Journal of Materials Science & Technology  2019 , 35 (9): 2058-2063 https://doi.org/10.1016/j.jmst.2019.05.013

Orginal Article

Atomic structure and enhanced thermostability of a new structure MgYZn4 formed by ordered substitution of Y for Mg in MgZn2 in a Mg-Zn-Y alloy

Lifeng Zhanga, Shangyi Maa, Weizhen Wangba, Zhiqing Yanga*, Hengqiang Yea

a Shenyang National laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, School of Materials Science and Engineering, University of Science and Technology of China, Shenyang, 110016, China
b School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China

Corresponding authors:   *Corresponding author.E-mail address: yangzq@imr.ac.cn (Z. Yang).

Received: 2018-09-18

Revised:  2018-11-5

Accepted:  2018-11-5

Online:  2019-09-20

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

A new phase MgYZn4 in Mg-Zn-Y alloy was studied using aberration-corrected scanning-transmission electron microscopy and first-principles calculations. Nanometer-sized MgYZn4 precipitates were formed through ordered substitutions of Y with 50% Mg atoms in MgZn2. MgYZn4 has an orthorhombic structure with a space group of Pmnn, and lattice parameters a =5.2965 Å, b =9.4886 Å, and c =8.5966 Å. Importantly, both size and structure of MgYZn4 are stable at 625 K for 5 h, showing higher thermostability than MgZn2, which should be important for applications at elevated temperatures. The enhanced thermostability of MgYZn4 is attributed to the lower formation energy and bonding enhancement due to Y substitution.

Keywords: MgYZn4 ; Thermostability ; Laves phase ; Precipitate ; Mg alloys

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Lifeng Zhang, Shangyi Ma, Weizhen Wang, Zhiqing Yang, Hengqiang Ye. Atomic structure and enhanced thermostability of a new structure MgYZn4 formed by ordered substitution of Y for Mg in MgZn2 in a Mg-Zn-Y alloy[J]. Journal of Materials Science & Technology, 2019, 35(9): 2058-2063 https://doi.org/10.1016/j.jmst.2019.05.013

1. Introduction

Magnesium (Mg) alloys are of great interest for industrial applications due to their high specific strength and low density. However, poor high-temperature mechanical properties limit their applications in components working at elevated temperatures [1]. Mg alloys containing rare-earth (RE) elements have promising high-temperature strength and creep-resistance, due to the formation of βʹ-Mg7RE and βʹʹ-Mg3RE precipitates in α-Mg matrix [[2], [3], [4], [5]]. But those precipitates can be coarsened and transformed into other phases at 473 K and above [6]. Zn addition can further improve Mg-RE alloys’ high-temperature mechanical properties, which was previously attributed to thermally stable secondary phases in Mg-Zn-RE alloys, including various Mg-Zn binary intermetallic compounds formed in Mg matrix, like Mg-Zn rods and C14 MgZn2 particles [7,8], γ' and γ” precipitates on the basal plane of Mg matrix compressing the formation of βʹʹ precipitates in Mg-RE alloys [[9], [10], [11]], and Mg3Zn6RE icosahedral quasicrystal (IQC) or long-period stacking order (LPSO) phases at grain boundaries (GBs) [[11], [12], [13], [14], [15], [16]]. While these large secondary phases at GBs are need to be fragmented into fine particles through thermo-mechanical processing, in order to achieve better strengthening effects [17,18]. Direct formation of nanometer-sized precipitates with better thermostability in Mg matrix, besides coarse IQC or LPSO phases at GBs, should benefit the high-temperature mechanical properties of Mg alloys.

In the present study, nanometer-sized MgYZn4 precipitates with size and structural stability at 625 K were formed in α-Mg matrix of a Mg-Zn-Y alloy. The atomic structure of MgYZn4 precipitates was determined by combination of aberration-corrected high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM) imaging and first-principles calculations. The presence of nanometer-sized MgYZn4 precipitates prevents the alloy from softening upon annealing at 625 K for several hours effectively.

2. Experimental

Nominal composition of the Mg alloy used in this study is Mg-3.6Zn-0.6Y (at.%). Samples cut from casting ingots were annealed at 595 K, 615 K and 625 K for different time ranging from 0.5 h to 50 h, in order to study the stability and phase transformation of MgYZn4 precipitates. Specimens for electron microscopy observations were prepared using standard ion milling techniques. Electron diffraction analyses were performed using a JEOL 2100 transmission electron microscope. Nano-probe energy dispersive X-ray spectroscopy (EDXS) measurements were used to detect alloying elements of new phases. HAADF-STEM observations were conducted on an aberration-corrected Titan G2 60-300 microscope operated at 300 kV. The beam convergence half-angle was 25 mrad, and the collection half-angles of the HAADF detector ranged from about 60 mrad to about 290 mrad. Image simulations were carried out using a multi-slice method [19].

