As-cast microstructure of Mg-Al-Zn magnesium alloys

ZHANG Jing(张 静), PAN Fu-sheng(潘复生), YANG Ming-bo(杨明波), LI Zhong-sheng(李忠盛)

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China

Received 28 July 2006; accepted 15 September 2006

Abstract: The research achievements on as-cast microstructure in Mg-Al-Zn alloy were summarized. Under permanent mould cast condition, there are four kinds of primary compounds with distinct crystallographic morphology, Mg₁₇Al₁₂(γ)，Mg₃₂(Al,Zn)₄₉ (τ), MgZn (ε) and a ternary icosahedral quasi-crystalline compound (Q). Accordingly, Mg-Al-Zn alloy can be grouped into γ-, τ-, ε- and Q-type alloy by each characteristic compound. The volume fraction of γ-Mg₁₇Al₁₂ in commercial γ-type alloy increases with increasing Al content. MgZn and Mg_xAl_yZn_z ternary complex compounds emerge with the change of the element content Al and Zn and Zn/Al concentration ratio. A practical phase diagram showing microstructure constituent change with composition was proposed. The addition of micro-alloying elements Y and Sr results in not only obvious refinement of eutectic cluster but also eutectic morphological change from block to granule.

Key words: Mg alloys; Mg-Al-Zn alloys; microstructure; intermetallic compounds; solidification

1 Introduction

Mg-Al-Zn based magnesium alloy is the most widely used alloy system currently. Typical commercial alloys include AZ31, AZ61, AZ91, etc. Al and Zn are both effective elements in improving the strength and ductility of magnesium[1-2], in addition, they are relatively in low price and easy for recycle. Therefore, the development of Al and Zn alloyed magnesium based alloys and the modification of existing Mg-Al-Zn alloys is always an important aspect in magnesium research area, especially in recent years, alloys with low cost, high strength, good creep resistance, and good forming capacity becomes more and more demanding in order to satisfy the need of mass reduction in automobile industry. Studied alloys include AZ84[3], ZA84[4], ZA85[5], ZA52[6], and ZA104[7]. However, since the Mg rich Mg-Al-Zn ternary phase diagrams are very complex and are not well established[8], in addition, phases in alloys solidified under practical casting conditions are often at variance with those in equilibrium diagram, the phase constituents and the microstructural change with composition in Mg-Al-Zn alloy have not been cleared. Discrepancy exists among different investigations [4, 9-10]. Attempts of new alloy design and composition optimization through microstructure modification by identifying more promising chemical constitution are thus restricted. A systematic study on phases, microstructure, and their relationship with composition has been carried out in Chongqing University. This paper summarizes the related research results, which hopefully could be used for reference to alloy development.

2 As-cast microstructures of typical Mg- Al-Zn alloys

Typical commercial alloys include AZ31, AZ61, AZ91, in which Zn is less than 1% and mainly dissolved in α-Mg solid solution and binary Mg-Al compounds.

According to the Mg-Al equilibrium phase diagram[11], the maximum solid solubility at the eutectic temperature of 438 ℃ is about 13% Al, and a eutectic between Mg and the intermetallic γ-Mg₁₇Al₁₂(BCC, a=1.056 nm) appears at about 33% Al. The aluminium contents used in the commercial alloys are all below the maximum solid solubility limit and the alloys therefore solidify with a primary phase. The equilibrium microstructure is 100% α-Mg, but non-equilibrium, metastable, eutectic normally forms during solidification and is present in the as-cast microstructure in Mg-Al alloys down to about 2% Al. Fig.1 shows the as-cast microstructures of some AZ alloys under normal permanent mould cast condition. It can be seen that they all contain a significant volume fraction of eutectic. With the increase of Al content, the amount of grain boundary blocky γ-Mg₁₇Al₁₂ increases. Furthermore, the alloys manifest typical partially divorced eutectic morphology, ie., γ-Mg₁₇Al₁₂ distributes along grain boundaries, the black island within the γ-Mg₁₇Al₁₂ phase is high Al content eutectic α-Mg, whereas the bulk of the α-Mg is still outside the γ-Mg₁₇Al₁₂ particles. Lamellar precipitates are often found around the eutectic γ-Mg₁₇Al₁₂ phase, which is the secondary γ-Mg₁₇Al₁₂ precipitated out from the supersaturated α-Mg area during cooling. The lower the cooling rate, the higher the supersaturate degree of Al in eutectic α-Mg, the more obvious the lamellar secondary precipitation around γ- Mg₁₇Al₁₂ phases.

