Viscosity and crystallization in a series of Zr-based bulk amorphous alloys

bulk amorphous alloys;crystallization;metallic glass

Waniuk, Theodore Andrew ; Johnson, William Lewis

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: bulk amorphous alloys;    crystallization;    metallic glass;   

Others  :  https://thesis.library.caltech.edu/2538/1/Waniuk_dissertation_2004.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

The various influences on glass-forming ability and crystallization in a series of alloy compositions ranging along a tie-line from Zr41.2Ti13.8Cu12.5Ni10Be22.5 to Zr46.75Ti8.25Cu7.5Ni10Be27.5 are studied using multiple techniques. Both the kinetic and thermodynamic aspects of crystallization are examined experimentally and used to explain why, in this series, thermal stability is greatest in those alloys with the poorest glass-forming ability. Overall, the investigations reveal that a single criterion like thermal stability or high viscosity is an insufficient indicator of the glass-forming ability in certain complex bulk glass-forming systems. Instead, in these complex systems, multiple criteria must be combined to get a complete picture.The equilibrium viscosity of Zr41.2Ti13.8Cu12.5Ni10Be22.5 and three other alloys was determined for temperatures near the glass transition using three-point beam bending. The results of these measurements indicate that the alloys in this system are moderately strong liquids, and relaxation behavior is directly influenced by a chemical decomposition process which occurs in the supercooled liquid.This decomposition process also appears to affect the crystallization behavior in the same series of alloys. Critical cooling rates were determined by observing crystallization upon cooling from the molten state with different rates, and these were compared to thermal analysis performed under constant heating conditions. The latter measurements show that the supercooled liquid region is largest for compositions midway between the two endpoints; in contrast, the critical cooling rate increases continuously from 1.4 K/s to 28 K/s for each successive alloy in this series.In order to examine the crystallization process more closely, low temperature time-temperature-transformation diagrams for the alloy series were measured during isothermal annealing of initially amorphous specimens. For all investigated alloys, a primary quasicrystalline phase forms at a rate which varies substantially with alloy composition. Describing the complicated influences on crystallization required not only the previous thermal analysis but also constant heating measurements and x-ray diffraction data obtained after various states of annealing.