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Wear-related energy and material loss cost over 2500 Billion Euro per year. Traditional wisdom suggests that high-strength materials reveal low wear rates, yet, their plastic deformation mechanisms also influence their wear performance. High strength and homogeneous deformation behavior, which allow accommodating plastic strain without cracking or localized brittle fracture, are crucial for developing wear-resistant metals. Here, we present an approach to achieve superior wear resistance via in-situ formation of a strong and deformable oxide nanocomposite surface during wear, by reaction of the metal surface with its oxidative environment, a principle that we refer to as ‘reactive wear protection’. We design a TiNbZr-Ag alloy that forms an amorphous-crystalline oxidic nanocomposite surface layer upon dry sliding. The strong (2.4 GPa yield strength) and deformable (homogeneous deformation to 20% strain) nanocomposite surface reduces the wear rate of the TiNbZr-Ag alloy by an order of magnitude. The reactive wear protection strategy offers a pathway for designing ultra-wear resistant alloys, where otherwise brittle oxides are turned to be strong and deformable for improving wear resistance., Wear-resistant metals have long been a pursuit of reducing wear-related energy and material loss. Here the authors present the ‘reactive wear protection’ strategy via friction-induced in situ formation of strong and deformable oxide nanocomposites on a surface.

Severe plastic deformation leads to cementite decomposition in pearlitic and martensitic alloys, resulting in high-strength nanocrystalline ferrite. This effect can be employed to strengthen pearlitic wires but it can also be associated with material failure by white etching cracks (WECs) that are primarily known to concern bearings or rails. We investigate the decomposition of spheroidal cementite in the martensitic bearing steel 100Cr6 with 62 Rockwell hardness during high pressure torsion at 9.5 GPa applied pressure. The hard martensitic matrix and the even harder spheroidal cementite precipitates behave plastically very differently. The enforced macroscopic plastic deformation is almost entirely carried by the matrix. Plastic material flow of the matrix around the spheroidal cementite leads to wear of the spheroidal cementite as indicated by continuously increasing levels of chromium in the matrix. Plastic deformation of spheroidal cementite via dislocation gliding supposedly accelerates this process as slip steps generated thereby are preferential sites of wear at the matrix/cementite interface. Larger spheroidal cementite precipitates are more prone to plastic deformation and to decomposition than smaller ones. The mechanism likely holds true in general for multiphase materials with large strain difference between phases subjected to high pressure torsion. Although the formation of WECs in 100Cr6 bearings might be slowed down by reducing the size of the spheroidal cementite precipitates, it is unlikely that this could entirely prevent this failure mechanism.

Material failure by white-etching-cracks (WECs) can cause enormous economic costs. The formation of WECs emerges from the decomposition of the original, usually cementite-containing, microstructure...

Failure in engineering materials like steels is strongly affected by in-service deleterious alterations in their microstructure. White Etching Layers (WELs) are an example of such in-service alterations in the pearlitic microstructure at the rail surface. Cracks initiate in the rails due to delamination and fracture of these layers and propagate into the base material posing severe safety concerns. In this study, we investigate the microscale fracture behaviour of these WELs. We use in situ elastic-plastic fracture mechanics using J-integral to quantify the fracture toughness. Although usually assumed brittle, the fracture toughness of 21–25 MPa√m reveals a semi-brittle nature of WELs. Based on a comparison of the fracture toughness and critical defect size of WELs with the undeformed pearlitic steels, WELs are detrimental for rails. In the micro fracture tests, WELs show crack tip blunting, branching, and significant plasticity during crack growth due to their complex microstructure. The fracture behaviour of the WELs is governed by their microstructural constituents such as phases (martensite/austenite), grain size, dislocation density and carbon segregation to dislocations and grain boundaries. We observed dislocation annihilation in some martensitic grains in the WELs which also contributes to their fracture behaviour. Additionally, the strain-induced transformation from austenite to martensite affects the crack growth and fracture.

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.

