Precipitation behavior and effect of new precipitated β phase in AZ80 magnesium alloy

TANG Wei(唐 伟), HAN En-hou(韩恩厚), XU Yong-bo(徐永波), LIU Lu(刘 路)

Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Granular precipitate that was a new kind of β-Mg₁₇Al₁₂ phase found in aged AZ80 wrought Mg alloy at all aging temperature was studied. The structure and precipitation behavior of this granular β-Mg₁₇Al₁₂ precipitate were studied by environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM). The effect of the granular precipitate on mechanical properties of AZ80 alloy was also studied. The new precipitate that was granular and nucleated both on grain boundaries (GBs) and twin boundaries, has the same crystal structure and lattice parameter as those of the continuous or discontinuous precipitated β-Mg₁₇Al₁₂. And the nucleation and growth of the granular precipitate are faster than those of the other two precipitates at higher temperatures (above 583 K), but are suppressed at lower temperatures (below 423 K). At lower temperatures, the discontinuous β-Mg₁₇Al₁₂ precipitates firstly and the granular β-Mg₁₇Al₁₂ precipitates after aged more than 40 h. The crack is easily nucleated on the phase boundaries of granular phase and matrix because of the weak binding force. As a result, the strength and ductility of AZ80 Mg alloy are decreased by the granular β-Mg₁₇Al₁₂ precipitate.

Key words: magnesium alloy; AZ80; granular β-Mg₁₇Al₁₂ precipitate, precipitation behavior; aging

1 Introduction

Magnesium alloys with attractive properties such as low density which is 1/4 of steel, 2/3 of Al alloys and leads to high specific mechanical properties, good thermal conductivity, good damping, electromagnetic shielding characteristics, dimensional stability, machinability and low cost, are expected to be the most promising structural materials for application in aerospace, automotive and portable electrical products  [1-2]. Magnesium aluminum alloys are the most popular magnesium alloys and have been well studied. Mg₁₇Al₁₂ (β phase) is only precipitate generated magnesium aluminum alloys during aging after solution treatment in previous work[3-9].  The β phase can precipitate in either continuous way or discontinuous way.  The continuous precipitate that leads to a progressive change in matrix composition nucleates and grows within the parent grains homogeneously. The discontinuous precipitate which leads to a discontinuous change in chemical composition and lattice parameter nucleates on grain boundaries and grows as nodules. The two kinds of β phases are both in lamellar form[3-4]. According to the research results of Refs.[3-5], the precipitation hardening effect of β phase in Mg-Al based alloys is small compared with other alloys such as aluminum alloys. This is because of the unfavourable orientation of β phase plates and large inter-precipitate spacing. The basal slip cannot be blocked effectively.

The continuous and discontinuous precipitated β phases are the only precipitates in Mg-Al based alloys based on Refs.[3-13].  However, a new precipitated phase is found in this study, which is granular and has the same crystal structure and lattice parameter as that of the other two β phases. And the granular β phase that has the different effects on mechanical properties of Mg alloys has not been reported in any other documentation. In this paper, the structure of the granular β phase, the precipitation behavior and its effect on properties of AZ80 alloy were studied.

2 Experimental

The alloy was prepared by vacuum casting under a protective atmosphere of argon. The ingots were then homogenized at 683 K for 24 h, cooled in furnace, then solid solution treated at 683 K for 2 h followed by conventional extruded at 653 K into the alloy sheets.  The chemical composition is listed in Table 1.

Table 1 Chemical composition of AZ80 alloy(mass fraction, %)

The alloy was solution treated at 683 K for 2 h, followed by water quenching, then aged at 583 K and 423 K for 0.5 h to 128 h, air cooled. Tension tests were carried out on a Shimadu universal material testing ma- chine at room temperature and 1.04×10^-3 s^-1(cross speed: 2 mm/min). Tensile specimens with a gauge of 25 mm in length, 6 mm in width and 3 mm in thickness were cut by linearly cutting machine with the tensile axis parallel to the extrusion direction.  All specimens were grinded carefully using waterproof abrasive paper to 1000^# before tension, and some of them were polished and etched for SEM observation, the etching solution was 5% nital.

Microstructure and surface morphology of deformed specimens were examined by XL30 FEG environment scanning electron microscope (ESEM), and the interior microstructure of grain was examined by JEOL 2000FXII transmission electron microscope (TEM).  Specimens for TEM test were firstly ground- polished to about 80 μm, then thinned by two-jet electrolytic machine followed by ion polishing.  The electrolyte was 1?3(volume ratio) nitric methanol, temperature was  –20 ℃.

