Effect of cooling rate on microstructure and compressive performance of AZ91 magnesium alloy

YANG Lin(杨 林), FENG Hui(冯 辉), QIU Ke-qiang(邱克强), CHEN Li-jia(陈立佳), LIU Zheng(刘 正)

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

Shenyang 110023, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Effect of cooling rate on both microstructure and room temperature compressive performance of the AZ91 magnesium alloy was investigated. The experimental results show that with increasing cooling rate, the quantity of the solid solution phase increases and the fraction of secondary phase Mg₁₇Al₁₂ decreases. The almost single solid solution phase can be obtained with using liquid nitrogen as a coolant. The compressive strengths of the rapid solidified AZ91 magnesium alloys are higher than those of normal cast alloy, and decrease with increasing cooling rate. After artificial aging treatment for 14 h at 168 ℃, the compressive strength of the rapidly solidified AZ91 magnesium alloy cooled in liquid nitrogen increases from 253.5 to 335.3 MPa, while the compressive yield strength increases from 138.1 to 225.91 MPa. The improvement in the compressive strength of the rapidly solidified AZ91magnesium alloys can be attributed to the hardening effect from fine secondary phase.

Key words: magnesium alloy; cooling rate; microstructure, compressive performance

1 Introduction

Compared with the conventional casting, the rapid solidification provides the better alloying, increases the solid solubility, reduces micro-segregation and forms the metastable phases. At the same time, the formation of a finer microstructure will lead to an improvement in the mechanical and physical properties of the alloys.

So far, a number of attempts have been made to use rapid solidification in design and study of magnesium alloys[1-4]. It was found that for the rapidly solidified EA55R wrought magnesium alloy prepared by Allied Signal Company using PFC, both absolute value and relative value of mechanical property are higher than many advanced light wrought magnesium alloys[5]. The rapidly solidified magnesium alloys with addition of rare-earth elements, including the RSP AZ91+ 3%-5%RE and RSP ZK60 (Mg-Zn-Zr)+3%MM alloys, were already investigated[6]. In addition, the rapidly solidified magnesium alloys can also be prepared by spray deposition[7-8]. However, there are so many cavities in those alloys, which will lead to a deterioration of mechanical properties.

In this study, the effect of cooling rate on both microstructure and room temperature compressive performance of the AZ91 magnesium alloy was investigated to add microstructural and mechanical research aspects of rapidly solidified magnesium alloys.

2 Experimental

The material used in this investigation was the commercial AZ91 magnesium alloy, and its chemical composition is listed in Table 1.

The AZ91 magnesium alloy ingot was firstly put in a stainless steel pipe whose one end was connected with the vacuum-quenching furnace (see Fig.1). When the vacuum degree was below 100 Pa, the AZ91 alloy ingot was heated up to 720 ℃ and held for 20 min. After the AZ91 alloy melted down completely, the argon gas was filled up. When the AZ91 alloy melt flowed down completely, the melt was cooled to 650 ℃ and held for 5 min, and then quenched into water, salt solution or liquid nitrogen respectively.

The rapidly solidified AZ91 alloy bar through the above fabricating procedures has a diameter of 10 mm and a length of 100 mm. The compressive samples with a diameter of 7.5 mm and a gage length of 15 mm were machined from the rapidly solidified AZ91 alloy bar. The artificial aging treatment was performed at 168 ℃ for 12, 14, 16, 18 and 20 h. The compressive properties of the rapidly solidified and artificial aged AZ91 magnesium alloys were investigated on a MTS testing machine. Microstructures of some rapidly solidified AZ91 magnesium alloys were characterized using a Leica optical microscopy. The observations on fracture surfaces were performed on a S-3400N scanning electron microscopy.

Table 1 Chemical composition of AZ91 alloy (mass fraction, %)

Fig.1 Schematic diagram of vacuum-quenching equipment

3 Results and discussion

3.1 Microstructures of as-cast and rapidly solidified AZ91 magnesium alloys

The microstructures of the as-cast and rapidly solidified AZ91 magnesium alloys are shown in Fig.2. For as-cast AZ91 magnesium alloy, the typical divorced eutectic microstructure, which is made up of primary α-Mg and eutectic Mg₁₇Al₁₂ phases, and coarse grains can be noted (see Fig.2(a)). For the rapidly solidified AZ91 magnesium alloy by quenching in water, the typical divorced eutectic feature and dendrites structure can be clearly observed, but the microstructure has gotten remarkably refined, as shown in Fig.2(b). It can be seen from Fig.2(c) that for the rapidly solidified AZ91 magnesium alloy by quenching in salt solution, the grains and precipitates are obviously refined, compared with as-cast AZ91 magnesium alloy. For the rapidly solidified AZ91 magnesium alloy by quenching in liquid nitrogen, the quantity of the precipitates gets evidently decreased, and the precipitates mainly distribute on the grain boundaries, as show in Fig.2(d).

For the AZ91 magnesium alloy, the α-Mg dendrites firstly develop under the condition of equilibrium solidification. During the growth of the dendrites, the content of Al in front of the interface between the solid and liquid phases increases, and subsequently in the inter-dendrites, the eutectic transformation will occur in the remnant liquid phase rich in Al. Because of the small content of eutectic phase, the eutectic α-Mg phase generally nucleates and growths by adhering to the primary α-Mg phase, and the finally solidified Mg₁₇Al₁₂ phase has to be pushed to the boundary of α-Mg dendrites. Thus, the divorced eutectic microstructure forms. It is obvious that the microstructure of as-cast AZ91 magnesium alloy consists of the coarse α-Mg dendrites and Mg₁₇Al₁₂ phases distributed in the inter-dendrites. Under the condition of rapidly solidification, however, the diffusion of solute atoms in the front of the interface between the solid and liquid phases is blocked, and the eutectic transformation is restrained so that the supersaturated single-phase solid solution forms[9].

