Defective Microstructures Research Articles

Study of deformation mechanism of structural anisotropy in 4H–SiC film by nanoindentation

4H–SiC film is strongly anisotropic in terms of deformation behavior. This study systematically investigated the deformation mechanism of structural anisotropy in 4H–SiC film on three indentation planes (i.e. base plane, a-plane and m-plane) by nanoindentation. The results of molecular dynamics (MD) study show that the hardness and Young’s modulus of 4H–SiC film are different on three indentation planes. The morphology of the defective microstructures and the nucleation and extension of dislocations are strongly dependent on the indentation planes. The deformation behavior of 4H–SiC film is related to the crystal direction, atomic displacement, atomic strain and stress distribution. Further analysis reveals that the formation of prismatic dislocation half-loops on the basal plane (0001) can be attributed to the interaction of one full dislocation loop on the basal plane (0001) and two full dislocation loops on the prismatic plane {11‾00}. Whereas the prismatic dislocation loops on the a-plane (112‾0) and m-plane (11‾00) are formed by the “lasso"-like mechanism.

Predicting elastic strain fields in defective microstructures using image colorization algorithms

In this work, an image colorization algorithm based on convolutional neural networks is explored as an approach to predict tensile plane-strain field components of microstructures featuring porosity defects. For the same, microstructures featuring porosity of various shapes, sizes, area fractions and number densities were sampled on the gage section of ASTM-E8 sized numerical specimens whose tensile deformation was simulated in plane strain mode using commercial finite element analysis package Abaqus. Subsequently, the image colorization algorithm was trained by treating the microstructure featuring porosity defects as the gray scale image, and its strain field components as its color layers, analogous to the red-green-blue color components of traditional digital representations of images. Towards the same, various CNN frameworks were tested for optimization of its parameters, viz. number of layers, number of filters in each layer, stride, padding, and activation function. An optimized CNN framework is presented that is able to predict strain fields on randomly sampled microstructures with high accuracy R2>0.91 at a fraction of the time that finite element analysis would take. Various cross-validation tests were performed to test the accuracy and robustness of the CNN in learning features of various microstructures. Results indicated that the CNN algorithm is extremely robust and can provide near-accurate strain fields in generic scenarios.

Thermal Transport in Graphene Oxide Films: Theoretical Analysis and Molecular Dynamics Simulation.

As a derivative material of graphene, graphene oxide films hold great promise in thermal management devices. Based on the theory of Fourier formula, we deduce the analytical formula of the thermal conductivity of graphene oxide films. The interlaminar thermal property of graphene oxide films is studied using molecular dynamics simulation. The effect of vacancy defect on the thermal conductance of the interface is considered. The interfacial heat transfer efficiency of graphene oxide films strengthens with the increasing ratio of the vacancy defect. Based on the theoretical model and simulation results, we put forward an optimization model of the graphene oxide film. The optimal structure has the minimum overlap length and the maximum thermal conductivity. An estimated optimal overlap length for the GO (graphene-oxide) films with degree of oxidation 10% and density of vacancy defect 2% is 0.33 μm. Our results can provide effective guidance to the rationally designed defective microstructures on engineering thermal transport processes.

Defects as a root cause of fatigue failure of metallic components. III: Cavities, dents, corrosion pits, scratches

Abstract This third part of the review on defects as root cause of fatigue failure addresses cavities (pores, micro-shrinkages, unmelted regions), defective microstructures and microcracks as material defects and defects due to local damage during manufacturing, service and maintenance such as dents, scratches and localized corrosion. In addition, damage due to contact fatigue and the effect of surface roughness are discussed in the context of fatigue failure. Also addressed is the competition between different kinds of defects in controlling the initiation and early growth of fatigue cracks.

Inorganic-metallic bilayer on Mg alloy via wet and dry plasma treatments

The present study investigated the formation of inorganic-metallic bilayer on Mg alloy by combination of wet and dry plasma treatments. For this purpose, Mg alloy subjected to plasma electrolytic oxidation (PEO) in a silicate-based electrolyte was treated subsequently by dry plasma treatment via radio frequency (RF) sputtering to deposit Ni-Cr alloy on the porous inorganic layer. From microstructural observation, Ni-Cr layer modified the morphologies of the porous inorganic layer owing to the conformal coverage. Per electrochemical assessments, sample with bilayer exhibited superior corrosion protection due to the less defective microstructures and the presence of stable oxides. The electrochemical mechanism underlying the superior corrosion resistance of bilayer coating was discussed in the context of an equivalent circuit model by taking the surface morphologies into account.

