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The effect of applied stress on the reversible strain in CuZnAl shape memory alloys
Pergamon Journals Ltd All rights reserved
Vol. 20, pp. 995-1000, 1986 Printed in the U.S.A.
Scripta METALLURGICA
THE EFFECT OF APPLIED STRESS ON THE REVERSIBLE SHAPE MEMORY ALLOYS. Royal Military
C M Friend College of Science,
(Received i.
February
STRAIN
Shrivenham,
IN CuZnAl
U.K.
24, 1986)
Introduction
There is now a reasonable understanding of the source of the Reversible Shape-Memory (RSM) effect in alloys (1), however in many applications exploiting this effect the memory actuator is subjected to applied stress during transformation cycling (2). There is some understanding of the macroscopic effects of applied stress during transformation (3) but no detailed understanding of its effect on the martensite microstructure. This paper reports a study conducted on RSM-trained CuZnAI alloys to identify the effect of stress on both the macroscopic strain response and the microstructure of RSM actuators during thermal transformation cycling. 2.
Experimental
Procedures
The actuators employed were commercially produced polycrystalline C u Z n A 1 R S M coil-spring actuators. Their compositions and temperatures of onset of shape-change (T z) are shown in table 1. TABLE Alloy Alloy No
Weight Cu
I.
Compositions. Percentaqe Zn
T z (oc)
A1
1
74.09
20.30
5.61
52
2
73.53
20.83
5.64
46
3
70.26
26.55
3.19
42
The RSM was introduted into the alloys by thermomechanical orocessing and the resulting actuators exhibited a reversible memory between a closed-coil and open-coil configuration. The effect of applied stress on the reversible shape-strain was studied in three ways (i) by macroscopic measurement of axial shape-strain of the actuator (6)(ii) by the resistivity changes during transformation and (iii) by in-situ optical microscopy. These measurements were carried out simultaneously in a custom-built optical microscope hot-stage where to simulate the applied bias-stress spectrum in RSM devices the actuators were subjected to an increasing compressive bias force during reversion, provided by conventional springs of varying spring rates. The temperatures of the actuators were controlled within the range O-IO00C + Ioc. 995 0036-9748/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals
Ltd.
996
CuZnAl
SHAPE M E M O R Y ALLOYS
3. The RSM behaviour of freely transformed (no applied bias) actuators is illustrated in figure 1. Under these conditions the actuators exhibited a reversible shape-change with a narrow reversion/formation range (lO-12oc) and narrow transformation hysteresis. These observations were consistent with the microstructural behaviour which exhibited classical thermoelastic reversion and transformation. Figure 2 shows a cross-plot of the s i m u l t a n e o u s l y measured deflection and fraction of martensite reverted (derived from the resistivity measurements). The resistivity measurements showed w e l l - d e f i n e d A s temperatures and the deflection behaviour well-defined onsets of shape-strain T z. The T z temperatures were higher than the A s in all the alloys (by 6°C for the 3 wt~ A1 alloy in figure 2). The effect of stress opposing the transformation shape-strain is illustrated in figure 3. Increasing the bias-rate, which essentially increased the a p p l i e d stress at any temperature, resulted in an extension of the reversion temperature range. The maximum strain-output and T z were unaffected by the stress but the extension of the reversion range decreased the strain-output at any temperature. Figure 4 illustrates the effect of bias-rate on the transformational resistivity change. The A s remained unaffected since with this bias arrangement no stress was developed below T z. However above T z a transformation tail developed with a stabilisation of the reversion. The magnitude of this stabilisation increased with increasing bias-stress. Figure 5 shows the result of applied bias on the martensite microstructure. At temperatures up to T z there was reversion of non shape-
Vol.
20, No.
Results 5(ram)
10
6-
4-
2-
Fig i. Actuator S h a p e - s t r a i n response during unbiased thermal cycling. ~mm) I
FRACTION OF MARTENSITE REVERTED
10.0
1.0
0,9,
/ / I
0.8'
0.7.
x
eo
DEFLECTION
0.6'
0.5'
0.4,
0.3"
..... - ?
j,
i~
/I
....
0,2-
g/',
0,1-
0"038--As
~
T~
,.o.
I ,,'5
50
55
Fig 2. C r o s s - p l o t of shape-strain and reversion during an unbiased reversion
(~0
T{°C)
7
Vol.
