Towards a Dependable TEM Characterization of Hot-Dip Galvanized Steels with Low and High Si Content

Designing Advanced High-Strength Steels assumes fine-tuning of alloying elements to enhance performance properties. This affects the kinetics of Fe-Zn interaction during further galvannealing. A key role here belongs to the inhibition layer at the steel/coating interface. Applying mutually-reinforcing TEM techniques, we traced the evolution of Zn-Fe phases in low- and high-Si steels during galvannealing. It has been shown that conventional room temperature Ga FIB for TEM sample preparation is applicable only to annealed low-Si steel, a coating of which comprises equilibrium Zn-Fe compounds. The phases were validated via electron microscopy and diffraction methods. In contrast, even relatively long annealing of the high-Si steel was not sufficient to form stable Zn-Fe phases pointing to a hampering of the interphase reactions. As-galvanized and annealed high-Si specimens contain almost pure Zn and are affected by Ga via a eutectic reaction during FIB preparation. Thus, low-temperature FIB has been applied for reliable Zn-coated steel preparation. We refined with HRTEM the inhibition layer structures specific for low- and high-Si steel grades. While the typical Fe2Al5-xZnx interface layer has been found in the low-Si samples, for high-Si ones the formation of a quite stable Si-Mn mixed oxide membrane, which drastically hinders Fe-Al reaction, has been disclosed.