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Variation of long-period stacking order structures in rapidly solidified Mg97Zn1Y2 alloy
Materials Science and Engineering A 393 (2005) 269–274
Variation of long-period stacking order structures in rapidly solidified Mg97Zn1Y2 alloy M. Matsudaa,∗ , S. Iib , Y. Kawamuraa , Y. Ikuharab , M. Nishidaa a
Department of Materials Science and Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan b Engineering Research Institute, University of Tokyo, 2-11-16 Yayoi Bunkyo-ku, Tokyo 113-8656, Japan Received 23 August 2004; received in revised form 12 October 2004; accepted 12 October 2004
Abstract The long-period stacking order (LPSO) structures in rapidly solidified Mg97 Zn1 Y2 alloy have been studied by conventional and highresolution transmission electron microscopes (HRTEMs). There are four kinds of stacking sequences in the LPSO structures, i.e., 18R of ABABABCACACABCBCBC, 14H of ACBCBABABABCBC, 10H of ABACBCBCAB and 24R of ABABABABCACACACABCBCBCBC. The 18R structure is dominantly observed in the present study. The rest three are occasionally observed in places. The 10H and 24R structures ¯ and 24R(1 1¯ 1 1¯ 1 1¯ 2)3 structures are are recently discovered. The lattice constants of 18R(1 1¯ 1 1¯ 2)3 , 14H(2¯ 1 2¯ 1 1¯ 1 1¯ 2 1¯ 2), 10H(1 3¯ 1 1¯ 3 1) estimated to be a = 0.320 nm and c = 4.678 nm, a = 0.325 nm and c = 3.694 nm, a = 0.325 nm and c = 2.603 nm, a = 0.322 nm and c = 6.181 nm for the hexagonal structure, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Long-period stacking order structure; Stacking sequence; Magnesium–zinc–yttrium; Transmission electron microscopy
1. Introduction Superior performance of rapidly solidified (RS) powder metallurgy Mg97 Zn1 Y2 (at.%) alloy with extremely high tensile yield strength of 610 MPa and elongation of 5% has been recently developed [1]. These excellent properties are considered to be due to the hcp(2H)-Mg fine grain matrix of 100–200 nm in diameter with a novel long-period stacking order (LPSO) phase and homogeneously dispersed Mg24 Y5 fine particles of less than 10 nm in diameter [2–4]. The morphological, crystallographic and chemical characterizations of the LPSO phase have been performed by conventional transmission electron microscope (CTEM) [2], a high-angle annular dark-field scanning TEM with Z-contrast [3] and a three-dimensional atom probe [4]. Recently, the LPSO phase has been also observed in the Mg97 Zn1 Y2 alloy produced by Cu-mold casting [5] and the melt spun Mg97 Zn1 Ln2 (Ln = Lanthanide metal) alloys [6]. It has been found that ∗
Corresponding author. Tel.: +81 96 342 3707; fax: +81 96 342 3710. E-mail address: [email protected] (M. Matsuda).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.10.040
there are several varieties of the LPSO structures showing different electron diffraction phenomena in those alloys. However, the accurate structural analysis of those LPSO structures has not been performed yet, especially in numbers of stacking layers and stacking sequence of the basal plane. The purpose of the present study is to clarify the stacking sequence of various LPSO structures in the melt spun Mg97 Zn1 Y2 ribbon by CTEM and high-resolution transmission electron microscope (HRTEM) observations.
2. Experimental procedure An Mg97 Zn1 Y2 ingot was prepared by high-frequency induction melting of pure metals in an argon atmosphere. Ribbons were prepared by a single-roller melt spinning method at circumferential speed of 42.0 m/s. The obtained ribbons were annealed at 573 and 673 K for 3.6 ks in vacuum of 2.5 × 10−3 Pa, since the LPSO phase is observed in the whole area of many grains by annealing at 573 K and the LPSO phase is coagulated by annealing at 673 K [2]. TEM
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observed and the rest are occasionally observed in places. The 10H and 24R structures have never been observed in this alloy system within the authors knowledge. The details of each structure are described in the following sections. 3.1. 18R structure
Fig. 1. Bright field image of a RS Mg97 Zn1 Y2 ribbon annealed at 573 K for 3.6 ks.
specimens were prepared by the argon ion milling technique. CTEM and HRTEM observations were carried out in JEOL-2000FX, JEOL-4010 and TECNAI F20 microscopes, respectively.
