Modification of Mg2Si in Mg–Si alloys with gadolinium
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:
Their Gibbs free energy can be calculated by using the following method:
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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