Cross-sectional TEM observation of iron–zinc intermetallic Γ and Γ1 phases in commercial galvannealed IF steel sheets

The Γ phase and Γ1 phase in commercial galvannealed interstitial free (IF) steel sheets were studied by transmission electron microscopy (TEM). The cross-sectional TEM specimen was prepared using focused ion beam (FIB) technique which was also introduced briefly. The results show that there exist superlattice reflections in the selected diffraction patterns of both Γ phase and Γ1 phase, which indicate that both of the Γ phase and Γ1 phase are ordered intermetallic compounds. A Γ phase cell can be regarded as a α-Fe cell, which enlarges lattice constant three times in the x, y, z directions and keeps the same crystal structure with the Zn atoms occupying some positions of Fe atoms. A Γ1 phase can be formed by doubling the Γ phase cell in the x, y, z directions, while the crystal structure transforms from BCC to FCC.