Trans. Nonferrous Met. Soc. China 23(2013) 3161-3166

Modification of Mg₂Si in Mg-Si alloys with neodymium

Ji-long HU^1,2,3, Chang-ping TANG^1,2, Xin-ming ZHANG^1,2, Yun-lai DENG^1,2

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

2. Key Laboratory of Nonferrous Metal Materials Science and Engineering of Ministry of Education, Central South University, Changsha 410083, China;

3. Beijing Automotive Co., Ltd, Beijing 101300, China

Received 13 December 2012; accepted 17 May 2013

Abstract: The modification effect of neodymium (Nd) on Mg₂Si in the hypereutectic Mg-3%Si (mass fraction) alloy was investigated by optical microcopy, scanning electron microscopy and X-ray diffraction. The results indicate that the morphology of the primary Mg₂Si transforms from coarse dendrite into fine polygon with increasing Nd content. The average size of the primary Mg₂Si significantly decreases to about 10 μm with increasing Nd content up to 1.0%, and then becomes coarser again. The modification and refinement of the primary Mg₂Si are mainly attributed to the poisoning effect. The NdMg₂ phase in the primary Mg₂Si transforms into NdSi and NdSi₂ compounds as the Nd content exceeds 3.0%. Therefore, it is reasonable to conclude that the proper Nd (1.0%) addition can effectively modify and refine the primary Mg₂Si.

Key words: Mg-Si alloy; modification; Mg₂Si; Nd

1 Introduction

Recently, Mg alloys have a great potential application to the lightweight components of transportation vehicles due to their low density and high specific strength. Among them, Mg-high Si alloys are Mg matrix composites (MMCs) containing in-situ synthesized Mg₂Si particles [1]. According to the Mg-Si phase diagram, the solubility of Si in Mg matrix is only 0.003% (mole fraction), extra Si atoms react with magnesium and are precipitated as an intermetallic compound of Mg₂Si. Many researchers [1-3] reported that Mg alloys containing Mg₂Si particles have high potential because Mg₂Si exhibits a low density (1.99×10³ kg/m³) , high melting point (1085 °C), high hardness (4.5×10⁹ Pa), reasonably high elastic modulus (120 GPa), and a low coefficient of thermal expansion (7.5×10^-6 K^-1). However, the large and brittle Mg₂Si particles will greatly deteriorate the mechanical properties of Si-containing magnesium alloys, which is a principal obstacle for their application [2,4]. Therefore, how to modify and refine the coarse Mg₂Si particles in Mg-high Si alloys has attracted considerable attention. It has been reported that some processing techniques (rapid solidification, hot extrusion) [2,5,6], heat treatment [4,7] and alloying addition (Ba, Sb, Bi, Sr or Y) [1,3,8-10] were able to produce positive modification effect on the morphology of Mg₂Si in Mg-Si alloys. Up to date, only XU et al [11] proposed that Nd element can effectively modify the primary and eutectic silicon in hypereutectic Al-Si alloy. Rather limited information is available regarding the modification effect of Nd on the primary Mg₂Si in hypereutectic Mg-Si alloys.

Considering the similarity between Si modification in Al-Si alloy and Mg₂Si modification in Mg-Si alloy [3], we attempt to apply the Nd element to modifying the primary Mg₂Si in the hypereutectic Mg-Si alloys. The aim of this work is to develop an effective modifier on the primary Mg₂Si for hypereutectic Mg-Si alloys and explore the modification mechanism. It is also expected that the preliminary results can be significant in promoting the fabrication of high quality Mg-Gd- Y-Nd-Si system alloys as demonstrated [12].

2 Experimental

Mg-3%Si (mass fraction, the same below if not mentioned) alloy was prepared by melting pure Mg (>99.93%) and Si (>99.95%) in a steel crucible placed in an electrical resistance furnace at 760 °C under the protection of Ar atmosphere. After about 20 min, the desired amounts of Mg-30.15%Nd master alloys with 0.5%, 1.0%, 2.0% and 3.0% Nd were added into the Mg-Si melts, respectively. The melts were stirred for about 90 s at a speed of 300 r/min, then poured into a preheated (250 °C) permanent low-carbon steel mold (d55 mm×150 mm).

