Creation Of Free Surface Research Articles

The effects of free surfaces on deformation twinning in HCP metals

Deformation twinning is a predominant mode of plastic deformation for hexagonal close packed metals, like Mg and Ti. The heterogenous microstructure and the local stresses associated with twinning play a key role in their mechanical response and fracture. Surface analyses, like electron microscopy, are frequently employed to spatially map microstructure and micromechanical fields in order to study twinning behavior. However, these measurements are inherently influenced by the vicinity of the free surface. Here, an elasto-visco-plastic fast-Fourier-transform (EVP-FFT) polycrystal modeling approach is employed to investigate the effects of free surfaces on twin development before and after loading. We compare calculated micromechanical fields on free surfaces with those calculated inside the bulk and, in some cases, experimental surface measurements. The results indicate that the creation of free surfaces can promote twin propagation and growth and can influence twin morphology by causing a twin lamella to become larger, more blunted and irregular. The structure along the twin boundaries are also affected, due to the higher driving stresses that extend prismatic-basal and basal-prismatic facets. Furthermore, free surfaces invoke different slip activities in the twin and the surrounding parent crystal by enhancing basal, prismatic and pyramidal slip in some localized regions, while reducing slip in others. We demonstrate that the simulated free-surface effects lead to better qualitative and quantitative agreement with experimental measurements from scanning electron microscopy and digital image correlation.

The Effects of Free Surfaces on Deformation Twinning in HCP Metals

Deformation twinning is a predominant mode of plastic deformation for hexagonal close packed metals, like Mg and Ti. The heterogenous microstructure and the local stresses associated with twinning play a key role in their mechanical response and fracture. Surface analyses, like electron microscopy, are frequently employed to spatially map microstructure and micromechanical fields in order to study twinning behavior. However, these measurements are inherently influenced by the vicinity of the free surface. Here, an elasto-visco-plastic fast-Fourier-transform (EVP-FFT) polycrystal modeling approach is employed to investigate the effects of free surfaces on twin development before and after loading. We compare calculated micromechanical fields on free surfaces with those calculated inside the bulk and in some cases, experimental surface measurements. The results indicate that the creation of free surfaces can promote twin propagation and growth and can influence twin morphology by causing a twin lamella to become larger, more blunted and skewed. The structure along the twin boundaries are also affected, due to the higher driving stresses that extend prismatic-basal and basal-prismatic facets. Furthermore, free surfaces invoke different slip activities in the twin and the surrounding parent crystal by enhancing basal, prismatic and pyramidal slip in some localized regions, while reducing slip in others. We demonstrate that the simulated free-surface effects lead to better qualitative and quantitative agreement with experimental measurements from scanning electron microscopy and digital image correlation.

Capturing the Effects of Free Surfaces on Threading Dislocation Density Reduction

The big concern with using silicon as a substrate for making Ge and III-V devices is the dislocation density in the epilayers. Dislocations degrade device performance by trapping the photo-generated carriers, dragging down efficiency.In this presentation, we show that dislocation engineering using nanovoids in the heteroepitaxy of Ge/Si can be a plausible path for monolithic integration of high-quality GaAs on Si platform. Possible mechanisms responsible for threading dislocation (TD) density reduction through free surfaces located, either in the Ge layer or in Si substrate that lead to their annihilation or fusion are discussed. By combining various scanning and transmission electron microscopy techniques, it has been possible to link different effects of either of the free wedge surfaces on dislocation, particularly, threading arm pinning by nanovoids. The results suggest that introducing nanovoids favors recombination of TDs by increasing the characteristic interaction distance between neighboring dislocations. In fact, the creation of free surfaces could facilitate interactions between dislocations, enabling the dislocation network to change its connectivity in a way, which facilitates the subsequent annihilation of TD segments. In addition, the voids formed in the silicon substrate could potentially capture and thereby combine many more TDs. The TDs bend towards the voids in Si in order to minimize their length, causing their recombination and elimination at the nearest free surface thus leading to the creation of an almost defect-free Ge layer on Si [1].The use of such a simple, industry friendly, inexpensive and self-organised process through electrochemical etching and thermal annealing could be crucial in cutting manufacturing costs and enabling market penetration of III–V on Si-based devices.[1] Y. A. Bioud et al., “Uprooting defects to enable high-performance III–V optoelectronic devices on silicon,” Nat. Commun., vol. 10, no. 1, 2019.

Interactions of KLA Amphipathic Model Peptides with Lipid Monolayers

Interactions of the cationic amphipathic peptide KLALKLALKALKAALKLA-NH(2) (KLAL) and its double D-amino acid replacement analogues l(11)k(12)-KLAL and k(9)a(10)-KLAL with lipid monolayers of anionic POPG, zwitterionic POPC and mixtures thereof at the air/water interface were investigated by infrared reflection- absorption spectroscopy (IRRAS). At high surface pressure (>30 mN m(-1)) all peptides incorporated into lipid monolayers containing at least 25 % anionic POPG, and adopted an alpha-helical conformation. Creation of free surface by expansion of the monolayers resulted in an additional adsorption of peptides from the subphase, but now in a beta-sheet conformation; this led to the coexistence of peptides in two distinctly different conformations within the lipid monolayer. The beta-sheets bound to the free surface could be squeezed out of the film by compressing the film to low surface areas, whereas the alpha-helices remained bound to the lipids until the film collapsed. When bound to the lipid monolayer, the helical axis of the peptides is oriented almost parallel to the surface of the monolayer.

Effect of crack shape and size on estimating the fracture strength and crack growth fatigue life of bridge cable steel wires

Macrofracture entails the creation of free surface. The process can be enhanced or impeded depending on the ways in which the material microstructures are designed to react against the operational conditions. Undesirable chemical reactions can be the main source of strength degradation for load-supporting large structural members. Assessment of the remaining strength of damaged cable wires for cable-stayed or suspension bridges can rely on a knowledge of strength and/or fatigue life depending on the prevailing stress amplitude and whether the cables are stayed or not. Although large-scale computer schemes are available for making detailed failure analyses, they are not conducive to retrieving technical information within a relatively short time. Computer simulation has not advanced to the stage where experimental validation could be spared. To this end, effective analytical/experimental methodologies are much in need of development. The fracture mechanics approach appears to have gained ground in recent t...

Strain energy considerations in the effect of a class of catalyst promoters

It is shown that the crystallite size refining and maintaining phenomena evidenced by the singly promoted ammonia catalyst can be attributed to a delicate balance between strain energy and surface energy effects. In the model presented the promotor acts as the point defect source of a strain field. The strain field of the point defects can be partially relaxed by the propinquity of a free surface. This comminuting effect is resisted by the creation of free surface. It is shown that for a certain narrow range of material parameters the strain and surface terms achieve a balance which produces an equilibrium particle size. It is shown, finally, that the material parameters of the alumina-promoted iron catalyst lie in a range in which the strain-surface energy effect is feasible.