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Microstructures and Properties of As-Cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 Alloys with LPSO Phase
Rare Metal Materials and Engineering Volume 44, Issue 7, July 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(7): 1617-1622
ARTICLE
Microstructures and Properties of As-Cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 Alloys with LPSO Phase Shi Fei1,2, 1
Wang Chunqing1,2,
Guo Xuefeng3
Harbin Institute of Technology, Weihai 264209, China;
2
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of
3
Technology, Harbin 150001, China; Henan Polytechnic University, Jiaozuo 454000, China
Abstract: Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys with long period stacking ordered (LPSO) structure phases were prepared by conventional solidification process. By OM, SEM, EDS, XRD and TEM analysis the phases and 14H-LPSO structures of the two alloys were characterized. The results show that as-cast Mg-alloy with the atomic ratio of Zn/RE = 1 will lead to LPSO phase; adding of Gd element to Mg92Zn4Y4 alloy can facilitate the formation of LPSO phase, and its volume fraction increases from 12.1% to 30.4%; Mg dendrites are split and refined during the precipitation of LPSO phase formed at high temperature, resulting in that the average grain size of -Mg decreases from 50 µm to 10 µm; the solidification microstructure of as-cast Mg92Zn4Y4 alloy is -Mg solid solution + Mg12ZnY + Mg3Zn3Y2 + Mg-Y; In Mg92Zn4Y3Gd1 alloy, the as-cast microstructure is confirmed to be composed mainly of -Mg solid solution, Mg12Zn(Y, Gd) and Mg3Zn3(Y, Gd)2; at room temperature, the compression ratio and the thermal conductivity of Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys are 12.4% and 15.5%, and 99.233 W·(m·K)-1 and 88.639 W·(m·K)-1, respectively. Key words: as-cast Mg-Zn-Y(-Gd) alloy; long period stacking ordered structure; rare earth; thermal conductivity; compression plasticity
Magnesium alloy has great potential applications in the fields of aerospace, marine engineering, precision machining, biological equipment, etc, due to its high strength, good electromagnetic shielding property, thermal conductivity, regeneration and biological compatibility[1-3]. The microstructure of Mg-Zn binary alloy can be refined by adding rare elements; meanwhile the properties of magnesium alloys and antioxidant capacity in smelting process can also be improved. The key point is that different reinforced phases can be obtained by adjusting alloy composition, such as the quasicrystal phase and long-period stacking ordered (LPSO) structure phase. It is believed that quasicrystal reinforced magnesium alloy which exhibits good mechanical properties can be obtained by adjusting the ratio of Zn/Y to be 6 [4,5]. In recent years, many efforts have been focused on studying the LPSO structure phase. Magnesium alloy with LPSO structure
phase was initially found by Luo Z P and marked by the Х-phase[6]. In 2001, Kawamura Y[7] prepared Mg97Zn1Y2 alloy with LPSO by a rapidly-solidified powder/metallurgy method (RSP/M). The chemical formula of LPSO structure phase is Mg12ZnY (at%) and it has 610 MPa yield strength and 5% elongation at room temperature. Currently, magnesium alloy with LPSO phase can be prepared by RS P/M, heat treatment and the conventional casting methods. However, it is generally believed that RE/Zn atomic ratio of 2 is the ideal chemical composition to obtain LPSO phase[8], for the reason that the magnesium alloy can acquire good mechanical properties due to its special crystal structure. The cost of manufacturing will be undoubtedly increased by a rapid solidification method or heat treatment process for preparing LPSO phase reinforced magnesium alloy. In recent years, the researchers have found that LPSO-magnesium alloy
Received date: July 25, 2014 Foundation item: National Natural Science Foundation of China (51374084/E041607) Corresponding author: Wang Chunqing, Ph. D., Professor, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China, Tel: 0086-451-86418725, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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can be obtained by a conventional casting method, while it is usually believed that the rare earth content in the alloy must be higher (or significantly higher) than that of Zn [9-11]. This investigation chose Mg92Zn4Y4 and Mg92Zn4Y3Gd1 (at%) alloys, namely the RE/Zn atomic ratio of 1, and used Gd to replace Y. The purpose is to analyze whether the low RE/Zn ratio or the as-cast condition can acquire the LPSO phase in magnesium alloy, and to investigate the effect of Gd on phase formation, the thermal conductivity and compression plasticity. Moreover, thermal conductivity and compression mechanical properties were tested so as to determine the feasibility of LPSO self-reinforced magnesium base composites as a power electronic packaging shell and a heat sink carrier.
1
Experiment
Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys were prepared from the high purity Mg (99.9wt%), analytical purity Zn (99.9wt%), the master alloy Mg-30wt% Y and Mg-30wt% Gd by melting in graphite crucibles in a 5 kW electric resistance furnace. During melting process, the mixed salt of NaCl and KCl was used to be a protective agent. After the solution treatment, the molten alloy was poured in the metal mould at 830 °C. Standard metallographic specimens were prepared and etched with 3% nitric acid and 97% alcohol. The microstructure of as-cast alloys was observed by OLYMPUS LX 50 optical microscope (OM) and AMREY-100B scanning electron microscope (SEM). And the chemical composition of each phase and its structure were analyzed by JSM-6700F cold field emission scanning electron microscope and JEM-2100 transmission electron microscopy (TEM), respectively. Samples for TEM observation were mechanically polished to 60 μm, and then further thinned by a twin-jet electro polishing method and an ion thinning method. Twin-jet treatment was performed in a solution of 10% nitric acid and 90% alcohol at –20 °C. The phase transformation process of sample was tested by NETZSCH STA 449F3 type differential scanning calorimeter (DSC). High pure Ar was selected as a protective atmosphere during the experiment (Ar gas flow rate was 60 mL/min), and the heating rate was 10 °C/min from RT to 700 °C. The thermal diffusivity and the thermal conductivity of samples were measured between 25 °C and 225 °C with the laser flash thermal conductivity instrument (NETZSCH LFA447). The testing samples with the size of 10 mm×10 mm × 2 mm were cut by electro-discharge machining, and alumina was selected for comparison. At room temperature, the compressive property of samples was tested on INSTRON 2382 universal material testing machine with a constant strain rate of 5.74×10-4 s-1. And the sample size was Φ8 mm × 12 mm.