Theoretical evaluation of the role of crystal defects on local equilibrium and effective diffusivity of hydrogen in iron

ABSTRACTHydrogen diffusion and trapping in ferrite is evaluated by quantum mechanically informed kinetic Monte Carlo simulations in defective microstructures. We find that the lattice diffusivity is attenuated by two to four orders of magnitude due to the presence of dislocations. We also find that pipe diffusivity is vanishingly small along screw dislocations and demonstrate that dislocations do not provide fast diffusion pathways for hydrogen as is sometimes supposed. We make contact between our simulations and the predictions of Oriani's theory of ‘effective diffusivity’, and find that local equilibrium is maintained between lattice and trap sites. We also find that the predicted effective diffusivity is in agreement with our simulated results in cases where the distribution of traps is spatially homogeneous; in the trapping of hydrogen by dislocations where this condition is not met, the Oriani effective diffusivity is in agreement with the simulations to within a factor of two.This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Nanodiamond stability with friction and heat

The structural stability of nanodiamonds (ND) has been investigated as a function of abrasion/friction and thermal annealing. Nanodiamonds examined before and after abrasion by transmission electron microscopy (TEM) are found to have modified surface characteristics. Frictional rubbing generates smaller, non-spherical particles with rougher, cleaner surfaces, and debris consisting of compacted ND+amorphous carbons. Annealing of ND and abraded ND at 1000°C demonstrates that abrasive pre-treatment increases proportion of transformation to onion-like-carbon (OLC) structures with generation of 3-30nm diameter OLC. The OLC achieved are of similar dimensions to those grown from ND annealed with a Ni catalyst but with more defective microstructures.

An in-process measurement method for repair of defective microstructures by using a fast tool servo with a force sensor

This paper presents an in-process measurement method, which is capable of conducting real-time position identification and on-machine profile characterization of micro-defects in defective microstructures by using a fast tool servo with a force sensor (FS-FTS) on an ultra-precision lathe. A real-time thrust force map is captured in the process of cutting the microstructures by the FS-FTS to indicate the cutting status with respect to the cutting tool position in the coordinate system of the lathe. Based on the thrust force map, the positions of the defects are identified in real time from the locations of the abnormal variations in the map associated with the occurrences of the defects. Characterization of the defects is then conducted by employing the force-controlled cutting tool as the measuring probe to measure the sectional profiles of the defective microstructures. The identified positions and the characterized profiles of the defects are then fed back to carry out the repair fabrication of the defective microstructures with an accurate tool path. Experiments of in-process detection and repair of the micro-defects in the microstructures over the outer surface of a roll mold were carried out to confirm the feasibility of the proposed method.

Ambient-stable tetragonal phase in silver nanostructures

Crystallization of noble metal atoms usually leads to the highly symmetric face-centred cubic phase that represents the thermodynamically stable structure. Introducing defective microstructures into a metal crystal lattice may induce distortions to form non-face-centered cubic phases when the lateral dimensions of objects decrease down to nanometre scale. However, stable non-face-centered cubic phases have not been reported in noble metal nanoparticles. Here we report that a stable body-centred tetragonal phase is observed in silver nanoparticles with fivefold twinning even at ambient conditions. The body-centered tetragonal phase originates from the distortion of cubic silver lattices due to internal strains in the twinned nanoparticles. The lattice distortion in the centre of such a nanoparticle is larger than that in the surfaces, indicating that the nanoparticle is composed of a highly strained core encapsulated in a less-strained sheath that helps stabilize the strained core.

Variations of the impact toughness of 12Kh1MF steel in operating steam pipelines of thermal power plants

It is shown that, in the intact state, the impact toughness of 12Kh1MF steel at room temperature is, as a rule, higher than at the elevated temperature (545°C) for metals both with fair and defective microstructures. It is demonstrated that, under the conditions of long-term operation of the steam pipelines of thermal power plants at room temperature, the impact toughness of steel usually decreases. However, at the working temperature, its changes are insignificant. Hence, the brittle-fracture resistance of the metal at room temperature becomes lower than that at the elevated (working) temperature. We reveal the cases of anisotropy of the impact toughness of the metal cut out from a bend of the steam pipeline with cracklike defects. A correlation dependence of the impact toughness at room temperature on the relative narrowing obtained in the tests for static tension is established.