In this article, we probe the strain partitioning between the microstructural features present in a continuously cooled carbide-free bainitic steel together with damage nucleation and propagation. These features mainly comprise of phases (bainitic ferrite, martensite, and blocky/thin film austenite), interfaces between them, grain size and grain morphology. A micro Digital Image Correlation (μ-DIC) technique in scanning electron microscope is used to quantify the strain distribution between these microstructural features. The results show a strong strain partitioning between martensite, bainitic ferrite and retained austenite that provides weak links in the microstructure and creates conditions for the crack initiation and propagation during deformation. Blocky austenite islands accommodate maximum local strains in the global strain range of 0–2.3% and undergo strain-induced austenite to martensite transformation governing the local strain evolution in the microstructure. However, the local strains are minimum in martensite regions during entire in-situ deformation stage. Narrow bainitic ferrite channels in between martensitic islands and martensite-bainitic ferrite interfaces are recognised as primary damage sites with high strain accumulation of 30 ± 2% and 20 ± 3% respectively, at a global strain of 9%. The inclination of these interfaces with the tensile direction also affects the strain accumulation and damage.

When metals are mechanically loaded at elevated temperatures for extended periods of time, creep damage will occur in the form of cavities at grain boundaries. In the present experiments it is demonstrated that in binary iron-tungsten alloys creep damage can be self healed by selective precipitation of a W-rich phase inside these cavities. Using synchrotron X-ray nano-tomography the simultaneous evolution of creep cavitation and precipitation is visualized in 3D images with a resolution down to 30 nm. The degree of filling by precipitation is analysed for a large collection of individual creep cavities. Two clearly different types of behaviour are observed for isolated and linked cavities, where the isolated cavities can be filled completely, while the linked cavities continue to grow. The demonstrated self-healing potential of tungsten in iron-based metal alloys provides a new perspective on the role of W in high-temperature creep-resistant steels.

Many materials science phenomena require joint structural and chemical characterization at the nanometer scale to be understood. This can be achieved by correlating electron microscopy (EM) and atom probe tomography (APT) subsequently on the same specimen. For this approach, specimen yield during APT is of particular importance, as significantly more instrument time per specimen is invested as compared to conventional APT measurements. However, electron microscopy causes hydrocarbon contamination on the surface of atom probe specimens. Also, oxide layers grow during specimen transport between instruments and storage. Both effects lower the chances for long and smooth runs in the ensuing APT experiment. This represents a crucial bottleneck of the method correlative EM/APT. Here, we present a simple and reliable method based on argon ion polishing that is able to remove hydrocarbon contamination and oxide layers, thereby significantly improving APT specimen yield, particularly after EM.

Observing solute hydrogen (H) in matter is a formidable challenge, yet, enabling quantitative imaging of H at the atomic-scale is critical to understand its deleterious influence on the mechanical strength of many metallic alloys that has resulted in many catastrophic failures of engineering parts and structures. Here, we report on the APT analysis of hydrogen (H) and deuterium (D) within the nanostructure of an ultra-high strength steel with high resistance to hydrogen embrittlement. Cold drawn, severely deformed pearlitic steel wires (Fe–0.98C–0.31Mn–0.20Si–0.20Cr–0.01Cu–0.006P–0.007S wt%, e = 3.1 ) contains cementite decomposed during the pre-deformation of the alloy and ferrite. We find H and D within the decomposed cementite, and at some interfaces with the surrounding ferrite. To ascertain the origin of the H/D signal obtained in APT, we explored a series of experimental workflows including cryogenic specimen preparation and cryogenic-vacuum transfer from the preparation into a state-of-the-art atom probe. Our study points to the critical role of the preparation, i.e. the possible saturation of H-trapping sites during electrochemical polishing, how these can be alleviated by the use of an outgassing treatment, cryogenic preparation and transfer prior to charging. Accommodation of large amounts of H in the under-stoichiometric carbide likely explains the resistance of pearlite against hydrogen embrittlement.

Millions worldwide suffer from arthritis of the hips, and total hip replacement is a clinically successful treatment for end‐stage arthritis patients. Typical hip implants incorporate a cobalt alloy (Co–Cr–Mo) femoral head fixed on a titanium alloy (Ti‐6Al‐4V) femoral stem via a Morse taper junction. However, fretting and corrosion at this junction can cause release of wear particles and metal ions from the metallic implant, leading to local and systemic toxicity in patients. This study is a multiscale structural‐chemical investigation, ranging from the micrometer down to the atomic scale, of the underlying mechanisms leading to metal ion release from such taper junctions. Correlative transmission electron microscopy and atom probe tomography reveals microstructural and compositional alterations in the subsurface of the titanium alloy subjected to in vitro gross‐slip fretting against the cobalt alloy. Even though the cobalt alloy is comparatively more wear‐resistant, changes in the titanium alloy promote tribocorrosion and subsequent degradation of the cobalt alloy. These observations regarding the concurrent occurrence of electrochemical and tribological phenomena are vital to further improve the design and performance of taper junctions in similar environments., Corrosion and wear at taper junctions in metallic implants can release toxic metal ions into the body. Although biomedical cobalt alloys have a higher hardness than titanium alloys, this study finds that changes on the titanium alloy during fretting can promote tribocorrosion in the cobalt alloy. Such insights can be used to improve material design in taper junctions and implants.

