On the Microstructure and Isothermal Oxidation of the Si-22Fe-12Cr-12Al-10Ti-5Nb (at.%) Alloy

Nb-silicide based alloys are new ultra-high temperature materials that could replace Ni-based superalloys. Environmentally resistant coating system (s) with αAl2O3 or SiO2 forming bond coat alloys that are chemically compatible with the Nb-silicide based alloy substrates are needed. This paper makes a contribution to the search for non-pesting bond coat alloys. The microstructure and isothermal oxidation at 800 °C of the silicide-based alloy Si-22Fe-12Cr-12Al-10Ti-5Nb (OHC2) were studied. The cast alloy exhibited macrosegregation of all elements. The microstructures in the cast alloy and after the heat treatment at 800 °C consisted of the same phases, namely TM6Si5, TM5Si3 (TM = transition metal), FeSi2Ti, Fe3Al2Si3, (Fe,Cr)(Si,Al), and an unknown phase of dark contrast. The latter two phases were not stable at 950 °C, where the TMSi2 was formed. There was evidence of endothermic reaction(s) below 1200 °C and liquation at 1200 °C. The alloy followed parabolic oxidation kinetics after the first hour of isothermal oxidation at 800 °C, did not pest, and formed a self-healing scale, in which the dominant oxide was Al2O3. The alloy was compared with other alumina or silica scale-forming intermetallic alloys and approaches to the design of bond coat alloys were suggested.

Thermal Fatigue Damage Evaluation of EB-PVD TBC Specimens - Its Potential and Problems

Thermal fatigue damage evolution behavior in thermal barrier coatings (TBCs) was studied, by employing the originally designed two dimensional ring-shape TBC specimen. The TBC specimen consisted of Ni-based superalloy IN738LC substrate, bond coat, and 8 wt.% Y2O3-stabilized ZrO2 (YSZ) top coat. The top coat was fabricated by electron-beam physical vapor deposition (EB-PVD) method with 250 micron-meters in thickness. Three kinds of MCrAlY bond coat alloys were specified as an experimental variable. Through the work, special attention was paid not only to the failure life of TBC specimen, but also to the underlying failure mechanisms. Some problems have been also pointed out, on feeding back these experimental findings to engineering applications.

Stress Relaxation of MCrAlY Bond Coat Alloys as a Function of Temperature and Strain

The tensile stress relaxation behavior of two NiCoCrAlY bond coat alloys was examined at several temperatures between 25 and 899°C (1650°F) and at 0.1, 0.3, 0.5, and 0.8% strain. One alloy was made from Praxair’s CO211 powder and served as the reference alloy, while the other was a Westinghouse-developed, oxide-dispersion-strengthened alloy. The specimens were loaded to the desired tensile strain at a constant strain rate, and the elastic modulus, yield strength, and yield strain were determined as a function of temperature for the two alloys using the stress/strain information from this loading segment. There was not a statistically significant difference in the high temperature elastic properties between the two alloys, although the oxide-dispersion-strengthened alloy tended to exhibit larger yield strengths. The relaxation data for both alloys were reduced into a form in which instantaneous stressing rate during relaxation was examined as a function of stress and temperature using an Arrhenius power-law model. The oxide-dispersion-strengthened alloy exhibited a larger stress exponent and activation energy than the reference alloy between 677–899°C (1250–1650°F), and was generally more creep resistant. The results from this study demonstrate that bond coat relaxation should occur during engine operation. Bond coatings fabricated from the oxide-dispersion-strengthened alloy have the potential to reduce residual stresses in the TBC ceramic top coating.