3. Results and discussion Fig. 1 shows typical microstructure of a RS Mg97 Zn1 Y2 ribbon annealed at 573 K for 3.6 ks. It is apparent that most of the grains are with the characteristic striations of the LPSO phase developing from grain boundaries to interior as reported so far [2]. In the present study, four kinds of the LPSO structures, i.e., 18, 14, 10 and 24 layers structures, are confirmed by electron diffraction experiments, CTEM and HRTEM observations. The expression of R and H is based on the Ramsdell notation [7]. The 18R structure is dominantly
Fig. 2(a and b) show a two-dimensional lattice image and the corresponding electron diffraction pattern of the 18 layer structure, respectively. One can recognize that the LPSO structure consists of 18 stacking sequence of ABABABCACACABCBCBC with 4.678 nm as shown in (a). This is consistent with 18 spots with regular intervals between the transmission beam and the spot corresponding to the {0 0 0 2} plane of pure Mg as shown in (b) and (c). The stacking sequence does not have mirror symmetry with respect to the basal plane. In addition, the spot distribution on both sides of the c* axis is asymmetric as indicated by dotted lines in (b). From these features, the 18 layer structure is determined to be of the 18R(1 1¯ 1 1¯ 2)3 structure and the same as the structure of the X Mg12 YZn phase reported by Luo [8], where the expression of (1 1¯ 1 1¯ 2)3 is based on the Zhdanov symbol [7]. The lattice constants are estimated to be a = 0.320 nm and c = 4.678 nm for the hexagonal structure. It is considered that the origin of the 18R structure is attributed to the localized segregation of Zn and Y, i.e., chemical ordering of these atoms [2–4]. The 18R structure is dominantly observed among the four LPSO structures in the present study. It has been reported that the LPSO phase shrinks and/or coagulates by increasing the annealing temperature [2]. The coagulated phase is suitable for microstructural observation from different incident beam direction. Fig. 3 shows successive TEM micrographs along columnar to basal plane
Fig. 2. (a and b) Two-dimensional lattice image and the corresponding electron diffraction pattern of the 18R(1 1¯ 1 1¯ 2)3 structure, respectively. (c) Enlargement of the framed area in (b).
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Fig. 3. (a, c, e and g) Successive bright field images along columnar to basal plane normals of hexagonal structure at the same position in the ribbon annealed at 673 K for 3.6 ks. (b, d, f and h) Electron diffraction patterns of (a, c, e and g), respectively. (b) Electron beam // [2 1¯ 1¯ 0]. (d) Electron beam // [1 8 9¯ 9¯ 1]. (f) Electron beam // [6 3¯ 3¯ 1]. (h) Electron beam // [0 0 0 1].
normals of hexagonal structure at the same position in the ribbon annealed at 673 K for 3.6 ks. It has been confirmed by HRTEM that the stacking sequence along the c axis of the 18R structure is invariant during coagulation. There are no extra spots along the a* axis in a series of diffraction patterns in Fig. 3. Morphology of the coagulated phase observed here is columnar in shape with about 100 nm diameter and 150 nm height. Fig. 4(a) is a bright field image taken along the basal plane normal, and Fig. 4(b) is a two dimensional lattice image of the framed area in (a). The coherent interface between the LPSO phase and the matrix is clearly observed along the solid line in (b). No periodic structure is recognized in the coagulated phase. These facts indicate that there are no modulation and/or ordering along the a axis in the LPSO phase.
