a beta - phase nickel aluminide overlay coating and method for modifying the grain structure of the coating to improve its oxidation resistance . the coating is deposited by a method that produces a grain structure characterized by grain boundaries exposed at the outer coating surface . the grain boundaries may also contain precipitates as a result of the alloyed chemistry of the coating . during or after deposition , the overlay coating is caused to form new grain boundaries that , though open to the outer surface of the coating , are free of precipitates or contain fewer precipitates than the as - deposited grain boundaries . new grain boundaries are preferably produced by causing the overlay coating to recrystallize during coating deposition or after deposition as a result of a surface treatment followed by heat treatment .