Samples for microstructure observation were initially polished using different grades of polishing papers and finally polished with 0.25 μm diamond paste. Polished samples were chemically etched in a solution of 4% (volume fraction) nital for 10 s. Microstructure observation was performed on an optical microscope (OM) and scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). The size of primary Mg₂Si particles was measured by a mean linear intercept method. X-ray diffraction (XRD) analyses were carried out using a Rigaku D/max 2500 diffractometer (Cu K_α radiation) with a scanning angle from 10° to 80° and a scanning speed of 2 (°)/min. The phases were identified by the ICDD PDF2—2004 database in the Jade 6 software.

3 Results

3.1 Microstructure of Mg-3%Si alloy

According to the Mg-Si binary phase diagram, the composition of Mg-3%Si alloy is a hypereutectic alloy with a solidified microstructure of primary Mg₂Si and eutectic α-Mg+Mg₂Si phases. The SEM microstructure of the unmodified Mg-3%Si alloy is shown in Fig. 1(a). It should be mentioned that the coarse dendritic phases are primary Mg₂Si (as shown by arrow A), while the rod-like shaped phases are eutectic Mg₂Si (as shown by arrow B), and the gray areas are Mg matrix (as shown by arrow C). The average size of the primary Mg₂Si particles is about 65 μm. Moreover, the primary Mg₂Si dendritic phases are surrounded by Mg dendritic halos, around which the eutectic α-Mg+Mg₂Si phases are formed, which is similar to the microstructure feature that was observed in other hypereutectic systems, such as Al-Si and Mg-Si alloys [1,13]. Figure 1(b) shows the XRD pattern of the Mg-3%Si alloy. It is evident to note that the components of the alloy are only α-Mg and Mg₂Si phases, which agrees fairly well with the results of SEM observation.

3.2 Microstructures of Mg-3%Si alloys with Nd addition

Figure 2 shows the OM images of Mg-3%Si alloys with 0, 0.5%, 1.0%, 2.0% and 3.0% Nd, respectively.

Fig. 1 SEM image (a) and XRD pattern (b) of as-cast Mg-3%Si alloy

Fig. 2 OM images of Mg-3%Si alloys with different Nd additions

Fig. 3 SEM image (a), EDS line scanning result of Mg, Si and Nd (b), XRD pattern (c) and EDS results of points A (d) and B (e) of Mg-3%Si alloy with 1.0% Nd

When the Nd content is 0.5%, the primary Mg₂Si is only slightly refined to about 50 μm and its morphology is changed from coarse dendrite to polygon (Fig. 2(b)), which can be named sub-modified alloy. When the Nd content is 1.0%, the average size of the primary Mg₂Si is the smallest, which is about 10 μm, and its morphology still keeps the irregular and fine polygon (Fig. 2(c)). It can be named a fully modified alloy. However, when the Nd content is further increased to 2.0% and 3.0%, the primary Mg₂Si becomes coarser again, although there is no obvious change in its morphology (Figs. 2(d) and (e)), which means that the modification effect of Nd is weakened, and consequently it can be named an over modified alloy. Compared with the primary Mg₂Si, it should be noted that the Nd addition has little effect on the morphology of the eutectic Mg₂Si.

The SEM image, EDS line scans of Mg, Si and Nd, XRD pattern and EDS results of points A and B of the Mg-3%Si alloy with 1.0% Nd are shown in Fig. 3. In Fig. 3(a), some tiny white particles are found in the primary Mg₂Si. It can be seen from Fig. 3(b) that the white particle (as shown by arrow A) is a Mg-Nd compound. However, it is hard to identify this phase by the XRD pattern (Fig. 3(c)) due to its low intensity.

