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High resolution electron microscopy imaging of the habit plane in CuZnAl shape memory alloys
Scripta
METALLURGICA
Vol. 21, pp. 1 6 2 7 - 1 6 3 1 , 1987 P r i n t e d in t h e U . S . A .
Pergamon Journals, Ltd. All rights reserved
HIGH RESOLUTION ELECTRON MICROSCOPY IMAGING OF THE HABIT PLANE IN Cu-Zn-AI SHAPE MEMORY ALLOYS
F.C. Lovey*, G. Van Tendeloo # and J. Van Landuyt # Centro At6mico Bariloche~ Comisi6n Nacional de Energ~a At6mlca, 8400 Bariloche, Argentina # University of Antwerp, RUCA, Groenenborgerlaan 171, B-2020 Antwerp (Belgium). ( R e c e i v e d J u n e 30, 1987) (Revised September 21, 1987) Introduction The presence of a macroscopically undistorted interface (the habit plane) between the martensite and the parent phase is a necessary characteristic of a martensitic transformation. Depending on the alloy system and its composition, the habit plane can be obtained either by twinning the martensite (composite interface) or by sllp on a particular plane (single interface). In the former case the interface exhibits a zlg-zag shape on a micrometer scale. Conversely, the single interface case shows a quite planar boundary even when imaged by transmission electron microscopy at very high magnification. Very few attempts have been made to study the habit plane structure by high resolution electron microscopy. The first contribution (by one-dlmensional lattice imaging) to this area has been performed by Sinclair and Mohamed (i) who were able to observe interracial dislocations at the habit plane in a Ni-Ti alloy. On the other hand, Knowles (2) and Knowles et al. (3) have shown (by using the two-dimensional lattice fringe imaging) that no Interfaclal dislocations were present at the parent-msrtenslte interface in a Ti-Mn alloy. The structure of the habit plane in a Cu-Zn-Al alloy is analysed in the present report. The projected structure of both the parent and the martensite phases could indeed be resolved, allowing a detailed study of the interface. Experimental A ~ Cu-14.8%Zn-16.6%Al
(atomic
per cent)
single crystal, with an MS= 260 K, was thermally
treated at 1073 K for 30 min followed by water quenching. This material was partially transformed into several plates of one variant of martenslte by tensile stressing at room temperature. The sample was kept under tension for 48 h; in this way some martenslte plates could be retained at room temperature after unloading. Samples suitable for studying the matrlx-martenslte interface were sllced by spark-erosion. The foll orientation was chosen in order to enable imaging along the [ IIIJB U [210] directions.
(Throughout the paper we shall use the subscript B when referring
to the B phase; no subscript will be used for the 18R, or 9R, Miller indices). Under this geometrical condition the habit plane is almost edge-on (about 3 ° inclined, as shown in Fig. i), and, moreover, the atom columns of both the 18R and the ~ structures can be imaged as white dots with the same conditions of defocus and thickness (4,5). This allows a direct interpretation of the high resolution images in terms of the projected structure model. The specimens were studied with a Philips EM 300 microscope with a high angle tilting gonlometer and a JEOL 200 CX microscope equipped with a high resolution top entry goniometer. Results and Discussion Two conventional transmission electron microscopy images of the habit plane are shown in Fig. 2. In Fig. 2a the specimen was tilted in order to image the intersections of the basal plane stacking faults with the habit plane (striations parallel to the white dashed line in Fig. 2a arise from these intersections). Fig. 2b shows an area where the "type I" defects intersect the matrix-martensite interface. The "Type I" defect has been described earlier by Andrade et al. (6), and in more detail by Lovey et al. (5). It constitutes the boundary between two differently stacked parts of a martenslte plate. The "Type I" defect is parallel to a hypothetical crystallographically equivalent habit plane (EHP), which is indicated in Fig. i.
1627 0036-9748/87 $ 3 . 0 0 + .00 Copyright (c) 1 9 8 7 P e r g a m o n J o u r n a l s
Ltd.
1628
Cu-Zn-Al
1
0
SHAPE MEMORY ALLOYS
~
Vol.
21 No.
010e
Fig. 1 Stereographic projections showing the relationship between the matrix (Bcc) and the martensite (18R). After Tas et al. (9).
Fig. 2 Conventional bright field electron micrographs of the habit plane. a) The basal plane stacking faults Intersecting the habit plane b) The "Type I" defect intersecting the habit plane
12
Vol.
