Elsevier

Materials Characterization

Volume 66, April 2012, Pages 104-110
Materials Characterization

Effects of Sc content on the microstructure of As-Cast Al-7 wt.% Si alloys

https://doi.org/10.1016/j.matchar.2011.11.005Get rights and content

Abstract

The effects of Sc content on the microstructure of as-cast Al-Si alloys were investigated by adding 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, and 0.4 wt.% Sc to hypoeutectic Al-7 wt.% Si alloys. The results show that there are significant changes in the microstructure when the Sc content is increased to 0.15%. The volume fraction of the primary α-Al phase increased nearly monotonically with increasing Sc content in the as-cast state. As the Sc content increased, the average length of the eutectic Si in the as-cast Al-7Si-xSc alloy decreased sharply from 150 μm (without Sc content) to 20 μm (0.4 wt.% Sc content). The Sc atoms are concentrated in the interdendritic regions. A fish-bone structure composed of Al, Si, Sc, and Fe, which might be the Fe-rich phase precipitated on the AlSc2Si2 (V-phase) was detected adjacent to the grain boundary by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

Highlights

► Significant changes in the microstructure when the Sc content is increased to 0.15%. ► Volume fraction of the primary a-Al phase increased with increasing Sc content. ► Average length of Si decreased sharply from 150 to 20 μm with increasing Sc content. ► The Sc atoms are concentrated in the interdendritic regions.

Introduction

Al-Si alloys are widely used for casting because of their low-density, good mechanical properties, and casting performance. Their microstructure comprises a mixture of the α-Al phase and eutectic Si, as well as various intermetallic phases formed from other alloying additions. Grain refinement plays a crucial role in improving the mechanical properties of cast alloys. Generally, Al-Ti and Al-Ti-B master alloys are added to Al alloys to refine the solidified product [1], [2], [3], [4]. The morphology and size of the Si during solidification can have a significant effect on the mechanical properties of the cast alloy; thus, the size must be controlled to achieve the desired properties. The effects of modifiers [5], [6], [7], [8], such as Na, Sr, Sb, and rare earth metals, among others, on Al-Si systems have been the subject of considerable work. These previous studies suggest the important role of the modifier content on the microstructures and properties.

Scandium, one of the most effective refiners of the cast grain structure in Al alloys [9], has been successfully used as a trace alloying treatment that is applied to wrought Al alloys. The addition of trace amounts of Sc may act as a hardener in Al alloys because of its influence on the formation of precipitate phases in most of the important Al alloys [10]. Verma et al. [11] concluded that the addition of Sc to pure Al results in a 50% reduction of grain size compared to equivalent addition of Al-5Ti-1B refiner. Combined additions of Sc and Zr can also reduce the grain size of Al-Mg-Si-Mn alloys [12], [13].

Only a few studies in the literature have reported the use of Sc in cast Al-Si alloys, especially in hypoeutectic Al-Si alloys. The AlSc2Si2 (V-phase) reportedly exists along with the eutectic Si [14]. Cousineau et al. [15] demonstrated that for Al-7Si-0.4 Mg alloys, a Sc content of 0.39%–0.71% is required to achieve any significant grain refinement. Recently, Sc has also been reported to increase the fluidity [16] and modify the eutectic Si in the A319 alloy [17]. In this paper, the effect of different levels of Sc addition on primary α-Al, as well as the morphology and size of the eutectic Si phase, was examined in detail. The preliminary results are expected to be significant for the promotion of Sc in future high-strength and high-toughness Al-Si alloy designs.

Section snippets

Experimental Procedure

The investigated alloys of Al-7Si-xSc (mass fraction, %) alloys were prepared by melting commercial purity Al (> 99.7 wt.%) and Al-12Si (mass fraction, %) master alloy in a resistance heating furnace with a graphite crucible. The Al-2Sc master alloy was added into the Al-Si melts at 750 °C for 30 min before pouring into a sand mold (see Fig. 1) at 710 °C. The Sc was added to constitute 0, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, and 0.4% (by mass fraction) of the alloy. The real chemical composition

Microsegregation In The As-Cast Alloy

The as-cast microsegregation of Sc elements was determined from linear composition profiles measured by EDS analysis on traverse spans of multiple dendritic cells across the regions of the ternary alloy Al-7Si-0.4Sc (Fig. 3), as evidenced by the periodic variation of solute concentration. The wavelengths of the variations presumably correspond to the secondary dendrite arm spacing. The Sc peak is accompanied by the Si peak. The Sc atoms are concentrated in the interdendritic regions, which are

Conclusions

The following conclusions are drawn in the present investigation:

  • 1.

    The microstructure changes significantly when the Sc content increases to 0.15%, and the volume fraction of the primary α-Al phase increases nearly monotonically with the increasing Sc content of the as-cast state.

  • 2.

    Sc can modify eutectic Si from a coarse, plate-like, and acicular form to a finely branched and somewhat fibrous one. With increasing Sc content in the alloy, the average length of the eutectic Si in the as-cast

Acknowledgements

Financial support from the scientific and technological research project of the Shanxi province (No. 20090321075) is gratefully acknowledged. The authors express their sincere gratitude to Professor S.R. Cao and Dr. Y.J. Shi for their fruitful discussions. Special thanks go to Mr. S.Z. Gao for his laboratory assistance and cooperation.

References (24)

  • X. Chen et al.

    Study on the eutectic modification level of Al–7Si Alloy by computer aided recognition of thermal analysis cooling curves

    Mater Sci Eng A

    (2006)
  • B.S. Murty et al.

    Grain refinement of aluminium and its alloys by heterogeneous nucleation and alloying

    Int Mater Rev

    (2002)
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