Mitigating delayed fracture in AlSi-coated 2 GPa press-hardened steel via surface plasticity-induced hydrogen delocalization

Abstract

AlSi-coated 2 GPa press-hardened steel (PHS) has great potential for improving fuel efficiency in the automotive industry owing to its ultra-high strength. However, the ultra-high strength increases the hydrogen embrittlement risks and thus hinders its broad application. To mitigate its hydrogen embrittlement risks, a novel mechanism that is surface plasticity-induced hydrogen delocalization is proposed. While the conventional surface structure of industrial PHS composed of brittle intermetallics and interdiffusion ferrite (IF), exhibits negligible plasticity during the bending test, the newly developed PHS features a surface structure composed of ductile low-Al/Si IF, thereby activating substantial surface plasticity capable of enhanced plastic deformation energy storage. The enhanced surface plasticity enables the elimination of pre-existing cracks during thermal processing, coordinating multiple mechanisms including blunting of crack tip, expansion of plastic zone, twisting of crack propagation path, and the multiplication of hydrogen trapping sites in the surface structure during bending fracture. Consequently, both stress and hydrogen concentration near the main crack are noticeably alleviated, thereby mitigating the hydrogen embrittlement risks. Experiment results demonstrate that the hydrogen embrittlement resistance of the newly developed PHS is improved by more than twofold without sacrificing the tensile properties. This work provides a new pathway to mitigate the HE risks of 2 GPa AlSi-coated PHS and promotes its application in the automotive industry.