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We investigate the effect of direct electric current (DC) on grain growth in 100 nm thick iron-carbon films with carbon concentrations between 0.7 to 4.4 at%. The application of DC-current during annealing at 550 °C confirms the expected transport of carbon in the direction of the electric current and the unexpected formation of elongated, abnormally large carbide and ferrite grains along the current direction in the carbon-rich regions. The formation of elongated grains is explained by electromigration-induced carbon flux divergences that result from the carbide precipitates. This presents a possible scenario for controlling microstructure evolution in iron by using DC electric currents. Changes for alternating current (AC) pulses had been observed before.

Transient and steady state electrotransport and diffusion of ions being produced and annihilated by internal reactions are discussed and exemplified for Yttria Stabilized Zirconia (YSZ). These phenomena are important for understanding flash sintering. They will also play a role in solid oxide fuel cell (SOFC) and solid oxide electrolysis cells (SOEC), where current densities may exceed the reaction rates with gases at the porous electrodes. The characteristic time for attaining steady state transport contains two parts, one depending on the length of the sample and one depending on the field strength. This characteristic time is derived in this study for a linear increase of temperature for the first time. By assuming that the characteristic time is a measure of the onset of flash sintering, yields – without fitting parameters – incubation times or onset temperatures of flash sintering in good agreement with experimental results for YSZ. Thus concentration changes building up during the incubation period are driving forces for internal reactions generating or consuming holes and electrons. A concomitant increase of conductivity leads to Joule heating which further accelerates reaction rates and ion mobility.

Hydrogen trapping at metal/carbide interfaces is changed with changing interfacial composition. This has been exemplified for the iron/vanadium carbide (VC) interface. It will be assumed that the carbide interface is a (100)-plane having a composition between the limiting cases of either 50% V and 50% C for excess carbon in iron or 50% V and 50% carbon-vacancies for excess vanadium in iron. The latter will provide strong traps for hydrogen. For validating the simple model a modified interpretation of published atom probe and thermal desorption data became necessary as discussed in the present work.

Taking advantage of in-situ fracture testing method inside a transmission electron microscope, crack evolution in a low alloy steel under low triaxiality conditions is studied and the interaction between cementite particles and the crack is revealed. It is found that the ferrite matrix is the major void initiation site due to the low stress triaxiality in the TEM sample, which contrasts the behavior under plane strain conditions in bulk specimens, where voids are typically found to initiate by decohesion at the particle/matrix interface. This work reveals that fracture behavior proceeds differently under low triaxiality conditions, such as in thin films, and demonstrates the possibility to avoid interface decohesion and thereby to enhance ductility in steels.

Taking advantage of in-situ fracture testing method inside a transmission electron microscope (TEM), crack evolution in a low alloy steel under low triaxiality conditions is studied and the interaction between cementite particles and the crack is revealed. It is found that the ferrite matrix is the major void initiation site due to the low stress triaxiality in the thin TEM sample (plane stress condition), which contrasts the behavior under plane strain conditions in bulk specimens, where voids are typically found to initiate by decohesion at the particle/matrix interface. This work reveals that fracture behavior proceeds differently under low triaxiality conditions, such as the shear lip region of fractured bulk sample, and demonstrates the possibility to avoid interface decohesion and thereby to enhance ductility in steels.

Grain growth in nanocrystalline vanadium films was studied at 600 and 700 °C in vacuum and in hydrogen atmosphere with partial pressures ranging from 1 to 1000 Pa. It is shown that grain growth is significantly increased in the presence of hydrogen. Thus the expected effect of retarding grain growth either by reducing grain boundary energy due to hydrogen segregation or by hydrogen drag on the moving boundary did not occur. Two explanations are given for the accelerated grain growth. First, hydrogen reduces the formation energy of ledges, assuming these ledges are required for initiating and advancing boundary motion. Second, grain growth requires the annihilation of excess volume which may be enhanced by reducing the formation energy of vacancies in the presence of hydrogen.

Electric transport in oxides with mixed conduction, being exposed to dc electric fields and being placed between blocking electrodes is discussed and exemplified for yttria stabilized zirconia (YSZ). This is relevant for cases of large current densities in solid oxide fuel cell (SOFC), solid oxide electrolysis cells (SOEC) and during flash sintering of oxide powders. In the present study equilibrium of defect reactions is not attained in the presence of blocking electrodes leading to a continuous generation of holes in p-regions and electrons in n-regions feeding the external current. In the remaining region between n- and p-region ion conduction is dominant. Thus a p-i-n junction is formed. The electronic species may be continuously generated by reaction with gaseous oxygen or by the creation and annihilation of single-charged vacancies. For both cases current voltage-relations are derived by assuming that the gradient of the chemical potential of double-charged vacancies is reduced below its value given by a zero electrochemical potential. The reduction is introduced as a consequence of the Le Chatelier Principle, which requires reactions to occur, which counteract the accumulation and depletion of double-charged vacancies by the applied electric field and corresponding deviations from charge neutrality. Scenarios are discussed where electrons and holes recombine with a concomitant emission of light. This explains why the spectrum of the emitted light deviates from that of black body radiation for both examples of flash sintering and the Nernst glower.

