Thesis (Ph. D.)--University of Washington, 1991. / Vita. Includes bibliographical references (leaves [117]-123).

Co-based bulk metallic glasses (BMGs) are well known for their excellent mechanical properties with high fracture strength, hardness and elastic modulus. Since the first report of A. Inoue with co-workers in 2003 on Co43Fe20Ta5.5B31.5 BMG with fracture strength up to 5 GPa, a series of Co-based BMGs including Co-Fe-B-Si-Nb, Co-Fe-Cr-Mo-C-B-Er, Co-Ta-B systems have been developed. Co-Ta-B ternary BMGs, discovered recently, are characterized by even higher fracture strength of up to about 6 GPa. These BMGs with outstanding mechanical behavior are interesting for applications as advanced structural materials and coatings. Due to a relatively simple constitution (only three components), Co-Ta-B BMGs are very attractive for investigations of relationships between composition, structure, undercoolability, glass-forming ability, thermal and mechanical properties. However, there have been published just a few papers on Co-Ta-B BMGs focusing on the glass-forming ability in terms of the critical diameter and mechanical properties so far. In present work, a systematic study of the structure and properties of Co-Ta-B BMGs has been carried out on four intentionally chosen compositions  Co61Ta6B33, Co59Ta8B33, Co57Ta10B33 and Co53Ta10B37. Glass formation, thermal stability, crystallization kinetics upon isochronal and isothermal annealing, mechanical and magnetic properties were investigated. Co-Ta-B BMGs studied in this work are characterized by high thermal stability, ultrahigh fracture strength in compression, large Vickers hardness and high values of elastic constants. Increasing of B and Ta content is beneficial to the improvement of both thermal and mechanical properties. Based on the study of the short-range atomic order in Co57Ta10B33 BMG, Co-Ta, Co-B and B-B bonds are supposed to play an important role in the thermal and mechanical properties. A comprehensive picture on structure-composition-property relationship was established. In order to better understand the glass formation, non-equilibrium solidification of the undercooled alloys was investigated using electromagnetic levitation, high-energy X-ray diffraction and high-speed video observation. Three compositions with bulk glass-forming ability (Co61Ta8B31, Co59Ta8B33, Co55Ta8B37) were chosen to study the phase formation during non-equilibrium solidification. In addition, one ternary near-eutectic alloy Co64Ta5.5B30.5 and two binary alloys Co67B33 and Co63B37 with poor glass formation were comparably investigated using the same method. The phase formation, dendrite growth velocity and microstructure of the solidified samples were analyzed in detail as function of undercooling. The alloy composition, maximum undercooling and growth velocity were related closely with the glass-forming ability of the Co-Ta-B alloys studied.

The primary focus of the present work is the development of macro-models for numerical simulation of binary alloy solidification processes, consistent with microscopic phase-change considerations, with a particular emphasis on capturing the effects of non-equilibrium species redistribution on overall macrosegregation behaviour. As a first step, a generalised macroscopic framework is developed for mathematical modelling of the process. The complete set of equivalent single-phase governing equations (mass, momentum, energy and species conservation) are solved following a pressure-based Finite Volume Method according to the SIMPLER algorithm. An algorithm is also developed for the prescription of the coupling between temperature and the melt-fraction. Based on the above unified approach of solidification modelling, a macroscopic numerical model is devised that is capable of capturing the interaction between the double-diffusive convective field and a localised fluid flow on account of solutal undercooling during non-equilibrium solidification of binary alloys. Numerical simulations are performed for the case of two-dimensional transient solidification of Pb-Sn alloys, and the simulation results are also compared with the corresponding experimental results quoted in the literature. It is observed that non-equilibrium effects on account of solutal undercooling result in an enhanced macrosegregation. Next, the model is extended to capture the effects of dendritic arm coarsening on the macroscopic transport phenomena occurring during a binary alloy solidification process. The numerical results are first tested against experimental results quoted in the literature, corresponding to the solidification of an Al-Cu alloy in a bottom-cooled cavity. It is concluded that dendritic arm coarsening leads to an increased effective permeability of the mushy region as well as an enhanced eutectic fraction of the solidified ingot. Consequently, an enhanced macrosegregation can be predicted as compared to that dictated by shrinkage-induced fluid flow alone. For an order-of-magnitude assessment of predictions from the numerical models, a systematic approach is subsequently developed for scaling analysis of momentum, heat and species conservation equations pertaining to the case of solidification of a binary mixture. A characteristic velocity scale inside the mushy region is derived, in terms of the morphological parameters of the two-phase region. A subsequent analysis of the energy equation results in an estimation of the solid layer thickness. It is also shown from scaling principles that non-equilibrium effects result in an enhanced macro-segregation compared to the case of an equilibrium model For the sake of assessment of the scaling analysis, the predictions are validated against computational results corresponding to the simulation of a full set of governing equations, thus confirming the trends suggested by the scale analysis. In order to analytically investigate certain limiting cases of unidirectional alloy solidification, a fully analytical solution technique is established for the solution of unidirectional, conduction-dominated, alloy solidification problems. The results are tested for the problem of solidification of an ammonium chloride-water solution, and are compared with those from existing analytical models as well as with the corresponding results from a fully numerical simulation. The effects of different microscopic models on solidification behaviour are illustrated, and transients in temperature and heat flux distribution are also analysed. An excellent agreement between the present solutions and results from the computational simulation can be observed. The generalised numerical model is subsequently utilised to investigate the effects of laminar double-diffusive Rayleigh-Benard convection on directional solidification of binary fluids, when cooled and solidified from the top. A series of experiments is also performed with ammonium chloride-water solutions of hypoeutectic and hypereutectic composition, so as to facilitate comparisons with numerical predictions. While excellent agreements can be obtained for the first case, the second case results in a peculiar situation, where crystals nucleated on the inner roof of the cavity start descending through the bulk fluid, and finally settle down at the bottom of the cavity in the form of a sedimented solid layer. An eutectic solidification front subsequently progresses from the top surface vertically downwards, and eventually meets the heap of solid crystals collected on the floor of the cavity. However, comparison of experimental observations with corresponding numerical results from the present model is not possible under this situation, since the associated transport process involves a complex combination of a number of closely interconnected physical mechanisms, many of which are yet to be resolved. Subsequent to the development of the mathematical model and experimental arrangements for macroscopic transport processes during an alloy solidification process, some of the important modes of double-diffusive instability are analytically investigated, as a binary alloy of any specified initial composition is directionally solidified from the top. By employing a close-formed solution technique, the critical liquid layer heights corresponding to the onset of direct mode of instability are identified, corresponding two a binary alloy with three different initial compositions. In order to simulate turbulent transport during non-equilibrium solidification processes of binary alloys, a modified k-8 model is subsequently developed. Particular emphasis is given for appropriate modelling of turbulence parameters, so that the model merges with single-phase turbulence closure equations in the pure liquid region in a smooth manner. Laboratory experiments are performed using an ammonium chloride-water solution that is solidified by cooling from the top of a rectangular cavity. A good agreement between numerical and experimental results is observed. Finally, in order to study the effects of three-dimensionality in fluid flow on overall macrosegregation behaviour, the interaction between double-diffusive convection and non-equilibrium solidification of a binary mixture in a cubic enclosure (cooled from a side) is numerically investigated using a three-dimensional transient mathematical model. Investigations are carried out for two separate model systems, one corresponding to a typical metal-ally analogue system and other corresponding to an actual metal-alloy system. As a result of three-dimensional convective flow-patterns, a significant solute macrosegregation is observed in the transverse sections of the cavity, which cannot be captured by two-dimensional simulations.