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Shape memory properties of a CuZnAl multivariant martensite single crystal under stress and its mechanical behaviour
Volume 9, number 7,8
MATERIALS LETTERS
April 1990
SHAPE MEMORY PROPERTIES OF A Cu-Zn-Al MULTIVARIANT MARTENSITE SINGLE CRYSTAL UNDER STRESS AND ITS MECHANICAL BEHAVIOUR
J.M. GUILEMANY and F.J. GIL Metalurgia Fisica, Facultad de Quimica. Vniversidad de Barcelona, CMarti i Franquh I, 08028 Barcelona, Spain
Received 19 January 1990
The behaviour of a single crystal of a Cu-Zn-Al shape memory alloy with a multivariant martensitic structure has been studied. Shape memory strains and recovery values have been determined, under different compressive stresses. The martensitic structures obtained under stress have been observed by optical microscopy.
1. Introduction
If a stress is applied to the parent (p) phase above its Af temperature, a mechanically elastic martensite is stress induced in alloys which exhibit thermoelastic behaviour. When the p phase transforms into martensite by cooling, twenty four variants of martensite form and there is no macroscopic shape change. In a stress-induced transformation a particular variant is preferentially formed since stress has a directionality in contrast to temperature. Formation of a particular variant depends upon which shear system interacts with the applied stress and, it is generally accepted that the shape strain interacts with the applied stress. The selected variant is that which gives the largest Schmid factor first among all possible variants [ 11.
mation temperatures of M,=43”C, Mr=25”C, A,=30”C aad A,=46”C. Compression tests were carried out in a Hounsfield W testing machine on a single specimen to stresses of 58.1, 116.3, 174.3, 232.5, 290.7, 348.8, 407.0, 465.0, 532.0 and 581.4 MN/m2. Only one cycle was used for each stress. Prior to the compression test, the dimensions of the sample were measured as 0 = 5 mm and 1,= 6.5 mm. After both unloading, and a heat treatment consisting of 10 min at 150” C followed by air cooling, the lengths of the sample 12,and l3 respectively were measured again. This process was carried out for each initial stress. In addition, the following values were calculated. Strain at zero stress after unloading: e2(%)= [(l(j-l2)/z~]loo; final strain after heating: 63(0/o)= [(lo-1,)/l,]
2. Experimental procedure
100 ;
shape recovery: A single crystal obtained by the Bridgman method [ 2 ] has been used with the following chemical composition (in weight percentage): 77.OW Cu, 8.14% Al and 14.86% Zn (electron to atom ratio e/a= 1.477). The structure of the sample at room temperature is completely martensitic, the alloy having transfor242
3. Results and discussion
Strains e2 and Ed,as well as shape recovery values
0167-577x/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
Volume 9, number
MATERIALS
7,8
Rz obtained in the compression test, were determined on a single sample for all the applied stresses, as shown in table 1. Strains ranged from a minimum value of 1.5% for 58.1 MN/m2 to a maximum value of 6.6% for 581.4 MN/m*. Fig. 1 shows e2 as a function of stress. e2 is seen to be linearly related to stress up to a stress of 290.7 MN/m’. For higher stresses, t2 increases to a constant value of 6.6%. e3 strain values are zero for stresses below 290.7 MN/m*. At this point, defects are created in some of the martensite plates, which pin the plates (stabilized martensite) [ 3 1. Retransforming a deformed martensite into p phase by heat Table 1 Strains e2, tj and shape recovery values R2,obtained compression test for different applied stresses
58.1 116.3 174.4 232.5 290.7 348.8 407.0 465.0 523.0 581.4
1.50 1.78 2.47 3.17 3.30 6.07 6.60 5.65 5.65 6.61
0 0 0 0 0.14 0.55 1.38 0.56 1.13 1.86
April 1990
LETTERS
treatment results in a permanent partial strain, and hence the shape recovery values are not 1OO%, as can be seen in fig. 2 where it should be noted that from 290.7 MN/cm*, shape recovery decreases linearly. Shape recovery as a function of strain e2, is represented in fig. 3. This figure exhibits the same shape as fig. 2 and indicates that shape recovery starts to
in the
100 100 100 100 96 91 79 90 80 72
50
Fig. 2. Shape recovery
600
Stress (MN/m21
R2as a function of the applied stress.
