|About this Abstract
||2023 TMS Annual Meeting & Exhibition
||Hume-Rothery Symposium on First-Principles Materials Design
||Origin of the Invar effect
||Brent T. Fultz, Stefan Lohaus, Pedro Guzman
|On-Site Speaker (Planned)
||Brent T. Fultz
We found that Fe64Ni36 Invar has zero thermal expansion over modest pressures near 300K. This enabled an assessment of thermal expansion by a thermodynamic Maxwell relation, dS/dP=-dV/dT, using separate entropy contributions. Specifically, the pressure dependence of phonon entropy and magnetic entropy were obtained by nuclear resonant X-ray scattering measurements on isotopic 57Fe64Ni36 in a diamond anvil cell. The contributions to entropy from phonons and spins have opposite signs, and both increase by more than a factor of two over the pressures of Invar behavior. They have opposite signs, however, and cancel precisely during Invar behavior. At pressures above 3GPa this cancellation is lost, but consistently with the positive thermal expansion. The cancellation of the dS/dP from phonons and spins seems to be aided by a magnon-phonon interactions. This precise cancellation causes the Invar effect in Fe64Ni36.
This work was supported by the National Science Foundation under Grant No. 1904714.
||Magnetic Materials, Phase Transformations, Other