| The fatigue fracture behaviors of metallic are studied in micro scale combined with modified analytic embedded atom method(MAEAM) by molecular dynamics(MD) for iron, nickel and nickel-base alloy. The fatigue fracture mode and deformation mechanisms at crack tip in metal are discussed. The crack growth rate is investigated by computing the variation of the crack length and stress intensity factor, and discuss the effects of the crystal orientation, temperature and grain boundary on the fatigue crack propagation.By studying the behaviors of fatigue crack propagation in single crystal iron, the results show that, at room temperature of 300 K, the cracks of different orientation have the different crack propagation mode and the deformation mechanisms at crack tip.[001](010) crack present ductile rupture, the shearing slip bands and cross slip are formation at crack tip and the slip systems are <111>{110}. But, [110]( 110) crack show brittle fracture, a few slip bands are formation at crack tip and the slip systems are <110>{110}. For the [11 2 ](111) crack, the voids and vacancies are the main deformation mechanisms at crack tip, and the crack propagated in the form of blunting.As the temperature increases from 300 K to 600 K, [001](010) crack appear voids and local FCC phase except slip bands, and the crack growth rate increase as the increasing temperature. But, for [110](110) crack, the dominate deformation of crack tip are voids and micro-crack, and the crack growth rate decrease as the temperature rising. For the[11 2](111) crack, the crack present brittle fracture as the temperature rising, and the crack growth rate also increase with the temperature increasing. The grain boundary of(50) 3 and(50) 5 are used to study the crack propagation, the results show that the(50) 3 have no effects on hindering the crack growth, and the crack present brittle fracture. In(50) 5,two symmetric slip bands appear in grain boundary so that the crack growth rate decrease and the local plastic deformation of crack include FCC slip bands and HCP structure phase.The metals of different crystal structures have the different crack propagation.Through the studies of fatigue crack growth in single crystal nickel, it is found that the mode of crack propagation and deformation mechanisms at crack tip in FCC nickel are different from that of BCC iron. In single crystal nickel, at 300 K, the deformation mechanisms of [001](010) crack at crack tip are blunting and edge dislocations duringgermination, but in the crack propagation, the persistent slip bands are the main deformation mechanisms at crack tip and the slip system is [110](11 1). For the[110](110) crack, the dislocation line is formation at crack tip, and grow along with [011] direction and form on(111) plane. For [11 2](111) crack, two symmetric slip bands are formation at crack tip, the slip directions are [123] and [132], respectively, and the slip plane is(111), including many vacancies and HCP phase in local plastic deformation. As the temperature increases from 300 K to 600 K, the crack growth rate in all crack models increase with the temperature increasing. Compared with the 300 K,the high temperatures lead to the increase of slip bands, but there is no change of deformation mechanisms at crack tip. The studies of grain boundary on fatigue crack propagation are investigated. The simulation results show that the(50) 3 model occur deformation under cyclic loading before crack propagation, and the slip bangs of crack tip go through the grain boundary into another part resulting in the increasing of crack growth rate. In(50) 5, the slip bands of crack tip are [112](110) and [112](110), and a lot of slip bands are formation in the two sides of grain boundary, and the grain boundary link the two side slip bands together so that the structure have excellent plastic and effectively hindered crack propagation.Relative to the single crystal metal, the fatigue crack propagation of alloys are different. Through simulating the fatigue crack propagation in Ni3 Al find that, at 300 K,the new triangular crack is formation at crack tip in(010)[001] crack and induce the crack propagation accompany with the formation of voids. The plastic deformation of(010)[001] crack at crack tip are dislocations and slip bands, and the slip systems are{110}<111>. In [110]( 110) crack, the stress intensity factor is lower comparing with(010)[001] crack, the crack propagate in the form of blunting sharp crack, and observe slip bands at crack tip, the slip system is(110)[110]. For(111)[11 2] crack, the ductile phenomenon of blunting occur at the crack tip, no sharp crack, resulting in the minimum crack growth rate in comparison with the other crack models, and the dominant deformation mechanism of crack tip is the slip bands, the slip direction is along [132] and [312], the slip planes are {111} plane. As the temperature increased from 300 K to 600 K, the crack growth rate of Ni3 Al increase with the temperature increasing comparing with the room temperature, the slip systems have no change in high temperature, and a large number of dislocations occur around the crack tip. The influence of Ni/Ni3 Al boundary on the crack propagation is studied. The obtained from the results present that the blunting atoms prompt the crack to deform and result in the formation of sharp crack along [110] orientation at crack tip, and the sharp crackinduce the extension of blunting. This moment the grain boundary transforms the slip bands of crack tip to the phase of Ni3 Al so that the crack growth is limited, so it is conclusion that the Ni/Ni3 Al grain boundary by transforming the plastic deformation has effectively hindered the fatigue crack propagation.In addition to being influence by crystal orientation, temperature and grain boundary, the crack also is influence by the defects in materials. So the effects of irradiation defects on the fatigue crack propagation have investigated at 100 K and300K. The obtained from the simulation results presented that the yield stress and yield strain of irradiated metal decrease comparing with un-irradiated as the irradiation temperatures are 100 K and 300 K. However, the elastic modules increase before and after irradiation at 100 K and 300 K. By analyzing the microstructure, the defects that formed at irradiation of 100 K hinder the dislocation movement, thus limiting the extension of slip bands so that the density of slip bands decrease. When the irradiation temperature is 300 K, the defects atoms induce the formation of many vacancies around the crack tip so that the structure is hardening, and the extension of slip bands have also been limited. The influence of irradiation defects on the fatigue crack propagation is performed. The results of interaction between the defects and crack present that the irradiation defects hinder the formation of blunting and slip bands at crack tip at irradiation temperature of 100 K, and decrease the crack growth rate. As the irradiation temperature is up to 300 K, the crack attract the defects together resulting in the rapid growth of slip band, and induce a lot of defects formation around the crack tip. |
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