Crystallization behaviors of Zr50Cu50-based bulk metallic glasses (BMGs) upon isothermal annealing in the supercooled liquid regime were studied. By minor addition of Al, six alloys with compositions of Zr48Cu48Al4, Zr47Cu47Al6, Zr46.25Cu46.25Al7.5, Zr46Cu46Al8, Zr45.75Cu45.75Al8.5, Zr45.5Cu45.5Al9 were prepared by copper mold casting method and used for investigation. The crystallization mechanisms were demonstrated.

Chapter 1 gives a general introduction of metallic glass and Chapter 2 provides details on the sample preparation, experimental methods, and data reduction and analysis.

In Chapter 3, simultaneous wide- and small-angle X-ray scattering (WAXS/SAXS) measurements were performed to investigate the low-temperature crystallization kinetics of Zr46Cu46Al8 BMG during isothermal annealing. Kolmogorov-Johnson-Mehl-Avrami (KJMA) analysis of the WAXS intensity data indicated that the crystallization process was dominated by homogeneous nucleation and three-dimensional diffusion-controlled growth. This observation was corroborated by quantitative analysis of the SAXS data showing a saturation of the growth of nanoscale crystallites. Comparison of the simultaneously measured WAXS and SAXS data showed evidence of composition fluctuations prior to crystallization, suggesting that the crystallization was facilitated by nanoscale phase separation which established regions of favorable compositions catalyzing crystallization. Temperature dependence of crystallization mechanisms and the resulting microstructures in metallic glass are discussed.

In Chapter 4, a three-stage structural evolution mode for the isothermal crystallization of Zr45.5Cu45.5Al9 BMG in the supercooled liquid region was well characterized by a new approach: in-situ, simultaneous differential scanning calorimetry (DSC) and small angle neutron scattering (SANS). It was found that the crystallization process was dominated by diffusion-controlled growth mechanism. The mean radius of the crystalline particles in the annealed sample derived from model fitting of SANS data was 15.7 nm. Further structural evolution involving chemical redistribution occurred at the late stage of isothermal annealing, consistent with the observation from atom probe tomography. Electron microscopy confirmed the size deduced from SANS model fitting and revealed the presence of high number density of near-spherical particles embedded in the residual amorphous matrix.

In Chapter 5, simultaneous differential scanning calorimetry (DSC) and small angle neutron scattering (SANS) experiments were performed to investigate the structural evolution of (Zr50Cu50)100-xAlx (x = 4, 6, 7.5, 8, 8.5, 9) BMGs upon isothermal annealing in the supercooled liquid region, and to reveal the effect of Al addition on thermal stability and crystallization behavior of these BMGs. Although all samples exhibited a three-stage structural evolution during isothermal annealing, two main observations were found. Firstly, the thermal stability showed compositional dependence. The incubation time for crystallization increased with Al content x increased from 4 to 8 and decreased with further addition of Al. Secondly, the crystalline behavior showed compositional dependence. On the one hand, the crystallization kinetics showed compositional dependence. The incubation time for crystallization increased with Al content x increased from 4 to 8 and decreased with further addition of Al. One the other hand, the crystalline products showed compositional dependence. While Cu10Zr7 phase which existed in all annealed alloys, Zr2Cu phase disappeared gradually with increasing Al content.

In Chapter 6, the summary of this thesis is presented.