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The isothermal oxidation tests were conducted on HR120 alloy in the Fe-40Ni-24Cr system to investigate oxide growth characteristics in terms of oxidation kinetics, oxide phases, and oxide scale morphology. The alloy underwent heat treatment at 1050 °C and 1150 °C for 3 hours, followed by rapid water quenching. The samples were categorized based on grain size as having either low grain diameter (from 1050 °C treatment) or high grain diameter (from 1150 °C treatment). The average grain diameter was measured using the line intercept method in accordance with ASTM E112. The heat-treated samples were subjected to isothermal oxidation at 500 °C for 500 hours in laboratory air. Post-oxidation, the samples were characterized for oxidation kinetics via weight change measurements, oxide phase identification using X-ray diffraction (XRD), and oxide scale morphology using scanning electron microscopy (SEM) and field emission SEM (FESEM). The results showed that both samples followed a parabolic oxidation rate law. The sample with low grain diameter exhibited a lower parabolic rate constant, indicating a slower oxidation rate. XRD analysis revealed the formation of several oxide phases on the alloy surface. Morphological analysis showed the presence of protruding Nb-rich oxide particles. The high grain diameter sample exhibited an overgrown, non-adherent oxide scale prone to exfoliation. These findings suggest a diffusion-controlled oxidation mechanism influenced by grain size, particularly affecting the development and distribution of Nb-rich oxide structures.