2
Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys. It is commonly demonstrated in the two alloys that there are white block phase, needle phase and eutectic organization. The average grain size of Mg92Zn4Y4 alloy is about 50 μm, and the dendrite is relatively coarse. Compared with Mg92Zn4Y4 alloy, the average grain size of Mg92Zn4Y3Gd1 is less than 10 μm, which is smaller by an order of magnitude, and there are only a small amount of dendrite with 20~30 μm. The SEM images of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys are shown in Fig.2. Combined with the analysis of SEM, EDS and XRD, the micro area composition of block phase and dendrites in as-cast Mg92Zn4Y4 alloy is about 98.46 at%Mg, 0.79 at%Zn and 0.75 at%Y, confirming that it is -Mg solid solution. The composition of white eutectic secondary phase at grain boundary is about 64.26 at%Mg, 21.81 at%Zn and 13.94 at%Y (that is 36.96 wt%Mg, 33.73 wt%Zn and 29.31 wt%Y). The needle phase composition is 87.12 at%Mg, 6.23 at%Zn and 6.65 at%Y. This component is similar to Mg12ZnY which is the chemical formula of LPSO phase; presumably this phase is LPSO in the tested alloy, as shown in Fig.2a and Fig.2b. In addition, there are some particle reinforced phases with less than 1 μm in as-cast Mg92Zn4Y4 alloy, and result of EDS analysis of this phase is Mg-Y with the atomic ratio of Mg/Y to be 3. In as-cast Mg92Zn4Y3Gd1 alloy, matrix phase contains 97.53 at%Mg, 1.38 at%Zn, 0.81 at%Y and 0.28 at%Gd; thus it can be identified as -Mg solid solution as well. Eutectic phase of the alloy includes 63.35 at%Mg, 23.10 at%Zn, 9.78 at%Y and 3.78 at%Gd (that is 34.13 wt%Mg, 33.46 wt%Zn, 19.26 wt%Y and 13.16 wt%Gd). The composition of the slender needle phase is 84.87 at%Mg, 7.72 at%Zn, 5.25 at%Y and 2.15 at%Gd. The atomic percentage of (Y+Gd) is 7.4 at%, chemical formula for Mg12Zn(Y,Gd). The volume fraction of LPSO phase in the two alloys is about 12.1% and 30.4%, respectively. The maximum width of the LPSO phase in Mg92Zn4Y4 alloy is about 10 μm, while the width of LPSO phase in Mg92Zn4Y3Gd1 alloy is less than 2 μm. For Mg92Zn4Y4 alloy, -Mg matrix and the eutectic phase can be refined by adding Gd, and the LPSO phase as well. The EDS patterns of Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys are shown in Fig.3, corresponding position A, B, C and D in Fig.2b and Fig.2d. In Fig.4, XRD patterns show that -Mg, W-Mg3Zn3Y2 and 14H-LPSO are main phases in as-cast Mg92Zn4Y4 alloy. a
20 μm
Results and Discussion
2.1 Microstructure characteristics Fig.1 shows the optical microstructures of the as-cast
b
Fig.1
Optical microstructures of as-cast Mg alloys: (a) Mg92Zn4Y4 and (b) Mg92Zn4Y3Gd1
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b
A
B 5 μm
50 μm c
d
C D 50 μm Fig.2
5 μm
SEM backscatter electron images of as-cast Mg-Zn-Y(-Gd) alloys: (a, b) Mg92Zn4Y4 and (c, d) Mg92Zn4Y3Gd1
Intensity/a.u.
Mg
a
Point A Element Mg Zn Y
Zn
Mg
Intensity/a.u.
8
12
4
4
Content/at% 97.53 1.38 0.81 0.28
8
12
16 0
4
Energy/keV
16 d
Element Mg Zn Y Gd
Mg Zn Gd Y
12
8
Point D
c
Element Mg Zn Y Gd
Mg Gd Zn
Zn
16 0
Point C
Fig.3
Content/at% 87.12 6.23 6.65
Y
4
0
Element Mg Zn Y
Zn
Y
0
b
Point B
Content/at% 98.46 0.79 0.75
Content/at% 84.87 7.72 5.25 2.15
8
12
16
Energy/keV
EDS analysis of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys: (a) -Mg solid solution and (b) Mg12ZnY phase in Fig.2b for Mg92Zn4Y4
alloy;
-Mg
(c)
solid
solution
and
(d) Mg12Zn(Y, Gd) phase in Fig.2d for Mg92Zn4Y3Gd1 alloy
-Mg W-Mg3Zn3Y2 14H-LPSO
Intensity/a.u.
