Predicting The Forming-Region And Thermal Stability Of Bulk Amorphous Alloys

As a novel material, metallic glass has been studied intensely in both theory and experiment, and has shown wide application respect in the field of engineering. However, due to its limited thermal stability and size, the utilization of metallic glass as structural material is greatly limited. A general approach with predictability for thermal stability of bulk metallic glasses and bulk metallic glass-forming compositions, however, is still lacking, which made the development of novel bulk metallic glasses trapped in the mire of trial-and-error.The thermal stability of bulk metallic glasses was investigated, based on the analysis of the crystallization process during heating. The Gibbs free energies of formation of amorphous alloys have been calculated, in Mg-Cu-Y, Pd-Cu-Si, Cu-Zr-Al, Gd-Ni-Al, Gd-Co-Al, La-Al-Ni-Cu, Sc-Al-Co-Y, Gd-Y-Al-Co, Er-Al-Co-Y, Pr-Cu-Ni-Al, Ce-Al-Cu, Ce-Al-Ni-Cu and Ce-Al-Cu-Zn multicomponent alloy systems. Significant negative linear correlations between Gibbs free energies and the corresponding crystallization temperatures were found. Based on the correlations, crystallization temperature of any alloy can be predicted by calculating Gibbs free energy of formation of amorphous alloy using fundamental physical and chemical data at a least cost. Compared with the available approaches used to predict thermal stability of amorphous alloys, the present approach developed by the author has unique advantages, which is not only valid in wide composition regions, but also in more alloy systems, and is of great significance in both theory and experiment. To develop a new approach which could predict and help to design metallic glass with larger size, the bulk metallic glass-forming region were investigated.Considering the atomic packing and thermodynamic factors, a simple approach was developed to rapidly predict the bulk metallic glass-forming composition region, in Cu-Zr binary alloy system, Cu-Zr-Ti, Cu-Zr-Al, Cu-Zr-Ni-Ti, Cu-Zr-Ni-Al-Ti, Ca-Mg-Zn, Mg-Cu-Y, Gd-Ni-Al, Gd-Co-Al, La-Al-Co and Ce-Al-Co multicomponent alloy systems, by quantitative calculations using fundamental physical and chemical data.The chemical bonding has significant effect on bulk metallic glass formation in three aspects. Firstly, only those atomic pairs with strong chemical bonding can form stable clusters. Secondly, the bulk metallic glass-forming regions of the alloy systems with similar topological structure and chemical bonding are similar, while those of the alloy systems with similar topological structure and different chemical bonding are quite different. Finally, bulk metallic glasses could not form in the alloys with very low negative chemical mixing enthalpy, even though the atoms in the alloys are efficiently packed.The bulk metallic glass-forming compositions predicted by the author are close to the eutectic compositions, and are all in good agreement with experimental results, when the effect of chemical bonding on bulk metallic glass formation is taken into account.In terms of thermodynamics, high negative mixing enthalpy favors the formation of metallic glass for an alloy. However, it was found in this dissertation that bulk metallic glasses can not always be obtained for alloys with higher negative mixing enthalpy, because atomic pairs with higher negative mixing enthalpies and lower packing efficiency can not appears abundantly in bulk metallic glass. On the other hand, from the point of view of topology, the atoms in the alloy within the predicted composition are densely packed, and the alloy also is an easy metallic glass former. However, it was found in this dissertation that bulk metallic glass-forming region is not always in well consistent with the predicted composition, because atomic pairs with high negative mixing enthalpies also appear in bulk metallic glass, even though their packing efficiencies are slightly lower than the domain ones.Both topological and chemical effects play key role and compete with each other during the formation of bulk metallic glass. The efficient cluster packing of constituents could stabilize an alloy topologically and bulk metallic glass could be prepared for the alloys with densely packed atomic structures, even though they are not the most thermodynamically stable ones. On the other hand, strong bonding between constituents could stabilize an alloy thermodynamically and bulk metallic glass could be obtained for the alloy with high negative mixing enthalpy, even though it is not the most efficiently packed ones.The widely valid approaches developed in this dissertation have true predictability for thermal stability of bulk metallic glasses and bulk metallic glass-forming compositions. They show promise for rapidly developing novel bulk metallic glasses.