Combination and Interface Structure of 9R Martensite Plate Variants in Ti 50.0 Pd 43.0 Fe 7.0 Shape Memory Alloy

Transcription

1 Materials Transactions, Vol. 43, No. 5 (2002) pp. 902 to 907 Special Issue of Smart Materials-Fundamentals and Applications c 2002 The Japan Institute of Metals Combination and Interface Structure of 9R Martensite Plate Variants in Ti 50.0 Pd 43.0 Fe 7.0 Shape Memory Alloy Sei-ichiro Ii 1,, Minoru Nishida 1, Toru Hara 2 and Kazuyuki Enami 3 1 Department of Material Science and Engineering, Graduate School of Science and Technology, Kumamoto University, Kumamoto , Japan 2 National Institute for Material Science, Tsukuba , Japan 3 Department of Mechanical and System Engineering, Ryukoku University, Otsu , Japan The combination of 9R martensite plate variants in Ti 50.0 Pd 43.0 Fe 7.0 shape memory alloy has been investigated by conventional transmission electron microscopy (CTEM). Three fundamental combinations of plate variants are identified in the plate group. These are designated as A:B, A:C and A:D types, which correspond to 591 9R Type II, {11 4} 9R Type I and {105} 9R compound twins, respectively. They show the same morphological characteristics of 9R and 18R martensite in Cu-base shape memory alloys, i.e., wedge, spear and fork (or kink) types. The Type II and compound twins are new findings. Irrational nature of the A:B interface is also studied in the edge-on state by high resolution electron microscopy (HREM). The boundary is gradually and randomly curved with strain contrast. (Received November 22, 2001; Accepted February 8, 2002) Keywords: titanium-palladium-iron shape memory alloy, martensitic transformation, 2H martensite, 9R martensite, transformation twin, variants accommodate twin, transmission electron microscopy (TEM), high resolution electron microscopy (HREM) 1. Introduction Ti Pd alloy of near equi-atomic composition has been one of promising materials for high temperature shape memory alloys, since it undergoes a thermoelastic martensitic transformation from B2 to B19 (orthorhombic 2H) type structure around 800 K upon cooling. The microstructures of the martensite have been studied by conventional transmission electron microscopy (CTEM) and electron diffraction. The internal defects of the 2H martensite confirmed so far are {111} 2H Type I, 1 3) 121 2H Type II 4) and {101} 2H compound twinnings. 4) The {111} 2H Type I and 121 2H Type II twinnings which are conjugate to each other coexisted in a same martensite variant. 4) The {111} 2H Type I twinning is considered to be a lattice invariant shear of the present transformation and the rest are defined as a deformation twin due to the elastic interaction during the transformation rather than a lattice invariant shear. 4) The effect of transition metal (TM: V, Cr, Mn, Fe, Co) substitution for Pd in Ti 50 Pd 50 X TM X alloys with X up to about 20.0 at% has been extensively studied by Enami and coworkers, especially in Ti 50 Pd 50 X Fe X alloys. 1, 5, 6) They reported that 9R martensite appears beside the ordinary 2H martensite when Fe contents are over 6.3 at%. The obtained microstructural aspects are summarized as follows. The 9R martensite variant consisting of {114} 9R Type I twins is sandwiched by the two 2H martensite variants and vice versa, i.e., two-in-one structure. In the two-in-one structure, the {111} 2H Type I twin plate in the 2H martensite variant penetrates into the 9R martensite variant across the zigzag boundary and directly connects to the {114} 9R Type I twin plate. 6) From the above aspects, the {114} 9R Type I twin is considered to be a lattice invariant shear of the B2 to 9R transformation. This is also supported by the fact that both {111} 7) 2H and {114} 8) 9R planes are derived from {110} B2 plane. However, it has been widely recognized that the lattice invariant shear of martensite with long periodic stacking structure such as 9R or 18R martensite is stacking faults on (001) basal plane. Therefore, internal twins are boundaries of martensite plate variants, which are so-called variant accommodation twinning. The variant accommodation twin is introduced as a result of mutual accommodation of shear strains between variants in the martensite. For 9R and/or 18R martensite, many researchers have reported that there are four plate variants commonly designated as A, B, C and D, and three fundamental plate variant combinations can be identified in a given plate variants; designated as A : B, A : C and A : D types. 9 11) The intervariant boundaries of these three types are in Type II, Type I and compound twin relations, respectively. 12) The interface structures of the 9R and 18R martensite in Cu Zn based alloys have been studied by many researchers using CTEM and high resolution electron microscopy (HREM). Fukamachi et al. observed the {11 4} Type I twinning, i.e. A : C type interface, of 9R martensite in Cu Zn Si alloy by using one-dimensional lattice image, and found that the twin boundary structure is coherently connected with each other across the boundary, except the places where stacking faults exist. 13) Adachi et al. observed the A : C type interface of 18R martensite in Cu Zn Al alloy by one-dimensional lattice imaging, and found that the interface structure is straight and essentially the same as the result of Fukamachi et al., while the A : D type interface has tendency to wander and to form curved segments. 14) Lovey et al. 15) and Wang et al. 16) reported a similar result for A : C and A : D type interface, by using two-dimensional lattice imaging. Zhang et al. examined A : B interface and found that the boundary structure is randomly and gradually curved with strain contrast, by using two-dimensional lattice imaging. 17) However, there is no observation of irrational A : B type interface in the edge-on state. The crystallography of twinning is described by either the Graduate Student, Kumamoto University.

