Hydrogen segregation in the area of threefold junctions of grain boundaries

The process of hydrogen embrittlement of metals in the area of threefold junctions of grain boundaries at the expense of molecular hydrogen pressure in the voids and as a result of hydride-phase formation has been considered. The physical base of these mechanisms is diffusion migration of hydrogen atoms in the stress fields of structural imperfections. The threefold junctions of grain boundaries, which stress field is simulated by wedge disclination, are considered as the main structural imperfections. The exact analytical solution of a diffusion equation in the stress fields of the wedge disclination has been obtained. The analytical dependencies for the molecular hydrogen pressure in the voids near the threefold junctions of grain boundaries have been given. This pressure favors opening the microcracks along the grain boundaries. Hydrogen embrittlement of metals has been observed in the macroscopic scale. Kinetics of hydride-phase formation in the area of the threefold junctions of grain boundaries has been studied. The considerable volumetric changes of hydride in comparison with the basic metal lead to the void formation along the interphase boundaries. The results of a theoretical analysis are used for explaining the hydrogen embrittlement of metals within a wide temperature range.     

Role of grain boundaries in hydrogen embrittlement of alloy 725: single and bi-crystal microcantilever bending study

In situ electrochemical microcantilever bending tests were conducted in this study to investigate the role of grain boundaries (GBs) in hydrogen embrittlement (HE) of Alloy 725. Specimens were prepared under three different heat treatment conditions and denoted as solution-annealed (SA), aged (AG) and over-aged (OA) samples. For single-crystal beams in an H-containing environment, all three heat-treated samples exhibited crack formation and propagation; however, crack propagation was more severe in the OA sample. The anodic extraction of H presented similar results as those under the H-free condition, indicating the reversibility of the H effect under the tested conditions. Bi-crystal micro-cantilevers bent under H-free and H-charged conditions revealed the significant role of the GB in the HE of the beams. The results indicated that the GB in the SA sample facilitated dislocation dissipation, whereas for the OA sample, it caused the retardation of crack propagation. For the AG sample, testing in an H-containing environment led to the formation of a sharp, severe crack along the GB path.     

Effect of pre-deformation on the fatigue crack initiation life of X60 pipeline steel

It is impossible to keep petroleum and natural gas transmission pipelines free from defects in the manufacturing, installation and servicing processes. The damage might endanger the safety of pipelines and even shorten their service life; gas or petroleum release due to defects may jeopardise the surrounding ecological environments with associated economic and life costs.Pre-tensile deformation of X60 steel is employed to experimentally simulate the influence of dents on the fatigue crack initiation life. The investigation indicates that the fatigue crack initiation life of pre-deformed X60 pipeline steel can be assessed by a previously proposed energetic approach. The threshold for crack initiation increases with the pre-deformation due to a strain hardening effect, while the fatigue resistant factor exhibits a maximum with pre-deformation owing to its special dependence on fracture strain and fracture strength. The result is expected to be beneficial to the understanding of the effect of damage on the safety of pipelines and fatigue life prediction.     

Evidence for internal diffusion of sulphide from groundwater into grain boundaries ahead of crack tip in Cu OFP copper

Abstract

Copper canister is a central technical barrier for radioactive release from high level nuclear waste in the so called KBS-3 concept planned to be used in Finland and Sweden for disposal of spent nuclear fuel. Canisters will be placed in the granitic bedrock at about 400–500 m depth and surrounded by a layer of bentonite clay planned to protect the canister from any chemical and mechanical damage, and especially acting as a diffusion barrier. While researching for the possibility of stress corrosion cracking in phosphorus microalloyed copper in presence of sulphides in the groundwater, indications were found for a new potential degradation mechanism involving internal diffusion of sulphide. This paper describes the evidence for the new mechanism and discusses the scenarios involving diffusion of sulphide onto the copper canister surface.     

Microcrystalline silicon, grain boundaries and role of oxygen

The authors report on the correlation of the oxygen content for three high growth-rate series of thin Si films crossing the boundary between amorphous and microcrystalline growth together with the evolution of the prefactor and the activation energy of the dark d.c. conductivity. The different roles of oxygen, such as doping, alloying or defect passivation, are discussed in the framework of the model of transport based on the formation of large grain boundaries with an increased band gap due to hydrogen and/or oxygen alloying.     

Electrical properties of grain boundaries and size effects in samarium-doped ceria

Ce_0.85Sm_0.15O_1.925 samples have been prepared using the glycine-nitrate process. The effects of microstructure on their electrical properties were investigated by X-ray diffraction, density measurements, scanning electron microscopy and AC impedance spectroscopy. The grain-boundary conductivities and total conductivities increase with decreasing grain size for sintered densities of 95% or greater. The bulk conductivities are nearly independent of grain size for sintered densities of 95% or greater. It is found that the space-charge potential is nearly independent of grain size and the increase of the conduction path width is responsible for the increase of the apparent specific grain-boundary conductivity. The power densities and current densities of single cells based on Ce_0.85Sm_0.15O_1.925 electrolytes with different grain size are evaluated.     