8
)C ””
I
I
1
I
I
600
Stress lMNlm2) Fig. 1. Strain t2 as a function
of the applied
stress.
t2
Fig. 3. Shape recovery
Strain I%1
R2as a function of the strain e2. 243
Volume 9, number 7,8
MATERIALS LETTERS
April 1990
decrease linearly above 3.30% strain. Comparing figs. 2 and 3, we observe that the stress value 290.7 MN/ m* has associated a strain value of 3.30% as can be seen in table 1. There is no training effect in these tests due to the fact that the imposed stress increases gradually causing the creation and interaction of movements of new and pre-existing defects (dislocations) [ 3 1. The morphology of martensitic structures has been observed as a function of increasing stress values. As the imposed stress increases there is a tendency to form specific variants which grow in size at the expense of other variants as can be seen in fig. 4. Fig. 4a shows thermal martensite which presents different variants without showing any preferential orientation (OM x 200). By stressing the same sample, we can observe some preferential orientations of the plates (fig. 4b, OM X 200). By increasing the stress even further, thin martensite plates grow at the expense of others, producing thick plates with preferential orientations, as shown in fig. 4c (OMx200).
Acknowledgement
The present research was supported by CAICYT project PA 0085-84.
References [ 1 ] B.C. Mellor, Martensitic transformation and the shape memory effect, COMETT Project: The Science and Technology of Shape Memory Alloys (Barcelona, 1989), in press. [2] J.M. Guilemany, F.J. Gil and J.R. Miguel, Rev. Metal. 24 (1988) 175. [ 31 F.J. Gil, Doctoral Thesis, Universidad de Barcelona ( 1989). Fig. 4. Morphologies of martensites: (a) thermal martensite; (b) martensite plates with preferential orientations; (c) thick martensite plates with preferential orientations.
244
MATERIALS LETTERS
April 1990
SHAPE MEMORY PROPERTIES OF A Cu-Zn-Al MULTIVARIANT MARTENSITE SINGLE CRYSTAL UNDER STRESS AND ITS MECHANICAL BEHAVIOUR
J.M. GUILEMANY and F.J. GIL Metalurgia Fisica, Facultad de Quimica. Vniversidad de Barcelona, CMarti i Franquh I, 08028 Barcelona, Spain
Received 19 January 1990
The behaviour of a single crystal of a Cu-Zn-Al shape memory alloy with a multivariant martensitic structure has been studied. Shape memory strains and recovery values have been determined, under different compressive stresses. The martensitic structures obtained under stress have been observed by optical microscopy.
1. Introduction
If a stress is applied to the parent (p) phase above its Af temperature, a mechanically elastic martensite is stress induced in alloys which exhibit thermoelastic behaviour. When the p phase transforms into martensite by cooling, twenty four variants of martensite form and there is no macroscopic shape change. In a stress-induced transformation a particular variant is preferentially formed since stress has a directionality in contrast to temperature. Formation of a particular variant depends upon which shear system interacts with the applied stress and, it is generally accepted that the shape strain interacts with the applied stress. The selected variant is that which gives the largest Schmid factor first among all possible variants [ 11.
