooling to room temperature, l.t. and h.t. forms were double exposed. (e) After 15 cycles of heating and cooling.
Fig. 2 R e v e r s i b l e shape memory e f f e c t in t h e Fe-30%Ni a l l o y . (a) I n i t i a l form o f the specimen. (b) Deformed i n l i q u i d n i t r o g e n , o (c) H e a t e d above Af (above a b o u t 400 C), low t e m p e r a t u r e and h i g h t e m p e r a t u r e forms were d o u b l e - e x p o s e d . (d) Cooled t o l i q u i d n i t r o g e n , 1 . t . and h . t . forms were d o u b l e - e x p o s e d . (e) A f t e r 12 c y c l e s o f h e a t i n g and cooling. A l l t h e p h o t o g r a p h s were t a k e n a t room t e m p e r a t u r e .
Vol. 9, No. 9
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945
Fig.3 is a schematic representation of the RSM effect. of the specimen can be either austenitic or martensitic.
The initial state
Condition for RSM to
occur is at least deformation temperature is below Md temperature.
When the
specimen is initially in the ~ustenitic state or at least in a partially transformed state, deformation occurs accompanied by the martensitic transformation (i.e. SIM occurs).
.
.
.
.
O
Thls is Indlcated. as A
I
-MTI (or M~I4'''" ) _ O is the stressin Fig.3, where A is the retained (metastable) austenite and MII induced martensite formed by deformation whose crystal structure is the same as the thermal martensite M I but somewhat different in the configuration of lattice defects involved.
This schema should correspond to such alloys as the Fe-base
alloys in which the austenite phase is retainable.
When the deformation is
severe, the austenite obtained after the reverse transformation should contain dislocations or other lattice defects such as twins, stacking faults and APBs. call it A'. o 4'MII (or
In order to differentiate it from the initial, retained austenite we The reverse transformation in this case is indicated as "'" )
• A' .
When the specimen in the state A' is cooled to Ms
(not Md), stress-assisted region of A' due to the strain field of lattice
o
defects involved is transformed to the deformed state (MII or M
A'
o (or MI; . MII _- 4 '
).
I 4. "''),
i.e.
This situation corresponds to the Fe=base alloys.
Repeating the cycles of heating and cooling, transformation cycles of o (or MII "') ._:" A' appear repeatedly. When the specimen is initially in fully martensitic state, the RSM effect will occur along the manner of the lower schema of Fig.5, i.e. J M ~ I' M ~ I' M ~ I' or M ~ I' ... . " A' (or M~) In A', as in the case of MI the above schema, several types of lattice defects are involved. In addition to them, the SIM which is stabilized_ even above the Af temperature of M I also acts aS
such defects
(2).
M~I~ is the stress-induced martensite whose crystal
structure is different from M I.
M~I is the deformed martensite in which the
reorientation of the initial variants occurs along the most favourable orientation for the applied stress. M~I is the deformed martensite in which the twins are newly generated or the one of the initial i n t e r n a l t w i n s grows at the expense of another twin-related crystal. M~I is the deformed martensite in A
which the stacking faults are newly formed, etc.
The lower part of this
schema is the case for the 8-brass type alloys, where both the RSM and the OSM are found to occur more completely when they are deformed in fully martensitic state. M~ in parentheses means the martensite whose crystal structure is the same as M I but configuration of lattice defects involved is different from M I , similar to the case A' vs. A.
It is probable that the parent phase of M~41 .... ,
especially of M~I is M I (or M~) in some cases.
In the case of the elastic shape
memory effect (pseudo-elasticity), the above schema, i.e. M~I to M~ has been observed (9). According to Nakanishi et al.(9), the stress-induced martensitic
Schematic
-
(OS~
structure from MI)
9
stabilized
APB
twin
stacking
SIM
fault
*dislocation
defects*
with lattice
l
A'Cor MI]
A'
t h e RSM e f f e c t
m
$ooling
Heating,
Fig. 5 of the mechanism of
stacking faults]
M~i(Generation of
twins)
tion of internal
M~i(Growth or genera-
variants]
M~i(Reorientation of
representation
IMartensite(thermal): MI]
sm/"
//
"% ss~q- .....
M~I(SIM different in
AI----ISIM: MIIo (or M~I 4'" "')
l
Deformation below Md
%0
Z 0
%0
0
<
0
~n
~n
>
~n
<
4~
%0
Vol. 9, No. 9
REVERSIBLE SHAPE MEMORY
947
transformation from M I to M~I has occurred in the Au-Cd alloys.
