ON THE USE OF NANO-INDENTATION FOR TENSILE PROPERTY CORRELATION OF FERROUS METALS

摘要

Micro- and nano-mechanical test techniques represent a powerful tool for the derivation of mechanical properties and microstructure-mechanical property correlations, provided that, besides validated experimental methods, robust frameworks for the analysis of scale-dependent results are developed and validated. The paper reports new results in the field of nano-indentation (NI) testing and data analysis based on different NI methods (force control, depth control and force-controlled progressive multicycling), which have been applied to various ferrous metals of interest to nuclear structural materials applications: the ferritic/martensitic steel T91, a ferritic Fe-9%Cr model alloy and pure Fe. These materials cover a wide range of microstructures in terms of grain sizes, martensite lath structure, and dislocation densities, which is probed by NI. For the interpretation of results a modification to the Nix-Gao framework is used to describe the breakdown of the scaling regime of the indentation size effect (ISE) at small indentation depths that is associated with a cut-off of the density of geometrically necessary dislocations (GNDs). Application of the model to F/M steels sheds new light on the role of GNDs and statistically stored dislocations, thereby rationalizing a material's propensity to developing pile-up behaviour. Extrapolation of the ISE to large indentation depths, as they are characteristic of the materials' micro-hardness, results in proper values for the tensile yield stress and characteristic tensile strength according to the models by Gao et al. [3] and Johnson [2], respectively, provided that pile-up effects are corrected and scale-dependent corrections of the Hall-Petch contribution to the flow stress are applied. A critical discussion of the methodologies and results demonstrates the capabilities and limitations of NI as a tool for the assessment of mechanical properties and microstructural features, as well as radiation-induced changes thereof.