Thermodynamics and Kinetics of Deeply Supercooled Multicomponent Metallic Glass Forming Melts

Oregon State University, Corvallis OR

Investigators

Abstract

The objective of this proposal is to study the thermodynamics and kinetics of supercooled bulk metallic glass forming liquids in conjunction with the glass transition as well as the thermal stability against crystallization. It focuses on the question why these liquids are superior glass formers compared to previous metallic glass formers. Together with the high viscosity, the Zr-Ti-Cu-Ni-Be alloys show small heats of fusion and a small Gibbs free energy difference between supercooled liquid and crystalline mixture. This indicates that at least this alloy group is a rather dense liquid with a small free volume. In the proposed research, a selection of different alloy groups will be examined with respect to their kinetics, thermodynamics and crystallization behavior. Three groups of alloys will be investigated. One group consists of early transition metal (ETM) based alloys that are free of Beryllium, in which the ETM(TM)s may serve as a rigid backbone. The second group includes bulk metallic glass forming metalloid containing liquids of the Pd-Cu-Ni-P type and the last group is based on simple metals such as Mg and Al. The goals of the study are to measure the viscosity of supercooled liquids as a function of temperature, alloy composition and shear rate, the specific heat capacities, heats of fusion and heats of crystallization to determine the thermodynamic functions of the alloys as well as the kinetics of the glass transition probed by viscosity and enthalpy relaxation experiments as well as the heating rate dependence of the glass transition temperature. In addition, the crystallization kinetics and microstructure for different degrees of undercooling will be investigated. Various models will be used and developed to analyze the results on viscosities. The thermodynamic functions are calculated as a function of temperature from the experimental data and the functional form of the entropy curve is compared with the viscosity via the Adam Gibbs theory. Nucleation and growth simulations will be applied to the kinetic and microstructural data and the measured viscosities and the thermodynamic driving forces are incorporated in these calculations. The educational outreach involves exposing undergraduate students as well as local high school seniors in various experimental aspects of the program. Bulk metallic glasses represent an exciting new class of structural materials, because of their properties like high strength, large elastic strain limit, corrosion resistance and formability. Furthermore, the high thermal stability of these alloys to resist crystallization allows investigation of supercooled metallic liquids from the melting point down to the glass transition. This region has not been experimentally accessible in the past for random closed packed liquids. The proposed work will have significance in the area of crystallization and glass formability of complex alloy systems, which can have technological underpinning, specifically in regard to the processing of bulk metallic glasses.