Modification of Mg2Si in Mg–Si alloys with gadolinium

https://doi.org/10.1016/j.matchar.2013.02.005Get rights and content

Abstract

The modification effect of gadolinium (Gd) on Mg2Si in the hypereutectic Mg–3 wt.% Si alloy has been investigated using optical microscope, scanning electron microscope, X-ray diffraction and hardness measurements. The results indicate that the morphology of the primary Mg2Si is changed from coarse dendrite into fine polygon with the increasing Gd content. The average size of the primary Mg2Si significantly decreases with increasing Gd content up to 1.0 wt.%, and then slowly increases. Interestingly, when the Gd content is increased to 4.0 and 8.0 wt.%, the primary and eutectic Mg2Si evidently decrease and even disappear. The modification and refinement of the primary Mg2Si is mainly attributed to the poisoning effect. The GdMg2 phase in the primary Mg2Si is obviously coarsened as the Gd content exceeds 2.0 wt.%. While the decrease and disappearance of the primary and eutectic Mg2Si are ascribed to the formation of vast GdSi compound. Therefore, it is reasonable to conclude that proper Gd (1.0 wt.%) addition can effectively modify and refine the primary Mg2Si.

Highlights

► Proper Gd (1.0 wt.%) addition can effectively modify and refine the primary Mg2Si. ► We studied the reaction feasibility between Mg and Si, Gd and Si in Mg–Gd–Si system. ► We explored the modification mechanism of Gd modifier on Mg2Si.

Introduction

Mg–Si alloys are reinforced with in situ Mg2Si particles [1], [2]. It has been shown that Mg–Si alloys have high potential as a structural material due to the Mg2Si particles that exhibit low density (1.99 × 103 kg/m3), high melting point (1085 °C), high hardness (4.5 × 109 N · m 2), reasonably high Young's modulus (120 GPa), and low coefficient of thermal expansion (7.5 × 10 6 K 1) [1], [3], [4]. However, the large and brittle Mg2Si particles will greatly deteriorate the mechanical properties of Mg–Si alloys [3], [5]. Therefore, how to modify and refine the coarse Mg2Si particles in Mg–Si alloys has attracted considerable attention. It has been reported that some processing techniques (rapid solidification, hot extrusion [3], [6], [7], heat treatment [5] and alloying addition (Y [1], Ba [4], Sb [8], P and Ca [9])) were able to produce positive modification effect on the morphology of Mg2Si in Mg–Si alloys. However, rather limited research has been carried out on the modification effect of Gd on the Mg2Si in hypereutectic Mg–Si alloys.

Xu et al. [10] and Yi and Zhang [11] reported that RE elements (such as Nd and La) can effectively modify the primary and eutectic silicon in hypereutectic Al–Si alloys. Considering the similarity between Si modification in Al–Si alloys and Mg2Si modification in Mg–Si alloys [4], we attempt to apply the Gd element to modify the Mg2Si in hypereutectic Mg–Si alloys. The aim of this work is to develop an effective modifier 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 the high quality and properties of Mg–Gd–Y–Nd–Si–Zr system alloys [12].

Section snippets

Experimental

The alloys required for this study were prepared by melting pure Mg (> 99.93%) and Si (> 99.95%) in an electrical resistance furnace at 760 °C under the protection of Ar atmosphere. After about 20 min, the desired Mg–31.25% Gd (wt.%) master alloys were added into the Mg–Si melts. The melts were stirred about 90 s at a speed of 300 rpm, then poured into a preheated (250 °C) permanent low carbon steel mold (Φ 55 mm × 150 mm).

Samples for microstructure observation were initially polished using different

Microstructure of Mg–3 wt.% Si Alloy

According to the Mg–Si binary phase diagram, Mg–3 wt.% Si alloy is a hypereutectic alloy with a solidified microstructure of primary Mg2Si and eutectic α-Mg + Mg2Si phases. The SEM image of the unmodified Mg–3 wt.% Si alloy is shown in Fig. 1(a), which demonstrates that the coarse dendritic phases are the primary Mg2Si (as shown by arrow A), while the rod-like shaped phases are the eutectic Mg2Si (as shown by arrow B), and the gray areas are Mg matrix (as shown by arrow C). The average size of the

Thermodynamic Analysis

The reactions between Mg and Si, Gd and Si, which may occur in the Mg–Gd–Si system, are conducted as follows:23Mg+13Si=13Mg2SiΔG158Gd+38Si=18Gd5Si3ΔG259Gd+49Si=19Gd5Si4ΔG312Gd+12Si=12GdSiΔG438Gd+58Si=18Gd3Si5ΔG513-xGd+2-x3-xSi=13-xGdSi2-xΔG6.

Their Gibbs free energy can be calculated by using the following method:ΔGT=ΔHTTΔSTΔHT=ΔH298.15+298.15TmνΔCp1dT+ΔHTm+TmTνΔCp2dTΔST=ΔS298.15+298.15TmνΔCp1TdT+ΔHTmTm+TmTνΔCp2TdTCp=a+b103T+c105T2.

The standard Gibbs free energy change ΔG for Eqs. 

Conclusions

  • (1)

    Proper Gd can effectively modify and refine the primary Mg2Si in the Mg–3 wt.% Si alloy. The average size of the primary Mg2Si significantly decreases with increasing Gd content up to 1.0 wt.% and then slowly increases. Meanwhile, its morphology is changed from coarse dendrite into fine polygon.

  • (2)

    The optimal modification effect is obtained when the Gd content is 1.0 wt.%, which is mainly attributed to the poisoning effect. The GdMg2 phase in the primary Mg2Si obviously becomes coarser and

Acknowledgment

The authors would like to appreciate the financial supports from The National Basic Research Program, China.

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