Magnesium Alloys Strengthened by Nanosaucer Precipitates with

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Magnesium alloys strengthened by nano-saucer precipitates with confined new topologically close-packed structure Hongbo Xie, Hucheng Pan, Yuping Ren, Shineng Sun, Liqing Wang, Hong Zhao, Boshu Liu, Xixi Qi, and Gaowu Qin Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00542 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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Crystal Growth & Design

Magnesium alloys strengthened by nano-saucer precipitates with confined new topologically close-packed structure Hongbo Xie, Hucheng Pan, Yuping Ren*, Shineng Sun, Liqing Wang , Hong Zhao, Boshu Liu, Xixi Qi, Gaowu Qin* Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China

The γ’’ nano-saucer precipitates in many Mg-RE-Zn(Ag) alloys, also named as the G. P. zones in some cases such as in the Mg-Ca-Al(Zn) alloys, play the critical role in strengthening α-Mg matrix and enhancing their creep resistance. However, the previous reports on the crystal structure of γ’’ phase is still controversial at present, and thus hard to correlate the γ’’ phase with mechanical properties of Mg alloys. In this study, we confirmed a new topological close-packed (TCP) structure for the γ’’ precipitate in a typical peak-aged Mg-Gd-Zn alloy using Cs-corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and computational simulation. The new structure is totally different from the commonly accepted structure for the γ’’ phase that was consisted of three atomic layers. In contrast, this TCP nano-saucer precipitate is composed of the monolayer atomic icosahedral clusters with five (0001)γ’’ atomic layers (hexagon structure, space group: P6/mmm, a = 5.56 Å, c = 5.21 Å; stacking sequence: ABCBA; grid structure: A → 63(Mg), B → 36(Gd), C → 3636(Zn)). Moreover, the chemical formula of the γ’’ nano-saucer precipitate is also identified as Mg2Gd2Zn3 (A2B2C3), well consistent with previous 3D atom probe results. The orientation relationship between the ത 0ሿ α. The γ’’ precipitate and α-Mg matrix is also determined as (0001)γ’’ // (0001)α; ሾ011ത0ሿ γ’’ // ሾ112 finding would not only shed light on deeper understanding of the confined existence of monolayer icosahedral in the field of crystallography, and of the γ’’

nano-saucer precipitates strengthening

mechanism in Mg alloys, but also guides the further design of new high-strength and creep resistant Mg alloys.

1. INTRODUCTION

Magnesium alloys, usually with density lower than 2 g/cm3, are the most potential weight-saving structural materials. So far, they have been manufactured into various industrial products, such as portable electronic devices, automobile and aerospace components1-4. However, the relatively weaker mechanical properties, as compared with steel and Al alloys, restrict their further wider-spread applications. Therefore, many effective ways, such as grain refinement by different deformation processing and precipitation hardening by aging, have been used to improve mechanical properties of Mg alloys1-3,

5, 6

. Recently, the Mg-RE / Mg-Ca based alloys contain Al, Zn, or Ag elements have ACS Paragon Plus Environment

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received particular interests since the γ’’ nano-saucer precipitates7-28, also named by the GP zones in some cases9,

14-16, 23-27

, can be effectively induced by proper alloying and heat treatment. Therefore,

excellent age hardening response can be achieved in the alloys mentioned above, involving the Mg-NdZn7, 8, Mg-Nd-Ce-Zn9, Mg-Sm-Zn10, 11, Mg-Gd-Zn12-18, Mg-Gd-Ag19, 20, Mg-Gd-Y-Ag21, Mg-Y-Ag-Zn22, Mg-Ca-Zn23,

