Application of nucleation theory to the rate dependence of incipient plasticity during nanoindentation
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We propose a nucleation theory-based analysis for incipient plasticity during nanoindentation and predict the statistical distribution of rate-dependent pop-in events for many nominally identical indentations on the same surface. In the framework of stress-assisted, thermally activated defect nucleation, we quantitatively rationalize new nanoindentation measurements on 4H SiC and extract the activation volume of the nucleation events that mark the onset of plastic flow. We also illustrate how this statistical approach can differentiate between unique nucleation events for different indenter tip geometries.
I. INTRODUCTION
During nanoindentation experiments, the onset of plastic deformation is frequently associated with a displacement burst, or “pop-in,” corresponding to the operation of a discrete strain accommodation mechanism. For example, work by Gerberich and co-workers1–3 on nanoindentation of passivated metal surfaces has ascribed very large pop-in events to breakage of the oxide and subsequent plastic deformation. Other authors have associated pop-in events with the formation of cracks (e.g., in brittle ceramics4) or shear bands (e.g., in metallic glasses5–8), or with the breakaway of dislocations from solute pinning points (e.g., in solid solution Al-alloys9,10). For plastic crystalline materials the most common interpretation for pop-in events, especially the first pop-in associated with the elastic-plastic transition, is the nucleation of dislocations.11–16 Atomistic simulations have also corroborated this interpretation for several different metals,17–21 illustrating that pop-in signals in the load−displacement curve correlate with discrete nucleation of dislocations beneath an indenter. One aspect of incipient plasticity that has not been elucidated in detail is the rate or time dependence of the pop-in phenomenon. For pure metals without significant surface films, where pop-in signals the nucleation of dislocations, one might expect a significant dependence on time and temperature since defect nucleation is thermally activated.2,22,23 There are several prior reports that suggest this kind of effect, beginning with the work of Gerberich et al.,3 who observed that at subcritical loads (below the first pop-in event), cyclic loading eventually
DOI: 10.1557/JMR.2004.0276 2152
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 7, Jul 2004 Downloaded: 09 Apr 2015
caused a displacement burst. Syed-Asif and Pethica24 indented GaAs at very low loads below the first displacement burst, and observed that pop-in occurred after a delayed period of holding in the elastic regime. Similar behavior has since been reported in tungsten2 and nickel aluminides.23,25 These observations suggest that even at subcritical loads, pop-in displacements can be triggered if sufficient time is allowed for a favorable thermal vibration to induce defect nucleation. The recent ratedependent observations of Wang et al.25 support this view, with pop-in being delayed by rapid indentation velocities. The above observations all
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