The Contribution of Dislocation Density and Velocity to the Strain Rate and Size Effect Using Transient Indentation Meth
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THE tendency for some metals, especially FCC metals, to exhibit a size effect under specific loading conditions including indentation and torsion has been the focus of intensive research to understand the fundamental mechanisms that control this phenomenon.[1–5] The indentation size effect (ISE) is widely accepted to be governed by dislocation-based mechanisms in crystalline materials, whereas in bulk metallic glasses, ceramics, and semiconductor materials, the size effect is controlled by non-dislocation mechanisms, phase and shear transformations, and cracking.[6–9] Although an ISE is unlikely to occur in glasses and
D.E. STEGALL and A.A. ELMUSTAFA are with the Department of Mechanical and Aerospace Engineering, Old Dominion University, 5115 Hampton Boulevard, Norfolk, VA, 23529 and also with the Applied Research Center, Thomas Jefferson National Accelerator Facility, Newport News, VA, 23606. Contact e-mail: aelmusta@ odu.edu Manuscript submitted February 27, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
ceramics,[10,11] interestingly, other researchers have concluded that an ISE in bulk metallic glasses and ceramic materials exists.[12–16] In crystalline materials, the ISE is caused by the accumulation of geometrically necessary dislocations (GNDs) resulting from a large strain gradient for shallow indents.[17,18] For single crystal metals and alloys, transmission electron microscopy (TEM) and electron beam scattering diffraction (EBSD) techniques were extensively used to scan and examine deformation maps within the plastic zone.[19–23] At smaller indentation depths, the hardness may decrease with the increasing indentation depth due to the classical ISE where the motion of dislocations nucleated below the indenter is not influenced by the surrounding interfaces.[24] It is therefore understood that the ISE is not influenced by grain size for polycrystalline materials due to the nucleation of dislocations that interact directly with the surrounding interfaces below the indenter tip for shallow depths of indentation.[24] Yang and Vehoff[24] observed in their experiments of nanocrystalline nickel that the dislocations, which nucleate below the indenter tip, only interact directly with the surrounding interfaces for grains below 900 nm,
where the nanohardness and pop-in width are grain size dependent. From EBSD kernal average misorientation maps, regions with large orientation gradients typically indicate significant lattice curvature, and hence high density of GNDs which are key to the presence of an ISE.[20] Initially, Elmustafa and Stone[25,26] hypothesized that the ISE was influenced by the SFE, which led to their testing of an FCC in Al (high SFE) and an alloy in alpha brasses (low SFE) materials. The stacking fault energy (SFE) of a material is directly related to its capacity to work harden. FCC materials with low SFE such as Ag work harden faster than materials with high SFE such as aluminum. Elmustafa and Stone (2003) investigated the possible effect of SFE on the magnitude of the ISE for aluminum (high SFE) an
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