S. Venkataraman

On the fragility of Cu47Ti33Zr11Ni8Si1 metallic glass

This study reports on calorimetric measurements of the glass transition at various heating rates stretching over three orders of magnitude for Cu47Ti33Zr11Ni8Si1 metallic glass powders synthesized by gas-atomization and subjected to varying thermal treatments. A Vogel-Fulcher-Tammann-type relation was fitted to the data recorded for the heating rate dependent shift of the glass transition. The fragility of the alloy is evaluated in terms of the fragility parameter. Its value is 44 for the as-prepared Cu47Ti33Zr11Ni8Si1 metallic glass powder and decreases with increasing annealing treatment temperature. The decrease in the fragility parameter is due to partial crystallization, which shifts the composition of the remaining supercooled liquid to that of a strong glass-forming alloy.

Atom Probe Tomography Characterization of a Gas Atomized Metallic Glass

Summary form only given. A number of metallic glasses that exhibit a wide supercooled liquid region before crystallization and a high glass-forming ability have been discovered. These metallic glasses exhibit useful properties such as high strength and high stiffness and can be fabricated from the melt in a bulk form with a thickness of ~ 10 mm. The high glass-forming ability facilitates the formation of metallic glass powders by conventional gas-atomization technique. Subsequent consolidation of the powders to any dimensions is possible due to the viscous flow of the material in the supercooled liquid region. Hence, the synthesis of bulk metallic glasses using gas atomization coupled with subsequent consolidation holds a promising future. Atom probe tomography, X-ray diffraction and differential scanning calorimetry (DSC) characterizations of gas atomized powder particles of a Cu47 Ti33Zr11Ni8Si1 metallic glass have been performed. The needle-shaped specimens required for the local electrode atom probe were fabricated from individual 10-40 mum diameter particles with the use of a dual beam focused ion beam miller. The microstructure of the alloy was investigated from the as-atomized powder and annealing treatments up to the completion of the first and second exothermic events at 785 and 838 K. Atom probe tomography revealed that the microstructure consisted of an interconnected network structure of two amorphous phases after an annealing treatment of 360 min. at 623 K. A fine-scale multiphase microstructure of an irregularly shaped copper-enriched and titanium-, nickel- and silicon-depleted phase that was 10-20 nm in extent, a higher number density of smaller, ~10 nm diameter, and roughly spherical titanium-enriched and copper- and zirconium-depleted phase and a matrix phase was found after continuous heating in a DSC to 785 and 838 K. The scanning electron microscope also revealed 4 distinct coarser phases consistent with Cu51Zr14, CuTi, (Cu,Ni)Ti and an unidentified structure after annealing for 24 h at 1073 K