In this case,
when the stress is released M~I moves back to MI, or more precisely to M~ , as observed in Fig.5 in ref.(9). Therefore, it can be considered that the parent phase of M~I is M I and the product phase obtained after releasbng the stress is M~, and the high temperature S 1 phase should be named "grandmother phase", in this case. If the deformation is not severe, the OSM effect occurs along the manner M ~ 1-4 .... to A or MI, as indicated by broken lines in Fig.3. Lattice defects within A' (or M~) can arise from both deformation process and thermal cycle of transformation itself. Sandrock (10), and Kajiwara (11) have observed the generation of tangled dislocations within the austenite during thermal cycles of martensite ~ austenite transformation in TiNi and Fe3Pt. I n t h e c a s e o f t h e RSM e f f e c t , the specimen is deformed severely before the reverse transformation, so that chances of the generation of excess lattice defects in the austenite are expected to be more frequent than the thermal cycle alone. Lattice defects induced by thermal cycles will also act as the origin
of the
induced If type
back stress
lattice
any other
of strain
(12),
Some o t h e r
et
their
in metastable
obtained
in addition
to the deformation-
It
as the
and Perkins
source
(14).
and their
state
RSM e f f e c t
of the
Delaey
proposal
in general,
incorrect
or
(15). of course,
to be clarified
al.,
have recently however,
(i)
the
alloy
schema of Fig.3),
and his
in view of the
RSM e f f e c t .
can be included
when
the upper
et
only propose what sort
necessarily
understanding
facts
or another
second phase
RSM e f f e c t
His complex model cannot
is
is yet
(i.e.
defects
such as the
on t h e m e c h a n i s m o f t h e
nonuniform.
(1,2)
act
some l a t t i c e
phase,
model can only be applied
The a b o v e a r g u m e n t s , RSM e f f e c t .
RSM e f f e c t
austenite
can also
a1.(13),
austenitic is
RSM e f f e c t in
the
model precisely,
mechanism of the of the
they
Perkins'
deformation
into
proposals
made b y D e l a e y arguments.
the
ways, which can introduce
field
are possible,
developed
for
defects.
been
have not in our
is and
deformed (ii)
describe
the
on t h e
features
on t h e RSM e f f e c t
already
the
general
of lattice
outlook defects
of the act
most
effectively as the origin of strain field, or what conditions, e.g. crystal structure, orientation, grain size, and so on, govern the reversibility and the degree of shape change, and the stability of the RSM effect. Acknowledgement The authors wish to express their thanks to Dr. T.Saburi of Osaka University for his helpful discussion on the subject.
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References 1. A.Nagasawa, K.Enami) Y . I s h i n o , Y.Abe and S.Nenno, S c r i p t a Met. 8,
1055(1974).
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
T . S a b u r i and S.Nenno, S c r i p t a Met. 8, 1363(1974). H.Schumann, Arch. EisenhUttenwes. 38, 647(1957). T.Honma, J . J a p a n I n s t . M e t a l s , 21, 263(1957). T e t s u r o Suzuki and M.Wuttig, Met. Trans. 3, 1555(1972). A.Nagasawa, J . Phys. Soc. Japan, 30, 1505(1971). C.W.Wayman and K.Shimizu, Met. S c i . J . 6, 175(1972). A.Nagasawa, Phys. S t a t . S o l . ( a ) , 8, 531(1971). N.Nakanishi, T.Mori, S.Miura, Y.Murakami and S . K a c h i , P h i l . Mag. 28,
277(1973).
G.D.Sandrock, Ph.D.Theses, Case Western Reserve Univ. 1971. S.Kajiwara and W.S.Owen, Met. Trans. ~, 1988(1975). R.Oshima and K.Adachi, Japanese J . appl. Phys. 14, 555(1975). L.Delaey, R.V.Krishnan, H.Tas and H.Warlimont, J. M a t e r i a l s Sci. 9, 1521(1974). 14. J.Perkins, Scripta Met. 8, 1469(1974}. 15. H.Tas, L.Delae7 and A.Deruyttere, J. Less-Common Metals, 28, 141(1972).
Vol. 9, pp. 941-948, 1975 Printed in the United States
Pergamon Press,
Inc.
