The formation of White (WEL) and Brown Etching Layers (BEL) on rail raceways during service causes the initiation of microcracks which finally leads to failure. Detailed characterization of the WEL ...and the BEL in a pearlitic rail steel is carried out from micrometer to atomic scale to understand their microstructural evolution. A microstructural gradient is observed along the rail depth including martensite, austenite and partially dissolved parent cementite in the WEL and tempered martensite, ultrafine/nanocrystalline martensite/austenite, carbon saturated ferrite and partially dissolved parent cementite in the BEL. Plastic deformation in combination with a temperature rise during wheel-rail contact was found to be responsible for the initial formation and further microstructural evolution of these layers. The presence of austenite in the WEL/BEL proves experimentally that temperatures rise into the austenite range during wheel-rail contact. This is in agreement with finite element modelling results. Each wheel-rail contact must be considered as an individual short but intense deformation and heat treatment cycle that cumulatively forms the final microstructure, as shown by diffusion length calculations of C and Mn. The presence of secondary carbides in the BEL indicates that the temperature in the BEL during individual loading cycles reaches levels where martensite tempering occurs. Partially fragmented primary cementite laths, enriched in Mn, depleted in Si, and surrounded by a C-gradient and dislocations were found in the BEL. The initial step in the formation of BEL and WEL is the defect- and diffusion-assisted decomposition of the original microstructure.
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Hydrogen induced cracks were introduced in an ultra-low carbon (ULC) steel by subjecting it to electrochemical hydrogen charging. The damage characteristics were investigated for three material ...conditions, i.e. cold deformed, recovered, and recrystallized state. The aim of the work was to understand the effect of deformation induced defects on the hydrogen induced cracking of this material. Additionally, the effect of the charging conditions, i.e. charging time and current density, on the cracking characteristics were verified. The blister surfaces and related hydrogen induced cracks were studied by optical microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Deformed samples were considerably more sensitive to hydrogen induced cracking, which demonstrates the important role of dislocations in hydrogen induced damage mechanisms. Permeation tests were performed in order to elucidate the role of hydrogen diffusion in the process. Charging conditions had a clear influence on the hydrogen induced cracking behavior of the material. This should be taken into account when designing experimental parameters in order to obtain results valid under real life conditions.
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Recently, lattice titanium manufactured by additive manufacturing (AM) techniques has been utilized in various applications, including biomedical. The effects of topological design ...and processing parameters on the fatigue behaviour of such meta-biomaterials have been studied before. Most studies show that the fatigue life of additively manufactured lattice structures is limited. Post-processing techniques could play a major role in improving the fatigue of these promising biomaterials. This study aims to provide an in-depth investigation into the effects of heat treatments, hot isostatic pressing (HIP), sand blasting, and chemical etching on the microstructure, surface morphology, strength and fatigue resistance of selective laser melted titanium meta-biomaterials. It was found that the combination of microstructural design and surface engineering, induced by HIP and sand blasting respectively, allows to increase the endurance limit of these lattice meta-biomaterials by a factor of two. HIP treatment substantially decreased the internal porosity and transformed the microstructure to a more ductile mixture of α + β phases. Sand blasting allowed to eliminate surface imperfections and induced favourable compressive stress in the surface layer of the struts.
Additively manufactured metallic meta-biomaterials are progressively being used as bone replacement orthopedic implants. While there is a great amount of research related to topological designs and their effect on mechanical (e.g. stiffness), physical (e.g. mass transport), and biological (e.g. osseointegration) properties, fatigue lifetime of such structures remains limited. This study provides fundamental investigation into the combined effect of microstructural design and surface engineering of titanium meta-biomaterial, enabled through various post treatment methods ranging from heat treatments to physical and chemical surface modifications. The findings show that fatigue life is significantly improved by applying developed herein novel method, which effortlessly can be used on other bone-mimicking metallic meta-biomaterials.
