Characterization of plastically graded nanostructured material

Nanostructured materials have attracted extensive research interest in the past decades due to their exceptionally high yielding strength. Among different processing technologies, one way is to induce surface nanostructures with the consequence of gradually changed microstructures and mechanical properties from the surface to the interior layer. In order to accurately quantify the depth-dependent constitutive law, the instrumented nanoindentation associated with the FEM-based inverse algorithm was developed in this paper. The linear relationship between the recovery energy ratio and the elastic representative strain is noted, in which the slope corresponds to the specific hardening coefficient. Therefore, besides the calculation of the representative stresses, the hardening coefficient can also be explicitly calculated from the energy recovery ratio. The whole flow behavior of a linear hardening material can then be quantified by a single Berkovich indent. The computational algorithm is well verified by virtual indentations and has been successfully applied to the surface mechanical attrition treated (SMATed) stainless steel for quantify its graded mechanical properties.