REVERSIBLE SHAPE MEMORY EFFECT IN Fe-BASE ALLOYS
K.Enami*, A.Nagasawa** and S.Nenno* *Department o f M e t a l l u r g y , F a c u l t y o f E n g i n e e r i n g , Osaka U n i v e r s i t y , Yamadakami, S u i t a , Osaka 565, J a p a n **Department o f P h y s i c s , Osaka C i t y U n i v e r s i t y , S u g i m o t o - c h o , Osaka 558, J a p a n (Received June 18, 1975)
Introduction As r e p o r t e d i n t h e p r e v i o u s p a p e r s ( 1 , 2 ) , we have f o u n d t h e r e v e r s i b l e shape memory (RSM) e f f e c t o c c u r r i n g i n t h e 8 - b r a s s t y p e a l l o y s such as N i T i , CuZn, NiA1 and Cu-Zn-Ga. The RSM e f f e c t i s a phenomenon i n which a s p e c i m e n o f each a l l o y d e f o r m e d below Ms, o r a t l e a s t below Md t e m p e r a t u r e , r e c a l l s r e v e r s i b l y and r e p e a t e d l y b o t h n o n d e f o r m e d and d e f o r m e d s h a p e s when h e a t e d above As (or more p r e c i s e l y above Af) and c o o l e d below Ms ( o r below Mf), r e s p e c t i v e l y . We have p r o p o s e d t h a t t h e n e c e s s a r y c o n d i t i o n f o r t h e RSM e f f e c t t o o c c u r i s t o d e f o r m s e v e r e l y t h e sample o v e r t h e r e c o v e r a b l e l i m i t o f t h e o r d i n a r y shape memory (OSM) e f f e c t . According to our p r o p o s a l , the r o l e of the severe d e f o r m a t i o n i s t o i n d u c e t h e s t r e s s f i e l d i n t o t h e s p e c i m e n , by which t h e g r o w t h d i r e c t i o n s o f t h e m a r t e n s i t e c r y s t a l s and h e n c e t h e mode o f t h e s p o n t a n e o u s deformation are controlled. These r e s u l t s s t r o n g l y s u g g e s t t h a t t h e RSM e f f e c t w i l l be f o u n d i n t h e m a r t e n s i t i c a l l o y s o t h e r t h a n t h e 8 - b r a s s t y p e o n e s , i f t h e above c o n d i t i o n
is satisfied.
In f a c t , t h e RSM e f f e c t o c c u r r i n g i n some F e - b a s e m a r t e n s i t i c a l l o y s has been f o u n d by our r e c e n t s t u d y . The p u r p o s e o f t h e p r e s e n t p a p e r i s t o r e p o r t t h e o b t a i n e d r e s u l t s on t h e RSM e f f e c t i n F e - b a s e a l l o y s d i s c u s s i o n on t h e g e n e r a l o u t l o o k o f t h e RSM e f f e c t .
and g i v e a b r i e f
Experimental The m a t e r i a l s
used in the p r e s e n t
work a r e an F e - 1 8 . 5 wt%Mn a l l o y
Fe-30 at%Ni a l l o y . The Fe-Mn a l l o y was a r c - m e l t e d ~ f o r g e d and hot-rolled, and f i n a l l y c o l d - r o l l e d t o a b o u t 0.1mm t h i c k . The c h e m i c a l 941
and an
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composition in wt% is as follows: C 0.002, Si 0.002, Mn 18.5, P 0.002, S 0.011, Fe bal.. The Ms temperature of this alloy is estimated to be about 100--150°C, based on the data of Schumann (5). The As and Af temperatures were about 190 ° and 240°C respectively which were measured by a dilatometric method. The Fe-Ni alloy was prepared using Fe and Ni of 99.99% purity in a plasma-arc furnace under an argon atmosphere containing 5% hydrogen. After repeated melting for homogenization of the alloy, no measurable weight loss was found, therefore the composition of the alloy was estimated to be the nominal one. The Ms and Af temperatures of the alloy are estimated to be about -50 ° and 560°C respectively, based on the data of Honma (4). Specimens of the Fe-18.SMn and the Fe-50Ni alloys with the narrow ribbeRshape of about 0.1mm thick were annealed at 1050 ° and 1000°C respectively for i hr in vacuum-sealed quartz capsules followed by quenching into ice-water. Then they were deformed below their Ms temperatures, i.e. at room temperature for the Fe-18.SMn alloy and at liquid nitrogen temperature for the Fe-50Ni alloy. Results and Discussion Fig.l shows the RSM effect in the Fe-18.5Mn alloy. A specimen of the alloy with a straight form (l(a)) was severely bent at room temperature (below Ms) as shown in Fig.l(b). By heating to about 280°C (well above Af), the shape of the specimen was partially recovered (l(c)). This phenomenon is the OSM effect. When the specimen was cooled to room temperature without any mechanical treatment, however, its shape partially returned towards the low temperature, bent shape (l(d)); the amount of recovery is clearly seen in this double-exposed photograph. Although the spontaneous shape change is small, this phenomenon means that the