Nanomechanical characterization of nanostructured bainitic steel: Peak Force Microscopy and Nanoindentation with AFM

Abstract

The full understanding of the deformation mechanisms in nanostructured bainite requires the local characterization of its mechanical properties, which are expected to change from one phase, bainitic ferrite, to another, austenite. This study becomes a challenging process due to the bainitic nanostructured nature and high Young’s modulus. In this work, we have carried out such study by means of the combination of AFM-based techniques, such as nanoindentation and Peak Force Quantitative Nanomechanical Mapping (PF-QNM) measurements. We have addressed critically the limits and advantages of these techniques and been able to measure some elastoplastic parameters of both phases. Specifically, we have analyzed by PF-QNM two nanostructured bainitic steels, with a finer and a coarser structure, and found that both phases have a similar Young’s modulus.

Ductility of Nanostructured Bainite

“…The increase of the bainitic formation temperature leads to a coarser microstructure, as can be seen by comparing the microstructures in Figure 1 (the same steel treated at two different temperatures, 250 °C and 350 °C) [15], which can be characterized by morphological parameters such as the bainitic ferrite plate thickness and the austenite block/film size. In addition, due to the transformation mechanisms, higher transformation temperatures result in higher retained austenite contents.…”

“…This enrichment of austenite in C in solid solution after the bainitic transformation, ranging from 0.5-1.5 wt %, is responsible for the reduction of the martensitic start temperature, Ms, below room temperature. Thus, austenite remains untransformed, having two very distinguishable morphologies: thin films trapped between the plates of bainitic ferrite and coarser blocks [13][14][15], Figure 1a,b.…”

“…Figure 1. Secondary-electron scanning electron micrographs (SE-SEM) of two nanostructured bainites from the same steel, with chemical composition in reference [15], treated at 250 °C (a) and 350 °C (b). γ stands for austenite, and α PT, for bainitic ferrite plate thickness.…”

“…On the other hand, the higher observed C content of retained austenite as the treatment temperature increases might contribute to enhance its strength, which becomes closer to the bainitic ferrite values or even exceed them. In order to confirm that possibility, local mechanical measurements in the phases present in the initial microstructure were performed by means of AFM (atomic force microscopy)-based nano-indentation, as shown in Figure 15 [15], where the phase indentation, either a plate of bainitic ferrite or an austenite feature, is clearly identified. Figure 15.…”

Facile measurement of single-crystal elastic constants from polycrystalline samples

“…14,15 Nanoidentation modulus measurement coupled with orientation measurement using electron backscatter diffraction (EBSD) have recently been applied to obtain orientation-dependent Young's modulus, but the method has yet demonstrated the capability of evaluating the full elastic constant values. 16 Atomic force microscopy (AFM)-based methods have extremely high spatial resolutions and can quickly produce qualitative modulus maps, 17,18 but extraction of accurate elastic constants from the AFM modulus maps is still far from a reality, especially for hard materials such as metals, inorganic semiconductors, and ceramics.…”