a
the action of Gd atoms in Mg92Zn4Y3Gd1 alloy, the diffraction peaks show a slightly offset to the left. Fig.5 shows DSC curves of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys during heating process. With temperature increasing, there are 3 endothermic peaks in the two kinds of alloys, which are located at 511, 523 and 579 °C for Mg0.2Zn4Y4 alloy, while 510, 518 and 580 °C for Mg0.2Zn4Y3Gd1 alloy. Inferred by the phase diagram analysis, 3 endothermic peaks correspond to 14H-LPSO, W-Mg3Zn3Y2 and matrix phase transition peaks, respectively. In Mg92Zn4Y4 alloy, melting temperature of matrix solution starts at 552 °C. While in the Mg92Zn4Y3Gd1 alloy, the beginning of melting temperature of matrix phase increases to 561 °C due to Gd dissolved into -Mg solid solution. In addition, the reaction peaks of 14H-LPSO and W phases are close, and the W peak of Mg92Zn4Y3Gd1 alloy is weaker than that of Mg92Zn4Y4 alloy. It suggests that the number of W phase in Mg92Zn4Y3Gd1 alloy is less than that in Mg92Zn4Y4 alloy. This results are consistent with the analysis of OM and SEM, namely the addition of Gd will promote adequately the reaction of L + W→14H-LPSO, and at the same time in the consumption of eutectic W phase more LPSO phase, is generated. The phases and structures of as-cast Mg 92 Zn 4 Y 4 and Mg92Zn4Y3Gd1 alloys were further investigated by TEM as presented in Fig.6. In Mg92Zn4Y4 alloy, there are a large number of black precipitation phases with the size less than 20 nm on matrix. The SAED pattern shows a series of diffraction ring and the calibration for polycrystalline magnesium, as shown in Fig.6a. The bright-field (BF) image and corresponding SAED of LPSO phase are shown in Fig.6b with electron beam (EB) paralleling to the direction of [11 2 0]. The lighter diffraction spot is calibrated for -Mg. It has 13 equal interval diffraction spots between (0002) and (0002) 14H, which indicates that the lamellar phase is 14H-LPSO structure phase with the lattice constants of a = 0.3253 nm and c = 3.6946 nm. Fig.6c shows the BF TEM and corresponding SAED pattern of LPSO phase in Mg92Zn4Y3Gd1 alloy. It can be seen that the LPSO has 14H-type structure, and the lattice constants are essentially the same as 14H-LPSO phase in Mg92Zn4Y4 alloy. Moreover, Fig.6d shows the TEM image and SAED pattern of
0.0
20
Fig.4
30
40 50 2θ/(°)
60
Heat Flow/mW·mg
-1
b
70
XRD patterns of as-cast Mg92Zn4Y4 (a) and Mg92Zn4Y3Gd1 (b) alloys
Because the volume fraction of Mg-Y phase is relatively few, the diffraction peaks is rather small. Judging from the type of the XRD diffraction peaks, the distribution is consistent with that of Mg92Zn4Y3Gd1 and Mg92Zn4Y4. However, because of
–-0.4 –-0.8
502 °C
–-2.0
501 °C 552 °C
Mg92Zn4Y4 Mg92Zn4Y3Gd1 511 °C 510 °C
–-1.2 –-1.6
Endo.
a
Phase Mg92Zn4Y4 LPSO 511 °C W 523 °C Matrix 579 °C
300
400
561 °C 523 °C 518 °C
579 °C
Mg92Zn4Y3Gd1 510 °C 580 °C 518 °C 580 °C
500
600
700
Temperature/°C Fig.5
DSC curves of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys
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a
α
b
α
200 nm
100 nm EB//[1120]α c
d
α
α
α
EB//[1120]α Fig.6
α
100 nm
50 nm EB//[0001]α
_
BF-TEM images and corresponding SAED patterns of the precipitations (a) and 14H-LPSO structure with EB taken along [1120]α (b) in _
as-cast Mg92Zn4Y4 alloy; 14H-LPSO structure with EB paralleling to the direction of [1120]α (c) and [0001]α (d) in as-cast Mg92Zn4Y3Gd1 alloy
14H-LPSO phase with EB//[0001] for the lamellar phase in Mg92Zn4Y3Gd1 alloy. The experimental results show that, with high content of Zn (4 at%) and Zn/RE=1, 14H-LPSO structure phase can be fabricated by the conventional solidification method in Mg92Zn4Y4 (Mg-9.2Zn-12.6Y, wt%) alloy and Mg92Zn4Y3Gd1 (Mg-9.0Zn-9.2Y-5.4Gd, wt%) alloy, with the chemical formula corresponding to Mg12ZnY and Mg12Zn(Y,Gd), respectively. Adding a small amount of Gd can refine the matrix and the eutectic secondary phases, promoting the formation of long period stacking order structure phase in the alloy. According to the solidification process analysis, the maximum solubility of Zn, Y and Gd in Mg are 6.2 wt%, 12.4 wt% and 23.5 wt%, respectively[12]. That is to say, the solid solubility of Gd in Mg is about 2 times than that of Y in Mg. However, in the solidification of Mg-Zn-Y(-Gd) alloy, solid solubility of Zn, Y and Gd in Mg are very low. As a result, on one hand, solute segregation can cause a concentration fluctuation during the solidification process; on the other hand, the addition of heavy rare earth element Gd can enhance the nucleation sites so as to promote the nucleation and growth of LPSO structure phase. Eventually, the volume fraction of 14H-LPSO phase is higher in Mg92Zn4Y3Gd1 alloy. From Fig.5, it can be seen that the phase transformation temperature of LPSO is ~510 °C, and the formation of LPSO phase at this temperature can divide -Mg dendrites, so as to refine the matrix phase. Comparing a large number of polycrystalline magnesium on the matrix in as-cast Mg92Zn4Y4 alloy, precipitates cannot be found on the matrix in as-cast Mg92Zn4Y3Gd1 alloy. Analysis
shows that, when Gd promotes LPSO nucleation, the precipitates involve the formation of LPSO phase. Because Y, Gd and Zn can form YZn clusters and GdZn clusters, the sites of Y atoms will be substituted by part of Gd atoms and the instability of LPSO structure arrangement will be increased, so as to make the finer LPSO phase in as-cast Mg92Zn4Y3Gd1 alloy. The atomic ratio of Gd/(Y+Gd) in LPSO phase is about 2.15/(5.25+2.15)=0.2905, namely that 29% Y in Mg12ZnY phase is replaced by Gd to form Mg12Zn(Y,Gd) 14H-LPSO structure phase. 2.2 Thermal performance Fig.7 shows the temperature dependence of thermal conductivity of the as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd alloys at 25~225 °C. At room temperature, the thermal diffusivity values of Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys are 30.261×10-6 m2·s-1 and 26.097×10-6 m2·s-1, and the thermal conductivities of them are 99.233 W·(m·K)-1 and 88.639 W·(m·K)-1, respectively. The thermal conductivities of reference alloys Mg96Zn2Y2 and Mg97Zn1Y2 are 53 W·(m·K)-1 and 59 W·(m·K)-1, respectively (in Fig.7, expressed by dotted lines)[13]. The thermal conductivity of pure Mg is about 156 W·(m·K)-1[14]. Along with the test temperature, the thermal conductivities of all as-cast samples rise slowly. Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys exhibit good thermal properties. There exists a good linear relationship between the test temperature and the thermal conductivity, [W·(m·K)-1], in as-cast Mg92Zn4Y4 alloy, and the fitting formula is as follows: =0.12299(T 273) 99.233 (298 K T 498 K) (1)