Metallurgical and materials transactions / A 51(11), 5517-5586 (2020). doi:10.1007/s11661-020-05947-2, Published by Springer, Boston

For several decades, the formation of carbon(C)-rich domains upon room temperature aging of supersaturated martensite has been a matter of debate. C-rich tweed-like patterns are observed to form after short aging times at room temperature and coarsen upon further aging. Here, we present a systematic atomic-scale investigation of carbide formation in Fe-15Ni-1C (wt.%) martensite after two to three years of isothermal room temperature aging by a combination of atom probe tomography and transmission electron microscopy. Owing to the sub-zero martensite start temperature of −25 °C, a fully austenitic microstructure is maintained at room temperature and the martensitic phase transformation is initiated during quenching in liquid nitrogen. In this way, any diffusion and redistribution of C in martensite is suppressed until heating up the specimen and holding it at room temperature. The microstructural changes that accompany the rearrangement of C atoms have been systematically investigated under controlled isothermal conditions. Our results show that after prolonged room temperature aging nanometer-sized, plate-shaped η-Fe2C carbides form with a macroscopic martensite habit plane close to {521}. The orientation relationship between the η-Fe2C carbides and the parent martensite grain (α′) follows [001]α’//[001]η, ( 1 ¯ 10 ) α’//(020)η. The observation of η-Fe2C–carbide formation at room temperature is particularly interesting, as transition carbides have so far only been reported to form above 100 °C. After three years of room temperature aging a depletion of Fe is observed in the η carbide while Ni remains distributed homogenously. This implies that the substitutional element Fe can diffuse several nanometers in martensite at room temperature within three years.

Performing electron microscopy and atom probe tomography at the same location on the same specimen combines the strengths of electron microscopy, which is primarily the analysis of defects and crystal structures, with the strengths of atom probe tomography, which is primarily the robust, accurate and sensitive three dimensional compositional analysis. This viewpoint article provides a summary of the broad range of electron microscopy techniques that have been performed on atom probe specimens to date. It describes what technique is best suited to address a specific materials science question and finishes with an outlook on possible future developments in the field.

Nanoscale solute segregation to or near lattice defects is a coupled diffusion and trapping phenomenon that occurs in superalloys at high temperatures during service. Understanding the mechanisms underpinning this crucial process will open pathways to tuning the alloy composition for improving the high-temperature performance and lifetime. Here, we introduce an approach combining atom probe tomography with high-end scanning electron microscopy techniques, in transmission and backscattering modes, to enable direct investigation of solute segregation to defects generated during high-temperature deformation such as dislocations in a heat-treated Ni-based superalloy and planar faults in a CoNi-based superalloy. Three protocols were elaborated to capture the complete structural and compositional nature of the targeted defect in the alloy.

In this study, we investigate the fundamentals of deformation-driven cementite decomposition during rolling contact fatigue in a binary Fe-0.74C (wt.%) steel with pearlitic microstructure. To reveal the involved nano-scale mechanisms, we apply transmission electron microscopy and atom probe tomography, as well as the combination of both techniques on the same probe volume for correlative structural and chemical analysis. After ~32,500 individual ball contacts under a nominal Hertzian contact stress of ~1,250 MPa, cementite lamellae are consistently depleted in carbon (C) down to ~20 at.%, which is a reduction of 20% in comparison to the original stoichiometric composition of 25 at.% before deformation. By electron diffraction on atom probe tip volumes, we show that this under-stoichiometric cementite still maintains its crystal structure. The potential effect of temperature increase during rolling contact fatigue on cementite decomposition is addressed by carrying out annealing experiments at 150°C and 250°C for 30 minutes after rolling contact fatigue. Our results show that slip transmission across cementite lamellae is primarily responsible for the observed C-depletion in cementite. We quantitatively discuss slip transmission from ferrite to cementite using slip trace analysis and correlate the amount of slip activity with the observed C-depletion. In this way, we explain the formation of under-stoichiometric cementite by combined slip transmission and solute drag by dislocations, where C is transported out of cementite by dislocation gliding through cementite lamellae. Only afterward, cementite decomposition in terms of phase dissolution is suggested to occur by the dislocation-shuffle mechanism.