A melt pocket in the bonding zone of medium carbon steel bonded on low-carbon steel by explosive welding was investigated by laser assisted atom probe tomography. It was found that the structure is nanocrystalline and (sub-) grain boundaries are enriched with carbon. High hardness values in the melt pocket are attributed to a combination of (sub-) grain size and carbon distribution.

Cold-drawn pearlitic steel wires have attracted considerable interest because of their excellent combination of strength and ductility. The physical interpretation of these properties has been a subject of repeated controversial discussions in the literature. Unquestioned is the fact that during the process of cold drawing, cementite is partially dissolved, while the ferrite is obviously enriched in carbon. The debate is related to the questions why cementite is decomposed and where the carbon is accommodated. It is the aim of this work to review the relevant literature and to conciliate the controversies. Special attention is paid to the microstructural evolution during progressive cold-drawing, which eventually attains nanometer-scale in two dimensions and is essential for the evolution of the mechanical as well as electrical properties. This is all the more important as recent developments in atom probe tomography allowed to study the chemical composition on the atomic scale. A further important aspect is the path of recovery and recrystallization, accompanied by softening, during post-draw annealing. A consolidated view indicates that carbon-defect interactions play a major role in all aspects of the wire properties.

Purified iron was loaded electrochemically with hydrogen in the presence of a hydrogen promoter, leading to the formation of cracks inside of the bulk and blisters on the surface. The mechanism for the crack initiation was investigated using SEM cross-section images and by investigating the fracture surface of a ruptured sample, where preexisting cracks were exposed for observation. Cracks were found to originate at inclusions. It was observed that blisters grow with time, leading to the conclusion that the underlying growth process is discontinuous. The surface morphology of the blisters consists of steps and in the underlying microstructure investigated by TEM shear bands were found. Hydrogen gas pressures in the range of half of the yield strength of iron were determined directly after hydrogen loading using density measurements. Therefore, the hydrogen gas pressure in the cracks was concluded to be the driving force for crack advance.

Cold-drawn pearlitic steel wires with a drawing true strain of 3 were annealed at temperatures (Tann) ranging from 423 K to 723 K (150 °C to 450 °C) with an interval of 50 K. Recovery of the lattice defects in the severely deformed ferrite lamellae were characterized by means of high-energy X-ray diffraction and positron annihilation techniques (including positron annihilation spectroscopy and coincidence Doppler broadening spectroscopy). Accordingly, the impact of defect recovery on the softening of the annealed wires was investigated. It is found that at low temperatures [Tann ≤ 523 K (250 °C)], the recovery of the lattice defects in ferrite lamellae is dominated by the agglomeration and annihilation of vacancy clusters, while at Tann > 523 K (250 °C), the recovery process is controlled by the annihilation of dislocations. Further analyses on the softening of the annealed wires indicate that the evolutions of dislocation density and concentration of vacancy clusters, and the strain age hardening in ferrite lamellae play important roles in changing the strength of the wires. The strain aging hardening leads to a maximum strength at 473 K (150 °C). Above 523 K (250 °C), the annihilations of vacancy clusters and dislocations in ferrite lamellae cause a continuous softening of the wires, where the decrease in dislocation density plays a major role.

Bulk diffusion-controlled thermal desorption spectroscopy (TDS) is studied by solving the corresponding transport equations numerically as well as analytically with appropriate approximations. The two solutions are compared in order to validate the derived equations including the Kissinger equation. Besides the diffusion of the desorbed species through the sample, trapping of the species at special lattice sites within the sample is included in the numerical and approximate analytical solutions. Trapping energies are mono-energetic, multi-energetic, or are described by a box-type distribution. TDS-peaks were simulated for different heating rates, sample thicknesses, trap concentrations, and initial degrees of trap saturation. It is shown that for the case of mono-energetic traps, Kissinger’s equation is obeyed for both numerical and analytical results. This widely used equation for reaction rate-controlled studies is derived in an explicit form for diffusion-controlled processes. Together with a newly derived relation between maximum desorption rate and temperature, TDS-spectra yield information about diffusion coefficient, trap energies, and trap concentration as well as trap saturation. This is exemplified using data of two experimental studies. Although the numerical and analytical treatment is in general applicable to all diffusion species, hydrogen in iron alloys is used as a model system because of its technological importance and the increasing number of experimental work with this material.