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Thermal Conductivity/W ·(m·K)
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Shi Fei et al. / Rare Metal Materials and Engineering, 2015, 44(7): 1617-1622
120
Fig.7
100
Mg92Zn4Y4
90
Mg97Zn1Y2
Stress/MPa
110 Mg92Zn4Y3Gd1 Mg96Zn2Y2
80 70 60 50
50
100 150 Temperature/°C
200
Thermal conductivity curves of as-cast Mg92Zn4Y4 and
Fig.8
Mg92Zn4Y3Gd1 alloys
400 350 300 250 200 150 100 50 0
Mg92Zn4Y4 Mg92Zn4Y3Gd1
0
2
4
6
8 10 Strain/%
12
14
16
Compressive stress-strain curves of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys
Thermal conductivity of metal materials mainly depends on the lattice vibration and the free electron movement. Phonon is the energy quantum of harmonic oscillators in the lattice vibration, and the movement of free electrons can have an effect on the thermal conductivity[15]. Generally speaking, the thermal conductivity of alloy is lower than that of pure metal, because more solute elements in alloy will obstacle the movement of free electrons and phonons, leading to reduce the mobility of free electrons and phonons. Contribution of electrons to heat capacity is much smaller than the phonon contribution to the heat capacity in the range of 298~523 K. Thus the influence of phonon on the thermal properties can only be considered in this paper. However, there are many factors that can affect the thermal conductivity of alloy, such as the solidification microstructure and phase distribution, solute element, grain boundary scattering and dislocation scattering. In Mg97Zn1Y2 and Mg96Zn2Y2 alloys, the maximum volume fraction of W phase is only ~2%, and the LPSO phases are ~26% and ~25%[13]. The lower thermal conductivity is mainly caused by fewer W phase and LPSO grain boundary scattering. Thus, the eutectic W phase with developed and net-like continuous distribution in Mg92Zn4Y4 and Mg92Zn4Y3Gd1 can promote the long distance movement of phonons. Comparing Mg92Zn4Y4 alloy, the grain size becomes finer in as-cast Mg92Zn4Y3Gd1 alloy, so the number of grain boundary in unit volume will increase greatly. Therefore the heat transfer of phonons can be hindered significantly, forming a greater degree of grain boundary scattering and reducing the thermal conductivity.
strength are about 201 and 304 MPa, respectively. Affected by more dispersed distribution of LPSO phase, Mg92Zn4Y3Gd1 alloy obtains much higher compressive yield strength, which is about 215 MPa. With the increase of strain, the strength of the alloy increases more significantly, and its compressive strength reaches 374 MPa. As-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys exhibit good mechanical property due to the polycrystalline (Mg), fine grain Mg-Y and 14H-LPSO phases. Among them, the special stacking mode of LPSO can cause the lattice distortion and enhance the elastic energy, so as to improve the potential energy of the inhibition dislocation movement. During plastic deformation, LPSO phase will hinder the dislocation motion of Mg matrix. With the increase of LPSO phase volume fraction, the amount of obstacles to dislocation motion also will increase; LPSO structure itself is a kind of stacking fault structure, and it has many dislocation and slip. Under loading, this kind of dislocation slip needs to overcome a greater force in LPSO phase. In addition, grain refinement can not only improve the yield strength of magnesium alloy, but also form a large number of grain boundary, leading the energy increase of dislocation around the grain boundary, which play the role of grain boundary strengthening. Eventually, as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys have outstanding compressive strength and compressive plasticity, owing to the coupling effect of the second phase dispersion strengthening, fine-grain strengthening and grain boundary strengthening.
2.3 Compressive property The compressive stress-strain curves of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys at room temperature are shown in Fig.8. Both of them exhibit excellent plasticity, in which the compression rate of Mg92Zn4Y4 alloy is about 12.4%, while Mg92Zn4Y3Gd1 alloy compression rate increases to 15.5%. This result is due to the addition of Gd, which can significantly refine the matrix. On the strength of as-cast Mg92Zn4Y4 alloy, compressive yield strength and compressive
1) In as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys, 14H-LPSO phase can be prepared directly by the conventional solidification method. 2) Adding of a small amount of Gd element can improve the nucleation rate and growth of LPSO phase. The volume fraction of LPSO phase increases from 12.1% to 30.4%, and about 29% Gd atoms replace Y atoms to form Mg12Zn(Y, Gd). 3) Due to the grain boundary scattering and dislocation scattering, the transport of phonons in as-cast Mg92Zn4Y3Gd1
3
Conclusions
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alloy can be hampered greatly, leading to its lower thermal conductivity than that of Mg92Zn4Y4 alloy. 4) Good mechanical properties of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 magnesium alloys are attributed to polycrystalline Mg, particle reinforced Mg-RE phase and 14H-LPSO phase.