20, No. 7
CuZnAI
SHAPE MEMORY ALLOYS
generating variants u n t i l at Tz the m i c r o s t r u c t u r e consisted of p a r a l l e l - s i d e d predominant v a r i a n t s . Above T z there was slow reversion of these predominant variants and some stress-induced transformation from grain-boundaries. The volume fraction of stabilised and stress-induced variants at any temperature increased with the level of applied bias-stress.
997
ON TfX--
12.0-
1O.0 •
8.0-
6.0"
4.0"
4.
Discussion 2.0"
Melton and Mercier (4) in attempting to explain the difference between the strain and resistivity changes during biased transformation cycling speculated t h a t there was only a s m a l l volume f r a c t i o n o f a c t i v e shape-generating v a r i a n t s p r e s e n t in RSM t r a i n e d a l l o y s . The present work confirms t h e i r h y p o t h e s i s . Figure 2 shows t h a t Tz l i e s above the As and t h e r e must t h e r e f o r e be some t r a n s f o r m a t i o n which develops no macroscopic shape-change p r i o r t o Tz. I t i s also c l e a r from f i g u r e 2 t h a t t h i s volume f r a c t i o n o f m a r t e n s i t e accounts f o r up t o 70 vol~ o f the m a t e n s i t e morphology since t h e r e i s no e x t e n s i v e s t r a i n development u n t i l 70% m a r t e n s i t e r e v e r s i o n . Thus the a c t i v e s h a p e - g e n e r a t i n g m a r t e n s i t e s in a RSM-trained a l l o y are a r e l a t i v e l y s m a l l volume f r a c t i o n o f the m a r t e n s i t e s present ( i n t h i s case -30 vol%) and are the last variants to revert on heating.
O.O
•0
,8
~0
8~
~0
6~
,0 T(°C)
Fig 5. The effect of applied bias on shapestrain development.
kot O'9t
\
0.8"
0.7"
Ak UNBIASED X 1,6Nmrn'~ ~) 7Nmm-'
OS'
0.5
0.4-
0.3-
The effect of bias stress on the shape-strain behaviour can be identified by comparing figures 3 and 4. It is clear that the shape-strain reduction on the application of stress is due to the stabilisation tail which develops during the reversion of the last 20-30 vol% of martensite. Thus the applied stress must be interacting
0.2-
O.1-
O.O
,'o
Fig 4. The effect of applied bias on the transformational restivity change.
998
CuZnAI
SHAPE M E M O R Y A L L O Y S
A - 20°C
B - 47°C
C - 75°C Fig 5. The effect of applied bias (7Nmm- I ) on the martensite microstructure of a l l o y 2 during reversion.
Vol.
20, No.
7
Vol.
20, No. 7
CuZnAl
SHAPE MEMORY ALLOYS
with the active shape-strain generating martensites such t h a t they are s t a b i l i s e d with respect to reversion, thus reducing the s t r a i n output of the a c t u a t o r . The source of this s t a b i l i s a t i o n can be established by considering the stress dependence of the biased Af temperatures (Af*). Plotting Af* against the calculated (5) maximum shear stress at the end of reversion results in the behaviour shown in figure 6. This linear relationship is typical of a stress-induced martensitic transformation from bcc parent phase to martensite. Arnedo and Ahlers (6) have shown that in single crystals of CuZnA1 the slope of dTP~m/dMs is independent of orientation and composition. Thus an analysis of ~max/Af* for the present actuators using the Clausius-Clapeyron equation should produce the entropy change associated with t h i s s t a b i l i s a t i o n . An analysis of t h i s type produces a value of &S = 1.48 Jmol-:K -~. This compares well with ASB÷B' for a v a r i e t y of CuZnAI and CuZnMn a l l o y s ( t a b l e 2).
(MPa)
90
80
70
60
50 =
CuZnAI (3 wt% AI) Cu-21.6 Z n - l l . 9 A1 Cu-16.1 Zn-16.0 A1 Cu-26.6 Zn-8.1 AI Cu.36.2 Zn-3.5 Mn Cu-30.O Zn-9.2 Mn
do (B÷B') dT [MPa(OC)- I ] 2.19 2.17 2.10 2.00
.
)"
40.
30.
20,
10
0
, 46
~ 50
,
,
,
,
,
,
,
,
,
,
,
54
58
62
66
70
74
78
82
86
90
94
B, ~° (-0)
Fig 6. The stress dependence of the Af temperature for the 3wt % A1 alloy.
