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This dissertation, The Nanoindentation Size Effects of Creep by Han, Li, 李晗, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled: The Nanoindentation Size Effects of Creep Submitted by LI, Han for the degree of Master of Philosophy at The University of Hong Kong in December, 2004 Nanoindentation provides convenient and localized measurements of the mechanical properties of materials, and is an especially useful technique for the study of thin films and materials of small volumes. Previous investigations showed that the submicron contacts with the rigid indenting punch could yield creep at an enormous speed at low homologous temperatures, where the same material in bulk condition would normally not exhibit any creep. The influence of this time dependent deformation on the calculation of material properties has been investigated, and compensation methods have been proposed, but the fundamental mechanisms leading to such a high creep rate remain unknown. It would also be interesting to establish if creep behavior and its mechanisms change as the indent size diminishes towards incipient plasticity, and if so, how. The aim of this study is therefore to investigate the characteristics and mechanisms of the nanoindentation creep in different materials over a wide range of indent sizes (from several nm to above one micron). A depth-sensing, constant load creep test was used to measure the stress exponent, chosen to quantify indentation creep behavior, at different indentation sizes on both Ni Al (111) single crystal, polycrystalline Al, amorphous fused quartz and nanocrystalline Ni-at25%Al alloy films with a spectrum of grain sizes. bExperimental results demonstrated that the stress exponents in all materials increased significantly as the indent size increased, exhibiting an intense size effect. The near unity stress exponent of Al in the smallest indent suggests that the creep mechanism is likely to be diffusional flow, and a simple pipe-diffusion model is proposed as a theoretical explanation. For the amorphous fused quartz, the size effect on the stress exponent can be explained by a reduction of the localized shear volume as the indent size decreases. For the nanocrystalline thin films with different average grain sizes, the stress exponent was also found to increase with grain size at the same nominal peak load. This is ascribed to the operation of increasing dislocation activities near grain boundaries at larger grain sizes. A preliminary study of the initial contact behavior of a nanocrystalline film during nanoindentation is also presented. It is well known that single crystals with low defect densities can support stresses near the theoretical limit without yielding, and incipient plasticity is usually manifested in the indentation curve as a sudden displacement burst, associated with dislocation nucleation and multiplication. The initial contact deformation of nanocrystalline materials appears to be more complicated and less well documented. Through nanometer scale indentation tests on a nanocrystalline Ni-25at.%Al alloy thin film, three representative deformation modes were observed by immediate pre and post-indentation atomic force microscopy. One exhibited near elastic deformation, with elastic modulus equal to the upper limit for all loads and even close to the bulk value. In the other two modes, the deformation was elastoplastic from the very beginning. Strain burst was rarely observed. 473 words (main body) DOI: 10.5353/th_b3069638 Subje

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