6 Luo Z P, Zhang S Q. Journal of Materials Science Letters[J],
References
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ARTICLE
Microstructures and Properties of As-Cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 Alloys with LPSO Phase Shi Fei1,2, 1
Wang Chunqing1,2,
Guo Xuefeng3
Harbin Institute of Technology, Weihai 264209, China;
2
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of
3
Technology, Harbin 150001, China; Henan Polytechnic University, Jiaozuo 454000, China
Abstract: Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys with long period stacking ordered (LPSO) structure phases were prepared by conventional solidification process. By OM, SEM, EDS, XRD and TEM analysis the phases and 14H-LPSO structures of the two alloys were characterized. The results show that as-cast Mg-alloy with the atomic ratio of Zn/RE = 1 will lead to LPSO phase; adding of Gd element to Mg92Zn4Y4 alloy can facilitate the formation of LPSO phase, and its volume fraction increases from 12.1% to 30.4%; Mg dendrites are split and refined during the precipitation of LPSO phase formed at high temperature, resulting in that the average grain size of -Mg decreases from 50 µm to 10 µm; the solidification microstructure of as-cast Mg92Zn4Y4 alloy is -Mg solid solution + Mg12ZnY + Mg3Zn3Y2 + Mg-Y; In Mg92Zn4Y3Gd1 alloy, the as-cast microstructure is confirmed to be composed mainly of -Mg solid solution, Mg12Zn(Y, Gd) and Mg3Zn3(Y, Gd)2; at room temperature, the compression ratio and the thermal conductivity of Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys are 12.4% and 15.5%, and 99.233 W·(m·K)-1 and 88.639 W·(m·K)-1, respectively. Key words: as-cast Mg-Zn-Y(-Gd) alloy; long period stacking ordered structure; rare earth; thermal conductivity; compression plasticity
Magnesium alloy has great potential applications in the fields of aerospace, marine engineering, precision machining, biological equipment, etc, due to its high strength, good electromagnetic shielding property, thermal conductivity, regeneration and biological compatibility[1-3]. The microstructure of Mg-Zn binary alloy can be refined by adding rare elements; meanwhile the properties of magnesium alloys and antioxidant capacity in smelting process can also be improved. The key point is that different reinforced phases can be obtained by adjusting alloy composition, such as the quasicrystal phase and long-period stacking ordered (LPSO) structure phase. It is believed that quasicrystal reinforced magnesium alloy which exhibits good mechanical properties can be obtained by adjusting the ratio of Zn/Y to be 6 [4,5]. In recent years, many efforts have been focused on studying the LPSO structure phase. Magnesium alloy with LPSO structure
phase was initially found by Luo Z P and marked by the Х-phase[6]. In 2001, Kawamura Y[7] prepared Mg97Zn1Y2 alloy with LPSO by a rapidly-solidified powder/metallurgy method (RSP/M). The chemical formula of LPSO structure phase is Mg12ZnY (at%) and it has 610 MPa yield strength and 5% elongation at room temperature. Currently, magnesium alloy with LPSO phase can be prepared by RS P/M, heat treatment and the conventional casting methods. However, it is generally believed that RE/Zn atomic ratio of 2 is the ideal chemical composition to obtain LPSO phase[8], for the reason that the magnesium alloy can acquire good mechanical properties due to its special crystal structure. The cost of manufacturing will be undoubtedly increased by a rapid solidification method or heat treatment process for preparing LPSO phase reinforced magnesium alloy. In recent years, the researchers have found that LPSO-magnesium alloy
Received date: July 25, 2014 Foundation item: National Natural Science Foundation of China (51374084/E041607) Corresponding author: Wang Chunqing, Ph. D., Professor, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China, Tel: 0086-451-86418725, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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can be obtained by a conventional casting method, while it is usually believed that the rare earth content in the alloy must be higher (or significantly higher) than that of Zn [9-11]. This investigation chose Mg92Zn4Y4 and Mg92Zn4Y3Gd1 (at%) alloys, namely the RE/Zn atomic ratio of 1, and used Gd to replace Y. The purpose is to analyze whether the low RE/Zn ratio or the as-cast condition can acquire the LPSO phase in magnesium alloy, and to investigate the effect of Gd on phase formation, the thermal conductivity and compression plasticity. Moreover, thermal conductivity and compression mechanical properties were tested so as to determine the feasibility of LPSO self-reinforced magnesium base composites as a power electronic packaging shell and a heat sink carrier.
1
Experiment
Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys were prepared from the high purity Mg (99.9wt%), analytical purity Zn (99.9wt%), the master alloy Mg-30wt% Y and Mg-30wt% Gd by melting in graphite crucibles in a 5 kW electric resistance furnace. During melting process, the mixed salt of NaCl and KCl was used to be a protective agent. After the solution treatment, the molten alloy was poured in the metal mould at 830 °C. Standard metallographic specimens were prepared and etched with 3% nitric acid and 97% alcohol. The microstructure of as-cast alloys was observed by OLYMPUS LX 50 optical microscope (OM) and AMREY-100B scanning electron microscope (SEM). And the chemical composition of each phase and its structure were analyzed by JSM-6700F cold field emission scanning electron microscope and JEM-2100 transmission electron microscopy (TEM), respectively. Samples for TEM observation were mechanically polished to 60 μm, and then further thinned by a twin-jet electro polishing method and an ion thinning method. Twin-jet treatment was performed in a solution of 10% nitric acid and 90% alcohol at –20 °C. The phase transformation process of sample was tested by NETZSCH STA 449F3 type differential scanning calorimeter (DSC). High pure Ar was selected as a protective atmosphere during the experiment (Ar gas flow rate was 60 mL/min), and the heating rate was 10 °C/min from RT to 700 °C. The thermal diffusivity and the thermal conductivity of samples were measured between 25 °C and 225 °C with the laser flash thermal conductivity instrument (NETZSCH LFA447). The testing samples with the size of 10 mm×10 mm × 2 mm were cut by electro-discharge machining, and alumina was selected for comparison. At room temperature, the compressive property of samples was tested on INSTRON 2382 universal material testing machine with a constant strain rate of 5.74×10-4 s-1. And the sample size was Φ8 mm × 12 mm.