Fig.2 Microstructures of AZ91 magnesium alloys under different conditions: (a) As-cast; (b) Quenched in water; (c) Quenched in salt solution; (d) Quenched in liquid nitrogen

The previous investigation[10-11] has shown that the solid solubility of Al in α-Mg increases greatly from 11.5% for as-cast AZ91 magnesium alloy to 21.6% for the rapidly solidified AZ91 magnesium alloy. Only supersaturated α-Mg solid solution can form under the condition of rapid solidification, and thus the strength, toughness and erosion resistance of the AZ91 alloy can be greatly improved. However, if the cooling rate is not very great, the complete supersaturated solid solution cannot be attained, and only the solid solubility of Al in α-Mg was increased. In addition, the precipitation and growth of the second phase were restrained, which leads to the refinement of the Mg₁₇Al₁₂ phases. For the rapidly solidified AZ91 magnesium alloy quenched in liquid nitrogen, the eutectic transformation is completely restrained so that the amount of the supersaturated solid solution remains the greatest[11]. Based on the present results, it can be concluded that with the cooling rate increasing, the amount of the α-Mg solid solution increases, and the fraction of the Mg₁₇Al₁₂ precipitates decreases.

To investigate the distribution of Zn in as-cast and rapidly solidified AZ91 magnesium alloys, the analysis of Zn element was conducted using SEM and EDS. The results are given in Fig.3. For the conventional cast AZ91 magnesium alloy, Zn distributes in both second phase and α-Mg matrix, while for the rapidly solidified AZ91 magnesium alloys, Zn appears only in the second phase. It implies that with an increase in the cooling rate, Zn tends to distribute mainly in the second phase.

3.2 Microstructures of rapidly solidified AZ91 magnesium alloys subjected to artificial aging

The microstructures of the rapidly solidified AZ91magnesium alloys subjected to artificial aging for different time are shown in Fig. 4. It can be noted that many β-Mg₁₇Al₁₂ phases precipitate after aging treatment. With prolonging aging time, the size of the β-Mg₁₇Al₁₂ phase increases gradually.

3.3 Compressive strengths of rapidly solidified AZ91 magnesium alloys with different conditions

The compressive fracture strength and yield strength of as-cast AZ91 magnesium and those rapidly solidified AZ91 magnesium alloys with different processing conditions are shown in Fig.5. The compressive fracture strength and yield strength of the rapidly solidified AZ91 magnesium alloys quenched in water, salt solution, or liquid nitrogen are much higher than those of as-cast AZ91 magnesium alloy. Compared with the conventional cast, the rapid solidification can refine the microstructure and enhance the solubility of the solid solution, and thus the compression strength of the materials gets improved.

Fig.3 Distribution of Zn in AZ91 magnesium alloys under different conditions: (a) As-cast; (b) Quenched in water; (c) Quenched in salt solution; (d) Quenched in liquid nitrogen

Fig. 4 Microstructures of rapidly solidified AZ91magnesium alloys after artificial aging for different time: (a) 14 h; (b) 16 h; (c) 18 h

Fig.5 Compressive strengths of rapidly solidified AZ91 magnesium alloys under different conditions

Furthermore, it can be found from Fig.5 that for the rapidly solidified AZ91 magnesium alloys, the compressive strengths decrease with increasing the cooling rate. It can be thought that the decrease in the compressive strengths is related to the change in the quantity of the second phase. Because the amount of the β-Mg₁₇Al₁₂ phase in the AZ91 magnesium alloy quenched in liquid nitrogen decreases, the effect of the second phase strengthening is weaken, and thus, the compressive strengths decrease.

3.4 Fractographs

During the compressive test, the samples will rupture under shearing stress. Thus, the angle between the rupture surface and compressive axes is about 45?, which belongs to the brittle rupture. The SEM observations on fracture surfaces have been performed. The typical fractoghraphs are shown in Fig.6. It can be seen that there exist cleavage-type facets like the stream pattern on the fracture surfaces of as-cast and rapidly solidified AZ91 magnesium alloys. In addition, many traces of β-Mg₁₇Al₁₂ phases can be noted. For the rapidly solidified AZ91 magnesium alloys, the faster cooling rate, the trace of β-Mg₁₇Al₁₂ phase is smaller, and the size of the cleavage crack is also much smaller. For the AZ91 magnesium alloys quenched in liquid nitrogen and subjected to artificial aging, many big cleavage-type facets appear on the compressive fracture surfaces. From the microscopic view, the cleavage rupture of the materials is related to the intragranular slipping of dislocations.

4 Conclusions

1) With increasing the cooling rate, the quantity of the solid solution phase increases, and the fraction of the Mg₁₇Al₁₂ phase decreases.

2) For the rapidly solidified AZ91 magnesium alloys, Zn appears only in the second phase while for the conventional cast AZ91 magnesium alloy, Zn distributes in both second phase and α-Mg matrix.

3) After artificial aging treatment for 14 h at 168 ℃, the compressive strength of the rapidly solidified AZ91 magnesium alloys quenched in liquid nitrogen increases from 253.5 to 335.3 MPa, while compressive yield strength gets increases from 138.1 to 225.9 MPa.

Fig.6 Typical fractographs of AZ91 magnesium alloys with different statuses:  (a) As-cast; (b) Quenched in liquid nitrogen;      (c) Aged for 14 h; (d) Aged for 20 h

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(Edited by YANG Hua)

Foundation item: Project (2001BA311A03) Supported by National Science and Technique Foundation during the 10th Five-Year Plan Period

Corresponding author: YANG Lin; Tel: +86-24-25691316; E-mail:yanglin318@126.com