Effects of Y and Zn additions on electrical conductivity and electromagnetic shielding effectiveness of Mg-Y-Zn alloys

1. Introduction

Electromagnetic radiation pollution has become a problem to be solved with the rapid development of electronic science and technology. Therefore, electromagnetic interference (EMI) shielding has attracted much attention due to the tremendous demand for aerospace, medical, electrical apparatus and military applications. Shielding effectiveness (SE) is a commonly used parameter for characterization of shielding performance and measured in decibels (DB) [1], [2], [3], [4], [5]. As such shielding materials are commonly used in the shell of electronic equipment to prevent electromagnetic leakage or as a protected body shell to prevent the entry of electromagnetic waves. They are generally plates, strips or castings [6], [7], [8], [9]. Materials in the field of electromagnetic protection usually require light weight, so density of the selected materials should be as low as possible under the stable shielding structure in practical engineering applications [10,11]. Magnesium (Mg) alloys not only possess low density, high specific strength, high specific stiffness and excellent damping capacity, but also exhibit relatively good conductivity and shielding capacity [12], [13], [14], [15], [16], [17], [18]. Therefore, Mg alloys can be considered as promising EMI shielding materials.

Mg-Zn-based alloys have high strength at room temperature and addition of some elements such as Zr, Mn, Al, Sn, RE (La, Y, Gd, Nd, Ce, etc.) can further improve the overall mechanical properties [19], [20], [21], [22], [23]. Among them, Mg-Zn-Y ternary alloys have attracted much attention due to the existence of quasicrystalline phase and long period stacking ordered phase (LPSO) [24,25]. It has been reported that Mg alloys with LPSO phase have excellent mechanical properties [26,27]. For example, a new nanocrystalline Mg-1Zn-2Y alloy prepared by Kawamura et al. exhibits ultimate tensile strength exceeding 600 MPa [28]. These excellent properties are derived not only from grain refinement but also from the precipitates with LPSO structure. So far, some LPSO phases with different stacking orders have been found in Mg-Zn-Y alloys, including 10H, 18R, 14H and 24R, which are formed by stacked blocks composed of five, six, seven and eight closely packed atomic layers [29,30]. Liu et al. studied microstructure, electromagnetic shielding effectiveness and mechanical properties of Mg-Zn-Y-Zr alloys. They pointed out that the precipitation of W, β₁^’ and β₂^’ phases can improve EMI SE [31]. In a word, Mg-Zn-Y alloys are a relatively mature system and their mechanical properties have been extensively studied. However, research on electrical conductivity and electromagnetic shielding property is still relatively rare.

Therefore, in the present work, the effects of Y/Zn ratio on electrical conductivity and electromagnetic shielding properties of cast Mg-Zn-Y alloys were investigated. Such an investigation will provide an important basis for developing high-performance Mg alloys for shielding applications and broaden the practical applications of Mg alloys.

2. Experimental

Mg alloys with nominal compositions of Mg-xZn-yY (x = 2-5 wt%, y = 1-10 wt% and desi Mg (99.95 wt%), Zn (99.9 wt%), and Mg-30 wt% Y master alloy (99.9 wt%) under a protective atmosphere of CO₂ and SF₆ (99:1). The Zn content of 2-5 wt% and Y content of 1-10 wt% were chosen to get different Y/Zn rations and phases, including I-, W- and X-phases. Chemical compositions and main phases of cast alloys determined by a photo electricity spectrum analyzer (APL4460) are listed in Table 1.gnated as ZW alloys hereafter) were prepared through conventional casting with pure

Microstructural and compound compositions of the alloys were examined by VEGA II LMU scanning electron microscopy (SEM). For SEM observation, polished specimens were etched with a mixture comprising 2 ml acetic acid, 14 ml ethanol and supersaturated picric acid. Phase analysis was performed with a Rigaku D/MAX-2500PC X-ray diffractometer (XRD). Electrical conductivity was measured with a conductivity meter (Sigmascope SMP 10) at ambient temperature. Each mean value was received based at least 8 replicates.

EMI SE was measured using the standard coaxial cable method in accordance with ASTM D4935-2010. The setup is consisted of a DN 1015A SE tester with input and output connected to an Agilent 8753ES network analyzer [32]. Disc specimens of 115 mm in diameter and 2 mm in thickness were prepared for EMI SE measurements and the frequency range was from 30 MHz to 1.5 GHz. A plane wave electromagnetic field was used for vertically firing.

