Investigation of Geometric Necessary Dislocation and Indenter Size Effect Using Atomistic Simulation
Date Issued
2012
Date
2012
Author(s)
Chen, Yi-Pin
Abstract
Nanoindentaiton is the most useful test method to probe the strength of materials that are manufactured at micro or nano scales. Unlike the continuum behavior, the mechanical properties exhibit a strong dependency with characteristic length scale, which is also referring to the nanoindentation size effect. Nix and Gao proposed the strain gradient plasticity theory to interpret the size effect by introducing a geometrically necessary dislocation density to overcome the strain incompatibility. Atomistic simulations were conducted to elucidate the relationship between size effect and the geometrically necessary dislocation density in this study.
In this study, spherical indenters with their radius from 20A to 100A and Berkovich indenter were exploited to examine the FCC single crystal thin firm of Nickel. Hardness was directly obtained from the atomistic simulation that hardness is inversely proportion to the square root of indenter radius and indentation depth respectively. The findings agree with the well-known strain gradient theory.
In order to calculate the geometry necessary dislocation density, an equivalent plastic zone size was chose to meet the theoretic requirement. In present work, diverse radius of spherical indenter and Berkovich indenter indicated that the geometry necessary dislocation density showed a great agreement with the theory proposed by Swadener et al and Nix&Gao respectively. The hardness and geometric necessary dislocation density extracted directly from atomistic simulation were both agreed with the theory. It can be concluded that the strain gradient plasticity of size effect and geometric necessary dislocation density were valid at atomistic scale.
Subjects
Nano indentation size effect
Strain gradient plasticity
Geometrically necessary dislocation density
Type
thesis
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