Intergranular Surface Research Articles

Influence of deformation degree on grains size and borders between them in metal alloys during cold rolling of sheets

The internal structure of metal products was investigated and it was identified that it affects quality and workability of equipment parts. Changes that occur in metal products structure during cold treatment by pressure, which lead to changes in physical and mechanical properties of products, have been analyzed. It was established that grains and borders between them, which determine internal structure of metal products, change their shape and size during cold rolling, depending on percentage reduction value. It is offered to consider grain borders as amorphous layer. Methodology has been developed to study influence of deformation degree during cold rolling to grain size of metal internal structure and intergranular layer volume. Intergranular surface area, its volume and part of surface volume from metal volume were determined. Parameters of internal structure of low-carbon steel were calculated depending on reduction value during cold rolling. According to study results, interconnection between grains size and deformation degree was established; and percentage reduction influence to grain size and borders between them was determined, obtaining a minimum standard grain size with percentage reduction of over 80 %; yield strength and its growth for metal alloy during cold rolling are established, which allows controlling the strength of steel by adjusting the grain size and their junction surface. It is shown that with an increase in deformation degree, yield strength of metal product material increases in hyperbolic dependence, and fraction of amorphous layer and strength properties of low-carbon steel change almost equally in exponential functions. It is revealed that during cold rolling, it is possible to control internal properties of obtained product by adjusting reduction value.

Temperature dependence of hydrogen-assisted damage evolution and fracture behavior in TRIP-aided bainitic ferrite steel

We systematically investigated the effects of hydrogen and deformation temperature on the micro-damage evolution and fracture behavior of the TRIP-aided bainitic ferrite steel using slow strain rate tensile tests and microstructure examinations. For hydrogen-uncharged condition, the micro-damage evolution behavior was not dependent on temperature, except for −100 °C. The exceptional behavior at −100 °C was attributed to the occurrence of brittle fracture. Hydrogen uptake significantly increased the number density of micro-damage in all deformation temperatures. The fracture mode of hydrogen-charged specimens changed from quasi-cleavage and ductile fractures to cleavage, quasi-cleavage, and intergranular fractures with decreasing deformation temperature. Quasi-cleavage fracture features were attributed to the crack growth process consisting of repetition of small crack initiation/blunting (or void initiation) and crack coalescence, whereas the cleavage and intergranular surfaces with smooth facets were due to the occurrence of hydrogen-reduction cohesive strength of cleavage planes and prior austenite grain boundaries.

On mechanism of hydrogen generation at oxidation of activated aluminum in aqueous solutions

To clarify the mechanism of hydrogen generation upon aluminum oxidation by water, the behavior of the mechanical alloys {Al+20%Bi} and {Al+7%Ga+3%In} in various salt solutions has been studied. Analysis of the reaction rate and the overall hydrogen yield in the studied systems allows us to suggest that, unlike the oxidation of magnesium-based materials, the hydrogen generation by activated aluminum is not mainly controlled by electrochemical processes. Additives of activating metals, bismuth and gallium-indium alloys, primarily act as surfactants, making an unoxidized intergranular surface accessible to water according to the Rebinder effect.

Hydrogen environment embrittlement in nickel-base superalloy Alloy 617

Hydrogen environment embrittlement (HEE) in a nickel-base superalloy, INCONEL® Alloy 617, was scrutinized using a notch tensile test in a 99 MPa hydrogen atmosphere to determine the function of grain boundary carbides. The fracture surface ratio of the intergranular surface increased monotonically with increasing grain boundary coverage, which is defined as the ratio between the length of the grain boundary covered with carbide and the total length of the grain boundary. Strain distribution analysis clarified that more strain locally accumulated near the grain boundary covered with grain boundary carbide compared to the uncovered one. The suggested mechanism of HEE in the alloy is that a large amount of plastic strain near the grain boundary covered with grain boundary carbide triggers crack formation at the interface between the matrix and the grain boundary carbide, followed by crack growth along the grain boundary, ultimately leading to fracture.