3.2. 14H structure Fig. 5(a and b) shows a two-dimensional lattice image and the corresponding electron diffraction pattern of the 14 layer structure, respectively. The stacking sequence is ACBCBABABABCBC as apparent from (a). This stacking has mirror symmetry with respect to the basal plane. In the diffraction pattern in Fig. 5(b and c), the 14 spots with regular intervals are observed between the transmission beam and the spot corresponding to the {0 0 0 2} plane of pure Mg. The spot distribution on both sides of the c* axis is symmetric as indicated by dotted lines in (b). Therefore, this LPSO structure is determined to be of the 14H(2¯ 1 2¯ 1 1¯ 1 1¯ 2 1¯ 2) structure. The lattice constants are estimated to be a = 0.325 nm
Fig. 4. (a) Bright field image taken along the basal plane normal. (b) Two-dimensional lattice image of the framed area in (a).
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Fig. 5. (a and b) Two-dimensional lattice image and the corresponding electron diffraction pattern of the 14H(2¯ 1 2¯ 1 1¯ 1 1¯ 2 1¯ 2) structure, respectively. (c) Enlargement of the framed area in (b).
and c = 3.694 nm for the hexagonal structure. It is difficult to distinguish the microstructural difference between 18R and 14H structures such as morphology, formation sites and so on. Itoi et al. have reported [5] that the 18R structure transforms to the 14H structure by annealing at 773 K for 18.0 ks and the subsequent slow cooling, and the morphology of the 14H structure is needle-like with 1 m in thickness distributed randomly in Mg grains with 20–50 m in diameter in a Mg97 Zn1 Y2 alloy prepared by Cu-mold casting. Amiya et al. have reported [6] that the 14 layered packing is observed in an as-melt spun Mg97 Zn1 Y2 ribbon, and it changes to the six layered packing by annealing at 573 K for 1.2 ks. The stacking sequence of the former is ABABABACBCBCBC and that of the latter is ABABABACACACAC. Both sequences do not have mirror symmetry with respect to the basal plane and different from that in the present study. Contrary to the previous studies, the 18R and 14H structures coexist not only within the same specimen but also in the same grains in the present observation. It is considered that the different stacking sequence between these three 14H type LPSO structures is due to the differences in processing and subsequent treatment, since the LPSO structures are chemically ordered as well as stacking ordered structures [3]. 3.3. 10H and 24R structures In the present study, we have recently found out other two stacking sequences as described below. The 10 stacking sequence of ABACBCBCAB with mirror symmetry with respect to the basal plane is seen in Fig. 6(a). In the diffraction pattern in Fig. 6(b and c), the 10 spots with regular intervals are recognized between the transmission beam and the spot
corresponding to the {0 0 0 2} plane of pure Mg. The spot distribution on both sides of the c* axis is symmetric as indicated by dotted lines in (b). From these features, this new stacking ¯ strucsequence is determined to be of the 10H(1 3¯ 1 1¯ 3 1) ture. The lattice constants are estimated to be a = 0.325 nm and c = 2.603 nm for the hexagonal structure. The other recently found one is the 24 stacking sequence of ABABABABCACACACABCBCBCBC as shown in Fig. 7(a), which is not mirror symmetric with respect to the basal plane. The minimum interval of the extra spots corresponds to 124th of 0 0 0 2* of pure Mg as clearly seen in Fig. 7(b and c). The spot distribution on both sides of the c* axis is not symmetric as indicated by dotted lines in (b). From these results, it is concluded that this LPSO structure is of the 24R(1 1¯ 1 1¯ 1 1¯ 2)3 structure with lattice constants of a = 0.322 nm and c = 6.181 nm for the hexagonal structure. Finally, four kinds of LPSO structures were found in the RS Mg97 Zn1 Y2 ribbon annealed at 573 K for 3.6 ks. However, no differences in the morphology and formation sites are detected between these four structures in the present study. It is considered that there are two possibilities about the variety of stacking sequences. One is that those are polymorph such as that in the SiC crystal [9]. The other is that some of those are metastable phases, since the chemical composition might be different in each structure. The LPSO phase nucleates at grain boundaries where Zn and Y are segregated during solidification, and then it grows to the grain interior along the basal plane [2]. Therefore, the LPSO phase is the product of diffusional transformation. Abe et al. have pointed out [3] that the LPSO phase is not only stacking ordered but also chemically ordered structure. These facts suggest that the stacking sequence is sensitive to the Zn and Y concentrations in the Mg matrix. Itoi et al. have reported
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¯ structure, respectively. (c) Enlargement Fig. 6. (a and b) Two-dimensional lattice image and the corresponding electron diffraction pattern of the 10H(1 3¯ 1 1¯ 3 1) of the framed area in (b).