TABLE 2 Typical values of A S of transformation stress/strain data. Alloy (at %)
999
calculated
B÷B ' &S EJmol- IK-I ] 1.48 1.47 1.45 1.36 J l.~o I 1.33 J
]
from
Ref
This Work (7)
(S)
The stabilisation is therefore associated with B÷B ~ transformation. This mechanism is consistent with the microstructural observations shown in figure 5 which show that the stabillsation of the shape-strain results from the stress-stabillsation of B ~ predominant variants. It is also clear that in addition there is the formation of new stress-induced B ~ variants - a phenomenon not included by Melton and Mercier (4) in their hypothesis. This results in the presence of additional martensites at elevated temperatures (compared with the unbiased Af) and a further reduction in the reversible strain-output.
i000
CuZnAI
5.
SHAPE MEMORY ALLOYS
Vol.
20, No. 7
Conclusions
The shape-strain in RSM-trained actuators arises from the presence of a small volume fraction of shape-strain generating predominant variants which account for 30 vol% of the martensite morphology. these are the last variants to revert on heating which accounts for the difference in temperature between A s and T z. The reduction in the reversible strain when an actuator is subjected to an applied stress opposing the transformation strain results from two processes (i) the stress-stabilisation of 6' variants to temperatures well above their unstressed Af, and (ii) the presence of limited stress-induced martensitic transformation from reverted 6 phase. The result of both processes is the presence of unreverted martensite variants whose transformation strains reduce the effective straln-output of the actuator. AcknowleOgements SERC and the Delta Memory Metal Company for financial support. Professor A P Miodownik for useful discussions and the provision laboratory facilitles.
of
References (i) (2) ()) (4) (5) (6) (7) (8)
L Delaey, R V Krishnan, H Tas and H Warlimont. J.Mat.Sci.9, 1521 (1974). C M Wayman. 3.Met. June 1980. A D Michael and W B Hart. Met.Mat.Tech. 12, 434 (1960). K N Melton and D Mercier. Scripta Met. 12, 5 (1978). J E Shlgley and L D Mitchell. Mechanical Engineering Design (4th Edition) p.444 McGraw-Hill. W Arnedo and M Ahlers. Acta Met. 22, 1475 (I974). K Takezawa and S Sato. Proc. Int.Conf. on Mart.Transf. ICOMAT 79 Cambridge, Mass. p.661 (I979). L Chandrasekaran. PhD Thesis (April 1980) University of Surrey.
Vol. 20, pp. 995-1000, 1986 Printed in the U.S.A.
Scripta METALLURGICA
THE EFFECT OF APPLIED STRESS ON THE REVERSIBLE SHAPE MEMORY ALLOYS. Royal Military
C M Friend College of Science,
(Received i.
February
STRAIN
Shrivenham,
IN CuZnAl
U.K.
24, 1986)
Introduction
There is now a reasonable understanding of the source of the Reversible Shape-Memory (RSM) effect in alloys (1), however in many applications exploiting this effect the memory actuator is subjected to applied stress during transformation cycling (2). There is some understanding of the macroscopic effects of applied stress during transformation (3) but no detailed understanding of its effect on the martensite microstructure. This paper reports a study conducted on RSM-trained CuZnAI alloys to identify the effect of stress on both the macroscopic strain response and the microstructure of RSM actuators during thermal transformation cycling. 2.
Experimental
Procedures
The actuators employed were commercially produced polycrystalline C u Z n A 1 R S M coil-spring actuators. Their compositions and temperatures of onset of shape-change (T z) are shown in table 1. TABLE Alloy Alloy No
Weight Cu
I.
Compositions. Percentaqe Zn
T z (oc)
A1
1
74.09
20.30
5.61
52
2
73.53
20.83
5.64
46
3
70.26
26.55
3.19
42
The RSM was introduted into the alloys by thermomechanical orocessing and the resulting actuators exhibited a reversible memory between a closed-coil and open-coil configuration. The effect of applied stress on the reversible shape-strain was studied in three ways (i) by macroscopic measurement of axial shape-strain of the actuator (6)(ii) by the resistivity changes during transformation and (iii) by in-situ optical microscopy. These measurements were carried out simultaneously in a custom-built optical microscope hot-stage where to simulate the applied bias-stress spectrum in RSM devices the actuators were subjected to an increasing compressive bias force during reversion, provided by conventional springs of varying spring rates. The temperatures of the actuators were controlled within the range O-IO00C + Ioc. 995 0036-9748/86 $3.00 + .00 Copyright (c) 1986 Pergamon Journals
Ltd.