Intergranular space, specific surface area, grain size distribution and distribution of macro, meso and micropores of multiphase microstructure in active microalloyed multifunctional ceramics

As a complex multiphase heterogeneous system in solid state, multifunctional active microalloyed alumo-silicate ceramics has an inhomogeneous structure with intergranular space, which is reflected in a number of structurally sensitive properties. A very complex intergranular space and numerous interactions between individual phases and grains create new boundaries and an even more complex space with much smaller micrograins, which are formed by grain fragmentation by dislocations displacement. In addition to reducing macro and meso porosity, densification of intergranular space increases the number of micro pores. Intergranular surface area and volume are considered as dislocation space. Quantitative metallography method was applied to determine grain size distribution using software for automatic analysis. Specific surface tests and pore distribution were performed on special samples of multifunctional ceramics. Standard methods for determining specific surface area of samples in vacuum were used. Obtained results, which were relevant in terms of theoretical and practical implications, confirmed that multifunctional active microalloyed ceramics had a developed surface with significant number of meso and micro pores. Due to constancy of grain fragmentation process, there were significant changes in micromorphology and all multifunctional properties, as well as movement of dislocations, which made a significant contribution to contemporary research in this field.

Modeling mesoscale fission gas behavior in UO2 by directly coupling the phase field method to spatially resolved cluster dynamics

Fission gas release within uranium dioxide nuclear fuel occurs as gas atoms diffuse through grains and arrive at grain boundary (GB) bubbles; these GB bubbles grow and interconnect with grain edge bubbles; and grain edge tunnels grow and connect to free surfaces. In this study, a hybrid multi-scale/multi-physics simulation approach is presented to investigate these mechanisms of fission gas release at the mesoscale. In this approach, fission gas production, diffusion, clustering to form intragranular bubbles, and re-solution within grains are included using spatially resolved cluster dynamics in the Xolotl code. GB migration and intergranular bubble growth and coalescence are included using the phase field method in the MARMOT code. This hybrid model couples Xolotl to MARMOT using the MultiApp and Transfer systems in the MOOSE framework, with Xolotl passing the arrival rate of gas atoms at GBs and intergranular bubble surfaces to MARMOT and MARMOT passing evolved GBs and bubble surface positions to Xolotl. The coupled approach performs well on the two-dimensional simulations performed in this work, producing similar results to the standard phase field model when Xolotl does not include fission gas clustering or re-solution. The hybrid model performs well computationally, with a negligible cost of coupling Xolotl and MARMOT and good parallel scalability. The hybrid model predicts that intragranular fission gas clustering and bubble formation results in up to 70% of the fission gas being trapped within grains, causing the increase in the intergranular bubble fraction to slow by a factor of six. Re-solution has a small impact on the fission gas behavior at 1800 K but it has a much larger impact at 1000 K, resulting in a twenty-times increase in the concentration of single gas atoms within grains. Due to the low diffusion rate, this increase in mobile gas atoms only results in a small acceleration in the growth of the intergranular bubble fraction. Finally, the hybrid model accounts for migrating GBs sweeping up gas atoms. This results in faster intergranular bubble growth with smaller initial grain sizes, since the additional GB migration results in more immobile gas clusters reaching GBs.

STRUCTURE-FUNCTIONAL SELF-ORGANIZATION OF ZrO2–SiO2:Sn(IV) SYSTEM

The study is devoted to the solution of one of the actual problems of materials science – the conscious management of the fundamental properties of solids. It is based on the development of an algorithm for creating both on intergranular surfaces and in the volume of particles nanosized inclusions, crystallites, structural defects. Taking into account the accumulated results of systematic studies of simple, binary systems as previous experience for further design of more complex systems, for correctly overcome the fundamental disadvantages, associated with the inconsistency of multicomponent systems, the sequence of physico-chemically substantiated technolo­gical stages on the way of formation of functional architecture has been formulated. The coevolutionary concept of self-organization of chemical systems is also formulated, according to which the regulation of the course of structural-functional reorganization processes takes place by two mechanisms: adaptation and bifurcation. Taking into account the phy­sicochemical properties, optimal conditions for the formation of element oxide clusters and the peculiarities of interparticle interaction, the course of structural and functional self-organization – response of colloidal solutions of a multicomponent system to directionally initiated changes in the characteristics of the dispersed reaction medium and, accordingly, the parameters of particles that are deliberately designed in this way (size, shape, composition, structure of their ensembles), as well as the effect on interparticle distances, hierarchy of structural levels, the action of concentration and temperature factors and the introduction of a modifying reagent were diagnosed by va­rious measurements. The driving forces (electro­negativity, competitive rearrangements) and tendencies of energy-supplied bifurcation formation of coordination polyhedra of structure-forming ions in multicomponent ensembles were identified, namely, the pathways of directed initiated rearrangement of the atomic architecture with the organization of oxygen-unsaturated zirconium-containing sites, which determined the matrix formation with practically significant catalytic activity.

