Microstructure and High Damping Properties of Mg-Zn-Y Alloys Containing LPSO Phase and I Phase

Rare Metal Materials and Engineering Volume 44, Issue 11, November 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(11): 2651-2655.

ARTICLE

Microstructure and High Damping Properties of Mg-Zn-Y Alloys Containing LPSO Phase and I Phase Wan Diqing1,2, 1

Li junjie1,

Yu Tian2

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China;

2

East China Jiaotong

University, Nanchang 330013, China

Abstract: A high damping Mg-Zn-Y alloy containing LPSO phase and I phase was investigated. The microstructure of a high damping Mg80Y4Zn16 (at%) alloy was examined by optical microscopy (OM) and SEM. Furthermore, the differential thermal analysis (DTA) was used to analyze its phase transformation. Microstructure evolution and damping properties under the conditions of the different solid solution time (3, 6, 9 h) were studied. The results show that the main phase component of the Mg80Y4Zn16 alloy is α-Mg, Mg12ZnY (X phase), Mg3Zn6Y (I phase) and Mg10ZnY2 phase (LPSO phase), in which I phase and LPSO phase coexistence is firstly reported. The average damping value of the alloy is 0.03, indicating that it is a high damping alloy. The damping is strain amplitude dependent. The high damping mechanism was also discussed. Key words: Mg-Zn-Y alloy; I phase; LPSO; high damping

Magnesium alloys are one of the lightest structural materials with high specific strength, high specific elastic modulus and excellent damping property, which can be used to eliminate unwanted noise and vibration, significantly reducing the energy consumption of vehicles, improving vehicle structure and stability of the vehicle and equipment. Therefore, magnesium alloys are remarkably attractive to the applications for various structures in automotive, aerospace industries, and so on. Hence, they are considered as the most promising metals in the 2lst century[1-3]. However, their poor mechanical properties, especially those at the elevated temperature restrict their broader applications. Elements Y and Zn can be added into magnesium alloys to improve their mechanical properties, which gives their good prospects in the aviation, aerospace and civilian transport and other manufacturing fields[4-9]. Mg-Zn-Y alloy system could produce several phases including Mg-Y-Zn compounds, quasicrystal phase and LPSO phase (long period stacking ordered structure)[10,11]. I-phase and LPSO phase are of special crystal structure, which can

play a special role as the secondary phase[12]. For the Mg-Y-Zn alloy, under certain conditions of heat treatment, when the atomic ratio of Zn and Y is in the range from 1:2 to 1:1 , it was possible to obtain a LPSO[13-15]; while the Zn/Y ratio is between 5 and 7, I-phase could be obtained by conventional casting methods[16-19]. However, as the two phase coexistence is limited to alloy composition, previous studies were mostly confined to a single phase reinforced materials, and rare research was referred to two special phase coexistence of Mg-Zn-Y alloy. Therefore, the aim in the present study is to explore the relationship between the microstructure and damping properties of a high damping magnesium alloy containing LPSO phase and I phase.

1

Experiment

The alloy with a nominal composition (at%) in atomic percent of Mg80Y4Zn16 was prepared by melting high purity Mg and Zn and a Mg-25wt%Y master alloy in an electric resistance furnace. Ingots were cast at 700 °C and then a

Received date: November 15, 2014 Foundation item: National Natural Science Foundation of China (51361010); Natural Science Foundation of Jiangxi Province (196 20114BAB216015); the Foundation of Jiangxi Educational Committee (GJJ12320); the State Key Laboratory of Solidification Processing in NWPU (SKLSP201321) Corresponding author: Wan Diqing, Ph. D., Associate Professor, School of Mechanical and Electrical Engineering, East China Jiaotong University, Nanchang 330013, P. R. China, Tel: 0086-791-87046102, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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solution treatment of 325 °C (determined by DTA) was carried out for different time (3, 6, 9 h) followed by quenching in water. Microstructure was examined by a COOLPIX-4500 optical microscope (OM) and a JSM-6360LA scanning electron microscope (SEM) equipped with EDS (energy dispersive X-ray spectrometry) and the crystallography was analyzed by X-ray diffraction (XRD, Bede D1). Damping capacity (Q-1) of as-cast specimens was measured by TAP-8 equipment with a single cantilever. Testing frequency was held at 1 Hz. All the specimens had a rectangular shape of 50 mm×4 mm×1 mm.

