**5. Conclusion**

232 Materials Science and Technology

the specimen with many defects on nano scale, which results in the decreasing of shear intensity. It also can be seen from Fig. 15 that the yield stress of scratched specimen also decreases with the increasing specimen size. The yield stress is observed to decrease with the increasing specimen size, which is due to the enhanced opportunities for dislocation motion to occur at a larger size. For small specimen sizes, PSS increases at the same scratch depths, which show size effect (namely the scale dependence, that the size and quantity of defects inside the nanostructure decrease and the intensity improve when its scale becomes smaller and smaller, and thus the nanostructure will show different mechanical properties

For an ideal specimen or a specimen with shallow scratch depths, the stress gradients is flat near the groove, the dislocations resulting from the stress field at the corner are the main sources of plastic deformation, and the breakpoint of failure is close to the either end of the specimen. With the increasing scratch depths, the gradient of stress is steep near the groove, are likely to overtake the bafement to cause the movement of dislocations, and thus the stress gradients near the corners and/or the groove induce the dislocations to determine the yielding point. In this way, the yield plastic deformation is likely to occur near the groove, and the breakpoint of failure is close to the groove of the specimen. Fig.17 shows the breakpoints of specimen under shear loads. It can be seen from Fig. 17 that the breakpoint is close gradually to the groove as the depth of groove increases. At the same scratched depths, the breakpoint is closer to the groove for a small specimen. It is noted that the breakpoint of specimen can be determined by groove when the ratio of the depths of groove to the height of specimen is up to one third (scratched depths 5*a*0) regardless of size and

When the specimen is continuously subjected to the shear load, the specimen begins to yield and break down. The snapshots of the rupture processes of specimen are as shown in Fig. 18. From Fig.18 we can see that the formations of rupture of nanostructure are that the development of vacant clusters in the neck region which causes the rupture of specimen, or gradual thinning of theneck region of the scratched specimen into a single

a) b)

c) d)

Fig. 18. (a) and (b) the formation of rupture of specimen with no defects for shear

simulation, (c) and (d) that of specimen with scratching depths of 5*a*0.

compared with body material)

strain rates [36].

chain of atoms.

Because of the smaller size of the nanostructure and the lack of related instrument, the traditional experimental method is not suitable to study the mechanical behavior of nanostructure. However, lots of researching work proves that the MD simulation method is a feasible method to study the mechanical and physical properties and plastic deformation of materials in nanoscale. based on the simulating results we provide above, we find that the removal mechanism of material, and states of the stress and strain of structure in nanoscale show obvious differences compared with macrostructures, such as the size effects, surface effects, and interface effects, two yield peaks et al. and the defects induced by machining play an important role during the dislocations moving and material yielding, and lead to the rupture of nanostructure loaded at last. MD simulation method is able to make the researchers to observe directly and clearly this phenomenon and find the hiding regulations in micro processing. The deep and wide study will improve the development of modern electrical industry and the whole manufacturing industry in the future.
