Study of the Yield Stress and Ductility of Hardened Surface Layers of Metal Alloys by Indentation

The mechanical properties possessed by surface layers of metal alloys hardened by severe plastic deformation methods were studied by indentation. The test materials were AISI O2 and AISI 316L steels and D16 aluminum alloy. Most scientific papers evaluate the mechanical properties of hardened surface layers and thin coatings only by hardness measurement. Hardness alone cannot characterize the ductility of a material without using modern concepts for determining the physical ductility from hardness. The mechanical behavior of a material can only be characterized by determining both the ductility and the yield stress. These quantities can be found by micro- or nanoindentation. We developed indentation methods to find the physical ductility δ_H and yield stress σ_SH of the surface layers of metal alloys hardened by various techniques (with analysis of changes in these characteristics across the hardened layer) to select the optimal hardening method. The hardening characterized by the ratio between the yield stresses of the hardened layer and the starting material $\left({\upsigma}_{{\mathrm{SH}}_{\mathrm{hard}}}/{\upsigma}_{{\mathrm{SH}}_{\mathrm{start}}}\right)$ was much greater for all studied alloys than the hardening characterized by the hardness ratio (HV_hard/HV_start). Thus, the use of δ_H and σ_SH obtained from standard microhardness measurements enhanced the informative nature and efficiency of the indentation method in studying the mechanical behavior of metal alloy layers hardened by various techniques.