Title: Diffusion coatings of steels: Formation mechanism and microstructure of aluminized heat-resistant stainless steels
Abstract: The effect of base-alloy composition on the microstructure and mechanical and thermal stabilities of aluminum diffusion coatings has been studied for 316, 310, and I800H stainless steels, by optical, microprobe, transmission/scanning transmission electron microscopy, and microhardness testing. In all the diffusion aluminized alloys, two distinct coating layers form: an outer aluminide layer and an inner, interdiffusion layer. The substrate austenite stability is the single most important parameter affecting the thickness, phase distribution, and microchemistry of these two layers. TEM/STEM analyses showed that the interdiffusion layer is a ‘‘natural composite’’ made up of a uniform dispersion of the hard nickel aluminide phase (B2) in a soft ferrite matrix. Formation of this layer involves ‘‘ferritization’’ of the substrate, a process akin to pearlitic transformation in carbon steels. The interdiffusion layer demonstrated high hardness with good mechanical integrity and its thermal stability with the substrate depends strongly on the ease of ferritization, which depends in turn on the degree of austenite stability of the substrate.