Structure evolution of Zr-based glass-forming alloys and composition design methodology for good glass-forming abilities
Student thesis: Doctoral Thesis
The thesis is primarily devoted to the understanding of bulk metallic glass (BMG) formation phenomenon. Phase transformations have been studied first in a family of multicomponent Zr-based alloys. Attempt is made to decipher the BMG structure with the structure evolution evidence revealed in the phase transformations. By considering the short-range atomic clustering effect and the Hume-Rothery stabilization mechanism for the structural stability of BMG, a structural way is proposed for designing BMG compositions. The electrical transport properties of BMGs and the related quasicrystals have also been studied to examine the responses of their structures to the applied external field, which provides valuable information on the structures themselves. Phase transformations in (Zr65Al10Ni10Cu15)100-xNbx (at.%; x = 0, 1, 2, 3, 5, 7, 10, and 12) alloys cooled at different rates, and the structural evolution during the glass devitrification process were investigated. When the alloys were cooled from the liquid state, the component phases of the final solidification microstructures exhibited structural metastability. By means of copper mould casting, BMGs were made within the Nb-content range of 0 ~ 7 at.%. The largest glass-forming ability (GFA) was found at the (Zr65Al10Ni10Cu15)95Nb5 composition. It was found that the well recognized GFA indicators failed in the assessment of GFA. High resolution transmission electron microscopy and electron diffraction provided evidence that Zr-based BMGs bear a more disordered structure in nanometer scale than that of the first generation metallic glasses, for instance, Fe-Si-C. A P-type metastable icosahedral quasicrystal (I-phase) (aR _ 5.36 A) formed in the alloys containing 5 ~ 12 at.% Nb, and the formation was associated with a limiting cooling rate span at each composition. Diffraction evidence revealed that the I-phase structure is intrinsically defective, similar to the first generation Al-Mn I-phases. The diffraction spectrum of the defective quasicrystalline structure is interpreted with the icosahedral glass model. Experimental results also indicated that oxygen played a role in the BMG and I-phase formations. In-situ and ex-situ TEM demonstrated that, upon heating, the as-cast I-phase decomposed into two crystalline phases, which are isostructural with the tI-Zr2Cu and hP-Al2NiZr6 equilibrium phases, respectively. The in-situ observation further revealed sluggish diffusive transformation kinetics of the I-phase.
- Metallic glasses, Alloys