Effects of Cu addition on resistance to hydrogen embrittlement in 1 GPa-grade duplex lightweight steels

Hydrogen embrittlement (HE) has arisen as one of main issues for developing high-strength lightweight steels. The precipitation of fine particles providing as stable H-trapping sites is preferred to overcome the intrinsic HE of high-strength steels. However, studies on HE in high-strength lightweight steels along with roles of B2 particles have not been reported yet. In this study, three Fe–0.8C–15Mn–7Al–(0,1,3)Cu duplex lightweight steels were fabricated, and their resistance to HE was evaluated. Roles of Cu addition were investigated by the loss of elongation measured from slow-strain-rate tensile tests and by the concentration and diffusivity of H measured from electrochemical H permeation tests. The Cu addition results in the decreased elongation loss and the lower concentration and effective diffusivity of reversible H, indicating the higher resistance to HE. The unraveled mechanism is that the Cu addition increases the fraction of austenite, where the diffusivity of H is much lower than ferrite, and decreases the strain localization along ferrite grains to reduce the internal diffusion of H during deformation. In addition, it promotes the formation of complex semi-coherent Cu-rich B2 particles, which provides misfit dislocations at interfaces as stable and irreversible H-trapping sites. The B2 particles preferentially nucleated at reversible sites such as grain boundaries and phase interfaces promote a transition from reversible to irreversible sites, which further reduces the diffusivity of H. The present work, thus, would suggest the Cu addition to enhance both tensile properties and resistance to HE for designing high-strength lightweight steels.