3. Results

Fig. 1 displays typical XRD patterns of ZW alloys. Three phases are appeared in the alloys: icosahedral quasicrystalline phase Mg₃Zn₆Y (I-phase); cubic phase Mg₃Zn₃Y₂ (W-phase) and LPSO phase Mg₁₂ZnY (X-Phase) [33]. The type of phases changes similarly in different ZW series alloys. These alloys are mainly consisted of α-Mg and I-phase when Y/Zn ratio is less than 1. W-phase and X-phase begin to appear, and I-phase disappears gradually with increasing Y/Zn ratio. Finally, the main second phase is only X-phase when Y/Zn ratio reaches 2.

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Fig. 1.   XRD patterns of the (a) ZW2x; (b) ZW3x; (c) ZW4x and (d) ZW5x alloys.

SEM images were observed to analyze the amount and dispersion of second phases in primary Mg matrix. Fig. 2 presents SEM micrographs of Mg-2Zn-xY alloys. In Fig. 2, it reveals that the quantity of second phase gradually increases with the addition of Y in ZW2x alloys, and the morphology of second phases also changes evidently. In combination with the XRD patterns (Fig. 1), ZW20 alloy contains I-phase; ZW21 alloy contains I-phase and W-phase; ZW22 alloy contains W-phase and X-phase; and ZW24 alloy contains X-phase. I-phase disperses as a discontinuous net on α-Mg matrix in ZW20 and ZW21 alloys, and gradually disappears with the Y content. W-phase is an irregular blocky phase which has a dense and fine lamellar structure in the interior, and part of W-phase is attached to I-phase. X-phase is a long strip shaped phase.

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Fig. 2.   SEM images of the (a, e) ZW20, (b, f) ZW21, (c, g) ZW22, (d, h) ZW24. (e), (f), (g), and (h) are the magnified images of the (a), (b), (c) and (d), respectively.

SEM micrographs of ZW3x alloys are shown in Fig. 3, it can be found that there are more second phases with the addition of Y. Especially in ZW36 alloy, the second phase has become a continuous reticulation structure. I-phase, W-phase and X-phase appeare with the addition of Y and morphologies are thin rod-like, fish-bone-like and plate-like, respectively. For each alloy, ZW30 alloy contains I-phase; ZW31 alloy contains I-phase and W-phase; ZW33 alloy contains W-phase and X-phase; and ZW36 alloy contains X-phase.

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Fig. 3.   SEM images of the (a, e) ZW30, (b, f) ZW31, (c, g) ZW33, (d, h) ZW36. (e), (f), (g), and (h) are the magnified images of the (a), (b), (c) and (d), respectively.

Fig. 4, Fig. 5 display SEM micrographs of ZW4x and ZW5x alloys. With a constant Zn content, the type and change rules of second phase in ZW4x and ZW5x alloys are same as those of ZW2x and ZW3x alloys. The difference is that the quantity of second phase increases obviously in ZW4x and ZW5x alloys containing a high content of Zn. Even when the content of Y is low, the second phase with semi-continuous network distribution has begun to appear.

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Fig. 4.   SEM images of the (a, e) ZW40, (b, f) ZW41, (c, g) ZW44, (d, h) ZW48. (e), (f), (g), and (h) are the magnified images of the (a), (b), (c) and (d), respectively.

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Fig. 5.   SEM images of the (a, e) ZW50, (b, f) ZW52, (c, g) ZW55, (d, h) ZW5-10. (e), (f), (g), and (h) are the magnified images of the (a), (b), (c) and (d), respectively.

The electrical conductivity of ZW alloys is summarized in Fig. 6(a). When Zn content is fixed, the electrical conductivity of ZW alloys decreases with Y content. When Y content is constant, the electrical conductivity of ZW alloys decreases with increasing Zn, but the reduction range of conductivity caused by the increase in the same content of Zn is lower than that of Y. For example, when Y content is less than 1 wt% and Zn content increases from 2 wt% to 5 wt%, conductivity of the alloys decreases from 19 MS/m to 17.2 MS/m; Similarly, when Zn content is 2 wt% and Y content increases from 0.5 wt% to 4 wt%, conductivity of the alloys decreases from 19 MS/m to 11.8 MS/m. Same conclusions can be drawn from Fig. 6b that the electrical conductivity of ZW alloys decreases as Y/Zn ratio increases, which means the conductivity decreases more obviously when adding same amount of Y.

