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 sytems 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 fee 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 t(2g) symmetry and AF exchange to non-bonding orbitals with e(g) 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 asa function of temperature is a question of the different strength of the t(2g)- and e(g)-electron-phonon coupling.