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

DETAILED DESCRIPTION What I claim as my invention is: 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.
degree.
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.
2.
The mold of claim 1 wherein the average particle size of the alumina in the mixture is greater than about 10 microns.
3.
The mold of claim 2 wherein the pre-reacted fired mold resists sagging up to about 1800.
degree.
C.
4.
The mold of claim 2 wherein the average chemical composition of the mullite phase is from about 72 weight percent alumina to about 74 weight percent alumina, balance silica.
5.
The mold of claim 4 wherein the melting point range of the mullite phase is about 1825.
degree.
C.
to 1830.
degree.
C.
6.
The mold of claim 2 wherein, after heating the alumina-silica material mixture of the mold at least about 1800.
degree.
C.
for one hour, the microstructure of the material of the mold comprises a mixture of mullite and alumina and a substantial absence of an Al.
sub.
2 O.
sub.
3 -SiO.
sub.
2 liquid phase.
7.
The mold of claim 6 wherein the average particle size of the alumina is .
about.
3.
5 microns.
8.
The mold of claim 6 wherein the average chemical composition of the mullite phases has a range from 72 weight percent alumina, balance silica to 78 weight percent alumina, balance silica