2 Combination and Interface Structure of 9R Martensite Plate Variants in TiPdFe SMA 903 Table 1 Twinning elements of twinning modes discovered in 9R martensite in Ti Pd Fe alloy. Type K 1 η 1 K 2 η 2 s {11 4} Type I A:C (11 4) [1, 1.604, 0.151] (1, 0.839, 2.555) [ 591] Type II A:B (1, 0.839, 2.555) [ 591] (144) [1, 1.604, 0.151] {105} Compound A:D (105) [ 501] (401) [ 104] {001} Compound (001) [ 100] ( 100) [001] K 1 plane (twinning plane) and η 2 direction (the intersection of the K 2 plane and the plane of shear) or by the K 2 plane (another undistorted plane) and η 1 direction (twinning shear direction). 18) Type I twinning has the rational K 1 plane and η 2 direction. Type II twinning has the rational K 2 plane and η 1 direction. The two twin crystals in the former and the latter are related by the mirror symmetry with respect to the K 1 plane and by the rotation of π around the η 1 axis, respectively. All indices of the four elements are rational in compound twinning. In order to determine the twinning mode of Type I and compound twins directly by electron diffraction, the incident electron beam is required to be parallel to the K 1 plane. The obtained pattern consists of two sets of reflections, which are in mirror symmetry to each other with respect to the K 1 plane. On the other hand, the diffraction pattern obtained along η 1 direction of Type II twin shows a single pattern. 4, 19 21) These incident beam directions for each twinning mode are so-called the edge-on state. The edge-on state is also required to exactly analyze the twin boundary structure on atomic scale by HREM. It is apparent from the above description that there are plural electron beam directions in the edge-on state for the Type I and the compound twinnings. However, the edge-on state for the Type II twinning is the unique axis of η 1. In the present study, we find out the three fundamental plate variant combinations of 9R martensite in Ti 50.0 Pd 43.0 Fe 7.0 alloy. In addition, the boundary structure of A : B and A : C type interfaces in the 9R martensite is also investigated by HREM in the edge-on state, and then the irrational nature of A : B type interface is discussed. 2. Experimental Procedure Ti 50.0 Pd 43.0 Fe 7.0 alloy was prepared from 99.7 mass%ti and 99.8 mass Pd and 99.5 mass%fe by arc melting in argon atmosphere. The ingot was homogenized in vacuum at 1273 K for 36 ks. The rod of 3 mm in diameter was spark cut from the ingot. The rod was cut into disks of 0.2 mm in thickness. They were solution-treated in vacuum at 1273 K for 3.6 ks. These were lightly mechanically polished to remove the surface scale. Ms and Mf temperatures measured by DSC were about 543 K and 530 K, respectively. Specimens for CTEM and HREM studies were electropolished using the twin jet method in an electrolyte with an approximate composition of glacial acetic acid; 72%, methanol; 12%, ethylene glycol; 8% and perchloric acid; 8%, in volume, around 283 K. CTEM observations were carried out in JEM-2000FX microscope. Selected area electron diffraction experiments were carried out to determine the twinning mode. HREM observations were carried out in JEM-2010F microscope. Both the microscopes were operated at 200 kv. The following lattice parameters were used for the analysis in the 2H and 9R martensite phase; a = nm, b = nm, c = nm 22) and a = nm, b = nm, c = nm, β = 85.6 degrees, 6) respectively. 3. Results and Discussion 3.1 Combination of 9R martensite plate Four twinning modes in the 9R martensite as listed in Table 1 were confirmed in the present study. The 591 9R Type II and the {105} 9R compound twins which correspond to A : B and A : D boundaries, respectively, are newly found out in the present study. The twinning elements were calculated by the Bilby-Crocker theory. 