­­­ Effects of grain refinement on mechanical properties and

microstructures of AZ31 alloy

YU Kun(余 琨), RUI Shou-tai(芮守泰), SONG Jue-min(宋觉敏), LI Wen-xian(黎文献), GUO Long(郭 隆)

School of Materials Science and Engineering, Central South University, Changsha 410083, China

Received 12 June 2008; accepted 5 September 2008

Abstract: Cerium was added in AZ31 alloy with the contents of 0.4%, 0.8% and 1.2% respectively to produce experimental alloys. The grain refinement of Ce in the as-cast and rolled AZ31 alloy were studied by using Polyvar-MET optical microscope with a VSM2000 quantitative analysis system, KYKY2000 SEM and Tecnai G² 20 TEM. And the mechanical properties of AZ31+Ce alloy were tested on a CSS-44100 testing system with computerized data acquisition. The results show that the cerium has a good grain refinement effect on the as-cast AZ31 alloy because cerium can build up a solute enriched zone rapidly during the solidification process. The dynamic recrystallization (DRX) grains less than 10 μm can be obtained in hot rolled AZ31+Ce alloy. A cold rolling deformation degree over than 20% and a following annealing at 400 ℃ for 1 h will lead to refine and uniform grains with the sizes of about 25 μm. The cerium can form dispersed and thermally stable Al₄Ce phase that can prohibit the coarsening of grains in AZ31+Ce alloy during the hot rolling and annealing process.

Key words: AZ31 magnesium alloy; grain refinement; rare earth

1 Introduction

Magnesium alloys have a low density, good specific strength and excellent recycling capabilities so that they are attractive materials for using in a wide range of structural applications recently[1-2]. Control of the grain size and microstructure is a key factor for magnesium alloys because the uniform fine grains are preferred for both the cast and the wrought magnesium alloys[3-4]. The strength and ductility of the refined-grain magnesium alloys will be improved obviously[5-7]. Therefore, finding an effective grain refining method and studying the properties of refined alloy play an important role in producing high quality magnesium alloys. Some researches have been done on the grain refinement of magnesium alloys during the casting process. For example, rapid cooling and stirring of the solidifying melt effectively refine the grain structure in some high-pressure die casting process[8-10]. And adding 0.3%-0.8% zirconium can obtain an extremely fine grain in pure Mg or Mg alloys without Al and Mn[11]. Superheating of the melt, which is used extensively inthe past for grain refining of Mg-Al system alloy, is probably induced by the formation of Al₄C₃ precipitates as the grain nucleus[12-13]. In the present work, the rare earth element cerium was added in the AZ31alloy to study its grain refinement effect. The microstructures and the mechanical properties of the experimental alloy were also researched.

2 Experimental

The experimental alloy belongs to Mg-Al-Zn-Mn system. The cerium was added in AZ31 alloy (Mg-3%Al-1%Zn-0.4%Mn) with the contents of 0.4%, 0.8% and 1.2% respectively by using the Mg-20%Ce master alloys. The experimental alloys with addition of cerium were melted by the protection of a flux at 760-800 ℃ and cast into iron mould. Then the ingots of experimental alloys were hot rolled with different reduction rates. The hot rolled sheets were annealed at 300 ℃ for 1 h, then cold rolled with different reduction of 5% and 22%, respectively.

The microstructures of the ingots were observed by using Polyvar-MET optical microscope with a VSM2000quantitative analysis system, KYKY2000 SEM and Tecnai G² 20 transmission electron microscope(TEM). TEM foils were prepared by standard twin-jet electropolishing in a solution of 97% methanol and 3% nitric acid at about 16 V and 253 K. Tensile test was performed on a CSS-44100 testing system with computerized data acquisition.

3 Results and discussion

3.1 Grain refinement of cerium on as-cast AZ31 alloy

Different contents of cerium were added in the AZ31alloy during the melting. The microstructures of as-cast AZ31 alloy with three different contents of cerium are shown in Fig.1. The grain size of AZ31 alloy without cerium is 200-300 μm. But the grains are refined obviously with adding 0.4%Ce. Especially, with the increase of cerium content in the alloy, the grain size of alloy becomes smaller. The average grain size is only about 40 μm with an adding of 0.8%Ce. And it can be found that the content of second phases on the grain boundaries increases with the Ce adding to 1.2%.

Fig.1 Grain refinement of as-cast AZ31 alloy with rare earth cerium: (a) AZ31 Alloy; (b) AZ31+0.4%Ce; (c) AZ31+0.8%Ce; (d) AZ31+1.2%Ce

With the analysis of X-ray diffraction (Fig.2) and the scanning electron microscopy (Fig.3), the compounds on the boundaries of AZ31Ce alloy are β(Mg₁₇Al₁₂) and Al₄Ce phases. Based on the Mg-Ce phase diagram, the solubility of cerium in Mg is so small that the maximum solution content is less than 0.5%. So, the cerium will build up a solute enriched zone rapidly during the solidification process. Such solute enriched zone exists on the boundaries between solid and liquid phase and causes a supercooling zone ahead the solid grain growth front. Therefore, a heterogeneous crystal nucleus forming mechanism occurs in such zones during the solidification as shown in Fig.4.

