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 arebecoming increasingly difficult because of the complex alloycomposition. In this study, we focus on the initial oxidationbehavior of Ni-base superalloys, and suggest a new oxidation mapwith using Al and Cr chemical potentials to distinguish the initialoxide type of Ni-base superalloys with seventy binary, ternary,and multicomponent Ni-base single crystal superalloys at 1100 °C.As a comparison of observed and calculated weight changes afterone cycle at 1100 °C obtained by a regression analysis, seventyalloys demonstrated two distinct behaviors, which are dividedheretofore into group A and group B. Microstructural observationrevealed that an oxide layer in the group A alloys consists ofAl2O3 and/or spinel or complex oxide, whereas an oxide layer inthe group B alloys consists of a thick NiO layer with an Al2O3internal subscale. Thermodynamic properties can reflect moreeffects of alloy elements in Ni-base superalloys, and Al and Cractivities, calculated by Thermo-Calc, were used as factors todistinguish the type of initial oxides. Group A and B alloys canclearly be divided according to Al and Cr activities. This wassuggested as a new oxidation map to distinguish the initial oxidetype of Ni-base superalloys, and possibly it can apply for anygeneration of Ni-base superalloys. In addition, empiricalequations obtained from regression analysis were suggested to beused for predicting the weight change of alloys after one cycle at1100 °C, and these could also be applied to all generations of Nibasesuperalloys. One equation exhibited excellent agreementbetween observed and calculated weight changes. The alloys thatwere applicable for this equation therefore had a compositionaldependence.