Atomistic Study for Geometrically Necessary Dislocation and Nanoindentation Size Effects
| Autor: | Kuan-Po Lin, 林冠伯 |
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| Rok vydání: | 2015 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 103 Indentation experiment is one of the most useful method to probe the strength of materials that are manufactured at micro or nano scales. When indentation depth decrease to micro meters, the hardness increase as the indentation depth decrease. It is known as the indentation size effect. Nix and Gao present a theoretical model to explain the indentation size effect in microindentation. However, it overestimate the hardness in nanoindentation. For years, many studies tried to explain and modify Nix and Gao model for nanoindentation with free parameters. In this study, a method is presented to measure the dislocation density directly in atomistic simulation, and using the Taylor dislocation theory to compare with the hardness from atomistic simulation. Atomistic simulation were conducted to elucidate the relationship between size effect and the geometrically necessary dislocation density. In this study, spherical indenters with their radius from 20Å to 60Å was exploited to examine the FCC single crystal thin film of nickel, copper and gold. Indentation experiments methods of measuring dislocation density and hardness also works well in atomistic simulation. Geometrically necessary dislocation density measuring by our method works well in simulation if the size of the model is large enough to ignore the boundary effects. In present work, diverse radius of spherical indenter indicated that the geometry necessary dislocation density is much smaller than the theory proposed by Swadener et al., which is in agreement of the findings of Feng[1]. Hardness derived by dislocation density is close to the hardness directly computed from atomistic simulation. Hardness directly from simulation is inversely proportion to the square root of indenter radius which is agreed with the theory of strain gradient plasticity. It can be concluded that the strain gradient plasticity of size effect and geometric necessary dislocation density were valid at atomistic scale. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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