- Email: [email protected]
order structure in Mg-Ni-Y alloys
Journal of Alloys and Compounds 788 (2019) 277e282
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A novel long-period stacking/order structure in Mg-Ni-Y alloys K. Yamashita a, T. Itoi b, M. Yamasaki c, Y. Kawamura c, E. Abe a, d, * a
Department of Materials Science & Engineering, University of Tokyo, Tokyo, 113-8656, Japan Department of Mechanical Engineering, Chiba University, Chiba, 263-8522, Japan c Magnesium Research Center, Kumamoto University, Kumamoto, 2-39-1, Japan d Research Center for Structural Materials, National Institute for Materials Science, 305-0047, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2018 Received in revised form 31 January 2019 Accepted 18 February 2019 Available online 20 February 2019
We have identified a novel long-period stacking/order (LPSO) phase in a Mg-Ni-Y alloy, which provides novel LPSO structural features regarding both the stacking/order aspects; the 12R-type stacking sequence and the in-plane modulation of approximately 7 ð1210Þhcp with respect to the fundamental hexagonalclosed-packed Mg structure. The ideal 7 M model for the present 12R-type LPSO phase is constructed based on the ordered arrangements of the L12-type Ni6Y8 clusters embedded in the local ABCA stacking layers, resulting in the stoichiometry Mg77Ni9Y12 (Mg78.6Ni9.2Y12.2) that appears to be extremely close to the experimentally determined composition Mg78.4Ni9.0±0.2Y12.6±0.3 (at.%). This in turn suggests the robustness of the solute cluster with a specific ratio (Ni6Y8), emerging the pseudo-binary formation behaviors of the LPSO phase in the Mg ternary alloys. © 2019 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloys Crystal structure Long-period order Electron microscopy
1. Introduction Mg alloys containing a few atomic percent of transition metal (TM) and rare-earth elements (RE) have attracted attentions because of their excellent mechanical properties [1] provided by unique long-period structures [2e8], which are termed as longperiod stacking/ordered (LPSO) phases [7,8]. Fundamental LPSO structures are stacking polytypes of an original hexagonal-closedpacked (hcp) Mg structure and constructed by a combination between the 2H-stacking (AB …) and the intrinsic-II type stackingfault unit represented as the ABCA stacking [7]. To date, four stacking polytypes, 10H, 18R, 14H and 24R [7], have been identified for the LPSO phases. In the LPSO structures, TM/RE atoms distribute across the ABCA stacking layers that form local face-centered cubic (fcc) environments, where the TM/RE atoms form L12-type clusters (TM6RE8) to give rise a superlattice order with an ideal dimension of 6 (1210)hcp (6 M) [8,9]. We note in particular that, after the discovery of the cluster-based characteristics, tuning and/or exploring the novel LPSO phases [10,11] have been guided along the particular TM/RE ratio (3/4) line [8] in the ternary phase diagram. During the LPSO and its related phase investigations in the Mg-
* Corresponding author. Department of Materials Science & Engineering, University of Tokyo, Tokyo, 113-8656, Japan. E-mail address: [email protected] (E. Abe). https://doi.org/10.1016/j.jallcom.2019.02.219 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Ni-Y alloys around the Mg-rich corner [12,13], we have found the novel LPSO phase with a 12R-type stacking structure (recently, it has been also reported in Ref. [14]). Furthermore, the present 12Rtype LPSO phase also reveals a novel in-plane order of approximately 7M-type. In the present paper, we describe details of the structural characteristic of the novel 12R-LPSO phase. 2. Experiment A master alloy ingot (approximately 300 g in weight) with a nominal composition Mg-9at.%Ni-12 at.%Y (hereafter compositions are denoted as Mg79Ni9Y12 in at.%) was prepared by high-frequency induction melting of Mg (99.99 wt%), Ni (99.9 wt%) and Y (99.9 wt%) pure metals in a carbon crucible. The molten alloys were kept at 1023 K and cast in an argon atmosphere, during which there were no significant mass reductions (less than ~3% in total mass). A piece of the master ingot was sealed in a Pyrex tube filled with an argon atmosphere after evacuation to pressures lower than 3 103 Pa, and then annealed at 793 K for 100 h followed by air-cooling to room temperature. Scanning electron microscope (SEM) equipped with a silicon drift detector was used to perform energy-dispersive spectroscopy (EDS) analysis. Thin foils for transmission electron microscope (TEM) observations were prepared by mechanical polish and a standard argon ion milling. For atomic resolution highangle annular dark-field (HAADF)-scanning TEM (STEM) observations, we used an aberration-corrected microscope (JEM-
278
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