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Fig. 6.   (a) Electrical conductivity of Mg-Zn-Y alloys; (b) electrical conductivity variation with the Y/Zn ratio.

SE values of ZW alloys at 400 and 600 MHz are summarized in Fig. 7. At 400 MHz SE values decrease gradually with increasing Y content and SE values are generally higher for the alloys containing more Zn. Taking ZW5x alloys as example, when the content of Y increases from 1 wt% to 10 wt%, SE of the alloys decreases from 97 dB to 75 dB. It can be inferred that SE of ZW alloys decreases as Y/Zn ratio increases combined with a linear fit of SE in Fig. 7(b). The variation trend of SE is consistent with that of conductivity. The variation trend of SE in two frequencies are similar. However, the reduction in SE values at 600 MHz is greater than that of 400 MHz. SE values of ZW alloys are higher than that of pure Mg (about 60-65 dB at 400-600 MHz) and 2024 Al alloy (about 35 dB at 30-1500 MHz) [4].

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Fig. 7.   Electromagnetic shielding effectiveness of Mg-Zn-Y alloys at (a) 400 and (c) 600 MHz; electromagnetic shielding effectiveness variation with the Y/Zn ratio at (b) 400 and (d) 600 MHz.

4. Discussion

When electromagnetic wave transmits through a shielding material, there are three mechanisms to attenuate the electromagnetic wave: (1) reflection loss (SE_R), (2) absorption loss (SE_A) and (3) multiple reflection loss (SE_B) [34]. The total EMI SE of a shielding material can be expressed as the following equations:

SE(dB) = R + A + B (1)

SE_R=108.1-10lg(fμ_r/σ_r) (2)

SE_A=1.314t(fμ_r/σ_r)^1/2 (3)

SE_B=20lg(1-e^2t/δ) (4)

δ=(πμσf) ^-1/2 (5)

where f, μ_r, σ_r, t, μ and σ are electromagnetic radiation frequency, relative magnetic permeability, relative electrical conductivity, shield thickness, magnetic permeability and electrical conductivity, respectively. On the basis of Eqs. (2), (3), (4), (5), SE of a material depends on its electrical conductivity, magnetic permeability and thickness when the frequency of electromagnetic waves is constant. In this work, magnetic shielding properties are believed to be limited, since Mg alloys are relatively good as electrical conductors but poor in magnetic properties. According to above-mentioned shielding equations, the shielding properties in the ZW alloys can be intimately related to electrical conductivity.

For ZW alloys, their electrical conductivity decreases with the increasing Y/Zn ratio due to the different influence of Y and Zn elements on electrical conductivity. The effect of solute atoms on the resistivity of the alloy is affected by various factors, including the volumetric difference of solute atoms and Mg atoms ΔV/V_Mg, the valence of the solute atoms, and the configuration of extranuclear electron. The above properties of Zn and Y are presented in Table 2 [35]. The high ΔV/V_Mg value means serious lattice distortion in α-Mg, leading to obstruction in electron transmission and intensive lattice vibration wave scattering which increase alloy resistivity. The influence of solute atomic valence on Mg alloy resistivity is mainly based on Linde’s rule [36]: the increasing resistivity caused by each addition of 1 at.% solid solution element is proportional to the square of the difference value of valence between solvent element and solute element. The difference value of valence between Y³⁺ and Mg²⁺ is bigger than those of Zn²⁺ and Mg²⁺, so it is more likely for Y to cause Mg alloy resistivity increases. The influence of the configuration of extranuclear electron on resistivity is mainly affected by the number of extra nuclear electron vacancies. According to Hund’s rule, the less electrons are filled in each sublayer, the more vacancies in the sublayer, and it is more likely to absorb conduction electron [37]. The number of vacancies in outer orbit of Zn(3d¹⁰4s²) is less than Y(4d¹⁵s²). Maximum solid solubilities of Zn and Y in Mg are 8.4 at.% and 12.5 at.%, respectively [38]. The cooling method adopted for these alloys after casting is rapid water cooling so the quantity of Zn and Y in the Mg matrix is large. Given all that, the resistivity increases more obviously when the same content of Y element is added compared with Zn element.

