Nucleation and Growth of Intermetallic Compounds Formed in Boron Steel Hot-Dipped in Al–Ni alloy

The formation mechanism of intermetallic compounds formed in boron steel hot-dipped in Al–7Ni (wt %) at 690 °С for 10–120 s was studied by identifying the intermetallic phases and investigating the growth process. Initially, a Fe3O4 oxide layer formed on the steel. The oxide layer separated into multiple layers sporadically; following this, the Al–Ni molten alloy permeated into the region of the oxide layer breakdown and formed the Al9FeNi (T, monoclinic, space group: P21/c) phase on the steel surfaces. The Al9FeNi (T) phase formed from the reaction between the Al–Ni molten alloy and Fe eluted from the steel; this phase not only acts as an Al interdiffusion channel, but also as a barrier for Fe; and facilitates only grain growth without a significant change in thickness. Inside the steel, the Fe2Al5 (η, orthorhombic, space group: Cmcm) phase grows along the c-axis in the [001] direction; and has a long columnar structure. The Fe3AlC (κ, Cubic, space group: Pm3m) phase is formed owing to a reduction in the Al concentration and the simultaneous diffusion and discharge of C toward the steel interface, as C cannot dissolve in the Fe2Al5 (η) phase.