LAUSR

Abstract The Inconel 718 is a promising Ni-based alloy for a wide range of high-temperature industrial applications. However, the radiative characteristics of Inconel 718 alloy change due to surface oxidation and cause large deviation in temperature field computation and radiation thermometry. Herein, the Inconel 718 alloy has been oxidized at different temperatures, ranging from 700 K to 1100 K, for different time (up to 100 h) to systematically study the influence of oxidation on radiative properties. The highly oxidized alloy samples have been prepared, whose oxidation degrees are more similar to the engineering applications. The normal spectral emittance of as-prepared and surface-oxidized Inconel 718 alloys has been measured in the range of 2–15 μm at different temperatures (400–1400 K). The spectral data have been integrated to analyze the effect of temperature on total emittance. In the case of as-prepared Inconel 718, the normal spectral emittance increased with decreasing wavelength, whereas the total emittance linearly increased with increasing temperature. By comparing the emittance data of oxidized and as-prepared samples, it is clear that the high-temperature oxidation improves the emittance of Inconel 718 alloy. The effect of oxidation on radiative characteristics of Inconel 718 can be divided into three stages: 1. the spectral emittance slightly increases in short wavelength range; 2. obvious emittance fluctuation occurs due to radiation interference; 3. emittance oscillations disappear and the spectral emittance curves shows more radiative characteristics of the main oxide (Cr2O3). Moreover, the total emittance of Inconel 718 alloy, with compact oxide scales, decreased with increasing temperature, and during the 2nd stage, a knee point is observed on the total emittance curve near the Neel temperature of NiO. To unveil the mechanism of oxidation-induced emittance variations, scanning electron microscope (SEM) and energy disperse spectroscopy (EDS) are employed. The structure of a multilayered oxide scale is demonstrated and the process of oxidation is explained. According to the analysis, the oxidation degree determines the weight of influence of substrate alloy and surface oxides on radiative characteristics of oxidized alloy, and this is also the reason why the influence of oxidation on radiative characteristics shows staged. Moreover, NiO was detected on the surface of oxidized samples, and combined with the emittance results, it can be concluded that the magnetic ordering of oxides will influence the radiative behavior of Inconel 718 alloy.