21 No.
12
Cu-Zn-Al
SHAPE
MEMORY
ALLOYS
1629
Fig. 3 High resolution electron micrograph of the habit plane (vertical in between the arrows). The projected structure of both the matrix and the martensite are clearly resolved. A hexagonal fault (h) and a double hexagonal fault (dh) are observed in the martensite. The perfect coherency of the boundary, even at places where the stacking faults intersect, is striking.
1630
Cu-Zn-Al
A high resolution
SHAPE
MEMORY
ALLOYS
Vol.
21 No.
12
image of the habit plane, observed along the "-'LIlIJBII L210J direction, is
shown in Fig. 3. The white dots in Fig. 3 can he interpreted as the images of the atom columns projected along the direction of observation (4,5). Hence, a hexagonal arrangement of white dots is observed in the B phase, while the image of the 18R martensite, along the [210] direction, is shown in Fig. 4. Using this labelling code the stacking sequence of the martensite has been indicated in Fig. 3. Some basal plane stacking faults can be identified in the martensite area of Fig. 3. The fault located in the lower part of Fig. 3 is a double hexagonal fault ((according to the definition given by Nishlyama et al. (7)], while the other lault (upper part) is a single hexagonal fault. With respect to Fig. 3 the following remarks can be made: a) Every ~ II0}B plane of the [~II]B zone joins a low index plane in the martenslte. (0]I) B planes continue
Thus the
into the (0 0 18) upon crossing the boundary. A rotation of about 4 ° is
observed between the (0~i) B and the martensite basal plane in agreement with (8,9). On the other hand,
the (ll0)B planes correspond
to the (i 2 10) planes
in the martensite, and the (I01) B
planes change into the (128) planes after crossing the interface. b) No significant difference of the structure along the boundary is observed whether the stacking sequence in the martensite is perfect or faults are present. c) The habit plane is a perfectly coherent boundary
[OOl] A B
• I)
•
A
•
C
• •
C
C
•
•
A
B
•
•
• •
• •
•
•
[230] A
Fig. 4 Projected structure of the 18R martensite along the [2~0] direction In some earlier studies (10-12), by conventional electron microscopy of the matrlx-gR(or 18R) martensite inter£aces, line strialtions observed along the boundaries were assoclatea with interracial dislocations with a ~ [ i00] Burgers vector (10,12). An interracial dislocation was assumed to exist at places where a shear type of stacking fault (in the 9R martensite) terminated.
Vol.
21 No.
12
Cu-Zn-AI
SHAPE
MEMORY
ALLOYS
1631
at the habit plane. However, more recently, reported results from high resolution electron microscopy (5,6,13, 14) @nd electron diffraction contrast analysis (15) have shown that shear type faults, i.e. w i t h a ~ [ i 0 0 ] d i s p l a c e m e n t vector, have not been o b s e r v e d in the 18R (or 9R) martensites. Hence, the earlier assumptions that interface dislocations with this type of Burgers vector exist, are no longer valid. Moreover, it is confirmed by the present observations that no interfacial dislocations (in the sense of loss of coherence) are observed at places where the hexagonal faults terminate. Coherency dislocations, as those defined by Olson and Cohen (16), could he present along the boundary and particularly where the stacking faults are. However, it appears difficult to define the Burgers vectors of such dislocations only from the information given by Fig. 3. This figure shows the image of the projected structure while in the actual crystal the atom columns are at different levels along the direction of observations. Although the basal plane stacking faults do not seem to affect the habit plane to a noticeable extent, a problem may arise when a "Type I" defect intersects the parent-martensite interface (Fig. 2b). Unfortunately, because of the unfavorable geometrical conditions no detailed information on the habit plane structure in these areas could be obtained by high resolution electron microscopy. In conclusion it should he emphasized that some hexagonal basal plane stacking faults are necessary for obtaining an undistorted habit plane. The required density of faults depends on the so called tetragonality of the martenslte, as it was recently shown elsewhere by one of the authors (14). On the other hand the "Type I" defects are a consequence of the mechanism of the transformation [as already pointed out in (5)) but they do not contribute to the formation of a microscopically undistorted habit plane. On the contrary, local distortions are expected in the areas where the "Type I" defects intersect with the habit plane. Acknowledgements The authors wish to thank Dr. M. Chandrasekaran, Dr. M. Sade and Licenciado J. Pellegrina of the Centro At6mlco Bariloche for preparlng the Cu-Zn-AI specimen. This work was partially supported by the Consejo Naeional de Investlgaclones Cient~ficas y T~cnicas of Argentina. References i. 2. 3. 4. 5. 6. 7. 8. 9. I0. ii. 12. 13. 14. 15. 16.