The present work evaluates hydrogen induced cracking in a TRIP (transformation induced plasticity) assisted steel and pure iron. The goal of this work is to understand the effect of the macroscopic ...stress distribution in the material on the hydrogen induced cracking phenomenon. Additionally, the effect of a complex multiphase microstructure on the characteristics of hydrogen induced cracking was investigated by comparing results for TRIP-assisted steel and pure iron as reference material. Tensile tests on notched and unnotched samples combined with in-situ electrochemical hydrogen charging were conducted. Tests were performed until the tensile strength was reached and until fracture. The resulting hydrogen induced cracks were studied by optical microscopy and scanning electron microscopy (SEM). Hydrogen induced cracks showed a typical S-shape and crack propagation was mainly transgranular, independently of the presence of a notch or the material's microstructure. This was also the case for the V-shaped secondary crack network and resulting stepped crack morphology characteristic for hydrogen induced damage. These observations indicate that the stress state surrounding the crack tip has a very large impact on the hydrogen induced cracking characteristics. The use of a notch or the presence of a different microstructure did not influence the overall hydrogen induced cracking features, but did change the kinetics of the hydrogen induced cracking process.
The present work evaluates hydrogen induced cracking by performing an elaborate EBSD (Electron BackScatter Diffraction) study in a steel with transformation induced plasticity (TRIP-assisted steel). ...This type of steel exhibits a multiphase microstructure which undergoes a deformation induced phase transformation. Additionally, each microstructural constituent displays a different behavior in the presence of hydrogen. The aim of this study is to obtain a better understanding on the mechanisms governing hydrogen induced crack initiation and propagation in the hydrogen saturated multiphase structure. Tensile tests on notched samples combined with in-situ electrochemical hydrogen charging were conducted. The tests were interrupted at stresses just after reaching the tensile strength, i.e. before macroscopic failure of the material. This allowed to study hydrogen induced crack initiation and propagation by SEM (Scanning Electron Microscopy) and EBSD. A correlation was found between the presence of martensite, which is known to be very susceptible to hydrogen embrittlement, and the initiation of hydrogen induced cracks. Initiation seems to occur mostly by martensite decohesion. High strain regions surrounding the hydrogen induced crack tips indicate that further crack propagation may have occurred by the HELP (hydrogen-enhanced localized plasticity) mechanism. Small hydrogen induced cracks located nearby the notch are typically S-shaped and crack propagation was dominantly transgranularly. The second stage of crack propagation consists of stepwise cracking by coalescence of small hydrogen induced cracks.
•Hydrogen induced cracking in TRIP-assisted steel is evaluated by EBSD.•Tensile tests were conducted on notched hydrogen saturated samples.•Crack initiation occurs by a H-Enhanced Interface DEcohesion (HEIDE) mechanism.•Crack propagation involves growth and coalescence of small cracks.•Propagation is governed by the characteristics of phases on the crack path.
The present work evaluates hydrogen induced cracking in a TRIP-assisted steel with a multiphase microstructure, containing ferrite, bainite, retained austenite, and some martensite. When deformed, ...the retained austenite transforms to martensite, which changes the phase balance in the alloy. Each microstructural constituent demonstrates a different behavior in the presence of hydrogen. The goal of this work is to understand the response of the hydrogen saturated multiphase structure to a mechanical load. Tensile tests on notched samples combined with in-situ electrochemical hydrogen charging were conducted. The test was interrupted at specific points, before the macroscopic failure of the material. Hydrogen induced crack initiation and propagation were examined by studying the microstructure at several intermediate elongations. Characteristic hydrogen induced cracks were only observed after reaching tensile strength and were located at the surface in a specific pattern. Finite element simulations indicated that the observed crack pattern coincides with the increased stress regions induced by the notch presence. This indicates that hydrogen induced crack formation is dominantly stress induced for this steel.
•Hydrogen induced cracking in TRIP-assisted steel is evaluated.•Tensile tests were conducted on notched hydrogen saturated samples.•TRIP steels are very sensitive to hydrogen embrittlement.•Hydrogen induced cracks were clearly present after reaching tensile strength.•Hydrogen presence promotes stress-induced cracks in specific regions.