24

, Mg-Ca-Al25-27 and so on. In addition, the γ’’ nano-saucer precipitates can also

significantly improve the creep strength of Mg alloys 5, 12, 13, 26, 28. However, the reported works on the crystal structure and atomic coordinates of γ’’ phase, which precipitates on the {0001}α basal planes, are still controversial at present. Previously, it is commonly accepted that crystal structure of the γ’’ phase was consisted of three atomic layers. For example, Nie et al. reported that the γ’’ precipitate in the Mg-Gd-Zn-Zr alloy is composed of the Gd/Zn/Gd enriched three atomic layers with an ordered hexagonal structure (space group P6ത2m, a = 5.60Å, c = 4.44Å), but the position of Zn in the unit cell is not determined13. In contrary, Saito et al. proposed that the γ’’ precipitate should be defined as a G. P.-zone, which is consisted of three atomic layers of Gd/Zn-MgGd/Zn15, 16. Recently, the crystal structure of γ’’ precipitate was characterized by using atomic-scale ത 0ሿα direction in the HAADF-STEM17, and it was claimed that there exists a displacement along the ሾ112 center layer. Through a translation operation along the ሾ112ത0ሿα direction (~ 0.8 Å), an ordered body centered rhombohedral structure was rebuilt for the γ’’ phase (lattice parameter: a = b = 4.4 Å, c = 4.0 Å). If so, however, it is hard to be consistent with one another when it is observed from six < 112ത0>α directions. More recently, Gu et al. reported the γ’’ precipitate, Mg2Zn3Gd1, was hexagonal structure with three atomic layers, but it was not consistent with their 3D atom probe results18. On the other hand, some other essential questions remain to be answered for the γ’’ nano-saucer precipitate. For example, why such nano-saucer precipitate can resist thickening and why they form in pairs or clusters. In order to clarify the structure of the essential aging hardening responded γ’’ phase, in present study, the γ’’ nano-saucer precipitate in a typical peak-aged Mg-Gd-Zn ternary alloy was investigated in detail by using atomic-scale high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) observation, select-area diffraction pattern (SADP) and computational simulation to re-establish its crystal structure. Our results clearly confirmed that the γ’’ nano-saucer precipitate is a topological close-packed (TCP) phase consisted of five atomic layers with a chemical formula of Mg2Gd2Zn3, and explained the atomic-scale reasons that the nano-saucer precipitate resist thickening and form in clusters. These findings are also applied to other ternary or multi-component magnesium alloy systems containing the γ’’ precipitates.

2. EXPERIMENTAL SECTION

The alloy with nominal composition of Mg-8 wt% Gd-2 wt% Zn (Mg-1.34 at.%Gd-0.81 at.%Zn) ACS Paragon Plus Environment

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was prepared by melting the pure Mg (99.9 wt%), Zn (99.9 wt%) and Mg-30Gd (wt%) master alloy in the induction furnace with protection of the argon atmosphere. The molten alloy was stirred and kept at 760 ºC for 5 minutes and poured into a steel mold preheated to 300 ºC. The chemical composition of ascast ingot was measured by OPTIMA 4300 DV composition analyzer, and the actual composition was determined to be Mg-8.37 wt% Gd-1.96 wt% Zn. The as-cast samples were solution treated at 520 ºC for 12 h, followed by water quenching and ageing at either 200 ºC or 300 ºC for different times in oil bath furnace. Vickers hardness testing was performed by using a hardness tester (W-W-450SVD) with a loading force of 30 N and dwell time of 15 s. The TEM specimens with a diameter of 3 mm were prepared by twin jet electro-polishing at - 40 ºC in mixture solution of 5.3 g lithium chloride, 11.2 g magnesium perchlorate, 500 ml methanol and 100 ml 2-butoxy ethanol, and subsequently ion milling with low energy electron beam. Finally, Gatan SOLARUS (950) Plasma Cleaning System was used to clean up the sample surfaces. TEM and STEM observation was carried out by using the JEM-ARM200F at an accelerating voltage of 200 kV, equipped with probe Cs corrector and cold field emission gun. Crystal structures of the precipitate-plate were reconstructed by CrystalMaker.CrystalMaker.v2.2.4, and then the electron diffraction patterns of the precipitate-plate detected from the [0001], ሾ11ത00ሿ, and ത 0ሿ directions were simulated by using CrystalMaker.SingleCrystal.v2.0.1. ሾ112

