Invar, AF-Invar, Anti-Invar and martensite in fcc Fe-based alloys: an attempt to order this chaos

Systematics concerning magnetic and structural phase transitions within Fe-based systems can be achieved if experimental data are analyzed as a function of composition and the atomic volume. The findings can be understood from calculations of the exchange coupling of pure fcc Fe as a function of the n.n. distance. Decisive is that the volume increase going along with Invar, AF-Invar, Anti-Invar as well as martensite transitions originates from a charge transfer between antibonding electronic orbitals with t2g symmetry and AF exchange to non-bonding orbitals with eg symmetry and FM exchange. Which of the effects is leading at a given composition is a question of the position of the Fermi energy relative to these electronic levels. The question of how an effect manifests itself as a function of temperature is a question of the different strength of the t2g- and eg-electron-phonon coupling. (orig.)