R. Sinclair and H.A. Mohamed, Acta Metall., 26, 623 (1978). K.M. Knowles, Proc. Roy. Soc. Lond A 380, 287 (1982). K.M. Knowles, J.W. CHristian and D.A. Smith, J. Physique, 43, C4-185 (1982). F.C. Lovey, W. Coene, D. Van Dyck, G. Van Tendeloo, J. Van Landuyt and S. Amelinckx, Ultramlcroscopy, 14, 345 (1984). F.C. Lovey, G. Van Tendeloo, J. Van Landuyt, L. Delaey and S. Amelinckx, Phys. Star. Sol. (a), 86, 994 (1984). M. Andrade, L. Delaey and M. Chandrasekaran, J. Physique, 43, C4-673 (1982). Z. Nishiyama, J. Kakinokf and S. Kajiwara, J. Phys. Soc. Japan, 20, 1192 (1965). Z. Nishiyama and S. Kajlwara, Japan J. Appl. Phys., 2, 478 (1963). H. Tas, L. Delaey and a. Deruyttere, Metallurg. Trans., 4, 2833 (1973). S. Chakravorty and C.M. Wayman, Acta Metall., 25, 989 (1977). S. Kajiwara and T. Kikuchi, Acta Metall., 30, 589 (1982). T. Tadaki, T. Kakeshlta and K. Shimlzu, J. Physique, 43, C4- 191 (1982). J.M. Cook, M.A. O'Keefe, D.J. Smith and W.H. Stobbs, J. Microscopy, 129, 295 (1983). F.C. Lovey, Acta Me tall. 35, 1103 (1987). M. Andrade, M. Chandrasekaran and L. Delaey, Acta Metall., 32, 1809 (1984). G.B. Olson and M. Cohen, Acta Metall., 27, 1907 (1979).
METALLURGICA
Vol. 21, pp. 1 6 2 7 - 1 6 3 1 , 1987 P r i n t e d in t h e U . S . A .
Pergamon Journals, Ltd. All rights reserved
HIGH RESOLUTION ELECTRON MICROSCOPY IMAGING OF THE HABIT PLANE IN Cu-Zn-AI SHAPE MEMORY ALLOYS
F.C. Lovey*, G. Van Tendeloo # and J. Van Landuyt # Centro At6mico Bariloche~ Comisi6n Nacional de Energ~a At6mlca, 8400 Bariloche, Argentina # University of Antwerp, RUCA, Groenenborgerlaan 171, B-2020 Antwerp (Belgium). ( R e c e i v e d J u n e 30, 1987) (Revised September 21, 1987) Introduction The presence of a macroscopically undistorted interface (the habit plane) between the martensite and the parent phase is a necessary characteristic of a martensitic transformation. Depending on the alloy system and its composition, the habit plane can be obtained either by twinning the martensite (composite interface) or by sllp on a particular plane (single interface). In the former case the interface exhibits a zlg-zag shape on a micrometer scale. Conversely, the single interface case shows a quite planar boundary even when imaged by transmission electron microscopy at very high magnification. Very few attempts have been made to study the habit plane structure by high resolution electron microscopy. The first contribution (by one-dlmensional lattice imaging) to this area has been performed by Sinclair and Mohamed (i) who were able to observe interracial dislocations at the habit plane in a Ni-Ti alloy. On the other hand, Knowles (2) and Knowles et al. (3) have shown (by using the two-dimensional lattice fringe imaging) that no Interfaclal dislocations were present at the parent-msrtenslte interface in a Ti-Mn alloy. The structure of the habit plane in a Cu-Zn-Al alloy is analysed in the present report. The projected structure of both the parent and the martensite phases could indeed be resolved, allowing a detailed study of the interface. Experimental A ~ Cu-14.8%Zn-16.6%Al
(atomic
per cent)
single crystal, with an MS= 260 K, was thermally
treated at 1073 K for 30 min followed by water quenching. This material was partially transformed into several plates of one variant of martenslte by tensile stressing at room temperature. The sample was kept under tension for 48 h; in this way some martenslte plates could be retained at room temperature after unloading. Samples suitable for studying the matrlx-martenslte interface were sllced by spark-erosion. The foll orientation was chosen in order to enable imaging along the [ IIIJB U [210] directions.