Premature failure of bearings during rolling contact fatigue is often associated with the formation of white etching cracks (WECs). Crack surface rubbing of WECs transforms the original bainitic/martensitic microstructure into white etching areas (WEAs), comprised of nanocrystalline ferrite. The grain size and carbon content vary within the WEA. Here, we show by atom probe tomography and scanning electron microscopy, that there is an inversely proportional relationship between grain size and carbon content in WEAs formed in 100Cr6 bearings that failed by WECs in service. We explain this phenomenon by the reduction of grain boundary energy through carbon segregation. Depending on the carbon content, this reduces the driving force for recrystallization and grain coarsening, thereby stabilizing the nanocrystalline microstructure. No such effect is observed for the substitutional element chromium. The smallest grain size (

We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 °C, nano-sized κ-carbides form, as characterized by TEM and APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate. In comparison to the as-quenched κ-free state, the precipitation of κ-carbides leads to a significant increase in yield strength (∼480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the κ-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111} , leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to understand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of κ-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, κ-free state.

We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions.

Atom probe tomography was employed to characterize the microstructure and C distribution in the white etching area (WEA) of a martensitic 100Cr6 bearing steel subjected to rolling contact fatigue. Different from its surrounding matrix where a plate-like martensitic structure prevails, the WEA exhibits equiaxed grains with a uniform grain size of about 10 nm. Significant C grain boundary enrichment (>7.5at.%) and an overall higher C concentration than the nominal value are observed in the WEA. These results suggest that the formation of WEA results from severe local plastic deformation that causes dissolution of carbides and the redistribution of C.

A density variation in shear bands visible by electron microscopy is correlated with compositionally altered locations measured by atom-probe tomography in plastically-deformed Al85.6Y7.5Fe5.8 metallic-glass ribbons. Twocompositionally distinct regions are identified along shear bands, one is Al-rich (~92 at.%), the other is Al depleted(~82.5 at.%) and both regions show marginal concentration fluctuations of Y and Fe. The elementalre-distribution is observed within shear bands only, and no chemical exchange with the surrounding glassy matrixis observed. Macroscopic deformation of metallic glasses (MGs) is mainly confinedto shear bands (SBs), which limits the ductility to often less thana fewpercent intension[1–6].Much effort has been devoted to enhancethe ductility by controlling SBs initiation and propagation [7–18]. Due tothe key role of SBs in the deformation, better understanding of theirstructure and chemical composition is of interest. Transmission electron microscopy (TEM) has been extensively usedto study the underlying mechanisms of vitrification and nanocrystallizationin Al-based MGs [19–22] in order to obtain glass/crystalcomposites with enhanced ductility [23,24]. High-angle annular darkfieldscanning TEM(STEM-HAADF) has revealed a periodic spatial variationof dark and bright regions of intensity along thin SBs in plasticallydeformedAl88Y7Fe5 [7], Pd40Ni40P20 [8] andZr52.5Cu17.9Ni14.6Al10Ti5 [25]MGs (all compositions are given in at.% throughout the text). Since thedark field intensity corresponds to a resultant atomic (Z) contrast, theobserved periodic variations can originate from the combination oflocal compositional and mass-density fluctuations. The variationswere also observed for Zr-, Pd- and Mg-based MGs [26] by a synchrotronX-ray tomography. For an Al88Y7Fe5 MG ribbon, cold-rolled to~28% thickness reduction, mass-density changes between −9% and+6% and an average fluctuation length of ~50–110 nm in b10 nmthick SBs were observed [7]. Periodic-density fluctuations along SBswere also reported for cold-rolled Pd40Ni40P20 MGs with up to 20%thickness reduction [8]. Some reports correlate the fluctuations to ahigh volumetric strain during SBs formation [11]. Although the structureof SBs has been described by atomistic simulations [27–29], it is unclearwhether density variation inside SBs is a generally-observedphenomenon. Unlike the observations reported in Ref. [8], no densityfluctuations inside SBs were observed in Pd40Ni40P20 bulk MGs compressedto an engineering plastic strain of 0.4 [30]. Despite the effortsto characterize the local low- and high-mass-density variations in SBsfor Al88Y7Fe5 glass, the corresponding compositional changes have notbeen rigorously analyzed except by Rösner et al. [25] using STEMenergy-dispersive spectroscopy (STEM-EDS). Nevertheless, varying foil thickness and a projection plane of SBs in the EDS analyzed volumecauses practical challenges to accurately quantify the composition insideSBs, which are clearly visible in the edge-on perspective only, andhence characterizing 3-D aspects of the elemental re-distribution bySTEM is difficult.Atom-probe tomography (APT) enables quantitative 3-D compositionalanalysis with the near atomic resolution with equal sensitivityin the range of 10 ppm for all elements [31], which makes it an idealprobe for chemical changes associated with SBs. Typically, APT hasbeen used to study solute partitioning [32], vitrification and crystallization [33,34] in Al-basedMGs. There have been relatively a fewattemptsto investigate both, the mass-density and the compositional fluctuationsalong SBs by APT. Hunter et al. [35] studied the origin of SBs initiationin a Ti-based glass/nanocrystalline composite and they observedno compositional fluctuations along SBs. For APT technique, theatomic-density and spatial resolutions are prone to artifacts due toion-trajectory aberrations and local magnification effects. It has beenargued that the apparent density change observed in SBs by APT couldbe an artifact due to the local lowering of the evaporation field at SBs positionsin a tip [35]. The presence of such aberration effects forbids to unambiguouslylocate SBs in terms of APT alone, especially when thechemical variations inside SBs and relative to the adjacent matrix aresubtle. In this paper, a combined TEM/APT characterization [36] is appliedto directly correlate density fluctuations along SBs visible bySTEM with chemical alterations analyzed by APT.

