Ni-Nb合金の高温初期酸化過程―低酸素圧酸化への高エネルギーイオン後方散乱分析法の応用

Abstract

High energy ion backscattering method is suitable to make nondestructive analysis of depth distribution of elements within one micrometer from a surface. However, this method has disadvantage of poor mass discrimination. In order to overcome this disadvantage, 16O ion particles accelerated up to an energy of 15 MeV were used instead of 4He ions. This improved backscattering method was applied to an analysis of the initial stage of the high temperature oxidation of Ni-Nb alloys.The 95Ni-5Nb alloy and 90Ni-10Nb alloy were prepared in a plasma-jet furnace, and the cold-rolled specimen coupons were oxidized under oxygen pressure of 6×10−2 Pa at 1023 K. Each specimen was oxidized until electrical microbalance reading showed that its surface oxide film could have a thickness of about one micrometer. The oxidized specimens were examined by using the backscattering method together with conventional X-ray diffraction, high energy electron diffraction and scanning electron microscopy. The 95Ni-5Nb specimen was covered mainly with a flat dense oxide layer of M-Nb2O5 and partially with granular particles of T-Nb2O5. On the other hand, the 90Ni-10Nb specimen was covered mainly with granular oxide particles. It was clarified with the backscattering method that the thickness of the oxide film formed on the 95Ni-5Nb alloy ranged from 20 to 120 nm. Whilst, the thickness of the oxide film on the 90Ni-10Nb alloy ranged from 40 to 200 nm. The weight gain due to oxidation was mainly attributed to the formation of an internal oxidation zone 6∼8 μm thick. The oxide particles in the outer part of the internal oxidation zone were tentatively identified to be T-Nb2O5, and those in the inner part was to be a lower niobium oxide.The backscattering analysis using 16O ions has been demonstrated to be highly useful in the studies of material surface phenomena, including high temperature alloy oxidation, on account of its improved mass discrimination compared to the 4He backscattering.