Shell investment molds embodying a metastable mullite phase in its physical structure

Claim 1

A shell investment mold suitable for the casting and directional solidification of superalloys therein comprising a ceramic material formed in situ as a reaction product of the chemical reaction between a mixture of alumina and a silica binder material which produces two solid phases consisting of mullite and unreacted alumina, the reaction product has a microstructure which exhibits irregular shaped alumina particles disposed in a mixture of a mullite phase and an alumina liquid phase when the alumina-silica mixture is fired at about 1725° C. for up to 2 hours, the composition of the alumina-silica material mixture when dried, and before firing, being from 80 to 90 weight percent alumina, the balance silica, the average particle size of the alumina in the mixture is less than 100 microns, and the average particle size of the silica in the mixture is less than 1000 A.