Abstract
The effects of individual and combined additions of Sr and Yb on the solidification behavior and microstructure of Al-7Si alloys were investigated using thermal analysis, optical microscopy, and scanning electron microscopy. The results showed that the nucleation temperature, minimum temperature, and growth temperature of eutectic solidification were lower than those of the unmodified alloys, and eutectic recalescence at 2°C in the Yb- and (Sr + Yb)-modified alloys was observed. The decrease in the nucleation temperature and an increase in recalescence obviously refined the eutectic Si morphology of the Sr-, Yb-, and (Sr + Yb)-modified alloys from a coarse plate-like structure to a fine fibrous structure. Moreover, the three-dimensional microstructure of eutectic Si in the (Sr + Yb)-modified alloy showed the finer fibrous structure, which was mainly attributed to formation of the Al2Si2Yb phase. The formation enthalpy of Al2Si2Yb was lower than that of Al-(Si)-Sr in the (Sr + Yb)-modified alloy, thus suppressing the formation of the Al2Si2Sr intermetallic compound. The more effective Sr together with Yb further improved the modification effect of eutectic silicon.
Highlights
Al-Si casting alloys have been used for a variety of purposes in the automotive and aircraft industries, due to their low density, superior castability, corrosion resistance, high strength to weight ratio, ease of recycling, high efficiency, and high wear resistance [1]
Experimental results indicate that the morphology of eutectic Si changes from a coarse plate-like structure to a fine fibrous morphology after adding such rare earth elements, significantly improving the mechanical properties of the Al-Si alloys. e effects of the combined modifications on eutectic Si and on the mechanisms of modification have been reported [15, 16]
Liao and Sun [17] investigated the effect of Sr and B on the refinement of eutectic Si in near eutectic Al-11.6% Si alloys. e results showed that adding 0.03% Sr and 0.028% B changes the dendritic structure from a columnar structure to an equiaxed structure; further, the eutectic silicon fibers had a fine texture
Summary
Al-Si casting alloys have been used for a variety of purposes in the automotive and aircraft industries, due to their low density, superior castability, corrosion resistance, high strength to weight ratio, ease of recycling, high efficiency, and high wear resistance [1]. Recent studies have discussed the effects of rare earth elements, including La [6], Nd [7], Eu [8], Ce [9], Er [10], Yb [11], Y [12], Sc [13], and Sm [14] on the morphology of eutectic Si and the modification mechanisms in cast Al-Si alloys. Experimental results indicate that the morphology of eutectic Si changes from a coarse plate-like structure to a fine fibrous morphology after adding such rare earth elements, significantly improving the mechanical properties of the Al-Si alloys. E formation enthalpy of the Al2Si2Yb phase was obtained via the first-principles method. e Perdew–Berke– Ernerhof genre of the generalized gradient approximation was introduced to portray the exchange correlation potential energy. e cutoff energy for the plane wave was set to 470 eV, and a 5 × 5 × 3 grid was utilized to sample the Brillouin zone with the Monkhorst–Pack scheme. e Broyden–Fletcher–Goldfarb Shanno minimization scheme was utilized for geometric optimization, and the elastic constants were determined by the stress-strain method [23]
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