Synthesis, Thermodynamic Stability and Diffusion Mechanism of Al5Fe2-Based Coatings

Abstract

Aluminide coating of steels enables more efficient power generation through higher operating temperatures. Low-temperature (T < 660 °C) pack cementation aluminide coatings form an Al5Fe2 phase which allows for the development of a large Al flux, but the mechanism is not clear. The coating structures and resultant oxides were examined in both austenitic and ferritic steels at 1,000 and 800 °C to evaluate the high temperature oxidation behavior in air. To understand the relatively fast Al diffusion, the stability of the Al5Fe2 phase and the defect structure have been examined by a cluster expansion method with density functional theory calculations. The Al5Fe2 phase has a low site occupancy and a high vacancy content that promotes rapid kinetics. The high vacancy concentration in the Al5Fe2 phase can be traced to the interaction between Al and vacancies along the [001] chains. The analysis offers useful guidance to enable an effective control of low temperature aluminizing.