With decreasing system sizes, the mechanical properties and dominant deformation mechanisms of metals change. For larger scales, bulk behavior is observed that is characterized by a preservation and significant increase of dislocation content during deformation whereas at the submicron scale very localized dislocation activity as well as dislocation starvation is observed. In the transition regime it is not clear how the dislocation content is built up. This dislocation storage regime and its underlying physical mechanisms are still an open field of research. In this paper, the microstructure evolution of single crystalline copper micropillars with a $\langle1\,1\,0\rangle$ crystal orientation and varying sizes between $1$ to $10\,\mu\mathrm{m}$ is analysed under compression loading. Experimental in situ HR-EBSD measurements as well as 3d continuum dislocation dynamics simulations are presented. The experimental results provide insights into the material deformation and evolution of dislocation structures during continuous loading. This is complemented by the simulation of the dislocation density evolution considering dislocation dynamics, interactions, and reactions of the individual slip systems providing direct access to these quantities. Results are presented that show, how the plastic deformation of the material takes place and how the different slip systems are involved. A central finding is, that an increasing amount of GND density is stored in the system during loading that is located dominantly on the slip systems that are not mainly responsible for the production of plastic slip. This might be a characteristic feature of the considered size regime that has direct impact on further dislocation network formation and the corresponding contribution to plastic hardening.

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Microstructure evolution of compressed micropillars investigated by in situ HR-EBSD analysis and dislocation density simulations

Influence of size effect on the dynamic mechanical properties of OFHC copper at micro-/meso-scales

• Generalized constitutive relations that incorporate the thermally activated and drag mechanisms. • Continuum description of crystal plasticity framework combined with the geometrically necessary dislocation densities and the mobile/immobile statistically stored dislocation densities. • Computation of the geometrically necessary dislocation densities based on the L 1 -norm minimization scheme to provide physical interpretation of the lower bound on overall GND population. • Dislocation mechanisms depending on generation, annihilation, interactions, trapping, and recovery. • Evolution laws of dislocation densities with mechanism-based material parameters. A new nonlocal crystal plasticity model is developed to study the mechanical behavior of face-centered cubic single crystals under heterogeneous inelastic deformation through a crystal plasticity finite element method. The main feature of this work is generalized constitutive relations that incorporate the thermally activated and drag mechanisms to cover different kinetics of viscoplastic flow in metals at a variety of ranges of stresses and strain rates. The constitutive laws are founded upon integrating continuum description of crystal plasticity framework with dislocation densities which is relevant to the geometrically necessary dislocation densities and the statistically stored dislocation densities. The model describes the plastic flow and the yielding of face-centered cubic single-crystal employing evolution laws of dislocation densities with mechanism-based material parameters passed from experiments or small-scale computational models. The geometrically necessary dislocation densities evolve on account of the curl of the plastic deformation gradient where its associated closure failure of the Burgers circuit exists. A minimization scheme termed L 1 -norm is utilized to secure the physical values of the geometrically necessary dislocation densities on slip systems. The evolution equations of statistically stored dislocation densities describe the complex interactions between two distinct dislocation populations, mobile, and immobile statistically stored dislocation dislocations, relying on generation, annihilation, interactions, trapping, and recovery. The experiments of a micropillar compression for the copper single crystal are compared to the computational results obtained using the formulation. The physics-based model clarifies the complex microstructural evolution of dislocation densities in metals and alloys, allowing for more accurate prediction. . 

A Physics-based crystal plasticity model for the prediction of the dislocation densities in micropillar compression

https://comptes-rendus.academie-sciences.fr/physique/item/10.5802/crphys.55.pdf

Stress state mechanism of thickness debit effect in creep performances of a Ni-based single crystal superalloy

Analysis of single crystalline microwires under torsion using a dislocation-based continuum formulation

Classification of slip system interaction in microwires under torsion

Plastic deformation of metals involves the complex evolution of dislocations forming strongly connected dislocation networks. These dislocation networks are based on dislocation reactions, which can form junctions during the interactions of different slip systems. Extracting the fundamentals of the network behaviour during plastic deformation by adequate physically based theories is essential for crystal plasticity models. In this work, we demonstrate how knowledge from discrete dislocation dynamics simulations to continuum-based formulations can be transferred by applying a physically based dislocation network evolution theory. By using data-driven methods, we validate a slip system dependent rate formulation of network evolution. We analyse different discrete dislocation dynamics simulation data sets of face-centred cubic single-crystals in high symmetric and non-high symmetric orientations under uniaxial tensile loading. Here, we focus on the reaction evolution during stage II plastic deformation. Our physically based model for network evolution depends on the plastic shear rate and the dislocation travel distance described by the dislocation density. We reveal a dependence of the reaction kinetics on the crystal orientation and the activity of the interacting slip systems, which can be described by the Schmid factor. It has been found, that the generation of new reaction density is mainly driven by active slip systems. However, the deposition of generated reaction density is not necessarily dependent on the slip system activity of the considered slip system, i.e. we observe a deposition of reaction density on inactive slip systems especially for glissile and coplanar reactions. 

/pdf/identification-of-dislocation-reaction-kinetics-in-complex-3sekf2hw.pdf

Kolja Zoller

Papers

Analysis of single crystalline microwires under torsion using a dislocation-based continuum formulation

Microstructure evolution of compressed micropillars investigated by in situ HR-EBSD analysis and dislocation density simulations

Microstructure evolution of compressed micropillars investigated by in situ HR-EBSD analysis and dislocation density simulations

Classification of slip system interaction in microwires under torsion

Identification of dislocation reaction kinetics in complex dislocation networks for continuum modeling using data-driven methods