First-principles calculations were performed with density functional theory as implemented in the Vienna ab initio Simulation Package (VASP) [20,21]. The electron exchange and correlation were described with the generalized gradient approximation of Perdew, Burke and Ernzerhof [22] and the electron-ion interaction was represented by the projector augmented wave method [23]. Combining with the Phonopy code [24], thermodynamic properties of the relevant structures and phases were calculated using the density-functional perturbation theory implemented in the VASP code.

Vickers microhardness was measured using a MVK-H300 microhardness tester equipped with a diamond pyramidal indenter. A load of 50 g was applied for 15 s for each measurement. Microhardness of Mg matrix in samples annealed at 625 K for 1 h is 82.7 ± 2.5 which is only 3.8% lower than that in as-cast samples (86.0 ± 2.6), demonstrating excellent resistance to softening upon annealing at elevated temperatures. However, microhardness of a control sample of ZK60 alloy was decreased from 70.3 ± 1.1 to 60. 0 ± 2.5 (i.e. a relative decrease of 14.3% which is much higher than that for the present Mg alloy) upon annealing at 625 K.

3. Results and discussion

Fig. 1(a) is a low-magnification HAADF-STEM image recorded along the 〈11$\bar{2}$0〉α zone axis in as-cast samples. Coarse second phases at GBs are IQCs, as shown by the inset 5-fold selected area electron diffraction pattern in Fig. 1(a). There are numerous small precipitates showing bright features about 10 nm thick and 200 nm to 400 nm wide parallel to basal planes in α-Mg matrix. Fig. 1(b) shows a low-magnification HAADF-STEM image for the α-Mg matrix by tilting about 47° around the 〈10$\bar{1}$0〉α axis, after recording the image shown in Fig. 1(a). The inset in Fig. 1(b) is an image for precipitates recorded along the [0001]α zone axis. A plate-like morphology can thus be determined for those precipitates parallel to basal planes in α-Mg matrix. Interestingly, sizes of those plate-like precipitates did not change after annealing at 625 K for 1 h, as shown in Fig. 1(c). Moreover, no changes in the crystal structure of the plate-like precipitates were detected, according to electron diffraction analyses (Fig. 1(d-h)). Besides plate-like basal precipitates, there are [0001] nano-rods and particles of binary Mg-Zn compounds, such as β1' -MgZn, MgZn2 and Mg4Zn7 in α-Mg matrix [25], as indicated by white and red arrows respectively in Fig. 1(a) and (b). However, no [0001] nano-rods of binary Mg-Zn compounds could be observed, while some larger particles with diameters of about 200 nm appeared in samples annealed at 625 K for 1 h, as indicated by a yellow arrow in Fig. 1(c). It should be pointed out that no binary Mg-Zn precipitates were observed even in samples annealed at 625 K for only 0.5 h. Therefore, the plate-like precipitates show thermostability much better than other binary Mg-Zn compounds of [0001] nano-rods and particles, playing an important role in preventing Mg matrix in the Mg-Zn-Y alloy from softening effectively upon annealing at 625 K.

Fig. 1.   Low-magnification HAADF-STEM images showing morphology of plate-like phases in as-cast (a, b) and annealed at 625 K for 1 h (c) samples of Mg-3.6Zn-0.6Y alloy. The inset in Fig. 1c is Zn and Y profiles of EDXS line-scanning along the red line. The yellow arrow in Fig. 1c points to a newly formed IQC precipitate, which will not be discussed in the present paper. (d-h) Electron micro-diffraction patterns of plate-like precipitates recorded along [0001]α, [10$\bar{1}$0]α, [11$\bar{2}$0]α, [01$\bar{1}$0]α and [2$\bar{11}$0]α zone axes, respectively.

