  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, atom probe tomography and
transmission electron microscopy on the nanometer scale. Microcracks at prior
austenite grain boundaries were observed and structures that developed after
microcrack formation were identified. Zn/Fe intermetallic phases with grain
sizes smaller than 100 nm in diameter 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 micrometers away
from the actual Zn/Fe phases in the coating. Such a grain boundary weakening by
Zn wetting of prior austenitic grain boundaries during austenitization and/or
hot forming is a necessary condition for microcrack formation.
