Elastic Constants and Rheology of Glass Forming Metallic Liquids

Graduate students Mary Laura Lind and Gang Duan have combined experimental ultrasonic studies and Molecular Dynamics Simulations (MD) to determine how the high frequency elastic moduli of glass forming metallic liquids depend on temperature as the glass transition is approached. The experimental work [1] shows that the shear modulus, G, of a glass exhibits a slow linear drop with temperature as predicted by the traditional Debye-Gruneisen model of solids while the liquid above the glass transition shows a precipitous drop in G associated with the configurational excitation of the liquid “inherent states”. An example of data on the glass forming alloy Vitreloy 1, are shown in the Figure below. The experimental results confirm earlier observations made by analyzing MD simulations . Combined with a recent Cooperative Shear Model [2] for liquid flow developed in collaboration with the group of Prof. K. Samwer (U. of Gottingen, Germany), the data provide direct evidence that flow in the liquid is controlled by energy barriers whose height scales with G. This significant finding will be reported in an upcoming issue of Physical Review Letters. The work provides the basic building blocks for a novel theory of liquid flow as well as the flow and fracture properties of metallic glasses.

The temperature dependence of the “isoconfigurational elastic constants of liquid and glassy Zr_46.25Ti_8.25Cu_7.5Ni₁₀Be_27.5 as determined by ultrasonic “pulse-echo” measurements at 25 Mhz. The drop of the liquid shear modulus with temperature is attributed to excitation of “inherent state” configurations of the liquid. The glass shows a weaker linear temperature dependence as expected from the Debye-Gruneisen theory of solids.