Development of methods for studying intergranular surface features in semiconductor heterogeneous polycrystals of bismuth-antimony tellurides with the imposition of electric and deformation fields

The article discusses the technique and tools for studying the role of the effective density of electronic surface states innanocrystalline semiconductor films when cyclic deformation is applied directly from the analysis of experimental data. The surface electron states play the role of recombination and trapping centers depending on the number of carriers, the electron capture section and hole, the concentration of surface states of their type and energy position. To determine the effective density of surface states, we found both a change in the Fermi level and a change in the density of the effective surface charge. The effective density of electronic surface states was determined from the measured variations in the active resistance and capacity of nanocrystalline films of bismuth-antimony tellurides upon the application of irreversible deformation, and its strain dependence was found. From data analysis, one can judge the irreversibility of heterogeneous structures during deformation, that is, the electronic structure of a nanocrystalline semiconductor film changes greatly when cyclic deformation is applied.

Characteristics of hydrogen-related fatigue fracture in 2Mn-0.1C martensitic steel

This study investigated the hydrogen-related fatigue fracture in 2Mn-0.1C steel having a lath martensite microstructure. The presence of hydrogen significantly reduced the fatigue life. The transgranular surface was a main component in each stress range of the uncharged specimen, while the intergranular surface was frequently observed in the hydrogen-charged specimen. The crystallographic orientation analysis by electron backscattering diffraction revealed that the cracks mainly propagated along {011} planes regardless of the presence of hydrogen. Compared with the uncharged specimen, however, plastic deformation was localized near the fatigue crack in the hydrogen-charged specimen. According to the reconstructed fracture process by fracture surface topography analysis, the hydrogen-related fatigue fracture was discontinuous and composed of isolated nucleation of intergranular cracks and quasi-cleavage crack propagation initiated at the pre-existing intergranular cracks.

Effect of environment-assisted cracking on the premature fatigue failure of high-strength valve springs

In this work, the failure mechanism of four high-strength valve springs were investigated. It shows that the premature fatigue failure of valve spring can be attributed to the presence of an intergranular surface crack formed by the environment-assisted cracking (EAC) mechanism. The surface cracks significantly reduces the fatigue strength to a level of 320–350 MPa, which is much lower than the designed stress (720 MPa). Detailed analysis reveal that the reduced shot peening effect caused by the soft and brittle PbO adhesion contributes to the occurrence of EAC. Furthermore, it suggests that the nucleation and propagation of EAC crack controls the failure process.

A comparative study in different rare earth ions in multiferroic nanocomposites: Low temperature resistivity minima and low field magnetoresistance

Low temperature magnetoresistance and resistivity properties are systematically studied in rare earth based multiferroic nanocomposites La0.7Sr0.3MnO3-TMnO3 (T = Er,Ho) under magnetic fields up to 5 Tesla. Resistivity with temperature exhibits metal-insulator transition. At very low temperature, the sharp upturn of resistivity is observed, which is found to be suppressed by field. The upturn of resistivity is characterized with spin fluctuation of the system. The reduction of the upturn of resistivity with magnetic field ensures the progress of the electrons tunneling. Effective reduction of charging energy is obtained upon an applied magnetic field. Temperature dependent resistivity shows minima at low temperature under different applied magnetic fields. The resistivity fit at low temperature is made with consideration of equally electron-electron interactions concerning (temperature)1/2 term and the Kondo-identical spin dependent scattering for ln(temperature) dependence. Lessening of the spin fluctuation by an applied magnetic field plays a dynamical role in defining the magnetoresistive behavior in samples. Experimental data are analyzed and demonstrated through a simple proposed phenomenological model to provide an insight on the physical mechanism behind the low temperature transport phenomena. The derived surface spin susceptibility is accordance with the temperature dependence of magnetoresistance involving an intergranular surface magnetization. Temperature dependent field cooled and zero field cooled dc magnetization curve having a large bifurcation between them, is a suggestion of a glassy like phase that may be originated from the challenging magnetic exchange interactions. The low temperature glassy magnetic behavior is endorsed to a spin-glass identical frozen state for the surface of the nanoparticles. Magnetic field dependent magnetization study shows an increase in magnetization without saturation in decreasing temperature. The multiferroic nanocomposites exhibit the magnetoelectric coupling, which arises due to the strain mediated magnetostriction behavior of the piezomagnetic phase (La0.7Sr0.3MnO3) in the nanocomposites.

