Abstract
Nanoindentation has been used intensively during the last decades to characterize experimentally the elastic and plastic material properties of phases at the microscale. Accompanying simulations have investigated the plastic mechanisms during nanoindentation. While experiments and simulations have led to a thorough understanding of most mechanisms during nanoindentation, the plasticity on positively and negatively inclined slip planes is still not completely clear. In this work, {1 0 0}-, {1 0 1}- and {1 1 1}-grains of an austenitic stainless steel are indented to better understand the dislocation mediated plasticity through slip step analysis. We observe that slip occurs on positively and negatively inclined slip planes during nanoindentation and we propose methods to differentiate between both types of planes. We find that slip steps on positively inclined slip planes form preferentially during the early stage as compared to the formation of slip steps on negative inclination, which occurs during the later deformation stage due to the change in surface topography. By calculating the resolved shear stress in the presence and absence of pile-ups, we reveal the origin of slip on positively and negatively inclined planes as well as the reason for the sequence of occurrences. We conclude that accounting for the surface topography evolution in experiments and simulations is essential in predicting the plastic slip activation during nanoindentation.
Highlights
Metal plasticity is associated with dislocation motion on specific crystallographic planes
This small-scale behavior is consistently described by the Nix-Gao model [8] and this behavior occurs in this study but that mechanism does not address the observed inversion of slip-plane activity: i.e. the indentation size effect explains a decrease/increase of strain gradients; but the indentation size effect does not explain a change of the stress state
Towards the goal of this study, we investigate the deformation of {1 1 1}, {1 0 1} and {0 0 1} grains and use combinations of different tools to distinguish between the slip plane inclinations: (1) Electron Backscatter Diffraction (EBSD), (2) EBSD þ Electron Channeling Contrast Imaging (ECCI), (3) EBSD þ atomic force microscopy (AFM)
Summary
Metal plasticity is associated with dislocation motion on specific crystallographic planes. During nanoindentation the indenter spontaneously moves into metals without an increase in the applied load This displacement-burst is called ‘pop-in’ [3] and is connected to the dislocation nucleation and multiplication [17,18]. Tromas et al [19,20] stopped the nanoindentation in MgO after the pop-in to study the incipient plasticity They reconstructed the dislocation structures by nanoetching and atomic force microscopy (AFM). Velednitskaya et al [21] determined the dislocation structure and the Burgers vectors of the dislocations around the MgO indentation through transmission electron microscopy (TEM) and scanning electron microscopy (SEM) They mentioned that the dislocations, which originate from underneath the indenter, do not explain fully the pile-up that surrounds the indentation
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