The Role of Grain Boundary Wetting for Metal Induced Embrittlement in Press Hardened Galvanized 20MnB8 Steel: A Nanoscale Investigation

Grain boundary wetting as a preliminary stage for zinc induced grain boundary weakening and embrittlement in a Zn coated press hardened 20MnB8 steel was analyzed by means of electron backscatter diffraction, Auger electron spectroscopy, energy dispersive X-ray analysis and transmission electron microscopy on the nanometer scale. Microcracks at prior austenite grains boundaries were observed and structures developed after microcrack formation were identified. Zn/Fe intermetallic phases, smaller than 100 nm in size, are present at the crack surfaces and at the wedge-shaped crack tips. In order to get a complete picture, including the microstructure before cracking, an undeformed, electrolytically coated reference sample which underwent the same heat treatment as the press hardened material was investigated. Here, Zn, in the order of one atomic layer or less, could be found along prior austenite grain boundaries several micrometer away from the actual Zn/Fe phases in the coating. From this, we conclude that grain boundary weakening by Zn wetting of prior austenitic grain boundaries cannot be ruled out as necessary condition for microcrack formation from a physical characterization point of view.