Nanoindentarion Pop-in Behaviors in Steel

Abstract

Recently, nanoindentation has been employed to probe the small-scale mechanical behavior of materials for a wide range of academic or engineering applications. The response of a material to the nanoindentation is usually represented as a form of the load-displacement curve. When metallic materials undergo irreversible permanent deformation, the discrete physical events, such as dislocation nucleation, dislocation source activation, phase transformation or mechanically-induced twinning, can be detected as discontinuities of displacement or load during nanoindentation. These can produce geometrical softening accompanied by a sudden displacement excursion during load-controlled nanoindentation, which referred to in the literature as a pop-in. In this study, several physical events which cause pop-ins during nanoindentation of steel will be reported and discussed. First, experimental results of nanoindentation and microstructural studies of metastable austenite in TRIP steel is reported to provide its micromechanical insight into the strain-induced phase transformation and deformation behavior. Sequential experiments were carried out, first using electron backscattered diffraction (EBSD) to map the phase and orientation distributions of the grains, followed by nanoindentation of individual austenite grains in the mapped region, then sectioning through an indent using focused ion beam (FIB) milling and finally transmission electron microscopy (TEM) to confirm the formation of martensite from austenite under the indent. The load-displacement curve obtained from nanoindentation revealed two types of pop-in events on the loading segment. The first type was attributed to the elastic-to-plastic transition of austenite based on a Hertzian analysis of the elastic portion of the load-displacement curve. A second type of pop-in can be described as resulting from geometrical softening due to the selection of a favorable martensite variant based on the mechanical interaction energy between the externally applied stress and lattice deformation during nanoindentation. The existence of martensite after nanoindentation was confirmed by TEM analysis of the cross-section of an indented sample. The TRIP strain calculated by simple considering of crystal geometry chage during phase transformation was in good agreement with the measured pop-in depth. Multiple pop-ins in less stable austenite was considered as a result of sequentially transformed multiple martensite from austenite. Second, nanoindentation and microstructural studies are reported to provide experimental evidence of the relationship between the formation of ε martensite and pop-in behavior in metastable austenite in high nitrogen TRIP steel. Sequential experiments of EBSD, SPM, nanoindenter, FIB, and high resolution TEM (HR-TEM) were also carried out in order to directly observe ε martensite under the indent. The load-displacement curve obtained from nanoindentation revealed stepwise pop-ins in the early stage of plastic deformation. Considering that the stress-induced ε martensite transformation is the predominant deformation mode in the early stage of plastic deformation and its monopartial nature as well, geometrical softening can also occur by ε martensite formation. From analyses of high resolution TEM images, a cluster of banded structure under the indent turned out a juxtaposition of (111) planes of γ austenite and (0001) planes of ε martensite. The most favorable slip system predicted by simple calculations based on the Schmids law was the same one that experimentally observed by TEM. It was also calculated that formation of more than just 10 single ε martensite layers in this slip system can introduce several nanometers of pop-in. These microstructural investigations strongly suggest that the pop-in behavior in the early stage of plastic deformation of austenite is closely related to the formation of ε martensite. Lastly, pop-ins on nanoindentation load–displacement curves of a ferritic steel were correlated with yield drops on its tensile stress–strain curves. To investigate the relationship between these two phenomena, nanoindentation and tensile tests were performed on annealed specimens, prestrained specimens, and specimens aged for various times after prestraining. Clear nanoindentation pop-ins were observed on annealed specimens, which disappeared when specimens were indented right after the prestrain, but reappeared to varying degrees after strain aging. Yield drops in tensile tests showed similar disappearance and appearance, indicating that the two phenomena, at the nano- and macroscale, respectively, are closely related and influenced by dislocation locking by solutes (Cottrell atmospheres).