Fig.1 As-cast microstructures of permanent mould cast alloys: (a) AZ30; (b) AZ31; (c) AZ50; (d) AZ51

3 Effects of Al and Zn on as-cast microstruc- tures and alloy phases

Useful range of aluminium and zinc composition in Mg-Al-Zn alloy system should be within the two castable domains[12]. In low zinc concentration range, commercial Mg-Al-Zn alloys are located, Zn remains below approximately 1%. With increasing Zn content, here comes a hot cracking area. Beyond this there is a wider high zinc castable domain.

The results on the as-cast microstructure of typical castable alloys show that the microstructural constituent not only relies on the element content, but also on the concentration ratio of Zn/Al. The practical phase constituent diagram showing phase type versus composition is given in Fig.2[13]. Phase constituent is indicated for a given composition, depicted by its Zn/Al concentration ratio (X-axis) and Al element content (Y-axis). The two curves indicate the borders of the hot cracking area between the two castable areas. The very left part of the diagram locates the low zinc castable area, where commercially available Mg-Al-Zn alloys are limitrophe. The intermetallic phase present in these alloys are γ-Mg₁₇Al₁₂. Comparatively, there are three different phases existing in high zinc castable domain. The central part of the phase diagram (zone I), with approximately 2-4 Zn/Al concentration ratio and 2%-4% Al content occupies a ternary phase, τ, with a formula of Mg₃₂(Al,Zn)₄₉ (BCC, a=1.416 nm). Such alloys include ZA104, ZA73. In right corner (zone II) with higher Zn/Al concentration ratio (＞4) and lower Al content (＜2%) there exists ε phase with a formula of MgZn (HCP, a=25.66 nm, c=18.18 nm), alloys including ZA102, ZA122. The arrow in this zone indicates the trend that higher Zn/Al ratio leads to ε as the only intermetallic compound, otherwise there is also a variable fraction of τ present. Whereas decrease of Zn/Al ratio (＜2) and increase of Al content (zone III), such as alloy ZA75 and ZA84, lead to an icosahedral quasi-crystalline phase, denote as Q, distinguishing with I phase formed under extreme nonequilibrium condition in Mg-Al-Zn system[14]. The nature and stability of Q have not yet determined, as well as the existence range in term of Al upper limit. Therefore, the boundary in high Al side is drawn by dash line. Further investigation is being carried on.

Accordingly, Mg-Al-Zn alloys can be grouped into four types according to the dominate phase presented, which are γ-type, τ-type, ε-type, and Q-type, respectively, as shown in Fig.2.

Fig.2 Practical phase constituent diagram showing microstruc- ture constituent change with change of Zn/Al concentration ratio and Al content

Typical as-cast microstructures of τ-, ε-, and Q-type alloys display a mostly dendritic morphology with the second phases distributed in interdendritic spacings and along grain boundaries. Further detailed observation reveals that these primary intermetallic particles manifest different crystallographic characteristics, as can be seen from Fig.3. τ phase shows itself a fish bone like cramp lump crystallography (Fig.3(a)), whereas ε phase consists of many approximately paralleled white laths (Fig.3(b)).

Fig.3(c) shows the morphology of Q phase, which is a cluster of corn-like phase marked with eyelike spots. In order for comparison, the morphology of the characteristic compound in γ-type alloy is also given in Fig.3(d), which shows the grain boundary blocky eutectic compound and the surrounding lamellar precipitate cluster, as described before.

