Effect Of Nanoscale Amorphization On Crack Blunting And Fracture Toughness In Nanocrystalline Materials

To alleviate the conflicting behavior of strength and toughness of nanocrystalline materials,researchers have devoted a lot of time and effort.At the same time,the researchers found that a small number of nanocrystalline materials have excellent strength,hardness,tensile ductility,and even superplasticity due to the special plastic deformation modes,including intergrain sliding,grain boundary(GB)sliding or GB migration,nano-twinning,nanoscale amorphization and diffusion creep.These deformation modes can relax the local stresses and release elastic energy,thereby hindering the crack nucleation and propagation.As one of these deformation modes,nanoscale amorphization is proved to serve as a special mode of local plastic deformation near crack tips.So it is very meaningful to study the specific effects of nanoscale amorphization on crack blunting and fracture toughness of nanocrystalline materials.In this paper,the nanoscale amorphization is taken as the research object,and the related continuum mechanical model is established.The effect of the nanoscale amorphization on critical stress intensity factors of dislocation emission,and the effect of the nanoscale amorphization and dislocation emission on the fracture toughness of pre-cracked materials are investigated.On one hand,the dislocation emission from the crack tip is an important plastic deformation for crack blunting.In this paper,the stress field generated by the interaction between the nanoscale amorphization and the crack is derived.The influence of the nanoscale amorphization on the critical stress intensity factors of dislocation emission is discussed.The factors involved include the load type,the crack length,the parameters of the amorphous region and the distance between the crack tip and the amorphous region.On the other hand,the stress field caused by the interference between the nanoscale amorphization,the crack and the emitted dislocations is obtained.The maximum numbers of emitted dislocations for different grain sizes are calculated.The effect of emitted dislocations and nanoscale amorphization on the fracture toughness of materials is studied.The fracture toughness of materials with or without the amorphous region is compared with the consideration of dislocation emission.The fracture toughness of materials before and after dislocation emission is also compared under the influence of amorphous region.The results show that when the crack is sharp and of finite length,nanoscale amorphization may promote or hinder the dislocation emission,depending on the distance between the amorphous region and the crack tip.When the crack is elliptically blunted,nanoscale amorphization may also promote or hinder the dislocation emission.In addition to the distance between the amorphous region and the crack tip,the curvature radius of the crack is also an influencing factor.The dislocation emission and the amorphization obviously play an important role in the material toughening.Specifically,the toughening effect of nanoscale amorphization near a blunted crack is more obvious for smaller grain size.