Thermodynamic potentials of metallic alloys in the undercooled liquid and solid glassy states

TL;DR

This paper compares thermodynamic potentials of metallic alloys in undercooled liquid and glassy states, introduces a structural order parameter, and explores its correlation with critical cooling rates.

Contribution

It presents a novel comparative analysis method for thermodynamic potentials in metallic alloys and introduces a new order parameter to characterize structural evolution.

Findings

Thermodynamic potentials coincide in supercooled liquid range when melting entropy is properly calculated.

The order parameter $\xi_{scl}$ correlates with the melt critical cooling rate $R_c$.

Smaller $\xi_{scl}$ indicates a structure closer to the equilibrium liquid and a lower critical cooling rate.

Abstract

We first present a comparative analysis of temperature evolution of the excess thermodynamic potentials (state functions), the enthalpy $\Delta H$, entropy $\Delta S$ and Gibbs free energy $\Delta \Phi$, determined for \textit{i}) undercooled melts using literature data and \textit{ii}) solid glassy state calculated on the basis of calorimetry measurements using an approach proposed recently. Three metallic alloys were taken as an example for data analysis. It is found that temperature dependences $\Delta H(T)$, $\Delta S(T)$ and $\Delta G(T)$ calculated with both approaches coincide in the supercooled liquid range (i.e. at temperatures $T_g<T<T_x$, where $T_g$ and $T_x$ are the glass transition and crystallization onset temperatures, respectively). However, the necessary conditions for this coincidence is the introduction of important changes to the above approach \textit{i}), which are related to the calculation of the melting entropy. We also introduce and calculate a dimensionless order parameter $\xi$, which changes in the range $0<\xi<1$ and characterizes the evolution of the structural order from liquid-like to crystal-like one. It is shown that the order parameter $\xi_{scl}$ calculated for the end of the supercooled liquid range (i.e. for a temperature just below $T_x$) correlates with the melt critical cooling rate $R_c$: the smaller the order parameter $\xi_{scl}$ (i.e. the closer the structure to that of the equilibrium liquid), the smaller $R_c$ is.