Microstructural investigations on the hot-dip aluminized AISI 321 stainless steel after diffusion treatment

Abstract

The microstructure of the hot-dip aluminized AISI 321 stainless steel was studied after diffusion treatment at 900 °C for 3 h. The microstructural characterization was carried out by scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry. The microstructure of the as aluminized steel consisted of two regions, viz.; aluminum topcoat and aluminide layer. During the diffusion treatment, the coating transformed into a layered structure consisting of four layers. The Fe<inf>2</inf>Al<inf>5</inf> phase was formed in the outermost layer and the presence of Al<inf>13</inf>Fe<inf>4</inf> quasicrystalline approximant phase was observed. The innermost layer adjacent to the base metal transformed to ferrite phase with NiAl precipitates. Next, to this layer, a disordered FeAl phase was observed. The lattice parameter of the disordered FeAl phase was found to be larger than that of the ordered B2 FeAl phase. The layer between outer Fe<inf>2</inf>Al<inf>5</inf> phase and disordered FeAl phase consists of a mixture of three phases, namely Fe<inf>2</inf>Al<inf>5</inf>, disordered FeAl and a new phase with the simple cubic structure. The phase with simple cubic structure shares cube on cube crystallographic orientation relationship with the disordered FeAl phase. © 2019 Elsevier B.V.