Fig. 7. (a and b) Two-dimensional lattice image and the corresponding electron diffraction pattern of the 24R(1 1¯ 1 1¯ 1 1¯ 2)3 structure, respectively. (c) Enlargement of the framed area in (b).
[5] that the 18R structure transforms to the 14H structure by annealing at 773 K for 18.0 ks and the subsequent slow cooling as mentioned above. Therefore, the mutual relationship and stability of the four structures must be examined further. The morphology, changes in the stacking sequence and stability of the four structures with increasing annealing time and/or temperature are now under study. The systematic results will be reported such as a kind of the time–temperature–transformation diagram in due course.
4. Conclusions The atomic arrangement of the LPSO structures in the RS Mg97 Zn1 Y2 alloy annealed at 573 and 673 K for 3.6 ks have been studied by CTEM and HRTEM. The obtained results are summarized as follows: (1) There are four kinds of stacking sequences in the LPSO structures, i.e., 18R of ABABABCACACABCBCBC,
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14H of ACBCBABABABCBC, 10H of ABACBCBCAB and 24R of ABABABABCACACACABCBCBCBC. The 18R structure is dominantly observed in the present study. The rest three are occasionally observed in places. The 10H and 24R structures are recently discovered. (2) The lattice constants of the 18R(1 1¯ 1 1¯ 2)3 structure are estimated to be a = 0.320 nm and c = 4.678 nm for the hexagonal structure. The 18R structure shrinks and/or coagulates to columnar shape by annealing at 673 K. The stacking sequence along the c axis of the 18R structure is invariant during the coagulation, and there is no modulation and/or ordering along the a axis. (3) The lattice constants of the 14H(2¯ 1 2¯ 1 1¯ 1 1¯ 2 1¯ 2), ¯ 10H(1 3¯ 1 1¯ 3 1) and 24R(1 1¯ 1 1¯ 1 1¯ 2)3 structures are estimated to be a = 0.325 nm and c = 3.694 nm, a = 0.325 nm and c = 2.603 nm, and a = 0.322 nm and c = 6.181 nm for the hexagonal structure, respectively. No difference in the morphology and formation sites in all the structures can be distinguished. Acknowledgments This study was supported by a Grant-in-Aid for Scientific Research on Priority Area (B), “Platform Science and
Technology for Advanced Magnesium Alloys” from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and by METI, Japan, as part of the Regional Research & Development Consortium Project for Development of High Strength Mg Alloys. The authors would like to express their sincere appreciation to Prof. R. Tomoshige of Sojo university for their support in the HRTEM experiments.