According to the EDS result (Fig. 3(d)), the chemical composition of the white particle in the primary Mg₂Si is Mg-28.75%Si-4.12%Nd (mole fraction), which indicates that its stoichiometry is near (Mg₂Si)(NdMg₂). Therefore, it is reasonable to conclude that the Mg-Nd compound is NdMg₂ phase. Furthermore, it is also observed that there are some tiny white particles (as shown by arrow B) located in the matrix, and the chemical composition is Mg-10.87%Si-10.23%Nd (mole fraction) according to the EDS result (Fig. 3(e)). This indicates that its stoichiometry is near NdSi, which is different from that in the primary Mg₂Si.

Figure 4 shows the SEM image and XRD spectrum of the Mg-3%Si alloys with 3.0%Nd. Compared with Fig. 3(a) and Fig. 4(a), it is evident that the white particles (as shown by arrow A) in the primary Mg₂Si are obviously coarsened from 0.5 μm to 5.0 μm and agglomerate. According to the EDS result (Fig. 4(c)), it can be seen that its chemical composition is Mg-34.10%Si-29.14%Nd (mole fraction). This indicates that its stoichiometry is near NdSi_2-x. While the chemical composition of the white particle (as shown by arrow B) in the matrix is Mg-39.91%Si-35.27%Nd (mole fraction) according to the EDS result (Fig. 4(d)), which is also near NdSi_2-x. It is also evident that more diffraction peaks of NdSi and NdSi₂ appear in the alloy according to the XRD spectrum (Fig. 4(b)), which means that the white NdSi_2-x particles are NdSi and NdSi₂ compounds. Therefore, it is concluded that the white particles in the primary Mg₂Si transform from NdMg₂ phase into the NdSi and NdSi₂ compounds as Nd content increases.

4 Discussion

CHEN et al [3] reported that there are two major mechanisms of modification and refinement of the Mg₂Si particles. One is the increase of nucleation, which is the simplest and most effective. The formation of large amount of nuclei in the melts leads to the refinement of the Mg₂Si particles [14]. The other is inhibition of crystal growth through changing the solidification condition and the modification is attributed to the poisoning effect. Rare earth (RE) elements, which are surface-active elements, are prone to adsorbing into the crystal growth front to changing the surface energy of the crystal, effectively poison the growth steps, resulting in the isotropic growth of the crystals. JIANG et al [1] reported that the adsorption of Y atoms on the Mg₂Si growing surface front can change the solidification condition and modify the primary Mg₂Si particles.

Since Nd element has a high solubility (3.6%) in magnesium matrix, it is unlikely to form any Nd-containing compounds in Mg–Si melts at the early solidification stage. And the primary Mg₂Si becomes coarser again (Fig. 2(d) and Fig. 4(a)) as more Nd-containing compounds form, which rules out the possibility of the Nd-containing compounds acting as heterogeneous nucleation sites for the primary Mg₂Si. Meanwhile, the faceted growth of Mg₂Si crystals is very sensitive to the solidification conditions [15]. LU and HELLAWELL [16] proposed the impurity-induced twinning mechanism in which atoms of the modifier are absorbed onto the growth steps of the silicon surface. A growth twin will be created at the interface when the ratio (the atomic radius ratio of the modification elements relative to silicon) is close to 1.6457 (1.54-1.85). The radius ratio of Nd atom (r_Nd=0.18214 nm) to Si atom (r_Si=0.11720 nm) is 1.55 [16]. Therefore, it is reasonable to deduce that the modification effect of Nd on Mg₂Si is attributed to the poisoning effect.

Fig. 4 SEM image (a), XRD spectrum (b) and EDS results of points A (c) and B (d) of Mg-3%Si alloy with 3.0% Nd

According to the impurity-induced twinning mechanism, the Nd atoms may be adsorbed onto the growth steps of the Mg₂Si crystal and change the surface energy of the Mg₂Si crystal by lattice distortion during solidification. Then, this will effectively poison the growth steps of Mg₂Si and promote multiple twinning to cause structural modification. Consequently, the preferred growth manner of the primary Mg₂Si is suppressed, while another type of growth, isotropic growth type, is accelerated [1,11,16].

