13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 3. Results and Discussion 3.1 Strain rate effect on tensile property Figure 1 shows tensile properties of the alloy aged with different parameters at various strain rates. It shows a similar strain-rate dependence on yield strength: shallow at the strain rates lower than101 s-1, and even steeper with increasing of strain rate, especially when the strain rate is higher than 102 s-1. It indicates that the plastic deformation mechanism of the alloy begins to change when the strain rate is higher than 101 s-1. The ductility of an alloy aged for different time also shows variation with the increasing of tensile strain rates, as shown in Fig.1b. When the strain rate is increased from 10-3 s-1 to 102 s-1, the fracture elongation decrease obviously, while, then the fracture elongation increased significantly. From the results, it can be concluded that the plastic deformation mechanism of the alloy changed at the strain rate of 102 s-1. When the alloy aged with different aging parameters, the increasing of ductility in the alloy aged for 500 h and 1000 h is less than that for 100 h and 200 h. Figure 1. Variation of mechanical properties with the tensile strain rate of the alloy aged by different parameters: (a) yield strength, (b) fracture elongation 3.2 Strain rate effect on tensile fracture morphology Figure 2 shows the morphologies in fibrous zones of tensile fracture surface of the alloy by aged for 100 h and 100 h with different strain rates. The alloys aged for 100 h and 1000 h exhibit mixed fracture characteristics with ductile and intergranular when the strain rate is lower than 10-3 s-1. The small changes can be observed from the fracture morphologies of the alloy with different aging time when the strain rates are increased to 103 s-1. It can be found that the percentage of ductile fracture region decreases obviously in the alloy aged for 1000 h as compared with that aged for 100 h at the same strain rate. In addition, the percentage of ductile fracture region in the alloy aged for 1000 h further decreases with the increasing of strain rate and the intergranular fracture characteristics is more apparent. 3.3 Dislocation configurations near fracture surface Dislocation configurations near the fracture surface in the alloy aged for 200 h at various strain rates are shown in Figure 3. It can be noticed that the dislocation density is not high and it seems no remarkable change with the strain rate of 10-3 s-1 to 101 s-1, while when the strain rate is increased to 102 s-1, the dislocation density significantly increases and activation of slip systems can be found with TEM observation. However, when the strain rate is higher than 102 s-1, the dislocation density remains stable but dislocation tangles become more severe. (a) (b)
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