Rheological Behavior And Atomic Diffusion Of Pd-based And Zr-based Bulk Metallic Glasses

In this dissertation, the structure relaxation, rheological behavior, atomic diffusion andcrystallization of the Pd-based (Pd40Cu30Ni10P20) and Zr-based (Zr41.2Ti13.8Cu12.5Ni10Be22.5,Zr55Al10Cu30Ni5) bulk metallic glass forming systems with the best glass-forming ability(GFA) so far, have been systematically investigated by X-ray diffraction (XRD), scanningand transmission electron microscopy (SEM and TEM), thermal analysis(DSC/TMA/DMA), positron annihilation technique (PAT) and secondary ion massspectroscopy (SIMS). In the first part of the dissertation, a historic progress of metallic glass over past 40years has been critically overviewed, which includes the develop history, excellentproperties and wide applications, forming mechanics and preparation technique, the GFAand its criterion of bulk metallic glasses (BMGs). Based on investigation actuality, theobjective and significance of this dissertation were proposed. It was found that the structure relaxation of BMG Pd40Ni10Cu30P20 (PNCP), can bedivided into two regions based on temperature, namely, the low and the high temperatureregions. Furthermore, based on annealing time, the relaxation of the alloy also follows atwo-stage process. In the initial annealing stage, the isothermal relaxation kinetics obeys aKohlrausch law and can be well explained by the free-volume theory. However, in the laterstage, the isothermal relaxation kinetics cannot be described by the Kohlrausch law but canbe well understood by the percolation theory. The transformation processes of microstructure for Zr41Ti14Cu12.5Ni10Be22.5 (Vit1)BMG have been examined using the static extension method. A few strain rate peaks, whichcorrespond to the glass transition and multi-step crystallization in DSC curves, wereobserved in relative curves between strain rate and temperature. It is found that therheological behaviour of this alloy changes from elasticity to anelasticity, finally to theNewtonian viscous flow along with increasing temperature. Above the glass transitiontemperature Tg, the temperature dependences of viscosity in supercooled liquid region canbe fitted well with the Vogel-Fulcher-Tammann (VFT) equation. The sample exhibited thefragility parameter D* value of 36, the VFT temperature T0 of 319K and the fragility mvalue of 30. It was shown that the Vit1 alloy is a strong liquid, similar to sodium silicateliquids. However, below Tg, the relation between temperature and viscosity deviated fromVFT equation due to structure relaxation. The undercooled liquid of BMG PCNP isrelatively fragile as compared to Vit1. The viscosity in supercooled liquid region for BMG IIIPCNP is about one order in magnitude lower than that for Vit1. It suggests that the nature ofGFA for BMG PCNP is different with that for Vit1. The deformation of BMG Zr55Cu30Al10Ni5 in the supercooled liquid region exhibits asignificant superplasticity with the elongation of 200%, so much as 600%. The superplasticdeformation is strongly dependent upon strain rate (or stress) and testing temperature. Atlower strain rate, it takes place that homogeneous Newtonian deformation, i.e. the strain ratesensitivity exponent, m≈1. However, at higher strain rates, the plastic flow becamesimultaneous existing of homogeneous Newtonian and inhomogeneous non-Newtonianwith m<1, which due to generation of nanocrystallites is induced by stress. It was obtained that the data of some atomic diffusion, such as Si, Ti atom diffusion inZr55Al10Cu30Ni5, and Si atom diffusion in PNCP BMG. It was found that the diffusionmechanisms of these alloys in both the glassy and supercooled liquid states were a highlycollective hopping process, and where is a critical temperature Tc in the deeply supercooledstate, which lends new experamental support to view of the mode-coupling theory. For the sameSi atom, the diffusion coefficient for BMG PCNP is about one order in magnitude higher...