Microstructure and oxidation behaviour of chromium–molybdenum silicides

In the present study the results regarding a microstructural investigation carried out on mixed Cr–Mo silicide alloys are presented. A powder metallurgy route was adopted for the preparation of the specimens. The green compacts obtained from powder mixtures were hot-pressed at 1400°C for several hours. Two batches were prepared, in one of which 15 vol.% of Pt–6%Rh fibres were introduced. The majority phases present in the materials are: Cr3Si and Cr5Si3 with molybdenum in solution. Scanning electron microscopy observations were carried out in order to better characterise the microstructure of the alloys. Isothermal oxidation treatments were conducted on metallographically polished alloy coupons. The treatments were carried out in air at temperatures ranging from 900 up to 1100°C. The oxide scales were examined with X-ray diffractometry and electron microscopy to obtain information on the oxidation mechanisms. At 900°C all the analysed samples revealed excellent oxidation resistance determined by the formation of a continuous protective Cr2O3 surface layer. At 1000°C Cr2O3 is still the main oxide phase in the scale even though, particularly for the specimen without Pt fibres, it starts to be destabilised with the formation of the volatile CrO3 phase. At 1100°C some cristobalite is also present in the oxide scale. The oxidation kinetics are largely dominated by the formation of CrO3.