Reversible shape memory in CuZnGa

Scripta METALLURGICA

Vol. 8, pp. 1363-1368, 1974 Printed in the United States

Pergamon

Press,

Inc.

REVERSIBLE SHAPE MEMORY IN C u - Z n - G a

T. Saburi and S. Nenno Department

of Metallurgy,

Faculty of Engineering,

Osaka University, Yamadakami,

Suita, Osaka, Japan.

(Received August

19, 1974)

Introduction The shape memory effect observed be associated

with a reverse

parent phase.

These alloys, when deformed

back to the parent phase, inthe

parent phase.

their deformed

shapes

remember

However,

found a reversible state,

cycles.

of the reversible mechanism

state;

to the parent phase, the effect

and examined

which occurred

to a relatively

the persistency

to its

state and heated

their original undeformed

shapes

they forget

is of only one way. alloys,

we

in a very persistent

severe deformation

in the

of the effect during extended

of the present paper is to report on the finding

shape memory

effect

in a Cu-Zn-Ga

alloy and to discuss

of the effect briefly on the basis of metallographic

microscopic

is known to

martensite

of the shape memory effect of Cu-Zn-Ga

shape memory effect,

The purpose

in the martensitic

once transformed

manner when a specimen was subjected thermal

(1-12)

of a thermoelastic

to recover

in the martensitic

During an investigation

martensitic

in a number of alloys

transformation

the

and electron

observations. Procedure

Several Cu-Zn-Ga 50°C were prepared by referring

weight

(99.99%Cu,

to the work by Delaey and Warlimont

was maintained capsules,

alloys with different M s temperatures

from high purity metals

at 1.45.

between

99.999%Zn

(13).

-50°C and

and 99.99%Ga)

The electron

atom ratio

The alloys were melted and cast in evacuated quartz

and then homogenized

for 24h at 800°C in the capsules.

after melting were less than 0.1%.

alloy with a M s temperature

around 35°C

The alloy was forged to 4mm in thickness,

For the present

The losses

investigation

(Cu-20.4at.%Zn-12.Sat.%Ga)

in

an

was used.

then held for 2h at 800°C in an

evacuated quartz capsule and quenched into iced brine. A sample with a dimension of 2 x 0.7 x 50mm was prepared by mechanical cutting and subsequent handpolishing,

and then electropolished

treatment was performed

to a thickness

to avoid the vaporization 1363

of 0.4mm. of Zn.

No further heat

The shape memory effect

1364

SHAPE MEMORY IN Cu-Zn-Ga

Vol. 8, No. 12

was e x a m i n e d i n a w a t e r b a t h . Reversible The o r i g i n a l

straight

Shape Memor7

s h a p e o f t h e s p e c i m e n i s shown i n F i g .

l(a).

s p e c i m e n was t h e n b e n t t o an a n g l e o f 44 ° i n a c o m p l e t e l y m a r t e n s i t i c 0°C ( F i g . original

l(b-1)). shape

.When t h e s p e c i m e n was h e a t e d up t o 100°C,

(Fig.

l(b-2)),

change

(Fig.

l(b-3)).

l(c-1)),

it

recovered

and on s u b s e q u e n t c o o l i n g

the original

However, i t

deflected

direction

on s u b s e q u e n t c o o l i n g

pronounced with increasing

s h a p e a g a i n by h e a t i n g

slightly

up t o 100°C ( F i g .

towards the previous

t o 0°C ( F i g .

l(c-3)).

0°C ( F i g .

the original

shape but a slight

On s u b s e q u e n t c o o l i n g

l(d-1))

external

t o 0°C,

25 ° t o w a r d s t h e p r e v i o u s

value after

(Fig.

the specimen straightened

and b e n t

it

of

almost recovered (Fig.

l(d-2)).

t o an a n g l e o f

l(d-3)).

During

figure

30 c y c l e s

that

the deflection

S0 t o 100 c y c l e s

(21°).

a r e shown i n F i g .

angle decreased

( f r o m 25 ° t o 21.3 ° ) b u t i t

the

in a s p o n t a n e o u s manner.

a n g l e s a t 0°C d u r i n g t h e t h e r m a l c y c l e s

c a n be s e e n i n t h i s during the early

of curvature

spontaneously

bending direction

became more

b e n d i n g a t 0°C.

o f a b o u t 2 ° was l e f t

the specimen bent

external

bending

t h e r m a l c y c l e s b e t w e e n 0°C and 50°C t h e s p e c i m e n r e p e a t e d

same s h a p e c h a n g e s ; deflection

and t h e n h e a t e d up t o 50°C, deflection

external

This effect

t h e amount o f t h e i n i t i a l

a b o u t 3mm a t

The

the

t o 0°C t h e s h a p e d i d n o t

When t h e s p e c i m e n was b e n t t o an a n g l e o f 133 ° w i t h a r a d i u s

continued

at

recovered

When t h e s p e c i m e n was b e n t t o an a n g l e o f 90 ° a t 0°C ( F i g .