RSM effect occurs in the Fe-18.SMn alloy as is the case in the B-brass type alloys (1,2). Even after 15 cycles of heating and cooling, the RSM effect of the alloy shows no hysteresis as shown in Fig.l(e). The RSM effect in the Fe-50Ni alloy is demonstrated in Fig.2. A specimen of the alloy with a straight form (2(a)) was severely bent in a liquid nitrogen bath (2(b)). When the bent specimen was heated to about 400°C (above Af), a small shape recovery occurred (2(c)). By subsequent immersion into liquid nitrogen, the Specimen showed a spontaneous bending (2(d)): this is also the RSM effect. Even after 12 cycles of heating and cooling, the RSM effect was still found as shown in Fig.2(e). All the photographs in Fig.2 were taken at room temperature after the martensitic and the reverse transformations had occurred. As clearly shown in Figs. I and 2, the present study proves evidently that the RSM effect occurs also in the Fe-base alloys. The RSM effect in the present case is characterized by a small amount of the spontaneous shape change compared to the cases of the B-brass type alloys (1,2). However, it must be emphasized here that the essential natures of the RSM effect, i.e. spontaneous, reversible and repeatable shape changes corresponding to the repeated cooling
Vol. 9, No. 9
REVERSIBLE SHAPE MEMORY
943
and heating cycles, are also observed in the Fe-base martensitic alloys, as is the case in the 8-brass type alloys. As was proposed in our previous papers
(1,2), some sort of residual strain
or stress field should remain in the specimen even above Af in order for the RSM effect to occur. be responsible,
In the case of Fe-base alloys, dislocations are thought to
at least in part, for residual strain or stress field.
When the
specimen is deformed below Md (or Mr), dislocations due to slip deformation are introduced in the retained austenite or in the martensite.
When the specimen is
heated above Af and then cooled again below Ms, these dislocations not only act as the nucleation sites of the martensitic transformation the direction of growth of the martensite crystals.
(5) but also control
In these Fe-base alloys,
the RSM effect does not occur when deformation is light (6).
Thus, severe
deformation is also the necessary condition for Fe-base alloys to introduce excess amount of dislocations.(or other lattice defects) which act as the origin of the RSM effect.
Therefore,
the necessary condition for the RSM effect
proposed in (i) is confirmed again by the present study. The next step to discuss is the difference in the amount of shape recovery associated with the RSM effect between the Fe-base and the 8-brass type alloys. This difference may be explained in connection with the deformation mechanism. Firstly,
in the Fe-base martensitic alloys, deformation below the Md temperature
proceeds with both transformation to martensite
(SIM) and slip.
of deformation is responsible for the irreversible shape change. Mf temperature,
The latter part Even below the
deformation will proceed with slip in addition to twinning or
reorientation of variants of martensite crystals.
Secondly, on the other hand,
in the 8-brass type alloys, irreversible part of deformation due to slip is very small even when the deformation is performed above Ms (below Md), because stress-induced martensitic transformation can easily occur without slip when the specimen &s stressed and deformation-induced or possibly stacking faults are mobile.
lattice defects such as twins (7)
The differences in these deformation
mechanisms are mainly responsible for the difference in the RSM manner in the two types of alloys. considered,
Other factors for this difference, of course, can be
such as crystal structure, the number of the crystals
(variants) involved in the martensite phase or grain size of the parent phase. The above arguments can also be applied to explaining the difference in the OSM manner in the two types of alloys (8).
However, dislocations or other lattice
defects which act as the barrier to reduce the OSM effect become the origin of the RSM effect, and the existence of enough dislocations or other lattice defects and therefore enough stress from these defects is needed to control the growth direction of the martensite crystals in the RSM effect. We now consider the outline of the relation between deformation process, lattice defects and the RSM effect of the martensitic alloy in g e n e r a l .
944
REVERSIBLE SHAPE MEMORY
Vol. 9, No. 9
Fig.
