Dislocation Nucleation and Source Activation during Nanoindentation Yield Points
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INTRODUCTION
INSTRUMENTED indentation experiments are a standard method to measure the mechanical properties of small volumes of materials, particularly properties such as hardness and elastic modulus. An indenting tip made of a hard material and having a defined geometrical shape (often assumed to be spherical during the initial loading) is placed in contact with the surface of the material being studied. In some metals, the initial indentation behavior is completely elastic, with fully reversible loading. At some point, as the load increases, the material undergoes irreversible plastic deformation that, in load-controlled-instrumented indentation, manifests as a ‘‘pop-in’’ or excursion in depth. Gane and Bowden were the first to observe the excursion phenomena on an electropolished surface of gold and also on copper and aluminum surfaces.[1] A fine tip was pressed on the gold surface, but no permanent penetration was observed until some critical load was reached. Pethica and Tabor, using an electrical resistance technique, observed that a material could withstand high stresses, possibly reaching the theoretical stress, in the presence of an oxide layer on top of the indented surface.[2]
A.A. ZBIB, Graduate Student, and D.F. BAHR, Professor, are with the School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920, United States. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. GerberichÕs 70th Birthday,’’ which occurred during the TMS Annual Meeting, March 12–16, 2006 in San Antonio, Texas and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS. Article published online July 31, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
With increasing development of load and depth sensing technology, it is possible to clearly identify the onset of plasticity during indentation by analyzing the loaddisplacement curve. Many experiments have observed that the excursions or the discontinuity in the load-depth curves corresponds to the transition between elastic and plastic behavior of the material. These observations were thus related to dislocation motion in general, and in particular to dislocation nucleation[3–7] or to dislocation source activation.[8,9] One of the major challenges in assessing the behavior of the elastic-plastic transition during nanoindentation is the difficulty in ascribing the behavior to homogeneous dislocation nucleation,[10] the activation of well-spaced dislocation sources, the activation of a point defect source (i.e., a vacancy),[9] and the tensile fracture of a surface film,[11–14] or some other surface film relationship with the underlying dislocation structure.[15] For instance, in a well-annealed metallic single crystal, it is not unreasonable to assume a dislocation density on the order of 1011 m-2. Slight strain and deformation in processing (particularly in grinding and polishing samp
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