Influence of aging on microstructure and properties of ZK60 magnesium alloy
ZHANG Zhi-min(张治民)1,2, ZHANG Xing(张 星)1, YANG Ya-qin(杨亚琴)1
1. School of Materials Science and Engineering, North University of China, Taiyuan 030051, China;
2.Engineering Center for Precision Forming of Shanxi Province, Taiyuan 030051, China
Received 20 April 2006; accepted 30 June 2006
Abstract: The different aging process was investigated for ZK60 magnesium alloy to get the ideal synthetic properties. The results show that the values of strength, hardness and plasticity of ZK60 magnesium alloy increase at first and then decrease with increasing aging temperature, the suitable aging temperature of ZK60 alloy is from 160 to 180 ℃. At the meantime, the microstructures of appear mesh texture under high ageing temperature, this is the main reason why the mechanical properties decrease.
Key words: ZK60 magnesium alloy; aging; mechanical properties
1 Introduction
Magnesium alloys are the lightest metallic structural materials, their strength-to-mass is higher than aluminum alloy and steel, so they are very attractive in such applications as automobile, railway and aerospace industries in order to decrease mass[1,2]. However, as a consequence of hexagonal crystal structure, they generally exhibit only lower ductility and yield strength at ambient temperatures. The ductility of magnesium alloy limits their applications. To date, there has been attempt to improve the room temperature mechanical properties of magnesium alloys through the use of equal-channel angular pressing (ECAP)[3], rapidly solidified (RS)[4], and so on.
The mechanical properties of alloys can be improved by solution treatment and the aging afterwards[5,6]. In order to get high performance magnesium alloy, the microstructure and mechanical properties in aging process of magnesium alloys has been investigated in this paper. Experiments were conducted to evaluate the potential for improving the mechanical properties of ZK60 alloy at room temperature by aging treatment.
2 Experimental
ZK60 alloys were chosen for investigation, the state is as-cast, and the chemical compositions are listed in Table 1.
Table 1 Chemical compositions of ZK60 alloy (mass fraction, %)
The specimens were axially cut out from casting bars. Then the solution treatment was done, the temperature of solution was 500 ℃, all specimens were held for 4 h then air-cooled. Aging process was processed afterwards, the aging temperatures were 110, 140，170，200 and 230 ℃, and the time was 12 h. Then the mechanical properties was measured.
Tensile specimens were machined with the diameter of 5mm. The test was performed with a WDW-E100D material testing machine at room temperature, and the tensile speed is 3 mm/s. Then the hardness test was carried out with the HB 3 000 Brinell hardness machine, the diameter of the head of delivery is 5 mm, the load is 2.5 kN, and the loading time is 12 s. The micrograph was observed with the Image A1 metallographic microscope, and the corrosive agent was made up of 2% oxalic acid and water.
3 Results and discussion
3.1 Mechanical properties
Fig.1 shows the relationship between mechanical properties and aging temperatures.
3.1.1 Tensile properties
From Figs.1(a-c), we can see that ZK60 magnesium alloy variation trend in tensile strength, yield strength and reduction of area, namely, they increase firstly and then drop with the rising aging temperature. The mechanical properties of aged alloys are higher in the range of 140-200 ℃, and the peak value is at about 170 ℃.
This curves shows that ZK60 alloys have the better synthetic mechanical properties in the range of aging temperature from 140 to 200 ℃, namely, higher strength and better plasticity[7].
3.1.2 Hardness
Fig.1(d) shows that the hardness of ZK60 magnesium alloy has a rising tendency with the increase of aging temperature. The hardness increases slowly at the lower aging temperature from 110 to 140 ℃, and increases rapidly when the temperature is up to 170 ℃, it shows the character of the age-hardening[8].
However, the hardness decreases with the temperature further increasing. The precipitated phase is little under the lower aging temperature; the second-phase increases with increasing temperature, so the hardness reaches the maximum[9]. The precipitated phase will grow up under higher temperature(Fig.2), and the hardness decreases with the appearance of over aging.
3.2 Evolution of microstructure
The microstructure of as cast ZK60 appears as coarse mesh and the eutectic (Fig.3), and mainly distributes along the grain boundary. The eutectic microstructure dissolves on the whole by the solution treatment. Firstly, the precipitated phase along the grain boundary discontinuous by the aging, the amount of precipitation phase is less at low temperature, and the shape displays slice. But the amount of precipitated phase increases with increasing aging temperature, at the same time, the second-phase transfers from the discrete precipitation to the continuous precipitation[10], and the microstructure changes into mesh texture at last. That is to say, the microstructure is mesh structure under the as-cast and ageing at higher temperature.
The microstructure of the ZK60 alloy changes into coarse block and mesh texture, and the value of mecha-
Fig.1 Relationship between mechamical properties and aging temperature: (a) Yield strength; (b) Tensile strength; (c) Reduction of area; (d) Hardness
Fig.2 Micrographs of ZK60 alloy at different aging temperatures: (a) 110 ℃; (b) 140 ℃; (c) 170 ℃; (d) 230 ℃
Fig.3 Initial as-cast micrograph of ZK60 alloy
nical parameter decrease above 200 ℃[11]. So it can be indicated that the mesh structure make the alloy mechanical property worse.
References
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(Edited by LONG Huai-zhong)
Foundation item: Project (2004CCA04600) by the Early-stage Special Research on Key Basic Research Projects of China; project supported by the High-school Outsanding Innovation Talented Person of Shanxi Province, China
Corresponding author: ZHANG Zhi-min; Tel:+86-351-3922081; E-mail:zhangzhimin@nuc.edu.cn