18) In these twinning modes, since the {001} compound twinning was separately and less frequently observed and was not included in the plate group as discussed below, we do not describe here. The same result on the {001} compound twinning was reported by Enami et al. 6) So, the details of rest three twinning modes corresponding to the variant accommodation twin are described in the following sections. In the present study, we discover the region consisting only 9R martensite plate as shown in Fig. 1(a), besides the two-inone structure above mentioned. Typical electron diffraction patterns obtained are shown in Figs. 1(b) to (e), which are taken from areas 1 to 4 in (a), respectively. We can recognize easily that there are 9R martensite in (a), since diffraction spots at 1/3 position are seen along the row of 001 reflections in the patterns which can be indexed consistently with the lattice parameter of 9R martensite mentioned above. Since both patterns in (b) and (c) are identified as (11 4) 9R Type I twinning pattern, the plate boundary at the region 1 is A : C combination and that at the region 2 is B : D one, respectively. Both the A : C and B : D combinations are of A : C type as listed in Table 1. The most probable explanation for the plate boundaries at the regions 3 and 4 is considered to be the conjugate twinning mode of the {11 4} Type I twin. That is the 591 Type II twin. As might be expected, both patterns in (d) and (e) are identified to be [ 591] 9R Type II twinning. The [ 110] 9R and [ 192] 9R zone axes are not exactly parallel but angle between these two zone axes is 0.74 degrees, if the alternate platelets are related by a rotation of π around [ 591] 9R. The inclined angle between the trace of K 1 planes for the {11 4} 9R Type I and 591 9R Type II twins is about 14 degrees in average and almost compatible with the calculate value from the indices of each K 1 plane listed in Table 1. This fact also supports the existence of 591 9R Type II twin. From these results it is confirmed that the plate boundary at the region 3 is A : B combination and that at the region 4 is C : D one, respectively. These two combinations are of A : B type as apparently from Table 1. Generally,

3 904 S. Ii, M. Nishida, T. Hara and K. Enami Fig. 1 (a) Bright field image showing plate group configuration at the region consisting of only 9R martensite. (b), (c), (d) and (e) Corresponding electron diffraction patterns taken from areas 1, 2, 3 and 4 showing A : C, B : D, A : B and C : D pairs, respectively. (Electron beam (b) [ 110] 9R A,C, (c) [ 192] 9R B,D, (d) [ 110] 9R A, [ 192] 9R B and (e) [ 110] 9R C, [ 192] 9R D ). Subscripts of indices show the corresponding martensite plate variants, respectively. Fig. 2 Schematic illustration of plate group configuration in Fig. 1(a). the diffraction pattern derived from the Type II twinning can be easily analyzed with the aid of stereographic projection as described in previous reports. 4, 12, 21) In the same way, all plate boundaries are characterized as illustrated in Fig. 2. We can clearly see the spear and wedge morphologies at A : C and A : B type boundaries, respectively. In addition to the above plate boundaries, fine striations are seen in plates A and C as indicated by the double arrows in Fig. 1(a). These are stacking faults on the (001) 9R basal plane which is generally considered to be a lattice invariant shear of B2 to 9R transformation, since streaks along the c axis in (b), (d) and (e) are perpendicular to the striations. These results indicate that this image orientation is in the edge-on state for the A : C boundary plane, B : D boundary plane and the basal planes of plates A and C except for A : B boundary. Another example of plate group configuration only consisting of 9R martensite is presented in Fig. 3. A bright field image in Fig. 3(a) is observed along [ 591] 9R, which is the η 1 direction and the unique edge-on state of the [ 591] 9R Type II twinning, i.e., A: B boundary plane. This orientation is also edge-on state of A : C and A : D boundary plane. Figures 3(b), (c) and (d) are electron diffraction patterns taken from the