Prediction of Initial Oxidation Behavior of Ni-Base Single Crystal Superalloys: A New Oxidation Map and Regression Analysis

摘要

Predictions for oxidation behavior of Ni-base superalloys are becoming increasingly difficult because of the complex alloy composition. In this study, we focus on the initial oxidation behavior of Ni-base superalloys, and suggest a new oxidation map with using Al and Cr chemical potentials to distinguish the initial oxide type of Ni-base superalloys with seventy binary, ternary, and multtcomponent Ni-base single crystal superalloys at 1100 °C. As a comparison of observed and calculated weight changes after one cycle at 1100 °C obtained by a regression analysis, seventy alloys demonstrated two distinct behaviors, which are divided heretofore into group A and group B. Microstructural observation revealed that an oxide layer in the group A alloys consists of A1203 and/or spinel or complex oxide, whereas an oxide layer in the group B alloys consists of a thick NiO layer with an A1203 internal subscale. Thermodynamic properties can reflect more effects of alloy elements in Ni-base superalloys, and Al and Cr activities, calculated by Thermo-Calc, were used as factors to distinguish the type of initial oxides. Group A and B alloys can clearly be divided according to Al and Cr activities. This was suggested as a new oxidation map to distinguish the initial oxide type of Ni-base superalloys, and possibly it can apply for any generation of Ni-base superalloys. In addition, empirical equations obtained from regression analysis were suggested to be used for predicting the weight change of alloys after one cycle at 1100 °C, and these could also be applied to all generations of Nibase superalloys. One equation exhibited excellent agreement between observed and calculated weight changes. The alloys that were applicable for this equation therefore had a compositional dependence.