13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- stress, l* is the length of edge dislocation line at the energy saddle point of the dislocation motion which are required to surmount for dislocation motion. kB is the Boltzman’s constant, T is the absolute temperature, νd is the attempt frequency of dislocation motion. On the other hand, in the presence of hydrogen, energy barrier landscape becomes complex depending on the correlation between the positions of dislocation and hydrogen atoms. According to the case analyses of the competition between dislocation motion and hydrogen atom diffusion, dislocation velocity vary from softening to hardening depends on applied shear stress [4] under the lower hydrogen concentration (CH = 0.49 /nm). These results indicated that the softening attribute the decrease of energy barrier from 0b to 1b from the position of hydrogen atom, and hardening attribute the increase of energy barrier from 1b to 2b[4]. Moreover, the softening does not occur under high hydrogen concentration (CH = 1.24 /nm) due to the increment of energy barrier from 0b to 2b[5]. These results of dislocation velocity obtained by atomistic simulations are employed as material properties for dislocation dynamics analyses. Here, the energy barrier is approximated as following function[13]: ΔE= A⋅ 1− τ/ τ ath ( ) n , (8) where, τath is the athermal stress for dislocation motion, A and n are the fitting parameters. The parameters used in this study are shown in Table 1. Here, the dislocation velocity at higher hydrogen concentration condition (CH = 1.24 /nm) is approximated as shown in Table 1. The energy barrier obtained in these results, however, neglected the contribution of the enthalpy effects. The effect of enthalpy is approximately considered as following equation[14], therefore the activation free energy for dislocation motion is written as: ΔEact−free = A1−T/T m ( )1− τ/ τath ( ) n , (9) where, Tm is the surface disordering temperature, and half of the melting temperature (Tm = 904 K) is adopted. In this study, the activation free energy as shown in equation (9) is employed as an energy barrier of dislocation motion, the obtained relationship between dislocation velocity and applied shear stress at 300 K are shown in Figure 2. At the low hydrogen concentration, dislocation velocity increase (compared with that in the absence of hydrogen) occurs when the applied stress is lower than 29 MPa, and decrease occur when the stress takes higher than 29 MPa. Moreover, applied stress takes over 81 MPa, dislocation velocity does not show the effect of hydrogen. Table 1. Fitting parameters of the energy barrier for dislocation motion n τath A CH=0.49 /nm ( τ≦28MPa) CH=0.49 /nm ( τ≧81MPa) CH=1.24 /nm Hygrogen free 1.48 660 1.48 660 1.41 680 0.0821 0.0795 0.0821 0.0795 680 1.41
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