Alloy castings having protective layers and methods of making the same

1. A method of forming a protective layer on a surface of a casting of an alloy, comprising:

disposing the alloy in a mold; and

oxidizing an element of the alloy, while the alloy is in the mold, to form a protective layer on the surface of the casting, such that the protective layer is attached to the surface of the casting;

wherein a thickness of the protective layer is in a range from about 1.2 microns to about 50 microns.

2. The method of claim 1 , wherein the mold comprises an inner coating that is reactive to at least one element of the alloy and relatively less reactive or inert to other elements of the alloy.

3. The method of claim 2 , wherein the inner coating of the mold comprises chromia, niobia, titania, tantala, silica, zircon, yttria, or combinations thereof.

4. The method of claim 2 , wherein oxidizing comprises:

reacting at least one material from the inner coating of the mold with aluminum present in the alloy.

5. The method of claim 1 , wherein oxidizing comprises:

exposing the alloy or the casting to an oxidizing atmosphere to facilitate oxidation of aluminum metal present in the alloy.

6. The method of claim 5 , wherein the oxidizing atmosphere comprises an oxygen source diluted by an inert carrier.

7. The method of claim 6 , wherein the oxygen source comprises carbon monoxide, oxygen, carbon dioxide, moisture, or combinations thereof.

8. The method of claim 6 , wherein the inert carrier comprises argon.

9. The method of claim 6 , wherein the oxidizing atmosphere comprises oxygen source in a range of about 2 percent to about 20 percent of the total amount of a mixture of the oxygen source and the inert carrier.

10. The method of claim 6 , wherein the oxygen source comprises about 10 percent carbon monoxide and about 90 percent argon.

11. The method of claim 1 , wherein the mold comprises zirconia, zircon, silica, alumina, or combinations thereof.

12. The method of claim 1 , wherein at least a portion of the protective layer is formed simultaneously with a casting of the alloy.

13. The method of claim 1 , wherein at least a portion of the protective layer is not coupled to a surface of the mold.

14. A method of forming an aluminum oxide layer on a casting, comprising:

providing a mold having a coating disposed on an inner surface of the mold;

disposing an alloy in the mold, wherein at least a portion of aluminum in the casting reduces a portion of constituents in the coating to form the aluminum oxide layer, wherein the aluminum oxide layer is attached to a surface of the casting, wherein at least a portion of the aluminum oxide layer is not coupled to a surface of the mold, and wherein a thickness of the aluminum oxide layer is in a range from about 1.2 microns to about 50 microns; and

immersing the mold and the casting into a liquid metal coolant.

15. The method of claim 14 , wherein the liquid metal coolant comprises at least one molten metal from the group consisting of lithium, sodium, magnesium, aluminum, potassium, zinc, indium, tin, antimony, cesium, and tellurium.

16. A method of forming an aluminum oxide layer on a casting, comprising:

providing a mold;

disposing an alloy in the mold to form the casting;

providing an oxidizing atmosphere in the mold to oxidize at least a portion of aluminum present in the alloy, forming the aluminum oxide layer, wherein the aluminum oxide layer is attached to an outer surface of the casting, and wherein a thickness of the aluminum oxide layer is in a range from about 1.2 microns to about 50 microns; and

immersing the mold and the casting into a liquid metal coolant.

17. The method of claim 16 , wherein the liquid metal coolant comprises at least on molten metal from the group consisting of lithium, sodium, magnesium, aluminum, potassium, zinc, indium, tin, antimony, cesium, and tellurium.