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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.

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.

The subsurface dislocation content in a Ti-5Al-2.5Sn (wt%) uniaxial tension sample deformed at ambient temperature was characterized by peak streak analysis of micro-Laue diffraction patterns collected non-destructively by differential aperture X-ray microscopy, and with focused ion beam transmission electron microscopy of material in the same volume. This comparison reveals that micro-Laue diffraction streak analysis based on an edge dislocation assumption can accurately identify the dominant dislocation slip system history (Burgers vector and plane observed by TEM), despite the fact that dislocations have predominantly screw character. Other dislocations identified by TEM were not convincingly discernible from the peak streak analysis.

Recrystallization during hot compression of the Ti5553 alloy in the two-phase α+β regime has been studied with the help of fine scale, orientation image mapping techniques. We find two distinct recrystallization processes. The first is associated with the well-known α globularisation process, and results in the loss of the Burgers orientation relationship between α and β, with the consequent randomization of the of the sharp, starting transformation texture. An alternative, dynamic recrystallization process is also observed in which newly recrystallized α and β grains form with the Burgers orientation relationship with each other. We call this epitaxial recrystallization.

An approach for producing ultrahigh spatial resolution selected area electron channeling patterns (UHR-SACPs) using the FEI/Thermo Elstar electron column is presented. The approach uses free lens control to directly assign lens and deflector values to rock the beam about precise points on the sample surface and generate the UHR-SACPs. Modification of the lens parameters is done using a service application that is preinstalled on the microscope or using the iFast scripting interface to run a short program to assign lens and deflector currents. Using the approach outlined here, the UHR-SACPs are collected at normal instrument scanning rates and pixel densities, resulting in rapid collection times and sharp patterns with simple push button changes in instrument mode. UHR-SACPs with spatial resolutions of 300 nm with angular ranges of 20° are demonstrated, as are patterns approaching 125 nm spatial resolution with angular ranges of 4°. Such spatial resolution/angular range combinations are significantly better than any reported previously. This approach for rapidly collecting high accuracy crystallographic information greatly enhances the ability to carry out electron channeling contrast imaging (ECCI) for a broad range of materials applications.

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.

Crystallographic slip in hexagonal metals involves a number of geometrically distinct slip families characterized by their slip direction and slip plane (basal, prismatic, and pyramidal). Owing to the low symmetry of hexagonal lattices, each of these slip families only have a few symmetrically equivalent slip systems (family members). Furthermore, different slip families become active at different resolved shear stress, i.e., they have different critical resolved shear stress values (CRSS). The plastic anisotropy of hexagonal materials makes the numerical prediction of their plastic behavior challenging and depends critically on the knowledge of CRSS values. The present contribution assesses the reliability of three proposed methods (with additional variations) to quantify CRSS values of the different hexagonal slip families. Those methods (a to c) rely on: (a) the statistics of observed surface slip traces in a (slightly) deformed polycrystal; (b) an iterative adjustment of CRSS values until a simulated single crystal indentation matches the corresponding experiment in terms of load–displacement response and residual surface topography of the indent; (c) in-situ high-energy X-ray diffraction to measure the evolution of resolved stress (from lattice strains) in grains for which single-family slip is probable based upon specific lattice reorientation conditions. Virtual experiments are performed on synthetic microstructures such that the (extracted) CRSS values resulting from simulating the different methodologies can be rigorously compared against the (target) CRSS values that are installed in the phenomenological constitutive material description used in the simulations. The resulting CRSS values of methods (a) exhibit a strong dependence on, and deterioration with, decreasing level of slip trace observability, which is an uncertain quantity in experimental measurements. For the inverse indentation method (b), the predicted CRSS values are within 8 % of their reference CRSS values for the two investigated cases. The high-energy X-ray diffraction method (c) most reliably determines CRSS values for basal and prism slip, but lacks a strict grain selection criterion to assess pyramidal slip.