ARM200F) operated at an accelerating voltage 200 kV, providing a probe size of approximately 0.8 Å with a convergence semi-angle of 22 mrad. For HAADF imaging the annular detector was set to collect electrons scattered at angles higher than 90 mrad, which is sufficiently high to obtain the atomic-number dependent Z-contrast. 3. Results and discussion Fig. 1a shows a SEM image obtained from the annealed Mg79Ni9Y12 alloy. Although the contrast differences are weak, there appears two distinct phases as indicated by X and Y. SEM-EDS analyses of the X and Y regions revealed average compositions to be Mg76.1Ni9.9±0.2Y14.0±0.3 and Mg78.4Ni9.0±0.2Y12.6±0.3, respectively. Note that the both phases occur with a Ni/Y ratio close to 3/4 [8],
implying that these are the LPSO phases. In fact, we have identified two types of the LPSO phases in the present alloy, as shown by their electron diffraction (ED) patterns in Fig. 1bee. By tracing along the c*-axis, there are three and four extra reflections toward the (0002)hcp fundamental reflections for Fig. 1bee, respectively, suggesting formation of 12R-type and 10H-type stacking polytypes in the LPSO series [7]. This is directly confirmed by the relevant HAADF-STEM atomic images shown in Fig. 2 a and b, where the model structures of the 12R- and 10H-type (Fig. 2c) are successfully inserted. As seen in the images, for the both structures the brightest HAADF contrast commonly occur at the B and C layers within the ABCA stacking units, representing the Ni/Y distribution characteristics of the LPSO structure (although they are not obvious in these HAADF images, Y atoms are sparsely located at the A layers owing to
Fig. 1. (a) Back-scattered scanning electron microscopy image of the annealed Mg79Ni9Y12 alloy, showing the two different composition regions indicated by X and Y. Small white particles are likely to be the remaining alumina abrasives used for the surface polishing. (bee) Electron diffraction patterns of the two LPSO phases obtained from the present Mg79Ni9Y12 alloy, taken with the incident beam along (b), (d) 〈1210〉hcp and (c), (e) 〈1010〉hcp directions, respectively.
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
279
Fig. 2. HAADF-STEM images of the (a) 12R-type and (b) 10H-type LPSO phases, taken with the electron diffraction patterns of Fig. 1(b) and (d), respectively. (c) Structure models of the 12R-type and 10H-type LPSO polytypes projected along the corresponding 〈1210〉hcp direction. ABC … columns represent the stacking sequences, together with the h and c notations that show the local hcp/fcc environments [see Ref. [7] for details]. Significant Ni/Y enrichments occur at the c layers (red circles) of a local fcc environment. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
their L12-cluster configurations, as described later in Fig. 3). By further comparison between the present 12R- and 10H-type LPSO phases, we find that the order features parallel to the close-packed planes (i.e., in-plane order) are different; see again Fig. 1 c and e. Although being not exactly at the commensurate positions, toward the fundamental (1210)hcp reflections there appear six and five diffuse/spot lines for the 12R- and 10H-type LPSO phases, respectively (note that the LPSO in-plane order reported so far hardly reaches an ideal commensurate order and appears to be intrinsically incommensurate, as explained by formation of nanometerscale domains [15]). This immediately indicates that the average in-plane order is 7 (1210)hcp (7 M) for the present 12R-LPSO phase, while that of the 10H-LPSO phase is 6 M as commonly observed for all the previous LPSO phases [8e11]. Consequently, it is turned out that the present phase provides the novel LPSO structural features both in the stacking/order aspects; 12R-type stacking sequence as well as the in-plane order of approximately 7 M. It should be remembered here that the observed phase compositions are Mg76.1Ni9.9±0.2Y14.0±0.3 and Mg78.4Ni9.0±0.2Y12.6±0.3; the
former value appears to be close to the stoichiometry composition of the 10H-LPSO with the 6 M in-plane order (Mg23Ni3Y4) [10], while the latter is significantly deviated from that of the 12R-LPSO if assuming the same 6 M order (Mg17Ni3Y4). This fact suggests that the present 7 M in-plane order occurs with the less-dense Ni6Y8cluster condition and hence at further Ni/Y dilute compositions. Details of the phase compositions will be discussed later. We now attempt to construct the 7 M in-plane order model for the present 12R-LPSO phase, based on the L12-Ni6Y8 cluster arrangements within the fcc environment layers (i.e., ABCA-stacking unit). As shown in Fig. 3a, for the 6 M in-plane order the L12-clusters are arranged to avoid the RE-RE nearest-neighbor configurations which are energetically unstable [8,15]. Along this line, it turns out that the Ni6Y8-clusters can be placed up to three within the 7 M superlattice framework as shown in Fig. 3d; note that the cluster arrangement cannot be uniquely determined since there are a number of possible cluster positions, such as indicated by dashedhexagon in Fig. 3d. By considering all the possible three Ni6Y8cluster arrangements within the 7 M superlattice framework, we find there converge into three variant structures shown in Fig. 4 a-c.
