Coating of Nickel Aluminide by Pack Cementation to Improve Oxidation Resistance of Nickel-Based Superalloy, Grade IN 738

Nickel-based superalloys grade IN 738 were aluminized and annealed to improve oxidation resistance. The sample surface was analyzed by GIXD after aluminizing, annealing and oxidation testing. The microstructure of the coating layer was observed by optical microscope and SEM. EDS was employed to investigate distribution of elements in the coating layer. The thickness of the coating layer increased with increasing time and temperature. The growth rate of the nickel-aluminide layer increased from 0.95 to 1.55 and 3.47 mu m/s(1/2) when the aluminizing temperature increased from 1073 to 1173 and 1273K. The aluminizing temperature played an important role in the structure of coating. When aluminizing at 1073 and 1173 K, a fine structure of Ni2Al3 formed as a coating layer by inward diffusion of aluminum. For 1273 K aluminizing, large columnar grains of Ni2Al3 coexisting with AlCr2 were formed by both outward and inward diffusion. Annealing can homogenize the coating layer. Ni2Al3 can be changed to the NiAl phase by annealing at 1273 K for 36 ks under argon atmosphere. Oxidation resistance of IN738 was markedly increased by aluminizing and annealing because of the formation of aluminum-oxide film as a protective film.