Conference Tools for 2023 TMS Annual Meeting & Exhibition

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About this Abstract
Meeting	2023 TMS Annual Meeting & Exhibition
Symposium	Hume-Rothery Symposium on First-Principles Materials Design
Presentation Title	Origin of the Invar Effect
Author(s)	Brent T. Fultz, Stefan Lohaus, Pedro Guzman
On-Site Speaker (Planned)	Brent T. Fultz
Abstract Scope	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.
Proceedings Inclusion?	Planned:
Keywords	Magnetic Materials, Phase Transformations, Other

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