13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Figure 2. Tensile fracture morphologies in the alloy aged for different time at various strain rates Figure 3. Dislocation configurations near fracture surface in the alloy aged for 200 h at various strain rates 3.4 Mechanism of strain rate effect on dynamic deformation behavior The stress-strain curves of the alloy aged for 200 h with different strain rates are plotted in Figure 4. It indicates that there is no obvious change of the Young’s modulus with the increase of the strain rate. Elastic limit and 0.2% offset yield strength of the alloy increase as the strain rate increase. From Figure 4, it is clear that the stress-strain curves in plastic deformation stage shows strong strain rate-dependence, which is consistent with the results shown in Figure 1. The strength of the alloy exhibits stronger strain rate sensitivity, when the strain rate is upto 102 s-1. It is known that when the force component at a slip system reaches the critical value, the dislocation begins moving. In fact, the dislocation motion is the atom motion in lattice and thus the dislocation motion is time-dependent. As the applied stress increases, the dislocation motion speed increases rapidly. When the specimen undergoes a high loading rate, the acceleration which the dislocation motion needs will be also increased [11]. The dislocation will meet with obstacles continually when it begins moving and this resistance of dislocation motion presents the deformation resistance during plastic deformation from the macroscopic point of view [12]. It is clear that the higher the speed of movable dislocation become, the higher acceleration and resistance of dislocation motion will be.
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