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Self-accommodating martensite plate variants in shape memory CuAlNi alloys
Journal of Materials Processing Technology 123 (2002) 498–500
Self-accommodating martensite plate variants in shape memory CuAlNi alloys Yildirim Aydogdu, Ayse Aydogdu, Osman Adiguzel* Department of Physics, Firat University, 23169 Elazig, Turkey Received 14 March 2000; accepted 5 March 2002
Abstract In noble metal copper-based b-phase alloys, which exhibit the shape memory effect, martensitic transformation occurs by lattice invariant shears on close-packed {1 1 0}b planes of the matrix, called the basal plane of the martensite. The parent single crystal undergoes the martensitic transformation on cooling in a self-accommodating manner minimising the total shape change at the advanced stage of the transformation. These alloys are deformed by variant-to-variant transformation when stressed in the martensite state. The reversible shape memory effect is also considered in terms of the correspondence-variants and the formation of each martensite is accompanied by a definite shape deformation. The so-called self-accommodation is achieved by combining suitable correspondence-variants. In a single crystal of the parent phase, 24 martensite variants grouped around six {1 1 0}b planes can be formed at temperatures lower than Ms. In the present study, configurations of the martensite variants in CuAlNi alloys have been investigated by an electron microscope technique. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Self-accommodation; Martensite plate variants; CuAlNi alloys; Shape memory effect
1. Introduction Noble metal copper-based alloys undergo a diffusionless phase transition called martensitic transformation on cooling from the b-phase region and exhibit an unusual property called superelasticity or shape memory effect [1]. In copperbased b-phase alloys, shape deformation in the martensitic condition is essential for the shape memory effect. If an external stress is applied on an alloy sample which contains twin related martensite variants, deformation proceeds by reorientation of the variants through the movement of their boundaries. When the stress is removed and temperature is increased up to the reverse Af temperature, the boundaries move back to initial positions and the original shape of the sample is recovered [2]. The fundamental mechanism of the shape memory effect is characterised by the correspondence-variants. The socalled self-accommodation during the thermal transformation is achieved by combining suitable correspondence-variants.
*
Corresponding author. Tel.: þ90-424-2122707; fax: þ90-424-2330062. E-mail address: [email protected] (O. Adiguzel).
On the other hand, martensitic transformation is a first order transition associated with shape change, and the selfaccommodation mechanism is also known to operate to minimise total shape change [3]. The parent phase in b-phase alloys has a symmetric structure, while the product martensite has a complex microstructure and consists of several variants of martensite [4]. The formation of martensite which has a long period stacking sequence in shape memory alloys takes place by the localised formation of four self-accommodating martensite variants in a plate group. Self-accommodating martensite transformation involves plate groups, each of which contains four ‘‘cooperating’’ martensite variants. The single crystal of parent phase undergoes the martensite transformation in a self-accommodating manner to minimise the total shape change and six groups of four variants formed from the parent, and four variants in a plate group have habit plane normal clustered about one of the six {1 1 0}b planes [5]. The mechanism of self-accommodating martensite plate variants have attracted considerable attention in many studies [6–10]. In the present contribution, self-accommodating martensite plate variants in two shape memory CuAlNi alloys have been investigated by means of optical and transmission electron microscopy (TEM).
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 1 4 0 - 1
Y. Aydogdu et al. / Journal of Materials Processing Technology 123 (2002) 498–500
2. Experimental Two shape memory CuAlNi alloys investigated in the current study were supplied by Trefimetaux Centre’ de Recherche (France). These alloys designated as Alloy 1 and Alloy 2 have the following chemical compositions: Alloy 1: Cu–11.00%Al–3.82%Ni (in weight); Alloy 2: Cu–11.00%Al–3.38%Ni (in weight). All specimens obtained from these alloys were solution treated in the b-phase fields (930 8C for 30 min for the Alloy 1 and 920 8C for 30 min for the Alloy 2) and then quenched in iced brine to retain the b-phase. Metallographic observation and transmission electron microscope studies were carried out on samples prepared as follows: (a) Metallographic observation: For optical microscopy, the specimens were polished using conventional procedures and etched in a solution composed of 50 g FeCl36H2O and 960 ml methanol in 200 ml HCl. These specimens were examined in a Reichert MeF2 optical microscope. (b) TEM observations: For the TEM observations, the specimens were cleaved to 2 mm 2 mm in size and back-thinned using a chemical etch, 20% nitric acid in methanol, following mechanical polishing. The TEM specimens were examined in a Philips EM430 microscope at 250 kV. 3. Results and discussion Optical and transmission electron micrographs have been taken from the etched specimens of CuAlNi alloys in the as-quenched condition. Two optical micrographs exhibiting plate martensites and self-accommodating morphology are