Assessment of a creep-fatigue crack initiation and crack propagation for a welded cylindrical shell

An assessment of the creep-fatigue crack bahaviour for a cylindrical structure with weldments has been carried out by using a structural test and an evaluation with an assessment procedure. The structural specimen with a diameter of 600 mm and thickness of 7 mm is a welded cylindrical shell made of 316L stainless steel (SS) for one half of the cylinder and 304 SS for the other half. Eight artificial defects were machined and the defect behaviours were examined. In the creep-fatigue test, the hold time was 1 h at 600 °C and the primary nominal stress was 45 MPa. The evaluation results for the creep-fatigue crack initiation and crack propagation were compared with those of the observation images from the structural test. The assessment results for the creep-fatigue crack behaviour by using the French A16 procedure showed that the A16 guide is reasonably conservative but overly conservative for the creep-fatigue crack propagation in the case of a short hold time. It was shown that the crack initiation and propagation were dominated by a creep.     

Optical response of grain boundaries in upgraded metallurgical-grade silicon for photovoltaics

Using upgraded metallurgical-grade silicon (UMG-Si) is a cost-effective and energy-efficient approach for the production of solar cells. Grain boundaries (GBs) play a major role in determining the device performance of multicrystalline Si (mc-Si) solar cells. In this study two UMG-Si wafers, one from the middle part of a brick and the other from the top part of the same brick, were investigated. An excellent correlation was found between the grain misorientation and the corresponding optical response of GBs as indicated by photoluminescence (PL) imaging, electron backscattered diffraction (EBSD), and cross-sectional transmission electron microscopy (TEM). In addition, the PL features at random GBs depend also on the impurity levels in the wafer. In particular the PL emission was greatly enhanced in the narrow regions close to the random GB in the top wafer, which is an interesting phenomenon that may have potential application in high efficiency light-emission diodes (LEDs) based on Si.     

Comparison of creep crack initiation and growth in four steels tested in HIDA

In power generation plants and the chemical industries there is a need to assess the significance of defects which may exist in high temperature equipment operating in the creep range. For the life prediction methodology for cracked components developed under the HIDA (High Temperature Defect Assessment) Brite/Euram project, it is necessary to have a verifiable data-base of crack initiation and growth data in order to obtain relevant material properties for use in calculations. This paper examines the methods of analysis used. Four types of steels were tested in the programme. These were P22, 1CMV, 316LN and P91 in the parent, as welded and overaged conditions. The data have been obtained from seven participating laboratories. All the results were analysed in the same way using a programme called zrate developed to follow the ASTM E1457-98 testing standard. The results are compared with other crack initiation and creep crack growth data in the literature and with predictions produced from creep data using a model of the cracking process. It has been found that in all cases the scatter in the data is less than for the generic data in the literature. It has also been found that creep crack initiation and growth data can be correlated most satisfactorily in terms of the creep fracture mechanics parameter C¹.     

Two-dimensional LBIC and Internal-Quantum-Efficiency investigations of grooved grain boundaries in multicrystalline silicon solar cells

Grain boundaries (GBs) in multicrystalline silicon (mc-Si) degrade the solar cells performances. To enhance the conversion efficiency of photovoltaic cells, preferential grooving was achieved in GBs in order to reduce their area and then their highly recombination effect. For this purpose, a chemical etching method in HF/HNO₃ solution was used. In this study, we show how grooved GB enables deep penetration of phosphorus and metallic contacts, which lead to a P-Al-co-gettering of undesired impurities. As a result, we observe a decrease in the recombination activity in regions close to the GBs as compared to ungrooved sample; this was investigated by the two-dimensional Light-Beam-Induced-Current (LBIC) imaging. In addition, we carried out a two dimensional representation of the Internal-Quantum-Efficiency (IQE), where images show an improvement of the electrical activity in grooved GBs.     

Fatigue crack initiation and propagation as a consequence of thermal striping

An analysis is given of the initiation and subsequent propagation of surface cracks in a stainless steel section subjected to high frequency thermal shocks (thermal striping) at metal temperatures up to 650°C.Estimates of the maximum sustainable metal temperature range allowable to avoid crack nucleation in the surface layers have been made using fatigue endurance data corrected for the thermally induced equibiaxial strain field. Using these methods, it is apparent that a metal temperature difference of 63°C may be allowed. This represents the application of a safety factor of two, in the conventional manner, to the uniaxial fatigue endurance data in order to account for scatter in fatigue data arising from thermal ageing, surface finish and sodium corrosion effects. A discussion of the assessment is given.The extent of crack propagation under thermal striping has been analysed using a linear elastic fracture mechanics approach and it is shown that, in certain circumstances, crack arrest may occur. Methods are outlined which allow the inclusion of multiple surface cracking (and its effect of reducing the peak stress intensity factor) into the analysis.Consideration is also given to the possibility of arrested thermal fatigue cracks being further propagated by creep crack cracking under conditions of high mean stress.     