Fig.2 XRD patterns of AZ31Ce alloy sample

Fig.3 Morphology of AZ31Ce alloys

Fig.4 New nucleus in supercooling zone formed by enriched Ce

3.2 Grain refinement of Ce on hot rolling AZ31 sheet

The experimental AZ31 alloys with different contents of cerium were hot rolled at 400 ℃ under a reduction of 40%-50% then annealed at 300 ℃. The total reduction is about 90%. There are different micro- structure morphologies in the hot rolled alloys, and all of them are shown in Fig.5(a). The deformation micro- structure morphology and dynamic recrystallization

(DRX) structures are both observed in the hot rolled alloy sheets[14]. Such DRX grains of less than 10 μm are refined during the hot deformation process and can provide a good plastic deformation ability to alloy sheets. The addition of cerium in alloy can form Al₄Ce phases that disperse in the Mg matrix homogeneously and can impede the growth of grain (Figs.5(b) and (c)). Therefore, the refined DRX grains of AZ31Ce alloy sheet get a better deformation property than the alloy without cerium.

Fig.5 Microstructures of hot-rolled AZ31Ce Alloy: (a) Deformation structures and DRX grains during hot rolling(OM); (b) Refined DRX grains(TEM); (c) Al₄Ce particles in AZ31Ce alloy(TEM)

 

3.3 Grain refinement of cold rolled and annealed AZ31Ce sheet

The annealed AZ31Ce sheet after hot rolling can be cold rolled with a reduction from 5% to 22% at the room temperature. During the rolling deformation process, the main deformation behaviors are dislocation slip and twinning[15]. They can be observed with the TEM as shown in Fig.6. But the grain sizes under different rolling reductions after annealing are distinguished obviously. It can be seen in Figs.7(a) and (b) that the average grain size is larger than 50 μm with a small rolling reduction of about 5%. And such annealed grains are not uniform. Some of them are even larger than 100 μm. So it is un-suitable to roll the AZ31Ce alloy under such a small reduction. But the microstructures after cold rolling over 20% can obtain very fine grains under a suitable annealing process. The annealing temperature should be higher than 300 ℃ and the time should be shorter than 2 h. With the comparison of different annealing process (Figs.7(c) and (d)), the optimal annealing process of the cold rolled AZ31Ce alloy is about 400 ℃ for 1 h. The average grain size is only 25 μm and very uniform. Such fine grain is obtained due to the fact that the Al₄Ce phase has a good thermal stability and can prohibit the grain coarsening.

Fig.6 Dislocation and twinning in cold rolled AZ31Ce alloy

3.4 Mechanical properties of refined AZ31Ce alloys

The tensile strength and elongation of AZ31Ce with different deformation degree and annealing temperatures are shown in Fig.8. It can be seen that the deformation degree is the main influence factor to the tensile strength at room temperature. But annealing at high temperature (400 ℃) for a short time (0.5-1 h) can provide a good elongation for the alloy, due to the fine recrystallization grains.

4 Conclusions

1) Cerium has a good grain refining effect on the as-cast AZ31alloy because it can build up a solute enriched zone rapidly during the solidification process. Such solute enriched zone exists on the boundaries between the solid and liquid and causes a supercooling zone ahead the grain growth front. Therefore, the cast grain can be refined obviously.

Fig.7 Grains morphologies after different deformation degrees and annealing temperatures: (a) 5% reduction, annealing at 250 ℃ for 1 h; (b) 5% reduction, annealing at 400 ℃ for 1 h; (c) 22% reduction, annealing at 250 ℃ for 1 h; (d) 22% reduction, annealing at 400 ℃ for 1 h

Fig.8 Mechanical properties of AZ31Ce alloy: (a) Ultimate tension strength(UST); (b) Elongation

2) The DRX grains less than 10 μm can be obtained in hot rolled AZ31Ce alloy. The cerium can form dispersed and thermally stable Al₄Ce particles that can prohibit the coarsening of AZ31Ce alloy grains during the hot deformation process.

3) A cold rolling deformation degree over than 20% and a following annealing at 400 ℃ for 1 h will lead to a refined and uniform grains about 25 μm. Such small recrystallization grains can provide a good elongation for the AZ31Ce alloy.

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(Edited by ZHAO Jun)

Foundation item: Project(2006BAE04B02-3) supported by the National Science and Technology Support Program during the 11th Five-Year Plan of China

Corresponding author: YU Kun; Tel: +86-731-8879341; E-mail: kunyugroup@163.com