499
seen from Fig. 1a and b. As seen in these micrographs, CuAlNi martensites have typical plate configuration. The most characteristic feature of the martensitic microstructures in noble metal copper-based ternary alloys such as CuZnAl, CuAlMn or CuAlNi is the prevalence of groups of essentially parallel-sided plates and occurrence of comparatively few, large groups of unique orientation within the grains of the parent phase [8,10]. Bright field and dark field electron micrographs of parallel-sided martensites taken from as-quenched specimens of Alloy 1 are seen from Fig. 2a and b, respectively. As seen from the optical and transmission electron micrographs, self-accommodated martensite plate groups form in the as-quenched condition. It has been reported that grains grow with further annealing in martensitic condition [10]. Many studies report that thermal martensite appears in self-accommodated stacking form while the stress-induced martensite occurs in monopartial stacking form [11–14]. The morphology exhibiting self-accommodating martensite plates is also called diamond-shaped morphology in copper-based b-phase alloys, which produces the martensites which have the long period layered structure [15]. In the self-accommodating morphology, six kinds of variant groups are generated from a single crystalline b-phase and each of them consists of four different variants, each of which contains four cooperating martensite variants which when combined produce nearly zero macroscopic shape change [6,15,16]. This enables the materials to deform under low stresses by means of variant coalescence, and this is also an important necessity to provide the shape memory effect. The individual martensite plates are initially either very thin and parallel sided or grow in broader units of two or four plates which constitutes the self-accommodating system whose junction planes with the matrix are the conventional habit planes, whereas their internal junction planes are twin planes of the martensite lattice [6,17,18].
Fig. 1. Optical micrographs of etched samples of: (a) Alloy 1 and (b) Alloy 2. Typical martensite structures and self-accommodation morphology are clearly seen in grains.
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Y. Aydogdu et al. / Journal of Materials Processing Technology 123 (2002) 498–500
Fig. 2. Electron micrographs of parallel sided plate-type martensites taken from the as-quenched specimen of Alloy 1: (a) dark field, (b) bright field.
In conclusion, the growth units of two or four plates constitute the self-accommodating phenomena and junction planes of these plates with the parent phase are the conventional habit plane. The self-accommodation through plate group formation may be comprehended with the clarification of the crystallography of the variant combination.
References [1] K. Tsuchiya, K. Marukava, J. Phys. IV 5 (1995) C8-901–C8-905. [2] S. Sugimoto, H. Sakamoto, T. Hara, H. Tsuchiya, J. Phys. IV 5 (1995) C8-925–C8-930. [3] O. Adiguzel, Tr. J. Phys. 13 (1989) 171–179. [4] S. Cakmak, N. Kayali, E. Artunc, O. Adiguzel, Tr. J. Physics 17 (1993) 847–855. [5] K. Adachi, J. Perkins, C.M. Wayman, Acta Metall. 34 (1986) 2471– 2485. [6] T. Saburi, C.M. Wayman, Acta Metall. 27 (1979) 979–995. [7] S. Cakmak, Ph.D. Thesis, Firat University, Elazig, Turkey, 1992 (in Turkish). [8] N. Kayali, S. Ozgen, O. Adiguzel, Mater. Res. Bull. 32 (1997) 569–578.
[9] Y. Aydogdu, A. Aydogdu, Y. Atici, O. Adigu¨ zel, Variant morpholgy at martensite formation in shape memory CuAlNi alloys in: Proceedings of the 14th International Congress on Electron Microscopy (ICEM-14), Symposium U, Vol. II, Cancun, Mexico, August 31–September 4, 1998, pp. 97–98. [10] A. Aydogdu, Y. Aydogdu, O. Adiguzel, Electron microscopy of layered structures in copper based shape memory alloys, in: Proceedings of the 13th National Electron Microscopy Congress, ODTU/METU, Ankara, Turkey, September 1–4, 1997, pp. 676–683. [11] N. Bergeon, G. Guenin, C. Esnouf, J. Phys. IV 7 (1997) C5-125–C5130. [12] T. Maki, K. Tsuzaki, in: Proceedings of the International Conference on Martensitic Transformation (ICOMAT’92), Monterey, 1992, Monterey Institute for Advanced Studies, 1993, pp. 1151–1162. [13] J.L. Putaux, J.P. Chevalier, Acta Mater. 44 (1996) 1701–1716. [14] H.J. Yang, C.M. Wayman, Mater. Characterization 28 (1992) 37–47. [15] N. Kayali, S. Ozgen, O. Adiguzel, J. Phys. IV 7 (1997) C5-317–C5322. [16] T. Saburi, S. Nenno, S. Kato, K. Takata, J. Less Comm. Met. 50 (1976) 223–226. [17] C.M. Friend, J. Phys. IV 1 (1991) C4-25–C4-30. [18] T. Waitz, H.P. Karnthaler, C. Rentenberger, J. Phys. IV. 7 (1997) C5185–C5-190.