3. RESULTS AND DISCUSSION

When the Mg-8Gd-2Zn alloy was solution treated at 520 ºC for 12 h, followed by water quenching and aging at 200 ºC for different times, its hardness change with aging time is shown in Fig.1a. The hardness increases from ~ 61 HV in the initial solution treated state to the highest value of ~ 91 HV at the peak-ageing time of 64 h, and then starts to decrease with prolonged aging time. The aging hardening behavior is similar as the previous reports in this alloy system and other Mg-RE or Mg-Ca based alloy systems5, 11, 12, 19-25. In this sense, the peak-aged sample with the maximum hardness was selected for the following TEM and HAADF-STEM characterizations. Fig. 1b and 1c provide the bright-field TEM images of the peak-aged Mg-8Gd-2Zn alloy with the ത 00ሿα and ሾ112ത0ሿα, respectively. There are high density needleelectron beam direction parallel to ሾ11 shaped precipitates formed along the {0001}α planes, whose lengths are approximately 30~50nm. It indicates that the observed needle-shaped precipitate is actually the γ’’ phase, which is the same as the previously reported γ’’ phase in literature5, 7-28.

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Figure 1. (a) Age hardening response curve of the Mg-8Gd-2Zn (wt.%) alloy during isothermally aging at 200 ºC. (b and c) TEM bright-field images of the γ’’ nano-saucer precipitates in the Mg-8Gd-2Zn (wt.%) alloy after aged at 200 ºC ത 00ሿα in (b), and ሾ112ത0ሿα in (c). for 64 h. The electron beam is parallel to ሾ11

Fig. 2 further exhibits the typical HAADF-STEM images of the peak-aged Mg-8Gd-2Zn alloy, as observed along the incident directions of [0001]α, ሾ11ത00ሿα, and ሾ112ത0ሿα zone axis, respectively. In the low magnification micrographs (Figs. 2a, 2d, and 2g), the bright-contrast nano-saucer precipitates with a diameter in the range of 30 ~ 50 nm and a thickness less than 1 nm lying on the {0001}α basal planes can be observed, which is consistent with the BF-TEM images show in Fig. 1b-c. It should be pointed out that the overall morphology of the γ’’ precipitate with saucer-shape is clearly demonstrated from the three perpendicular directions of [0001]α, ሾ11ത00ሿ α, and ሾ112ത0ሿ α, not the previously reported bandshaped or streak-shaped ones16, 17. In order to determine the crystal structure and the atomic coordinates of the nano-saucer precipitates, atomic imaging in HAADF-STEM was be used. In the micrograph observed along the [0001]α direction (Figs. 2b and 1c), numerous bright dots inside the precipitate can be observed, and each dot represents an atomic column enriched in the Gd element because the brightness of individual atomic-columns in HAADF-STEM images approximates the square of the average atomic numbers29 (the atom number is 12 for Mg, 30 for Zn, and 64 for Gd). The Gd-enriched atomic columns distribute along the α directions and are separated by a constatnt distance. In this sense, the precipitate-


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saucer has an order hexagonal structure, as marked by the red lattice image in Fig. 2c. The crystal lattice constant of a can also be measured to be ~5.56 Å. In some studies10, 11, 13, 14, 16-18, 20, the nano-scale monolayer saucer-precipitate were characterized by ത 00ሿα and ሾ112 ത 0ሿα directions, and it is generally accepted that the γ’’ precipitate HAADF-STEM from ሾ11 with three atomic layers thickness and fully coherent with the α-Mg matrix, in which the positions of Mg are ordered substitute by solute atoms.

Figure 2. HAADF-STEM images of the γ’’ nano-saucer precipitates in the Mg-8Gd-2Zn (wt.%) alloy after aged at 200 ºC for 64 h. These precipitate-saucers are obtained from three perpendicular zone axis of [0001]α (a, b, and c), ሾ11ത00ሿα ത 0ሿα (g, h, and i). The lattice image of the precipitate-saucer shown in Fig. 2c, showing an order (d, e, and f), and ሾ112



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hexagonal structure. The insets in Fig. 2f and 2i show the enlarged images of some local parts of the precipitate-saucers, and Mg atoms in outmost layers are marked with red circles, and Gd (in sub-outer layers) and Zn (in center layer) atoms in these insets are marked with blue and yellow circles, respectively.

