Thermal stability enhancement of hybrid Ni2Al3/Ni coatings on creep resistant ferritic steels by a mechanism of thermodynamically constrained interdiffusion

Based on the thermodynamic analysis, experimental work is undertaken to investigate the thermal stability of the hybrid Ni2Al3/Ni coating with a thin inner Ni layer formed on creep resistant ferritic steel. The results showed that once the coating is transformed by interdiffusion to a NiAl/Al-rich-Fe-base-alloy layer structure, thermal stability of the newly formed outer NiAl layer at 650 °C can be enhanced by the constraint imposed on inward Al diffusion by Al activity difference at the outer/inner layer interface. The Fe diffusion into the newly formed outer NiAl layer is restrained by its solubility limit in the newly formed outer NiAl layer.