Rare-earth doping of aluminosilicate refractory compositions has been shown to impart improved resistance to attack by molten aluminum alloys. The basis for this improved alloy attack resistance was investigated. The addition of rare-earth oxide to bauxite was found to alter the microstructure to form large, uniformly shaped corundum crystals interlinked with cerium-containing strands. It was concluded that this unique microstructure was responsible for the enhanced attack resistance of the refractories. Other possible causes for improved alloy attack resistance, such as enhanced sintering, excess oxygen provided by the Ce02 dopant, or higher thermodynamic stability of the phases formed in the doped refractory compositions, were not consistent with the experimental findings. [

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Several aluminosilicate compositions were tested for molten 7075 aluminum alloy attack with and without the addition of 10% Na2C03. The extent of corrosion was proportional to the silica content of the refractory and especially of the matrix phase. The addition of rare earths up to 15% by weight to the aluminosilicates virtually eliminated corrosion. Bastnasite, a rare-earth concentrate, was found to be a more effective additive at low concentrations than either CeOz or LazO3 alone in rendering the aluminosilicates corrosion resistant. [