Some local parts of the nano-saucer precipitate viewed along ሾ11ത00ሿα direction in Fig. 2d is further enlarged and shown in Figs. 2e and 2f. The atomic-scale HAADF-STEM images show that the brightest dots located in the sub-outer layer correspond to the Gd atomic columns and the bright dots distributing on the central layer correspond to the Zn atomic columns with the help of the Z-contrast of Gd and Zn, as represented by the blue and yellow circles in Fig. 2f, respectively. As a result, it can be seen that the nano-saucer precipitate is consisted of five atomic layers, i.e., the Mg / Gd / Zn / Gd / Mg atomic stacking structure, as displayed in the inset of Fig. 2f. The present result is obviously different from the conventional understanding that the out-layer Mg atoms and sub-layer Gd atoms locate on the same plane. In other words, it was previously accepted that the γ’’ precipitate is consisted of three atomic layers and is fully coherent with the α-Mg matrix. Actually, it is demonstrated in this work that the outmost Mg atomic layers and sub-outer Gd atomic layers aren’t on a plane, and the central-layer of Zn atoms is not coherent with the (0001)α atomic layer, since the atomic distribution of the precipitatesaucer is not the same as that of the Mg matrix (Fig. 2f). Specifically speaking, the atomic site pointed by green arrow in Fig. 2f can’t be found the corresponding location in the α-Mg matrix. ത 0ሿ α More evidences can be found from the HAADF-STEM images, detected along the ሾ112 direction (Figs. 2h and 2i). The Zn atomic columns, as marked as the yellow solid-circles, always locate in the middle of four neighboring Gd atomic columns, and the Zn-dots wih weaker contrast can also be detected and hide between the two brighter contrast Zn atomic columns, as indicated in the yellow dashed-circles in inset of Fig. 2i. The results above indicate that the central-layer Zn atoms has a new grid structure that incoherent with (0001)α atomic layer. In addition, the atomic inter-distances of hGd-Gd and hMg-Mg can be measured to be ~ 3.92 Å and ~ 5.21 Å, respectively. Table 1. Structure parameters of the γ’’ precipitate. Chemical formula Mg2Gd2Zn3

Atom number in cell 7

Space group P6/mmm

Lattice parameter α = β = 90º, γ = 120º; a = 5.56 Å, c = 5.21 Å

Atom site Mg: 2c (0.3333, 0.6667, 0); Gd: 2e (0, 0, 0.1238); Zn: 6i (0.500, 0, 0.500)

Based on the atomic-scale HAADF-STEM results mentioned above, a new crystal structure for the γ’’ nano-saucer precipitate is reconstructed. Fig. 3 displays the modeled atomic arrangements viewed ത 0ሿγ’’ and ሾ11ത00ሿγ’’ directions, and the Mg, Gd and Zn atoms are presented by the along the ሾ0001ሿγ’’, ሾ112 red, blue and yellow spheres, respectively. It can be clearly seen that the γ’’ nano-saucer precipitate is ACS Paragon Plus Environment

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actually composed of the topologically close-packed structure with five atomic layers. The outmost “A” layer corresponds to the Mg atoms, and the distance between adjacent Mg atoms is ~ 3.21 Å, as shown in Fig. 3a. Moreover, one Mg atom is surrounded by three equilateral hexagons, which means that the grid type of the close-packed plane is 63. In the sub-outer “B” layer, inter-distance of the Gd atoms is ~ 5.56 Å, and a 36 grid structure can be detected. In the central layer “C”, as occupied by the Zn atoms, a 3636 grid can be found, which corresponds to the “equilateral triangle - equilateral hexagon - equilateral triangle - equilateral hexagon” structure. Inter-distance between the Zn atoms is ~ 2.78 Å. The five layers, with three types of grid structure, form a “63+36+3636+36+63” topologically close-packed structure with thickness of ~ 5.21 Å in the stacking sequences of “ABCBA”. More structural information for the present TCP phase, as well as the atomic coordinates can be found in Table 1.