areas 1, 2 and 3 in (a), respectively. The pattern in (b) consists of two sets of reflection of [ 591] 9R zone axis which are mirror symmetry with respect to the (11 4) 9R plane. The alternating platelets in the area 1 can be concluded to be the (11 4) 9R Type I twins, i.e., A: C type. On the other hand, the pattern in (c) shows a single pattern of [ 591] 9R zone axis. The single pattern taken from the alternating platelets is characteristic of Type II twin obtained along η 1 direction. In other words, the patterns from the matrix and twin are related by the rotation of π around the η 1 =[ 591] 9R axis. The platelets in the area 2 are considered to be in the [ 591] 9R Type II twins, i.e., A: B type. The pattern in (d) consists of two sets of reflection of [ 591] 9R zone axis which are mirror symmetry with

4 Combination and Interface Structure of 9R Martensite Plate Variants in TiPdFe SMA 905 Fig. 3 (a) Another example of plate group configuration at the region consisting of only 9R martensite. (b), (c) and (d) Corresponding electron diffraction patterns taken from areas 1, 2 and 3 showing A : C, A : B and A : D pairs, respectively. (Electron beam (b), (c) and (d) [ 591] 9R A,B,C and D. ) (e) Schematic illustration of plate group configuration in (a). Fig. 4 (a) Bright field image of two-in-one structure. (b), (c), (d) and (e) Corresponding electron diffraction patterns taken from areas 1, 2, 3 and 4, respectively. (Electron beam (b) [ 110] 2H M,T, (c) [ 110] 9R M,T and (d) [ 110] 9R M, [ 192] 9R T and (e) [ 110] 2H M,T ). respect to the (105) 9R plane. The plane defects in the area 3 can be considered to be the (105) 9R compound twin, i.e., A : D type. In the same way, the plate variant configuration in Fig. 3(a) is determined as illustrated in Fig. 3(e). In addition to the spear and wedge morphologies, we can see fork and kink ones at A : D type boundary. Three fundamental plate variant combinations are identified in the plate group, which shows essentially the same configuration of 9R and/or 18R martensite plate variants in Cu-base shape memory alloy. The typical example of two-in-one structure is shown in Fig. 4(a). Figures 4(b) to (e) are corresponding electron diffraction patterns taken from areas 1, 2, 3 and 4 in (a). We can recognize easily that there is 9R martensite in the area 2 between two 2H martensites in the areas 1 and 4 separated by zigzag boundary in (a), since there are diffraction spots at 1/2 and 1/3 position in the row of 001 reflections in the corresponding patterns. The width of 9R martensite plate at the area 2 in (a) is about half that in Fig. 1(a) and Fig. 2(a).

5 906 S. Ii, M. Nishida, T. Hara and K. Enami The internal defects of 2H and 9R martensites can be identified to be (11 1) 2H Type I twinning and (11 4) 9R Type I twinning from the patterns (b), (e) and (c), respectively. Since the both the twinning planes are originally derived from {110} B2 plane, 7, 8) the parallel continuation of both the twinning plates is quite natural. It is likely that both the martensites form at the same moment during the transformation, although they are separated by the zigzag boundary. Further studies are required to clarify the nature and role of zigzag boundary. These features are so-called two-in-one structure which has been termed by Enami et al. 6) Apparently from the pattern in (d), alternate platelets in the area 3 are identified to be [ 591] 9R Type II twinning as described in Figs. 1(d) and (e). It is clearly seen that dominant twinning mode, i.e., internal defect, is {11 4} 9R Type I twin in the 9R martensite which coexists with 2H martensite. In other words, the martensitic shape strain is reduced by the formation of a lot of A : C type and a few of A : B type in the 9R martensite coexisting with 2H martensite in the present alloy. The coexistence of multiple martensitic phases with different stacking sequence is quite interesting, though the origin is not clear at present. It has been