Analysis and understanding of the role of hydrogen in metals is a significant challenge for the future of materials science, and this is a clear objective of recent work in the atom probe tomography (APT) community. Isotopic marking by deuteration has often been proposed as the preferred route to enable quantification of hydrogen by APT. Zircaloy-4 was charged electrochemically with hydrogen and deuterium under the same conditions to form large hydrides and deuterides. Our results from a Zr hydride and a Zr deuteride highlight the challenges associated with accurate quantification of hydrogen and deuterium, in particular associated with the overlap of peaks at a low mass-to-charge ratio and of hydrogen/deuterium containing molecular ions. We discuss possible ways to ensure that appropriate information is extracted from APT analysis of hydrogen in zirconium alloy systems that are important for nuclear power applications.

The process of building an open source library of simulated field desorption maps for differently oriented synthetic tips of the face-centered cubic, body-centered cubic, and hexagonal-close-packed crystal structures using the open source software TAPSim is reported. Specifically, the field evaporation of a total set of 4 × 101 single-crystalline tips was simulated. Their lattices were oriented randomly to sample economically the fundamental zone of crystal orientations. Such data are intended to facilitate the interpretation of low-density zone lines and poles that are observed on detector hit maps during Atom Probe Tomography (APT) experiments. The datasets and corresponding tools have been made publicly available to the APT community in an effort to provide better access to simulated atom probe datasets. In addition, a computational performance analysis was conducted, from which recommendations are made as to which key tasks should be optimized in the future to improve the parallel efficiency of TAPSim.

Ti has a high affinity for hydrogen and is a typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along {\alpha} grain boundaries and {\alpha}/\b{eta} phase boundaries in commercial pure Ti and {\alpha}+\b{eta} binary model alloys. No hydrides are observed in the {\alpha} phase in alloys with Al addition or quenched-in Mo supersaturation., Comment: Published in Acta Materialia in 2018

Atomic-scale investigation was performed on 51CrV4 steel, isothermally held at different temperatures within the bainitic temperature range. Transmission electron microscopy (TEM) analysis revealed three different morphologies: lower, upper, and inverse bainite. Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, which showed no evidence of partitioning of substitutional elements; only carbon partitioned into cementite to the equilibrium value. Carbon in the bainitic ferrite was found to segregate at dislocations and to form Cottrell atmospheres. The concentration of carbon remaining in solution measured by APT was more than expected at the equilibrium. Upper bainite contained cementite as well. Chromium and manganese were found to redistribute at the cementite-austenite interface and the concentration of carbon in the ferritic matrix was found to be lower than the one measured in the case of lower bainite. After isothermal treatments close to the bainite start temperature, another austenite decomposition product was found at locations with high concentration of Mn and Cr, resembling inverse bainite. Site-specific APT analysis of the inverse bainite reveals significant partitioning of manganese and chromium at the carbides and at the ferrite/martensite interfaces, unlike what is found at isothermal transformation products at lower temperatures.

