| Al-based and Fe-used bulk metallic glasses (MGs) are of engineering and scientific interest due to their unique combination of attributes. The objective of the present dissertation is to provide fundamental insight into the influence of synthesis on the microstructure and physical behavior of Al-based and Fe-based MGs. Two non-equilibrium synthesis techniques are selected for study, gas atomization and laser deposition via laser engineered net shaping (LENSRTM), which provide different thermal and solidification conditions. Thermal behavior studies involving numerical simulation and experimental validation, microstructure characterization, and mechanical behavior studies are combined in an effort to understand the influence of process parameters as well as to optimize microstructure and performance. Details of the approach used in the present dissertation are described below.; First, a numerical approach was implemented to simulate and analyze heat transfer and cooling rates of individual droplets during flight in gas atomization. The calculated results, along with the experimental validation, were used to interpret the formation of amorphous structure and crystal phase development in the atomized powders of different sizes, and to optimize process parameters.; Second, four different types of Al-based MG powders were gas-atomized with distinct process parameters. The microstructure, thermal stability, phase transformation sequences and micro-hardness were investigated with SEM, XRD, TEM, and DSC as a function of powder size. Fully amorphous powders were nominally <25mum in diameter. With increasing of powder size, nanocrystalline fcc-Al particles precipitated in an amorphous matrix. The micro-hardness increases with increasing volume fraction of amorphous phase.; Third, the thermal behavior and cooling rate during LENSRTM process was simulated numerically using the alternate-direction explicit (ADE) finite difference method (FDM) and the results were compared to those obtained experimentally. The thermal history associated with the LENS RTM process involves numerous reheating cycles. The evolution of microstructure was analyzed as a function of the deposition conditions.; Fourth, LENSRTM was implemented to fabricate net shaped Fe-based MG components. The glass transition, crystallization behavior, and physical properties of the glassy alloy, as processed under different process parameters, are analyzed to provide fundamental insight into the underlying physical mechanisms. |
