Generation and Propagation Mechanisms of Cracks in Galvannealed Coating

The generation and expansion of cracks in galvannealed coating were analyzed under the condition of compressive or tensile stress, and the interaction between the coating and the steel substrate was analyzed during galvannealed deformation by environmental scanning electron microscope (ESEM) with in-situ investigation method in tension test and three point bending test. The results showed that the damaging process of the galvannealed coating under tensile stress could be divided into two stages. In the first stage the major damage form was powdering, in the second stage, the major damage form was flaking. Under the composite tensile stress, the fracture of galvannealed coating was not along with the fixed interface but arised with different types of fractures and flakes at different stages of the loading; and the interface fracture was different in different regions. When the galvannealed coating was subjected to tensile stress, cracks occurred at the Г/δ interface and extended perpendicularly to coating plane in the δ phase, and then propagated along with the Г/Fe interface. The signs of plastic deformation of coating was not obvious under tensile stress, but showed apparent plastic deformation under compressive stress. During the deformation of coating and steel substrate, there was substantial interaction and inhomogeneity.