Effect of grain boundaries and grains on electrical properties of FeS2 films

Abstract

Because of its band gap and excellent optical absorption polycrystalline pyrite is a candidate material for absorber layers of solar cells. Pyrite films with different thicknesses were obtained by sol–gel method and following sulphurisation. The structural characteristics of the films were determined by X-ray diffraction, scanning electron microscopy and atomic force microscopy, while the electrical properties were investigated by Hall instrument and impedance spectroscopy. The grain and grain boundary contributions to the electrical transport mechanism were studied using the brick layer model. The equivalent circuit parameters were calculated by simulating experimental data. According to the results, the pyrite films were observed to crystallise more effectively, and the root mean square roughness tended to increase corresponding with increased film thickness. The grains govern the conductivity mechanism of FeS₂ film with a thickness of 109 nm. Both the grains and grain boundaries contribute to the electrical transport, and the grain contribution to the electrical transport mechanism has been further enhanced as the film thickness increases from 230 to 400 nm.     

Impurity thermovoltaic effect in the grain boundaries of a polycrystalline silicon solar cell

The experimental data on the implementation of the impurity thermovoltaic effect arising at polycrystalline silicon grain boundaries are presented. The temperature curve of the dark short-circuit current in a polycrystalline silicon solar cell, whose shape is determined by the effect revealed, is qualitatively discussed.     

Electrical activity of grain boundaries in silicon bicrystals and its modification by hydrogen plasma treatment

Electrical activity of grain boundaries (GBs) and its transformation under the influence of low-energy hydrogen plasma treatment in p-type silicon bicrystalline samples cut from EFG silicon polycrystals were investigated. Comprehensive studies have enabled one to investigate the electrical activity of GBs relative to the minority (MIC) and majority (MAC) carriers and to demonstrate the possibility of controlling this activity by different processing methods. These studies allowed for establishing the correlation between the type, structure and individual electrical activity of GBs and also thermal pre-history of samples. Among the tested modes, hydrogenation was found to be the most radical method of electrical activity modification for all types of GBs. In the process, results on hydrogenation of GBs in EFG silicon crystals depend essentially on three factors: type of GBs, state of ribbons (as-grown or annealed) and concurrence of grain boundary dangling bonds and boron passivation effects.     

Crack initiation and crack growth assessment of a high pressure steam chest

Extensive cracking had occurred in a number of high pressure steam chests. An assessment was undertaken based on the R5 British Energy methodology to assess the components for both creep–fatigue damage initiation and crack growth analysis to determine fitness for purpose. The analysis determined that the remaining base rupture endurance life of the component was greater then 1 million hours, however, due to the start-up and shutdown ramp rates, creep–fatigue damage greater then unity has occurred leading to crack initiation in a number of locations. These cracks were confirmed during internal inspection of the steam chest. A subsequent crack growth analysis determined that the component could safely be returned to service for the expected future life of the station.     

Fatigue crack initiation and propagation in austenitic stainless steels for light water reactors

The determination of fatigue life of components containing defects usually takes into account crack propagation only. In a real situation, a number of cycles are often required to reach fatigue crack initiation and predictive evaluation of fatigue crack initiation phases of real defects in austenitic stainless steel welded joints are presented. Fatigue crack growth rates in wrought and cast austenitic stainless steels and associated welds are also presented. Effects of various mechanical parameters (R ratio and variable amplitude loading) of a PWR environment and of metallurgical factors (δ ferrite content and ageing in cast austenitic stainless steels) are discussed.     

Creep crack growth in welds: a damage mechanics approach to predicting initiation and growth of circumferential cracks

The results of damage mechanics finite element analyses have been used to estimate the initiation and growth of type IV cracks in a series of internally pressurised circumferential pipe welds, in main steam pipelines made of 1/2CrMoV steel. The material properties used, for the various zones of new, service-aged and repaired welds, were produced from creep test data at 640°C. Damage distributions and accumulation with time within the HAZ are presented, from which the crack initiation times and positions for these welds, under a closed-end condition, and with additional axial (system) loading, were identified. By investigating the propagation of damage through the wall thickness, the remaining lives of the various weld types were estimated. The method provides a means for predicting the initiation and growth of type IV cracks in these CrMoV weldments, and for estimating the length of time a weld can safely be left in service, after damage, or type IV cracking, is identified during inspection.     

Hydrogen-assisted failure in a bimodal twinning-induced plasticity steel: Delamination events and damage evolution

The effect of the bimodal grain size distribution on the hydrogen susceptibility of a high-Mn fully austenitic twinning-induced plasticity (TWIP) steel was investigated by tensile testing under ongoing electrochemical hydrogen charging. Observation of the surface microstructure of the hydrogen-charged specimen yielded a correlation between the microstructure, crack initiation sites, and crack propagation path. The observed embrittlement arose from crack initiation/propagation along the grain and twin boundaries and delamination governed crack growth. In the present bimodal TWIP steel, the fine grained regions mostly showed intergranular cracking along the grain boundaries between the fine and coarse grains. By contrast, the coarse grained region exhibited transgranular cracking along the twin boundaries. The delamination cracking phenomena is rationalized by the evident nucleation, growth, and coalescence of microvoids in the tensile direction. The results reveal that the bimodal grain size distribution of TWIP steel plays a major role in hydrogen-assisted cracking and the evolution of delamination-related damage.