Eric Bakke

Viscosity of the supercooled liquid and relaxation at the glass transition of the Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass forming alloy

Abstract The flow and relaxation of the Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass forming alloy was investigated in the supercooled liquid state and the glass transition region using parallel plate rheometry, three-point beam bending as well as differential scanning calorimetry. The results indicate that this bulk metallic glass former is a strong liquid that, kinetically, behaves in a similar way as a silicate melt. The relaxation of the viscosity and specific heat capacity was investigated in isothermal experiments and with constant heating rates. The studies reveal that the relaxation of the viscosity into its equilibrium state is directly related to the calorimetric glass transition. The observed calorimetric glass transition in this strong glass former is of a purely kinetic nature and a thermodynamically metastable amorphous state cannot be observed in this bulk metallic glass forming liquid on a laboratory time scale. The heating rate dependence of the calorimetric glass transition reflects the fragility of the liquid.

The viscosity of the Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass forming alloy in the supercooled liquid

The viscosity of the Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass forming alloy in the supercooled liquid was measured using parallel plate rheometry. The measurements were carried out with different heating rates between 0.0167 and 1.167 K/s as well as isothermally. Because of the high thermal stability above the glass transition of this bulk metallic glass former with respect to crystallization, it was possible to measure viscosities in the range from 1010 to 106 poise. This region of viscosities has not been previously measured for supercooled metallic melts. Our measurements suggest that the viscosity of the supercooled liquid of this bulk glass former exhibits a small Vogel–Fulcher temperature relative to the glass transition temperature, similar to silicate glasses.

Supercooled melting in multicomponent Zr–Al–Cu–Ni diffusion couples

Diffusion couples combining a hcp Zr90Al10 supersaturated solid solution with a fcc Cu64Ni36 solid solution were annealed at 410 °C for different times. The reaction at the interface was investigated by transmission electron microscopy and energy dispersive x-ray spectroscopy. The investigations show the formation of a noncrystalline layer at the interface between the two solid solutions that grows to a maximum thickness of more than 0.3 µm. Concentration profiles reveal that two noncrystalline phases coexist in the diffusion couple. One is Ni-rich and was in the amorphous state during the reaction at 410 °C. The other phase is Zr-rich and grew in its supercooled liquid state. This novel supercooled melting process has not been previously observed in a solid state reaction. Kinetic and thermodynamic aspects that contribute to the high thermal stability of the Zr–Al–Ni–Cu in the supercooled liquid are discussed.

Viscosity, Relaxation and Crystallization Kinetics In Zr-Ti-Cu-Ni-Be Strong Bulk Metallic Glass Forming Liquids

The high thermal stability of bulk metallic glass (BMG) forming liquids in the undercooled state allows for measurements of thermophysical properties in a large time and temperature window. In this contribution, results on viscous flow, relaxation and crystallization of Zr-Ti-Cu- Ni-Be BMG forming alloys are presented. The data are compared with the kinetics of other metallic and non-metallic liquids. BMG formers are relatively strong liquids with melt viscosities that are about three orders of magnitude larger than in pure metals and other alloys. The strong liquid behavior of these alloys is also reflected by a small entropy of fusion and a weak temperature dependence of the thermodynamic functions upon undercooling. The high viscosity and small driving force for crystallization are major contributing factors to the high glass forming ability and low critical cooling rate. The upper portions of experimental timetemperature- transformation diagrams down to the crystallization nose can be described well using the kinetics deduced from the viscosity data. For lower temperature the viscosity can not describe the crystallization kinetics. The time scale for structural relaxation becomes larger than for diffusive hopping processes. Diffusion stays relatively fast, whereas viscosity and structural relaxation time upon undercooling follow a Vogel-Fulcher-Tammann relation.

Solid State Reactions in the ZR-AL-CU-NI Bulk Metallic Glass Forming Alloy System

Diffusion couples combining a h.c.p. Zr 90 Al 10 supersaturated solid solution with a fc.c. Cu 64 Ni 36 solid solution were annealed at 410°C for different times. The reaction at the interface was investigated by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDAX). Cross sectional TEM images and electron diffraction patterns reveal the formation of a non-crystalline layer at the interface between the two solid solutions which grows to a maximum thickness of more than 0.3 μm. Using EDAX, the concentration profiles across the layerswere determined. The glassy layer consists of a Zr-Ni-Al alloy with a small amount of Cu.The concentration profiles reveal that two non-crystalline phases coexist in the diffusion couple. One phase is Ni-rich and the other is Zr-rich. A small gradient of the Ni concentration in the Ni-rich amorphous phase and a steeper gradient in the Zr-rich non crystalline phase indicate that the Ni diffusion constant in the Ni-rich phase is larger than the Ni diffusion constant in the Zr-rich phase. In the late stage of the reaction, the growth of a nanocrystalline layer with an average concentration of Cu 90Nij 0 is observed on the Cu-Ni side of the diffusion couple. Crystallization starts at the Zr-Al side of the diffusion couple.