Fabrication and mechanical properties of alpha-Al2O3/beta-Al2O3/Al/Si composites by liquid displacement reaction

Al2O3/metal composites were fabricated by heating three kinds of commercial mullite refractories in contact with Al, and their mechanical properties w ere investigated. Aluminum reacted with the mullite and SiO2-glass constitu ting the mullite refractories and changed them into alpha-Al2O3 and Si. Sim ultaneously, beta-Al2O3 was formed by the reactions among alpha-Al2O3 and s odium and potassium oxides in the glassy phase. Also, Al penetrated into th e alpha-Al2O3/beta-Al2O3/Si composite by partly dissolving Si. Finally, the mullite refractories were changed into alpha-Al2O3/beta-Al2O3/Al/Si compos ites. The phase contents, microstructures and mechanical properties of the resulting composites varied with the composition of the refractories. The c ontent of beta-Al2O3 in the composite was lowest at the lowest Na2O and K2O contents in the refractories. Silicon in the composite had its highest con tent at the highest SiO2 content. The composite fabricated from SiO2/Al2O3 (in mol) (SAR) = 1.85 consisted of 2-5 mu m Al2O3 grains embedded in metal, but that from SAR = 1.05 showed a complicated microstructure with small an d large grains. The bending strength of the composites fabricated from the refractories of SAR = 1.85, 1.24, and 1.05 were 327, 405 and 421 MPa, respe ctively. Also, the corresponding fracture toughness values were 5.2, 6.1, a nd 5.5 MPa m(1)/2, respectively. (C) 2000 Kluwer Academic Publishers.