When the content of Nd is 0.5%, the concentration of Nd atoms is comparatively low in melts, so the Nd atoms absorbed upon the growth steps of the Mg₂Si crystal are less. Therefore, the morphology and size of the primary Mg₂Si are only slightly modified. When the content of Nd is up to 1.0%, more Nd atoms can be adsorbed onto the growth steps of the Mg₂Si crystal. In this case, the enriched Nd layer encloses the growing primary Mg₂Si and changes its surface energy, which makes the external Mg and Si atoms difficult to diffuse into the internal Mg₂Si, and further effectively depress the preferred growth manner of the primary Mg₂Si. Consequently, the coarse primary Mg₂Si is fully modified.

It is well known that the possibility of the compound formation among elements generally depends on the electronegativity difference. The larger the electronegativity difference, the higher the binding force between elements and the easier the formation possibility. The electronegativities of Mg, Nd and Si are 1.31, 1.14 and 1.80, respectively. The electronegativity difference between Nd and Si (0.66) is larger than that between Mg and Si (0.49) or Mg and Nd (0.17), which means that the tendency of Nd atom reacting with Si is greater than Mg atom. Therefore, when the Nd content is increased to 2.0%, especially 3.0%, large Nd atoms are absorbed onto the growth steps of the Mg₂Si crystal, which provides more Nd atoms to form NdSi and NdSi₂ compounds. Nd atoms cannot effectively depress the diffusion of Mg and Si atoms into the growing front of the primary Mg₂Si any more, which finally makes the primary Mg₂Si coarser again. This is also the major reason for the white particles (NdMg₂ phase) in the primary Mg₂Si transforming into NdSi and NdSi₂ compounds.

5 Conclusions

1) Nd can effectively modify and refine the primary Mg₂Si particles in the Mg-3%Si alloy. The average size of the primary Mg₂Si particles significantly decreases to 10 μm with increasing Nd content up to 1.0% and then becomes coarser again. And its morphology changes from coarse dendrite to fine polygon.

2) The optimal modification effect is obtained when the content of Nd is 1.0%, which is mainly attributed to the poisoning effect. The NdMg₂ phase in the primary Mg₂Si transforms into NdSi and NdSi₂ compounds as the Nd content increases to 3.0%.

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稀土元素Nd对过共晶Mg-Si合金中Mg₂Si粒子的变质作用

胡继龙^{1, 2, 3}，唐昌平^{1, 2}，张新明^{1, 2}，邓运来^{1, 2}

1. 中南大学 材料科学与工程学院，长沙 410083；

2. 中南大学 有色金属材料科学与工程教育部重点实验室，长沙 410083；

3. 北汽股份汽车研究院，北京 101300

摘  要：利用光学显微镜、扫描电镜及XRD物相分析研究稀土元素Nd对过共晶Mg-3%Si合金中Mg₂Si粒子的变质作用与机理。结果表明：随着Nd含量的增加，初生Mg₂Si粒子的形貌由粗大的树枝状转变为细小的多面体状。当Nd含量增至1.0%时，初生Mg₂Si粒子被完全细化，尺寸约为10 μm。然而，随着Nd含量的进一步增加，初生Mg₂Si粒子反而又出现了粗化的现象。其变质机理主要是Nd元素富集于初生Mg₂Si相的生长表面并抑制其优先生长晶向的生长，即中毒效应。当Nd含量超过3.0%时，初生Mg₂Si粒子中的白色粒子由NdMg₂相转变为NdSi和NdSi₂化合物。因此，适量的Nd元素可以有效地细化初生Mg₂Si粒子。

关键词：Mg-Si合金；变质；Mg₂Si；Nd

(Edited by Xiang-qun LI)

Foundation item: Project (A0420110401) supported by the National Security Basic Research, China

Corresponding author: Xin-ming ZHANG; Tel/Fax: +86-731-88830265; E-mail: xmzhang_cn@yahoo.cn

DOI: 10.1016/S1003-6326(13)62847-2