Figure 3. Modeled atomic arrangements of the γ’’ nano-saucer precipitate. The direction is viewed along the ሾ0001ሿγ’’ ത 00ሿγ’’ (c), respectively. Unit cell of the γ’’ precipitate is marked with rhombus frame in Fig. 3a. (a), ሾ112ത0ሿγ’’ (b), and ሾ11 Atomic icosahedral clusters in the γ’’ precipitate are marked with round frames in Fig. 3a and 3b.



In order to fully understand the crystal structure of the γ’’ nano-saucer precipitate, the modeled 3D unit cell of present TCP phase is provided in Fig. 4a, the corresponding 3D view is shown in Fig. 4b. The TCP phase exhibits the space group of P6/mmm, and lattice parameters of a = √3 a0 = 5.56 Å, and c = c0 = 5.21 Å, where a0 and c0 are the lattice parameters of α-Mg lattice, without considering any size effect of solute atoms. And the chemical formula of the γ’’ precipitate-plate is determined to be Mg2Gd2Zn3. To our joyful, this chemical formula is well consistent with the recent atom probe results18.

Figure 4. Schematic of three-dimensional atomic scale crystalline structure. (a) The 3D view of unit cell. (b) The 3D view of γ’’ nano-saucer precipitate. (c) 3D view of the atomic icosahedral cluster in the γ’’ precipitate, image 1 showing the pure Zn atomic plane in this atomic icosahedral cluster, image 2 showing the Zn-Mg atomic plane, and image 3 showing the Zn-Gd atomic plane, which are mutual perpendicular as shown in image 4.

Topologically close-packed phase is a kind of intermetallic compounds, in which the structure is composed of metal atoms with different atomic radius, the small radius atoms formed close-packed planes, and the larger radius atomic-layers embedded in them, thus, formed a layered structure that has a high space utilization and with tetrahedral gaps. Up to date, many kinds of TCP phases have been found, such as Laves phase (MgZn2, MgCu2, MgNi2), σ phase (FeCr, FeMo, CrCo, WCo), µ phase (Fe7W6, Co7Mo6), δ phase (MoNi), R phase (Cr18Mo31Co51), P phase (Cr18Ni40Mo42), and so on30-32. The TCP


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phases are usually detrimental to the mechanical properties of alloys due to their hard and brittle characteristics. For example, the presence of σ phase in stainless steel would results in intergranular corrosion and brittleness; if the composition or heat-treatment is improper, the material properties would be seriously deteriorated with the precipitation of lamellar σ phases in Ni-based superalloys or heatresisting steel. However, the nano-saucer TCP phase with an A2B2C3 chemical formula precipitation in magnesium alloys can significantly enhance the mechanical properties and creep strength5, 12, 13, 26, 28.

Figure 5. Electron diffraction results. (a, d, and g) Simulated electron diffraction patterns of the α-Mg matrix from [0001]α (a), ሾ11ത00ሿα (d), and ሾ112ത0ሿα (g). (b, e, and h) Simulated electron diffraction prtterns of the γ’’ nano-saucer precipitate from [0001]r’’ (b), ሾ2ത110ሿ r’’ (e), and ሾ011ത0ሿ r’’ (h). (c, f, and i) Select-area electron diffraction (SAED) ത 00ሿα + ሾ2ത110ሿr’’ (f), and patterns of the γ’’ nano-saucer precipitate in the Mg-Gd-Zn alloy from [0001]α + r’’ (c), ሾ11 ത 0ሿα + ሾ011ത0ሿr’’ (i). ሾ112



