Dislocation mechanisms of radius effect on displacement bursts during spherical nanoindentations
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Sanghoon Shim Steel Structure Research Laboratory, Research Institute of Industrial Science and Technology, Gyunngi-do, 445-813, Korea (Received 23 September 2011; accepted 10 May 2012)
Indentation load–displacement curves for Mo (100) single crystals reveal clear displacement bursts from spherical indenters with various radii from ;0.1 to ;130 lm. There are two different size-dependent mechanisms for dislocation evolution involved during the displacement bursts. It has been postulated that these bursts are triggered by the nucleation of dislocations for a small indenter radius and the activation of preexisting dislocations for a large indenter radius. We present a simple model with which the displacement bursts from a larger indenter radius can be rationalized. This model relates the load and the excursion length during the first displacement burst. The correspondence between the model and experimental data indicates that the displacement bursts are initiated by the activation of preexisting dislocations and the model can accurately describe the mechanism for the displacement bursts from large indenters. I. INTRODUCTION
Nanoindentation probes small volumes of material and measures the mechanical properties with nanoscale precision.1 Nanoindentation contributes significantly to the investigation of plasticity at decreasing scales of interest.2,3 Small-scale contacts are frequent during the fabrication and/or the operation of microelectronic and photonic as well as micro-electromechanical systems (MEMS) devices.4 The contact size can vary depending on the asperities of the two surfaces in contact. The understanding of plasticity at various contact sizes is hence important not only from a theoretical viewpoint but also for a practical application. In a nanoindentation on a well-prepared surface of crystalline materials (without defect or oxide contamination), indentation load–displacement (P–h) curves initially show an elastic deformation response up to a certain critical indentation load (Pc). Above Pc, the P–h curves show sudden increases in an indenter displacement (bursts) or sharp decreases in a load (dips) depending on load-controlled or displacement-controlled tests, respectively.5 These phenomena have been believed to indicate dislocation nucleation and propagation within a defect-free zone, which induces a sudden transition from elastic to plastic deformation.6 Thus, it has so far been assumed that the displacement bursts (pop-ins) are only probable when no mobile dislocations exist in the volume beneath an indenter.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.183 J. Mater. Res., Vol. 27, No. 16, Aug 28, 2012
Recently, Shim et al.7 reported a new type of indentation size effect (ISE) in single crystal Ni. Conventional ISE in the indentation literature is based on hardness, which is found to increase with decreasing indentation depth for Berkovich indenters8 and decreasing indenter radius for spherical indenters.9 The new type of ISE discovered by Shim et a
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