OXIDATION OF GAMMA-NI-AL-SI ALLOYS AT 1073-K

The formation and development of oxides in Ni-4 Al and Ni-4Al-xSi (at.%, x = 1, 3, 5) alloys at 5-9 x 10(-6) and 1 atm oxygen pressure at 1073 K have been studied The oxidation rate increased with an increase of silicon content in the alloy at the early stage of oxidation, but decreased after longer time exposure due to formation of an intermediate layer composed of NiO and spinel (NiAl2O4 and Ni2SiO4) between the top NiO layer and the internal-oxidation zone. This intermediate layer became a barrier for releasing stress, generated by the volume expansion associated with oxidation of solute atoms, resulting in high dislocation density and severe distortion in the internal-oxidation zone for the Ni-Al-Si alloys. In Ni-4 Al alloy where no complete intel mediate-layer formation occurred, stress was easily released by an enhanced vacancy gradient, and therefore an enhanced vacancy-injection rate into the alloy, resulting in a higher, oxidation late than the situation where a sample was oxidized at an oxygen pressure associated with the dissociation of NiO.