Microstructure evolution and hardness of hot dip aluminized coating on pure iron and EUROFER 97 steel: Effect of substrate chemistry and heat treatment

Abstract

EUROFER 97 steel, a variation of reduced activation ferritic-martensitic (RAFM) steel, is considered to be a potential material for the first wall of fusion reactor. Aluminide diffusion barrier coatings offer an effective solution to mitigate corrosion and tritium permeation in the first wall. However, the performance of the coating is influenced by the nature of intermetallic phases and their microstructure. In order to critically understand the phase evolution during both hot dip aluminizing (HDA) process and subsequent heat treatment, experiments were performed on (i) pure iron and (ii) EUROFER 97 steel. The samples were hot dip aluminized at 973 K in an inert Ar-5% H2 atmosphere. Detailed metallurgical analysis using backscattered electron detector, electron probe microanalyzer and electron backscattered diffraction technique was performed to examine the evolution of phases and microstructure. It was noted that Fe2Al5 is the major phase forming in both the cases during HDA. Subsequent heat treatment involving solution treatment at 1253 K and ageing at 1033 K led to transformation of Fe2Al5 to FeAl2, FeAl(Cr) and α-Fe(Al, Cr) in HDA EUROFER steel due to interdiffusion of Fe, Al and Cr. However, no trace of FeAl2 could be detected in HDA iron. Thicker and wavy interdiffusion layers were formed in HDA iron, whereas in HDA EUROFER steel near-planar interfaces were present. Micro-pores were observed in all heat treated samples. It was found that the hardness of the phases determined using nanoindentation are in the order of FeAl2(Cr)>Fe2Al5(Cr)>Fe2Al5>FeAl(Cr)>FeAl>α-Fe(Al, Cr)>α-Fe(Al).