Characterization And Structural Tuning Of Metallic Glasses Studied By Nanoindentation And High Pressure Compression

Glass is a class of disordered materials.Without long-range translational periodic symmetry in the atomic structure and common crystallographic defects,glass possesses many unique properties and plays an important role in modern industry and civilization.Metallic glass（MG）has emerged as a new generation of advanced metallic materials with great potential in widespread applications.With densely-packed disordered atomic structure,the mechanical strength of MGs is surprisingly close to the theoretical value.MGs also show good wear resistance,corrosion resistance,and excellent soft magnetic properties.The structure of a material determines its properties,and disordered materials should by no means be an exception.As a simple atomic glass model system,MGs have attracted extensive research interest not only for the design of better MGs at the atomic scale but also for deepening our understanding of glass materials and glass phenomena in general.The disordered structure of MGs poses a challenge to their research.The glass structure determines properties,in turn,properties also can reflect the structural information.Therefore,the study of the properties has also offered a valuable approach to a better understanding of glass structures.Mechanical property is one of the most unique properties of MGs.The mechanical response of MG is usually closely related to its microstructures.MGs show very interesting intrinsic heterogeneity in the spatial domain and dynamic relaxation in the time domain,therefore,MGs usually present complex structural evolution across multiple scales with external stimulation.The complex and diverse deformation behaviors challenge the classical deformation theory of metal materials.Therefore,it is of great interest to study the mechanical properties of MGs with quantitative and high-precision means.In this thesis,nanoindentation and diamond anvil cell were used as the mechanical loading methods,and multiple MGs with different energy states were studied systematically.Elastic and plastic deformation behaviors of MGs were investigated by combining nanoindentation testing,in situ synchrotron x-ray diffraction（XRD）,thermal analysis technologies,etc.Specifically,the spatial resolution limit of nanoindentation mapping was detected for characterizing MGs heterogeneity.The influence factors during nano-indenting were discussed and analyzed;The La-based MGs with different energy states were characterized and compared with their micromechanical properties and heterogeneity characteristics.To overcome the limitation of the pressure range and in situ characterization means during nanoindentation testing,in situ high-pressure mechanical compression technique was further employed.A Fe-based MG was studied to detect the changes of its atomic structure,electronic structure,and magnetic moment versus hydrostatic pressure.The main contents and conclusions are as follows:（1）A comprehensive nanoindentation mapping study was carried out on four representative MGs(Zr₅₀Cu₅₀,Fe₇₈Si₉B₁₃,Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3,and Ce₆₀Al₁₅Cu₁₀Ni₁₅)with different mechanical properties and glass transition temperatures.The spatial resolution for characterizing MGs heterogeneity using nanoindentation was investigated.The results suggest that,compared with crystalline metals,smaller normalized spacing can be used in 2D mapping nanoindentation experiments on MGs.When the ratio of indentations spacing d and indenting depth h decreases to d/h=10,the measured hardness and elastic modulus are almost independent of d/h.And the hardness and elastic modulus would increase slightly when d/h<10.However,the hardness and elastic modulus decrease obviously when d/h further decreases below 5.5-6.0.By optimizing the experimental parameters,a spatial resolution of～200 nm can be achieved for nanoindentation mapping on MGs.It can also be clarified that the difference between MGs and crystalline metals is mainly due to the relatively localized affecting zone of plastic deformation dominated by MGs’shear band.（2）Nano-indenting deformation behaviors under room temperature and high temperatures were investigated in the La₆₂Al₁₄Ag_2.34Ni_10.83Co_10.83 and Zr_64.13Cu_15.75Ni_10.12Al₁₀ MGs.According to the results,it is revealed that there is a critical loading rate for the emergence of the displacement“pop-ins”.The critical loading rate and the size of“pop-ins”vary among different MGs.By using in situ high-temperature nanoindentation testing on the La₆₂Al₁₄Ag_2.34Ni_10.83Co_10.83 MG,it can be found that there is an obvious transition from inhomogeneous to homogeneous deformation when the temperature reaches 0.71T_g.This temperature coincides with the onset temperature of relaxation enthalpy during the thermal analysis of the MG.（3）The La₆₂Al₁₄Ag_2.34Ni_10.83Co_10.83 MG samples with different energy states were obtained by tuning with isothermal annealing,cryogenic thermal cycling,and high-pressure torsion treatment.The mechanical properties and spatial heterogeneity of the samples were investigated by nanoindentation mapping with a spherical indenter,and the structural information and relaxation behavior of these samples were investigated by synchrotron XRD and thermal analysis.Based on the theory of free volume and shear transition zone,the structure and deformation mechanism were analyzed and discussed.Compared with the initial state,the relaxation enthalpy of the samples with high-energy states increases obviously.Meanwhile,the mechanical properties were obviously softened,and the samples show higher spatial heterogeneity.The increase of the energy state also means higher content of free volume.The decreased volume of the shear transition zone indicates that there might be fewer atoms involved in the formation of the units for plastic deformation in the MG samples with high energy states.These results would deepen our understanding of the effect of different energy states on the structure and properties of MGs.（4）By using hydrostatic compression in a diamond anvil cell,the pressure range can be further extended,and the structure and properties can be characterized by many in situ probes.In situ high-pressure synchrotron XRD was used to study the structural evolution of Fe₇₈Si₉B₁₃ MG,and the atomic structure and sample volume（average atomic distance）show continuous and reversible elastic densification under pressure;no obvious sharp structural transition is detected.Moreover,the bulk modulus of Fe₇₈Si₉B₁₃ MG at atmospheric pressure can be fitted as 162.5±2.1 GPa by using the equation of state fitting with the compression data.A pressure-induced reversible continuous spin crossover was observed in the Fe₇₈Si₉B₁₃ MG by in situ high pressure X-ray emission spectroscopy（XES）.The spin crossover is sensitive to applying pressure but very sluggish without completion up to～51 GPa.Interestingly,the spin reduction could be linearly scaled to perfectly overlap with the average atomic distance shrinkage during compression,which suggests that the magnetic and elastic variation may be synergistic and coupled.Furthermore,the in situ high pressure resistance test proves that the resistance also changes continuously during the compression process.The reversible change of the electron spin states in Fe₇₈Si₉B₁₃ MG during isostatic compression is similar to that of typical Fe-bearing crystalline materials,but without the sharp atomic volume collapse and resistivity mutation.Therefore,Fe-based MG might be an ideal material for special applications under extreme conditions requiring avoiding structure and property mutations.