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 degrees C and 1200 degrees 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)(6)Si-5, FeSi2Ti and (Fe,Cr)Si (OHC1), and the (Nb,Ti)(Si,Al)(2), (Nb,Cr,Ti)(6)Si-5, (Cr,Ti,Nb)(Si,Al)(2) (Si)(ss) and (Al)(ss) (OHC5) phases. The same compounds were present in OHC1 at 1200 degrees C and the (Nb,Ti)(Si,Al)(2) and (Nb,Cr,Ti)(6)Si-5 in OHC5 at 1400 degrees C. In OHC1 the (TM)(6)Si-5 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 degrees C both alloys did not pest. The scale of OHC1 was composed of SiO2, TiO2 and (Cr,Fe)(2)O-3. The OHC5 formed a very thin and adherent scale composed of Al2O3, SiO2 and (Ti(1-x-y),Cr-x,Nb-y)O-2. 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)(6)Si-5 was thin, and consisted of (Ti(1-x-y),Cr-x,Nb-y)O-2 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 degrees 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 -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 , and VEC.