The Research Of Deformation And Crack Propagation Of Metallic Glasses

Metallic glasses,as a kind of new metastable materials,are characterized as short-range order and long-range disorder in atomic structure,in which atoms are in a high energy state.When an external energy?such as strain-energy and heat-energy?is imposed,the structure of metallic glasses can be changed,which influences the application of metallic glasses.In this study,the mechanical properties of metallic glasses are investigated at different loading modes,and temperatures.The results will benefit to understanding the deformation behavior of metallic glasses.The research contents of this dissertation are shown as follows:The crack formation and propagation occurs after plastic deformation.Thus,in-situ tensile test of the notched plate of the Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ metallic glass is conducted in a high energy X-ray synchrotron source.The area in the front of the crack tip is observed by the X-ray diffraction pattern.The panoramic strain distribution ahead of a notch tip is mapped at different stresses.According to the characteristic of strain distribution and in-situ tensile test by SEM,the effects of strain components at loading direction?transverse direction and in-plane shear component on crack formation and propagation are discussed.Based on the maximum and minimum strains,which are profiled with the magnitudes and the azimuthal angles to map vector distributions,the effective strain is calculated.The oscillatory of the strain distribution in the front of crack can alternatively change the crack path.After the crack propagation,the fractography is analyzed by the fractal theory.To study the structural evolution of glassy phase during intermittent shear avalanches,in-situ compression tests in a high energy X-ray synchrotron source is carried out.The Cu₅₀Zr₅₀ metallic glass is fabricated to be micropillar for in-situ compression tests.Based on the diffraction patterns at different stresses,it can be found that disordering degree increases with increasing the stress in the elastic regime.After the elastic deformation,the serrated flow occurs.In each serration event,two cases,i.e.,the strain increases and the strain decreases,can be found.The strain increases in the serration events is usually accompanied with a reduction in the disorder degree of glassy phase.Vice versa,the strain decrease corresponds to an increase in the disorder degree of glassy phase.The structural evolution of a Zr_64.13Cu_15.75Ni_10.12Al₁₀ metallic glass is investigated by in-situ high-energy synchrotron X-ray radiation upon heating up to crystallization.The structural rearrangements on the atomic scale during the heating process are analyzed as a function of temperature,focusing on shift of the peaks of the structure factor in reciprocal space and the pair distribution function and radial distribution function in real space which are correlated with atomic rearrangements and progressing nanocrystallization.Thermal expansion and contraction of the coordination shells is measured and correlated with the bulk coefficient of thermal expansion.The characteristics of the microstructure and the yield strength of the metallic glass at high temperature are discussed aiming to elucidate the correlation between the atomic arrangement and the mechanical properties.The fracture morphologies of samples show that the adiabatic heating occurs on the fracture surface.The fracture angle changes with temperature.The fracture behavior of the metallic glass at different temperatures is discussed in the frame of the Mohr-Coulomb fracture criterion.Through introducing nanocrystals into the glassy phase,the plasticity of the Cu₅₀Zr₄₅Ti₅ metallic glass is improved,and the yield strength is decreased.Correspondingly,the nature of the shear avalanches in the plastic-strain regime is changed.Statistical analyses explicitly show that the universality of the scale-free intermittent flow can be applied to metallic glasses,and the nanocrystal/glass phase composite.The power-law distributions of the shear-avalanche size occur in the range covering the small shear-avalanche size,which gives evidence that the serrated flow dynamics is a self-organized critical system.Although the small nanocrystals?in size and volume fraction?do not significantly influence the deformation mechanism,i.e.,shear banding,they can postpone the emergence of the large shear avalanches,and,thus,enhance the ductility.Resultantly,the scale-free pattern,i.e.,the power-law regime,in the shear-avalanche-size distribution is enhanced.The mean-field theory?MFT?is used to describe the shear-banding behavior of the Cu₅₀Zr₄₅Ti₅ composite,when the nanocrystals are small in the size and the volume fraction.For the large nanocrystals embedded into the glassy phase,twinning can occur to cooperate with shear banding.The participation of the crystalline defects in the shear-banding behavior of the glassy phase causes the dynamics of the shear avalanches to deviate from the MFT.The large shear avalanches depend on the geometric distribution of the nanocrystals.The critical shear-avalanche value scales with the gap between the neighboring nanocrystals.The plasticity of the Cu₅₀Zr₄₅Ti₅alloy reflects the tuned criticality,showing a remarkably quantitative agreement with the slip statistics of dislocation motions in polycrystals.