Modelling of nanocrack evolution in body-centred-cubic iron
Kumanin et al. analysed the damage evolution in metals, and indicated that many types of heat treatment methods can be used to decrease the amount of defects in metals after a long-term operation at high temperature. The molecular dynamics method was used to simulate nanocrack healing in Al and Cu crystals, both of which are face-centred cubic (FCC), during heating or/and under compressive stress. The critical temperature for nanocrack healing depends upon the orientation of the crack plane. Body-centred-cubic (BCC) crystal is characterized by four close-packed <111> directions, and by the lack of a truly close-packed plane such as the octahedral plane of the FCC lattice. Wei et al. simulated the crack healing behavior in BCC-Fe crystal during heating. In this paper, a molecular dynamics model was further developed to investigate the nanocrack healing process in BCC Fe not only at elevated temperature but also under compressive stress. In the case of heating, a fixed boundary condition was adopted along the x- and y-axis directions. In the case of loading, displacement boundary condition was adopted along the x- and y-axis directions. In both cases, periodical boundary condition was used in the z-axis direction along which the atoms can move.
A parameter dy is introduced for comparing the healing process at different temperature, which represents the minimum vertical distance between the atoms on top crack surface and that on bottom crack surface. dy is equal to 1.526 a0 in the initial configuration. In the case of heating, temperature has a significant effect on the extent of nanocrack healing. The simulated temperature in the model in this study was set below 1185K for keeping the parameters of N-body potentials for BCC Fe valid. The time step Delta t was 1*10-16s. The size of crack decreased rapidly at the high temperature of 1173K. Only after 4.5ps, the crack was healed completely. The status of crack healing remained stable after that. The critical temperature of crack healing is approximately 673K in this study. In the case of loading, the temperature was 40K, the time step Delta t was 1*10-15s and the loading rate was 0.025MPa*m1/2/ps. When t=30ps and KI=0.75MPa*m1/2, the crack was healed completely.
Some defects such as dislocations and voids appeared during the healing process and their positions change continuously. In the case of heating, the distribution of the defects in the cell is not homogenous. In the case of loading, most defects are in the area of the original crack.