­­Influence of Sb modification on microstructures and mechanical properties of Mg2Si/AM60 composites

YAN Hong(闫 洪)¹, HU Yong(胡 勇)², WU Xiao-qian(吴孝泉)¹

1. Institute of Advanced Forming & Manufacturing and Die & Mold, Nanchang University, Nanchang 330031, China;

2. School of Mechanical and Electronical Engineering, East China Jiaotong University, Nanchang 330013, China

Received 23 September 2009; accepted 30 January 2010

Abstract: The refining effect and mechanism of Sb on Mg₂Si and the microstructure of the matrix were investigated. The results indicate that there are Mg₃Sb₂ particles in the composites with the addition of Sb, and Mg₃Sb₂ can promote the formation of fine polygonal type Mg₂Si by providing nucleation site. Meanwhile, the grain size of Sb modified alloy is finer than that of the matrix. The improved microstructure results in the improvement of mechanical properties. The ultimate tensile strength is increased by 12.2% with the addition of 0.8% Sb.

Key words: Mg₂Si; Sb modification; magnesium matrix composites; heterogeneous nucleation; mechanical property

1 Introduction

The composites prepared with in-situ reaction method are an important development direction of magnesium matrix composites. The reinforcement phase is produced in the matrix, which has many advantages such as small size, clean interface and no contamination, good thermal stability and good compatibility with the matrix[1]. There are reinforcement phases such as TiC[2], TiB₂[3], AlN[4] and Mg₂Si[1,5]. Especially, Mg₂Si is characterized by small density, high melting point, high hardness and strength and high elastic modulus, which becomes an ideal reinforcement phase of magnesium matrix composites. However, Chinese script type and plate-block Mg₂Si phases present in the matrix and dissever the matrix, which affect the mechanical properties of the matrix[6-12]. Therefore, the influences of Sb modification on the microstructure and mechanical properties of magnesium matrix composites Mg₂Si/ AM60 were studied.

2 Experimental

The experimental materials were AM60 alloy and crystallizing Si. First, AM60 alloy was melted in electric resistance furnace protected by self-made covering agent and Ar to avoid oxidative combustion of Mg alloy. The crystallizing Si powders packed in aluminum foil were added into the melt with the bell-jar at 800 ?C for 15 min. The melt was stirred for 5-10 min in order to make an ample dissolution and homogeneous diffusion of Si. After holding for 30 min, Sb was added into the melt at 700 ?C for 5 min. Then the melt was poured into a steel mold.

Metallographic specimens from the same section were corroded by HNO₃-alcohol solution, whose microstructures were examined by OM and SEM. The Brielle hardness was measured by HBE-3000A. The tensile tests were carried out on an electric universal testing machine.

3 Results and discussion

3.1 Influence of Sb on microstructure of composites

Fig.1 shows the influence of Sb on the microstructure of composites. α-Mg, β-Mg₁₇Al₁₂ and Chinese script type Mg₂Si phases present in the composites without the addition of Sb. It is obvious that Chinese script type Mg₂Si is changed into particles with 0.4% Sb addition and the matrix structures are refined. The matrix structures are further refined with 0.8% Sb. As Sb addition is increased up to 1.2%, Mg₂Si begins to grow and is finer than that of un-modified composites.

Fig.1 Effects of Sb on microstructures of composites: (a) w(Sb)=0; (b) w(Sb)=0.4%; (c) w(Sb)=0.8%; (d) w(Sb)=1.2%

3.2 Heterogeneous nucleation of Mg₂Si

The capability of heterogeneous nucleation depends on the interfacial energy between the nucleated substrate and crystalline phase. The factors of interfacial energy relate mainly to the lattice disregistry and electrostatic potential between the substrate and crystalline phase, chemical properties and morphology of substrate and so on[13-14]. TURNBULL and VONNDGUT pointed out that the factors promoting heterogeneous nucleation were determined by the lattice disregistry and a one- dimensional lattice model was supposed[15]. But BRAMFITT[16] found that the experimental results were different from the calculation ones with this one- dimensional lattice model. Therefore, a two-dimensional lattice model was put forward by him and the above problem was resolved. The two-dimensional lattice model was defined as:

                         (1)

where S is the lattice plane of substrate, n is the lattice plane of crystalline phase; (hkl)_S is the low-index plane of substrate, [uvw]_S is one direction of low-index plane on (hkl)_S; (hkl)_n is the low-index plane of crystalline phase, [uvw]_n is one direction of low-index plane on (hkl)_n; is the atomic distance in direction of [uvw]_n, is the atomic distance in direction of [uvw]_S; θ is the angle between [uvw]_S and [uvw]_n.

