Plastic deformation during indentation unloading in multilayered materials

The indentation behavior of a heterogeneous material with constituents having distinctly different mechanical properties was studied numerically. The microstructural heterogeneity was represented by an elastic-plastic finite element model featuring explicit metal/ceramic nanolayers. Internal deformation fields as well as stress and strain histories at selected points in the material were analyzed during the loading and unloading history. We show that the metallic layers experience plastic deformation even during the unloading part of indentation, while indentation contact is present. Thus, the unloading response in these heterogeneous materials is much more complex than the conventional purely elastic recovery process observed in single-phase metals.