Dislocation Microstructure and Surface Roughness Evolution in Single and Multi-phase Microcrystals

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Date
2015-08-20
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Johns Hopkins University
Abstract
A better understanding of the plastic deformation of metallic materials and their failure can greatly accelerate future material development. However, attaining such an understanding is difficult through only experimental investigations and/or simple analytical models. Recent advances in coarse grained simulations using large scale discrete dislocation dynamics (DDD) simulations provide detailed spatio-temporal descriptions of the evolution of complex dislocation networks in crystals having arbitrary microstructures and initial defect distributions, and under different loading conditions. In this thesis, a DDD framework has been developed and utilized to investigate the evolution of dislocation microstructures and surface roughness in single and multi-phase metal microcrystals. First, three atomistically identified cross-slip mechanisms, namely, bulk, intersection and surface cross-slip, have been implemented in the DDD framework and utilized to study monotonic axial deformation in single crystal Nickel microcrystals of different sizes and initial dislocation densities. It is concluded that cross-slip leads to significant dislocation density multiplication, strain hardening, the formation of dislocation cell-like structures in larger crystals and the formation and thickening of slip bands on the crystal surface. Surface cross-slip was found to be the most frequent, followed by intersection and then bulk cross-slip. These simulations are also shown to be in agreement with recent microcompression experimental observations in both Nickel and Aluminium microcrystals. Second, the evolution of dislocation microstructure and surface roughness during cyclic mechanical loading was investigated. The cyclic response was divided into two separate stages: the early stage response starting from a random dislocation microstructure, and the response of crystals having well-developed Persistent Slip Band (PSB). In random dislocation microstructure simulations, crystals having sizes larger that 5 μm showed cyclic hardening, significant dislocation density multiplication and the formation of dislocation cell-like structures. The evolution of surface roughness due to dislocations escape has been quantified. The atomic concentration of point defects generated from dislocation annihilation in PSB walls was quantified and was found to agree with experimental estimates. Finally, the DDD framework was extended to model the interaction of dislocations with an arbitrary distribution of precipitates that have an L12 crystal structure by explicitly tracking the creation and destruction of Anti-Phase Boundary (APB) regions. This framework was then utilized to perform the first DDD simulations of single crystal superalloy micropillars with different precipitate microstructures. The micropillar strength was found to vary linearly, follow a square-root relationship and an inverse square-root relationship with the precipitate volume fraction, APB energy and size respectively while the crystal size has no effect on its strength. The width of the inter-precipitate channels was found to be the main strength determining factor. The results were validated with detailed comparisons with recent microcompression experiments on single crystal Nickel-based superalloys.
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Keywords
Dislocation Dynamics, Plasticity, Crack Nucleation, Dislocation Microstructure Evolution, Surface Roughness Evolution, Dislocation Cross-Slip, Nickel Base Superalloys
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