Microstructure correlation in high-strength steels with continuous stiffness mode nanoindentation results. High-resolution vs. low-resolution nanoindentation

Abstract

This study utilizes instrumented nanoindentation to compare the mechanical response of a transformation-induced plasticity (TRIP) steel to that of a quench and partition steel (QP). The nanoindenter was operated using continuous stiffness mode, and the microstructure under the indent revealed by chemical etching was examined. Examination of the microstructure in the indents by scanning electron microscopy allows a direct correlation with nanoindentation properties. It is found that the hardness measured at an indentation depth of 1.1 μm is, on average, significantly greater in the QP steel than in the TRIP steel. The continuous hardness measurement also allows the microstructure observed at the center of the indent to be correlated with the hardness values when indentation depths are <0.1 μm. This high-resolution hardness measurement allows for a direct comparison of mechanical response for the individual retained austenite phase particles in the TRIP steel with the strengthening microconstituent in the QP steel. With this methodology, the surprising result is that the TRIP retained austenite particles have a higher hardness than the QP hardening microconstituent. It is proposed that the resolution of the apparent disagreement between the hardness measurements obtained at 1.1 μm depth (QP has the higher hardness) and the 0.1-μm depth hardness measurements of the microconstituents containing retained austenite (TRIP has the higher hardness) lies in the volume fraction of the microconstituent as the QP has a two- to threefold higher volume fraction of hardening phase as compared to TRIP.