Abstract
In this paper, we report research about the isothermal oxidation in air at 800 and 1200 °C for 100 h of the as-cast alloys (at.%) Nb-18Si-5Ge (ZF1), Nb-18Si-10Ge (ZF2), Nb-24Ti-18Si-5Ge (ZF3), Nb-24Ti-18Si-5Cr (ZF4), Nb-24Ti-18Si-5Al (ZF5), Nb-24Ti-18Si-5Al-5Cr-5Ge (ZF6), and Nb-24Ti-18Si-5Al-5Cr-5Ge-5Hf (ZF9), the microstructures of which were reported in previous publications. Only the alloys ZF1, ZF2, and ZF3 suffered from pest oxidation at 800 °C. The Ge addition substantially improved the oxidation resistance of the other alloys both at 800 °C and 1200 °C, which followed parabolic kinetics at 800 °C and in the early stages at 1200 °C and linear kinetics at longer times, where there was spallation of the scales. The Nb2O5 and TiNb2O7 oxides were formed in the scales of the alloys ZF4, ZF5, ZF6, and ZF9 at 800 °C and 1200 °C, the GeO2 was observed in all scales and the SiO2 in the scales of the alloys ZF4 and ZF5, the CrNbO4 in the scales of the alloys ZF4, ZF6, and ZF9 and the AlNbO4 in the scales of the alloys ZF5, ZF6, and ZF9. Diffusion zones were formed below the scale/substrate interface only in the alloys ZF4 and ZF5 where the Nbss and Nb5Si3 were contaminated by oxygen. However, these phases were not contaminated by oxygen in the bulk of the alloys ZF4, ZF5, ZF6, and ZF9. The alloys ZF9 and ZF6 exhibited the best oxidation behaviour at 800 °C and 1200 °C, respectively. The alloys were compared with Nb-silicide based alloys of similar compositions without Ge and Hf additions and the alloy ZF9 with refractory metal High Entropy Alloys. Compared with the former alloys, the addition of Ge reduced the vol % of the Nbss. Compared with the latter alloys, the superior oxidation behaviour of the alloy ZF9 was attributed to its higher and lower values respectively of the parameters δ and VEC.
Highlights
Niobium silicide based alloys are candidate materials to replace Ni-based superalloys in gas turbine engines in the beyond the Ni superalloys era because they have the potential to offer a balance of properties that will make them suitable for structural applications at T > 1200 ◦ C
We present the results of our preliminary study of the oxidation of as-cast Ge containing Nb-silicide based alloys the microstructures of which were discussed in [27,28,29,30,31]
) = kp t where ∆W is the mass change of the specimen, A is its surface area prior to oxidation, and t is the exposure time
Summary
Niobium silicide based alloys are candidate materials to replace Ni-based superalloys in gas turbine engines in the beyond the Ni superalloys era because they have the potential to offer a balance of properties that will make them suitable for structural applications at T > 1200 ◦ C. The basis for the development of these new alloys has been the Nb-Si-Ti-Al-Cr-Hf system. Properties of alloys with additions of other transition, refractory, and simple metals [1,2,3] have moved closer to property targets. The microstructures of the new alloys usually contain a bcc Nbss , and tetragonal and/or hexagonal. Tetragonal or metastable Nb3 Si, C14-NbCr2 Laves phase, and other intermetallics can be present [4,5,6]. The chemistry of the solid solution depends critically on the overall alloy constitution
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