In order to verify the validity of the structure that we reconstructed for the γ’’ nano-saucer precipitate, electron diffraction patterns were simulated by using CrystalMaker-SingleCrystal software. Fig. 5a, 5d and 5g show the simulated electron diffraction patterns for the α-Mg matrix from the [0001]α, ത 0ሿα directions, respectively. The simulated electron diffraction results of the γ’’ nanoሾ11ത00ሿα, and ሾ112 saucer precipitate from the [0001]r’’, ሾ2ത110ሿr’’ and ሾ011ത0ሿr’’ directions are shown in Fig. 5b, 5e, and 5h, respectively. The real select-area electron diffraction (SAED) patterns containing the γ’’ nano-saucer precipitate in the Mg-Gd-Zn alloy are shown in Fig. 5c, 5f, and 5i. From these images, it can be found that the simulation results are consistent with the experimental results, in which without considering the {0001}r’’ basal planes diffraction. Since the γ’’ nano-saucer precipitate only has a single unit cell height in the [0001]r’’ direction, and it has also no periodicity along the [0001]α direction, the diffraction streaks parallel to the [0001]α direction are observed in the actual electron diffraction patterns. In the SAED patterns, the extra diffraction intensity at 1/3{112ത0}α and 2/3{112ത0}α positions (Fig. 5c and 5f) are corresponding to the {011ത0}r’’ diffraction planes, and the diffraction pots of {11ത00}α and {2ത110}r’’ are ത 0}r’’ = 3d {112ത0}α, and d overlapped (Fig. 5c and 5i), which imply the interplanar spacing of d {011 {2ത110}r’’ = d {11ത00}α. In addition, the orientation relationship between the γ’’ precipitate and α-Mg matrix is also determine, i.e., (0001)γ’’ // (0001)α; ሾ011ത0ሿγ’’ // ሾ112ത0ሿα. It is different from the known TCP phase that has multiple or infinite times stacking cycles in the close-packed direction, the γ’’ nano-saucer precipitate in this study just has only one stacking sequence of “ABCBA”. More interestingly, it is found such nano-saucer precipitate strongly resist thickening, and they usually form in pairs or clusters. Atomic icosahedral clusters with the highest stacking density and the lowest free energy of system33, 34

, such as Laves phase, σ phase and µ phase, they all have the icosahedral atomic clusters with a

coordination number of 12. The situation also exists in the γ’’ precipitate. As shown in Fig. 3a and 3b, the icosahedral clusters structure formed by the Zn atom as the central have been marked with the dotted circle, the 3D view of the icosahedral cluster is shown in Fig. 4c. A chain structure model with a atomic number of “1-5-1-5-1” is be observed, which means that the central atom (Zn atom-sphere) is in contact with two sets of five atom-spheres and with one atom-sphere left and the other one right. The icosahedral clusters in the γ’’ nano-saucer precipitate have high structural symmetry, the Zn-Zn atomic plane (Fig. 4c-1), Zn-Mg atomic plane (Fig. 4c-2) and Zn-Gd atomic plane (Fig. 4c-3) are perpendicular to each other (Fig. 4c-4). It would exist in the matrix as a monolayer icosahedral clusters stably beneficial to the high symmetry. We consider that the icosahedral clusters structure wouldn’t growth along Z-axis due to the strong symmetry in the [0001] γ’’ direaction, it would only spread out along the ത 0ሿγ’’ and ሾ11ത00ሿγ’’ directions to form a saucer-like precipitate with one unit cell height. ሾ112 An important factor in controlling the formation of such nano-saucer precipitate is the atomic radius ACS Paragon Plus Environment