3 Results and discussion

3.1 Morphology and analysis of granular precipitate

The microstructure of AZ80 alloy aged at 583 K and 423 K are shown in Fig.1. At 583 K, there are two kinds of precipitates appear in the alloy, the lozenge shaped one is the continuous precipitated β phase, the other one is granular precipitate which precipitates on the grain boundaries and within parent grain in lines (Fig.1(b)). So this granular phase (GP) is neither continuous nor discontinuous β phase, and this precipitate has not been reported, it’s a new phase or a known one precipitated in a new way.  As can be seen in Fig.1, the precipitation of GP is affected by aging temperature greatly.  At 583 K, the precipitation of GP and continuous β phase are simultaneous, and the granular phase grows faster than the continuous one.  While at 423 K, the precipitation of both phases is suppressed.  There is no GP precipitated until aged for about 40 h (Fig.1(c)), and the discontinuous β phase more prefers to precipitate at this temperature.

Fig.2 shows the XRD pattern of AZ80 Mg alloy after aged at 583 K and the TEM results.  There are only α phase of magnesium matrix and β phase of Mg₁₇Al₁₂ existing in aged alloy.  According to TEM results, the crystal structure and lattice parameter of GP are the same as those of the continuous or discontinuous phase (Figs.2(b, c)), so the GP is a new kind of β Mg₁₇Al₁₂ phase.

The GP nucleates not only on grain boundaries but also within grains observed by TEM and SEM. Fig.3 shows the nucleation of GP. As can be seen in Fig.3(b), the place where GP nucleates is twin boundaries. It is well known that there are far more defects on the boundaries of grain and twin, and the speed of atomic diffusion is faster at these place compared with interior grain. At higher temperature (583 K), GP is easy to nucleate on the boundaries of grain and twin to decrease the energy and grows faster than continuous phase because of the fast diffusion speed of Al atomic on such boundaries. There are more defects such as dislocations or faults around grain boundaries and twin boundaries. Some of the GP particles grow about 2 μm aged for about 0.5 h at 583 K, and most of continuous β phase particles is very small (Fig.1(a)).  While at lower aging temperature (423 K), the nucleation of GP is more difficult, and it is substituted by the nucleation of discontinuous phase that prefers to nucleate at lower temperature.

Fig.1 Microstructures of AZ80 Mg alloy after aged under different conditions: (a) 583 K, 0.5 h; (b) 583 K, 4 h; (c) 423 K, 4 h; (d) 423 K, 128 h

Fig.2 XRD pattern of AZ80 Mg alloy aged at 583 K (a), TEM morphology of granular β phase (b) and SAED pattern (c)

Fig.3 Granular β phase nucleated on grain boundaries and twin boundaries

3.2 Mechanical properties

Fig.4 shows the mechanical properties of AZ80 Mg alloy aged at 583 K and 423 K for different time. At 583 K, the yield strength of alloy is almost the same but the ultimate strength and elongation decrease with aging time. At 423 K, the strength of alloy is much higher than that of solution treated alloy.

Fig.4 Effects of aging time on properties of AZ80 Mg alloy

As can be seen in Fig.1, the GP and continuous phase are the only precipitates in AZ80 Mg alloy aged at 583 K, so the precipitation hardening effects of GP and continuous β phase are small. As shown in Figs.5(a) and (b), the slip lines are not affected by GP and continuous β phase, this means the slip of dislocation can not be blocked much by both phases, so the yield strength increases very little.  On the other hand, the combine force between GP and matrix is weak, some of GP particles are split from matrix during deformation (Fig.5(b)) and this leads to cracks nucleate and expand along GP (grain boundary) at small strain. So the fracture is intergranular(Fig.5(c)), and the ultimate strength is even lower than that of solution treated alloy.

Fig.5 SEM morphologies of deformed alloy and fracture graphs

However, the discontinuous β phase precipitates from matrix when the alloy aged at 423 K. The discontinuous β phase is much easy to precipitate at lower aging temperature, the GP and continuous phases are both suppressed at this temperature.  Especially for GP, the discontinuous β phase nucleates on the grain boundaries and then inhibits the precipitation of GP.  The GP can only precipitate on the grain boundaries where there is no discontinuous β phase precipitate (Fig.1(d)). The strength of the alloy aged at 423 K is much higher than that of alloy aged at 583 K, so the precipitation hardening effect of discontinuous β phase is much higher than that of GP and continuous β phase. The fracture is transgranular because of the less amount of GP (Fig.5(d)).

4 Conclusions

1) The granular phase which has the same crystal structure and lattice parameter is a new kind of β phase and nucleates on grain boundaries and twin boundaries.  The precipitation of granular phase is affected greatly by aging temperature, the higher the aging temperature the easier the precipitation is.

2) Because of the weak binding force between the granular β phase and matrix, the crack is easy to nucleate in the granular β phase at small strain, and so the strength and ductility of AZ80 Mg alloy are decreased by the granular β phase precipitate.

3) The alloy can be aged hardened evidently at lower temperature (423 K) by discontinuous β phase.

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(Edited by LI Xiang-qun)

Foundation item: Project (2001AA331050) supported by the Hi-Tech Research And Development Program Of China

Corresponding author: TANG Wei; Tel: +86-24-23915897; E-mail: wtang@imr.ac.cn.