reported that the same phenomena is observed in Ni Al 23, 24) and Ni Al Mn shape memory alloys. The twin in martensite is generally classified into two categories. One is termed as the transformation twin which is introduced as a lattice invariant shear. The other is termed as the variant accommodation twin which is introduced as mutual accommodation of shear strains between plate variants. From above observations, it may be concluded that the twins in the present 9R martensite have both features. In other words, the (11 4) 9R Type I twin as shown in Fig. 4 is considered to be a lattice invariant shear morphologically, since the (11 1) 2H Type I twinning which is a lattice invariant shear of the 2H martensite 1) continuously connects with the above two twins in the 9R martensite with about the same spacing. However, the 591 9R Type II twin in Fig. 4 is considered to be a deformation twinning due to elastic interaction during the transformation, since there was no martensite variant consisting wholly of 591 Type II twins in the two-in-one morphology throughout the present observations. On the other hand, those twins and (105) 9R compound twin can be regarded as variant accommodation twin in the region only consisting of the 9R martensite as shown in Figs. 1, 2 and 3. To further understanding, the crystallographic analysis by phenomenological theory will be required. 3.2 Boundary structure of the A : C and A : B type martensite plate variants HREM observations of twin boundary in the above three twinning modes have been completed in the edge-on state. The boundary of the {105} 9R compound twinning curves and wanderes due to the presence of stacking faults on the basal plane. However, we do not reproduce it here, since the same boundary structure has been observed in the {1010} 18R compound twin of the 18R martensite in Cu Zn Al shape memory alloy ) Figures 5(a) and (b) show a two-dimensional lattice image of the {11 4} 9R Type I twin and 591 9R Type II twin boundary taken along η 1 =[ 591] 9R zone axis, which are A : C and A : B type interface of martensite plate variants. The bound- Fig. 5 Two-dimensional lattice images of (a) {11 4} Type I, i.e. A : C Type, and (b) 591 Type II, i.e. A : B Type, twin boundaries. (Electron beam (a) [ 591] 9R A,B and C ). ary lies between arrows in both the images in (a) and (b), respectively. Two twin crystals are related by mirror symmetry with respect to the K 1 plane in Type I twin as shown in (a), while they are rotated by π around η 1 in Type II twin as shown in (b). As far as we know, this is the first observation of the twins of 9R martensite in Ti Pd Fe alloy from the η 1 direction by HREM. The boundary of the {11 4} 9R Type I twin in (a) is straight and sharp as expected from the crystallographic characteristics of Type I twin mentioned above. The boundary of the [ 591] 9R Type II twin in (b) is gradually and randomly curved with strain. This observation suggests that a strain around the [ 591] 9R Type II twin boundary in the 9R martensite of Ti 50.0 Pd 43.0 Fe 7.0 alloy is elastically relaxed by gradual displacement of the atoms around the boundary. Knowles has observed 011 Type II twin boundary in Ti Ni B19 martensite by HREM and suggested that boundary structure of the irrational 011 Type II twin consists of low-indexed ledge and step. 25) Adachi et al. have reported a model of A : B type twin boundary consisting of ledge and step in 18R martensite in Cu Zn Al alloy. 12) On the other hand, Nishida et al. have succeeded the observation of 011 Type II twin boundary of Ti Ni B19 martensite in exactly edge-on state and reported there are strain contrast instead of ledge and step around twin boundary. 26) They also reported the same result in 121 Type II twin of B19 martensite in Ti Pd alloy. 27) Hara et al. observed 111 Type II twin of γ 1 martensite in Cu Al Ni alloy and reported neither ledge nor step. 28) The boundary structure

