Crystallization of zirconium-based bulk amorphous alloys and associated properties changes
Student thesis: Doctoral Thesis
Crystallization behaviors with associated properties changes of zirconium-based (Zrbased) bulk amorphous alloys (Terms of amorphous alloy, metallic glass and MG are used interchangeably and the composition of any MG is in atomic percentage throughout this thesis.) have been studied through continuous heating, isothermal annealing and friction joining. Two types of Zr-based bulk amorphous alloys were used in this study, namely Zr-Al-Ni and Zr-Ti-Cu-Ni-Be alloys. The ternary alloys have been used to study structural changes during continuous heating experiments. Particular attention is directed to compare glass forming ability and structural relaxation between the alloys. The other, Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass (BMG), has been used to illustrate the changes of the crystallization behaviors with the associated mechanical properties under different annealing time, temperature and atmospheric effects. Friction welding of the BMG has also been demonstrated so that the joining can further progress in various applications. Thermal stability and internal friction of Zr58.4Al20.8Ni20.8, Zr61.3Al19.4Ni19.3 and Zr63.0Al18.5Ni18.5 BMGs have been determined by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) for comparing their glass forming ability (GFA) and structural relaxation. The BMGs, which had the same e/a (the number of electrons per atom) value of 1.5 but with different compositions, were scanned from 573 K to 873 K at a heating rate of 10 K/min. A precise GFA indicator for BMG is still lacking so far, so the examination of the e/a rule is essential for developing better BMGs in the future. The measured supercooled liquid regions together with the e/a-based model supported the use of e/a rule for predicting GFA of the Zr-based BMGs. In addition, different excess volume was defined in the present alloys. The alloys having more excess volume exhibited lower thermal stability and faster relaxation kinetics upon heating. The effect of the excess volume on the relaxation kinetics of the glasses as well as the origin of the excess volume are also discussed. The correlation between the crystallization behaviors and mechanical properties of Zr41Ti14Cu12.5Ni10Be22.5 BMG has also been investigated under 5 Torr isothermal annealing in a temperature range from the glass transition temperature (613 K) to the first crystallization temperature (713 K). An in-depth investigation on the structural transformation from amorphous to nanocrystalline phases and finally reaching stable crystalline phases has been demonstrated during the annealing of the BMG at 613 K. Volume fraction and grain size of the evolved nanocrystalline phases increased with the annealing time. Microhardness measurements revealed a substantial increase with the volume fraction and grain size of the nanocrystals because of uniform distribution of the nanoscale particles in the amorphous matrix. Apart from 613 K annealing, other isothermal data was summarized and compared with those reported previously. Faster crystallization kinetics was identified for the BMGs annealed under 5 Torr than that under vacuum. Previous annealing studies only focused on the change of crystallization behaviors under different annealing environments. This study has not only reported the crystallization behavior varying with the air pressure, but also explained the faster kinetics on the basis of the oxidation behaviors of the BMGs. In order to understand the influence of the annealing atmosphere on the crystallization behavior of the BMG, the crystallization behavior has been studied by isothermal annealing between 5 and 760 Torr at 653 K. Microstructural evolution had been traced prudently with increasing annealing time. Crystallization kinetics increased with decreasing the air pressure. The incubation period for the crystal formation under 5 Torr was found almost sevenfold shorter than that under 760 Torr. Compositional depth profile revealed that oxygen penetrated deeper into the BMG in the 5 Torr case. The external oxygen level thus had a strong influence on the crystallization kinetics of the BMG. The relationship between the crystallization kinetics of the BMG and the external oxygen concentration under different air annealing atmospheres is discussed. The crystallization behavior and the mechanical properties of the BMG undoubtedly showed strong dependence on various annealing conditions. In order to avoid any crystallization on the BMG and retain its original mechanical properties, friction welding is one of the best ways to produce high quality joints by fabricating significantly large-sized BMG rather than using conventional casting method alone. Friction-welded joint of the Zr41Ti14Cu12.5Ni10Be22.5 BMG has been produced using a lathe with an applied pressure of 40 MPa, which is much lower than those reported previously. The examined joint not only retained amorphous nature, but also maintained the original mechanical properties.
- Analysis, Crystallization, Zirconium alloys