2
Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys. It is commonly demonstrated in the two alloys that there are white block phase, needle phase and eutectic organization. The average grain size of Mg92Zn4Y4 alloy is about 50 μm, and the dendrite is relatively coarse. Compared with Mg92Zn4Y4 alloy, the average grain size of Mg92Zn4Y3Gd1 is less than 10 μm, which is smaller by an order of magnitude, and there are only a small amount of dendrite with 20~30 μm. The SEM images of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys are shown in Fig.2. Combined with the analysis of SEM, EDS and XRD, the micro area composition of block phase and dendrites in as-cast Mg92Zn4Y4 alloy is about 98.46 at%Mg, 0.79 at%Zn and 0.75 at%Y, confirming that it is -Mg solid solution. The composition of white eutectic secondary phase at grain boundary is about 64.26 at%Mg, 21.81 at%Zn and 13.94 at%Y (that is 36.96 wt%Mg, 33.73 wt%Zn and 29.31 wt%Y). The needle phase composition is 87.12 at%Mg, 6.23 at%Zn and 6.65 at%Y. This component is similar to Mg12ZnY which is the chemical formula of LPSO phase; presumably this phase is LPSO in the tested alloy, as shown in Fig.2a and Fig.2b. In addition, there are some particle reinforced phases with less than 1 μm in as-cast Mg92Zn4Y4 alloy, and result of EDS analysis of this phase is Mg-Y with the atomic ratio of Mg/Y to be 3. In as-cast Mg92Zn4Y3Gd1 alloy, matrix phase contains 97.53 at%Mg, 1.38 at%Zn, 0.81 at%Y and 0.28 at%Gd; thus it can be identified as -Mg solid solution as well. Eutectic phase of the alloy includes 63.35 at%Mg, 23.10 at%Zn, 9.78 at%Y and 3.78 at%Gd (that is 34.13 wt%Mg, 33.46 wt%Zn, 19.26 wt%Y and 13.16 wt%Gd). The composition of the slender needle phase is 84.87 at%Mg, 7.72 at%Zn, 5.25 at%Y and 2.15 at%Gd. The atomic percentage of (Y+Gd) is 7.4 at%, chemical formula for Mg12Zn(Y,Gd). The volume fraction of LPSO phase in the two alloys is about 12.1% and 30.4%, respectively. The maximum width of the LPSO phase in Mg92Zn4Y4 alloy is about 10 μm, while the width of LPSO phase in Mg92Zn4Y3Gd1 alloy is less than 2 μm. For Mg92Zn4Y4 alloy, -Mg matrix and the eutectic phase can be refined by adding Gd, and the LPSO phase as well. The EDS patterns of Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys are shown in Fig.3, corresponding position A, B, C and D in Fig.2b and Fig.2d. In Fig.4, XRD patterns show that -Mg, W-Mg3Zn3Y2 and 14H-LPSO are main phases in as-cast Mg92Zn4Y4 alloy. a
20 μm
Results and Discussion
2.1 Microstructure characteristics Fig.1 shows the optical microstructures of the as-cast
b
Fig.1
Optical microstructures of as-cast Mg alloys: (a) Mg92Zn4Y4 and (b) Mg92Zn4Y3Gd1
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b
A
B 5 μm
50 μm c
d
C D 50 μm Fig.2
5 μm
SEM backscatter electron images of as-cast Mg-Zn-Y(-Gd) alloys: (a, b) Mg92Zn4Y4 and (c, d) Mg92Zn4Y3Gd1
Intensity/a.u.
Mg
a
Point A Element Mg Zn Y
Zn
Mg
Intensity/a.u.
8
12
4
4
Content/at% 97.53 1.38 0.81 0.28
8
12
16 0
4
Energy/keV
16 d
Element Mg Zn Y Gd
Mg Zn Gd Y
12
8
Point D
c
Element Mg Zn Y Gd
Mg Gd Zn
Zn
16 0
Point C
Fig.3
Content/at% 87.12 6.23 6.65
Y
4
0
Element Mg Zn Y
Zn
Y
0
b
Point B
Content/at% 98.46 0.79 0.75
Content/at% 84.87 7.72 5.25 2.15
8
12
16
Energy/keV
EDS analysis of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys: (a) -Mg solid solution and (b) Mg12ZnY phase in Fig.2b for Mg92Zn4Y4
alloy;
-Mg
(c)
solid
solution
and
(d) Mg12Zn(Y, Gd) phase in Fig.2d for Mg92Zn4Y3Gd1 alloy
-Mg W-Mg3Zn3Y2 14H-LPSO
Intensity/a.u.