Fig. 1(d-f) shows micro-diffraction patterns recorded from regions with a plate-like precipitate along the [0001]α, [10$\bar{1}$0]α and [11$\bar{2}$0]α zone axes, respectively. Besides the brighter diffraction disks from the α-Mg matrix, the weaker ones for plate-like precipitates could be indexed as [0001]p, [11$\bar{2}$0]p and [01$\bar{1}$0]p zone axes diffractions of C14 Laves MgZn2, as indicated respectively by black dots and white circles in Fig. 1(d)-(f). The orientation relationship between precipitates and the α-Mg matrix is [0001]p//[0001]α and ($\bar{1}$100)p//($\bar{1}$2$\bar{1}$0)α. After recording the diffraction patterns shown in Fig. 1(e) and (f), the samples were tilted by 60° around the [0001] axis to obtain diffraction patterns along the [11$\bar{2}$0]p and [01$\bar{1}$0]p zone axes. However, there are extra diffraction disks at positions of $\frac{2n+1}{2}$(10$\bar{1}$0)p and $\frac{2n+1}{2}$(1$\bar{2}$12)p when the [11$\bar{2}$0]p and [01$\bar{1}$0]p zone axes were tilted parallel to the incident electron beam for the plate-like precipitates, as indicated by arrows in Fig. 1(g) and (h), besides diffractions for C14 MgZn2 (Fig. 1(e) and (f)). It suggests occurrence of chemical or structural ordering in plate-like precipitates, resulting in the formation of a new structure based on C14 MgZn2.

In order to check the stability of plate-like precipitates more comprehensively, prolonged heat-treatments at different temperatures were performed, as shown in Fig. S1. No obvious changes in plate-like precipitates were observed in samples annealed at 625 K for 5 h; about 50% of plate-like precipitates were transformed into W-Zn3Mg3Y2 after annealing for 10 h (Fig. S2); and plate-like precipitates were fully transformed into W-Zn3Mg3Y2 after annealing for 25 h at 625 K. It was found that transformation from plate-like precipitates to W-Zn3Mg3Y2 started after annealing for 25 h at 615 K (eutectic point of Mg-rich Mg-Zn binary alloys [26]), and the transformation finished after annealing for 50 h. When the annealing temperature was decreased to 595 K, no changes in size and structure of plate-like precipitates were observed in samples annealed for 50 h. Additionally, no Mg-Zn binary intermetallic precipitates were observed in samples annealed at 615 K for 25 h and 595 K for 50 h.

EDXS analyses showed presence of Y in plate-like precipitates, as shown by line-scanning profiles overlapped on Fig. 1(c). It suggests presence of Y in Mg matrix played a role in the formation of plate-like precipitates during solidification process, instead of C14 MgZn2. In order to reveal the lattice occupation of Y in plate-like precipitates, atomic-resolution HAADF-STEM investigations were carried out. Fig. 2(a) and (b) shows atomic-resolution HAADF-STEM images of C14 MgZn2 recorded along 〈11$\bar{2}$0〉p and 〈10$\bar{1}$0〉p zone axes, respectively. It is seen that all strong intensity spots are produced by Zn atomic columns forming red rhombic units shown in Fig. 2(a), while atomic columns of Mg do not show intensity bright enough in the experimental images, which is consistent with image simulations, as shown by yellow squares in the insets of Fig. 2(a) and (b).

Fig. 2.   (a and b) Atomic-resolution HAADF-STEM images of C14 MgZn2 recorded along the 〈11$\bar{2}$0〉 and 〈10$\bar{1}$0〉 zone axes, respectively. The insets are the corresponding atomic projections and simulated images. (c-f) Atomic-resolution HAADF-STEM images of plate-like phase recorded along [11$\bar{2}$0]P, [01$\bar{1}$0]P, [$\bar{1}$2$\bar{1}$0]P and [10$\bar{1}$0]P zone axes, respectively.