Self-accommodating martensite plate variants in shape memory CuAlNi alloys Yildirim Aydogdu, Ayse Aydogdu, Osman Adiguzel* Department of Physics, Firat University, 23169 Elazig, Turkey Received 14 March 2000; accepted 5 March 2002
Abstract In noble metal copper-based b-phase alloys, which exhibit the shape memory effect, martensitic transformation occurs by lattice invariant shears on close-packed {1 1 0}b planes of the matrix, called the basal plane of the martensite. The parent single crystal undergoes the martensitic transformation on cooling in a self-accommodating manner minimising the total shape change at the advanced stage of the transformation. These alloys are deformed by variant-to-variant transformation when stressed in the martensite state. The reversible shape memory effect is also considered in terms of the correspondence-variants and the formation of each martensite is accompanied by a definite shape deformation. The so-called self-accommodation is achieved by combining suitable correspondence-variants. In a single crystal of the parent phase, 24 martensite variants grouped around six {1 1 0}b planes can be formed at temperatures lower than Ms. In the present study, configurations of the martensite variants in CuAlNi alloys have been investigated by an electron microscope technique. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Self-accommodation; Martensite plate variants; CuAlNi alloys; Shape memory effect
1. Introduction Noble metal copper-based alloys undergo a diffusionless phase transition called martensitic transformation on cooling from the b-phase region and exhibit an unusual property called superelasticity or shape memory effect [1]. In copperbased b-phase alloys, shape deformation in the martensitic condition is essential for the shape memory effect. If an external stress is applied on an alloy sample which contains twin related martensite variants, deformation proceeds by reorientation of the variants through the movement of their boundaries. When the stress is removed and temperature is increased up to the reverse Af temperature, the boundaries move back to initial positions and the original shape of the sample is recovered [2]. The fundamental mechanism of the shape memory effect is characterised by the correspondence-variants. The socalled self-accommodation during the thermal transformation is achieved by combining suitable correspondence-variants.
*
Corresponding author. Tel.: þ90-424-2122707; fax: þ90-424-2330062. E-mail address: [email protected] (O. Adiguzel).
On the other hand, martensitic transformation is a first order transition associated with shape change, and the selfaccommodation mechanism is also known to operate to minimise total shape change [3]. The parent phase in b-phase alloys has a symmetric structure, while the product martensite has a complex microstructure and consists of several variants of martensite [4]. The formation of martensite which has a long period stacking sequence in shape memory alloys takes place by the localised formation of four self-accommodating martensite variants in a plate group. Self-accommodating martensite transformation involves plate groups, each of which contains four ‘‘cooperating’’ martensite variants. The single crystal of parent phase undergoes the martensite transformation in a self-accommodating manner to minimise the total shape change and six groups of four variants formed from the parent, and four variants in a plate group have habit plane normal clustered about one of the six {1 1 0}b planes [5]. The mechanism of self-accommodating martensite plate variants have attracted considerable attention in many studies [6–10]. In the present contribution, self-accommodating martensite plate variants in two shape memory CuAlNi alloys have been investigated by means of optical and transmission electron microscopy (TEM).
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 1 4 0 - 1
Y. Aydogdu et al. / Journal of Materials Processing Technology 123 (2002) 498–500
2. Experimental Two shape memory CuAlNi alloys investigated in the current study were supplied by Trefimetaux Centre’ de Recherche (France). These alloys designated as Alloy 1 and Alloy 2 have the following chemical compositions: Alloy 1: Cu–11.00%Al–3.82%Ni (in weight); Alloy 2: Cu–11.00%Al–3.38%Ni (in weight). All specimens obtained from these alloys were solution treated in the b-phase fields (930 8C for 30 min for the Alloy 1 and 920 8C for 30 min for the Alloy 2) and then quenched in iced brine to retain the b-phase. Metallographic observation and transmission electron microscope studies were carried out on samples prepared as follows: (a) Metallographic observation: For optical microscopy, the specimens were polished using conventional procedures and etched in a solution composed of 50 g FeCl36H2O and 960 ml methanol in 200 ml HCl. These specimens were examined in a Reichert MeF2 optical microscope. (b) TEM observations: For the TEM observations, the specimens were cleaved to 2 mm 2 mm in size and back-thinned using a chemical etch, 20% nitric acid in methanol, following mechanical polishing. The TEM specimens were examined in a Philips EM430 microscope at 250 kV. 3. Results and discussion Optical and transmission electron micrographs have been taken from the etched specimens of CuAlNi alloys in the as-quenched condition. Two optical micrographs exhibiting plate martensites and self-accommodating morphology are
