Mechanics of multiphase materials

First the observation levels for materials are discussed. Then a material is modelled as a heterogeneous continuum consisting of grains, each of them being a tiny continuum in its turn. As demonstration material steel consisting of two phases is selected. In the case of duplex steel the material consists a priori of two phases at fixed volume fractions. Subsequently, the martensitic transformation in a Fe-Ni steel is investigated as an example for a material with changing volume fractions of the phases. It is shown how continuum mechanics and thermodynamics can help to explain the somewhat unexpected deformation behavior of those materials.