The Role of Grain Boundary Oxidation on Surface Crack Formation under Continuous Casting Conditions

This paper represents an extended abstract of a study presented at the ESTAD 2019 conference. In this contribution, the influence of different cooling strategies on the formation of intergranular surface cracks is observed with in-situ bending experiments under continuous casting conditions. The steel composition is equal to a 0.17 wt.% C construction steel. It is investigated with and without Al deoxidation at bending temperatures of 1100 °C to 700 °C. The results show the most critical situation prevailed for each testing condition at 900 °C. A holding temperature of 1200 °C leads to a selective grain boundary oxidation and therefore to the formation of notches at the austenite grain boundaries, which are the cause for stress concentrations and easier formation of cracks during a subsequent tensile deformation at critical temperatures. The experiments reveal a partially stronger influence of this phenomenon on the steel without Al deoxidation.

Early Time Evolution of Selective Oxidation in a CMnSi AHSS

The influence of dew point temperature (DPT) and oxidation time on the selective oxidation of a Fe-0.09C-2.02Mn-0.91Si (wt pct) advanced high-strength steel during the early stages of oxidation was investigated in this study. Samples were annealed at 1123 K (850 °C) in a high-temperature confocal scanning laser microscope for 0, 30, 60, 90, and 120 seconds in an N2 + 5 pct H2 atmosphere with 243 K (− 30 °C) or 273 K (0 °C) DPT. Oxide morphology and chemistry were examined with respect to time and atmosphere DPT using scanning electron microscopy, scanning transmission electron microscopy, focused ion beam (FIB) milling, FIB serial sectioning, and energy dispersive spectroscopy techniques. Significant internal and external oxidation for all samples, including those with a 0 second isothermal hold, was observed. This indicated significant oxidation occurred upon heating. Samples oxidized in the 273 K (0 °C) DPT atmosphere exhibited intra- and intergranular internal oxidation and surfaces covered in discrete iron-rich nodules. These were attributed to stress generation during oxidation. Samples oxidized in the 243 K (− 30 °C) DPT atmosphere had shallow internal oxidation, sparse intragranular oxides, and surfaces rich in silicon and manganese containing oxide. Internal oxides for both atmospheres and all isothermal hold times were comprised of a silicon-rich core and manganese-rich outer shell was frequently observed. This core/shell structure was related to evolving oxide stability during the high-temperature exposure.

A review of low-cycle fatigue of Alloy 617 for use in VHTR components: Experimental outlook

The effect of strain range and temperature on the low-cycle fatigue behaviour and microstructure change during cyclic deformation of Alloy 617 for use in very high temperature gas-cooled reactor components were studied at elevated temperature starting from ambient condition. Increasing the strain range and the temperature was noticed to reduce the fatigue resistance of nickel-based Alloy 617 due to facilitating the transformation behavior of the carbides in the grain interior, precipitates along the grain boundary, and oxidation behavior inducing surface connected precipitates cracking. Initial hardening behavior was observed at room temperature condition during cyclic due to the pile-up dislocation of micro-precipitates. The grain size was also taking a role due to the formation of an obstacle in the matrix. In the high temperature regime, the alloy 617 was found to soften for its entire life due to the fast recovery deformation, proved by its higher plasticity compared with lower temperature. The deformation behavior also showing high environmentally assisted damage. Oxidation behavior was found to become the primary crack initiation, resulting in early intergranular surface cracking.

Examination of Variable Frequency Drive Transformer Copper Cooling Line Failures from Nuclear Power Plants

The findings from a failure examination on copper cooling lines from nuclear power plant transformer cooling systems are discussed in this article. The failures occurred prior to regular service at two different nuclear power plants during installation and qualification testing. A failure examination including—light optical microscopy, scanning electron microscopy, metallography, and microhardness—was conducted on the fractured lines. The fractures predominately had fatigue striation morphologies, but were found to have initiated in areas exhibiting intergranular cracking, secondary cracking, and surface pitting. Additionally, evidence of phosphorus rich particles was found in the IG crack regions. The failures are hypothesized to be the result of outside diameter environmentally initiated cracking, assisted by residual stresses from brazing, and propagated by fatigue.