Mg₂Si is a face-centered cubic structure with the lattice constants a=0.634 7 nm. Mg₃Sb₂ is a close-packed hexagonal structure with lattice constants a=0.457 0 nm and c=0.723 0 nm. According to Eq.(1), the parameters of lattice disregistry of Mg₂Si and Mg₃Sb₂ are listed in Table 1. The relationships of lattice disregistries among low-index planes (0001) of Mg₂Si and low-index planes

Table 1 Lattice disregistry parameters of Mg₃Sb₂ and Mg₂Si

(100), (110) and (111) of Mg₃Sb₂ are shown in Fig.2.

Substitute the data in Table 1 in Eq.(1), then

100%=18.11%      (2)

100%=54.32%       (3)

Fig.2 Relationships of lattice disregistries among low-index plane (0001) of Mg₃Sb₂ and low-index planes (100), (110) and (111) of Mg₂Si: (a) Mg₃Sb₂ (0001) and Mg₂Si (100); (b) Mg₃Sb₂ (0001) and Mg₂Si (110); (c) Mg₃Sb₂ (0001) and Mg₂Si (111)

=1.83%        (4)

From Eqs.(2)?(4), the lattice disregistries between Mg₂Si (0001) and Mg₃Sb₂ (111) is 1.83% and less than 6%. So Mg₃Sb₂ can act as the heterogeneous nuclei of Mg₂Si. Fig.3(a) shows the SEM morphology of Mg₂Si particles in composites. Fig.3(b) shows the EDS spectrum of Mg₂Si core position, which contains the compounds of Mg, Si and Sb. It is inferred that Mg₃Sb₂ is nucleus core based on its atomic ratio.

Fig.1 shows that the grains can be refined by the addition of Sb. This phenomenon may be explained that the majority of Mg₃Sb₂ cores form during the solidification of alloy, and the Mg₂Si particles grow with Mg₃Sb₂ as the crystallization core. So a large number of finer Mg₂Si particles are obtained, which aggregate in front of solid-liquid interface so as to hinder the spread of atomic Al and obtain finer Mg₂Si particles and matrix. As shown in Fig.1, the modification effect becomes worse with 1.2% Sb. According to the theory of diffusion and phase transition, the coarsening rate of particles precipitated relates to the concentration of solute atoms. The higher the concentration of solutes atoms, the higher

Fig.3 SEM morphology (a) and EDS spectrum (b) of Mg₂Si particles the coarsening rate of particles. The Mg₃Sb₂ phase can aggregate and grow with increasing Sb content, and the heterogeneity of nuclear-point is reduced, which affects the nuclear-rate and makes Mg₂Si phase and the matrix enlarge. It is significant to affect Mg₂Si phase.

3.3 Effects of Sb on mechanical properties of composite

Fig.4 shows the effects of Sb on the mechanical properties of composites. The tensile strength of composites increases with increasing Sb content. The tensile strength is up to 221.6 MPa and improved by 12.2% when Sb content is 0.8%, compared with that of matrix. The tensile strength of composites decreases with further increasing Sb content, and the elongation rate of composites decreases with Sb content increasing.

Fig.4 Effects of Sb on mechanical properties of composites

As shown in Fig.4(b), the hardness of composites changes little with increasing Sb content. The hardness of composites is HB 55 with no addition of Sb and   HB 53-56.3 with the addition of Sb, which is mainly affected by the reinforcement of Mg₂Si. So the hardness has no change for the contents of Mg₂Si in composites maintain the original value even if Sb is introduced.

The mechanical properties of composites are improved with the addition of Sb. On one hand, Mg₂Si particles are refined and distributed at the matrix and grain boundaries, which increase the strength of composites for the pinning effect. On the other hand, the grains are refined by introducing Sb into composites. According to Hall-Petch formulation, the relationship between grain size and yield strength is expressed as follows.

                             (5)

where σ_s is the yield strength, σ₀ and k are constant, and D is the grain size.