280
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
Fig. 3. Structure models with different in-plane superlattice order, and the corresponding simulated electron diffraction patterns and the HAADF atomic-resolution images; (a)e(c) 10H-LPSO with 6 (1210)hcp (6 M) and (d)e(f) 12R-LPSO with 7 (1210)hcp (7 M). These are constructed by the different ordered arrangements of the L12-Ni6Y8 clusters embedded in the ABCA-unit, as shown in (a) and (d), where blue, green and red circles represent the Mg, Ni and Y atoms, respectively. The corresponding calculated electron diffraction patterns with a kinematical condition (i.e., structure factors for electron scattering) are shown for the (b) 10H-LPSO with 6 M [10] and (e) 12R-LPSO with 7 M; the latter is the averaged pattern of the three variant structures in Fig. 4. HAADF-STEM images of (c) and (f) are taken with the electron diffraction patterns of Fig. 1 (e) and (c), respectively, showing the cluster contrast in the AB’C’A stacking layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Here, for the each variant the corresponding highest-symmetry 12R-LPSO model can be constructed by a cyclic stacking (i.e., the manner described in Fig. 6e in Ref. [8]), resulting in the structures with R32, C2 and P3121 space group symmetries. As an example, the R32 model (Fig. 4a) tuned by first principles calculations is represented in Table 1. It is noteworthy here that, during the calculations, all the three structures (R32, C2 and P3121) converged into almost the same energy level, and hence there may be no structural preferences between the three (i.e., the present 7M-LPSO phase is likely to occur as a mixture of the possible variant structures). The ED patterns are then calculated for all the model structures, as shown in Fig. 4 d-f, and their averaged ED pattern is shown in Fig. 3e. As indicated by arrows in Fig. 3b and e, the ED patterns of the superlattice reflection lines are well reproduced for both the 6 M and 7 M order, respectively. In the corresponding HAADF atomic-resolution images in Fig. 3c and f, there indeed appear the L12-cluster contrast that shows up with the relevant in-plane intervals in some places, although the long-range inter-cluster order are not well developed for the present Mg-Ni-Y alloy (perhaps being due to formation of in-plane domains [15], which are seen in overlapped conditions in the HAADF images in Fig. 3).
Finally we describe the LPSO phase locations in the Mg-Ni-Y ternary diagram in Fig. 5, where the ideal compositions of the LPSO polytype series are shown for comparison. As described earlier, the observed composition for the present 10H-LPSO phase can be well reproduced along with the previous LPSO series of the conventional 6 M in-plane order. On the basis of the present model structures (e.g., Table .1), the ideal composition of the novel 12RLPSO phase with the 7 M in-plane order is estimated as being Mg77Ni9Y12 (Mg78.6Ni9.2Y12.2), which is very close to the observed composition of Mg78.4Ni9.0±0.2Y12.6±0.3 and significantly deviated from the composition estimated with the conventional 6 M order (Mg70.8Ni12.5Y16.7). Given the fact that the less-dense in-plane order is available with the LPSO phases, their formation ranges can possibly be extended into further Ni/Y dilute composition ranges by accompanying the stacking polytypes; see Fig. 5. The LPSO phases still occur bounded along the particular TM/RE ratio 3/4 due to the robust TM6RE8-cluster nature, whose extension into dilute TM/RE ranges provides an effective light-weight LPSO-Mg alloy design. This in turn implies strong interactions between the TM-RE atoms even at the dilute conditions in the LPSO-forming systems, and interestingly similar TM-RE robust configurations/networks can
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
281
Fig. 4. (a)e(c) Possible 7 M order arrangements of the L12-Ni6Y8 clusters embedded in the ABCA-stacking unit, where blue, green and red circles represent the Mg, Ni and Y atoms, respectively. For the each ABCA-unit, the 12R-LPSO models were constructed by the cyclic-displacement stacking [8], and the relevant electron diffraction patterns (structure factors) were calculated and shown in (d)e(f) with the relevant space group symmetries at the upper right. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Model structure of the 12R-LPSO with the 7 M in-plane order Mg77Ni9Y12; R32 (No.155), a ¼ 22.4 Å, c ¼ 33.1 Å. Atomic coordinates and the unit-cell dimensions were tuned based on first principle calculations using the Vienna Ab initio Simulation Package (VASP), with the cut-off energy 360 eV, 1 1 1 k-point mesh and Methfessel-Paxton smearing method with a width of 0.2 eV. Atom
Site
x
y
z
Atom
Site
x
y
z
Atom
Site
x
y
z
Mg1 Mg2 Mg3 Mg4 Mg5 Mg6 Mg7 Mg8 Mg9 Mg10 Mg11 Mg12
18f 18f 18f 18f 18f 18f 18f 18f 18f 18f 18f 18f
0.475 0.621 0.902 0.193 0.903 0.905 0.906 0.907 0.191 0.186 0.189 0.187
0.097 0.238 0.809 0.529 0.239 0.523 0.954 0.668 0.669 0.807 0.236 0.95
0.039 0.042 0.044 0.041 0.04 0.044 0.041 0.042 0.042 0.041 0.041 0.04
Mg13 Mg14 Mg15 Mg16 Mg17 Mg18 Mg19 Mg20 Mg21 Mg22 Mg23 Mg24
18f 18f 18f 6c 18f 18f 18f 18f 18f 18f 18f 18f
0.472 0.62 0.762 0 0.239 0.239 0.24 0.091 0.099 0.096 0.953 0.81
0.522 0.953 0.237 0 0.626 0.329 0.187 0.903 0.625 0.046 0.617 0.618
0.04 0.041 0.041 0.375 0.123 0.123 0.126 0.123 0.123 0.125 0.126 0.123
Mg25 Mg26 Mg27 Ni1 Ni2 Ni3 Y1 Y2 Y3 Y4
18f 18f 6c 18f 18f 18f 18f 18f 18f 18f
0.813 0.807 0 0.682 0.113 0.399 0.617 0.541 0.972 0.256
0.476 0.331 0 0.602 0.316 0.887 0.667 0.512 0.228 0.797
0.126 0.123 0.792 0.109 0.11 0.11 0.066 0.133 0.134 0.133
indeed be found in various ternary precipitates [16,17] and the other long-period structures [18e22] in the Mg-TM-RE alloys. We
will soon describe elsewhere further details of the robust TM-RE cluster behaviors in the dilute LPSO phases.
