Effect of intermetallic compound β-Mg₁₇Al₁₂ on ductility of AZ91D magnesium alloy

HUANG Wei (黄  巍), LI Di(李  荻)

School of Materials Science and Engineering,

Beijing University of Aeronautics and Astronautics, Beijing 100083, China

Received 20 April 2006; accepted 30 June 2006

Abstract: The microstructures of the AZ91D die-cast magnesium alloy were investigated by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and energy dispersion spectroscopy (EDS). Moreover, the microstructure change of AZ91D samples was observed during the process of heat treatment at 300 ℃ for different time. And the tensile testing was carried out for these samples and the fracture morphology of the tensile test samples was also examined. The results show that the microstructures of AZ91D magnesium alloy are mainly composed of α-Mg phase and β-Mg₁₇Al₁₂ phase. The morphology of the β phase alters from continuous distribution to discontinuous distribution during the process of heat-treatment. Meanwhile, the ductility of the materials reduces from 1.71% to 1.08% after a long time heat-treated at 300 ℃. Moreover, the quasi-cleavage fractures characters are also found in the fracture morphology. The granulation and discontinuous distribution of β-Mg₁₇Al₁₂ results in the deterioration of the ductility of AZ91D die cast magnesium alloy.

Key words: AZ91D magnesium alloy; microstructure; morphology; ductility; fracture texture

1 Introduction

Since magnesium and magnesium alloys have many advantages such as low density, high specific strength, good cast ability, suitable for high pressure die-casting, good electrical and thermal conductivity and so on, they are increasingly utilized in aerospace, military, nuclear energy, electronic and automotive industries[1-13]. In the past several years, some magnesium alloys have been developed to meet the need of structural applications. Among these magnesium alloys, the AZ91D die-cast is the most widely used[14]. However, to be used for structural components in industry fields such as automobiles, the material should exhibit sufficient ductility as well as other good mechanical properties because the material is often fractured by shear or tensile force. There are a lot of investigations conducted on the ductility of magnesium alloys. For instance, the structural evolution with increasing the content of solute atoms could result in the enhancement of ductility of magnesium alloy, and the ductility enhancement in magnesium could be achieved by refining its grain structure[15]. While it was similar to WE43 magnesium alloy[16]. MOHRI et al[17] reported the ductility enhancement of a Mg-Y-RE alloy by hot extrusion. They mentioned that the enhancement of ductility was due to the transition of fracture behavior with refining the microstructure. Thus refining microstructure enables to raise the possibility for the development of structural magnesium alloy with high ductility.

So far as the investigations on AZ91D die cast have not been widely made, the effect of different phases in microstructure on the ductility of AZ91D die cast should be explored. Since the AZ91D die-cast magnesium alloy has a two-phase microstructure typically consisting of a matrix of α grains with β phase (the intermetallic Mg₁₇Al₁₂) along the α grain boundaries[18, 19]. There are some investigations on the effects of β-Mg₁₇Al₁₂ on the materials’ mechanical properties during heat-treatment at different temperatures [20, 21]. While the microstructure and mechanical properties of AZ91D were greatly affected by the change of temperature[22-24]. It is not only necessary to study the effect of temperature on β phase in AZ91D but also to investigate the change of ductility of material with the alteration of β-Mg₁₇Al₁₂ phase. The objective of the present work is to disclose the relationship between β-Mg₁₇Al₁₂ and the ductility of AZ91D die-cast. Tensile test was conducted on the AZ91D die-cast consisting of different forms of β-Mg₁₇Al₁₂ to evaluate the relationship between microstructure and the ductility.

AZ91D die-cast alloy was employed for this study. Its chemical compositions are as follows (in mass fraction, %): Al 8.5-9.5, Zn 0.45-0.90, Mn 0.17-0.40, Mg balance.

The phase composition of the samples was investigated by means of energy dispersion spectroscopy (EDS) and D/MAX 2200PC X-ray diffraction (XRD) using monochromatic Cu K_α radiation.

The AZ91D samples were firstly ground using silicon carbide papers, secondly polished, then etched in 2% solution of nitric acid in ethanol. Following these procedures, the microstructure studies of these samples were carried out using OLYMPUS-BX51M metallo- graphic microscope and GSM-530 scanning electron microscope (SEM) for phase analysis.

The AZ91D die-cast samples were heated for 0, 20, 40, 60 h at 300 ℃ in electrical furnace respectively, then their mechanical properties were investigated by tensile tests at room temperature using an universal mechanical testing machine. The tests were conducted in air at room temperature. The maximum of load sensor was not beyond 100 kN, the scope of extensometer gage was 25 mm and the loading speed was 0.25 mm/min. The tensile test specimens are plates with the gauge length of 40 mm, width of 3 mm and thickness of 2 mm. The machine applied tensile load on the round shoulder of the samples by grip holder. These tested specimens were prepared by liner cutting machine and mechanically polished to remove the damaged surface layer prior to the tests. After the test, the fracture textures of the specimens were observed under SEM.

