Some Limitations of Dislocation Walls as Models for Plastic Boundary Layers

It has recently become popular to analyze the behavior of excess dislocations in plastic deformation under the assumption that such dislocations are arranged into walls with periodic dislocation spacing along the wall direction. This assumption is made plausible by the fact that periodic walls represent minimum energy arrangements for dislocations of the same sign, and it allows to use the analytically known short-ranged stress fields of such walls for analyzing the structure of plastic boundary layers. Here we show that unfortunately both the idea that dislocation walls are low-energy configurations and the properties of their interactions depend critically on the assumption of a periodic arrangement of dislocations within the walls. Once this assumption is replaced by a random arrangement, the properties of dislocation walls change completely.Comment: To appear in: Proceedings of the International conference on numerical analysis and applied mathematics (ICNAAM) 2011, 4 pages, to appear in APS proceeding

Efficient numerical method to handle boundary conditions in 2D elastic media

A numerical method is developed to efficiently calculate the stress (and displacement) field in finite 2D rectangular media. The solution is expanded on a function basis with elements that satisfy the Navier-Cauchy equation. The obtained solution approximates the boundary conditions with their finite Fourier series. The method is capable to handle Dirichlet, Neumann and mixed boundary value problems as well and it was found to converge exponentially fast to the analytical solution with respect to the size of the basis. Possible application in discrete dislocation dynamics simulations is discussed and compared to the widely used finite element methods: it was found that the new method is superior in terms of computational complexity.Comment: 21 pages, 10 figure

Changes in amorphous silica mechanical properties induced by femtosecond laser irradiation

Femtosecond laser irradiation is an efficient process to modify refraction index of silicate glasses in order to create waveguide in glass fibers. Depending pulse energy and pulse duration, different kind of modifications can happen such as densification to nanoporosity nucleation [1], which affects optical properties in different ways. However such modifications should also affect mechanical properties and could be prejudicial to glass durability. The aim of this paper is to investigate effects of femtosecond laser irradiation on mechanical properties of silica glass using nanoindentation and micropillar compression [2]. For that purpose linear waveguides are produced using different process parameters. Samples are cut perpendicular to these waveguides. Nanoindentations are performed on the resulting cross-sections. Pillars are fabricated using a FIB and are then compressed using a specific nanomechanical tester. Main results are presented on Fig 1. It is shown that the highest irradiation energy lead to decrease in mechanical properties. This effect is more pronounced with micropillar compression than with nanoindentation. This can be explained by the highest hydrostatic pressure in indentation experiments, which can limit damage of silica. Please click Additional Files below to see the full abstract

Plastic deformation of microsamples: Intermittent dislocation avalanches and their acoustic emission

On the micrometer scale, deformation properties of metals change profoundly: the smooth and continuous behavior of bulk materials is often replaced by jerky flow due to random strain bursts of various sizes. The reason for this behavior is the complex intermittent redistribution of lattice dislocations due to external loading. This process also leads to the formation of the uneven step-like surface upon deformation. Our highly sensitive micromechanical platform can detect the strain bursts caused by dislocation avalanches in three different ways: (i) by stress and strain measurements using a capacitive displacement sensor measuring the elongation of a spring, (ii) by detection of the emitted acoustic signal using a sensitive piezoelectric transducer and (iii) by visual images using the electron beam of the SEM. In my presentation, I will present two of our recent results obtained with the help of this toolbox. Please click Download on the upper right corner to see the full abstract

Statistical properties of fractal type dislocation cell structures

International audienceThe dislocation microstructure developing during plastic deformation strongly influences the stress-strain properties of crystalline materials. Resent theoretical investigations based on the 2D continuum theory of straight parallel edge dislocations were able to predict a periodic dislocation microstructure. The results obtained, however, can only be considered as a very first step toward the understanding of the origin of dislocation patterning. One of the most challenging problems is the modeling of the formation of the fractal like dislocation microstructure. So, it is crucial to determine the statistical properties of such a structure developing at ideal multiple slip orientation. In the paper, by x-ray line profile analysis and the method of high resolution electron backscatter diffraction (HR-EBSD) a complex experimental characterization of dislocation microstructure developing in uniaxially compressed Cu single crystals is presented. With these methods, the maps of the internal stress, the Nye tensor, and the geometrically necessary dislocation (GND) density were determined at different load levels. It is found from the fractal analysis of the GND maps that the fractal dimension of the cell structure is decreasing with increasing average spatial dislocation density fluctuation. Moreover, it is shown that the evolution of different types of dislocations can be successfully monitored with the HR-EBSD-based technique

In-situ characterization of continuous dynamic recrystallization during hot torsion of an Al–Si–Mg alloy

An extruded Al–Si–Mg alloy was deformed by torsion at 400 °C during in-situ high energy synchrotron radiation diffraction. This technique is used to prove, by analysing texture changes and the coherently scattering domain size evolution, that dynamic recovery followed by continuous dynamic recrystallization are the main restoration mechanisms. Moreover, the dislocation density types corresponding to each stage of deformation are discussed and the recrystallization grade is calculated