High Temperature Nanoindentation for the Study of Flow Defects
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High Temperature Nanoindentation for the Study of Flow Defects C. A. Schuh1, J. K. Mason1, A. C. Lund1 and A. M. Hodge2 1 Massachusetts Institute of Technology, Cambridge, MA 2 Lawrence Livermore National Laboratory, Livermore, CA ABSTRACT Our recent progress in elevated temperature nanoindentation is reviewed, with an emphasis on the study of discrete events (i.e., pop-in phenomena) observed during nanoindentation. For crystalline materials the incipient plasticity problem is associated with the nucleation of dislocations, an effect which we show to be significantly temperature dependent. For metallic glasses it is the operation of individual shear bands beneath the indenter that gives rise to pop-in events; here we also show this to be a temperature dependent phenomenon. Approaches to extract the activation volume and energy of defects involved in plastic flow beneath the indenter are also briefly described. INTRODUCTION The advent of instrumented nanoindentation has allowed the study of deformation physics in confined volumes, where the nucleation and motion of individual flow defects can be resolved. For example, the departure from purely elastic contact is usually associated with short burst of displacement (a ‘pop-in’ event) in the load-displacement (P-h) curve [1-4]. For nanoindentation of crystalline metal surfaces, there is a growing consensus that this initial pop-in is associated with homogeneous dislocation nucleation, while subsequent similar events often involve avalanches of dislocation activity. In metallic glasses a similar phenomenon is observed, but instead of dislocation activity occurring beneath the indenter, strain is accommodated via the nucleation and propagation of shear bands [5, 6]. The nucleation and motion of both dislocations and shear bands are understood to be thermallyactivated and stress-biased processes. Accordingly, one would expect significant variations in the measured P-h response with changes in indentation rate or temperature. Where some studies have observed a time or rate dependence of the pop-in phenomenon (see, e.g., [3, 6, 7]), temperature variations should also induce obvious changes in experimental P-h curves; elevated temperature nanoindentation testing could provide important experimental input to understanding the behavior of flow defects. Although some prior authors have employed nanoindentation at elevated temperatures, the study of pop-in phenomena at high temperatures has received little attention [8-13]. The purpose of this paper is to review our recent progress on nanoindentation testing at elevated temperatures, with a focus on pop-in phenomena in single crystals and metallic glasses. HIGH TEMPERATURE NANOINDENTATION Our experimental approach is based upon the Triboindenter instrumented nanoindentation platform (from Hysitron, Inc., Minneapolis, MN), which has several unique features suited for elevated temperature testing. First, discrete flow events are well-resolved in the Triboindenter, due to rapid instrument response and data acquisition rates
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