Formation of intermetallic compounds in aluminium-coated steel strip produced by the Elphal process

Aluminium-coated steel strip produced by the Elphal process has been heat treated in the temperature range 350°–640° C for times up to 480 h. The intermetallic compounds that form as a result of interdiffusion have been identified by X-ray diffraction and electron and optical metallography. The first phase to form at all temperatures has been shown to be θ (FeAl₃). At temperatures below 556°C the θ-phase remains dominant, but above 532°C η (Fe₂Al₅) develops and above 556°C becomes the dominant phase. The η-phase is observed to be complex and inhomogeneous with a needle-like precipitate. After extensive heat treatment a thin layer thought to be ζ (FeAl₂) can be observed at the Fe/η interface.