282
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
Fig. 5. Schematic Mg-Ni3Y4 cross-section around the Mg-rich corner in the Mg-Ni-Y ternary phase diagram at ~793 K. All the ideal LPSO compositions, including stacking polytypes and different in-plane order (6 M and 7 M), occur along the fixed Ni/Y ratio 3/4 owing to the Ni6Y8 -cluster nature. The present LPSO phases form with the compositions Mg76.1Ni9.9±0.2Y14.0±0.3 and Mg78.4Ni9.0±0.2Y12.6±0.3 (X and Y in Fig. 1 a), corresponding fairly well to the 10H-type with 6 M and the 12R-type with 7 M, respectively.
4. Conclusion In summary, we have identified the novel 12R-type LPSO phase in the Mg-Ni-Y alloy, providing extended LPSO structural features both in the stacking/order aspects; 12R-type stacking sequences and the in-plane 7 (1210)hcp (7 M) order. The model structure of the 12R-LPSO with the in-plane 7 M order is successfully constructed by proper arrangements of the L12-type Ni6Y8 clusters, resulting in the stoichiometric composition Mg77Ni9Y12 (Mg78.6Ni9.2Y12.2) that is very close to the experimental composition Mg78.4Ni9.0±0.2Y12.6±0.3. The present results strongly imply that, by tuning the in-plane cluster order, the occurrence of the LPSO phase may be possibly extended into further dilute compositions ranges. Acknowledgment This study is supported by JSPS KAKENHI for Scientific Research on Innovative Areas “MFS Materials Science (Grant Numbers JP18H05475, JP18H05476, JP18H05479)”, and “Nanotechnology Platform” of the MEXT, Japan. References [1] Y. Kawamura, K. Hayashi, A. Inoue, T. Matsumoto, Mater. Trans. 42 (2001)
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
1172. E. Abe, Y. Kawamura, K. Hayashi, A. Inoue, Acta Mater. 50 (2002) 3845. T. Itoi, T. Seimiya, Y. Kawamura, M. Hirohashi, Scripta Mater. 51 (2004) 107. Y. Kawamura, M. Yamasaki, Mater. Trans. 48 (2007) 2986. A. Ono, E. Abe, T. Itio, M. Hirohashi, M. Yamasaki, Y. Kawamura, Mater. Trans. 49 (2008) 990. Y.M. Zhu, A.J. Morton, J.F. Nie, Acta Mater. 58 (2010) 2936. E. Abe, A. Ono, T. Itoi, M. Yamasaki, Y. Kawamura, Philos. Mag. Lett. 91 (2011) 690. D. Egusa, E. Abe, Acta Mater. 60 (2012) 166. H. Yokobayashi, K. Kishida, H. Inui, M. Yamasaki, Y. Kawamura, Acta Mater. 59 (2011) 7287. M. Yamasaki, M. Matsushita, K. Hagihara, H. Izuno, E. Abe, Y. Kawamura, Scr. Mater. 78e79 (2014) 13. K. Kishida, K. Nagai, A. Matsumoto, A. Yasuhara, H. Inui, Acta Mater. 99 (2015) 228. Z. Wang, Q. Luo, S. Chen, K.-C. Chou, Q. Li, J. Alloys Compd. 649 (2015) 1306. M. Jiang, S. Zhang, Y. Bi, Hongxiao Li, Y. Ren, G. Qin, Intermetallics 57 (2015) 127. C. Liu, Y. Zhu, Q. Luo, B. Liu, Q. Gu, Q. Li, J. Mater. Sci. Technol. 34 (2018) 2235. H. Kimizuka, S. Kurokawa, A. Yamaguchi, A. Sakai, S. Ogata, Sci. Rep. 4 (2014) 7318. J.F. Nie, K. Oh-ishi, X. Gao, K. Hono, Acta Mater. 56 (2008) 6061. T. Koizumi, M. Egami, K. Yamashita, E. Abe, J. Alloys Compd. 752 (2018) 407. M. Egami, E. Abe, Scripta Mater. 98 (2015) 64. V.V. Shtender, et al., J. Alloys Compd. 737 (2018) 613. M. Matsushita, et al., Scripta Mater. 121 (2016) 45. N. Fujita, et al., Scripta Mater. 150 (2018) 78. V.V. Shtender, et al., Z. Kristallogr. 234 (2019) 19.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A novel long-period stacking/order structure in Mg-Ni-Y alloys K. Yamashita a, T. Itoi b, M. Yamasaki c, Y. Kawamura c, E. Abe a, d, * a
Department of Materials Science & Engineering, University of Tokyo, Tokyo, 113-8656, Japan Department of Mechanical Engineering, Chiba University, Chiba, 263-8522, Japan c Magnesium Research Center, Kumamoto University, Kumamoto, 2-39-1, Japan d Research Center for Structural Materials, National Institute for Materials Science, 305-0047, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2018 Received in revised form 31 January 2019 Accepted 18 February 2019 Available online 20 February 2019
We have identified a novel long-period stacking/order (LPSO) phase in a Mg-Ni-Y alloy, which provides novel LPSO structural features regarding both the stacking/order aspects; the 12R-type stacking sequence and the in-plane modulation of approximately 7 ð1210Þhcp with respect to the fundamental hexagonalclosed-packed Mg structure. The ideal 7 M model for the present 12R-type LPSO phase is constructed based on the ordered arrangements of the L12-type Ni6Y8 clusters embedded in the local ABCA stacking layers, resulting in the stoichiometry Mg77Ni9Y12 (Mg78.6Ni9.2Y12.2) that appears to be extremely close to the experimentally determined composition Mg78.4Ni9.0±0.2Y12.6±0.3 (at.%). This in turn suggests the robustness of the solute cluster with a specific ratio (Ni6Y8), emerging the pseudo-binary formation behaviors of the LPSO phase in the Mg ternary alloys. © 2019 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloys Crystal structure Long-period order Electron microscopy