From the DSC results of various Mg-Al-Zn alloys, there are two (sets of) peaks in each DSC curve recorded during solidification, corresponding to the melting of α-Mg matrix and the second phase transformations respectively. The characteristic temperatures of some Mg-Al-Zn alloys are listed in Table 1[13, 15]. It shows that the solidification range and liquidus temperature decrease with increasing Zn and Al content for τ- and Q-type alloys, whereas ε-type alloy shows reverse tendency. In addition to, the second phase transformation moves to higher temperature range when Al content goes up and Zn/Al ratio down. Since γ phase usually crystallizes at comparatively high temperature, it is reasonable to infer that further shift of the composition to high Al side will result in the emergence of γ phase in the as-cast microstructure.

Table 1 Characteristic temperatures of some Mg-Al-Zn alloys during solidification(℃)

Fig.3 SEM micrographs showing crystallographic characteristics of τ (a), ε (b), Q (c) and γ (d) phases

4 Effects of Y and Sr on as-cast microstruc- ture of Mg-3Al-1Zn alloy

Fig.4 shows the as-cast microstructure of Mg-3Al-1Zn alloy with different Y (0-0.9%) or Sr (0-0.10%) contents. The addition of micro-alloying elements Y or Sr results in an obvious refinement of the microstructures. The size of the eutectics cluster in interdendritic spacing and along grain boundaries becomes much smaller. Further observation under SEM (Fig.5) reveals that the morphology of the eutectic compounds changes from block to granule. The refinement degree of these micro-alloying elements on the microstructure is found to increase with the increase of the contents within the above mentioned range. In addition to, it is found[3] that a needle-like phase is formed in alloys containing higher micro-elements. Al₃Y is detected in alloy containing 0.90% Y (Fig.5(d)); and Al₄Sr in 0.10% Sr containing alloy.

5 Conclusive remarks

The as-cast microstructure of Mg-Al-Zn magnesium alloys consists of α-Mg dendrite with the secondary phases distributed in interdendritic spacing and along grain boundaries. Under permanent mould cast condition, there are four different kinds of primary compounds with

distinct crystallographic morphology: Mg₁₇Al₁₂(γ), Mg₃₂(Al,Zn)₄₉ (τ), MgZn (ε) and a ternary icosahedral quasi-crystalline compound (Q). Accordingly, Mg-Al-Zn alloys can be grouped into γ-, τ-, ε- and Q-type alloy by each characteristic compound. Commercial Mg-Al-Zn alloys, such as AZ31, AZ61 and AZ91, belong to γ-type alloy, whose as-cast microstructures are composed of α-Mg and γ-Mg₁₇Al₁₂ phases. In addition to, the volume fraction of γ-Mg₁₇Al₁₂ compounds increases with increa- sing Al content. MgZn and Mg_xAl_yZn_z ternary complex compounds emerge with the change of the element Al and Zn content and Zn/Al concentration ratio. The addition of micro-alloying elements Y and Sr results in not only obvious refinement of eutectic cluster but also eutectic morphological change from block to granule.

Commercial economically-viable Mg-Al-Zn alloys have been hitherto limited used in automobile industry largely because they have limited yield strength, creep resistance and formability. Research emphasis should be put on the development of multi-component and/or quasi-crystalline particle strengthened new alloys in order to enhance both ambient and elevated temperature properties, improve plasticity and ductility, and increase creep resistance, through proper micro-alloying or alloying, combined with appropriate deformation, heat treatment, or thermal mechanical operation to control the morphology, distribution, amount, and size of the compounds.

Fig.4 As-cast microstructures of Mg-3Al-1Zn alloy without (a), with 0.30%Y(b), 0.60%Y(c) and 0.10%Sr(d)

Fig.5 SEM Micrographs of compound morphology in Mg-3Al-1Zn alloys without (a), with 0.01Sr (b), 0.90%Y(c) and (d)

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(Edited by YANG Hua)

Foundation item: Project(2001AA331050) supported by the National High-Tech Research and Development Programme of China; Project(50301018) supported by the National Natural Science Foundation of China; Project supported by the Scientific Research Foundation for ROCS, Education Ministry of China

Corresponding author: ZHANG Jing; Tel: +86-23-65111167; E-mail: jingzhang@cqu.edu.cn