2

a

b

c

d

Results and Discussion

50 μm

2 0 –-2 –-4 –-6 –-8 –-10 –-12 –-14 –-16 –-100 0

Fig.1

DTA Onset temperature: 325 °C Onset temperature: 373 °C Onset temperature: 431 °C Onset temperature: 524 °C

Endo.

Heat Flow/mJ·(mg·s)

-1

Phase transformation of as-cast Mg80Y4Zn16 alloy was examined by differential thermal analysis (DTA), as shown in Fig.1. During continuous heating of Mg80Y4Zn16 alloy in DTA, the endothermic peak appears at an onset temperature of 325 °C, corresponding to complete dissolution of Mg12YZn phase; the initial endothermic peak is located at 350 °C, which is responded for the eutectic melting; the endothermic peak appears at an onset temperature of 373 °C, corresponding to dissolution of the Mg10Y2Zn phase; the endothermic peak appears at an onset temperature of 431 °C, corresponding to dissolution of the Mg3Zn6Y1 phase, which is consistent with the result that Farzadfar has reported (430 °C) [20]; the endothermic peak appears at an onset temperature of 531 °C, corresponding to melting of the α-Mg phase. According to the result of DTA analysis and the first phase transition point, the Mg80Y4Zn16 sample solution treatment temperature was selected at 325 °C. Fig.2a shows optical microscopy of as-cast Mg80Y4Zn16 alloy. As is shown, it mainly consists of α-Mg, stripy LPSO phase and a small amount of I phase. Stripy LPSO phase is of independent distribution in α-Mg matrix, which was not reported under the casting condition in previous works[5-8,12-15,21,22]. Fig.2b~2d show the microstructures of Mg80Y4Zn16 alloy after solid solution treatment at 350 °C for different time. As it can be seen, with solid solution treatment time increasing, α-Mg dendrite gradually fuses and spheroidizes, but the mor-

350 °C

100 200 300 400 500 600 700 Temperatrue/°C

DTA curve for as-cast Mg80Y4Zn16 alloy

Fig.2

Microstructures of as-cast Mg80Y4Zn16 alloy (a) and the alloy after 325 °C solid solution treatment for different time: (b) 3 h, (c) 6 h, and (d) 9 h

phology of stripy LPSO does not change significantly. It is very interesting to note that the alloy at cast state (as shown in Fig.2a), with stripy LPSO gathered more obviously, shows irregular and independent distribution of α-Mg dendrite. Generally, the LPSO phase nucleates at grain boundaries where Zn and Y are segregated during solidification, and then grows into the grain interior along the basal plane[21,22]. During the solidification, Zn and Y atoms would be pushed and enriched at the front of solid-liquid interface, which could bring constitutional undercooling and then effectively hinder the growth of α-Mg[23]. Moreover, as a surface-active element to magnesium, it is reported that Y and Zn can reduce the nucleation energy and the critical nucleation radius[24]. Therefore, the solidification procedure of the this alloy is the α-Mg first crystallized and then the I-phase and LPSO are formed, and lastly another LPSO would be generated by the eutectic phase formation. In order to reveal the phase components, the crystallographic analysis was carried out by XRD diffraction. Fig.3 shows the XRD patterns of the Mg80Y4Zn16 alloys, from the XRD patterns we can see that the alloy mainly consists of α-Mg phase, I phase (Mg3Zn6Y), X phase (Mg12ZnY) and LPSO phase (Mg10ZnY2). With the solution treatment time increasing, the main phases (I phase, LPSO phase and α-Mg) change rarely. For the Mg12ZnY phase, its intensity of XRD peak has a slight decrease with the solution time increasing, and at cast state its intensity peak reaches maximum; for I phase, when the solution treatment time is 9 h, its intensity peak reaches maximum; for Mg10ZnY phase, while solution treatment time is 6 h, its intensity peak reaches maximum. It is indicated that partly solution occurs in the alloy during solid solution treatment. Furthermore, it is found that Mg10ZnY2 phase exists under the as-cast and solid solution treatment condition, which is distinct from 18R(Mg10Zn1Y1) phase, which is rarely reported[25].