(Throughout the paper we shall use the subscript B when referring
to the B phase; no subscript will be used for the 18R, or 9R, Miller indices). Under this geometrical condition the habit plane is almost edge-on (about 3 ° inclined, as shown in Fig. i), and, moreover, the atom columns of both the 18R and the ~ structures can be imaged as white dots with the same conditions of defocus and thickness (4,5). This allows a direct interpretation of the high resolution images in terms of the projected structure model. The specimens were studied with a Philips EM 300 microscope with a high angle tilting gonlometer and a JEOL 200 CX microscope equipped with a high resolution top entry goniometer. Results and Discussion Two conventional transmission electron microscopy images of the habit plane are shown in Fig. 2. In Fig. 2a the specimen was tilted in order to image the intersections of the basal plane stacking faults with the habit plane (striations parallel to the white dashed line in Fig. 2a arise from these intersections). Fig. 2b shows an area where the "type I" defects intersect the matrix-martensite interface. The "Type I" defect has been described earlier by Andrade et al. (6), and in more detail by Lovey et al. (5). It constitutes the boundary between two differently stacked parts of a martenslte plate. The "Type I" defect is parallel to a hypothetical crystallographically equivalent habit plane (EHP), which is indicated in Fig. i.
1627 0036-9748/87 $ 3 . 0 0 + .00 Copyright (c) 1 9 8 7 P e r g a m o n J o u r n a l s
Ltd.
1628
Cu-Zn-Al
1
0
SHAPE MEMORY ALLOYS
~
Vol.
21 No.
010e
Fig. 1 Stereographic projections showing the relationship between the matrix (Bcc) and the martensite (18R). After Tas et al. (9).
Fig. 2 Conventional bright field electron micrographs of the habit plane. a) The basal plane stacking faults Intersecting the habit plane b) The "Type I" defect intersecting the habit plane
12
Vol.
21 No.
12
Cu-Zn-Al
SHAPE
MEMORY
ALLOYS
1629
Fig. 3 High resolution electron micrograph of the habit plane (vertical in between the arrows). The projected structure of both the matrix and the martensite are clearly resolved. A hexagonal fault (h) and a double hexagonal fault (dh) are observed in the martensite. The perfect coherency of the boundary, even at places where the stacking faults intersect, is striking.
1630
Cu-Zn-Al
A high resolution
SHAPE
MEMORY
ALLOYS
Vol.
21 No.
12
image of the habit plane, observed along the "-'LIlIJBII L210J direction, is
shown in Fig. 3. The white dots in Fig. 3 can he interpreted as the images of the atom columns projected along the direction of observation (4,5). Hence, a hexagonal arrangement of white dots is observed in the B phase, while the image of the 18R martensite, along the [210] direction, is shown in Fig. 4. Using this labelling code the stacking sequence of the martensite has been indicated in Fig. 3. Some basal plane stacking faults can be identified in the martensite area of Fig. 3. The fault located in the lower part of Fig. 3 is a double hexagonal fault ((according to the definition given by Nishlyama et al. (7)], while the other lault (upper part) is a single hexagonal fault. With respect to Fig. 3 the following remarks can be made: a) Every ~ II0}B plane of the [~II]B zone joins a low index plane in the martenslte. (0]I) B planes continue
Thus the
into the (0 0 18) upon crossing the boundary. A rotation of about 4 ° is
observed between the (0~i) B and the martensite basal plane in agreement with (8,9). On the other hand,
the (ll0)B planes correspond
to the (i 2 10) planes
in the martensite, and the (I01) B
planes change into the (128) planes after crossing the interface. b) No significant difference of the structure along the boundary is observed whether the stacking sequence in the martensite is perfect or faults are present. c) The habit plane is a perfectly coherent boundary
[OOl] A B
• I)
•
A
•
C
• •
C
C
•
•
A
B
•
•
• •
• •
•
•
[230] A
Fig. 4 Projected structure of the 18R martensite along the [2~0] direction In some earlier studies (10-12), by conventional electron microscopy of the matrlx-gR(or 18R) martensite inter£aces, line strialtions observed along the boundaries were assoclatea with interracial dislocations with a ~ [ i00] Burgers vector (10,12). An interracial dislocation was assumed to exist at places where a shear type of stacking fault (in the 9R martensite) terminated.