References [1] Y. Kawamura, K. Hayashi, A. Inoue, T. Masumoto, Mater. Trans. 42 (2001) 1172–1176. [2] M. Nishida, T. Yamamuro, M. Nagano, Y. Morizono, Y. Kawamura, Mater. Sci. Forum 419–422 (2003) 715–720. [3] E. Abe, Y. Kawamura, K. Hayashi, A. Inoue, Acta Mater. 50 (2002) 3845–3857. [4] D. Ping, K. Hono, Philos. Mag. Let. 82 (2002) 543–551. [5] T. Itoi, T. Seimiya, Y. Kawamura, M. Hirohashi, Scripta Mater. 51 (2004) 107–111. [6] K. Amiya, T. Ohsuna, A. Inoue, Mater. Trans. 44 (2003) 2151– 2156. [7] Z. Nishiyama, Martensitic Transformation, Academic Press, New York, 1978. [8] Z.P. Luo, S.Q. Zhang, J. Mater. Sci. Lett. 19 (2000) 813–815. [9] P. Pirouz, J.W. Yang, Ultramicroscopy 51 (1993) 189–214.
Variation of long-period stacking order structures in rapidly solidified Mg97Zn1Y2 alloy M. Matsudaa,∗ , S. Iib , Y. Kawamuraa , Y. Ikuharab , M. Nishidaa a
Department of Materials Science and Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan b Engineering Research Institute, University of Tokyo, 2-11-16 Yayoi Bunkyo-ku, Tokyo 113-8656, Japan Received 23 August 2004; received in revised form 12 October 2004; accepted 12 October 2004
Abstract The long-period stacking order (LPSO) structures in rapidly solidified Mg97 Zn1 Y2 alloy have been studied by conventional and highresolution transmission electron microscopes (HRTEMs). There are four kinds of stacking sequences in the LPSO structures, i.e., 18R of ABABABCACACABCBCBC, 14H of ACBCBABABABCBC, 10H of ABACBCBCAB and 24R of ABABABABCACACACABCBCBCBC. The 18R structure is dominantly observed in the present study. The rest three are occasionally observed in places. The 10H and 24R structures ¯ and 24R(1 1¯ 1 1¯ 1 1¯ 2)3 structures are are recently discovered. The lattice constants of 18R(1 1¯ 1 1¯ 2)3 , 14H(2¯ 1 2¯ 1 1¯ 1 1¯ 2 1¯ 2), 10H(1 3¯ 1 1¯ 3 1) estimated to be a = 0.320 nm and c = 4.678 nm, a = 0.325 nm and c = 3.694 nm, a = 0.325 nm and c = 2.603 nm, a = 0.322 nm and c = 6.181 nm for the hexagonal structure, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Long-period stacking order structure; Stacking sequence; Magnesium–zinc–yttrium; Transmission electron microscopy
1. Introduction Superior performance of rapidly solidified (RS) powder metallurgy Mg97 Zn1 Y2 (at.%) alloy with extremely high tensile yield strength of 610 MPa and elongation of 5% has been recently developed [1]. These excellent properties are considered to be due to the hcp(2H)-Mg fine grain matrix of 100–200 nm in diameter with a novel long-period stacking order (LPSO) phase and homogeneously dispersed Mg24 Y5 fine particles of less than 10 nm in diameter [2–4]. The morphological, crystallographic and chemical characterizations of the LPSO phase have been performed by conventional transmission electron microscope (CTEM) [2], a high-angle annular dark-field scanning TEM with Z-contrast [3] and a three-dimensional atom probe [4]. Recently, the LPSO phase has been also observed in the Mg97 Zn1 Y2 alloy produced by Cu-mold casting [5] and the melt spun Mg97 Zn1 Ln2 (Ln = Lanthanide metal) alloys [6]. It has been found that ∗
Corresponding author. Tel.: +81 96 342 3707; fax: +81 96 342 3710. E-mail address: [email protected] (M. Matsuda).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.10.040
there are several varieties of the LPSO structures showing different electron diffraction phenomena in those alloys. However, the accurate structural analysis of those LPSO structures has not been performed yet, especially in numbers of stacking layers and stacking sequence of the basal plane. The purpose of the present study is to clarify the stacking sequence of various LPSO structures in the melt spun Mg97 Zn1 Y2 ribbon by CTEM and high-resolution transmission electron microscope (HRTEM) observations.
