13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 4. Stress-strain curves of the alloy aged for 200 h at various strain rates Under the condition of static or quasi-static deformation, the speed of dislocation motion is higher than the deformation speed of materials by the low loading rate. Dislocations have relative enough time for acceleration. At the moment, the acceleration for dislocation moving is very lower, so the time-effect on deformation resistance is not obvious. Therefore, the increasing of yield strength is not high with the increasing of strain rate ( lower than101 s-1). When the strain rate is higher than 101 s-1, the acceleration of moving dislocation increases, meanwhile, the dislocation moving resistance increases. Consequently, the strength of the alloy increases significantly when the strain rate is higher than 102 s-1. It has been found[13] that the dislocation motion is faster at the higher strain rate and the short-range factors inhibiting the dislocation motion such as the Peierls-Nabarro force, thermal vibration of the atoms and electron cloud resistance increases significantly. When the dislocation motion speed exceeds a certain value of about 101 m·s-1, the controlling mechanism of dislocation motion is changed from thermo-activation to short-range damping. And the dislocation motion resistance shows an obvious increase and thus the deformation resistance of the alloy rapidly increases. It indicates that the obvious increasing of the strength as the strain rate higher than 102 s-1 is related to such factor. The compatible plastic deformation mechanism of dislocation pile-ups release and crystal rotating exhibits an obvious time-dependence in the alloy when tensile deformation under dynamic loadings [13-15]. And the plastic deformation coordinates by means of activation of dislocation slip systems, as shown in Figure 3. Dislocations are subjected to very high shear stress in a very short time and the shear stress reaches or exceeds the critical resolve shear stress (CRSS) of the dislocation slip systems when the dislocations are not yet ready to move at conventional dislocation slip systems. As a result, the ductility of the alloy shows a marked increase at the same strain rates range as that of the increasing strength. On the other hard, a large amount of deformation twins have been found near the fracture surface in the alloy, as shown in Figure 5. And the number of deformation twins increases with the increasing of strain rate. Also some dislocation pile-ups around the stacking faults in the crystal can be observed under dynamic loading which is due to the stress concentration. When the dislocation motion is easy, the stress concentration is small. On the contrary, the stress concentration becomes high. It is too late for the dislocations at the dislocation pile-up to move in the alloy during tensile deformation at high strain rate, and thus the stress around the stacking fault increases, which will promote the occurrence of twinning deformation. The existence of a large number of deformation twins in the alloy during tensile deformation at high strain rate makes obvious contribution to increasing of the plastic deformation ability.
RkJQdWJsaXNoZXIy MjM0NDE=