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930

α-Mg Mg3Zn6Y (I phase) Mg12ZnY (X phase) Mg10ZnY2 (LPSO phase)

620 310

a

structural phase could maintain the good damping capacity of the alloys[32]. a

0

2 1

1890 Intensity/cps

1260 630

α-Mg Mg3Zn6Y (I phase) Mg12ZnY (X phase) Mg10ZnY2 (LPSO phase)

b

α-Mg Mg3Zn6Y (I phase) Mg12ZnY (X phase) Mg10ZnY2 (LPSO phase)

c

3

0 1320 880 440 0 1320

α-Mg Mg3Zn6Y (I phase) Mg12ZnY (X phase) Mg10ZnY2 (LPSO phase)

880 440

C

d

60

b

•A

0 30

•

•

B

90

2θ/(°) XRD patterns of Mg80Y4Zn16 cast alloy (a) and 325 °C and (d) T4-9 h

Fig.4 show the SEM images of Mg80Y4Zn16 alloy solid-solutioned at 325 °C for 3 and 9 h and Table 1 is the corresponding EDS analysis. Combined with EDS analysis and XRD pattern, quasicrystals (I) and long period structure (LPSO) phase coexist in the alloy, which was rarely reported in the previous literatures. What’s more, two kinds of LPSO structures coexist under the as-cast and solid solution treatment conditions, one is stripy-like Mg12ZnY phase (marked as “1” point), and the other is fiber-like Mg10ZnY2 (marked as “2” point), as shown in Table 1. Fig.5 shows the strain amplitude-dependence on damping of Mg80Y4Zn16 alloy under different treatment conditions. It can be found that the damping values increase with the strain amplitude increasing. Obviously the as-cast alloy shows the best damping properties. During solution treatment, the value of Q-1 decreases slightly, which is mainly caused by microstructure evolution as shown in Fig.1. In terms of magnesium alloy, damping properties may be attributed to the interaction between the dislocation and the secondary phase or solute atoms[26]. Moreover, the interface damping is mainly attributed to LPSO and α-Mg between the matrix and the quasicrystal phase, as well as the occurrence of GBS (grain boundary sliding)[27,28]. However, GBS damping was usually suppressed by solute atoms[29]. Generally, the LPSO has a high internal energy stored by the lattice distortion and a change in the electronic structure can effectively ensure the damping[30,31]. Hence, the LPSO

Fig.4

SEM images of as cast Mg80Y4Zn16 alloy after solution treatment at 325 °C for 3 h (a) and 9 h (b)

Table 1 EDS spectrum of as cast Mg80Y4Zn16 alloy after solution treatment at 325 °C for 3 h and 9 h (at%) Position

Mg

Zn

Y

1

11.57

40.74

47.69

2

7.13

45.11

47.77

3

95.41

4.59

0

A

72.27

9.50

18.24

B

92.25

7.56

0.19

C

96.23

3.77

0

0.0304 0.0303

T4-9 h T4-6 h T4-3 h Cast

0.0302 -1

solution treatment for different time: (b) T4-3 h, (c) T4-6 h,

Q

Fig.3

0.0301 0.0300 0.0299 0.0298 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Strain

Fig.5

Strain amplitude-dependence on damping of Mg80Y4Zn16 alloy at different treatment conditions

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Icosahedral quasicrystal phase (I-phase) formed in situ as a secondary phase in the α-Mg matrix during solidification is thermally stable against coarsening owing to the low interfacial energy of the I-phase/matrix interface, providing improved bonding properties at the I-phase/matrix interface[33]. Hence, the I-phase can effectively form strong interfaces with the magnesium matrix[34]. Therefore, the interface between the I-phase and the matrix has some negative influences on interface damping of the Mg-Zn-Y alloy at room temperature, but the formation of high density dislocation plastic zoom among particle/matrix interface possibly due to the mismatch of lattice structure of the particles with the matrix[28] could benefit the overall damping. The damping properties at room temperature are influenced not only by the difference in the interface structures, i.e., coherent or incoherent interfaces, but also by the amount of solute atoms, The damping property of Mg-Zn-Y alloy is lower than that of pure magnesium due to the existence of solute atoms[28]. The content of solute atoms in α-Mg matrix increases due to the solution of precipitates during solid solution treatment. However, after solid solution treatment, the change of the damping properties is relatively slight, and the average value of internal friction is 0.03, which is larger than other magnesium alloys, thus indicating a high damping alloy[28]. In fact, the high damping mechanism is still needed to be further studied.

3

Conclusions

1) The solution treatment temperature of the Mg80Y4Zn16 alloy can be defined at 325 °C. 2) Quasicrystal (I phase) and LPSO phase can coexist in the alloy. 3) The average damping value of Mg-Zn-Y alloy is 0.03, which indicates that it is a high damping alloy. The damping values increase with the strain increasing. The as-cast alloy shows the best damping properties, which could be caused by LPSO phase. After solution treatment, the value of Q-1 decreases slightly.

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