Fig. 2(c-f) shows atomic-resolution HAADF-STEM images of plate-like precipitates recorded along [11$\bar{2}$0]P, [01$\bar{1}$0]P, [$\bar{1}$2$\bar{1}$0]P and [10$\bar{1}$0]P zone axes, respectively. All intensity spots at positions corresponding to Mg atomic columns in C14 MgZn2 become bright enough, when viewed along the [11$\bar{2}$0]P and [01$\bar{1}$0]P zone axes for the plate-like precipitates, as shown in Figs. 2(c) and (d). But only half of those intensity spots at positions corresponding to Mg atomic columns in C14 MgZn2 produced strong intensity spots, when viewed along the [$\bar{1}$2$\bar{1}$0]P and [10$\bar{1}$0]P zone axes for the plate-like precipitates, as shown in Fig. 2(e) and (f). It is seen that one of the two atomic columns shows intensity much higher than the other inside each rhombic units composed of Zn columns in Fig. 2(e). And strong elongated and weak round intensity spots corresponding to three neighboring atomic columns (one Zn atomic column and two neighboring Mg columns in C14 MgZn2) between pure Zn atomic layers are distributed alternatively in Fig. 2(f). The results shown in Fig. 2(c)-(f) imply that ordered substitution of heavy atoms (Y or Zn) for half of Mg atoms in C14 MgZn2 occurred. It is seen that the minimum projected repeat units for the plate-like precipitates shown in Fig. 2(c) and (d) are identical to those in C14 MgZn2 (Fig. 2(a) and (b)), although there are differences in image spots. But the minimum projected repeat units for the plate-like precipitates in Fig. 2(e) and (f) are doubled along the horizontal direction, compared with C14 MgZn2. The atomic-resolution imaging results demonstrate that the appearance of additional diffraction disks at positions of $\frac{2n+1}{2}$(10$\bar{1}$0)p and $\frac{2n+1}{2}$(1$\bar{2}$12)p in the [$\bar{1}$2$\bar{1}$0]P and [10$\bar{1}$0]P micro-diffraction patterns (Fig. 1(g) and (h)) should be attributed to ordered substitution of heavy atoms for Mg. And a unit cell of plate-like phase should be reconstructed based on a supercell composed of 2 × 2 × 1 (a1 × a2 × c) MgZn2 unit cells (16 Mg atoms and 32 Zn atoms), in which eight Mg atoms were substituted by Zn or Y atoms, as shown in Fig. 3(a). However, small precipitates in Mg matrix of Mg-Zn-Y alloys were mostly designated to be C14 MgZn2, based on a single electron diffraction pattern or high-resolution images [15,16]. The present results showed that systematic diffraction and imaging along several different zone axes may be necessary to determine properly the structure of small precipitates in alloys.

Fig. 3.   (a) Schematic diagram showing the substitution of heavy atoms for Mg in a 2 × 2×1 MgZn2 supercell. (b) [0001] projection of the 2 × 2×1 supercell with a red rectangular outlining a reduced unit cell of MgYZn4. (c) Unit cell of MgYZn4. Arrows “d” - “g” in (b) and (c) indicate the four observation directions of atomic-resolution imaging. The zone axes indicated by “d” - “g” which are respectively [11$\bar{2}$0]P, [01$\bar{1}$0]P, [$\bar{1}$2$\bar{1}$0]P and [10$\bar{1}$0]P in the four-number miller indices for hexagonal structures become [110], [310], [100] and [010], respectively, in the three-number miller indices for the orthorhombic MgYZn4. (d-g) Simulated images for orthorhombic MgYZn4 along zone axes [110], [310], [100] and [010], respectively.

In order to identify the substituted atoms (Zn or Y) in plate-like precipitates, formation energy of a series of structures with substitution of Zn or Y for Mg in the 2 × 2 × 1 supercell was calculated (Table 1). Substitution of Y for half of Mg atoms produces MgYZn4, with a formation energy of -0.313 eV/atom which is lower than that of MgZn2 (-0.152 eV/atom), while substitution of Zn for Mg or Y for Zn would result in increase in the formation energy (Table S1). Therefore, the plate-like precipitates with enhanced thermostability should be MgYZn4 formed by substitution of Y atoms for half of Mg atoms in MgZn2 (Fig. 3(a-c)), according to the thermodynamic theory [27,28]. This is consistent with elemental mapping results (Fig. S3).

Table 1   Formation energy Ef (eV/atom) for structures formed by substitution of Zn or Y for Mg in MgZn2.

StructureEf
Mg4Zn8-0.152
Mg3Zn9-0.139
Mg3YZn8-0.173
MgYZn4-0.313

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Fig. 3(b) shows the [0001] projection of the supercell shown in Fig. 3(a), from which a reduced unit cell of MgYZn4 can be deduced, as indicated by the red rectangle. Fig. 3(c) shows the three-dimensional structure of the unit cell of MgYZn4. MgYZn4 has an orthorhombic structure (space group Pmnn, No. 58). The transformation from P63/mmc to Pmnn is discussed in the Supplementary material [29]. First-principles calculations indicate that lattice parameters for MgYZn4 are a =5.2965 Å, b =9.4886 Å, c =8.5966 Å, α = β = γ = 90°. Lattice occupation and reflection conditions of orthorhombic MgYZn4 are shown in Tables 2, S2 and Fig. S4, respectively. And a CIF file is given in Supplementary Materials. Crystallographic relationship between MgYZn4 and MgZn2 can be described by Eq. (1):

Table 2   Wyckoff position and coordinates of atoms in MgYZn4, according to atom positions in the structure model after first-principles relaxation. The a and c axes of MgYZn4 are arbitrarily set parallel to those of C14 MgZn2, and the space group is thus described as Pmnn which is equivalent to Pnnm.

