Multiresolution mechanical characterization of hierarchical materials: Spherical nanoindentation on martensitic Fe-Ni-C steels

Abstract

Systematic length scale studies are required for understanding effects of microstructural features that determine the mechanical properties of hierarchical materials. Recent advances in spherical indentation stress-strain protocols have made it possible to characterize the local mechanical responses at different length scales, from hundreds of nanometers to hundreds of microns. In this paper, two model martensitic steels Fe-5.1Ni-0.13C (wt.%) and Fe-5.0Ni-0.30C (wt.%) with different carbon contents were investigated using spherical nanoindentation stress-strain curves to quantify the mechanical behavior of lath martensite at multiple length scales using different spherical indenter tip radii. The indentation yield strength is dominated by the nanoscale defect structure for all indenter radii (1 μm, 16 μm and 100 μm) and does not exhibit any discernible size effect in the measured yield strengths at different length scales. The work hardening rates measured in the indentation tests at the different length scales coincide until the indentation zones grow large enough, so that a significant increase of work hardening occurs which is attributed to the presence of high-angle block boundaries in the indentation probed volumes. Characteristic pop-ins were observed in the indentations performed with the 1 μm and 16 μm indenter tip sizes and have been attributed to the interaction of dislocations with lath boundaries and their eventual transmission. In addition, the correlations between the properties measured from these indentation protocols and those measured in uniaxial tensile tests are critically examined.