a
the action of Gd atoms in Mg92Zn4Y3Gd1 alloy, the diffraction peaks show a slightly offset to the left. Fig.5 shows DSC curves of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys during heating process. With temperature increasing, there are 3 endothermic peaks in the two kinds of alloys, which are located at 511, 523 and 579 °C for Mg0.2Zn4Y4 alloy, while 510, 518 and 580 °C for Mg0.2Zn4Y3Gd1 alloy. Inferred by the phase diagram analysis, 3 endothermic peaks correspond to 14H-LPSO, W-Mg3Zn3Y2 and matrix phase transition peaks, respectively. In Mg92Zn4Y4 alloy, melting temperature of matrix solution starts at 552 °C. While in the Mg92Zn4Y3Gd1 alloy, the beginning of melting temperature of matrix phase increases to 561 °C due to Gd dissolved into -Mg solid solution. In addition, the reaction peaks of 14H-LPSO and W phases are close, and the W peak of Mg92Zn4Y3Gd1 alloy is weaker than that of Mg92Zn4Y4 alloy. It suggests that the number of W phase in Mg92Zn4Y3Gd1 alloy is less than that in Mg92Zn4Y4 alloy. This results are consistent with the analysis of OM and SEM, namely the addition of Gd will promote adequately the reaction of L + W→14H-LPSO, and at the same time in the consumption of eutectic W phase more LPSO phase, is generated. The phases and structures of as-cast Mg 92 Zn 4 Y 4 and Mg92Zn4Y3Gd1 alloys were further investigated by TEM as presented in Fig.6. In Mg92Zn4Y4 alloy, there are a large number of black precipitation phases with the size less than 20 nm on matrix. The SAED pattern shows a series of diffraction ring and the calibration for polycrystalline magnesium, as shown in Fig.6a. The bright-field (BF) image and corresponding SAED of LPSO phase are shown in Fig.6b with electron beam (EB) paralleling to the direction of [11 2 0]. The lighter diffraction spot is calibrated for -Mg. It has 13 equal interval diffraction spots between (0002) and (0002) 14H, which indicates that the lamellar phase is 14H-LPSO structure phase with the lattice constants of a = 0.3253 nm and c = 3.6946 nm. Fig.6c shows the BF TEM and corresponding SAED pattern of LPSO phase in Mg92Zn4Y3Gd1 alloy. It can be seen that the LPSO has 14H-type structure, and the lattice constants are essentially the same as 14H-LPSO phase in Mg92Zn4Y4 alloy. Moreover, Fig.6d shows the TEM image and SAED pattern of
0.0
20
Fig.4
30
40 50 2θ/(°)
60
Heat Flow/mW·mg
-1
b
70
XRD patterns of as-cast Mg92Zn4Y4 (a) and Mg92Zn4Y3Gd1 (b) alloys
Because the volume fraction of Mg-Y phase is relatively few, the diffraction peaks is rather small. Judging from the type of the XRD diffraction peaks, the distribution is consistent with that of Mg92Zn4Y3Gd1 and Mg92Zn4Y4. However, because of
–-0.4 –-0.8
502 °C
–-2.0
501 °C 552 °C
Mg92Zn4Y4 Mg92Zn4Y3Gd1 511 °C 510 °C
–-1.2 –-1.6
Endo.
a
Phase Mg92Zn4Y4 LPSO 511 °C W 523 °C Matrix 579 °C
300
400
561 °C 523 °C 518 °C
579 °C
Mg92Zn4Y3Gd1 510 °C 580 °C 518 °C 580 °C
500
600
700
Temperature/°C Fig.5
DSC curves of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys
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a
α
b
α
200 nm
100 nm EB//[1120]α c
d
α
α
α
EB//[1120]α Fig.6
α
100 nm
50 nm EB//[0001]α
_
BF-TEM images and corresponding SAED patterns of the precipitations (a) and 14H-LPSO structure with EB taken along [1120]α (b) in _
as-cast Mg92Zn4Y4 alloy; 14H-LPSO structure with EB paralleling to the direction of [1120]α (c) and [0001]α (d) in as-cast Mg92Zn4Y3Gd1 alloy
14H-LPSO phase with EB//[0001] for the lamellar phase in Mg92Zn4Y3Gd1 alloy. The experimental results show that, with high content of Zn (4 at%) and Zn/RE=1, 14H-LPSO structure phase can be fabricated by the conventional solidification method in Mg92Zn4Y4 (Mg-9.2Zn-12.6Y, wt%) alloy and Mg92Zn4Y3Gd1 (Mg-9.0Zn-9.2Y-5.4Gd, wt%) alloy, with the chemical formula corresponding to Mg12ZnY and Mg12Zn(Y,Gd), respectively. Adding a small amount of Gd can refine the matrix and the eutectic secondary phases, promoting the formation of long period stacking order structure phase in the alloy. According to the solidification process analysis, the maximum solubility of Zn, Y and Gd in Mg are 6.2 wt%, 12.4 wt% and 23.5 wt%, respectively[12]. That is to say, the solid solubility of Gd in Mg is about 2 times than that of Y in Mg. However, in the solidification of Mg-Zn-Y(-Gd) alloy, solid solubility of Zn, Y and Gd in Mg are very low. As a result, on one hand, solute segregation can cause a concentration fluctuation during the solidification process; on the other hand, the addition of heavy rare earth element Gd can enhance the nucleation sites so as to promote the nucleation and growth of LPSO structure phase. Eventually, the volume fraction of 14H-LPSO phase is higher in Mg92Zn4Y3Gd1 alloy. From Fig.5, it can be seen that the phase transformation temperature of LPSO is ~510 °C, and the formation of LPSO phase at this temperature can divide -Mg dendrites, so as to refine the matrix phase. Comparing a large number of polycrystalline magnesium on the matrix in as-cast Mg92Zn4Y4 alloy, precipitates cannot be found on the matrix in as-cast Mg92Zn4Y3Gd1 alloy. Analysis
shows that, when Gd promotes LPSO nucleation, the precipitates involve the formation of LPSO phase. Because Y, Gd and Zn can form YZn clusters and GdZn clusters, the sites of Y atoms will be substituted by part of Gd atoms and the instability of LPSO structure arrangement will be increased, so as to make the finer LPSO phase in as-cast Mg92Zn4Y3Gd1 alloy. The atomic ratio of Gd/(Y+Gd) in LPSO phase is about 2.15/(5.25+2.15)=0.2905, namely that 29% Y in Mg12ZnY phase is replaced by Gd to form Mg12Zn(Y,Gd) 14H-LPSO structure phase. 2.2 Thermal performance Fig.7 shows the temperature dependence of thermal conductivity of the as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd alloys at 25~225 °C. At room temperature, the thermal diffusivity values of Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys are 30.261×10-6 m2·s-1 and 26.097×10-6 m2·s-1, and the thermal conductivities of them are 99.233 W·(m·K)-1 and 88.639 W·(m·K)-1, respectively. The thermal conductivities of reference alloys Mg96Zn2Y2 and Mg97Zn1Y2 are 53 W·(m·K)-1 and 59 W·(m·K)-1, respectively (in Fig.7, expressed by dotted lines)[13]. The thermal conductivity of pure Mg is about 156 W·(m·K)-1[14]. Along with the test temperature, the thermal conductivities of all as-cast samples rise slowly. Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys exhibit good thermal properties. There exists a good linear relationship between the test temperature and the thermal conductivity, [W·(m·K)-1], in as-cast Mg92Zn4Y4 alloy, and the fitting formula is as follows: =0.12299(T 273) 99.233 (298 K T 498 K) (1)
