Temperature dependence of the indentation size effect

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Citation

Franke, Oliver, Jonathan C. Trenkle, and Christopher A. Schuh. “Temperature Dependence of the Indentation Size Effect.” Journal of Materials Research 25.07 (2011): 1225–1229.

As Published

http://dx.doi.org/10.1557/jmr.2010.0159

Publisher

Cambridge University Press/Materials Research Society

Version

Author's final manuscript

Accessed

Thu May 30 04:58:08 EDT 2013

Citable Link

http://hdl.handle.net/1721.1/69887

Terms of Use

Creative Commons Attribution-Noncommercial-Share Alike 3.0

Detailed Terms

http://creativecommons.org/licenses/by-nc-sa/3.0/

Temperature dependence of the indentation size effect Oliver Franke, Jonathan C. Trenkle, and Christopher A. Schuh

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Abstract: The influence of temperature on the indentation size effect is explored experimentally. Copper is indented on a custom-built high-temperature nanoindenter, at temperatures between ambient and 200 ⁰C, in an inert atmosphere that precludes oxidation. Over this range of temperatures, the size effect is reduced considerably, suggesting that thermal activation plays a major role in determining the length scale for plasticity. Keywords: nanoindentation; hardness; high temperature deformation; work hardening; thermally activated processes

1

The indentation size effect (ISE) is a widely discussed phenomenon where the measured hardness of a crystalline material increases as the indentation depth is reduced 1-8. The ISE becomes more pronounced at very low displacements into the surface, such as are usually encountered in nanoindentation. The physical origins of the ISE are generally believed to lie in the strain gradients produced beneath the indentation and the associated geometrically necessary dislocations (GNDs). While strain gradient plasticity was extensively developed in the works of Fleck and Hutchinson

9-13

, the ISE was first

addressed in this context by Nix and Gao 7. Their work provided the basic explanation for the ISE as resulting from the surplus of dislocations needed to accommodate the impression geometry at low displacements. Support for this notion is provided by experiments showing that there is no ISE for materials with pre-existing high dislocation densities (such as ultra-fine grained materials produced by severe plastic deformation 3) or for amorphous materials (such as e.g. the fused quartz samples used to calibrate nanoindenters

14-15

).

More recent studies on the ISE by Durst and co-workers

1-4,16

have

emphasized how the size of the plastic zone exerts a significant influence on the ISE. Although there seems to be some consensus that the distribution of dislocations is central to the observation of the ISE, studies that specifically vary the dislocation properties (i.e. mobility, stacking fault width, etc.) and examine the result on the ISE are very few. For example, we

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