Effects of erbium on microstructure and mechanical properties of

as-cast Mg-7Zn-3Al alloy

ZHANG Jing(张 静), HE Qu-bo(何曲波), PAN Fu-sheng(潘复生),

ZHANG Xu-feng(张旭峰), LIU Chuan-pu(刘传镤)

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

Abstract: Mg-7Zn-3Al-xEr (x=0.1, 0.4, 0.7) magnesium alloys were prepared by permanent mould casting. The effects of rare earth element of erbium on the microstructure and mechanical properties of as-cast Mg-7Zn-3Al alloy at both room temperature and elevated temperatures were investigated with optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, differential scanning calorimetry, and tensile testing. The results show that the quasi-continuous grain boundary networked τ(Mg₃₂(Al,Zn)₄₉) phases are changed into discontinuous globular particles due to the addition of Er. Spherical Al-Er compounds are also identified in the matrix. The mechanical testing reveals that, with Er addition, the elevated temperature tensile properties can be remarkably improved. The microstructure evolution during tensile deformation under both temperature and stress and its effects on the mechanical properties were further discussed.

Key words: magnesium alloy; erbium; zinc; microstructure; mechanical properties

1 Introduction

Low elevated temperature property is still one of the shortcomings which hinder the extensive applications of magnesium. AZ and AM series magnesium alloys are the most popularly used alloys at present because of their good room temperature strength, sound casting performance, and low cost. However, the strength of these magnesium alloys deteriorates when the temperature exceeds 120 ℃[1]. Presently, there are two standpoints to explain this phenomenon[2-4]. Firstly, γ-phase(Mg₁₇Al₁₂) which strengthens the matrix at room temperature becomes unstable at elevated temperatures and has no strengthening effects any more. Secondly, γ-phase precipitates discontinuously at the grain boundary, deteriorating the elevated temperature properties. It was reported that rare earth additions can improve casting performance and increase mechanical properties both at room temperature and elevated temperatures[5-8].

The addition of zinc into Mg-Al alloys can inhibit the formation of γ-phase. The resultant Mg-Zn-Al alloys, ZA series alloy, such as ZA144, ZA104, ZA85, ZA84 [9-13], have improved elevated temperature strength, but undesirable depressed elongation, which decreases with the increase of the total element content[9]. A low alloying ZA alloy, ZA73, has been developed by the authors[10]. The compound existing in ZA73 alloy is τ-phase, which has a formula of Mg₃₂(Al,Zn)₄₉ (BCC, a=14.16 nm). In this work, rare earth erbium is added to ZA73 alloy, in odder to further improve the microstructure and mechanical properties, especially the plasticity. The effect of the content of erbium on the microstructure and mechanical properties, as well as the relationship between the microstructure and the property of as-cast ZA73 alloy are investigated.

Experimental ZA73 alloys with 0, 0.1%, 0.4%, 0.7% Er were prepared from commercial pure magnesium (99.95%), aluminum (99.35%), zinc (99.95%) and Mg-27%Er master alloy in electrical resistance furnace with the protection of 0.2% SF₆+CO₂ mixed gas. After all the elements were melted, the molten bath was heated to 750 ℃, then refinement agents (main constituent: C₂Cl₆, CaCO₃, MgCO₃) were added into the molten bath. After keeping for 30 min, the melt was cooled down to 680 ℃ and was cast into a permanent mould.

The microstructure of specimens was examined with optical microscope and scanning electron microscope(SEM) equipped with an EDX energy dispersive spectroscope system (Oxford). Metallographic specimens were etched in an 8% nitric acid solution in distilled water. Tensile mechanical property testing both at room temperature and elevated temperatures was conducted on a universal strength property test machine following Chinese National Standards GB/T228—2002 and GB/T4338—2006.

3.1 Optical microstructure

Fig.1 shows the as-cast microstructures of ZA73- xEr alloys under normal permanent mould cast condition. It can be seen that they all contain a significant volume fraction of eutectic phases. The quasi-continuous grain boundary networked τ-phase (Mg₃₂(Al,Zn)₄₉) is changed into discontinuous globular particle with the addition of 0.4%Er. The diameter of globular particle τ-phase increases when the content of Er reaches 0.7%, and the amount of τ-phase decreases.

