Thermal exposure induced microstructural evolution and oxidation-property coupling in liquid phase sintered Fe-Ni-P alloys

The thermal stability of liquid-phase sintered Fe-Ni-P alloys is critically influenced by the interplay between microstructural evolution and oxidation behavior during high-temperature exposure. In this study, Fe-Ni-P bulk alloys with distinct porosity levels were fabricated via electroless Ni-P coating followed by vacuum liquid-phase sintering. Dense and porous microstructures were deliberately designed to elucidate the coupled effects of phosphide phases and pore networks through isothermal exposure at 600°C in air for up to 200hrs. The results reveal that oxidation in dense alloys is primarily confined to a thin surface layer, while porous alloys exhibit severe internal oxidation due to rapid oxygen penetration through interconnected pores. The oxide scale evolves from Fe 3 O 4 to Fe 2 O 3 , while the interior matrix phases remain largely preserved. Meanwhile, (Fe,Ni) 3 P precipitates undergo coarsening and redistribution driven by diffusion-controlled processes. These microstructural and oxidation changes lead to pronounced mechanical embrittlement and magnetic deterioration, particularly in porous alloys. This study highlights the critical role of porosity and phosphide phases in governing oxidation resistance and property stability, providing insights for the design of cost-effective iron-based sintered materials for intermediate-temperature applications.