6 Combination and Interface Structure of 9R Martensite Plate Variants in TiPdFe SMA 907 of 591 9R Type II twins of 9R martensite in Ti 50.0 Pd 43.0 Fe 7.0 shape memory alloy investigated is the same as that of 2H martensite in the other thermoelastic alloys such as Ti Ni, 26) Ti Pd 27) and Cu Al Ni 28) alloy. This result suggests that the irrational nature of the Type II twin boundary in thermoelastic martensite is elastically relaxed by gradual displacement of the atoms at the boundary and irrespective of the martensite structure. 4. Conclusions The combination of 9R martensite plate variants in Ti 50.0 Pd 43.0 Fe 7.0 shape memory alloy has been investigated by CTEM. Three fundamental plate variant combinations are identified in the plate group at the region consisting of only 9R martensite. These are designated as A : B, A : C and A : D types, which correspond to 591 9R Type II, {11 4} 9R Type I and {105} 9R compound twins, respectively. They show essentially the same morphological characteristics of 9R and 18R martensite in Cu-base shape memory alloys, i.e., wedge, spear and fork or kink types. On the other hand, {11 4} 9R Type I and 591 9R Type II twins in the two-in-one structure with the 2H martensite is morphologically considered to be a lattice invariant shear. The Type II and compound twins are newly found out in the present study. Irrational nature of the A : B interface is also studied in the edge-on state by high resolution electron microscopy (HREM). The boundary is gradually and randomly curved with strain contrast instead of low-indexed ledge and step structure. Acknowledgements This work was partly supported by the Grant-in-Aid for Scientific Research from Ministry of Education, Science, Sports, and Culture, Japan under the project number REFERENCES 1) K. Enami, H. Seki and S. Nenno: Tetsu-to-Hagane 72 (1986) (in Japanese). 2) P. G. Lindquist and C. M. Wayman: MRS Int l. Mtg. on Adv. Mats. 9 (1989) pp ) D. Golberg, Y. Xu, Y. Murakami, S. Morito, K. Otsuka, T. Ueki and H. Horikawa: Intermetallics 3 (1995) ) M. Nishida, T. Hara, Y. Morizono, A. Ikeya, H. Kijima and A. Chiba: Acta Mater. 45 (1997) ) K. Enami and Y. Nakagawa: Proc. of Int. Conf. on Martensitic Transformations (ICOMAT-86), (The Japan Inst. Metals, 1986) pp ) K. Enami, T. Yoshida and S. Nenno: Proc. of Int.Conf. on Martensitic Transformations (ICOMAT-92), (Monterey Inst. Adv. Studies, 1993) pp ) K. Otsuka and K. Shimizu: Jpn. J. Appl. Phys. 8(1969) ) S. Kajiwara and Z. Nishiyama: Trans. JIM 17(1976) ) H. Tas, L. Deleay and A. Deruyttere: Metall. Trans. A 4A (1973) ) T. A. Schroeder and C. M. Wayman: Acta Metall. 25 (1977) ) T. Saburi and C. M. Wayman: Acta Metall. 27 (1979) ) K. Adachi, J. Perkins and C. M. Wayman: Acta Metall. 34 (1986) ) K. Fukamachi and S. Kajiwara: Jpn. J. Appl. Phys. 19 (1980) L479 L ) K. Adachi and J. Perkins: Metall. Trans. A 16A (1985) ) F. C. Lovey, G. Van. Tendeloo and J. Van Landuyt: Scr. Metall. 21 (1987) ) R. Wang, C. Luo, J. Gui, Q. Ren and J. Chen: J. Phys. Condensed. Matter. 4 (1992) ) J. X. Zhang, Y. F. Zheng and L. C. Zhao: Acta Mater. 47 (1999) ) B. A. Bilby and A. G. Crocker: Proc. of Roy. Soc. Ser. A 288 (1965) ) T. Onda, Y. Bando, T. Ohba and K. Otsuka: Mater. Trans., JIM 33 (1992) ) M. Nishida, H. Ohgi, I. Itai, A. Chiba and K. Yamauchi: Acta Metall. 43 (1995) ) T. Hara, T. Ohba, S. Miyazaki and K. Otsuka: Mater. Trans., JIM 33 (1992) ) P. Krautwasser, S. Bahn and K. Shcubert: Z. Metallk. 59 (1968) ) R. Kainuma, H. Ohtani and K. Ishida: Metall. Mater. Trans A, 27A (1996) ) S. Morito and K. Otsuka: Mater. Sci. Eng. A, A208 (1996) ) K. M. Knowles: Philos. Mag. A 45 (1982) ) M. Nishida, K. Yamauchi, I. Itai, H. Ohgi and A. Chiba: Acta Metall. 43 (1995) ) M. Nishida, T. Hara, A. Chiba and K. Hiraga: Proc. of Int. Conf. on Displasive Phase Trans. and Their Appl. Mater. Eng., (1996) pp ) T. Hara, T. Ohba, S. Miyazaki and K. Otsuka: Proc. of Int. Conf. on Martensitic Transformation (ICOMAT-92), (Monterey Inst. Adv. Studies, 1993) pp