The as-deformed microstructures of ZA73-0.4Er alloy after tensile testing are shown in Fig.2. It is seen that the distribution and amount of the second-phases remain almost unchanged below 250 ℃. The micro- structure after tensile testing at 150 ℃ shows no much difference from that undeformed (Fig.2(a)). In contrast, there are a few twins in samples tensile tested at 200 ℃ (Fig.2(b)) and 250 ℃ (Fig.2(c)). It is noted that there are large quantity of twins in the sample tensile tested at 200 ℃ if solution treatment is carried out before the test.

3.2 EDX analysis

At point A in Fig.3, Mg, Al, Er and Zn are detected by EDX in fine particles. The electronegativity difference between Mg and Er is smaller than that between Al and Er or Zn and Er. But the percentage of zinc at this point is rather smaller than that solved in the matrix nearby (point C). Thus the particle is deemed to be Al-Er compound. The results of EDX indicate there are Mg, Zn and Al elements in the globular compounds (point B) which are considered to be τ-phases. Fig.4 also reveals that the globular particles are τ-phases.

3.3 Thermal analysis

The DSC curves of ZA73 alloys with different Er contents are shown in Fig.5. There are two common peaks in each DSC curve, corresponding to the melting of eutectic phase (peak Ⅰ) and α-Mg matrix (peak Ⅱ). It is noted that there is an extra endothermic peak when Er content exceeds 0.4%, with the peak temperature increasing from 559.3 ℃ to 568.2 ℃ when Er content increases from 0.4% to 0.7%. It can be inferred that, combined with the microstructure, this extra peak may correspond to the transformation of the Al-Er phase formed in the Er alloyed ZA73 alloy.

Fig.1 Effects of Er on microstructures of as-cast ZA73 alloys: (a) 0; (b) 0.1%Er; (c) 0.4%Er; (d) 0.7%Er

Fig.2 Optical microstructures of as-cast ZA73-0.4Er alloy after tensile testing at temperature of 150 ℃ (a), 200 ℃ (b), 250 ℃ (c), and as-solution treated ZA73-0.4Er alloy at 200 ℃ (d)

Fig.3 SEM micrograph and corresponding EDX analysis results of compounds in as-cast ZA73-0.4Er alloy

3.4 Tensile mechanical properties

The tensile mechanical testing was conducted at 200 ℃, and the results are given in Table 1. It can be seen that with Er addition the tensile mechanical properties of the experimental alloys at 200 ℃ are improved. The yield strength and tensile strength increase gradually as the content of Er increases, while the elongation reaches its peak at 0.4% Er content, which is nearly five times the one without Er addition.

Table 1 Tensile mechanical properties of ZA73 alloys with different Er contents at 200 ℃

Table 2 shows the tensile properties of as-cast ZA73-0.4%Er alloy at the temperature from room temperature to 250 ℃. The yield strength changes little below 200 ℃. It is interesting to note that the strength doesn’t always decrease with increasing temperature. It reaches the peak at 150 ℃ (ultimate tensile strength) and 200 ℃ (yield strength) and then goes down. It seems that the alloy has a self strengthening effect at elevated temperature. When the temperature is higher than 200 ℃, the strengthening factor doesn’t bring into play any more, resulting in decline of the strength. It can also be seen that as the temperature increases, the strength changes little but the elongation increases dramatically.

Fig.4 SEM micrograph and corresponding EDX line scanning results of ZA73-0.4%Er alloy

Fig.5 DSC curves of ZA73 alloys with different Er contents:  (a) 0; (b) 0.1%Er; (c) 0.4%Er; (d) 0.7%Er

Table 2 Tensile properties of as-cast ZA73-0.4Er alloy at different temperatures

The maximal solid solubility of erbium in magnesium is 33.8%, which has a close-packed hexagonal structure. The solid solubility of erbium in magnesium decreases with the decrease of temperature. The atomic radius of Er is 175.7 pm which is 29.2% more than that of magnesium. So Er addition leads to intensive solution strengthening in magnesium. The electronegativity of Er is low (1.24), so stable compounds, which may lead to dispersion strengthening, may generate if higher electronegativity elements Al and Zn are added in magnesium as well. WANG et al[14] and XIAO et al[15] studied the effects of erbium in magnesium. They revealed that Mg-Zn-Er compounds exist in ZK60-Er and Al-Er compounds exist in AZ91-Er. In our work, Al-Er compound was also identified in ZA73-Er magnesium alloy.

