Entropy is an effective means to stabilize the high-temperature phase of a material. However, the origin of entropy is diverse, including configurational, vibrational, electronic, and magnetic, amongst others. It is still unclear, and sometimes controversial, which type of entropy plays a dominant role in a phase transition. Here, the individual entropy contributions during a hexagonal-close-packed (hcp) to face-centered-cubic (fcc) phase transition were quantified in transition metal based random solid solution alloys. We show, by inelastic neutron scattering measurements, that the vibrational entropy accounts for $\sim 80\%$ of the overall entropy change in a $\mathrm{C}{\mathrm{o}}_{75}\mathrm{N}{\mathrm{i}}_{25}$ binary alloy, whereas the contributions from electronic and magnetic entropy are small. First-principles calculations further identified that the excess vibrational entropy stems from the disappearance of optical phonon modes in the hcp phase. Analysis of a multicomponent $\mathrm{C}{\mathrm{r}}_{20}\mathrm{M}{\mathrm{n}}_{20}\mathrm{F}{\mathrm{e}}_{20}\mathrm{C}{\mathrm{o}}_{40}$ high-entropy alloy leads to the same conclusion, confirming the essential role of the vibrational entropy in driving the hcp-to-fcc phase transition in these alloys.

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• Entropy
• Lattice dynamics
• Phonons
• Structural phase transition
• Transition metal alloys
• Inelastic neutron scattering