Effect of Grain Size on High Temperature Oxidation Behavior of IN792 Superalloy

Abstract

It is known that Ni-based superalloys possess superior mechanical properties in high-temperature and high-pressure environments. One of the drawbacks of these alloys is their tendency to oxidize at high temperatures. Improving their oxidation properties and obtaining a fundamental understanding of the underlying mechanism of their oxidation behavior are critical for high temperature applications. In this work, we manufactured IN792 Ni-based alloys with two different grain sizes to examine the effect of grain size on oxidation behavior at 850<sup>o</sup>C and 980<sup>o</sup>C. The oxidation rate became faster as the grain size became finer, and at 850<sup>o</sup>C, the oxide layer was composed of external oxides (TiO<sub>2</sub>/NiO, Ni-Co-Cr-O, Cr<sub>2</sub>O<sub>3</sub>), internal oxides (TiTaO<sub>4</sub>, non-oxidized layer, Al<sub>2</sub>O<sub>3</sub>) and a precipitate-free zone (PFZ). At 980<sup>o</sup>C, this structure became unstable due to the diffusion of Ni, and in particular, a lot of exfoliation of external oxides occurred in the coarse grain. Regardless of the structure of the oxide layers, the thickness of the PFZ increased significantly in the fine grains with a low  fraction. Compared to the hardness of external Cr-rich oxides of ~500 HV, the hardnesses of the internal oxides consisting of Al-rich and Ni-Al-O were approximately 1000 HV and 1500 HV, respectively. Therefore, the most vulnerable position is expected to be the interfacial boundary between the external oxide and internal oxide, and this resulted in the ease of exfoliation of the external oxide in this location, by exposing the Ni-Al-O oxide to the surface in the coarse grain at 980<sup>o</sup>C. The current work can be a useful method in the design of Ni-based superalloys by controlling the grain size of the components.