Anisotropy and grain size dependence of the effects of hydrogen on the shock-induced spallation in iron

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Hydrogen (H) embrittlement in metals has long been a source of concern in academia and industry. However, the fundamental mechanisms remain not well understood in dynamic events. Using atomic simulations, shock responses of monocrystalline α-Fe along the [100] and [111] crystal orientations and nanocrystalline Fe with and without H atoms are investigated systematically. For the first time, the anisotropic dependences of the effects of H atoms on the spall strength are revealed. In the case of the [100] crystal orientation, H atoms prevent the phase transition of α-iron under shock compression, further slowing down the growth of voids during spall fracture and enhancing the spall strength at high strain rates. While in the case of [111] crystal orientation, H atoms promote dislocation formations under shock compression which is consistent with the known hydrogen-enhanced local plasticity (HELP) mechanism; H atoms accelerate the growth of voids during the spallation and reduce loads-bearing capacity, which finally lowers the spall strength of α-iron. In nanocrystalline samples, voids favor along GBs. The presence of H atoms would cause a local disturbance at the GB but block the long-distance movement of the surrounding atoms resulting in a slight increase in spall strength. The combined effects of H atoms and GBs influence the void evolution. This work provides deep insights into the hydrogen effect on metals under dynamic loading and should benefit a wide broad of research communities in materials and mechanics.