3 Results and discussion

3.1 Microstructure

Fig.1 shows the microstructure and XRD pattern of the AZ91D die-cast magnesium alloy. It is well known that the AZ91D die-cast magnesium alloy contains 8.5%-9.5% aluminimum. From the Mg-Al phase diagram, the Mg rich section of the binary phase diagram revealed a maximum solid solubility of 12.9% Al at the eutectic temperature of 437 ℃. Under equilibrium conditions, the Mg-Al casting alloys should solidify as a single phase α-Mg solid solution and further cooling should lead to the solid-state precipitation of Mg₁₇Al₁₂ (β-phase) around the α-Mg grains [25]. The microstructure of Mg-Al alloys under the die-cast condition, however, differs significantly from this prediction. Usually, the crystallization of alloy is proceeded at a fast cooling rate that is a non-equilibrium state. The procedure will induce a deviation from the thermodynamical equilibrium[26, 27]. After the solidification of the primary α phase, aluminum is enriched in the remaining liquid, which solidifies around the α grains at the eutectic temperature and generated the eutectic β′ phase (combination of mainly β phase and α phase). Eventually, the intermetallic compound β phase, distributed around the boundaries of α grains, forms in this way[28].

Fig.1 Metallographic microstructure(a) and XRD pattern(b) of AZ91D die-cast magnesium alloy

Referring to Fig.1(a), EDS was used to characterize their composition, the analysis results of the different phases observed are listed in Table 1. It can be seen that the two phases are mainly composed of the magnesium, aluminum and zinc element and they are differentiated by the element content. After the XRD analysis to the material, it is found that there are two phases in the microstructure including primary α-Mg and β-Mg₁₇Al₁₂. From Table 1, it can also be demonstrated that there are a solid solution of substitution of aluminum in magnesium (α-Mg phase) and intermetallic phase β-Mg₁₇Al₁₂ in an eutectic phase (α+β) of the AZ91D die-cast. In Fig.1(a), the β-Mg₁₇Al₁₂ distributes around the boundaries of α-Mg grains and it is continuous and uniform. Some studies show that the continuous distribution of β-Mg₁₇Al₁₂ phase can improve the anti-corrosion ability for AZ91D to some extent[14]. While, the investigations also show that β-Mg₁₇Al₁₂ plays an important role in the mechanical properties of material[29].

Table 1 EDS analysis results of constituting phases shown in Fig.1 (mass fraction, %)

3.2 Effect of heat-treatment on β-Mg₁₇Al₁₂ inter- metallic phase

Fig.2 shows the microstructures of AZ91D die-cast during the heat-treatment at 300 ℃ for different time. It can be seen that after 20 h heat-treatment, the change happens in the microstructures. The β′ phase begins to granulate and grow and this also means that the same change happens to β-Mg₁₇Al₁₂ phase. It is obviously different from the non-heat-treated samples in which the β phase is continuous and uniform distribution (see Fig.1(a)). However, after a long time heat treatment the β-Mg₁₇Al₁₂ phase forms a discontinuous distribution around the boundaries of α-Mg grains. Especially, in Figs.2(b) and (c), the granulation tendency of β-Mg₁₇Al₁₂ phase is more severe. This is due to the eutectic β′ phase of the microstructure which is not stable at elevated temperature especially the α-Mg phase in eutectic phase. During the process of the heat treatment, with the time of heat treatment extended the thermal diffusion of Mg atom exacerbates. Following this, the α-Mg in eutectic dissolves and the β-Mg₁₇Al₁₂ phase enriches accordingly. The morphology of β-Mg₁₇Al₁₂ phase is no longer thread form. Instead, it is more granulation shape.

3.3 Effect of β-Mg₁₇Al₁₂ on mechanical properties of AZ91D

The presence of β-Mg₁₇Al₁₂ significantly affects the mechanical properties of Mg-Al alloys[29]. As tensile fracture initiates by the cracking of the β-Mg₁₇Al₁₂ particles[30], the ductility of AZ91D alloy die-cast was determined by the volume fraction and spatial distribution of β-phase. In this regard, it is desired to form a continuous network of β-Mg₁₇Al₁₂ phase along the grain boundaries of the primary α-Mg grains for the better ductility of the material. After the different heat treatment time at 300 ℃, the shape of the β-Mg₁₇Al₁₂ was undergone a procedure of granulation and discontinuous distribution with the heat treatment time increasing at 300 ℃, whereas the primary α-Mg phase does not change much. To investigate the relation

Fig.2 SEM images of AZ91D microstructure heat-treated at 300 ℃ for 20 h(a), 40 h(b) and 60 h(c)

between the ductility and β-Mg₁₇Al₁₂ phase of AZ91D die cast, the tensile test was carried out at different heat treatment time that related to a certain form of β-Mg₁₇Al₁₂ phase. The results are shown in Fig.3.

