Abstract
The indentation-induced plasticity and roughness have been investigated intensively by experiments and simulations during the last decades. However, the precise mechanisms of how dislocation flow leads to pile-up formation are still not completely understood, although this is one of the initial steps causing surface roughening in tribological contacts at low loads. In this work, {001}-, {101}- and {111}-grain orientations in an austenite stainless steel [(face-centered cubic (FCC) phase]) are indented with varying load forces. By using scanning electron-based methods and slip plane analysis, we reveal: (1) how slip-steps show the change of pile-up formation, (2) how the slip-plane inclination determines the dislocation flow and (3) how slip-plane interactions result in the final pile-up shape during indentation. We find that the flow direction transforms from the forward flow to the sideway at a transition angle of 55^circ{-}58^circ between the slip-plane and the surface. We use large displacement finite element method simulations to validate an inversion of the resolved shear stress at this transition angle. We provide insights into the evolution of plasticity in dislocation-mediated FCC metal indentations, with the potential application of this information for indentation simulations and for understanding the initial stage of scratching during tribology in the future.
Highlights
Nanoindentation has become a mature technique for mechanical property identification and small lengthscale materials science research [1,2,3,4,5,6,7,8,9,10,11,12,13]
It should be noted that the nomenclature of positively and negatively inclined slip-planes was proposed by Nibur et al [20, 21], who studied the influence of grain orientation and tip geometry on slip-step formation in face-centered cubic (FCC) metals
Since post-deformation electron microscopy inspection of indentations made at these rates did not reveal any difference in the imprint patterns, see supplementary material, we focus on the results of the 0.5 mN/s experiments and do not address the results of the other loading–unloading rates
Summary
Nanoindentation has become a mature technique for mechanical property identification and small lengthscale materials science research [1,2,3,4,5,6,7,8,9,10,11,12,13]. As the indenter penetrates the sample, the material moves downward and experiences a strain gradient, which results in dislocation motion toward the surface, which depends on the resolved shear stress in the slip direction due to the particular grain orientation. A Positively inclined slip-plane is a plane on which the dislocations move from the highest resolved shear stresses (i.e., underneath the tip) to the surface and away from the tip. The dislocations travel on the negatively inclined slip-planes from the outside bulk material to the surface and closer to the tip during indentation (see Fig. 1)
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