Atomic Bonding And Fractal Structure Of Bulk Metallic Glasses

Metallic glasses exhibit complicated atomic-scale microstructure. Knowledge of structure feature for metallic glasses is crucial for better understanding the origin of glass formation and their outstanding physical properties. In this dissertation, Raman scattering, and X-ray photonelectron energy spectrum (XPS) have been employed to study the directional bonds in Ti-based bulk metallic glasses (BMGs). Based on the features of Raman spectra from Ti-based, Zr-based and La-based BMGs, the structural origin of glass forming ability (GFA) has been explored. Renormalization group method has been used to reveal the fractal features of Zr-based metallic glasses according to the packing configuration of medium range order (MRO).The local bonding structures (directional bonds) of Ti₄₀Zr₂₅Ni₃Cu₁₂Be₂₀ BMG were studied. XPS was employeed to demonstrate the formation of directional bonds. The experimental results indicate that the directional bonds are primarily comprised of Be-Ni and Be-Cu bonds. Raman spectra, together with vibration frequency simulation confirm the local bonding structures. Directional bonds can be used to interpret the enhanced GFA by the addition of Be in Ti-based BMGs and their anomalous temperature coefficients of resistance (TCR).Next, Raman spectra of Ti-based, Zr-based and La-based BMGs were studied. It has been found that both the optical modes and acoustic modes contribute to the Raman spectra in metallic glasses. Gaussian-shape and Lorentz-shape peak functions were adopted to fit the curve of acoustic modes and that of optical modes for Raman spectra, respectively. The fitting results indicate that acoustic mode vibration is closely correlated with optical mode vibration. An overlap of optical and acoustic mode vibration would occur at a certain frequency. Such overlap between optical and acoustic modes indicates the transition from isolated motions to collective motions. Based on Raman spectra, viewing from cooperative and local motions, GFA of BMGs was studied. Taking the Ti-based, Zr-based and La-based BMGs as examples, a simple but effective parameterη(=ITA/ITO , ITA and ITO represent the Raman intensity of transverse acoustic mode peak and transverse optical mode peak, respectively) was used to reflect the GFA of BMGs. It shows that there exists a linear correlation between GFA andη. The lower theηvalue, the higher the GFA. Furthermore, in Zr-based BMGs, a simple relationship betweenηand fragility coefficient m confirms the validity of the parameterη.Regarding the features of collective motions in BMGs, cooperative motions are primarily contributed by MRO. In metallic glass, there exist random fractals at The MRO in metallic glasses. The fractal scale extends from 5? to 20?. Beyond such scale, the fractal feature could vanish. Then, the renormalization models for MRO packing were constructed to calculate the critical exponents and fractal dimension. The results indicate the denser the MRO packing, the larger the fractal dimension. Furthermore, the fitting to pair distribution function (PDF) of Zr-based BMGs was carried out to calculate the fractal dimension (Df =2.475), which is consistent with the analysis of the first sharp diffraction peaks (FSDPs) by X-ray diffraction. Above all, experimental Df (2.475) from PDF approaches to the calculated Df (2.24) from renormalization models for FCC or HCP packing. Such consistency illustrated the validity of the models.