Effect Of Nanoscale Amorphization On Blunting Of A Crack In Crystalline Materials

Nanocrystalline metals and ceramics exhibit superior me chanical and physical properties, which present the subject of rapidly growing research efforts motivated by a wide range of their applications. However, in most cases, nanocrystalline materials have superior strength, strong hardness and good wear resista nce but with the costs of low tensile ductility and fracture toughness, which considerably limit their practical utility. Nevertheless, some works have been made to study the certain nanocrystalline materials with good tensile ductility or enhanced toughness. What’s more, the specific toughening mechanisms are attributed to specific deformation modes in nanocrystalline materials, such as nanoscale amorphization intergrain sliding, rotational deformation, as well as cooperative grain boundary sliding and mig ration. Meanwhile, Lots of experiments have testified that nanoscale amorphization(crystal-to-glass transition in nanoscale regions) often occurs in nanocrystalline materials and have great influence on the strength of the material. But, the people have not yet to reveal the phenomenon of the evolution of the microscopic and mechanical properties and microstructure of the quantitative correlation. Nanocrystalline materia ls in use and production process, will inevitably produce large Numbers of dislocation, micro defects such as micro cracks, and the microscopic defects and special deformation mode of nanocrystalline materials is becoming the basis of the study of nanocrystalline materials. Therefore, research on the effect of nanoscale amorphization on the dislocation emission from the crack tip not only beneficial to the crack tip of dislocation motion and nanocrystalline materials microstructure evolution and the relationship between the material fracture toughness, but also for fracture prevention provide a theoretical basis of nanocrystalline materials.Firstly, we have studied the influence of nanoscale amorphization on emission of dislocations from a finite length crack tip in nanocrystalline materials. A mechanical model describing the interaction between the edge dislocation, the finite length crack and the three disclination dipoles produced by the nanoscale amorphization region is established. The results show that nanoscale amorphization can suppress lattice dislocation emission. The most probable angle of dislocation emission decreases with increment of disclination strength and the shorter length crack drivers easier the dislocation emission from the crack tip.Secondly, we have established a mechanical model describing the interaction between the edge dislocation, the surface crack and the three disclination dipoles produced by the nanoscale amorphization region. The influence of disclination stresses, parameters of the three disclination dipoles as well as crack length on the critical SIFs are discussed in detail. The critical stress intensity factors for dislocation emission are calculated. The results show that for a certain nanoscale amorphization strength, the mode I loadings are easier than the mode II loadings to make the dislocation emission from the crack tip under the certain circumstance and the nanoscale amorphization can suppress the emission of the dislocation from the crack tip.Thirdly, we have established a mechanical model describing the interaction between the edge dislocation, the elliptically blunt crack and the three disclination dipoles produced by the nanoscale amorphization region. The complex form expressions of stress fields and forces acting on the dislocations are derived. The critical stress intensity factors for dislocation emission are calculated. The influence of disclination stresses, parameters of the three disclination dipoles as well as crack length on the critical SIFs are discussed in detail. The results show that the nanoscale amorphization releases the high stresses near the crack tip region and thereby enhances the SIFs for dislocation emission. The nanoscale amorphization has great influence on the most probable angle for dislocation emission. The size and the orientation of nanoscale amorphization have apparently influence on the variation of the critical SIFs. There is a critical normalized radius of curvature, which makes the edge dislocation emission from the crack tip the most difficult, and the dislocation emission from a short crack tip is much easier than that from a long crack tip.