1. Introduction Mg alloys containing a few atomic percent of transition metal (TM) and rare-earth elements (RE) have attracted attentions because of their excellent mechanical properties [1] provided by unique long-period structures [2e8], which are termed as longperiod stacking/ordered (LPSO) phases [7,8]. Fundamental LPSO structures are stacking polytypes of an original hexagonal-closedpacked (hcp) Mg structure and constructed by a combination between the 2H-stacking (AB …) and the intrinsic-II type stackingfault unit represented as the ABCA stacking [7]. To date, four stacking polytypes, 10H, 18R, 14H and 24R [7], have been identified for the LPSO phases. In the LPSO structures, TM/RE atoms distribute across the ABCA stacking layers that form local face-centered cubic (fcc) environments, where the TM/RE atoms form L12-type clusters (TM6RE8) to give rise a superlattice order with an ideal dimension of 6 (1210)hcp (6 M) [8,9]. We note in particular that, after the discovery of the cluster-based characteristics, tuning and/or exploring the novel LPSO phases [10,11] have been guided along the particular TM/RE ratio (3/4) line [8] in the ternary phase diagram. During the LPSO and its related phase investigations in the Mg-
* Corresponding author. Department of Materials Science & Engineering, University of Tokyo, Tokyo, 113-8656, Japan. E-mail address: [email protected] (E. Abe). https://doi.org/10.1016/j.jallcom.2019.02.219 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Ni-Y alloys around the Mg-rich corner [12,13], we have found the novel LPSO phase with a 12R-type stacking structure (recently, it has been also reported in Ref. [14]). Furthermore, the present 12Rtype LPSO phase also reveals a novel in-plane order of approximately 7M-type. In the present paper, we describe details of the structural characteristic of the novel 12R-LPSO phase. 2. Experiment A master alloy ingot (approximately 300 g in weight) with a nominal composition Mg-9at.%Ni-12 at.%Y (hereafter compositions are denoted as Mg79Ni9Y12 in at.%) was prepared by high-frequency induction melting of Mg (99.99 wt%), Ni (99.9 wt%) and Y (99.9 wt%) pure metals in a carbon crucible. The molten alloys were kept at 1023 K and cast in an argon atmosphere, during which there were no significant mass reductions (less than ~3% in total mass). A piece of the master ingot was sealed in a Pyrex tube filled with an argon atmosphere after evacuation to pressures lower than 3 103 Pa, and then annealed at 793 K for 100 h followed by air-cooling to room temperature. Scanning electron microscope (SEM) equipped with a silicon drift detector was used to perform energy-dispersive spectroscopy (EDS) analysis. Thin foils for transmission electron microscope (TEM) observations were prepared by mechanical polish and a standard argon ion milling. For atomic resolution highangle annular dark-field (HAADF)-scanning TEM (STEM) observations, we used an aberration-corrected microscope (JEM-
278
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
ARM200F) operated at an accelerating voltage 200 kV, providing a probe size of approximately 0.8 Å with a convergence semi-angle of 22 mrad. For HAADF imaging the annular detector was set to collect electrons scattered at angles higher than 90 mrad, which is sufficiently high to obtain the atomic-number dependent Z-contrast. 3. Results and discussion Fig. 1a shows a SEM image obtained from the annealed Mg79Ni9Y12 alloy. Although the contrast differences are weak, there appears two distinct phases as indicated by X and Y. SEM-EDS analyses of the X and Y regions revealed average compositions to be Mg76.1Ni9.9±0.2Y14.0±0.3 and Mg78.4Ni9.0±0.2Y12.6±0.3, respectively. Note that the both phases occur with a Ni/Y ratio close to 3/4 [8],
implying that these are the LPSO phases. In fact, we have identified two types of the LPSO phases in the present alloy, as shown by their electron diffraction (ED) patterns in Fig. 1bee. By tracing along the c*-axis, there are three and four extra reflections toward the (0002)hcp fundamental reflections for Fig. 1bee, respectively, suggesting formation of 12R-type and 10H-type stacking polytypes in the LPSO series [7]. This is directly confirmed by the relevant HAADF-STEM atomic images shown in Fig. 2 a and b, where the model structures of the 12R- and 10H-type (Fig. 2c) are successfully inserted. As seen in the images, for the both structures the brightest HAADF contrast commonly occur at the B and C layers within the ABCA stacking units, representing the Ni/Y distribution characteristics of the LPSO structure (although they are not obvious in these HAADF images, Y atoms are sparsely located at the A layers owing to
Fig. 1. (a) Back-scattered scanning electron microscopy image of the annealed Mg79Ni9Y12 alloy, showing the two different composition regions indicated by X and Y. Small white particles are likely to be the remaining alumina abrasives used for the surface polishing. (bee) Electron diffraction patterns of the two LPSO phases obtained from the present Mg79Ni9Y12 alloy, taken with the incident beam along (b), (d) 〈1210〉hcp and (c), (e) 〈1010〉hcp directions, respectively.
