Initial stages of yield in nanoindentation
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Initial stages of yield in nanoindentation J. D. Kielya) Sandia National Laboratories, Albuquerque, New Mexico 87185-1413
K. F. Jarausch North Carolina State University, Raleigh, North Carolina 27695-7531
J. E. Houston Sandia National Laboratories, Albuquerque, New Mexico 87185-1413
P. E. Russell North Carolina State University, Raleigh, North Carolina 27695-7531 (Received 1 September 1998; accepted 2 March 1999)
We have used the interfacial force microscope to perform nanoindentations on Au single-crystal surfaces. We have observed two distinct regimes of plastic deformation, which are distinguished by the magnitude of discontinuities in load relaxation. At lower stresses, relaxation occurs in small deviations from elastic behavior, while at the higher stresses they take the form of large load drops, often resulting in complete relaxation of the applied load. These major events create a relatively wide plastic zone that subsequently deepens more rapidly than it widens. We discuss these findings in terms of contrasting models of dislocation processes in the two regimes.
I. INTRODUCTION
One of the challenges of nanoindentation is developing an understanding of the detailed processes responsible for initial plastic yield. At larger scales of indentation, a number of models exist for plastic flow,1–5 but as experimental scales shrink, properties change,6–9 and these models must be reconsidered. It is clear that obtaining a detailed understanding of the atomic-level processes that contribute to the initial nucleation of dislocations, their motion and multiplication will greatly facilitate the design of tailored materials with specific mechanical properties. Two well-documented observations from nanoindentation experiments6,8–12 illustrate the potential of this technique for providing fundamental insight into initial yield process: (i) that single crystals with low defect density can support stresses near the theoretical limit without yielding and (ii) initial plasticity is usually observed as a sudden deviation from elastic behavior. Stresses near the theoretical limit can be supported because nanoindentation is an effective method of probing a volume sufficiently small that defects are unlikely. The sudden deviation from elastic loading, termed the “yield point,” has been attributed to the nucleation, multiplication, motion, and pileup of dislocations.9–14 Nanoindentation has also been established as a technique for measuring time-dependent deformation. It has a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 6, Jun 1999
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been shown that there is some time dependence to the phenomenon of the yield point,15 suggesting that creep of the material under the indenter may contribute to the onset of plasticity. Diffusion of material from beneath the indenter has been reported,16,17 which also suggests that surface diffusion within the contact area occurs and may contr
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