Vol.
21 No.
12
Cu-Zn-AI
SHAPE
MEMORY
ALLOYS
1631
at the habit plane. However, more recently, reported results from high resolution electron microscopy (5,6,13, 14) @nd electron diffraction contrast analysis (15) have shown that shear type faults, i.e. w i t h a ~ [ i 0 0 ] d i s p l a c e m e n t vector, have not been o b s e r v e d in the 18R (or 9R) martensites. Hence, the earlier assumptions that interface dislocations with this type of Burgers vector exist, are no longer valid. Moreover, it is confirmed by the present observations that no interfacial dislocations (in the sense of loss of coherence) are observed at places where the hexagonal faults terminate. Coherency dislocations, as those defined by Olson and Cohen (16), could he present along the boundary and particularly where the stacking faults are. However, it appears difficult to define the Burgers vectors of such dislocations only from the information given by Fig. 3. This figure shows the image of the projected structure while in the actual crystal the atom columns are at different levels along the direction of observations. Although the basal plane stacking faults do not seem to affect the habit plane to a noticeable extent, a problem may arise when a "Type I" defect intersects the parent-martensite interface (Fig. 2b). Unfortunately, because of the unfavorable geometrical conditions no detailed information on the habit plane structure in these areas could be obtained by high resolution electron microscopy. In conclusion it should he emphasized that some hexagonal basal plane stacking faults are necessary for obtaining an undistorted habit plane. The required density of faults depends on the so called tetragonality of the martenslte, as it was recently shown elsewhere by one of the authors (14). On the other hand the "Type I" defects are a consequence of the mechanism of the transformation [as already pointed out in (5)) but they do not contribute to the formation of a microscopically undistorted habit plane. On the contrary, local distortions are expected in the areas where the "Type I" defects intersect with the habit plane. Acknowledgements The authors wish to thank Dr. M. Chandrasekaran, Dr. M. Sade and Licenciado J. Pellegrina of the Centro At6mlco Bariloche for preparlng the Cu-Zn-AI specimen. This work was partially supported by the Consejo Naeional de Investlgaclones Cient~ficas y T~cnicas of Argentina. References i. 2. 3. 4. 5. 6. 7. 8. 9. I0. ii. 12. 13. 14. 15. 16.
R. Sinclair and H.A. Mohamed, Acta Metall., 26, 623 (1978). K.M. Knowles, Proc. Roy. Soc. Lond A 380, 287 (1982). K.M. Knowles, J.W. CHristian and D.A. Smith, J. Physique, 43, C4-185 (1982). F.C. Lovey, W. Coene, D. Van Dyck, G. Van Tendeloo, J. Van Landuyt and S. Amelinckx, Ultramlcroscopy, 14, 345 (1984). F.C. Lovey, G. Van Tendeloo, J. Van Landuyt, L. Delaey and S. Amelinckx, Phys. Star. Sol. (a), 86, 994 (1984). M. Andrade, L. Delaey and M. Chandrasekaran, J. Physique, 43, C4-673 (1982). Z. Nishiyama, J. Kakinokf and S. Kajiwara, J. Phys. Soc. Japan, 20, 1192 (1965). Z. Nishiyama and S. Kajlwara, Japan J. Appl. Phys., 2, 478 (1963). H. Tas, L. Delaey and a. Deruyttere, Metallurg. Trans., 4, 2833 (1973). S. Chakravorty and C.M. Wayman, Acta Metall., 25, 989 (1977). S. Kajiwara and T. Kikuchi, Acta Metall., 30, 589 (1982). T. Tadaki, T. Kakeshlta and K. Shimlzu, J. Physique, 43, C4- 191 (1982). J.M. Cook, M.A. O'Keefe, D.J. Smith and W.H. Stobbs, J. Microscopy, 129, 295 (1983). F.C. Lovey, Acta Me tall. 35, 1103 (1987). M. Andrade, M. Chandrasekaran and L. Delaey, Acta Metall., 32, 1809 (1984). G.B. Olson and M. Cohen, Acta Metall., 27, 1907 (1979).