Thermodynamics Studies On Forming Composition And Forming Ability Of Metallic Glasses

The preparation of new metallic glasses and the nature of glass transition are the key issues in the field of metallic glasses, for which the comprehensive understandings from the aspects of thermodynamics and kinetics are necessitated. However, compared with the abundant kinetic studies on the glass formation, the thermodynamic studies are relatively scarce. The earlier thermodynamic studies mainly involve the mixing enthalpies, configurational entropies and the Gibbs energy difference of the liquid and equilibrium crystalline phases(ΔG_l-c). In contrast, entropy is more closely related to the structure of materials and moreover the reports on the relationship of glass formation and entropy are quite limited. In addition, the metestable phase is of significance for glass formation, and the study for such phases is also important. In this study, the role of thermodynamics during the glass transition is investigated with the thermodynamic properties of the metastable phases being particularly focused.To clarify how the thermodynamics and kinetics affect the glass formation, the glass transition and glass forming ability of a binary system of methyl o-toluate（MOT） versus methyl p-toluate（MPT） are studied across the whole composition range by using the differential scanning calorimeter（DSC） and the broadband dielectric spectroscopy（BDS）. The phase diagram is constructed to explore the best glass forming composition when the characteristic temperatures for the glass transition, crystallization, and the melting processes are determined by DSC. The best vitrification region is found to locate between the eutectic and the mid-point compositions of the eutectic line, indicating a remarkable deviation from the eutectic composition. The compilation of various simple binary eutectic systems covering inorganic, metallic, ionic and molecular glass-forming liquids reproduces the rule. Kinetics and thermodynamics in binary systems are investigated to associate with the rule. It is found that the glass formation is kinetically favored at the eutectic compositions while thermodynamic contribution to the deviation of the best glass forming composition from eutectics is implied. For clarifying the thermodynamics of the partitionless solid solution on glass formation, four typical binary eutectic alloys with distinct glass-forming abilities and interatomic interactions, Ag60Cu40, Sb17.5Pb82.5, Au81.4Si18.6, and Ni24Zr76 are chosen. Here we focus on the thermodynamics involved in the liquid-solid solution transition at T₀ temperatures for the alloys of eutectic compositions, and the validity of the thermodynamic properties is evaluated. The comparison of the melting entropies for the equilibrium and partitionless solidifications reveals a basic relation that the difference between the melting entropies of the two phases is closely related with the glass forming ability, interatomic interactions, mixing enthalpies of the alloys. Based on the thermodynamics of the equilibrium phases and the solid solutions, an understanding of the glass formation of metallic alloys is proposed.In the light of the similar short range orders in supercooled liquid, metallic glasses and quasicrystals, two series metallic glasses are successively prepared based on the Zr₄₀Ti₄₀Ni₂₀ quasicrystal by adding Co and Be, and the best glass forming compositions are located. The studies on the crystallization of the glassy alloys reveal that the formation of metallic glasses is promoted by the initial precipitation of quasicrystals with specific thermodynamics. In addition, it is really interesting that both improvements in the formabilities for glasses and QCs are observed in a wide composition range for the alloys(Zr₄₀Ti₄₀Ni₂₀)_100-xBe_x. In order to clarify the microstructure changes in the alloy induced by Be-addition, the simulation of the alloy melts are conducted through Ab initio molecular dynamic simulations. The simulation results manifest the structural features for the optimized glass forming region, showing the effective development and distortion of the icosahedral clusters, which guarantee the primary precipitation of the quasicrystal but the increased difficulty for crystallization. The results deliver a fundamental bridge between quasicrystals and metallic glasses.