Effect of Mg-Zn-Y quasicrystals on microstructure and mechanical properties of AZ91 alloy
ZHANG Jin-shan(张金山), PEI Li-xia (裴利霞), DU Hong-wei(杜宏伟),
XU Chun-xiang (许春香), LU Bin-feng(卢斌峰), LIANG Wei(梁 伟)
School of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
Received 28 July 2006; accepted 15 September 2006
Abstract: The influence of the additions of Mg-Zn-Y quasicrystals-containing master alloy on the microstructures and mechanical properties of AZ91 alloy under conventional casting condition was studied using XRD, SEM equipped with energy dispersive spectrometer (EDS). The results show that the microstructure of Mg-Zn-Y quasicrystals reinforced AZ91 alloy consists of α-Mg supersaturating solid solution, β-Mg17Al12 phase and quasicrystals phase. Quasicrystals particles with excellent elevated temperature stability are dispersively distributed in the α-Mg matrix or at grain boundaries. After the addition of quasicrystals-containing master alloy, the matrix microstructure of AZ91 alloy is obviously grain-refined. The morphology of β-Mg17Al12 phase changes from continuous nets to discrete nets. At room and elevated temperatures mechanical properties of AZ91 alloy are also improved dramatically. The utility of Mg-Zn-Y quasicrystals as a reinforced phase provides new theoretical basis and technical support for the composite strengthening of magnesium alloys.
Key words: Mg-Zn-Y quasicrystals; AZ91 alloy; mechanical properties; composite strengthening
1 Introduction
At present, composite strengthening is one of the most effective ways to improve the mechanical properties of magnesium alloys at room and elevated temperatures. As compared to conventional magnesium alloys, particle reinforced magnesium matrix composites have superior properties such as high specific strength, specific stiffness, thermal stability and low thermal expansion coefficient, thus become a hot topic in research and development of new structure materials[1-7]. However, the poor wetting of ex-situ ceramic reinforced phase with metal matrix, which exists in the present engineering materials, restricts the improvement of mechanical properties of magnesium alloys and makes fabrication processes complicated, production cost higher and extensive application difficult. SINGH et al[8-11] reported that wrought Mg-Zn-Y alloy can be strengthened by in-situ quasicrystal particles. Therefore, provided that a particle reinforced phase, which has excellent wettability and bond strength with magnesium matrix alloys and which improves strength, toughness and heat resistance of magnesium alloys, could be found, it would bring about great significance on dramatic performance improvement as well as extensive application of magnesium alloys. The purpose of the present work is to investigate the effect of Mg-Zn-Y quasicrystals on microstructure and mechanical properties of AZ91 alloy, so as to provide new theoretical basis and technical support for composite strengthening of magnesium alloys.
2 Experimental
Initially, AZ91 magnesium mother melt was prepared by induction melting with magnesium (99.9%, mass fraction), aluminum (99.9%), zinc (99.9%) and manganese (99.9%) raw materials. The nominal and real compositions of alloys are listed in Table 1. Then, medium Mg-Zn-Y (MZY) quasicrystal-containing master alloy (0, 1.7%, 3.4%, 5.2%, and 7.0%) was added into the mother melt. Next, proper adjustment of composition was conducted at the temperature range from 1 013 to 1 033 K so as to obtain melt with nominal composition, followed by pouring the melt into various molds to form relevant samples. The mechanical properties were examined after the samples were prepared according to requirements of testing standard. Constituent phases were identified by X-ray diffracto metry (Y-2000) using monochromatic Cu Kα radiation. The phase compositions were analyzed by electron dispersive spectroscope(EDS) in scanning electron microscope (SEM, JSU-6700F).
Table 1 Chemical compositions of AZ91 alloy (mass fraction, %)
3 Results and discussion
3.1 Effect of MZY master alloy on as-cast micro- structure of AZ91 alloy
Microstructure evolution occurs with increasing addition level of MZY master alloy into AZ91 melt (shown in Fig.1). Fig.1(a) shows that the microstructure of un-reinforced AZ91 magnesium alloy is composed of black disconnecting eutectic β-Mg17Al12 phase and gray-white α-Mg matrix phase. After the addition of 5.2% MZY master alloy, the matrix microstructure of AZ91 alloy can be obviously refined and the morphology of β-Mg17Al12 phase changes from continuous nets to discrete nets(shown in Fig.1(b)). Meanwhile, the substantial quasicrystals particles distribute homo- geneously on the matrix or at grain boundaries (shown in Fig.1(c)). However, with the addition level up to 7.0%, the grain size of α-Mg matrix adversely increases and β-Mg17Al12 phase is in semi-continuous net-like shape (shown in Fig.1(d)). The grain refinement of MZY quasicrystals-containing master alloy on the matrix of AZ91 alloy may contribute to high-melting-point i-Mg30Zn60Y10 phase, which precipitates at or near grain boundaries during the solidification process, sets back the diffusion of Mg, Al and Zn atomics, and finally suppresses subsequent growth of α-Mg and β-Mg17Al12[12].
Fig.2 shows XRD patterns of the AZ91 alloy and AZ91+5.2%MZY master alloy. XRD analysis confirms that quasicrystals particles exist in AZ91+5.2%MZY master alloy (shown in Fig.2(b)). Therefore, MZY quasicrystals reinforced AZ91 alloy consists of α-Mg, β-Mg17Al12 phase and i-Mg30Zn60Y10 phase.
SEM image and EDS spectrum of AZ91+5.2%MAY master alloy are shown in Fig.3. Fig.3(a) shows that large amounts of white petal-like or particle-like quasicrystals phases distribute dispersively in the matrix or at grain boundaries, which can effectively strengthen the matrix and grain boundaries. Energy dispersive analysis of spot A in Fig.3(a), which is shown in Fig.3(b), has verified that spot A is a quasicrystal particle.
