High-speed loading of dislocations formed under

Source:   Date:2014-10-28 15:46:34   Views:

Mechanism for sealing and cutting machine high speed under load dislocations formed in the material is completely different from the low-speed loading, sealing and cutting machine high speed load when the shock wave propagation in the crystal is formed, it will generate a lot of dislocations organization. Moreover, dislocations generated at this time is more evenly distributed, are generally presented periodically arranged hierarchically. In past studies, the scholars have also made some very valuable model to analyze the organizational characteristics of dislocations at high speed loads sealer formed, mainly Smith model, Homboge model and Meyers model several.
Smith describes the first use of dynamic deformation theory of the formation of a shock wave impact load effect on the dynamics of dislocations in the material when the organization. He proposed the concept of an interface, and the interface is based on the presence of dislocations in the form of a column, the interface can compensate for differences in the crystal lattice parameter array in front of the rear shock. Meaning Smith interface and phase change interface of the two phases are very similar. There is no interface dislocations, shear stress at this time can not be balanced; the presence of dislocations Smith model interface, then the shock wave front dislocation network formed on the surface may shock front surface stress relaxation.
Smith model requires dislocation interface should move together with the shock front. Therefore, the speed of movement of dislocations must exceed the speed of sound, which is wrong velocity is the speed of sound for the ultimate position contrary. For the dislocation velocity reaches the speed of sound, the stress should be infinite. Therefore, the requirements to the supersonic motion of dislocations is the biggest flaw Smith model. Hornbogen model was modified for Smith, he found iron under dynamic loads, the (111) crystal plane direction can be seen screw dislocation. He proposed that shock people first entered into the crystal dislocation tangles formed, dislocations of edge components and compressed parts together to shock front velocity, and Screw dislocations component part does not move, along with bits component of edge dislocation motion, increasing the length of screw dislocation. Homhogen model built on only one metal - iron behavior observation. The difference between the sub-structures of various metals and alloys between large, Homhogen model does not apply. Also Homhogen same model and Smith model requires edge dislocations with supersonic motion.
Because Smith model and Homhogen model limitations in terms of speed and other aspects, Mereys proposed a new dislocation formation model, which has the following characteristics:
(1) uniaxial deformation under the effect of shear stress, dislocations in the shock wave front interview domain (or nearby areas) uniformly produced, the resulting dislocation leads to shear stress relaxation; dislocations (2) produced only need to move a small distance subsonic;
(3) With the spread of the new shock wave is generated in the material interface dislocations, dislocations generated in the original remain in the crystal.
Such Mereys model with respect to the Smith model and Hombogen Model T dislocations eliminate restrictions on the movement speed, supersonic throne mistake does not have to be, in addition Mereys can also be dynamically loaded after the estimated dislocation density. A simple schematic diagram is shown in wave propagation in the material at 14 in Figure 2 a. For cubic metals, in the very moment of partial stress shock wave into the material to make initial cubic lattice into a monoclinic lattice. When the stress reaches a certain critical value dislocations can even produce.
When the shear stress reaches a critical shear stress value Th wrong in the right direction host can even produce.
Shock wave front corresponding to the initial shock of dislocation interface. The dislocation density of the boundary surface can be calculated based on the shock wave front after the unit volume of the dot array of two dimensions. Expressed shock front face before exercise, stress can not re-produce partial compensation, resulting in a new dislocations interface. Thus the whole process is repeated. Mereys model can well explain many of the experimental results. As experiments show, unloading waves play a minor role in the formation of dislocation organization dynamic loading, ie unloaded wave into the existing high dislocation density material will not cause a large number of dislocations generated. This model is consistent, because pre-deformable material was dynamic impact, shear stress can rely on existing dislocations get loose, this time in the wavefront will not be re-produced in a large number of dislocations. The same can be generalized to the case of multiple dynamic loading. In this case, partial stress can rely on the movement of dislocations generated dynamically loaded when you first get relaxed, repeated load does not lead to further significant dislocations generated. Mereys model can also predict the dynamics of the formation adjacent staggered when loading a different Burgers vector sum is also equal to zero, is an ideal dislocation model.
In any case, when the closure of all dislocations formed dynamically loaded than normal load conditions for the formation of dislocation density is higher, evenly distributed. First, the main reason for the formation of such a situation is the role of dynamic loading loading time is short, the rate of dislocation formation, whereas the rate of dynamic recovery time is relatively much lower. In addition, when the high dynamic loading deviatoric stress, resulting in an increase in the number of bit error sources, and the number of dislocations per dislocation sources can produce has also increased. Dislocation distribution because of the impact of the pressure drop can be reversed so that the deformation caused by, there is a reason for this is to reduce the dislocation mean free path.