| Tungsten becomes one of the important candidates of the plasma facing materials(PFM),and is most probable to be used as the first wall material in the future fusion reactor.As PFM,tungsten is supposed to bear high thermal loads,which could induce high thermal stresses and the formation of crack.The crack propagation in tungsten will cause the fatal destruction of the components.Therefore,it is important to understand the fracture mechanisms of tungsten due to the crack propagation.In this paper,the microscopic deformation mechanism and macroscopic failure mechanism of a pre-existing center crack propagation in single crystal tungsten under cyclic loading was examined by molecular dynamics(MD)simulations at room temperatures.In order to further study the effects of temperature on the ductile-to-brittle transition of[001](010)crack and[10-1](101)crack,we conducted a simulation experiment at 600 K and 900 K,and the peak stress,strain and threshold stress intensity factor at different temperatures were calculated.Meanwhile,the strain in the plastic zone at different temperatures were also calculated to elucidate the effect of temperature on the crack propagation behavior in single crystal tungsten.Subsequently,we measured the crack growth rate at different temperatures.In order to further study the fracture behavior of crack propagation in tungsten,we simulated the fracture behavior of grain boundary crack at 5 K and 100 K and the corresponding crack growth rate was also given out.The results indicated that the deformation mechanism and fracture behavior at crack tip were differences for variously oriented cracks at room temperature.The[001](010)crack propagated as the form of the formation of slip before cycle 8,which was mainly expanded by ductile fracture.And the mechanism of deformation was mainly the blunting and void nucleation after cycle 8.While the deformation mechanisms of[10-1](101)crack were blunting voids and the fracture behavior showed brittleness in macroscopic.At higher temperature,many more slip systems were activated resulting in the change of mode of crack propagation.Simulated results showed that the effect of temperature on deformation mechanism and fracture behavior of[001](010)crack was more sensitive than that of[10-1](101)crack.At 900 K,the double {110} plane was observed and the deformation mechanism of the crack propagation was mainly slip and blunting.In addition,a large number of plastic deformation regions were observed near the crack tip.However,the deformation mechanism of[10-1](101)crack at high temperature was still blunting and void nucleation,which was characterized by micro-crack nucleation,growth and linked with the main crack.The corresponding threshold stress intensity factor was also calculated.The results showed that the calculated values were in good agreement with the experimental values.Meanwhile,the influence of a 5<310>{110} model grain boundary(GB)on crack propagation was also discussed.Detailed analysis showed that the grain boundary resisted the crack growth by changing the deformation mechanisms and the path of crack propagation at crack tip before the crack reached the grain boundary at 5 K.The crack could not pass through the GB because of the barrier.After several loading cycles,GB played an important role in the nucleation of microcracks due to the stress concentration near the GB.At 100 K,the main deformation mechanism was dislocation slip before the crack extended to the grain boundaries.It was observed that the crack passed through the grain boundary rapidly.Subsequently,the crack propagation as the form of the void nucleation and the fracture behavior showed brittleness.In a word,the grain boundaries resisted the crack growth before the crack reached the grain boundary,while it favored the nucleation of micro-cracks in later stage of loading.Finally,the crack growth rate was calculated,and the results showed that the grain boundary and temperature had significant influence on the crack growth rate. |
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