Fig.1 Microstructures of un-reinforced and reinforced AZ91 alloys: (a) AZ91 alloy; (b)AZ91+3.4%MZY master alloy;
(c) AZ91+5.2%MZY master alloy; (d) AZ91+7.0%MZY master alloy
Fig.2 XRD patterns of AZ91 alloy(a) and AZ91+5.2% MZY master alloy(b)
Fig.3 SEM image(a) and EDS spectrum(b) of AZ91+5.2% MZY master alloy
3.2 Effect of MZY master alloy on mechanical properties of AZ91 alloy
Fig.4 shows the effect of the addition level of MZY master alloy on the mechanical properties of AZ91 alloy. The hardness of AZ91 magnesium alloy increases as the addition level of MZY master alloy increases. This phenomenon may be attributed to high hardness of i-Mg30Zn60Y10 phase, which distributes inside α-Mg matrix or at grain boundaries after the introduction of MZY master alloy into AZ91 magnesium alloy. The micro-hardness of i-Mg30Zn60Y10 phase is HV 556.7, much higher than that of β-Mg17Al12 phase (HV 153), which improves macro-hardness of AZ91 magnesium alloy. Furthermore, because of the high-melting-point of i-Mg30Zn60Y10，more nucleating sites are provided for the solidification of AZ91 melt, the grain size of β-Mg17Al12 becomes smaller, and α-Mg matrix with finer size and higher strength can be obtained. Therefore, i-Mg30Zn60Y10 plays a very important role in strengthen- ing AZ91 alloy.
Fig.4 Influence of content of MZY master alloy on mechanical properties of AZ91 alloy
Impact toughness of MZY reinforced AZ91 magnesium alloy is distinctively higher than that of AZ91 base alloy. With the addition level of master alloy increasing to 5.2%, the impact toughness reaches the peak, which is nearly twice as many as that of AZ91 base alloy. However, when the addition level reaches 7.0%, the impact toughness decreases slightly from the peak. This phenomenon is mainly attributed to the coarse-crystalline tendency resulting from improperly high addition level of MZY master alloy. The smaller the grain size is, the more energy is consumed since the total interface area increases with finer grains. Therefore, grain refinement can effectively enhance toughness[13].
The tensile properties of alloys at room and elevated temperatures are listed in Table 2, which shows that the tensile strength at room temperature is significantly improved and elongation is reduced slightly. At elevated temperature (473 K), the ultimate tensile strength, the yield strength, and elongation of AZ91 alloy with the addition of 5.2% MZY master alloy are increased by 16%, 19% and 13%, respectively.
Table 2 Tensile properties of alloys at room and elevated temperatures
The primary strengthening phase of AZ91 magnesium alloy is i-Mg30Zn60Y10, distributing inside matrix as well as at the grain boundaries. Additionally, the excellent wetting between i-Mg30Zn60Y10 and magnesium can bring about strong interface bond[14]. With the excellent interface bond strength between i-phase and the α-Mg matrix, disintegrating effect of i-phase on the matrix will not occur; crack initiation and propagation are hard to generate; finally effective strengthening of i-phase on the matrix can be accomplished. Moreover, the dispersive distribution of i-Mg30Zn60Y10 and β-Mg17Al12 phases in the matrix of AZ91 alloy can effectively prevent the dislocation movement when deformed at room temperature, thus strengthening AZ91 alloy.
Similar to other metallic materials, the heat resistance of magnesium alloys depends on softening power of the matrix as well as the elevated-temperature stability of the second phase. The stable and distributed second phase will reduce the grain boundaries sliding and dislocation motion of the matrix at elevated temperature, then excellent heat resistance will be ensured. The reinforced phase in AZ91 base alloy is mainly β-Mg17Al12 phase, whose strengthening effect functions only at low temperature range (less than 373 K). At higher temperature, β-Mg17Al12 phase with melting point 710 K shows low thermal stability. The hardness of β-Mg17Al12 phase is reduced by 50%-60% when it is heated from room temperature to 473 K[15], and thus its strengthening effect will be weakened. The addition of MZY quasicrystals-containing master alloy results in dispersive distribution of high-melting-point quasicrystals particles on the matrix or at grain boundaries, which promotes the matrix strength, locks the grain boundaries and finally improves the elevated temperature properties. With the temperature increasing, β-Mg17Al12 phase will coarsen and soften, gradually losing its strengthening effects, while thermodynami-
cally-stable and homogeneously-distributed i-Mg30Zn60Y10 phase can still prevent sliding and improve the elevated strength.
4 Conclusions
1) With the addition of MZY master alloy, the matrix microstructure of AZ91 alloy can be obviously refined and the morphology of β-Mg17Al12 phase changes from continuous nets to discrete nets. Large amounts of i-Mg30Zn60Y10 phase distribute homogeneously in the
matrix or at grain boundaries.
2) The hardness of AZ91 magnesium alloy increases with the addition level of MZY master alloy. With the addition level of master alloy increasing to 5.2%, the impact toughness reaches the peak, which is nearly twice as much as that of AZ91 based alloy.
3) The ultimate tensile strength at room temperature is significantly improved by 24% and elongation is reduced slightly. The ultimate tensile strength, the yield strength and elongation of AZ91 alloy with the addition of 5.2% MZY master alloy at elevated temperature (473 K) are increased by 16%, 19% and 13%, respectively.
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(Edited by CHEN Wei-ping)
Foundation item: Projects(50571073) supported by the National Natural Science Foundation of China; project (20051052) supported by the Natural Science Foundation of Shanxi Province, China
Corresponding author: ZHANG Jin-shan; Tel: +86-351-6018154; Fax: +86-351-6018208; E-mail: jinshansx@tom.com