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difference. In the Mg-Gd-Zn system, the atomic radius of Mg, Gd, and Zn are 1.60 Å, 1.80 Å, and 1.39 Å, the approximation ratio of rGd : rMg : rZn is 9 : 8 : 7. According to the atomic radius ratio, the atomic layers are stacked in accordance with a “medium-large-small-large-medium” mode to follow the topologically close-packing, thus, formed the A2B2C3 type TCP phase (Fig. 6). Similarly, in the Mg-CaZn system, the atomic radius of Ca, 1.94 Å, is close to that of RE, so in this γ’’ precipitate-plate, the Ca atoms would occupy the positions of RE atoms (as shown in Figs. 6c and 6d). However, since the atomic number of Ca is close to the Mg (Mg atom number is 12, Ca is 20), so it can only be seen the bright contrast of Zn (atom number is 30) atomic layer when viewed along the ሾ11ത00ሿα or ሾ112ത0ሿα directions in the HAADF-STEM images24. In other Mg-Ca-Al and Mg-Gd-Ag alloys, the atomic radus of Al and Ag are 1.43 Å and 1.44 Å, respectively, and they are quite close to the Zn atomic radius, so they would occupy the position of the central layer in the γ’’ nano-saucer precipitates19-21, 25-27 (as shown in Figs. 6b and 6d).

Figure 6. Modeled atomic arrangements of the A2B2C3 type topological close-packed phases, viewed along the ത 0ሿγ’’ direction. (a) Mg2RE2Zn3 phase, (b) Mg2RE2Ag3 phase, (c) Mg2Ca2Zn3 phase, and (d) Mg2Ca2Al3 phase. ሾ112 4. CONCLUSIONS

In summary, the key nano-saucer precipitates, as an effectively strengthening phase, in the aged Mg-8Gd-2Zn (wt.%) alloy were investigated in detail. Based on the atomic-scale HAADF-STEM and ത 00ሿα and ሾ112ത0ሿα, respectively, the crystal SAED patterns results from three zone axes of [0001]α, ሾ11 ACS Paragon Plus Environment


structure, atomic coordinates and chemical formula of the γ’’ nano-saucer precipitates are fully unraveled. We determined that the nano-scale γ’’ precipitate-saucer in the Mg-Gd-Zn alloy is a new type of topologically close-packed structure. It has five (0001)γ’’ atomic layers in thickness, thus, formed a monolayer precipitate-saucer with icosahedral clusters. The TCP phase has an order hexagonal structure (space group: P6/mmm, a = 5.56 Å, c = 5.21 Å; stacking sequence: ABCBA; grid structure: A → 63/Mg, B → 36/Gd, C → 3636/Zn). In addition, chemical formula of the TCP phase is also been established, i.e., Mg2Gd2Zn3 (A2B2C3). This finding is an important breakthrough in the crystallography, a class of monolayer ternary TCP phases with A2B2C3 in composition would be established in follow-up studies in the near future. These results also deepen our understanding of the precipitate strengthening mechanism of γ’’ TCP phase in magnesium alloys, and it provides a meaningful theoretical guidance for designing and developing novel high-strength alloys.

AUTHOR INFORMATION Corresponding author. Email: * [email protected] (Y. P. Ren); * [email protected] (G. W. Qin).

ACKNOWLEDGEMENTS We thank Yonghui Sun and Yu Dong (Northeastern University) for the use of the Cs-corrected HAADF-STEM facility, and Dr. Bo Yang for constructive discussions. The authors acknowledge the National Key Research and Development Program of China (Grant No. 2016YFB0701202), the financial support from the National Natural Science Foundation of China (Grant No. 51371046, No. 51525101, No. 51501032, No. U1610253), and Fundamental Research Funds of the Central Universities (Grant No. N141008001).

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For Table of Contents Use Only Magnesium alloys strengthened by nano-saucer precipitates with confined new topologically close-packed structure Hongbo Xie, Hucheng Pan, Yuping Ren*, Shineng Sun, Liqing Wang , Hong Zhao, Boshu Liu, Xixi Qi, Gaowu Qin*

TOC Graphic. Schematic diagrams of the topological close-packed structure.

Brief Summary The topological close-packed (TCP) nano-saucer precipitate is composed of the monolayer atomic icosahedral clusters with five (0001)γ’’ atomic layers (hexagon structure, space group: P6/mmm, a = 5.56 Å, c = 5.21 Å; stacking sequence: ABCBA; grid structure: A → 63(Mg), B → 36(Gd), C → 3636(Zn)).


Magnesium Alloys Strengthened by Nanosaucer Precipitates with

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