996
CuZnAl
SHAPE M E M O R Y ALLOYS
3. The RSM behaviour of freely transformed (no applied bias) actuators is illustrated in figure 1. Under these conditions the actuators exhibited a reversible shape-change with a narrow reversion/formation range (lO-12oc) and narrow transformation hysteresis. These observations were consistent with the microstructural behaviour which exhibited classical thermoelastic reversion and transformation. Figure 2 shows a cross-plot of the s i m u l t a n e o u s l y measured deflection and fraction of martensite reverted (derived from the resistivity measurements). The resistivity measurements showed w e l l - d e f i n e d A s temperatures and the deflection behaviour well-defined onsets of shape-strain T z. The T z temperatures were higher than the A s in all the alloys (by 6°C for the 3 wt~ A1 alloy in figure 2). The effect of stress opposing the transformation shape-strain is illustrated in figure 3. Increasing the bias-rate, which essentially increased the a p p l i e d stress at any temperature, resulted in an extension of the reversion temperature range. The maximum strain-output and T z were unaffected by the stress but the extension of the reversion range decreased the strain-output at any temperature. Figure 4 illustrates the effect of bias-rate on the transformational resistivity change. The A s remained unaffected since with this bias arrangement no stress was developed below T z. However above T z a transformation tail developed with a stabilisation of the reversion. The magnitude of this stabilisation increased with increasing bias-stress. Figure 5 shows the result of applied bias on the martensite microstructure. At temperatures up to T z there was reversion of non shape-
Vol.
20, No.
Results 5(ram)
10
6-
4-
2-
Fig i. Actuator S h a p e - s t r a i n response during unbiased thermal cycling. ~mm) I
FRACTION OF MARTENSITE REVERTED
10.0
1.0
0,9,
/ / I
0.8'
0.7.
x
eo
DEFLECTION
0.6'
0.5'
0.4,
0.3"
..... - ?
j,
i~
/I
....
0,2-
g/',
0,1-
0"038--As
~
T~
,.o.
I ,,'5
50
55
Fig 2. C r o s s - p l o t of shape-strain and reversion during an unbiased reversion
(~0
T{°C)
7
Vol.
20, No. 7
CuZnAI
SHAPE MEMORY ALLOYS
generating variants u n t i l at Tz the m i c r o s t r u c t u r e consisted of p a r a l l e l - s i d e d predominant v a r i a n t s . Above T z there was slow reversion of these predominant variants and some stress-induced transformation from grain-boundaries. The volume fraction of stabilised and stress-induced variants at any temperature increased with the level of applied bias-stress.
997
ON TfX--
12.0-
1O.0 •
8.0-
6.0"
4.0"
4.
Discussion 2.0"
Melton and Mercier (4) in attempting to explain the difference between the strain and resistivity changes during biased transformation cycling speculated t h a t there was only a s m a l l volume f r a c t i o n o f a c t i v e shape-generating v a r i a n t s p r e s e n t in RSM t r a i n e d a l l o y s . The present work confirms t h e i r h y p o t h e s i s . Figure 2 shows t h a t Tz l i e s above the As and t h e r e must t h e r e f o r e be some t r a n s f o r m a t i o n which develops no macroscopic shape-change p r i o r t o Tz. I t i s also c l e a r from f i g u r e 2 t h a t t h i s volume f r a c t i o n o f m a r t e n s i t e accounts f o r up t o 70 vol~ o f the m a t e n s i t e morphology since t h e r e i s no e x t e n s i v e s t r a i n development u n t i l 70% m a r t e n s i t e r e v e r s i o n . Thus the a c t i v e s h a p e - g e n e r a t i n g m a r t e n s i t e s in a RSM-trained a l l o y are a r e l a t i v e l y s m a l l volume f r a c t i o n o f the m a r t e n s i t e s present ( i n t h i s case -30 vol%) and are the last variants to revert on heating.
O.O
•0
,8
~0
8~
~0
6~
,0 T(°C)
Fig 5. The effect of applied bias on shapestrain development.
kot O'9t
\
0.8"
0.7"
Ak UNBIASED X 1,6Nmrn'~ ~) 7Nmm-'
OS'
0.5
0.4-
0.3-
The effect of bias stress on the shape-strain behaviour can be identified by comparing figures 3 and 4. It is clear that the shape-strain reduction on the application of stress is due to the stabilisation tail which develops during the reversion of the last 20-30 vol% of martensite. Thus the applied stress must be interacting
0.2-
O.1-
O.O
,'o
Fig 4. The effect of applied bias on the transformational restivity change.
