Mechanical properties and microstructures of hot extruded AE42 alloy with addition of zinc

WANG Feng(王 峰), LIU Zheng(刘 正), CHEN Li-jia(陈立佳), YU Bao-yi(于宝义), LIN Li(林 立)

School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110023, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Mechanical properties and microstructures of AE42 magnesium alloy with addition of Zn and subjected to hot extrusion at 370 ℃ and an extrusion ratio of 8?1 were investigated. The results show that for the AE42 alloy, the addition of Zn can obviously improve its elongation as well as the ultimate tensile and yield strengths below 150 ℃. The addition of Zn can refine the microstructure of the AE42 alloy, and result in the precipitation of Mg₁₇Al₁₂ and MgZn₂ phases. Due to the addition of Zn to the AE42 alloy, the amount of Al₁₁RE₃ phase decreases, while the Al₁₁RE₃ phase becomes short rod-shaped from acicular and block, and distributes along the grain boundaries, which will have a stronger effect on the tensile properties of the alloy at elevated temperature. In addition, with the presence of MgZn₂ precipitates, the sliding of grain boundaries is restrained and the strength of the alloy gets enhanced.

Key words: AE42 magnesium alloy; zinc; extrusion; microstructure; mechanical property

1 Introduction

The AE42 alloy, a Mg-Al-RE series magnesium alloy, has the improved creep resistance at high temperatures compared with the more common AZ91 and AM50 alloys, and it is thought to be a kind of alloy with wide application for automotive components[1-4]. However, the poor castability and worse mechanical properties at room temperature restrain the actual application of the AE42 alloy.

Some recent researches have shown that both mechanical properties and microstructures of as-cast AE42 alloy can be influenced by adding such alloying elements as Ca and Sr[5-6]. The addition of Ca and Sr to AE42 alloy leads to an increase in the yield strength but a decrease in both ultimate tensile strength and elongation at 150 ℃, while the AE42 alloy containing Ca or Sr exhibits the higher ultimate tensile strength than the AE42 alloy above 150 ℃[7-8]. Zn element is thought to have an effective solution-strengthening effect on Mg-Al series alloys, and be able to combine with Mg to form MgZn₂ phase, which has a good dispersion-strengthening effect[9-10]. Meanwhile, small addition of Zn can decrease the effect of some impurity elements (Fe and Ni) on corrosion resistance property of magnesium alloy[11-12], but addition of Zn above 4% increase the hot-tear susceptility of Mg-9Al alloy[13]. In addition, hot extrusion is also found to be an effective method of refining microstructure and improving mechanical properties of the magnesium alloy[14-15], which suggests that near-net-shape forming for magnesium alloys is possible.

In this study, both microstructures and mechanical properties of the hot-extruded AE42 alloy with addition of zinc were investigated at room temperature and elevated temperatures.

2 Experimental

In the present investigation, the commercial AE42 (Mg-4Al-0.3Mn-2RE) alloy was chosen as master alloy. Firstly, the AE42 alloy was melted in a steel crucible resistance furnace using 0.1%SF₆+N₂ as protective gas. And then, 1.0%(mass fraction) Zn in form of pure metal was added into the molten AE42 alloy at 700 ℃. The melt was held at 700 ℃ for 10 min. Subsequently, the melt was poured in a metal mold with a diameter of 240 mm. The ingots of both AE42 and AE42+1Zn alloys were hot extruded into tube at temperature of 370 ℃ with an extrusion ratio of 8:1. The extruded tubes were machined into tensile specimens with a gauge length of 10 mm and a gauge section of 4 mm×6 mm. The displacement-controlled tensile tests were carried out on a CSS-55100 electric multiple-purpose testing machine at test temperature ranged from room temperature to  200 ℃. After held for 15 min to ensure the stability of test temperature, the tensile specimens were tested at a cross-head traveling rate of 1.0 mm/min and then air cooled in furnace following fracture. The scanning electron microscope samples were ground and polished before being etched with a solution of 4%(volume fraction) nitric acid+ethyl alcohol. Microstructures of the AE42 alloys with and without the addition of Zn were characterized using a Hitachi S-3400N scanning electron microscope (SEM) with an energy-dispersive spectros- copy (EDS). The phase constituents in the AE42 alloy with the addition of Zn were identified using a Rikakn D/maxr X-ray diffractometer.