1
Reversible shape memory effect in the Pe-18.S%Mn alloy. (a) Initial form of the specimen. (b) Deformed at from temperature. (c) Heating to 280vC, OSM occurred. Low temperature (l.t,) and high temperature (h.t.) forms of the specimen were double-exposed by use of an electronic flash. High temperature form~ were photographed at about 270-290 C. (d) Cooling to room temperature, l.t. and h.t. forms were double exposed. (e) After 15 cycles of heating and cooling.
Fig. 2 R e v e r s i b l e shape memory e f f e c t in t h e Fe-30%Ni a l l o y . (a) I n i t i a l form o f the specimen. (b) Deformed i n l i q u i d n i t r o g e n , o (c) H e a t e d above Af (above a b o u t 400 C), low t e m p e r a t u r e and h i g h t e m p e r a t u r e forms were d o u b l e - e x p o s e d . (d) Cooled t o l i q u i d n i t r o g e n , 1 . t . and h . t . forms were d o u b l e - e x p o s e d . (e) A f t e r 12 c y c l e s o f h e a t i n g and cooling. A l l t h e p h o t o g r a p h s were t a k e n a t room t e m p e r a t u r e .
Vol. 9, No. 9
REVERSIBLE SHAPE MEMORY
945
Fig.3 is a schematic representation of the RSM effect. of the specimen can be either austenitic or martensitic.
The initial state
Condition for RSM to
occur is at least deformation temperature is below Md temperature.
When the
specimen is initially in the ~ustenitic state or at least in a partially transformed state, deformation occurs accompanied by the martensitic transformation (i.e. SIM occurs).
.
.
.
.
O
Thls is Indlcated. as A
I
-MTI (or M~I4'''" ) _ O is the stressin Fig.3, where A is the retained (metastable) austenite and MII induced martensite formed by deformation whose crystal structure is the same as the thermal martensite M I but somewhat different in the configuration of lattice defects involved.
This schema should correspond to such alloys as the Fe-base
alloys in which the austenite phase is retainable.
When the deformation is
severe, the austenite obtained after the reverse transformation should contain dislocations or other lattice defects such as twins, stacking faults and APBs. call it A'. o 4'MII (or
In order to differentiate it from the initial, retained austenite we The reverse transformation in this case is indicated as "'" )
• A' .
When the specimen in the state A' is cooled to Ms
(not Md), stress-assisted region of A' due to the strain field of lattice
o
defects involved is transformed to the deformed state (MII or M
A'
o (or MI; . MII _- 4 '
).
I 4. "''),
i.e.
This situation corresponds to the Fe=base alloys.
Repeating the cycles of heating and cooling, transformation cycles of o (or MII "') ._:" A' appear repeatedly. When the specimen is initially in fully martensitic state, the RSM effect will occur along the manner of the lower schema of Fig.5, i.e. J M ~ I' M ~ I' M ~ I' or M ~ I' ... . " A' (or M~) In A', as in the case of MI the above schema, several types of lattice defects are involved. In addition to them, the SIM which is stabilized_ even above the Af temperature of M I also acts aS
such defects
(2).
M~I~ is the stress-induced martensite whose crystal
structure is different from M I.
M~I is the deformed martensite in which the
reorientation of the initial variants occurs along the most favourable orientation for the applied stress. M~I is the deformed martensite in which the twins are newly generated or the one of the initial i n t e r n a l t w i n s grows at the expense of another twin-related crystal. M~I is the deformed martensite in A
which the stacking faults are newly formed, etc.
The lower part of this
schema is the case for the 8-brass type alloys, where both the RSM and the OSM are found to occur more completely when they are deformed in fully martensitic state. M~ in parentheses means the martensite whose crystal structure is the same as M I but configuration of lattice defects involved is different from M I , similar to the case A' vs. A.
It is probable that the parent phase of M~41 .... ,
especially of M~I is M I (or M~) in some cases.
In the case of the elastic shape
memory effect (pseudo-elasticity), the above schema, i.e. M~I to M~ has been observed (9). According to Nakanishi et al.(9), the stress-induced martensitic
Schematic
-
(OS~
structure from MI)
9
stabilized
APB
twin
stacking
SIM
fault
*dislocation
defects*
with lattice
l
A'Cor MI]
A'
t h e RSM e f f e c t
m
$ooling
Heating,
Fig. 5 of the mechanism of
stacking faults]
M~i(Generation of
twins)
tion of internal
M~i(Growth or genera-
variants]
M~i(Reorientation of
representation
IMartensite(thermal): MI]
sm/"
//
"% ss~q- .....