The physical properties of magnesium are presented in Table 3 [39]. The relationships between SE and the relative electrical conductivity of ZW36 alloy at 400 and 600 MHz could be obtained, as presented in Fig. 8, indicating that SE decreases along with the decreasing relative electrical conductivity. On the basis of Eqs. (2) and (3), the increasing electrical conductivity will promote the interaction between the electromagnetic field and free electrons; this interaction can enhance SE_R. The higher the electrical conductivity, the greater the vortex electrical current in the shielding alloy, therefore, more energy of the radiation plane-wave field will be dissipated by ohmic electrothermal heat and a high reverse electromagnetic field will be produced, leading to stronger reflection and absorption of electromagnetic radiation based on Eqs. (3) and (4) and thus improved EMI shielding properties in the Mg alloy. In addition, the heat loss caused by the interaction will increase SE_A. SE_R decreases and SE_A improves as the frequency of the electromagnetic wave increases. For other ZW alloys which belong to good conductor, their SE_R is considerably higher than SE_A. As a result, SE decreases monotonically when the frequency of the electromagnetic wave increases. This also coincides with the calculation in Fig. 8.

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Fig. 8.   Relationship between SE and the relative electrical conductivity of ZW36 alloy at 400 and 600 MHz, respectively.

As the electrical conductivity and internal structure of the alloy are different, the amplitude of the reduction of SE value is different with the frequency of electromagnetic wave increasing. The variation tendency of the electromagnetic shielding performance of ZW alloys is similar to that of electrical conductivity. This is because the energy of incident electromagnetic wave is weak at 400 MHz, and most incident electromagnetic waves are reflected by the alloy surface. In this case, the alloy with high electrical conductivity, which impedance value Z is low, leads to an increase in the impedance ratio (K) of the alloy to air. Therefore, SE_R increases and the electromagnetic shielding performance of the alloy improves. When the conductivity of the alloy decreases rapidly, SE_R also decreases sharply. This led to the decline in SE value at 600 MHz which is greater than that of 400 MHz.

SE values of alloys with more Zn are generally higher, so it can be further speculated that the second phase will also have a certain effect on the electromagnetic shielding properties. For as-cast ZW alloys, the changes in morphology and distribution of second phase are not obvious, and main changes are reflected in the quantity and type. More second phase can split α-Mg matrix and enhance the discontinuity inside α-Mg matrix. The second phase can be regarded as an insulator due to the electrical conductivity of second phase is two orders of magnitude lower than that of pure Mg [3]. Severe impedance mismatch occurs because of the large difference between the impedance of the second phase and α-Mg matrix. The electromagnetic wave is more easily to be reflected when transmitting through the interface between magnesium matrix and second phase, thus improving the SE_B and SE_A. Because the wavelength of the electromagnetic wave is far greater than the size of a single precipitation phase, more attention is paid to the overall morphology of the precipitated phase and whether a continuous network structure is formed. In general, the change of internal structure of second phase that causes by the type of the second phase has negligible effect on the electromagnetic shielding properties of the alloy.

5. Conclusions

In this work, the effects of Zn and Y elements on shielding properties of cast Mg-Zn-Y ternary alloys were investigated. The following conclusions can be drawn:

(1) The quantity of second phase gradually increases with Y addition in the ZW alloys, and the morphology and type of second phase also change evidently. Second phase with semi-continuous network distribution has begun to appear even when Y content is low in the alloys containing more Zn.

(2) Electrical conductivity of the ZW alloys decreases with the increasing Y/Zn ratio. Valence of Y and Zn atoms, the configuration of extranuclear electron and the volumetric difference are the main reasons for this phenomenon. The conductivity decreases more obviously when adding the same amount of Y.

(3) The variation tendency of electromagnetic shielding property of ZW alloys at low frequency is consistent with that of their corresponding conductivity. SE of ZW alloys decreases as the Y/Zn ratio increases. The electrical conductivity is the main factor that affects the electromagnetic shielding performance of alloys. More second phases and the formation of semi-continuous network structure can also improve the SE value at high frequencies.

Acknowledgments

The authors thank the National Key R&D Program of China (2016YFB0301100), the National Natural Science Foundation of China (51571043 and 51531002), the Fundamental Research Funds for the Central Universities (2018CDJDCL0019 and cqu2018CDHB1A08) and Chongqing Technology Innovation and Application Demonstration (Social and Livelihood) Project (cstc2018jscx-msybX0090).

The authors have declared that no competing interests exist.