In-situ observation in a scanning electron microscope on the exfoliation behavior of galvannealed Zn-Fe coating layers

Exfoliation behavior of galvannealed (GA) coating layers was continuously observed in-situ under 3-point bending load in a scanning electron microscope (SEM). Two kinds of substrate, Al-killed steel and Ti-stabilized interstitial free (IF) steel, were used. For GA coating layers formed on Al-killed steel, powdering was dominant at the region under compressive stress during the bending. Exfoliation occurred at the Γ/δ₁ interface, and the amount of exfoliation decreased with an increase in Fe content in the coating layer. Meanwhile, for GA layers coated on IF steel substrate, flaking was dominant. One of the perpendicular cracks that were pre-induced during galvannealing grew and propagated up to the Γ/Fe matrix interface with increasing strain. On the other hand, the crack initiated at the Γ/Fe matrix interface grew and propagated along the interface. Then, this crack combined with a nearby perpendicular crack, resulted in flaking.