| In the present dissertation, the glass forming ability (GFA) of Ti-based metallic glasses, the microstructure and mechanical properties of Ti-based metallic powders fabricated by gas atomization method, the crystallization behavior during continuously post-heating process and the sintering behavior of the gas-atomized Ti-based metallic glassy powders have been systematically investigated.Based on thermodynamic characteristics of the stable metallic liquid at melting temperature and the supercooled liquid, the present work calculated the mixing enthalpyΔHmix, the mixing entropyΔSmix, and the Gibbs free energy difference between the supercooled liquid and the resulting crystalline phases,ΔG, of typical Ti-based glassy alloys. The results show that, for the case of largerΔSmix, moderateΔHmix for the stable liquid, and smallerΔG for the supercooled liquid, Ti-based alloys tend to achieve high GFA. A new parameter,β, defined as (Tg-Tk)/(Tl-Tg), has been introduced to evaluate the GFA of Ti-based bulk glassy alloys (wherein Tg, Tl, Tk represent the glass transition temperature, the liquidus temperature, and the Kauzmann temperature, respectively). Experiment data imply that the larger theβ, the better the GFA for Ti-based glassy alloys.Ti-based glassy and partially glassy powders were prepared using the argon gas atomization method. Experimental results show that the shape of powders is mainly regularly global with the cooling rate of 103～105°C/s. With increasing the powder size, the phase constituent alters from pure glassy phase to glassy phase with crystalline phases (fcc NiSnZr and hexagonal Ti3Sn phases). The composition analysis of these precipitates revealed that the crystals are Sn-rich phases when compared with the Cu-rich glassy matrix. The synergetic effect of the thermodynamics and kinetics during cooling determine the subsequent characteristics of the crystalline precipitations, such as the microstructure, grain size and composition.Under nanoindentation tests, the various sized powders exhibit different deformation behaviors. It is found that the small powders exhibit more pop-in events and better plastic deformability (larger pile-up ratio). The synergetic effect of the free volume and crystallization, determined by the cooling rate, has been found to be responsible for the plastic deformation of synthesized powders. The small powders formed due to the faster cooling rate should have the larger amount of free volume, leading to more shear band nucleation and thus more pop-in events, which make contribution to the plastic deformation. On the other hand, the crystals in the Ti-based metallic powders would deform difficultly, and the single shear band is expected to be deflected away from the crystals and extend continuously by rapid propagating in the matrix, leading to the poor microcosmic plastic flow ability of medium and large powders.During the post-heating crystallization, the Ti-based metallic glassy powders exhibit multi-step crystallization behavior. The glassy phase firstly crystallized into metastable nanocrystals, and then the nano-sized fcc Ni2SnZr and monoclinic TiNi0.8Cu0.2 phases formed by a solid state phase transition. Accompanying with the crystallization, the various composition regions were observed between the small atoms (Ti, Ni and Cu) and large atoms (Sn, Zr). The synergetic effect of chemical interaction (thermodynamics) and atomic mobility (kinetics) determine the subsequent characteristics of the crystalline precipitations. Comparing with cooling process, the nanocrystals caused by post-heating result from a higher nucleation rate and a lower growth rate, which is determined by the higher supercooled degree (lower transition temperature).A large size and high strength Ti-based bulk metallic glass is fabricated from the glassy powders by the spark plasma sintering (SPS) technique. For the sample sintered at 440°C, its fracture strength (1660 MPa) is quite close to that of the as-cast sample (1700 MPa). Based on the free volume model, the sufficient viscous flow induced by the temperature under a sintering stress is the main densification mechanism. During the SPS process, the atomic mobility and the flow defect concentration play the key roles for densification.During the SPS, it is expected that the microscopic temperature varies from the sample center to the die surface, thus forming a temperature gradient. The computed temperature in the sample center is 10~20°C higher than that at the sample edge in this work (sintered at 440°C), resulting in different relaxed microstructures. A new orthorhombic phase with the lattice parameters of a = 0.48 nm, b = 1.03 nm and c = 0.61 nm and a composition of Ti57.1Ni3.3Cu27.8Zr8.2Sn3.6 (at. %) has been found. Meanwhile, the microscopic temperature gradient can also affect the mechanical property of the sintered sample. The central region of sample sintered at 440°C exhibits the highest hardness and elastic modulus.
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