Effect of γ′ precipitation on oxide formation on the Ni-3Cr-20Al and Ni-10Cr-11Al-8Ti nanocrystalline coatings

Magnetron sputter deposition has been used to produce Ni-3Cr-20Al and Ni-10Cr-11Al-8Ti (at%) nanocrystalline coatings on substrates of the same materials. Oxide formation on these sputtered coatings at 1000 degrees C in air has been studied. gamma' phase precipitation was found to occur in the coatings during oxidation. Such precipitation caused various oxidation results to be observed. For the Ni-3Cr-20Al coating, on which an external layer of Al2O3 with some NiAl2O4 dispersions, was developed, the formation of NiAl2O4 was closely related to precipitation of the gamma' phase. Also, more Al was retained in the sub-lattice of the gamma' phase and, hence, the supply and diffusivity of Al was reduced markedly whereas the diffusivity of Ni did not show such a drastic effect. Therefore, NiO could be formed and react with transitional Al2O3 to form NiAl2O4. For the Ni-Cr-Al-Ti coating, with the precipitation of the gamma' phase, more Ti became ordered and the supply and diffusivity of Ti decreased. Thus, the growth rate of TiO2 formed in the early stages of oxidation decreased greatly, due to coalescence of Al2O3 into a complete scale. (C) 2000 Elsevier Science Ltd. All rights reserved.