The strain-rate dependence of the nanoindentation stress of gold at 300 K: A deformation kinetics-based approach
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Indentation tests involving a constant-loading rate stage followed by a constant-load stage were performed on annealed and 20% cold-worked Au to investigate the effect of indentation depth and initial dislocation density on the indentation deformation process. The indentation strain rate data were analyzed in terms of an obstacle-limited dislocation glide mechanism. The apparent activation energy was of the order of 0.16 mb3 and was neither a function of initial indentation depth nor cold work. The results of Haasen plot activation analysis and direct transmission electron microscopy (TEM) observations indicate that more mechanical work must be applied during the constant-loading rate stage due to the large amount of work hardening compared with the constant-load stage where considerably more dislocation recovery occurs. I. INTRODUCTION
Modern nanoindentation equipment has enabled researchers to probe the mechanical properties of materials at the micron and submicron length scales.1–4 This has resulted in an extensive analysis of the effects of indentation depth on the hardness of a variety of metals5–17; however, investigation of the effect of indentation strain rate upon the average indentation stress at different levels of indentation depth has received considerably less attention.18–23 It is widely accepted that because the magnitude of the local stress around the indentation is large, the underlying deformation mechanism, in most ductile metals, involves an obstacle-limited dislocation glide process.20–26 However, in cases where very small depth nanoindentation tests are performed, the actual operative plastic deformation mechanism may change. For example, Li and Ngan21 studied the size effects of nanoindentation creep in a range of materials including aluminum and Ni3Al and reported that the operative deformation mechanism changes from diffusional creep for shallow indents to dislocation glide-limited creep for deeper submicron scale indents. It is possible that, due to the limited number of available easy-slip systems to accommodate the deformation around submicron indentations, the activation energy associated with the deformation process may change with indentation depth even in the region where deformation occurs by obstacle-limited dislocation glide. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0161
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 4, Apr 2009 Downloaded: 13 Mar 2015
A recent investigation of the dependence of the average equivalent indentation shear strain rate g_ ind upon indentation depth during constant-load indentation creep of polycrystalline Au at 300 K indicate that although the average indentation stress, sind_t=0, at the start of the creep test is sensitive to indentation depth, the apparent activation energy of the deformation process is essentially independent of depth.23 This suggests that, for pure Au at 300 K, g_ ind is governed by dislocation-dislocation interactions at indentation depth from 100 to 5000 n
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