Normal and reverse indentation size effects in annealed and highly deformed metals

The normal and reverse indentation size effects for annealed and highly deformed Al, Al–Li alloy, Cu, and titanium VT1-0 were investigated. The normal indentation size effect (ISE) was revealed in all metals studied at large indentation depths. This behavior can only partially be described using the Nix and Gao model (NG) to interpret the depth-dependent hardness. The Feng and Nix correction to the NG model for small indentation depths works well for annealed coarse-grained (CG) metals. To explain the observed hardness dependence on the indentation depth, it is necessary to take into account the structure state of the surface layer. Grinding and mechanical polish of annealed fcc metals, such as aluminum, create a work-hardened layer that has higher hardness value compared to an electropolished sample as the load decreases and hence more pronounced ISE. Mechanical treatment of surfaces resulted in work-hardening of the thin layer in annealed CG metals and softening in naturally hard, ultrafine-grained, nano-crystalline, and hardening metals, changing the mechanical response of the metal at tension and contact deformation. In metals subjected to heavy plastic deformation, we first observed the reverse indentation size effect (RISE) phenomena where microhardness decreases with decreasing applied load.