1(c-2)).

further

it

The state

relatively

2.

It

quickly

reached almost a constant

The s h a p e a t 0°C on t h e 100 c y c l e s

is

shown

in Fig. l(d-4). The temperature4eflection Fig. 3.

curve during the 101st thermal cycle is shown in

It is clear in this figure that the reversibility of shape change is

almost perfect after 100 of thermal cycles. exhibits a reversible shape memory effect. recently been reported for Ni-Ti

In other words, the specimen Although similar observations have

(14,15), Cu-Zn-AI-Ni

(16), Cu-Zn (15) and Ni-AI

(15), the present work is the first to demonstrate the persistent reversibility of shape change. Metallography and Discussion The thermal martensite in the present alloy as observed by optical microscopy was characterized by the thermoelastic and the subsequent burst natures as was observed in Cu-Zn (17) and other Cu-Zn base ternary alloys

(18).

When the

specimen was cooled from the 8 state, thermoelastic thin plates of martensite formed at the M s temperature.

During further cooling the plates grew in the

lengthwise direction and additional plates formed parallel or at an angle to the original plates.

Finally a burst-type martensitic transformation occurred in a

very narrow temperature range. The crystal structure of the thermal martensite was observed to be of a

Vol. 8, No. 12

"= a w

SHAPE MEMORY IN Cu-Zn-Ga

c-10------,

26°C

b-1 ""

O°C'

"~

O°C, bent

d-1 ~ O o C

bent 90 °

44°\ c-2 ~

b-2 w ""

1365

100oc

d-2~ 100°C

c-3~

OOC

_

' bent 133

50°C

d-30~CO---~~.. ~

b-3 ~

1st cycle

Ooc

3 CM

d4 ( ~ - ~ . ~ . ~ . . . . ~ lOOth cycle

FIG. 1 Shape memory effect in Cu-Zn-Ga alloy. (a) Original shape. (b-l) Bent to 44 ° at 0°C. (b-2) Heated to 100°C. (b-3) Cooled to 0°C. (c-l) Bent to 90 ° at 0°C (c-2) Heated to I00°C. (c-3) Cooled to 0°C. (d-l) Bent to 133 ° at 0°C (radius of curvature = 3mm). (d-2) Heated to 50°C. (d-3) Cooled to 0°C (the first thermal cycle between 0°C and 50°C). The specimen bended spontaneously to an angle of 25 °. (d-4) Cooled to 0°C on the 100th thermal cycle. The specimen bended to an angle of 21 °.

.~ 25

==24

I

23 =0 22 :-r~ 21 20o.1 C~

I

I

I

I

I

I

I

I

I

I

I

0

10

20

30

40

50

60

70

80

90

100

Thermal Cycles FIG. 2 Angle of spontaneous deflection at 0°C plotted against thermal cycles.

1366

SHAPE MEMORY IN Cu-Zn-Ga

22 20 18 816

Vol. 8, No. 12

:

oup

-814 =12 0

'= 10 Q~ -ea 8 '-' 6

I

I

I

I

5

10

15

20

I

I

25 30 Temperature { °C}

I

I

I

I

35

40

45

50

FIG. 3 Temperature vs. deflection curve during the 101st thermal cycle.

faulted 18R type (with small amount of faulted 2H structure), agreement with the observation by Delaey and Warlimont

essentially in

(13, 19) who referred to

the martensite as 8~', but the c-axis direction was found to be not exactly 90 ° from the basal plane but deviated by l°40 ' . When the specimen was bent in a completely martensitic state, thin plates of stress-induced martensite formed at the expense of the thermally produced martensite.

When the angle of bend was relatively small both the thermally

produced and the stress-induced martensites disappeared completely on heating again above the Af temperature.

However, when the angle of bend was large as in

Fig. l(d-l), some of the stress-induced martensite (SIM) plates were stabilized somehow and they remained even after heating above the Af temperature. These plates appeared to be responsible for the slight deflection which was left at 50°C of Fig. l(d-2) and also for the shape change which occurred spontaneously on the subsequent cooling below the M s temperature.

The slight deflection at

50°C seemed to be amplified by the formation of the thermal martensite. The role of the stabilized SIM may be explained as follows. In the martensitic state the three dimensional strain minimization condition (11,21,22) must be satisfied taking both the stabilized SIM and the thermal martensite into account. Thus the residual SIM acts to determine the favorable configuration of

Vol. 8, No. 12

SHAPE MEMORY IN Cu-Zn-Ga

1367

the thermal martensite which forms on subsequent cooling,

i.e. to determine the

direction and the amount of shape change at temperatures below Ms.

For a better

understanding of the effect, however, we need more information such as those on the structure of the SIM, the mechanism of its stabilization, its configuration in 8 and the configuration residual SIM.

of the thermal martensite variants affected by the

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