Investigation Of The Plastic Deformation Of Metallic Glasses By Atomistic Simulations

Metallic glasses are promising structural materials owing to their excellent properties such as high strength,high elasticity and corrosion resistance.However,lacking of plasticity at room temperature limits its wide application.Exploring effective means to improve its plasticity is therefore one of the most active topics in the field.In recent years,it has been found that reducing the characteristic size of the metallic glass samples,or introducing appropriate notches,pores,or crystalline phase,might effectively improve the plastic deformation of metallic glasses.Nevertheless,there are still many basic scientific issues to be fully understood and resolved,such as the correlations between the deformation behavior,mechanical property and microstructure characteristics of metallic glass containing macroscopic structure defectsties.In addition,conventional experimental methods have many limitations in terms of structural characterization.Therefore,in this thesis,using molecular dynamics simulations,we systematically studied the deformation behavior of Cu₅₀Zr₅₀ metal glass,focusing on the influence of size,defects and deformation modes on ductile-brittle transition behavior and microstructure,as well as their underlying deformation mechanisms.The main results are summarized as follows.The mechanical behavior of metallic glasses shows similar size dependences as fine grained crystals,yet the strengthening mechanism is quite different.Tensile deformations of Cu₅₀Zr₅₀ metallic glass thin films by molecular dynamics simulation reveal that surface relaxation plays an important role in affecting both the deformation mode and the ductility of the films.Surface relaxation helps to annihilate the excess free volume generated in the film during deformation,in turn retards the nucleation and/or propagation of shear bands,and consequently enhances the ductility of the films.The surface area to volume ratio is more significant for thinner films in that the annihilation effect overwhelms the generation effect upon deformation,enabling the“smaller is stronger”phenomenon.Meanwhile,a transition of deformation mode from shear fracture dominated to mixed mode and then to homogeneous deformation is observed with the decreasing of film thickness.The findings suggest that the ductility and strength of metallic glass samples could be improved effectively by reducing the sample size or increasing its surface area to volume ratio.There is a certain correlation between the internal friction and the size of the metallic glass nano-films,although the correlation is non-monotoneous.The analysis on dynamical structural parameters reveals that the internal friction of metallic glasses is deeply affected by the rearrangement motion of the surface atoms.The smaller the size of the nano-glass,the greater the amount of the rearranged atoms in the surface region.In addion,the fraction of rearranged atoms in the surface region can be expressed in terms of the sample radius.Further study shows that a similar phenomenological model can also be derived to describe the correlation between the internal friction and the sample size.By analyzing the microstructure of the sureface region of the metallic glass samples with different sizes,it is shown that the smaller the sample size,the higher the content of HA bonded pairs with low index in the surface region.This correlation suggests that the amount of rearranged atoms is determined by the fraction of low index HA bonded pairs,which also determines the magnitude of surface relaxation and internal friction.Molecular dynamics simulations were also employed to examine the effect of internal notch depth on the mechanical behavior of Cu₅₀Zr₅₀ MGs.It is shown that both the strength and the deformation mode are sensitive to the notch depth,and in particular to the inter-void ligament distance.With increasing the notch depth,the notch intensification gets enhanced,while the deformation mode is seen to change from shear banding dominated to mixed mode and then to necking governed.Detailed stress field analysis reveals that the notch intensification and the deformation mode transition result mainly from the change of microscopic stress state induced by the presence of notches.The critical effective shear stress corresponding to local/atomic yielding in the MGs is identified to be around 1.9 GPa.The intricate interplay between the atomic mobility and the local microstructure is uncovered for shear bands and necked region in notched metallic glasses via molecular dynamics simulations.Simulation results show that the average atomic mobility inside the mature shear band is lower than that in necked region,which is closedly related to the atomic structures.The local microscturcture characterized by the Voronoi polyhedral for both the shear band and the necked region however exhibits quite similar distrubtion and comparable content,while the accumulated free volume content is found to be significantly higher in the necked region than that in the shear band.Such high content of free volume can promote the migration of atoms or clusters,and thus enhances the local plastic deformation.Further investigation suggests that the difference between the free volume content of the shear band and the necked region is closely related to the hydrostatic pressure involved.In addition,it is also found that the atomic mobility of the central atom in a cluster correlates with the local packing environment,such as the free volume content or the hydrostatic pressure,as well as the feature of the local cluster,e.g.,the symmetry or coordination number.In general,the lower the local fivefold symmetry,the higher the distortion of the polyhedra,the smaller the energy barrier to overcome for the shear transformation to occur and consequently the greater the atomic mobility upon shear transformation.