Low Temperature Specific Heat Of Bulk Metallic Glasses

As a kind of amorphous material, though the metallic glass does not have long-range atomic order unlike crystalline material, it does have pronounced short- or medium-range order at the atomic scale. Due to the unique atomic structure compared to their crystalline counterparts, metallic glasses have excellent mechanical, magnetic and chemical properties. Specific heat, which is known closely linked with the phase transformation, low-energy excitation, atomic vibrations, and electronic movement of the solids, can reflect the disordered atomic structural characteristics, such as local harmonic vibration and atomic clusters, etc. At low temperature, the vibration of atoms is weak in solid, and the specific heat contributions of other subsystems become more and more prominent, thus, the low temperature specific heat is considered to be an important parameter for studying many microscopic phenomena. In this thesis, the Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ and(Fe_0.5Co_0.5)₎72)B₂₀Si₄Nb₄ bulk metallic glasses were fabricated using copper mold casting method, and using XRD, DSC and PPMS, etc, the specific heats were discussed and the intrinsic correlation between microstructure and thermal properties at low temperature was further researched. Main researchful works include:（1） The low temperature specific heats of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ metallic glass and crystallized alloy were studied, and the effects of crystallization on the specific heats were discussed. It was found that metallic glass in the cast state has larger specific heat than that in the crystallized state in its curve of T-dependent Cp with the same composition and temperature, and the Cp in these two states has a rising trend with the temperature increasing. Meanwhile, the phenomenon of abnormal boson peak was observed in the plot of CPhonon/T3 at temperature of 1-20 K. Using analyzing the experimental data, it was shown that the Debye and Einstein modes co-exist, and the boson peak was interpreted primarily using local harmonic modes.（2） The effects of annealing on the specific heat and microstructure for(Fe_0.5Co_0.5)₎72)B₂₀Si₄Nb₄ metallic glass at low temperature were studied. It was found that the heat treatment affected not only the height of exothermic peak in DSC curve, but also the height of abnormal boson peak in specific heat curve, i.e. iso-annealing resulted in the peak height reducing and the peak width broadening. It was because that the free volume was reduced with iso-annealing, then the nature of continuum elastic medium of metallic glass was changed, thus the height and width of boson peak were changed. Further analyzing the specific heat, it obtained that two Einstein modes existed in metallic glasses, while one Einstein mode existed in crystallized alloys. Using the localized harmonic modes based on the vibrations of loose “rattler” atoms in the oversized cage structure, the origin of the boson peak was interpreted well.（3） Compared to the crystal specific heat model, the low temperature specific heat model of metallic glass was established using the solid state theory. The similarities and differences between two models were analyzed, and the validity and universality of metallic glass model were also verified using 14 kinds of metallic glasses. At the same time, it was also found that the electronic term coefficient γ for metallic glass was larger than that for crystallized one and the phonon term coefficient δ for metallic glass was reduced with the increases of annealing temperature. This common phenomenon was interpreted well using the electronic density of states, the microstructure of metallic glasses, and the phonon density of states.