998
CuZnAI
SHAPE M E M O R Y A L L O Y S
A - 20°C
B - 47°C
C - 75°C Fig 5. The effect of applied bias (7Nmm- I ) on the martensite microstructure of a l l o y 2 during reversion.
Vol.
20, No.
7
Vol.
20, No. 7
CuZnAl
SHAPE MEMORY ALLOYS
with the active shape-strain generating martensites such t h a t they are s t a b i l i s e d with respect to reversion, thus reducing the s t r a i n output of the a c t u a t o r . The source of this s t a b i l i s a t i o n can be established by considering the stress dependence of the biased Af temperatures (Af*). Plotting Af* against the calculated (5) maximum shear stress at the end of reversion results in the behaviour shown in figure 6. This linear relationship is typical of a stress-induced martensitic transformation from bcc parent phase to martensite. Arnedo and Ahlers (6) have shown that in single crystals of CuZnA1 the slope of dTP~m/dMs is independent of orientation and composition. Thus an analysis of ~max/Af* for the present actuators using the Clausius-Clapeyron equation should produce the entropy change associated with t h i s s t a b i l i s a t i o n . An analysis of t h i s type produces a value of &S = 1.48 Jmol-:K -~. This compares well with ASB÷B' for a v a r i e t y of CuZnAI and CuZnMn a l l o y s ( t a b l e 2).
(MPa)
90
80
70
60
50 =
CuZnAI (3 wt% AI) Cu-21.6 Z n - l l . 9 A1 Cu-16.1 Zn-16.0 A1 Cu-26.6 Zn-8.1 AI Cu.36.2 Zn-3.5 Mn Cu-30.O Zn-9.2 Mn
do (B÷B') dT [MPa(OC)- I ] 2.19 2.17 2.10 2.00
.
)"
40.
30.
20,
10
0
, 46
~ 50
,
,
,
,
,
,
,
,
,
,
,
54
58
62
66
70
74
78
82
86
90
94
B, ~° (-0)
Fig 6. The stress dependence of the Af temperature for the 3wt % A1 alloy.
TABLE 2 Typical values of A S of transformation stress/strain data. Alloy (at %)
999
calculated
B÷B ' &S EJmol- IK-I ] 1.48 1.47 1.45 1.36 J l.~o I 1.33 J
]
from
Ref
This Work (7)
(S)
The stabilisation is therefore associated with B÷B ~ transformation. This mechanism is consistent with the microstructural observations shown in figure 5 which show that the stabillsation of the shape-strain results from the stress-stabillsation of B ~ predominant variants. It is also clear that in addition there is the formation of new stress-induced B ~ variants - a phenomenon not included by Melton and Mercier (4) in their hypothesis. This results in the presence of additional martensites at elevated temperatures (compared with the unbiased Af) and a further reduction in the reversible strain-output.
i000
CuZnAI
5.
SHAPE MEMORY ALLOYS
Vol.
20, No. 7
Conclusions
The shape-strain in RSM-trained actuators arises from the presence of a small volume fraction of shape-strain generating predominant variants which account for 30 vol% of the martensite morphology. these are the last variants to revert on heating which accounts for the difference in temperature between A s and T z. The reduction in the reversible strain when an actuator is subjected to an applied stress opposing the transformation strain results from two processes (i) the stress-stabilisation of 6' variants to temperatures well above their unstressed Af, and (ii) the presence of limited stress-induced martensitic transformation from reverted 6 phase. The result of both processes is the presence of unreverted martensite variants whose transformation strains reduce the effective straln-output of the actuator. AcknowleOgements SERC and the Delta Memory Metal Company for financial support. Professor A P Miodownik for useful discussions and the provision laboratory facilitles.
of
References (i) (2) ()) (4) (5) (6) (7) (8)
L Delaey, R V Krishnan, H Tas and H Warlimont. J.Mat.Sci.9, 1521 (1974). C M Wayman. 3.Met. June 1980. A D Michael and W B Hart. Met.Mat.Tech. 12, 434 (1960). K N Melton and D Mercier. Scripta Met. 12, 5 (1978). J E Shlgley and L D Mitchell. Mechanical Engineering Design (4th Edition) p.444 McGraw-Hill. W Arnedo and M Ahlers. Acta Met. 22, 1475 (I974). K Takezawa and S Sato. Proc. Int.Conf. on Mart.Transf. ICOMAT 79 Cambridge, Mass. p.661 (I979). L Chandrasekaran. PhD Thesis (April 1980) University of Surrey.