Hydrogen Embrittlement Understood

The connection between hydrogen-enhanced plasticity and the hydrogen-induced fracture mechanism and pathway is established through examination of the evolved microstructural state immediately beneath fracture surfaces including voids, “quasi-cleavage,” and intergranular surfaces. This leads to a new understanding of hydrogen embrittlement in which hydrogen-enhanced plasticity processes accelerate the evolution of the microstructure, which establishes not only local high concentrations of hydrogen but also a local stress state. Together, these factors establish the fracture mechanism and pathway.

Hydrogen Embrittlement Understood

The connection between hydrogen-enhanced plasticity and the hydrogen-induced fracture mechanism and pathway is established through examination of the evolved microstructural state immediately beneath fracture surfaces including voids, “quasi-cleavage,” and intergranular surfaces. This leads to a new understanding of hydrogen embrittlement in which hydrogen-enhanced plasticity processes accelerate the evolution of the microstructure, which establishes not only local high concentrations of hydrogen but also a local stress state. Together, these factors establish the fracture mechanism and pathway.

Quantitative comparison between experimental measurements and CP-FEM predictions of plastic deformation in a tantalum oligocrystal

Quantitative comparisons of experimental measurements and model predictions are crucial to validate and improve material models as well as to understand underlying physical phenomena. In this work, we used a recently developed in situ technique combining high resolution digital image correlation (HR-DIC) and electron backscatter diffraction (EBSD) to obtain intergranular surface strain fields and crystal rotations. These measurements were compared to predictions from a crystal plasticity-finite element method (CP-FEM) simulation. To accurately reproduce the initial grain morphology in the CP-FEM simulation without assumptions regarding the sub-surface microstructure, a coarse columnar-grained tantalum specimen was used. Experimental data was projected onto the finite element mesh to compare measured and simulated data on a point-wise basis. It is shown that model predictions of both surface strain fields and crystal rotations agree reasonably well with HR-DIC and EBSD measurements at various applied strains. This procedure provides an objective and quantitative methodology to evaluate the model-experiment agreement and allows for testing of various model parameters to improve the model.

Fatigue lifetime and tearing resistance of AA2198 Al–Cu–Li alloy friction stir welds: Effect of defects

The fatigue strength and failure mechanisms of defect-free (“sound”) and flaw bearing friction stir butt-welds of 3.1mm-thick AA2198-T8 Al–Li–Cu alloy have been investigated via S–N curves at R=0.1 using cross weld specimens. The fatigue strength of sound welds is only reduced by 10–15% at the aimed lifetime of 105 cycles compared to the base material. Joint Line Remnant (JLR) bearing welds have a similar fatigue strength as sound welds and the JLR is not the crack initiation site. Kissing Bond (KB) bearing welds that have undergone a weld root polishing show a reduction in fatigue strength by 17% compared to sound welds. For specimens loaded at or above yield strength of the weld nugget the crack systematically initiates from the KB during the first cycle, which is interpreted further using fracture mechanics. The strongest reduction, about 28% in fatigue strength, is found for welds with an initial gap between the parent sheets (GAP welds) along with initiation at intergranular surface microcracks. Kahn tear tests show a reduction in tearing resistance for the flaw bearing welds with a similar ranking as for the fatigue strength.

Environmental creep intergranular damage and multisite crack evolution model for engineering alloys

In this paper a sub-grain size finite element meshing technique has been developed to employ in a combined creep continuum damage and environmentally assisted time dependent material depletion and crack growth model. The model can cover a wide range of failures allowing crack paths for intergranular and transgranular cracking as well as surface oxidation and depletion due to corrosion. It has been customised to predict time and stress induced intergranular surface damage and cracking for isotropic metallic grain structures which undergo oxidation at the surface and are under creep loading regime. It is shown that creep crack growth predictions using a remaining ductility multiaxial failure strain constraint-based model coupled with a time-dependent environmentally assisted corrosion/oxidation rate dependent parameter can realistically assess damage and cracking rates for the engineering alloys shown. Material depletion and multiple intergranular crack paths are modelled by assuming weaker grain boundaries having realistically a reduced critical damage index of a factor of 2 less than that of the grains. A probabilistic random crack extension criterion is also proposed which allows for statistically varying random damage and surface crack growth development during each run using the same input properties. This method can take into account the level of scatter that exists in the material characterisation test data and in long term component oxide and crack damage measurements. The results for a sample rectangular block under tension using representative material creep and oxidation properties are used to predict and compare the relative extent of creep to environmental damage under different loading conditions.