On the Microstructure and Isothermal Oxidation of Silica and Alumina Scale Forming Si-23Fe-15Cr-15Ti-1Nb and Si-25Nb-5Al-5Cr-5Ti (at.%) Silicide Alloys

An Nb-silicide based alloy will require some kind of coating system. Alumina and/or SiO2 forming alloys that are chemically compatible with the substrate could be components of such systems. In this work, the microstructures, and isothermal oxidation at 800 °C and 1200 °C of the alloys (at.%) Si-23Fe-15Cr-15Ti-1Nb (OHC1) and Si-25Nb-5Al-5Cr-5Ti (OHC5) were studied. The cast microstructures consisted of the (TM)6Si5, FeSi2Ti and (Fe,Cr)Si (OHC1), and the (Nb,Ti)(Si,Al)2, (Nb,Cr,Ti)6Si5, (Cr,Ti,Nb)(Si,Al)2 (Si)ss and (Al)ss (OHC5) phases. The same compounds were present in OHC1 at 1200 °C and the (Nb,Ti)(Si,Al)2 and (Nb,Cr,Ti)6Si5 in OHC5 at 1400 °C. In OHC1 the (TM)6Si5 was the primary phase, and the FeSi and FeSi2Ti formed a binary eutectic. In OHC5 the (Nb,Ti)(Si,Al)2 was the primary phase. At 800 °C both alloys did not pest. The scale of OHC1 was composed of SiO2, TiO2 and (Cr,Fe)2O3. The OHC5 formed a very thin and adherent scale composed of Al2O3, SiO2 and (Ti(1−x−y),Crx,Nby)O2. The scale on (Cr,Ti,Nb)(Si,Al)2 had an outer layer of SiO2 and Al2O3 and an inner layer of Al2O3. The scale on the (Nb,Cr,Ti)6Si5 was thin, and consisted of (Ti(1−x−y),Crx,Nby)O2 and SiO2 and some Al2O3 near the edges. In (Nb,Ti)(Si,Al)2 the critical Al concentration for the formation of Al2O3 scale was 3 at.%. For Al < 3 at.% there was internal oxidation. At 1200 °C the scale of OHC1 was composed of a SiO2 inner layer and outer layers of Cr2O3 and TiO2, and there was internal oxidation. It is most likely that a eutectic reaction had occurred in the scale. The scale of OHC5 was α-Al2O3. Both alloys exhibited good correlations with alumina forming Nb-Ti-Si-Al-Hf alloys and with non-pesting and oxidation resistant B containing Nb-silicide based alloys in maps of the parameters δ, Δχ and VEC.