Effects Of Heterogeneous Structures On The Deformation Behaviors Of Metallic Glasses

Metallic glasses are a kind of disordered materials prepared by rapidly quenching of supercooled liquids,which inevitably inherit intrinsically structural heterogeneities.During the deformation,these heterogeneities can evolve to form flow units at mircroscale to accommodate the macroscopic plastic strain.The concept of flow units provides a fundamental basis to understand the deformation mechanism of metallic glasses.However,up to date,although numerous studies focus on the observation and characterization of the structural heterogeneities and flow units,unlike the theory of crystallographic defects,how to effectively manipulate the size and distribution of flow units to improve the mechanical properties is still missing in metallic glasses.Therefore,in this thesis,utilizing the external fields(such as stress field,temperature field and ion irradiation),we attempt to stimulate the structure of metallic glasses,and then observe their responses.Accordingly,we expect to get insights into the structural evolution of the heterogeneities and investigate the role of flow units in dominanting the mechanical properties of metallic glasses.The main contents of this thesis are as follows:We investigated the effect of heterogeneous structure on the plastic flow behavior of metallic glass es with different strength.Mathematical and statistical methods were introduced to characterize the intermittently plastic flow of metallic glasses during nanoindentation.The evolution of the shear-strain field is studied.The maximum shear strain is obtained when the incipient plastic flow occurs.The cut-off of the strain burst size is associated with the deformation unit size in the glassy phase.The deformation unit can transit from jamming to unjamming with the hardness decreasing.The loading rates cannot influence the dynamic state of the plastic flow.Heavy-ion irradiation was used to manipulate the heterogeneous structure of a metallic glass.The experimental evidences from the structural characterization,the micro-pillar compression tests,the in-situ TEM tension,the nanoindentation tests,and the theoretical analysis based on the free-volume model indicate that the irradiation can increase the free-volume fraction,and improve the homogeneous structure of the glassy phase,and eventally enhance the plastic deformation ability of metallic glasses.This enlightens us that if an ion beam,with a low dosage,is able to modify the structure of the entire metallic glass,the metallic glasses would possess an excellent and lucrative plastic-deformation ability at room temperature.The effect of low temperature on the heterogeneous structure of a metallic glass was investigated.In-situ high-energy synchrotron X-ray diffraction results show that cryogenic temperature can induce the structural transformation.The activation volume of flow units linearly depends on temperature in a wide temperature regime(57 K-193 K).A plausible atomic deformation mechanism is proposed,considering that the activated flow units mediating the plastic flow originate from the medium-range order and transit to the short-range order with decreasing temperature.According to the finite element analysis of the Eshelby’s inclusion,a flow unit at cryogenic temperature experiences an abnormal volume expansion or dilation strain in addition to the pure shear strain.This pressure could alleviate the external tensile pressure during cryogenic-temperature tension,which then enhances the shear stability of shear bands,and finally enhances the shear rejuvenation of metallic glasses.We also studied the effect of heterogeneous structure on the crack propagation of metallic glasses.Through real-time,high-resolution in-situ TEM tension,we investigate the crack propagation of ductile and brittle metallic glasses.We show that intrinsic cavitation mechanism near the crack tip dominates the crack propagation.After crystalline phase precipitates in the front of crack tip,the crystalline phase can suppress the cavitation formation and sharpen of the crack tip,which can hinder the crack propagation.The current findings have important implications for better understanding the fundamental deformation and fracture mechanisms as well as the mechanical properties of various metallic glasses.Aspects of the above investigations can not only improve the understanding of the evolution laws of the heterogeneous structures and deformation units under the stimulation of external fields(ion-irradiation or temperature),and provide an experimental and fundamental basis to understand the deformation mechanism of metallic glasses,but also supply more experimental data for the performance of metallic glasses under different extreme conditions(high dosage of ion-irradiation and cryogenic temperature).Moreover,it is of crucial fundamental and practical importance for engineering applications of such advanced material.