Effects of grain sizes on the oxidation behavior of Ni-based alloy 230 and N

The effects of grain sizes on the oxidation behavior of the alloy 230 and alloy N were investigated with the consideration of texture. The grain refinement process only had a slight influence on the weight change of the alloy 230 while it significantly changed the oxidation behavior of the alloy N. Oxide spallation was observed after oxidizing the coarse-grained alloy N for 100 h while no spallation was observed on the fine-grained alloy N even after 1000 h. Furthermore, the oxide formed on the alloy 230 with a small grain size had similar surface morphology and chemical composition to that formed on the coarse-grained alloy 230. For the coarse-grained alloy N, thicker and cracked oxide layers formed in the grain interiors and thin Cr-rich oxide films were found on the grain boundary areas. A homogeneous Cr-rich oxide layer covered the entire surface of the alloy N with a small grain size which clearly implies that the diffusivity of Cr was enhanced owing to the grain refinement process. Schematic models were developed to manifest the oxidation mechanisms of the high and low Cr alloy with large and small grain sizes.