AtomWyckoff positionx/ay/bz/c
Mg4g00.159340.93678
Y4g0.50.328600.05781
Zn18h0.745280.418800.74757
Zn24g0.50.173510.74362
Zn32d0.500.5
Zn42b0.500

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Fig. 3(d-g) shows image simulated images and corresponding atomic projections of the proposed model (Fig. 3(c)) recorded along the four zone axes shown by “d”- “g” in Fig. 3(b) and (c), respectively. The simulated images match well with experimental images shown in Fig. 2(c-f). The simulated composite reciprocal lattice patterns of MgYZn4 and Mg along different zone axes are shown in Supplementary Fig. S5, which is also consistent with the experimental micro-diffraction patterns (Fig. 1(c-g)). Therefore, the proposed atomic structure of MgYZn4 is reasonable. MgYZn4 precipitates would be formed with half of the 4f lattice sites being occupied by Y in regions rich in both Zn and Y during solidification in the Mg-Zn-Y alloy, instead of c14 MgZn2, since substitution of Y for Mg atoms can reduce the formation energy (Table 1).

Formation energy of MgYZn4 is about -0.313 eV/atom at 0 K, 0.161 eV lower than in MgZn2 (Table 1), indicating that MgYZn4 is more stable at 0 K. And this relative stability is not changed with increasing temperature, as shown in Fig. 4(a). Table S3 shows the Bader charge of MgZn2 and MgYZn4. In MgZn2, there is 1.439 electron transferring from one Mg to Zn atoms, and per Zn atom obtains about 0.712 electron and thereby strongly bonding with Mg atom. Mg and Zn atoms in MgYZn4 obtain about 0.114 and 0.085 more electron per atom respectively, compared with that of MgZn2. This bonding enhancement due to Y substitution for Mg is also validated from their projected density of states (PDOS), as shown in Fig. 4(b). The bonding between Mg and Zn mainly stems from the interactions of Mg s and Zn s electrons in MgZn2, while Y atom not only strongly interacts with Mg atom through Y s and Mg s electrons, but also strongly interacts with Zn atom through Y d and Zn d electrons in MgYZn4. Therefore, the presence of Y enhances the microstructural stability of MgYZn4.

Fig. 4.   (a) Formation energy difference between MgYZn4 and MgZn2 from 0 K to 800 K. (b) PDOS of MgZn2 and MgYZn4.

According to the Mg-Zn binary phase diagram [26], binary Mg-Zn compounds, like MgZn2, should dissolve into α-Mg matrix during annealing at 625 K, since the eutectic temperature is about 615 K. Interestingly, no changes in size and structure of the ternary MgYZn4 precipitates were observed after annealing at 625 K for 5 h. The Addition of Y to Mg-Zn alloy can effectively improve the high-temperature mechanical properties [2,3], which was previously attributed to formation of IQC [14,30]. The present study shows that MgYZn4 precipitates in Mg-Zn-Y alloys have good stability at 625 K, so they are also expected to be able to play an important role in strengthening the Mg95Zn4Y1 alloys at elevated temperatures.

4. Conclusions

In summary, HAADF-STEM imaging and the first-principles theory analysis demonstrated that ordered substitution of Y for half of Mg atoms in 4f sites of MgZn2 led to the formation of MgYZn4 which has an orthorhombic structure with a space group of Pmnn, a =5.2965 Å, b =9.4886 Å, c =8.5966 Å. Nanometer-sized MgYZn4 precipitates show size and structural stability much better than binary Mg-Zn precipitates. The enhanced thermostability of MgYZn4 precipitates is attributed to the lower formation energy and bonding enhancement due to the substitution of Y for Mg atoms.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos.51390473, 51371178, 51771202), the Key Research Program of Frontier Sciences, CAS (No. QYZDY-SSW-JSC027).

The authors have declared that no competing interests exist.


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