The fracture behavior of a white etching layer formed on the rail surface in pearlitic steels during the rail-wheel contact is investigated using indentation-based microcantilever fracture tests. The sample thickness is in the order of 5 μm. The local fracture toughness of the white etching layer, its neighboring brown etching layer, martensite and pearlite with similar chemical composition are determined and compared to ferritic steels. All samples show stable crack growth accompanied by significant plasticity at the crack tip. The toughnesses scale inversely with the microhardness. The white etching layer exhibits a toughness of 16.0 ± 1.2 MPa m1/2 which is in the same range as the fully martensitic steel. It is shown that the local fracture toughness can be roughly estimated based on the Vickers hardness of the white etching layer. Also, an estimation of a critical defect size in white etching layers which considerably furthers the understanding of crack initiation is made in this study. Furthermore, various criteria for analyzing the elasto plastic fracture toughness are compared. Keywords: Rail steel, White etching layer, Elasto plastic fracture mechanics, Micro cantilever testing

Since white etching crack (WEC) phenomena primarily occur in high carbon steels, we elucidate the role of carbon in this failure mechanism in bearings. The nano-crystalline ferritic regions that make up the white etching area (WEA) are formed by crack surface rubbing leading to complete decomposition of the initial microstructure by severe plastic deformation. In order to analyze local carbon compositions on µm-nm length scales, we employ electron probe microanalysis, transmission electron microscopy and atom probe tomography. We focus on a 100Cr6 wind turbine gearbox bearing which failed in service due to extensive formation of WEC networks below the raceway surface and subsequent spalling. Our results show a significantly lower carbon content in the WEA as compared to the nominal alloy composition. At the same time, we find carbon deposits with a carbon content of > 85 at%, which are heterogeneously distributed across WEAs. We explain this observation by assuming segregation of excess carbon from the WEA to the open crack surfaces during crack surface rubbing. Further, the presence of a “lubricating” carbon film at the WEC surfaces might explain the accelerated failure by WECs as compared to classical rolling contact fatigue.

The strain hardening mechanism of a high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C (wt%)) is investigated by electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). The alloy is characterized by a constant high strain hardening rate accompanied by high strength and high ductility (ultimate tensile strength: 900 MPa, elongation to fracture: 68%). Deformation microstructures at different strain levels are studied in order to reveal and quantify the governing structural parameters at micro- and nanometer scales. As the material deforms mainly by planar dislocation slip causing the formation of slip bands, we quantitatively study the evolution of the slip band spacing during straining. The flow stress is calculated from the slip band spacing on the basis of the passing stress. The good agreement between the calculated values and the tensile test data shows dynamic slip band refinement as the main strain hardening mechanism, enabling the excellent mechanical properties. This novel strain hardening mechanism is based on the passing stress acting between co-planar slip bands in contrast to earlier attempts to explain the strain hardening in high-Mn lightweight steels that are based on grain subdivision by microbands. We discuss in detail the formation of the finely distributed slip bands and the gradual reduction of the spacing between them, leading to constantly high strain hardening. TEM investigations of the precipitation state in the as-quenched state show finely dispersed atomically ordered clusters (size

High-entropy alloys (HEAs) and metallic glasses (MGs) are two material classes based on the massive mixing of multiple-principal elements. HEAs are single or multiphase crystalline solid solutions with high ductility. MGs with amorphous structure have superior strength but usually poor ductility. Here, the stacking fault energy in the high-entropy nanotwinned crystalline phase and the glass-forming-ability in the MG phase of the same material are controlled, realizing a novel nanocomposite with near theoretical yield strength (G/24, where G is the shear modulus of a material) and homogeneous plastic strain above 45% in compression. The mutually compatible flow behavior of the MG phase and the dislocation flux in the crystals enable homogeneous plastic co-deformation of the two regions. This crystal-glass high-entropy nanocomposite design concept provides a new approach to developing advanced materials with an outstanding combination of strength and ductility.

The high nitrogen bearing steel X30CrMoN15-1, marketed under commercial names such as Cronidur30, VC444, NitroMax and N360, is well-known for its outstanding rolling contact fatigue and corrosion properties. More importantly, there are no reports about failure by white etching cracks for this alloy, although this premature failure mode is frequently observed in conventional high carbon bearing steels. To understand the origin of this superior performance, we analyze in the current paper its microstructure prior to rolling contact fatigue by means of scanning electron microscopy, atom probe tomography and nano-indentation, and relate this to its thermal processing history. The origin of the exceptional white etching crack resistance of this alloy is rationalized in terms of the mechanical and thermodynamic stability of the precipitates, the different grain boundary segregation behavior between nitrogen and carbon as indicated by the atom probe results, and the cleanliness of the steel.

