Study On The Mechanical Behavior Of Metallic Glasses By Molecular Dynamics Simulations

Metallic glasses, also named as amorphous alloy, is a relative new member of materials family. With special amorphous structure formed by metallic bonds, metallic glasses combine characters of glass, metal, solid and liquid, presenting very specially excellent properties. Metallic glasses could not only be one kind of new-structure alloy, but also an important model in the material science and condensed matter physics to investigate several important basic issues. Their mechanical properties including very large strength, elastic limit and wear resistance, especially attract much attention by persons. However, during the process to industrial application, several problems still need to be sloven, meanwhile, some basic scientific questions are still not very clear, including the method and effectiveness of enhance the plasticity, the special structure of shear bands, the size effect and the connection of microscopic deformation unit to the macroscopic mechanical properties and behaviors, and so on. In this thesis, using molecular dynamics simulations method to tune the structure, we systematically investigate several deformation behavior of metallic glasses and their underlying mechanisms, mainly in Cu-Zr binary system. The main results are summarized as follows.(1) The room temperature tensile deformation behavior of two Cu64Zr36 and Cu40Zr60 metallic glasses (MGs) is investigated, by employing molecular dynamics simulations, with a view to examine the evolution of plastic deformation from the atomic level perspective. It is found that after reaching the maximum stress, atoms in areas with lower packing efficiency, which have liquid-like polyhedral atomic configurations, exhibit larger displacement. On further deformation, atoms even at rigidly packed regions, which have solid-like polyhedral atomic configurations, also start partaking in the plastic deformation, especially within the shear band region. The average shear transformation zone (STZ) size, defined by the coordination neighborhood of highly strained atoms, was found to increase from 17±3 to 106±6 atoms within the strain range of 7-12%, which spans the shear band initiation to mature formation, in both MGs examined. A detail examination of the distributions of the number and the size of STZs as a function of strain reveals that the formation of shear band is linked with the occurrence of a few super-sized STZs.(2) Molecular dynamics simulations were employed to investigate the plastic deformation within the shear bands in three different metallic glasses. To mimic shear bands, MG specimens were first deformed until shear localization, and then volume of the material from within the localized regions was extracted and replicated. Homogeneous deformation that is independent of the size of the specimen was noted in specimen with shear band like structure, even at a temperature that is far below the glass transition temperature. Structural relaxation and rapid cooling were employed to examine the effect of free volume content on the deformation behavior. This was followed by detailed atomic structure analyses, employing both Voronoi polyhedra and ’liquid-like’ regions that contain high fraction of sub-atomic size open volumes. Results suggest that the total fraction of atoms in liquid like regions is key to control the plastic deformation in MGs. These are discussed in the context of reported experimental results and possible strategies for synthesizing monolithic amorphous materials that can accommodate large tensile plasticity are suggested.(3) Molecular dynamics simulations were employed to investigate the specimen thickness-dependent tensile behavior of a series of CuxZr100-x (x=20,40,50,64 and 80 at.%) metallic glass (MG) films, with a particular focus on the critical thickness, tc, below which non-localized plastic flow takes place. The simulation results reveal that while the transition occurs in all the alloys examined, tc is sensitive to the composition. We rationalize tc by postulating that the strain energy stored in the sample at the onset of plastic deformation has to be sufficient for the formation of shear bands. The composition-dependence of tc was found to correlate with the average activation energy of the atomic level plastic deformation events. From the result of tensile behaviors of CusoZrso metallic glass (MG) films with different thicknesses, we also found that the thinner the MG film, the lower Young’s modulus (E), the lower strength, the lower density and the higher Poisson’s ratio the MG film has, which are caused by the relative higher fraction of the lower-density surface layer (about 0.4 nm) in thinner MG films.(4) In bulk metallic glasses (MG) with amorphous structure, plastic deformation at room temperature is dominated by highly localized shear banding. Here we report non-localized deformation under tension in MG by constructing multiple layer structure with different composition of Cu-Zr system using large-scale atomistic simulations. It is demonstrated that the structure with layer number larger than seven, composed by Cu64Zr36 and Cu4oZr6o, or Cu64Zr36 and CusoZrso, presents obviously non-localized deformation behavior. The mechanism is attributed to their lower material strength and higher structure heterogeneity, confirming that a sufficient amount of glassy heterogeneity is help to enhance the plasticity in metallic glass. Our results could provide a promising strategy for designing tensile ductile MGs with pure amorphous structure at room temperature.