Impact of Nb Content on Oxide Growth Characteristics of HR120 Alloy in the Fe- 40Ni-24Cr System Under Isothermal Oxidation Conditions

Abstract

The impact of Nb content on the characteristics of oxide scale formation was studied during isothermal oxidation tests of the HR120 Fe-40Ni-24Cr alloy. The oxidation kinetics, phase analysis, and surface morphology were studied using samples with low grain diameter (LGD) and high grain diameter (HGD) obtained by heat treatment at 1050 °C and 1150 °C, respectively. The LGD and HGD samples were then subjected to isothermal oxidation tests at 500 °C for 500 hours. The oxidation kinetics were measured by tracking the time for weight changeovers. Phase analysis was performed using X-ray diffraction (XRD), and surface morphology was examined using scanning electron microscopy (SEM) and field-emission SEM (FESEM) with energy-dispersive X-ray (EDX) analysis. The oxidation kinetic plots exhibited an increasing weight gain trend for LGD and HGD samples, and followed a parabolic rate law. A lower parabolic rate constant was recorded by LGD sample of 7.38 × 10⁻⁹ mg²·cm⁻⁴·s⁻¹, indicating a lower oxidation rate, hence having good oxidation resistance. Phase analysis recorded Cr₂O₃ and MnCr₂O₄ oxides formed on both samples. Surface morphology analysis revealed a uniform oxide layer coexisting with a protruding Nb-rich oxide structure, further confirmed by EDX, and the LGD sample exhibited good scale integrity. The HGD samples displayed overgrown protruding oxide structures with non-adherent oxide scale, with evidence of oxide exfoliation. These findings support the proposed oxide diffusion mechanism, indicating that Nb content has the greatest impact on oxide growth in the HGD samples.