Microstructures of La1−xAx(A = Ca or Sr)MnO3−δthin films by liquid-delivery metalorganic chemical vapor deposition

The microstructure of La1–xAx(A = Ca or Sr)MnO3–δthin films grown by liquid-delivery metalorganic chemical vapor deposition on (001) MgO and (110)pseudo-cubicLaAlO3were studied by transmission electron microscopy. The La1–xCaxMnO3–δthin film on large lattice mismatched MgO exhibited very defective microstructures and consisted of two typical regions. The first region was close to the film–substrate interface and had an epitaxial relationship to the substrate with many differently oriented domains nucleated on the substrate surface. The second region consisted of columnar grains with some degree of texture. In contrast, the smaller lattice-mismatched La1–xSrxMnO3–δ/(110)pseudo-cubicLaAlO3film had good crystalline quality with highly oriented columnar grains but exhibited complicated dislocation structures. Apart from the misfit dislocations formed at the film–substrate interface, two types of anomalous dislocations with limited contribution to relieving misfit stresses were also observed. One type of dislocation had extra planes in the film and some climbed into the substrate. These dislocations were considered to form from dislocation loops during nucleation of the film. The other type of dislocations had extra planes parallel to the film–substrate interface and glided into the substrate side resulting in a 2° tilt of the film with respect to the substrate. The complicated dislocation configurations present in the sample were related to the complex strain field in the film. The relative strains along the interface measured in the film were heterogeneous. The variations of the strains in the film were related to the local Curie temperature changes and second-order phase transitions of the film.

A model for defective fibrous composites

We consider the problem of modeling fibrous composite materials which have defective regions in their microstructure. These defects take the form of either local fiber spreading or local fiber clumping. Due to the number of fibers, the problem requires an asymptotic analysis and so we develop a mathematical theory of homogenization for defective microstructures. In order to understand the effect of such defects on the overall behavior of the composite, we solve a global boundary value problem. We then combine the results from this global analysis with our homogenization to show that even when the defective region is very small, substantial increases in the constituent stresses occur.

Germanium Partitioning and Interface Stability During Rapid Solidification of Gesi Alloys

AbstractThe stability of laser processed GeSi heteroepitaxial alloys on Si to partitioning driven interface instabilities is examined. Existing stability models are extended to include nonequi-librium solidification effects for nondilute alloys and are examined under typical conditions of laser induced solidification. Ge diffusion and partitioning were measured for quantitative input to the models. The Ge liquid-phase diffusivity was determined to be 2.5 x 10-4 cm2/s. The measured velocity-dependent partition coefficients k(v) were fit to the Continuous Growth Model using an equilibrium k of 0.45 and a diffusive speed of 2.7 m/s. Stability calculations based on these values and our extended stability model are presented. Although instabilities at compositions comparable to those experimentally observed to give defective films are predicted, the growth rates of these instabilities appear too slow to destabilize an interface on laser processing time scales. These results suggest that strain or other effects play an important role in the observed defective microstructures.

TEM study of silicon carbide whisker microstructures

Β-SiC whiskers produced by a number of manufacturers have been examined in the transmission and scanning electron microscopes. In all cases defective microstructures were found with high densities of planar defects such as stacking faults and microtwins. Two distinct types of defective whisker can be identified. The first contains regions of very closely spaced twins on {111} planes arranged perpendicular to the whisker axis [111], these were sometimes separated by defect-free regions. In these whiskers a rough surface profile was normal with the roughness closely associated with the highly defective regions of the whisker. The second type of whisker contained stacking faults spaced relatively widely also on {111} planes but now on the planes inclined to the [111] axis of the whisker. This leads to a characteristic chevron contrast in the TEM. This second type of whisker had a much smoother surface profile than the first type with perpendicular defects. No whisker contained both defect types but some batches of whiskers contained populations of both types of whisker. The first type of whisker is shown to have defects similar to those reported as common during vapour-liquid-solid whisker growth. This is also consistent with the higher impurity content and the presence of voids found in these whiskers. The second type may be indicative of a different growth mechanism possible under certain conditions of SiC whisker synthesis.