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
279
Fig. 2. HAADF-STEM images of the (a) 12R-type and (b) 10H-type LPSO phases, taken with the electron diffraction patterns of Fig. 1(b) and (d), respectively. (c) Structure models of the 12R-type and 10H-type LPSO polytypes projected along the corresponding 〈1210〉hcp direction. ABC … columns represent the stacking sequences, together with the h and c notations that show the local hcp/fcc environments [see Ref. [7] for details]. Significant Ni/Y enrichments occur at the c layers (red circles) of a local fcc environment. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
their L12-cluster configurations, as described later in Fig. 3). By further comparison between the present 12R- and 10H-type LPSO phases, we find that the order features parallel to the close-packed planes (i.e., in-plane order) are different; see again Fig. 1 c and e. Although being not exactly at the commensurate positions, toward the fundamental (1210)hcp reflections there appear six and five diffuse/spot lines for the 12R- and 10H-type LPSO phases, respectively (note that the LPSO in-plane order reported so far hardly reaches an ideal commensurate order and appears to be intrinsically incommensurate, as explained by formation of nanometerscale domains [15]). This immediately indicates that the average in-plane order is 7 (1210)hcp (7 M) for the present 12R-LPSO phase, while that of the 10H-LPSO phase is 6 M as commonly observed for all the previous LPSO phases [8e11]. Consequently, it is turned out that the present phase provides the novel LPSO structural features both in the stacking/order aspects; 12R-type stacking sequence as well as the in-plane order of approximately 7 M. It should be remembered here that the observed phase compositions are Mg76.1Ni9.9±0.2Y14.0±0.3 and Mg78.4Ni9.0±0.2Y12.6±0.3; the
former value appears to be close to the stoichiometry composition of the 10H-LPSO with the 6 M in-plane order (Mg23Ni3Y4) [10], while the latter is significantly deviated from that of the 12R-LPSO if assuming the same 6 M order (Mg17Ni3Y4). This fact suggests that the present 7 M in-plane order occurs with the less-dense Ni6Y8cluster condition and hence at further Ni/Y dilute compositions. Details of the phase compositions will be discussed later. We now attempt to construct the 7 M in-plane order model for the present 12R-LPSO phase, based on the L12-Ni6Y8 cluster arrangements within the fcc environment layers (i.e., ABCA-stacking unit). As shown in Fig. 3a, for the 6 M in-plane order the L12-clusters are arranged to avoid the RE-RE nearest-neighbor configurations which are energetically unstable [8,15]. Along this line, it turns out that the Ni6Y8-clusters can be placed up to three within the 7 M superlattice framework as shown in Fig. 3d; note that the cluster arrangement cannot be uniquely determined since there are a number of possible cluster positions, such as indicated by dashedhexagon in Fig. 3d. By considering all the possible three Ni6Y8cluster arrangements within the 7 M superlattice framework, we find there converge into three variant structures shown in Fig. 4 a-c.