3 Results and discussion

3.1 Tensile properties

Fig.1 shows tensile properties of as-extruded AE42 alloys with and without the addition of Zn at different temperatures. It can be noted from Figs.1(a) and (b) that both ultimate tensile strength and yield strength of the AE42 alloys with and without the addition of Zn exhibit the similar trend, that is, the strengths of the alloys decrease with increasing temperature, and the AE42+1Zn alloy gives the higher strengths than AE42 alloy at room temperature, 100 ℃ and 150 ℃. However, the AE42 alloy exhibits the better ultimate tensile strength and yield strength than the AE42+1Zn alloy at 200 ℃. The elongation gets generally improved with increasing temperature. It is also observed that the elongation of the AE42 alloy can be considerably enhanced through adding Zn, as shown in Fig.1(c). Especially, the effect of Zn on the elongation is much more obvious below   150 ℃.

3.2 Microstructures

Fig.2 shows SEM micrographs of as-extruded AE42 alloys with and without the addition of Zn. It can be found that the addition of Zn to the AE42 alloy can refine the microstructure and result in an obvious increase in the amount of intergranular precipitates. After hot extrusion, the precipitated particles have the smaller size and distribute on grain boundaries with more dispersion.

The AE42 alloy is mainly composed of α-Mg solid solution and Al₁₁RE₃ phases, and acicular Al₁₁RE₃ phases distribute on grain boundaries[16]. Fig. 3 shows XRD pattern of the AE42+1Zn alloy. It reveals that the AE42+1Zn alloy contains Mg₁₇Al₁₂ and MgZn₂ phases besides α-Mg and Al₁₁RE₃ phases.

Fig. 1 Tensile properties at different temperatures: (a) Ultimate tensile strength; (b)Yield strength; (c) Elongation

Fig.4 shows SEM micrographs of AE42 and AE42+1Zn alloys. Microanalysis performed for the phases using EDS reveals that the Al-RE phase in the AE42 alloy is main Al₁₁RE₃ phase, which shows two kinds of morphologies, i.e. acicular and block. Some round Al-Mn-Ce particles can be also seen, as shown in Fig.4(a). In the AE42+1Zn alloy, the rod-shaped Al₁₁RE₃ and granular Mg₁₇Al₁₂ precipitates as well as the grey Al-Mn particle without Ce have been observed, as shown in Fig.4(b). The tiny particles distributing dispersively in the matrix, are thought as MgZn₂, which needs to be ascertained in further research.

Fig.2 SEM micrographs of as-extruded alloys: (a)AE42; (b) AE42+1Zn

Fig.3 XRD pattern of AE42+1Zn alloy

Fig.4 SEM micrographs of two alloys: (a)AE42; (b) AE42+1Zn

The tensile properties at different temperatures and microstructure of the AE42 alloy can be evidently influenced by the addition of Zn. The amount and shape of Al₁₁RE₃ phase in the AE42 alloy change due to the addition of Zn. With the presence of Mg₁₇Al₁₂ and MgZn₂ phases in the AE42+1Zn alloy, the amount of Al₁₁RE₃ phase decreases and the shape of Al₁₁RE₃ phase becomes short rod-shaped, which will have an significant improving effect on the tensile properties of the AE42 alloy. In addition, based on the dispersion-strengthening mechanism, the presence of the second phase particles distributed dispersively inside grains or on grain boundaries can restrain the movement of dislocations, and thus attain the strengthening effect. For the AE42+1Zn alloy, the MgZn₂ precipitates dispersively distributed in the matrix could contribute to the improvement in tensile properties below 150 ℃. When the testing temperature is above 150 ℃, the ultimate tensile strength and yield strength of the AE42+1Zn alloy are lower than those of the AE42 alloy. The probable reasons are the poor thermal stability of Mg₁₇Al₁₂ phase in the AE42+1Zn alloy, which can lead to a decrease in the effect of restraining the grain boundary sliding at elevated temperature.

4 Conclusions

1) The addition of 1% Zn to the AE42 alloy results in an improvement in both ultimate tensile strength and yield strength at room temperature, 100 ℃ and 150 ℃ as well as elongation.

2) The addition of Zn to AE42 alloy can refine the microstructure of the AE42 alloy, and leads to the precipitation of Mg₁₇Al₁₂ and MgZn₂ phases.

3) The improvement in the ultimate tensile strength and yield strength below 150 ℃ due to the addition of Zn is thought as comprehensive results of Al₁₁RE₃ and MgZn₂ phases dispersively distributed in the matrix. However, the lower ultimate tensile strength and yield strength of the AE42+1Zn alloy at 200 ℃ can be attributed to the poor thermal stability of Mg₁₇Al₁₂ phase.

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(Edited by YANG Hua)

Foundation item: Project (2002AA331120) supported by the National High-Technology Research and Development Program of China

Corresponding author: WANG Feng; Tel: +86-24-81555118; E-mail: wf9709@126.com