mation temperatures of M,=43”C, Mr=25”C, A,=30”C aad A,=46”C. Compression tests were carried out in a Hounsfield W testing machine on a single specimen to stresses of 58.1, 116.3, 174.3, 232.5, 290.7, 348.8, 407.0, 465.0, 532.0 and 581.4 MN/m2. Only one cycle was used for each stress. Prior to the compression test, the dimensions of the sample were measured as 0 = 5 mm and 1,= 6.5 mm. After both unloading, and a heat treatment consisting of 10 min at 150” C followed by air cooling, the lengths of the sample 12,and l3 respectively were measured again. This process was carried out for each initial stress. In addition, the following values were calculated. Strain at zero stress after unloading: e2(%)= [(l(j-l2)/z~]loo; final strain after heating: 63(0/o)= [(lo-1,)/l,]
2. Experimental procedure
100 ;
shape recovery: A single crystal obtained by the Bridgman method [ 2 ] has been used with the following chemical composition (in weight percentage): 77.OW Cu, 8.14% Al and 14.86% Zn (electron to atom ratio e/a= 1.477). The structure of the sample at room temperature is completely martensitic, the alloy having transfor242
3. Results and discussion
Strains e2 and Ed,as well as shape recovery values
0167-577x/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland)
Volume 9, number
MATERIALS
7,8
Rz obtained in the compression test, were determined on a single sample for all the applied stresses, as shown in table 1. Strains ranged from a minimum value of 1.5% for 58.1 MN/m2 to a maximum value of 6.6% for 581.4 MN/m*. Fig. 1 shows e2 as a function of stress. e2 is seen to be linearly related to stress up to a stress of 290.7 MN/m’. For higher stresses, t2 increases to a constant value of 6.6%. e3 strain values are zero for stresses below 290.7 MN/m*. At this point, defects are created in some of the martensite plates, which pin the plates (stabilized martensite) [ 3 1. Retransforming a deformed martensite into p phase by heat Table 1 Strains e2, tj and shape recovery values R2,obtained compression test for different applied stresses
58.1 116.3 174.4 232.5 290.7 348.8 407.0 465.0 523.0 581.4
1.50 1.78 2.47 3.17 3.30 6.07 6.60 5.65 5.65 6.61
0 0 0 0 0.14 0.55 1.38 0.56 1.13 1.86
April 1990
LETTERS
treatment results in a permanent partial strain, and hence the shape recovery values are not 1OO%, as can be seen in fig. 2 where it should be noted that from 290.7 MN/cm*, shape recovery decreases linearly. Shape recovery as a function of strain e2, is represented in fig. 3. This figure exhibits the same shape as fig. 2 and indicates that shape recovery starts to
in the
100 100 100 100 96 91 79 90 80 72
50
Fig. 2. Shape recovery
600
Stress (MN/m21
R2as a function of the applied stress.
8
)C ””
I
I
1
I
I
600
Stress lMNlm2) Fig. 1. Strain t2 as a function
of the applied
stress.
t2
Fig. 3. Shape recovery
Strain I%1
R2as a function of the strain e2. 243
Volume 9, number 7,8
MATERIALS LETTERS
April 1990
decrease linearly above 3.30% strain. Comparing figs. 2 and 3, we observe that the stress value 290.7 MN/ m* has associated a strain value of 3.30% as can be seen in table 1. There is no training effect in these tests due to the fact that the imposed stress increases gradually causing the creation and interaction of movements of new and pre-existing defects (dislocations) [ 3 1. The morphology of martensitic structures has been observed as a function of increasing stress values. As the imposed stress increases there is a tendency to form specific variants which grow in size at the expense of other variants as can be seen in fig. 4. Fig. 4a shows thermal martensite which presents different variants without showing any preferential orientation (OM x 200). By stressing the same sample, we can observe some preferential orientations of the plates (fig. 4b, OM X 200). By increasing the stress even further, thin martensite plates grow at the expense of others, producing thick plates with preferential orientations, as shown in fig. 4c (OMx200).
Acknowledgement
The present research was supported by CAICYT project PA 0085-84.
References [ 1 ] B.C. Mellor, Martensitic transformation and the shape memory effect, COMETT Project: The Science and Technology of Shape Memory Alloys (Barcelona, 1989), in press. [2] J.M. Guilemany, F.J. Gil and J.R. Miguel, Rev. Metal. 24 (1988) 175. [ 31 F.J. Gil, Doctoral Thesis, Universidad de Barcelona ( 1989). Fig. 4. Morphologies of martensites: (a) thermal martensite; (b) martensite plates with preferential orientations; (c) thick martensite plates with preferential orientations.
244