Abstract
Excellent inherent oxidation resistance is a prerequisite for the use of superalloys in many high-temperature applications. To achieve this goal, typically continuous alumina and chromia scale growths are assured through sufficient Cr and Al additions. Since the intended γ/γ′-microstructure of superalloys is only stable within a certain compositional window, the maximum concentrations of these protective scale forming elements are, however, dependent on the overall alloy composition. The latter is a severe drawback, especially for Co-rich superalloys, as for these the maximum content often is insufficient for reaching the desired continuous scale growth. In recent years, the addition of significant Ni levels was identified to improve the high-temperature oxidation properties in the case of simple model alloy systems. In this study, we compare the high-temperature oxidation behavior of two complex Co-rich multinary single-crystalline γ/γ′-strengthened superalloys that only differ regarding their Co/Ni ratios to the commercial Ni-base superalloy CMSX-4. Therefore, time-resolved isothermal gravimetric analysis (TGA) in synthetic air at 850 °C and 1050 °C for 100 hours, scanning electron microscopy analysis (SEM), and electron probe microanalysis (EPMA) were conducted. The results point out that a high Co-content beneficially affects the oxidation resistance at 850 °C, meaning that the Ni-base CMSX-4 is slightly outmatched by the Co-rich competitors. In contrast, at 1050 °C, the commercial (most Ni-rich) alloy performed best and, clearly, an increasing Co-content was identified to deteriorate the oxidation resistance. This temperature-dependent influence of the nominal Co/Ni ratio on oxidation resistance is shown to be especially pronounced for dendritic regions. Consequently, the latter could be identified to especially determine the overall oxidation kinetics.
Highlights
THE advanced multinary single crystal Co-base superalloy ERBOCo-1 with remarkable high-temperature properties above 1000 °C was recently introduced.[1]
It is noteworthy that the size of the c¢-precipitates within the dendritic regions is generally smaller compared to the interdendritic areas in both Heat Treatment of Short duration (HTS) and as-cast state (AC) state (see e.g., Figures 1(e) and (g) vs. f and h)
Comparing the microstructure in HTS state and after long heat treatment (HTL), it is apparent that the c¢-size after prolonged homogenization levels between the range depicted for the short heat treatment (see e.g., Figure 1(i) vs. (e) and (f) for ERBOCo-1)
Summary
THE advanced multinary single crystal Co-base superalloy ERBOCo-1 with remarkable high-temperature properties above 1000 °C was recently introduced.[1]. Selected experiments were conducted with the commercial Ni-base superalloy CMSX-4 to elucidate, if the oxidation properties of the ERBOCo alloys reach comparable levels of an established Ni-base disc material
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call