Effect of loading rate on the dislocation emission from crack-tip under hydrogen environment

Abstract

The interplay between H atoms and crack-tip plasticity is essential to understand hydrogen embrittlement. By formulating a novel closed-form energy-based analytical model, the effect of hydrogen on the emission of dislocations in the vicinity of a crack-tip is investigated. Using fcc Ni as model material, the analytical model predicts that the critical radius of the stably developed dislocation loop decreases with applied Mode-I stress intensity factor (SIF) KI. Combined with the transition state theory, it is found that the most probable Mode-I SIF of Ni-H system increases with the H concentration under lower loading rates, but is not sensitive to the H content under higher loading rates. Further atomistic simulations show that, 1) the critical SIF required for dislocation emission does not vary significantly with the H concentration; 2) the dislocation loop is emitted from the crack-tip along the (111) slip plane at ∼ KIe=0.58∼0.64MPam, which is consistent with the present theoretical prediction ∼ 0.63 MPam. These results thus indicate that the crack-tip plasticity is suppressed by the H insertion (i.e. the Song-Curtin nanohydrides formation mechanism) under lower loading rates, but might not be influenced under higher loading rates.