Download or read book INVESTIGATION OF THE DEFORMATI written by Pui-Ching Wo and published by Open Dissertation Press. This book was released on 2017-01-26 with total page 240 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation, "An Investigation of the Deformation Behaviour of Ni3AI Using Nanoindentation and Nanoscratch Methods" by Pui-ching, Wo, 胡佩晶, 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 An Investigation of the Deformation Behaviour of Ni Al Using Nanoindentation and Nanoscratch Methods submitted by Wo Pui Ching for the degree of Doctor of Philosophy at the University of Hong Kong in August 2005 Rapid advancements in micro- and nano-technology have made it vital for scientists to understand the mechanical behaviour of materials at the submicron length scale. The recent invention of nanoindenter and nanoscratch testers enables convenient testing of submicron volume deformation. Mechanical properties such as initial yield and subsequent plastic flow are found to exhibit size dependency and behave stochastically. The recent discovery that well-annealed crystal cannot sustain elastic loading indefinitely has re-focused attention on the origin and mechanism of incipient plasticity at the sub-micron length scale. In this study, constant-load nanoindentations were performed within a single Ni Al grain. The waiting time for plastic deformation after prolonged elastic contact was found to be statistically distributed. At high applied loads, the waiting time distribution was exponentially decaying, so that longer waiting times were rarer. This form of distribution is consistent with the picture of homogeneous nucleation of dislocations. At lower applied loads, peaked Poisson-like distributions were observed, and the most probable waiting time (the peak) decreased as the load increased. This form of distribution suggests that the mechanism is more complicated than homogeneous nucleation of dislocations. A mechanism which involves the climb growth of an atomic sized dislocation loop is proposed. i The yielding behaviour with an additional tangential traction is another interesting issue. In this study, constant-load scratching was performed after a period of static elastic load application. Within a narrow range of normal loads between elastic and plastic behaviour, incipient plasticity was observed during scratching. At this intermediate load, initial yielding was sensitive to spatial inhomogeneity, and was influenced by the elastically deformed volume prior to scratching. Plastic deformation on nanoscratches and cube-corner nanoindents were observed through a transmission electron microscope. The cube-corner indents were found to be surrounded by dense dislocation clouds, which were larger than the Oliver-Pharr equation would lead us to expect. This provides direct evidence for the hypothesis, already argued in several studies, that the breakdown of the Nix-Gao model for indentation size effect for hardness is due to the expansion of the geometrically-necessary-dislocation zone. Slip transmission behaviour at grain boundaries was also studied. Nanoindentations were performed at various distances from some selected grain boundary segments in pure and boron-doped Ni Al. Hardness was found to be ineffective in probing the intergranular slip transmission behaviour in undoped samples. However, topographical information of the indents in pure Ni Al obtained from SEM and AFM examination revealed a strong correlation between the intergranular slip behaviour with the misorientation between the slip systems across the grain boundary. This relationship was not observed in boron-doped Ni Al, probably due to preferential boron segrega

Download or read book An Investigation of the Deformation Behaviour of Ni3AI Using Nanoindentation and Nanoscratch Methods written by Pui-ching Wo and published by . This book was released on 2005 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Investigation Into Nanoindentation and Nanoscratch Behavior in Oxide Ceramics written by Lin Huang and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanoindentation and nanoscratch behavior has been systematically investigated in ultrafine-grained alumina ceramics, single crystal sapphire wafers and nanocrystalline ceramics, including yttria, calcium-doped-yttria, magnesia, yttria-stablized-zirconia and magnesia-yttria ceramic nanocomposites by using a Berkovich nanoindenter XPS system in indentation and scratch modes under ramp loading conditions, respectively. The dense ceramic samples were fabricated by conventional spark plasma sintering and high-pressure spark plasma sintering. For alumina ceramics, the nanoindentation and nanoscratch results suggest that grain refinement to the ultrafine grain size regime can improve both the mechanical behavior such as hardness, modulus and dynamic surface mechanical properties, and plastic deformation capability at the nano- and micro-scales. The evolution of plastic deformation in the M-plane sapphire sample during the nanoscratch process includes lattice disorder, dislocation loops, stacking faults on the basal plane, dislocation glide and finally basal twin formation from TEM characterization. Microcracks, suggesting brittle fracture, increased in frequency at the edge of the scratch groove in the high-load region. Crack deflection is observed due to the transformation of Mode II dominant fracture to Mode I dominant fracture. Elastic and residual stress field solutions have been utilized to explain the observed microstructural features. Intriguing plastic deformation features in terms of strain flow maps surrounding nanoindentation imprints and nanoscratch grooves have been observed in a dense, high purity nanocrystalline yttria ceramic sample. Investigation into the strain rate sensitivity reveals that the plastic deformation is primarily driven by dislocation activity. Grain size dependence on nanoscratch behavior in the nanocrystalline yttria, calcium-doped-yttria and yttria-stablized zirconia ceramic samples indicates that the samples with finer grain size show the higher potential for plastic deformation despite different crystal structures.

Download or read book Fundamentals of Nanoindentation and Nanotribology IV Volume 1049 written by Materials Research Society. Meeting and published by . This book was released on 2008-05-15 with total page 208 pages. Available in PDF, EPUB and Kindle. Book excerpt: The MRS Symposium Proceeding series is an internationally recognised reference suitable for researchers and practitioners. This book is a snapshot of the state of the art in nanoindentation and nanotribology, and highlights emerging topics including the development of new methods for characterizing nanoscale mechanical and tribological properties.

Download or read book Study of Deformation Behavior of Nanocrystalline Nickel Using Nanoindentation Techniques written by Changli Wang and published by . This book was released on 2010 with total page 94 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanocrystalline materials with grain size less than 100 nm have been receiving much attention because of their unparallel properties compared with their microcrystalline counterparts. Because of its high hardness, nanocrystalline nickel has been used for MEMS. Long term thermomechnical properties and deformation mechanism at both ambient and elevated temperatures need to be evaluated which is vital for reliability of its applications as structural material. In this thesis, nanoindentation creep of nanocrystalline nickel with an as-deposited grain size of 14 nm was characterized at elevated temperatures. The nanoindentation creep rate was observed to scale with temperature and applied load (or stress), and could be expressed by an empirical power-law equation for describing conventional crystalline solids. Creep activation energy was found to be close to that for grain boundary self-diffusion in nickel. The activation volume was also evaluated using a stress relaxation technique. The creep results were compared with those for fine-grained nickel in the literature. Possible mechanisms were discussed in light of the creep rate and temperature ranges. To provide a direct comparison, uniaxial creep tests were conducted on nanocrystalline nickel with an as-deposited grain size of 14 nm at 398 K. It was found that stress exponents under the two test conditions are almost the same, indicating a similar creep mechanism. However, the strain rate measured by nanoindentation creep was about 100 times faster than that by uniaxial creep. The rate difference was discussed in terms of stress states and the appropriate selection of Tabor factor. To further explore the time-dependent plastic behavior, multiple unload-reload tests were conducted on electrodeposited nanocrystalline nickel in both compression and tension. A hysteresis was observed during each unload-reload cycle, indicating irreversible energy dissipation. The dissipated energy was evaluated and the energy dissipation rate was found to increase with the flow stress to the third power and sensitive to the stress state (tension or compression). A mechanistic model based on grain boundary sliding was proposed to describe the unload-reload behavior. Experimental results were found to be in good agreement with the model predictions, suggesting the observed hysteresis was indeed caused by grain boundary sliding.