Basic Understanding of the Relations between Invar, Anti-Invar and Martensite in Fe-based Alloys

Based on experimental results of high temperature thermal expansion, paramagnetic neutron scattering, considerations of the relative stability of the various phases of elemental Fe and total energy calculations, we discuss the interrelated aspects of Invar, anti-Invar and martensitic phase transformations in Fe-rich alloys. The physical background to the discussions is provided by ab initio calculations of the magnetic and structural binding surfaces of pure Fe and Fe- based alloys. Decisive are charge transfers between electronic levels with different symmetry (eg and t2g) and their non-bonding and anti-bonding character. The variation with composition of the size of the Invar and anti-Invar effects is a question of the position of the Fermi energy relative to these levels. How an effect manifests itself as a function of temperature is a question of the energetic difference between these levels.