280
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
Fig. 3. Structure models with different in-plane superlattice order, and the corresponding simulated electron diffraction patterns and the HAADF atomic-resolution images; (a)e(c) 10H-LPSO with 6 (1210)hcp (6 M) and (d)e(f) 12R-LPSO with 7 (1210)hcp (7 M). These are constructed by the different ordered arrangements of the L12-Ni6Y8 clusters embedded in the ABCA-unit, as shown in (a) and (d), where blue, green and red circles represent the Mg, Ni and Y atoms, respectively. The corresponding calculated electron diffraction patterns with a kinematical condition (i.e., structure factors for electron scattering) are shown for the (b) 10H-LPSO with 6 M [10] and (e) 12R-LPSO with 7 M; the latter is the averaged pattern of the three variant structures in Fig. 4. HAADF-STEM images of (c) and (f) are taken with the electron diffraction patterns of Fig. 1 (e) and (c), respectively, showing the cluster contrast in the AB’C’A stacking layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Here, for the each variant the corresponding highest-symmetry 12R-LPSO model can be constructed by a cyclic stacking (i.e., the manner described in Fig. 6e in Ref. [8]), resulting in the structures with R32, C2 and P3121 space group symmetries. As an example, the R32 model (Fig. 4a) tuned by first principles calculations is represented in Table 1. It is noteworthy here that, during the calculations, all the three structures (R32, C2 and P3121) converged into almost the same energy level, and hence there may be no structural preferences between the three (i.e., the present 7M-LPSO phase is likely to occur as a mixture of the possible variant structures). The ED patterns are then calculated for all the model structures, as shown in Fig. 4 d-f, and their averaged ED pattern is shown in Fig. 3e. As indicated by arrows in Fig. 3b and e, the ED patterns of the superlattice reflection lines are well reproduced for both the 6 M and 7 M order, respectively. In the corresponding HAADF atomic-resolution images in Fig. 3c and f, there indeed appear the L12-cluster contrast that shows up with the relevant in-plane intervals in some places, although the long-range inter-cluster order are not well developed for the present Mg-Ni-Y alloy (perhaps being due to formation of in-plane domains [15], which are seen in overlapped conditions in the HAADF images in Fig. 3).
Finally we describe the LPSO phase locations in the Mg-Ni-Y ternary diagram in Fig. 5, where the ideal compositions of the LPSO polytype series are shown for comparison. As described earlier, the observed composition for the present 10H-LPSO phase can be well reproduced along with the previous LPSO series of the conventional 6 M in-plane order. On the basis of the present model structures (e.g., Table .1), the ideal composition of the novel 12RLPSO phase with the 7 M in-plane order is estimated as being Mg77Ni9Y12 (Mg78.6Ni9.2Y12.2), which is very close to the observed composition of Mg78.4Ni9.0±0.2Y12.6±0.3 and significantly deviated from the composition estimated with the conventional 6 M order (Mg70.8Ni12.5Y16.7). Given the fact that the less-dense in-plane order is available with the LPSO phases, their formation ranges can possibly be extended into further Ni/Y dilute composition ranges by accompanying the stacking polytypes; see Fig. 5. The LPSO phases still occur bounded along the particular TM/RE ratio 3/4 due to the robust TM6RE8-cluster nature, whose extension into dilute TM/RE ranges provides an effective light-weight LPSO-Mg alloy design. This in turn implies strong interactions between the TM-RE atoms even at the dilute conditions in the LPSO-forming systems, and interestingly similar TM-RE robust configurations/networks can
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
281
Fig. 4. (a)e(c) Possible 7 M order arrangements of the L12-Ni6Y8 clusters embedded in the ABCA-stacking unit, where blue, green and red circles represent the Mg, Ni and Y atoms, respectively. For the each ABCA-unit, the 12R-LPSO models were constructed by the cyclic-displacement stacking [8], and the relevant electron diffraction patterns (structure factors) were calculated and shown in (d)e(f) with the relevant space group symmetries at the upper right. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Model structure of the 12R-LPSO with the 7 M in-plane order Mg77Ni9Y12; R32 (No.155), a ¼ 22.4 Å, c ¼ 33.1 Å. Atomic coordinates and the unit-cell dimensions were tuned based on first principle calculations using the Vienna Ab initio Simulation Package (VASP), with the cut-off energy 360 eV, 1 1 1 k-point mesh and Methfessel-Paxton smearing method with a width of 0.2 eV. Atom
Site
x
y
z
Atom
Site
x
y
z
Atom
Site
x
y
z
Mg1 Mg2 Mg3 Mg4 Mg5 Mg6 Mg7 Mg8 Mg9 Mg10 Mg11 Mg12
18f 18f 18f 18f 18f 18f 18f 18f 18f 18f 18f 18f
0.475 0.621 0.902 0.193 0.903 0.905 0.906 0.907 0.191 0.186 0.189 0.187
0.097 0.238 0.809 0.529 0.239 0.523 0.954 0.668 0.669 0.807 0.236 0.95
0.039 0.042 0.044 0.041 0.04 0.044 0.041 0.042 0.042 0.041 0.041 0.04
Mg13 Mg14 Mg15 Mg16 Mg17 Mg18 Mg19 Mg20 Mg21 Mg22 Mg23 Mg24
18f 18f 18f 6c 18f 18f 18f 18f 18f 18f 18f 18f
0.472 0.62 0.762 0 0.239 0.239 0.24 0.091 0.099 0.096 0.953 0.81
0.522 0.953 0.237 0 0.626 0.329 0.187 0.903 0.625 0.046 0.617 0.618
0.04 0.041 0.041 0.375 0.123 0.123 0.126 0.123 0.123 0.125 0.126 0.123
Mg25 Mg26 Mg27 Ni1 Ni2 Ni3 Y1 Y2 Y3 Y4
18f 18f 6c 18f 18f 18f 18f 18f 18f 18f
0.813 0.807 0 0.682 0.113 0.399 0.617 0.541 0.972 0.256
0.476 0.331 0 0.602 0.316 0.887 0.667 0.512 0.228 0.797
0.126 0.123 0.792 0.109 0.11 0.11 0.066 0.133 0.134 0.133
indeed be found in various ternary precipitates [16,17] and the other long-period structures [18e22] in the Mg-TM-RE alloys. We
will soon describe elsewhere further details of the robust TM-RE cluster behaviors in the dilute LPSO phases.