2. Experimental procedure An Mg97 Zn1 Y2 ingot was prepared by high-frequency induction melting of pure metals in an argon atmosphere. Ribbons were prepared by a single-roller melt spinning method at circumferential speed of 42.0 m/s. The obtained ribbons were annealed at 573 and 673 K for 3.6 ks in vacuum of 2.5 × 10−3 Pa, since the LPSO phase is observed in the whole area of many grains by annealing at 573 K and the LPSO phase is coagulated by annealing at 673 K [2]. TEM
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observed and the rest are occasionally observed in places. The 10H and 24R structures have never been observed in this alloy system within the authors knowledge. The details of each structure are described in the following sections. 3.1. 18R structure
Fig. 1. Bright field image of a RS Mg97 Zn1 Y2 ribbon annealed at 573 K for 3.6 ks.
specimens were prepared by the argon ion milling technique. CTEM and HRTEM observations were carried out in JEOL-2000FX, JEOL-4010 and TECNAI F20 microscopes, respectively.
3. Results and discussion Fig. 1 shows typical microstructure of a RS Mg97 Zn1 Y2 ribbon annealed at 573 K for 3.6 ks. It is apparent that most of the grains are with the characteristic striations of the LPSO phase developing from grain boundaries to interior as reported so far [2]. In the present study, four kinds of the LPSO structures, i.e., 18, 14, 10 and 24 layers structures, are confirmed by electron diffraction experiments, CTEM and HRTEM observations. The expression of R and H is based on the Ramsdell notation [7]. The 18R structure is dominantly
Fig. 2(a and b) show a two-dimensional lattice image and the corresponding electron diffraction pattern of the 18 layer structure, respectively. One can recognize that the LPSO structure consists of 18 stacking sequence of ABABABCACACABCBCBC with 4.678 nm as shown in (a). This is consistent with 18 spots with regular intervals between the transmission beam and the spot corresponding to the {0 0 0 2} plane of pure Mg as shown in (b) and (c). The stacking sequence does not have mirror symmetry with respect to the basal plane. In addition, the spot distribution on both sides of the c* axis is asymmetric as indicated by dotted lines in (b). From these features, the 18 layer structure is determined to be of the 18R(1 1¯ 1 1¯ 2)3 structure and the same as the structure of the X Mg12 YZn phase reported by Luo [8], where the expression of (1 1¯ 1 1¯ 2)3 is based on the Zhdanov symbol [7]. The lattice constants are estimated to be a = 0.320 nm and c = 4.678 nm for the hexagonal structure. It is considered that the origin of the 18R structure is attributed to the localized segregation of Zn and Y, i.e., chemical ordering of these atoms [2–4]. The 18R structure is dominantly observed among the four LPSO structures in the present study. It has been reported that the LPSO phase shrinks and/or coagulates by increasing the annealing temperature [2]. The coagulated phase is suitable for microstructural observation from different incident beam direction. Fig. 3 shows successive TEM micrographs along columnar to basal plane
Fig. 2. (a and b) Two-dimensional lattice image and the corresponding electron diffraction pattern of the 18R(1 1¯ 1 1¯ 2)3 structure, respectively. (c) Enlargement of the framed area in (b).
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Fig. 3. (a, c, e and g) Successive bright field images along columnar to basal plane normals of hexagonal structure at the same position in the ribbon annealed at 673 K for 3.6 ks. (b, d, f and h) Electron diffraction patterns of (a, c, e and g), respectively. (b) Electron beam // [2 1¯ 1¯ 0]. (d) Electron beam // [1 8 9¯ 9¯ 1]. (f) Electron beam // [6 3¯ 3¯ 1]. (h) Electron beam // [0 0 0 1].
normals of hexagonal structure at the same position in the ribbon annealed at 673 K for 3.6 ks. It has been confirmed by HRTEM that the stacking sequence along the c axis of the 18R structure is invariant during coagulation. There are no extra spots along the a* axis in a series of diffraction patterns in Fig. 3. Morphology of the coagulated phase observed here is columnar in shape with about 100 nm diameter and 150 nm height. Fig. 4(a) is a bright field image taken along the basal plane normal, and Fig. 4(b) is a two dimensional lattice image of the framed area in (a). The coherent interface between the LPSO phase and the matrix is clearly observed along the solid line in (b). No periodic structure is recognized in the coagulated phase. These facts indicate that there are no modulation and/or ordering along the a axis in the LPSO phase.
