Chemical reactions between aluminum and fly ash during synthesis and reheating of Al-fly ash composite
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INTRODUCTION
SOLIDIFICATION processing has been used to make aluminum matrix composites, because this technique is the least expensive and best suited for fabrication of a variety of sizes and shapes of composite components.[1–7] During the last few years, fly ash, an industrial solid waste byproduct, has been combined with molten aluminum to make low-cost aluminum matrix–fly ash particulate composites.[8,9,10] During processing, the molten aluminum or aluminum alloy is stirred with the fly ash before the composite is cast. Alternatively, loosely packed beds or preforms of fly ash can be infiltrated by molten aluminum under low or high pressure to form composites containing high volume fractions of fly ash. During solidification processing, there is an opportunity for chemical reaction between the aluminum and fly ash particles. The ashes are derived as oxides of the mineral content of the coals used, principally, in the thermal generation of electricity. The major components of fly ash are the oxides (and mixed metal oxides) of silicon, aluminum, iron, and calcium. Small amounts of the oxides of other common elements, such as magnesium and titanium, are also present. The phases present in fly ash particles were identified by X-ray diffraction.[11–15] These are quartz, mullite, lime, spinel, hematite, and ferrite. Some of the constituents may be partly amorphous.[14,16] Based on thermodynamic considerations, there is the possibility of chemical reactions between the aluminum melt and the constituents of fly ash such as SiO2 and Fe2O3 or Fe3O4.[17,18,19] The elements reduced from fly ash (Si and Fe) would then alloy with the matrix aluminum. In sufficient quantity, these alloying elements can form intermetallic compounds with the aluminum and appear as second-phase precipitates in the castings. Such interfacial reactions between the metal
matrix and the reinforcement fly ash particles would significantly influence the properties of the metal–fly ash interface, as well as the properties of the composites.[20,21,22] In recent years, in situ synthesis has been reported for metal matrix–ceramic composites by reacting molten aluminum with amorphous silica,[23,24] aluminosilicate ceramic preforms,[25,26] and Al-Ti-C powder preforms,[27] or by reacting the molten Al-Ti alloy with CH4.[28] The main constituents of the fly ash used in this study are SiO2, Al2O3, and Fe2O3. The possible chemical reactions between molten aluminum and fly ash are shown as follows.[17,18,19] 2 Al(l) 1 3/2 SiO2(s) 5 3/2 Si(s) 1 Al2O3(s)
(931 to 1683 K)
[1]
2 Al(l) 1 Fe2O3(s) 5 2 Fe(s) 1 Al2O3(s)
(950 to 1033 K)
[2]
At the experimental temperature of 850 7C, the Gibbs free energy changes in these chemical reactions are negative (2302,261 J/mole for Eq. [1] and 2784,224 J/mole for Eq. [2]), indicating the possibility of chemical reactions between the molten aluminum and fly ash particles. Progress of the chemical reactions between the aluminum and fly ash particles was studied using differential thermal analysis (DTA). The microstructure of t
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