On the Microstructure and Isothermal Oxidation at 800, 1200, and 1300 °C of the Al-25.5Nb-6Cr-0.5Hf (at %) Alloy

Nb-silicide-based alloys have the potential to replace Ni-based superalloys in future aero engines to enable the latter to meet environmental and performance targets. These new alloys, like the Ni-based superalloys that are currently used, will require environmental protection with a coating system that should be chemically compatible with the substrate. A challenge for alloy development is to discover alpha Al2O3 scale forming coating alloys and in particular to find out whether such alloys could be compatible with other coating alloys for environmental coating systems for the Nb-silicide-based alloys. This paper focuses on these challenges. The alloy Al-25.5Nb-6Cr-0.5Hf (at %) was studied in the cast and heat-treated (1400 degrees C) conditions and after isothermal oxidation for 100 h in air at 800, 1200 and 1300 degrees C. The microstructure consisted of the alloyed NbAl3 and C14-NbCr2 compounds, both of which were stable at least up to 1400 degrees C, a eutectic of the two compounds and very small volume fractions of (Cr,Al,Nb)(ss) and HfO2. The prior eutectic microstructure was stable at T <= 1200 degrees C and the solid solution was not stable at T < 1200 degrees C. At 800 degrees C the alloy did not pest, but exhibited external and internal oxidation, with AlNbO4, CrNbAlO4, and alpha Al2O3 in the former and deeper oxidation along the NbAl3/Laves phase boundaries in the latter At 1200 and 1300 degrees C there was only external oxidation and the scale consisted of two layers, the outer was (Al,Cr)NbO4 intermixed with alpha Al2O3 and the inner was continuous alpha Al2O3. At all three oxidation temperatures, no Nb2Al was observed below the alloy/scale interface and Hf acted as a reactive element forming HfO2 that enhanced the adhesion of the scale. The alloy exhibited good correlations with alpha Al2O3 scale forming silicide and silicide + aluminide intermetallic alloys in maps of the parameters delta (related to atomic size), Delta chi (related to electronegativity), and VEC (number of valence electrons per atom filled into the valence band) that should assist the design of bond coats that do not pest and form alpha Al2O3 in their scales.