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Thermal Conductivity/W ·(m·K)
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120
Fig.7
100
Mg92Zn4Y4
90
Mg97Zn1Y2
Stress/MPa
110 Mg92Zn4Y3Gd1 Mg96Zn2Y2
80 70 60 50
50
100 150 Temperature/°C
200
Thermal conductivity curves of as-cast Mg92Zn4Y4 and
Fig.8
Mg92Zn4Y3Gd1 alloys
400 350 300 250 200 150 100 50 0
Mg92Zn4Y4 Mg92Zn4Y3Gd1
0
2
4
6
8 10 Strain/%
12
14
16
Compressive stress-strain curves of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys
Thermal conductivity of metal materials mainly depends on the lattice vibration and the free electron movement. Phonon is the energy quantum of harmonic oscillators in the lattice vibration, and the movement of free electrons can have an effect on the thermal conductivity[15]. Generally speaking, the thermal conductivity of alloy is lower than that of pure metal, because more solute elements in alloy will obstacle the movement of free electrons and phonons, leading to reduce the mobility of free electrons and phonons. Contribution of electrons to heat capacity is much smaller than the phonon contribution to the heat capacity in the range of 298~523 K. Thus the influence of phonon on the thermal properties can only be considered in this paper. However, there are many factors that can affect the thermal conductivity of alloy, such as the solidification microstructure and phase distribution, solute element, grain boundary scattering and dislocation scattering. In Mg97Zn1Y2 and Mg96Zn2Y2 alloys, the maximum volume fraction of W phase is only ~2%, and the LPSO phases are ~26% and ~25%[13]. The lower thermal conductivity is mainly caused by fewer W phase and LPSO grain boundary scattering. Thus, the eutectic W phase with developed and net-like continuous distribution in Mg92Zn4Y4 and Mg92Zn4Y3Gd1 can promote the long distance movement of phonons. Comparing Mg92Zn4Y4 alloy, the grain size becomes finer in as-cast Mg92Zn4Y3Gd1 alloy, so the number of grain boundary in unit volume will increase greatly. Therefore the heat transfer of phonons can be hindered significantly, forming a greater degree of grain boundary scattering and reducing the thermal conductivity.
strength are about 201 and 304 MPa, respectively. Affected by more dispersed distribution of LPSO phase, Mg92Zn4Y3Gd1 alloy obtains much higher compressive yield strength, which is about 215 MPa. With the increase of strain, the strength of the alloy increases more significantly, and its compressive strength reaches 374 MPa. As-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys exhibit good mechanical property due to the polycrystalline (Mg), fine grain Mg-Y and 14H-LPSO phases. Among them, the special stacking mode of LPSO can cause the lattice distortion and enhance the elastic energy, so as to improve the potential energy of the inhibition dislocation movement. During plastic deformation, LPSO phase will hinder the dislocation motion of Mg matrix. With the increase of LPSO phase volume fraction, the amount of obstacles to dislocation motion also will increase; LPSO structure itself is a kind of stacking fault structure, and it has many dislocation and slip. Under loading, this kind of dislocation slip needs to overcome a greater force in LPSO phase. In addition, grain refinement can not only improve the yield strength of magnesium alloy, but also form a large number of grain boundary, leading the energy increase of dislocation around the grain boundary, which play the role of grain boundary strengthening. Eventually, as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys have outstanding compressive strength and compressive plasticity, owing to the coupling effect of the second phase dispersion strengthening, fine-grain strengthening and grain boundary strengthening.
2.3 Compressive property The compressive stress-strain curves of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys at room temperature are shown in Fig.8. Both of them exhibit excellent plasticity, in which the compression rate of Mg92Zn4Y4 alloy is about 12.4%, while Mg92Zn4Y3Gd1 alloy compression rate increases to 15.5%. This result is due to the addition of Gd, which can significantly refine the matrix. On the strength of as-cast Mg92Zn4Y4 alloy, compressive yield strength and compressive
1) In as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys, 14H-LPSO phase can be prepared directly by the conventional solidification method. 2) Adding of a small amount of Gd element can improve the nucleation rate and growth of LPSO phase. The volume fraction of LPSO phase increases from 12.1% to 30.4%, and about 29% Gd atoms replace Y atoms to form Mg12Zn(Y, Gd). 3) Due to the grain boundary scattering and dislocation scattering, the transport of phonons in as-cast Mg92Zn4Y3Gd1
3
Conclusions
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alloy can be hampered greatly, leading to its lower thermal conductivity than that of Mg92Zn4Y4 alloy. 4) Good mechanical properties of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 magnesium alloys are attributed to polycrystalline Mg, particle reinforced Mg-RE phase and 14H-LPSO phase.
6 Luo Z P, Zhang S Q. Journal of Materials Science Letters[J],
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