MICROSTRUCTURE AND OXIDATION BEHAVIOR OF CHROMIUM-MOLYBDENUM SILICIDES

In the present study the results regarding a microstructural investiga tion carried out on mixed Cr-Mo silicide alloys are presented. A powde r metallurgy route was adopted for the preparation of the specimens. T he green compacts obtained from powder mixtures were hot-pressed at 14 00 degrees C for several hours. Two batches were prepared, in one of w hich 15 vol.% of Pt-6%Rh fibres were introduced. The majority phases p resent in the materials are: Cr3Si and Cr5Si3 with molybdenum in solut ion. Scanning electron microscopy observations were carried out in ord er to better characterise the microstructure of the alloys. Isothermal oxidation treatments were conducted on metallographically polished al loy coupons. The treatments were carried out in air at temperatures ra nging from 900 up to 1100 degrees C. The oxide scales were examined wi th X-ray diffractometry and electron microscopy to obtain information on the oxidation mechanisms. At 900 degrees C all the analysed samples revealed excellent oxidation resistance determined by the formation o f a continuous protective Cr2O3 surface layer. At 1000 degrees 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 degrees C some crist obalite is also present in the oxide scale. The oxidation kinetics are largely dominated by the formation of CrO3. (C) 1997 Elsevier Science S.A.