Evidence of Ordering in Cu-Ni Alloys from Experimental Electronic Entropy Measurements

TL;DR

This study demonstrates that short-range atomic order significantly influences the electronic entropy and thermodynamic behavior of Cu-Ni alloys, challenging the assumption of ideal mixing in phase diagram modeling.

Contribution

It introduces an experimental approach combining thermopower and resistivity measurements with an irreversible thermodynamic model to reveal atomic ordering effects.

Findings

Electronic entropy contributes significantly to mixing entropy.

Short-range order affects both solid and molten states.

Electronic transport measurements indicate atomic ordering presence.

Abstract

Phase diagrams exhibiting extended solid-solution and lens-like melting are often reproduced using ideal solutions, where ideal mixing considers a fully random configurational entropy of mixing. In the field of irreversible thermodynamics, experimental measurements of the composition variation of high-temperature electronic transport and molten-state properties suggest however a strong role for short-range atomic ordering in these systems. Herein, measurements of the thermopower and resistivity are reported for Cu-Ni solid-solutions as a function of temperature and composition. The electronic transport properties were interpreted with an irreversible thermodynamic framework, revealing a large electronic contribution to the entropy of mixing. Through appeal to a cluster model for the configurational entropy that uses the electronic contribution to inform the existence of ordered associates, we rationalize such contribution of the electronic entropy with the notion of an ideal entropy of mixing commonly used to model such systems. These results suggest that the short range order (S.R.O.) of the atoms plays a significant role in both the solid and molten states, even when there are no dominant intermetallic compounds in these alloys.