| Owing to the combination of superior magnetic, mechanical, and electrochemicalproperties, Fe-based metallic glasses (MGs) are widely used in the structural,functional and environmental protection fields. Research on MGs by dynamics is apoorly understood fundamental problem in mechanics field. It is very important tounderstand the glass transition, soft magnetic properties and mechanism of brittle toductile transition of Fe-based MGs. In this dissertation, the dynamic characteristicsand fracture-damage mechanism of Fe-based MGs were systematically investigatedby integrating dynamic theory with high resolution transmission electron microscope,Raman scattering, photoelectron spectroscopy. The main works are given as follows:(1) The glass transition behavior of (Fe0.71Dy0.05B0.24)96Nb4MGs wasinvestigated by dynamics and energy-dispersive spectrometry. It is confirmed that thisabnormal behavior is attributed to the transition process of an amorphous state with ahigher energy to another amorphous state with a relative lower energy. Theamorphous state with higher energy comes from the uneven distribution ofcompositions in glasses, which is mainly caused by the component with significantdifference in atomic size and nonnegative values of enthalpy of mixing. This kind ofstructural heterogeneity can lead to entropy increase, which results in decreasing ofdriving force of crystallization and improving the glass-forming ability. The structuralheterogeneity can also lead to the initiation of shear bands, which results in anincreasing number of shear bands, thus enhancing the ductility of MGs.(2) The grain growth dynamic model was proposed by crystallization dynamics.The relationships between grain size and annealing temperature for Fe-based MGswere investigated. The effects of structural heterogeneity on soft magnetic propertiesof as quenched Fe-based MGs were studied. The mechanism of permeability andeffective magnetostriction change with annealing time and temperature was analyzed.The effects of partial substitution of Fe by Ni, Co, Cu in Fe-based MGs on theirmagnetic properties were also studied.(3) The mechanism of low-frequency dynamic characteristics was investigated. Itwas found that the Boson peak (BP) originates from two local harmonic vibrationmodes that are associated with the lengths of short-range order (SRO) andmedium-range order (MRO) in MGs. The atomic packing in MGs was also found tofollow a universal scaling law, i.e., the ratios of SRO and MRO lengths to solvent atomic diameter are3and7, respectively, which exact match with length ratios of BPvibration frequencies to Debye frequency for the studied MGs. The clusters andsuperclusters in MGs may rotate slightly with respect to solute atoms like internalrotation in rubber, which is the origin of the MGs with large elastic strain limits. TheYoung’s moduli and fracture strength are inversely proportional to the cube ofdiameter of solvent atoms in MGs.(4) The self-similar characteristics of fracture surface in MGs were found. Thestrength theory of MGs was proposed by the fractal geometry. The fracture energy andstrength of brittle MGs are not only related to the plastic zone size, the length scalesof atomic order range and the surface energy, but also have significant relationshipwith the fractal dimension in fracture surface. Fe50Ni30P13C7MG which can be bendedwithout fracturing with unprecedented plasticity (~20%) at room temperature wasalso found. The atomic bonding model which can be described micro process ofdeformation and fracture was proposed by photoelectron spectroscopy, etc.experiments. The mechanism of brittle to ductile transition for Fe-based MGs wasrevealed. |
PDF Full Text Request