Abstract
The influence of Zr (3 at%) addition on non-isothermal and isothermal oxidation behaviour of the Nb-19Si-5Mo-20Ti (base alloy) has been investigated. During heating in a thermogravimetric analyser to 1300 °C in air, the mass-gain begins at ∼600 °C, and shows a more gradual rise as well as a lower peak value for the Zr-containing alloy compared to the base alloy. Furthermore, the Zr-containing alloy exhibits superior oxidation resistance in isothermal test for 24 h at 1100 °C, but not at 900 °C and 1200 °C, probably due to the differences in oxide scale growth kinetics and defect density. The oxidation products have been identified as Nb2O5, SiO2, TiO2, MoO2, TiNb2O7, Ti2Nb10O29 for both the alloys, with ZrO2 being additionally observed in the Zr-containing alloy. Analysis of the isothermal oxidation kinetics suggests an initial stage of interface-reaction controlled rapid mass increase followed by parabolic behaviour, where oxygen diffusion through the oxide scale is apparently rate-controlling. Slow down to parabolic kinetics is ascribed to the formation of a SiO2-rich compact oxide scale. The oxide scale becomes protective due to reduced oxygen anion diffusivity in Nb2O5 having Mo, Ti and Zr as substitutional solute, along with the formation of ZrO2 and TiO2 with Pilling-Bedworth ratio less than 2.
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