The stereology, variant distribution and coarsening behavior of semicoherent alpha(hcp) precipitates in a beta(bcc) matrix of a Ti5553 alloy has been analyzed, and a dominant 3-variant cluster has been observed in which the variants are related to each other by an axis-angle pair < 0 >/60 degrees. Shape and spatial distribution independent elastic self and interaction energies for all pairwise and triplet combinations of a have been calculated and it is found that the 3-cluster combination that is experimentally observed most frequently has the lowest energy for the semicoherent state. The coarsening behavior of the delta distribution follows LSW kinetics after an initial transient, and has been modeled by phase field methods. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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 objective of this contribution is to summarise the range of α crystallography and distribution that arises from the β to α transformation and the hot working of α/β alloys and their recrystallisation and relate these features to the slip character of these alloys. α morphology and distribution have been mapped as a function of β stabilizer content and aging temperature. A dominant transgranular grouping of α variants with a common close packed direction characterizes transformation patterns across the entire range of β stabilizer content. Recrystallization during thermomechanical processing in the α/β regime has been studied with the help of fine scale, orientation image mapping techniques. Two distinct recrystallization processes have been identified. The first is associated with the well-known α globularisation process. An alternative recrystallization process has also been described in which newly recrystallized α and β grains form with the Burgers orientation relationship with each other. This process has been called epitaxial recrystallization. The role of transformation and recrystallisation texture on the plasticity of titanium alloys is assessed through quantitative EBSD /TEM based analysis of slip systems in the α phase and slip transfer between the α and β phases.

A metastable β titanium alloy Ti–5Al–5Mo–5V–3Cr (Ti-5553) was friction stir processed (FSP) in both the β and α+β phase fields. Homogenous microstructures were obtained in both phase fields by adjusting FSP parameters. The mean β grain size after FSP varied from 15 to ~0.7 μm from the single-phase β to α+β conditions, respectively. At the α+β conditions, a near-globular α phase (50–200 nm in size) was observed. Microstructure and microtexture evolution in β and α+β conditions were investigated with the aid of scanning and transmission electron microscopy. Ultra-fine intra-granular α and duplex α microstructures were generated at both conditions by duplex aging of the FSP samples. The FSP alloys display excellent combinations of strength and ductility.

Titanium alloy Ti-5Al-5Mo-5V-3Cr (Ti-5553) is a near beta alloy used in structural aircraft components because of its excellent mechanical properties. Simulating the mechanical response of this material using constitutive models is an important step in understanding the relationship between its microstructure and properties. Uniaxial compression tests were conducted at temperatures both below and above the beta transus and at different loading rates. A viscoplastic self-consistent (VPSC) model was used to match the stress-strain response of the Ti-5553 alloy based on uniaxial compression tests across a range of temperatures and strain rates. Sets of parameter values were determined for two different hardening models, namely the modified Voce model, which is empirical, and the Mechanical Threshold Stress (MTS) model, which is based on dislocation theory. No consistent trends in the Voce parameter values were found as a function of temperature or strain rate. By contrast, the physically-based MTS model, which is explicitly designed to cover wide ranges of deformation conditions, was able to fit a wide range of temperatures and strain rates. It was further validated by comparison with experimental measurements with other b titanium alloys that had similar compositions. (C) 2017 Elsevier Ltd. All rights reserved.

The lifetime of moving metallic components is often limited by accelerated oxidation. Yet, the mechanisms and pathways for oxidation during tribological loading are not well understood. Using copper as a model system, tribologically-induced oxidation is systematically investigated by varying the sliding speed and test duration under mild tribological loading. We demonstrate that tribo-oxidation is controlled by test duration rather than the number of cycles or the sliding speed. Plastic deformation from tribological loading creates dislocations, grain and phase boundaries that act as high diffusivity pathways. A combination of electron microscopy and atom probe tomography revealed significantly enhanced atomic concentration of the diffusing species around dislocations. Oxygen diffusion into the bulk as well as of copper towards the free surface along these defects control the oxide formation kinetics. Our work paves the way for formulating a physics-based understanding for tribo-oxidation, which is crucial to develop strategies to slow or decrease oxidation and to strategically tailor surfaces to increase the lifetime of engineering systems.