3.2. 14H structure Fig. 5(a and b) shows a two-dimensional lattice image and the corresponding electron diffraction pattern of the 14 layer structure, respectively. The stacking sequence is ACBCBABABABCBC as apparent from (a). This stacking has mirror symmetry with respect to the basal plane. In the diffraction pattern in Fig. 5(b and c), the 14 spots with regular intervals are observed between the transmission beam and the spot corresponding to the {0 0 0 2} plane of pure Mg. The spot distribution on both sides of the c* axis is symmetric as indicated by dotted lines in (b). Therefore, this LPSO structure is determined to be of the 14H(2¯ 1 2¯ 1 1¯ 1 1¯ 2 1¯ 2) structure. The lattice constants are estimated to be a = 0.325 nm
Fig. 4. (a) Bright field image taken along the basal plane normal. (b) Two-dimensional lattice image of the framed area in (a).
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Fig. 5. (a and b) Two-dimensional lattice image and the corresponding electron diffraction pattern of the 14H(2¯ 1 2¯ 1 1¯ 1 1¯ 2 1¯ 2) structure, respectively. (c) Enlargement of the framed area in (b).
and c = 3.694 nm for the hexagonal structure. It is difficult to distinguish the microstructural difference between 18R and 14H structures such as morphology, formation sites and so on. Itoi et al. have reported [5] that the 18R structure transforms to the 14H structure by annealing at 773 K for 18.0 ks and the subsequent slow cooling, and the morphology of the 14H structure is needle-like with 1 m in thickness distributed randomly in Mg grains with 20–50 m in diameter in a Mg97 Zn1 Y2 alloy prepared by Cu-mold casting. Amiya et al. have reported [6] that the 14 layered packing is observed in an as-melt spun Mg97 Zn1 Y2 ribbon, and it changes to the six layered packing by annealing at 573 K for 1.2 ks. The stacking sequence of the former is ABABABACBCBCBC and that of the latter is ABABABACACACAC. Both sequences do not have mirror symmetry with respect to the basal plane and different from that in the present study. Contrary to the previous studies, the 18R and 14H structures coexist not only within the same specimen but also in the same grains in the present observation. It is considered that the different stacking sequence between these three 14H type LPSO structures is due to the differences in processing and subsequent treatment, since the LPSO structures are chemically ordered as well as stacking ordered structures [3]. 3.3. 10H and 24R structures In the present study, we have recently found out other two stacking sequences as described below. The 10 stacking sequence of ABACBCBCAB with mirror symmetry with respect to the basal plane is seen in Fig. 6(a). In the diffraction pattern in Fig. 6(b and c), the 10 spots with regular intervals are recognized between the transmission beam and the spot
corresponding to the {0 0 0 2} plane of pure Mg. The spot distribution on both sides of the c* axis is symmetric as indicated by dotted lines in (b). From these features, this new stacking ¯ strucsequence is determined to be of the 10H(1 3¯ 1 1¯ 3 1) ture. The lattice constants are estimated to be a = 0.325 nm and c = 2.603 nm for the hexagonal structure. The other recently found one is the 24 stacking sequence of ABABABABCACACACABCBCBCBC as shown in Fig. 7(a), which is not mirror symmetric with respect to the basal plane. The minimum interval of the extra spots corresponds to 124th of 0 0 0 2* of pure Mg as clearly seen in Fig. 7(b and c). The spot distribution on both sides of the c* axis is not symmetric as indicated by dotted lines in (b). From these results, it is concluded that this LPSO structure is of the 24R(1 1¯ 1 1¯ 1 1¯ 2)3 structure with lattice constants of a = 0.322 nm and c = 6.181 nm for the hexagonal structure. Finally, four kinds of LPSO structures were found in the