From Fig.3, it can be seen that the ductility of the material decreases with the time of heat treatment extended at 300 ℃ and the ductility of AZ91D die-cast is reduced from 1.71% for 0 h  to 1.19% for 20 h, 1.16% for 40 h to 1.08% for 60 h at 300 ℃, which is in accordance with the granulated degree of β-Mg₁₇Al₁₂ phase. The tensile test exerts the negative influence on the shape and distribution of the β-Mg₁₇Al₁₂ on the mechanical behavior of AZ91D die-cast Mg-alloys. It is because the α-Mg phase in eutectic phase β′ (α+β) can decrease the brittleness of β-Mg₁₇Al₁₂ phase to some

Fig.3 Stress—strain curve of AZ91D die-cast after heat-treated at 300 ℃ for different time

extent. The combination of α-Mg and β-Mg₁₇Al₁₂ eutectic phase can strengthen the binding force with the α-Mg grains. The absence of α-Mg in eutectic phase will cause the enrichment of β-Mg₁₇Al₁₂ which has weak cohesion with the α-Mg grains. It is well known that the β phase is brittle[24]. In the microstructure, it would restrain the deformation of the microstructure, which would reduce the alloy ductility severely. Therefore, the β-Mg₁₇Al₁₂ plays an important role in the reduction of ductility and the ductility is deteriorated by the enriching dispersion of the β-phase particles.

3.4 Fracture morphology of tensile test samples

There is no great difference between Figs.4(a) and

(b). They are all characterized by dimples with the character of plastic yield. This is because the AZ91D die-cast consists of α-Mg and β-Mg₁₇Al₁₂ phases. When the external stress acts on the material, the combining powder of the interface between primary α-Mg and β-Mg₁₇Al₁₂ is reduced. The strain at the interphase is produced, which causes the deformation of microstructure. Then, at the interphase some cavities are produced. After that, the coupled two phases at the interphase slide and the cavities begin to expand until the adjacent cavities meet each other. Eventually, the adjacent cavities integrate together and the coupling interphase is cracked. Meanwhile on the each side of the fracture texture, a strait shape of dimple face appears on the surface.

In Figs.4(c) and (d), the texture gradually changes into quasi-cleavage fracture with the character of certain brittlement (as the arrow designated). This indicates that the ductility of material is reduced with the extended heat treatment time at 300 ℃. It is also demonstrated that the granulation of β-Mg₁₇Al₁₂ influences the ductility of the heat-treated AZ91D die-cast. Compared with Fig.4, it can be seen that the plasticity is reduced with the enrichment of β-Mg₁₇Al₁₂. It can be concluded that once the β-Mg₁₇Al₁₂ is transformed from continuous shape to granulated shape, that is, the block deformation of material is enhanced, which results in the decrease of the plasticity of material. It can prove that the β-Mg₁₇Al₁₂ has an important effect on the properties of AZ91D. With the granulation of β-Mg₁₇Al₁₂ the plasticity is deteriorated. The more the granulated β-Mg₁₇Al₁₂, the more decrease of the AZ91D die-cast ductility.

Fig.4 Fracture morphologies of tensile test samples at 300 ℃ for different heat-treatment time: (a) 0 h; (b) 10 h; (c) 20 h; (d) 40 h

4 Conclusions

1) The AZ91D die-cast magnesium alloy consists of β-Mg₁₇Al₁₂ and α-Mg. And it is mainly the β-Mg₁₇Al₁₂ continuously distributed around the boundaries of α-Mg grains.

2) The morphology of β-Mg₁₇Al₁₂ can be altered from continuous and uniform state to discontinuous and granulation distribution by extending the heat treatment time at 300 ℃. Meanwhile, the α-Mg does not change much.

3) The morphology of β-Mg₁₇Al₁₂ in microstructure of AZ91D die-cast has effect on the ductility of the material. The ductility of material decreases with the degree of the granulation of β-Mg₁₇Al₁₂ steeply down.

4) The morphology of β-Mg₁₇Al₁₂ is greatly influenced by the long-time heat treatment at 300 ℃. It changes from the continuous and uniform shape to granulated shape because of the enrichment of β-Mg₁₇Al₁₂.

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(Edited by CHEN Wei-ping)

Corresponding author: HUANG Wei; Tel: +86-10-82317113; E-mail: huangwei@mse.buaa.edu.cn