The microstructure of ZA73 alloy is notably changed by Er addition. Erbium, which is a surface- active element, could restrain the growth of τ-phase. Besides, Al-Er compound, solidified first from the melt due to its high melting point, may act as the nucleus and refine the dendrite as well. Fine microstructure is realized when the addition amount increases to 0.4%. When the content of Er further increases to 0.7%, the formation of Al-Er compounds may consume part of Al element in the alloy, so the number of τ-phase decreases and the diameter increases.

Compared with the quasi-continuous grain boundary networked τ-phase, which may cause stress concentration and crack generation, the discontinuous globular τ-phase particle is not only an effective barrier of dislocation and grain boundary slipping, but also has the least harmful effect on the plasticity. As a result, the tensile properties of the experimental alloys are improved with Er addition. There are still some quasi-continuous grain boundary networked τ-phase existing in ZA73-0.1%Er sample, thus the strength property is not the best. Comparatively, the particles in 0.4%Er sample are discontinuous and homogeneous, which coordinate well with the matrix deformation, hence both the strength and elongation reach their peaks. When the content of Er further reaches 0.7%, the solution strengthening and dispersion strengthening increase, resulting in further increase of the strength but sacrificing the elongation heavily.

The flow stress depends on the characteristics of obstacles which hinder dislocations slipping. At elevated temperature, dislocation slipping occurs in the presence of both external stress and thermal activation process. Since the thermal activation process gets acute with the temperature rising, the needed external stress decreases, that is, the flow stress decreases. Thermal activation process is feeble and twinning does not occur in the 0.4%Er sample at 150 ℃, hence the strength is the highest but the elongation is rather low. In contrast, it is found from the microstructure that twinning occurs in the sample tensile tested at 200 ℃, which can cause grain rotation and promote dislocation slip. Besides, thermal activation starts up. As a result, dislocation slipping gets easier, leading to the increase of elongation. At higher temperature of 250 ℃, grain boundary softening occurs, resulting in dramatic decrease of the strength. In addition, grain boundary slipping and rotation may be one of the reasons for the elongation increase as well. ZA73-xEr series alloys would be suitable for usage in the temperature range of 150-200 ℃.

It is interesting to note that the strength of the as-cast ZA73-0.4%Er alloy doesn’t always decline with temperature rising, instead, the ultimate strength reaches its peak at about 150 ℃. We may call it self-strengthening effect. This may be attributed to dynamic age-hardening phenomena, probably due to the precipitation of the solution atoms from the matrix under both external stress and elevated temperature, which can anchor dislocations effectively. Stress concentration offers energy fluctuation for precipitate nucleating, and stretching strain promotes the decomposition of magnesium-based solid solution, consequently, precipitation occurs speedily. The tensile strength of ZA73-0.4%Er is the highest at 150 ℃, indicating that at this temperature dynamic age-hardening plays a major role rather than softening caused by temperature increasing. With the increase of temperature, softening effect exceeds hardening effects, resulting in strength reduction and elongation increase. After solution treatment, more elements dilute into matrix so more compounds precipitate. Therefore, the strength of the solution treated sample is higher than that of the untreated one.

1) The quasi-continuous grain boundary networked τ(Mg₃₂(Al,Zn)₄₉) phases are changed into discontinuous globular particles by the addition of Er. Spherical Al-Er compounds are also identified in the matrix.

2) With the Er addition, the elevated temperature tensile properties, both the elongation and strength, could be remarkably improved. The alloy modified by Er has a self-strengthening effect at elevated temperature.

3) ZA73-0.4%Er alloy has a good combined properties of strength and elongation in the temperature range of 150-200 ℃.

4) Dynamic age hardening occurs in the alloy containing erbium under external stress and elevated temperature.

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(Edited by YUAN Sai-qian)

Foundation item: Project(2007CB613704) supported by the National Basic Research Program of China; Project(50725413) supported by the National Natural Science Foundation of China; Projects(CSTC2006AA4012-9;  CSTC2006BB4023) supported by the Chongqing Science and Technology Commission, China

Corresponding author: ZHANG Jing, Tel: +86-23-65111167; Fax: +86-23-65102821; E-mail: jingzhang@cqu.edu.cn