499
seen from Fig. 1a and b. As seen in these micrographs, CuAlNi martensites have typical plate configuration. The most characteristic feature of the martensitic microstructures in noble metal copper-based ternary alloys such as CuZnAl, CuAlMn or CuAlNi is the prevalence of groups of essentially parallel-sided plates and occurrence of comparatively few, large groups of unique orientation within the grains of the parent phase [8,10]. Bright field and dark field electron micrographs of parallel-sided martensites taken from as-quenched specimens of Alloy 1 are seen from Fig. 2a and b, respectively. As seen from the optical and transmission electron micrographs, self-accommodated martensite plate groups form in the as-quenched condition. It has been reported that grains grow with further annealing in martensitic condition [10]. Many studies report that thermal martensite appears in self-accommodated stacking form while the stress-induced martensite occurs in monopartial stacking form [11–14]. The morphology exhibiting self-accommodating martensite plates is also called diamond-shaped morphology in copper-based b-phase alloys, which produces the martensites which have the long period layered structure [15]. In the self-accommodating morphology, six kinds of variant groups are generated from a single crystalline b-phase and each of them consists of four different variants, each of which contains four cooperating martensite variants which when combined produce nearly zero macroscopic shape change [6,15,16]. This enables the materials to deform under low stresses by means of variant coalescence, and this is also an important necessity to provide the shape memory effect. The individual martensite plates are initially either very thin and parallel sided or grow in broader units of two or four plates which constitutes the self-accommodating system whose junction planes with the matrix are the conventional habit planes, whereas their internal junction planes are twin planes of the martensite lattice [6,17,18].
Fig. 1. Optical micrographs of etched samples of: (a) Alloy 1 and (b) Alloy 2. Typical martensite structures and self-accommodation morphology are clearly seen in grains.
500
Y. Aydogdu et al. / Journal of Materials Processing Technology 123 (2002) 498–500
Fig. 2. Electron micrographs of parallel sided plate-type martensites taken from the as-quenched specimen of Alloy 1: (a) dark field, (b) bright field.
In conclusion, the growth units of two or four plates constitute the self-accommodating phenomena and junction planes of these plates with the parent phase are the conventional habit plane. The self-accommodation through plate group formation may be comprehended with the clarification of the crystallography of the variant combination.
References [1] K. Tsuchiya, K. Marukava, J. Phys. IV 5 (1995) C8-901–C8-905. [2] S. Sugimoto, H. Sakamoto, T. Hara, H. Tsuchiya, J. Phys. IV 5 (1995) C8-925–C8-930. [3] O. Adiguzel, Tr. J. Phys. 13 (1989) 171–179. [4] S. Cakmak, N. Kayali, E. Artunc, O. Adiguzel, Tr. J. Physics 17 (1993) 847–855. [5] K. Adachi, J. Perkins, C.M. Wayman, Acta Metall. 34 (1986) 2471– 2485. [6] T. Saburi, C.M. Wayman, Acta Metall. 27 (1979) 979–995. [7] S. Cakmak, Ph.D. Thesis, Firat University, Elazig, Turkey, 1992 (in Turkish). [8] N. Kayali, S. Ozgen, O. Adiguzel, Mater. Res. Bull. 32 (1997) 569–578.
[9] Y. Aydogdu, A. Aydogdu, Y. Atici, O. Adigu¨ zel, Variant morpholgy at martensite formation in shape memory CuAlNi alloys in: Proceedings of the 14th International Congress on Electron Microscopy (ICEM-14), Symposium U, Vol. II, Cancun, Mexico, August 31–September 4, 1998, pp. 97–98. [10] A. Aydogdu, Y. Aydogdu, O. Adiguzel, Electron microscopy of layered structures in copper based shape memory alloys, in: Proceedings of the 13th National Electron Microscopy Congress, ODTU/METU, Ankara, Turkey, September 1–4, 1997, pp. 676–683. [11] N. Bergeon, G. Guenin, C. Esnouf, J. Phys. IV 7 (1997) C5-125–C5130. [12] T. Maki, K. Tsuzaki, in: Proceedings of the International Conference on Martensitic Transformation (ICOMAT’92), Monterey, 1992, Monterey Institute for Advanced Studies, 1993, pp. 1151–1162. [13] J.L. Putaux, J.P. Chevalier, Acta Mater. 44 (1996) 1701–1716. [14] H.J. Yang, C.M. Wayman, Mater. Characterization 28 (1992) 37–47. [15] N. Kayali, S. Ozgen, O. Adiguzel, J. Phys. IV 7 (1997) C5-317–C5322. [16] T. Saburi, S. Nenno, S. Kato, K. Takata, J. Less Comm. Met. 50 (1976) 223–226. [17] C.M. Friend, J. Phys. IV 1 (1991) C4-25–C4-30. [18] T. Waitz, H.P. Karnthaler, C. Rentenberger, J. Phys. IV. 7 (1997) C5185–C5-190.