The Research On Deformation Mechanisms And Mechanical Behaviors Of Metallic Glasses

The mechanical properties of metallic glasses(MGs)play a decisive role in their engineering applications.However,owing to their disordered atomic structure,the mechanical problems of MGs such as deformation mechanisms and mechanical behaviors still require further study.The disordered structure without obvious features severely hampers the distinction to the deformation units as well as the control of the mechanical properties of MGs.So far,the researches on the deformation mechanisms of MGs are mainly focused on modeling and theory analysis,with an urgent need for more evidences from experiments.It must be hard work to structurally clarify the micro-deformation mechanisms of MGs with complex structure.Fortunately,the correlations among the various physical properties of materials enable us to conduct a lateral solution to the structural conundrum of MGs.In this dissertation,we investigated the deformation mechanism and mechanical behaviors of MGs,from the aspects of structural anisotropy,surface properties,and fracture behaviors.In an isotropic metallic glass elastically loaded in compression or tension,the atomic structure will become anisotropic.Therefore,the structure anisotropy,as a deformation result,can be taken as a structural feature that contains the information of deformation mechanisms.With a high-energy X-ray diffraction technique,we studied the deformation mechanisms of MGs via the investigation of the creep induced anisotropy in MGs.We found that MGs deformed through ?10 (?) sized local flow events embedded in an elastic matrix,accompanied by the breaking and formation of atomic bonds.We also investigated the mechanical properties of the crept MGs via the nanoindentation tests on the longitudinal section and cross-section of samples.We found that the longitudinal modulus is ?7.3% lower than the transverse modulus in the crept MGs.This indicates a mechanical anisotropy that origins from the structure anisotropy in the crept MGs.As a metastable material,MGs have a wide range of variations in internal energy state,such that turning properties of materials via energy is well applicable for MGs.The surface of MGs has been proved at a higher energy state with faster dynamics and liquid-like behaviors.As well known,the surface properties of metallic materials have a significant influence on their mechanical behaviors.However,the researches on the mechanical properties of the unique surface of MGs are quite a few.With a micro-torsion technique,we studied the mechanical properties of the MGs surface as well as its influence on the mechanical behaviors of MGs.We found that the surface region of MGs is much softer than the inner part,with a lower shear modulus and yield strength.Compared to the measured shear modulus of bulk MGs via the ultrasonic method,the maximum reduction in the surface shear modulus of a metallic glassy wire can be as large as ?44%,which is close to that of MGs in a supper-cooled liquid state.In addition,we propose a method that can calculate the thickness of the soft surface region.The calculated thickness of the soft surface region is ?400 nm-1000 nm,which is much larger than that evaluated from dynamics.With the Nano-CT technique,we observed a low-density surface layer with a thickness of ?480 nm,at the surface region of metallic glassy wires.On the fracture surface of samples,we also observed the peeled soft layer with a thickness of ?500 nm.In determining the thickness of the soft layer of MGs,the experimental results and calculated results show a good agreement.With an examination of the fracture morphology,we found that the surface softening can influence the mechanical behaviors of MGs,which is more significant when sample size down to micro-scale: with size reduction,the torsion fracture of MGs showed a brittle-to-ductile transition.Finally,after a comprehensive analysis,we argued that the properties of MGs can be affected via thermoplastic deformation,in which the triaxial tensile stress state helps MGs get into a higher energy state with a softer mechanical property.A majority of studies on the mechanical behaviors of MGs are mainly based on compression and tension tests,where the shear bands are constrained under normal stress.The studies in the situations without normal constraint on shear bands are few.In a cylindrical sample under torsion load,the maximum shear plane is in a free state without the constraint from normal stress.Therefore,the cylinder torsion model is applicable for studying the mechanical properties of MGs under the conditions without normal constraint on shear bands.With a series of torsion tests,we systematically investigated the composition-and size-dependence torsion fracture of MGs.We found that the torsion fracture of MGs can deviate the flat shear plane,and the fracture angle is dependent on sample composition and size.Based on the stress analysis,we showed that the torsion fracture of various MGs can be described by the ellipse fracture criterion as originally proposed for the tension fracture.Combined with data collected from literatures,the deviation from the circumferential shear plane under torsion is further shown to intrinsically related to the fracture toughness of MGs The tougher MG tends to have a smaller fracture angle,and vice versa.In addition,the fracture toughness is shown to correlate with the ratio of normal fracture strength to shear fracture strength,reflecting the underlying physical mechanisms that the fracture of MGs is mediated via a competition mechanism between normal and shear stress.This work mainly focuses on the mechanical behaviors of MGs from the aspects of “Structure anisotropy revealing the deformation mechanism of MGs”,“The surface-softening of MGs”,and “The composition-and size-dependent torsion fracture of MGs”.The discoveries and results of this work may help us to have a deeper insight into the physical nature of MGs,and better control of the mechanical properties of MGs.