282
K. Yamashita et al. / Journal of Alloys and Compounds 788 (2019) 277e282
Fig. 5. Schematic Mg-Ni3Y4 cross-section around the Mg-rich corner in the Mg-Ni-Y ternary phase diagram at ~793 K. All the ideal LPSO compositions, including stacking polytypes and different in-plane order (6 M and 7 M), occur along the fixed Ni/Y ratio 3/4 owing to the Ni6Y8 -cluster nature. The present LPSO phases form with the compositions Mg76.1Ni9.9±0.2Y14.0±0.3 and Mg78.4Ni9.0±0.2Y12.6±0.3 (X and Y in Fig. 1 a), corresponding fairly well to the 10H-type with 6 M and the 12R-type with 7 M, respectively.
4. Conclusion In summary, we have identified the novel 12R-type LPSO phase in the Mg-Ni-Y alloy, providing extended LPSO structural features both in the stacking/order aspects; 12R-type stacking sequences and the in-plane 7 (1210)hcp (7 M) order. The model structure of the 12R-LPSO with the in-plane 7 M order is successfully constructed by proper arrangements of the L12-type Ni6Y8 clusters, resulting in the stoichiometric composition Mg77Ni9Y12 (Mg78.6Ni9.2Y12.2) that is very close to the experimental composition Mg78.4Ni9.0±0.2Y12.6±0.3. The present results strongly imply that, by tuning the in-plane cluster order, the occurrence of the LPSO phase may be possibly extended into further dilute compositions ranges. Acknowledgment This study is supported by JSPS KAKENHI for Scientific Research on Innovative Areas “MFS Materials Science (Grant Numbers JP18H05475, JP18H05476, JP18H05479)”, and “Nanotechnology Platform” of the MEXT, Japan. References [1] Y. Kawamura, K. Hayashi, A. Inoue, T. Matsumoto, Mater. Trans. 42 (2001)
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
1172. E. Abe, Y. Kawamura, K. Hayashi, A. Inoue, Acta Mater. 50 (2002) 3845. T. Itoi, T. Seimiya, Y. Kawamura, M. Hirohashi, Scripta Mater. 51 (2004) 107. Y. Kawamura, M. Yamasaki, Mater. Trans. 48 (2007) 2986. A. Ono, E. Abe, T. Itio, M. Hirohashi, M. Yamasaki, Y. Kawamura, Mater. Trans. 49 (2008) 990. Y.M. Zhu, A.J. Morton, J.F. Nie, Acta Mater. 58 (2010) 2936. E. Abe, A. Ono, T. Itoi, M. Yamasaki, Y. Kawamura, Philos. Mag. Lett. 91 (2011) 690. D. Egusa, E. Abe, Acta Mater. 60 (2012) 166. H. Yokobayashi, K. Kishida, H. Inui, M. Yamasaki, Y. Kawamura, Acta Mater. 59 (2011) 7287. M. Yamasaki, M. Matsushita, K. Hagihara, H. Izuno, E. Abe, Y. Kawamura, Scr. Mater. 78e79 (2014) 13. K. Kishida, K. Nagai, A. Matsumoto, A. Yasuhara, H. Inui, Acta Mater. 99 (2015) 228. Z. Wang, Q. Luo, S. Chen, K.-C. Chou, Q. Li, J. Alloys Compd. 649 (2015) 1306. M. Jiang, S. Zhang, Y. Bi, Hongxiao Li, Y. Ren, G. Qin, Intermetallics 57 (2015) 127. C. Liu, Y. Zhu, Q. Luo, B. Liu, Q. Gu, Q. Li, J. Mater. Sci. Technol. 34 (2018) 2235. H. Kimizuka, S. Kurokawa, A. Yamaguchi, A. Sakai, S. Ogata, Sci. Rep. 4 (2014) 7318. J.F. Nie, K. Oh-ishi, X. Gao, K. Hono, Acta Mater. 56 (2008) 6061. T. Koizumi, M. Egami, K. Yamashita, E. Abe, J. Alloys Compd. 752 (2018) 407. M. Egami, E. Abe, Scripta Mater. 98 (2015) 64. V.V. Shtender, et al., J. Alloys Compd. 737 (2018) 613. M. Matsushita, et al., Scripta Mater. 121 (2016) 45. N. Fujita, et al., Scripta Mater. 150 (2018) 78. V.V. Shtender, et al., Z. Kristallogr. 234 (2019) 19.