White etching cracks (WECs) and the associated white etching areas (WEAs) are responsible for failure of widely spread engineering applications such as bearings and railways. Although the phenomenon is known for more than 100 years, the underlying mechanisms are still a matter of debate. In this work, we thoroughly investigate a 100Cr6 wind turbine gearbox bearing after failure in service operation. Based on our findings from detailed microstructure characterization on multiple length scales we formulate a new consistent explanation for the formation of WEAs during rolling contact fatigue. We propose a mechanism of moving WECs - not only in terms of conventional crack propagation but also as a movement of the crack normal to its plane. During cyclic loading the crack continuously changes its position and leaves behind a severely plastically deformed area consisting of ferritic nano-grains, i.e. the WEAs. The atomic-scale delocalization of the crack plane in a single loading cycle adds up to micron-sized WEAs during repetitive loading/unloading. After the initial formation of a fatigue crack around inclusions, crack face rubbing occurs during compressive loading cycles. This leads to the formation of WEA by local severe plastic deformation. It also leads to partial cohesion of the abutting crack faces and material transport between them. As a result, the WEC opens at a slightly shifted position with respect to its former location during unloading.

Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr wt. %) was electrochemically charged with deuterium to create deuterides and subsequently analysed with atom probe tomography and scanning transmission electron microscopy to understand zirconium hydride formation and embrittlement. At the interface between the hexagonal close packed (HCP) \alpha-Zr matrix and a face centred cubic (FCC) \delta deuteride (ZrD1.5-1.65), a HCP \zeta phase deuteride (ZrD0.25-0.5) has been observed. Furthermore, Sn is rejected from the deuterides and segregates to the deuteride/\alpha-Zr reaction front.

Carbides play a central role for the strength and ductility in many materials. Simulating the impact of these precipitates on the mechanical performance requires knowledge about their atomic configuration. In particular, the C content is often observed to substantially deviate from the ideal stoichiometric composition. In this work, we focus on Fe-Mn-Al-C steels, for which we determined the composition of the nanosized $\ensuremath{\kappa}$ carbides (Fe,Mn)${}_{3}\mathrm{AlC}$ by atom probe tomography in comparison to larger precipitates located in grain boundaries. Combining density functional theory with thermodynamic concepts, we first determine the critical temperatures for the presence of chemical and magnetic disorder in these carbides. Second, the experimentally observed reduction of the C content is explained as a compromise between the gain in chemical energy during partitioning and the elastic strains emerging in coherent microstructures.

X-ray diffraction contrast tomography is a recently developed, non-destructive synchrotron imaging technique which characterizes microstructure and grain orientation in polycrystalline materials in three dimensions. By combining it with propagation-based phase-contrast tomography it is possible to get a full picture description for the analysis of local crack growth rate of short fatigue cracks in three dimensions: the three-dimensional crack morphology at different propagation stages, and the shape and orientation of the grains around the crack. An approach has been developed on the metastable beta titanium alloy Ti 21S that allows for visualization and analysis of the growth rate and crystallographic orientation of the fracture surface. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We have investigated the autonomous repair of creep damage by site-selective precipitation in a binary Fe-Mo alloy (6.2 wt pct Mo) during constant-stress creep tests at temperatures of 813 K, 823 K, and 838 K (540 °C, 550 °C, and 565 °C). Scanning electron microscopy studies on the morphology of the creep-failed samples reveal irregularly formed deposits that show a close spatial correlation with the creep cavities, indicating the filling of creep cavities at grain boundaries by precipitation of the Fe2Mo Laves phase. Complementary transmission electron microscopy and atom probe tomography have been used to characterize the precipitation mechanism and the segregation at grain boundaries in detail.

The microstructure of complex steels can be manipulated by utilising the interaction between the local mechanical distortions associated with lattice defects, such as dislocations and grain boundaries, and solute components that segregate to them. Phenomenologically these phenomena can be interpreted in terms of the classical Gibbs adsorption isotherm, which states that the total system energy can be reduced by removing solute atoms from the bulk and segregating them at lattice defects. Here we show how this principle can be utilised through appropriate heat treatments not only to enrich lattice defects by solute atoms, but also to further change these decorated regions into confined ordered structural states or even to trigger localized decomposition and phase transformations. This principle, which is based on the interplay between the structure and mechanics of lattice defects on the one hand and the chemistry of the alloy's solute components on the other hand, is referred to as segregation engineering. In this concept solute decoration to specific microstructural traps, viz. lattice defects, is not taken as an undesired effect, but instead seen as a tool for manipulating specific lattice defect structures, compositions and properties that lead to beneficial material behavior. Owing to the fairly well established underlying thermodynamic and kinetic principles, such local decoration and transformation effects can be tuned to proceed in a self-organised manner by adjusting (i) the heat treatment temperatures for matching the desired trapping, transformation or reversion regimes, and (ii) the corresponding timescales for sufficient solute diffusion to the targeted defects. Here we show how this segregation engineering principle can be applied to design self-organized nano- and microstructures in complex steels for improving their mechanical properties.

Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation.

Three-dimensional (3D) short fatigue crack growth behaviour in a cast magnesium alloy, Elektron 21, was studied using a combination of X-ray diffraction contrast tomography (DCT) and microtomography at the European Synchrotron Radiation Facility. A 3D map of grain shapes and crystallographic orientations was produced in a miniature fatigue specimen using DCT. A focused ion beam instrument was used to introduce small notches in selected grains. Synchrotron microtomography was used to study the evolution of fatigue cracks from these notches during interrupted in situ fatigue cycling. Stage I crack growth occurred preferentially on the basal plane of the magnesium hexagonal close packed crystal, with local crack growth rates between 4 and 40 nm cycle. Retardations in the local crack growth rate were observed in certain grains and at certain grain boundaries. The observed interactions between the crack and the polycrystalline microstructure can be explained using Schmid factors and a development of the tilt-twist model of Zhai and Wilkinson (Zhai T, Wilkinson AJ, Martin JW. Acta Mater 2000;48;4917). Copyright 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The combination of synchrotron radiation x-ray imaging and diffraction techniques offers new possibilities for in-situ observation of deformation and damage mechanisms in the bulk of polycrystalline materials. Minute changes in electron density (i.e., cracks, porosities) can be detected using propagation based phase contrast imaging, a 3-D imaging mode exploiting the coherence properties of third generation synchrotron beams. Furthermore, for some classes of polycrystalline materials, one may use a 3-D variant of x-ray diffraction imaging, termed x-ray diffraction contrast tomography. X-ray diffraction contrast tomography provides access to the 3-D shape, orientation, and elastic strain state of the individual grains from polycrystalline sample volumes containing up to thousand grains. Combining both imaging modalities, one obtains a comprehensive description of the materials microstructure at the micrometer length scale. Repeated observation during (interrupted) mechanical tests provide unprecedented insight into crystallographic and grain microstructure related aspects of polycrystalline deformation and degradation mechanisms. © 2010 TMS.

Non-destructive, three-dimensional (3D) characterization of the grain structure in mono-phase polycrystalline materials is an open challenge in material science. Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for mapping 3D grain shapes and crystallographic orientations for certain categories of polycrystalline materials. Direct visualisation of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques may be possible, but is restricted to specific material systems. A recent extension of this methodology, termed X-ray diffraction contrast tomography (DCT), combines the principles of X-ray diffraction imaging, three-dimensional X-ray diffraction microscopy (3DXRD) and image reconstruction from projections. DCT provides simultaneous access to 3D grain shape, crystallographic orientation and local attenuation coefficient distribution. The technique applies to the larger range of plastically undeformed, polycrystalline mono-phase materials, provided some conditions on grain size and texture are fulfilled. The straightforward combination with high-resolution microtomography opens interesting new possibilities for the observation of microstructure related damage and deformation mechanisms in these materials. © 2009 Elsevier B.V. All rights reserved.

X-ray diffraction contrast tomography (DCT) is a technique for mapping grain shape and orientation in plastically undeformed polycrystals. In this paper, we describe a modified DCT data acquisition strategy which permits the incorporation of an innovative Friedel pair method for analyzing diffraction data. Diffraction spots are acquired during a 360 degree rotation of the sample and are analyzed in terms of the Friedel pairs ((hkl) and (hkl -) reflections, observed 180 degrees apart in rotation). The resulting increase in the accuracy with which the diffraction vectors are determined allows the use of improved algorithms for grain indexing (assigning diffraction spots to the grains from which they arise) and reconstruction. The accuracy of the resulting grain maps is quantified with reference to synchrotron microtomography data for a specimen made from a beta titanium system in which a second phase can be precipitated at grain boundaries, thereby revealing the grain shapes. The simple changes introduced to the DCT methodology are equally applicable to other variants of grain mapping. Copyright 2009 American Institute of Physics.