Oxidation Behavior of K4750 Alloy at Temperatures Between 750 °C and 1000 °C

Abstract

This study investigated the oxidation behavior of a new casting Ni-based superalloy K4750 at 750 °C-1000 °C in air for 100 h-1000 h by isothermal oxidation tests. The oxidation-kinetic curves were plotted by the static discontinuous weight gain method. Observation and identification of oxidation products were conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD). X-ray photoelectron spectrometer (XPS) was also used to analyze the chemical state of elements and the distribution in depth. The results showed that the oxidation-kinetic curves of K4750 alloy basically obeyed the parabolic law. The average oxidation rate below 900 °C was less than 0.1 g/m2·h which meant the alloy was at a complete anti-oxidation grade, and the alloy was at an anti-oxidation grade at 1000 °C. The predominant surface oxide was Cr2O3, and a double layer structure of the oxide scale was observed at all tested temperatures as time increased. The outer oxide layer contained continuous Cr2O3 and a small amount of oxides mixed TiO2 and NiCr2O4, while the inner oxide layer was composed with Al2O3. The oxidation process could be interpreted by the competitive oxidation of different elements. The diffusion rate of Ti through Cr2O3 layer increased with increasing temperature, and thus the generation of TiO2 was advantageous. The dispersed TiO2 reaching a certain amount destroyed the continuity of the surface oxide layer, which accounted for the reduction of oxidation resistance of K4750 alloy at high temperatures.