RS Mg97 Zn1 Y2 ribbon annealed at 573 K for 3.6 ks. However, no differences in the morphology and formation sites are detected between these four structures in the present study. It is considered that there are two possibilities about the variety of stacking sequences. One is that those are polymorph such as that in the SiC crystal [9]. The other is that some of those are metastable phases, since the chemical composition might be different in each structure. The LPSO phase nucleates at grain boundaries where Zn and Y are segregated during solidification, and then it grows to the grain interior along the basal plane [2]. Therefore, the LPSO phase is the product of diffusional transformation. Abe et al. have pointed out [3] that the LPSO phase is not only stacking ordered but also chemically ordered structure. These facts suggest that the stacking sequence is sensitive to the Zn and Y concentrations in the Mg matrix. Itoi et al. have reported
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¯ structure, respectively. (c) Enlargement Fig. 6. (a and b) Two-dimensional lattice image and the corresponding electron diffraction pattern of the 10H(1 3¯ 1 1¯ 3 1) of the framed area in (b).
Fig. 7. (a and b) Two-dimensional lattice image and the corresponding electron diffraction pattern of the 24R(1 1¯ 1 1¯ 1 1¯ 2)3 structure, respectively. (c) Enlargement of the framed area in (b).
[5] that the 18R structure transforms to the 14H structure by annealing at 773 K for 18.0 ks and the subsequent slow cooling as mentioned above. Therefore, the mutual relationship and stability of the four structures must be examined further. The morphology, changes in the stacking sequence and stability of the four structures with increasing annealing time and/or temperature are now under study. The systematic results will be reported such as a kind of the time–temperature–transformation diagram in due course.
4. Conclusions The atomic arrangement of the LPSO structures in the RS Mg97 Zn1 Y2 alloy annealed at 573 and 673 K for 3.6 ks have been studied by CTEM and HRTEM. The obtained results are summarized as follows: (1) There are four kinds of stacking sequences in the LPSO structures, i.e., 18R of ABABABCACACABCBCBC,
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14H of ACBCBABABABCBC, 10H of ABACBCBCAB and 24R of ABABABABCACACACABCBCBCBC. The 18R structure is dominantly observed in the present study. The rest three are occasionally observed in places. The 10H and 24R structures are recently discovered. (2) The lattice constants of the 18R(1 1¯ 1 1¯ 2)3 structure are estimated to be a = 0.320 nm and c = 4.678 nm for the hexagonal structure. The 18R structure shrinks and/or coagulates to columnar shape by annealing at 673 K. The stacking sequence along the c axis of the 18R structure is invariant during the coagulation, and there is no modulation and/or ordering along the a axis. (3) The lattice constants of the 14H(2¯ 1 2¯ 1 1¯ 1 1¯ 2 1¯ 2), ¯ 10H(1 3¯ 1 1¯ 3 1) and 24R(1 1¯ 1 1¯ 1 1¯ 2)3 structures are estimated to be a = 0.325 nm and c = 3.694 nm, a = 0.325 nm and c = 2.603 nm, and a = 0.322 nm and c = 6.181 nm for the hexagonal structure, respectively. No difference in the morphology and formation sites in all the structures can be distinguished. Acknowledgments This study was supported by a Grant-in-Aid for Scientific Research on Priority Area (B), “Platform Science and
Technology for Advanced Magnesium Alloys” from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and by METI, Japan, as part of the Regional Research & Development Consortium Project for Development of High Strength Mg Alloys. The authors would like to express their sincere appreciation to Prof. R. Tomoshige of Sojo university for their support in the HRTEM experiments.
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