|About this Abstract
||2021 TMS Annual Meeting & Exhibition
||Continuous Phase Transformations
||Thermodynamics of the Invar Transition: Phonons vs. Magnetism
||Stefan Lohaus, Pedro Guzman, Brent Fultz
|On-Site Speaker (Planned)
The invar effect, where the volume remains unchanged with increasing temperatures, is characterized by a continuous magneto-volume transition with competing thermodynamic effects. A change in magnetism from a ferromagnetic high spin state to a low spin state with temperature or pressure is accompanied by a decrease in volume, which counteracts the usual thermal expansion. For a fundamental thermodynamic understanding of this effect, the interaction between phonons, electrons and their spins was investigated. We performed nuclear inelastic X-ray scattering and synchrotron Mössbauer experiments at several pressure and temperature conditions across the invar transition, to measure the phonon spectrum and the magnetic evolution in Ni-Fe invar. These experiments allow us to quantify the vibrational and magnetic entropies and investigate their effect on the free energy of invar. The magnetic and vibrational entropies have opposing effects on the phase transition, helping to explain the unchanging volume and the thermodynamics behind the invar effect.