Abstract
We present the Mid-infrared stellar Diameters and Fluxes compilation Catalogue (MDFC) dedicated to long-baseline interferometry at mid-infrared wavelengths (3–13 $\mu$m). It gathers data for ...half a million stars, i.e. nearly all the stars of the Hipparcos-Tycho catalogue whose spectral type is reported in the SIMBAD data base. We cross-match 26 data bases to provide basic information, binarity elements, angular diameter, magnitude and flux in the near and mid-infrared, as well as flags that allow us to identify the potential calibrators. The catalogue covers the entire sky with 465 857 stars, mainly dwarfs and giants from B to M spectral types closer than 18 kpc. The smallest reported values reach 0.16 $\mu$Jy in L and 0.1 $\mu$Jy in N for the flux, and 2 microarcsec for the angular diameter. We build four lists of calibrator candidates for the L and Nbands suitable with the Very Large Telescope Interferometer (VLTI) sub- and main arrays using the MATISSE instrument. We identify 1621 candidates for L and 44 candidates for N with the Auxiliary Telescopes (ATs), 375 candidates for both bands with the ATs, and 259 candidates for both bands with the Unit Telescopes (UTs). Predominantly cool giants, these sources are small and bright enough to belong to the primary lists of calibrator candidates. In the near future, we plan to measure their angular diameter with 1 per cent accuracy.
The microstructure of a 9Cr-1W-0.22V-0.09Ta-0.11C reduced activation ferritic/martensitic (RAFM) steel has been investigated after thermo-mechanical rolling with subsequent annealing for 30 min at ...temperatures of 880 °C, 920 °C, 980 °C and 1050 °C, followed by water quenching. Scanning and transmission electron microscopy investigations and electron backscattered diffraction (EBSD) measurements were performed to determine the microstructural features after the different thermal treatments. Additionally, the microstructure and the mechanical properties of the materials were studied after tempering at 750 °C for 2 h. This study aims to understand microstructural processes that occur in the material during thermo-mechanical treatment and to assess the effect of the microstructure on its strength and toughness, with a view on improving its mechanical performance. Microstructural analysis together with the data from mechanical tests identified the beneficial effect of grain refinement obtained with adequate processing on the ductile-to-brittle transition temperature (DBTT) and on the delay of strength degradation at elevated temperatures.
•TM-rolling and micro-alloying with V and Ta lead to alternative precipitation state.•TM-rolling reduces precipitation of carbonitrides at grain boundaries.•The precipitation size and distribution can be optimized by appropriate annealing temperature.•TM-rolling and appropriate Q&T lead to a grain refinement of the martensitic structure.•The obtained refined martensite has improved toughness and a lower DBTT.
In this study, accumulative roll bonding (ARB) process was carried out on an AA1100 aluminum sheet up to 10 cycles. Electron backscattering diffraction (EBSD) method was utilized to investigate the ...microstructural evolution during the ARB process. It was observed that the ARB is a promising process for fabricating ultra-fine grained structures in aluminum sheets. The results indicate that several mechanisms are responsible for the microstructural changes at different levels of strain during this process. Grain subdivision as well as the development of sub-grains are the major mechanisms at the early stages of ARB. Strain induced transition of low angle to high angle grain boundaries and the formation of thin lamellar structure occurs at the medium levels of strain. Finally, the progressive break up of this thin lamellar structure into more equiaxed grains is the dominant mechanism at relatively high strains. By reducing the grain size, the yield stress and the tensile strength of the ARBed sheets increased significantly and reached the maximum values of 282 and 333
MPa after the tenth cycle. The strength held Hall–Petch relationship and was in a good conformity with the microstructural changes.
The softening effect in metals due to ultrasonic vibration is used in many industrial applications. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic ...treatment either imposes additional stress waves to supplement the quasi-static applied load or causes heating of the metal. In both cases the intrinsic deformation resistance and/or mechanisms of the metal are assumed to be unaltered by the ultrasound. In this study, the effect of an in situ ultrasonic treatment on the microstructure of low-carbon steel (Fe–0.051C–0.002Si–0.224Mn–0.045Al (wt.%)) under tensile deformation is reported. Detailed microstructural analyses reveal that the ultrasonic treatment intrinsically alters the deformation characteristics of the metal. The deformation microstructure underneath the area of treatment in the deformed samples was investigated by a combination of optical microscopy, scanning electron microscopy, crystal orientation mapping by electron backscattered diffraction and X-ray diffraction. The results show that the dislocation density and the fraction of low-angle grain boundaries decrease significantly, accompanied by preferential grain rotation. The softening effect of the ultrasound is found to drive recovery associated with a significant reduction in subgrain formation during deformation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, the reduction in subgrain formation is shown to occur due to the combined application of the quasi-static loading and the ultrasound, but is not a simple addition of the two factors acting separately. The effect of the ultrasound can be attributed to its ability to enhance dislocation dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances, thereby increasing the probability of annihilation.
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