It is known from Eq.(5) that the yield strength increases with decreasing grain size. And decreasing the grain size means increasing grain boundaries, which can improve the mechanical properties of magnesium alloy because the grain boundaries can hinder the dislocation motion.

The reason why the elongation of composites decreases is that the melting point is high and the plasticity of Sb modified alloy.

4 Conclusions

1) The Chinese script type Mg₂Si present in the Mg₂Si/AM60 composite is changed into the finer particles with the addition of Sb. Meanwhile, the grain size of Sb modified alloy is finer than that of the matrix.

2) There is a new phase formed with the addition of Sb, which provides the nucleation core of Mg₂Si.

3) The proper introducing of Sb results in the improvement in mechanical properties.

References

[1] HU Yong, YAN Hong, CHEN Guo-xiang. Effects of Si on microstructure and properties of in situ synthesized Mg₂Si/AM60 composites [J]. Rare Metal Materials and Engineering, 2009, 38(2): 343-347. (in Chinese)

[2] WANG Ji-jie, GUO Jin-hua, CHEN Li-qing. TiC/AZ91D composites fabricated by in situ reactive infiltration process and its tensile deformation [J]. Transactions of Nonferrous Metals Society of China, 2006, 16: 892-896.

[3] WANG H Y, JIANG Q C, ZHAO Y Q. In situ synthesis of TiB₂/Mg composites by self-propagating high-temperature synthesis reaction of Al-Ti-B system in molten magnesium [J]. Journal of Alloys and Compounds, 2004, 379: L4-L7.

[4] YE H Z, LIN X Y, BEN L. In situ synthesis of AlN particles in Mg-Al-alloy by Mg₃N₂ addition [J]. Materials Letters, 2004, 58: 2361-2364.

[5] LU L, THONG K K, GUPTA M. Mg-based composite reinforced by Mg₂Si [J]. Composite Science and Technology, 2003, 63: 627-632.

[6] NAM K Y, SONG D H, LEE C W. Modification of Mg₂Si morphology in as-cast Mg-Al-Si alloys with strontium and antimony [J]. Materials Science Forum, 2006, 510(3): 238-241.

[7] YUAN G Y, SUN Y S, DING W J. Effects of Sb addition on the microstructure and mechanical properties of AZ91 magnesium [J]. Scripta Mater, 2000, 43(11): 1009-1013.

[8] BALASUBRAMANI N, SRINIVASAN A, PILLAI U T S. Effect of antimony addition on the microstructure and mechanical properties of AZ84 magnesium alloy [J]. Journal of Alloys and Compounds, 2007, 455(1/2): 168-172.

[9] SRINIVASAN A, PILLAI U T S, PAI B C. Microstructure and mechanical properties of Si and Sb added AZ91 magnesium alloy [J]. Metallurgical and Materials Transactions A, 2005, 36(8): 2235-2243.

[10] WANG Li-guo, ZHANG Bao-feng, ZHU Shi-jie. Effect of silicocalcium on microstructure and properties of Mg-6Al-0.5Mn [J]. Transactions of Nonferrous Metals Society of China, 2006, 16: 551-555.

[11] JIANG Q C, WANG H Y, WANG. Y. Modification of Mg₂Si in Mg-Si alloys with yttrium [J]. Materials Science and Engineering A, 2005, 392: 130-135.

[12] KIM J J, KIN D H, SHIN K S. Modification of Mg₂Si morphology in squeeze cast Mg-Al-Zn-Si alloys by Ca or P addition [J]. Scripta Materialia, 1999, 41(3): 333-340.

[13] KAPRANOS P, LIU T Y, ATKINSON H V. Investigation into the rapid compression of semi-solid alloy slugs [J]. J Mater Process Technol, 2001, 111: 31-36.

[14] TSENG A A, HORSKY J, RAUDENSKY M. Deformation behavior of steels in mushy state [J]. Materials and Design, 2001, 22: 83-92.

[15] FLEMINGS M C. Solidification processing [M]. New York: McGraw-Hill, 1974: 45-89.

[16] BRAMFITT B L. The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron [J]. Metallurgical Transactions, 1970, 1(7): 1-9.

(Edited HE Xue-feng)

Foundation item: Project(50765005) supported by the National Natural Science Foundation of China

Corresponding author: YAN Hong, Tel: +86-791-3969633; E-mail: yanhong_wh@163.com