Impact of phonons on the phase stability of FCC alloys

Phase competition in multi component high-entropy alloys (HEAs) leads to the emergence of exceptional mechanical properties. However, the complex chemistry provides a huge composition space to explore, making it a great challenge to study the mechanisms of competing crystal structures. In this study, we focused on the Co-Ni binary alloy as a simplified model system for these complex alloys. Through inelastic neutron scattering (INS) and differential scanning calorimetry (DSC) measurements, we examine the impact of phonons (vibration) on the phase stability of Co₇₅Ni₂₅. Our first-principles calculations on a model Co-Ni random solid solution at 0K show a good agreement with our experimental phonon density of states data measured by the INS. Our work emphasizes the importance of vibrational entropy on the phase stability, where vibration rather than other contributions dominate the entropy term leading to the stability of the FCC phase in Co₇₅Ni₂₅.