M~I(SIM different in
AI----ISIM: MIIo (or M~I 4'" "')
l
Deformation below Md
%0
Z 0
%0
0
<
0
~n
~n
>
~n
<
4~
%0
Vol. 9, No. 9
REVERSIBLE SHAPE MEMORY
947
transformation from M I to M~I has occurred in the Au-Cd alloys.
In this case,
when the stress is released M~I moves back to MI, or more precisely to M~ , as observed in Fig.5 in ref.(9). Therefore, it can be considered that the parent phase of M~I is M I and the product phase obtained after releasbng the stress is M~, and the high temperature S 1 phase should be named "grandmother phase", in this case. If the deformation is not severe, the OSM effect occurs along the manner M ~ 1-4 .... to A or MI, as indicated by broken lines in Fig.3. Lattice defects within A' (or M~) can arise from both deformation process and thermal cycle of transformation itself. Sandrock (10), and Kajiwara (11) have observed the generation of tangled dislocations within the austenite during thermal cycles of martensite ~ austenite transformation in TiNi and Fe3Pt. I n t h e c a s e o f t h e RSM e f f e c t , the specimen is deformed severely before the reverse transformation, so that chances of the generation of excess lattice defects in the austenite are expected to be more frequent than the thermal cycle alone. Lattice defects induced by thermal cycles will also act as the origin
of the
induced If type
back stress
lattice
any other
of strain
(12),
Some o t h e r
et
their
in metastable
obtained
in addition
to the deformation-
It
as the
and Perkins
source
(14).
and their
state
RSM e f f e c t
of the
Delaey
proposal
in general,
incorrect
or
(15). of course,
to be clarified
al.,
have recently however,
(i)
the
alloy
schema of Fig.3),
and his
in view of the
RSM e f f e c t .
can be included
when
the upper
et
only propose what sort
necessarily
understanding
facts
or another
second phase
RSM e f f e c t
His complex model cannot
is
is yet
(i.e.
defects
such as the
on t h e m e c h a n i s m o f t h e
nonuniform.
(1,2)
act
some l a t t i c e
phase,
model can only be applied
The a b o v e a r g u m e n t s , RSM e f f e c t .
RSM e f f e c t
austenite
can also
a1.(13),
austenitic is
RSM e f f e c t in
the
model precisely,
mechanism of the of the
they
Perkins'
deformation
into
proposals
made b y D e l a e y arguments.
the
ways, which can introduce
field
are possible,
developed
for
defects.
been
have not in our
is and
deformed (ii)
describe
the
on t h e
features
on t h e RSM e f f e c t
already
the
general
of lattice
outlook defects
of the act
most
effectively as the origin of strain field, or what conditions, e.g. crystal structure, orientation, grain size, and so on, govern the reversibility and the degree of shape change, and the stability of the RSM effect. Acknowledgement The authors wish to express their thanks to Dr. T.Saburi of Osaka University for his helpful discussion on the subject.
948
REVERSIBLE SHAPE MEMORY
Vol. 9, No, 9
References 1. A.Nagasawa, K.Enami) Y . I s h i n o , Y.Abe and S.Nenno, S c r i p t a Met. 8,
1055(1974).
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
T . S a b u r i and S.Nenno, S c r i p t a Met. 8, 1363(1974). H.Schumann, Arch. EisenhUttenwes. 38, 647(1957). T.Honma, J . J a p a n I n s t . M e t a l s , 21, 263(1957). T e t s u r o Suzuki and M.Wuttig, Met. Trans. 3, 1555(1972). A.Nagasawa, J . Phys. Soc. Japan, 30, 1505(1971). C.W.Wayman and K.Shimizu, Met. S c i . J . 6, 175(1972). A.Nagasawa, Phys. S t a t . S o l . ( a ) , 8, 531(1971). N.Nakanishi, T.Mori, S.Miura, Y.Murakami and S . K a c h i , P h i l . Mag. 28,
277(1973).
G.D.Sandrock, Ph.D.Theses, Case Western Reserve Univ. 1971. S.Kajiwara and W.S.Owen, Met. Trans. ~, 1988(1975). R.Oshima and K.Adachi, Japanese J . appl. Phys. 14, 555(1975). L.Delaey, R.V.Krishnan, H.Tas and H.Warlimont, J. M a t e r i a l s Sci. 9, 1521(1974). 14. J.Perkins, Scripta Met. 8, 1469(1974}. 15. H.Tas, L.Delae7 and A.Deruyttere, J. Less-Common Metals, 28, 141(1972).