During fracture new surfaces are formed by a propagating crack. Depending on the chemical potential of the constituents of a material and their mobility the composition of the newly formed surfaces changes. Thus the surface energy as part of the work to fracture will be affected. This will be treated by combining the work to fracture representing the mechanical aspect and the Gibbs Adsorption Isotherm covering the chemical aspect. Compared to previous studies the present one provides a more generalized but also a simpler insight into chemomechanical effects. In extreme cases separation of lattice planes or separation of two crystals with a common interface occurs without applied external forces. Closed solutions for the work of fracture are derived for brittle fracture and surface segregation of solutes in the limit of a mean field approach. Chemomechanical effects including plastic deformation by dislocation or vacancy generation are discussed qualitatively.

Steel powders obtained by mechanical alloying of iron and graphite were compacted by high-pressure torsion. During high-pressure torsion, mean grain sizes rise from about 10 nm after mechanical alloying to about 20 nm. Vickers hardness reaches values of more than 10 GPa. Differential scanning calorimetry revealed superabundant vacancies present in concentrations up to 10 −3 . The results are discussed in terms of strain, strain rate, energy input and deformation mechanisms.

Well-annealed Pd and Pd–H alloys were subjected to tensile tests. It is found that the presence of H does not affect the yield strength but enhances the strain hardening of Pd remarkably. This is explained by a negligible solute drag effect of H on the movement of dislocations, and by H-induced enhanced multiplication of dislocations upon plastic deformation, respectively.

Electrochemical hydrogen permeation measurements and in situ gas-loading X-ray diffraction measurements were performed on polycrystalline Mg films. Hydrogen diffusion constants, the hydride volume content and the in-plane stress were determined for different values of driving forces at 300 K. For α-Mg–H, a hydrogen diffusion constant of D H Mg = 7 ( ± 2 ) · 10 - 11 m2 s−1 was determined. For higher concentrations, different kinetic regimes with reduced apparent diffusion constants D H tot were found, depending on the driving force, decreasing to about D H tot = 10−18 m2 s−1. This lowest measured diffusion constant is two orders of magnitude larger than that of bulk β-MgH2, and the difference is ascribed to a contribution from a fast diffusion along grain boundaries. The different kinetics regimes are attributed to the spatial distribution of hydrides. A heterogeneous hydride nucleation and growth model is suggested that is based on hemispherical hydrides spatially distributed according to the nuclei densities expressed as a function of the driving force. The model allows us to qualitatively explain the complex stress development, the different diffusion regimes and the blocking-layer thickness. As the blocking-layer thickness inversely scales with the driving force, small driving forces allow the hydriding of large film volume fractions. Maximum stress situations occur for hydride distances reaching four times the hydride radius and for hydride distances equaling the film thickness.

Carbon-supersaturated nanocrystalline hypereutectoid steels with a tensile strength of 6.35 GPa were produced from severely cold-drawn pearlite. The nanocrystalline material undergoes softening upon annealing at temperatures between 200 and 450 ° C. The ductility in terms of elongation to failure exhibits a non-monotonic dependence on temperature. Here, the microstructural mechanisms responsible for changes in the mechanical properties were studied using transmission electron microscopy (TEM), TEM-based automated scanning nanobeam diffraction and atom probe tomography (APT). TEM and APT investigations of the nanocrystalline hypereutectoid steel show subgrain coarsening upon annealing, which leads to strength reduction following a Hall–Petch law. APT analyzes of the Mn distribution near subgrain boundaries and in the cementite give strong evidence of capillary-driven subgrain coarsening occurring through subgrain boundary migration. The pronounced deterioration of ductility after annealing at temperatures above 350 ° C is attributed to the formation of cementite at subgrain boundaries. The overall segregation of carbon atoms at ferrite subgrain boundaries gives the nanocrystalline material excellent thermal stability upon annealing.

The effect of dissolved hydrogen on the dislocation density in cold-rolled palladium was investigated in order to provide evidence of a line energy reduction caused by hydrogen–dislocation interaction as proposed by the defactant concept. For this issue, palladium samples were electrochemically charged with hydrogen and subsequently cold rolled. Using conventional methods (X-ray diffraction, transmission electron microscopy) and a newly developed diffusion method, it was shown that the dislocation density after deformation increases with increasing hydrogen concentration.

Lattice discontinuities include lattice defects and surfaces both providing traps for hydrogen atoms. It will be discussed under which conditions discontinuities of a given distribution either release trapped hydrogen to become diffusible or capture diffusible H-atoms to become trapped. It will be shown that for any distribution, the self-diffusion coefficient of hydrogen is determined by the product of the H-diffusion in the perfect lattice times the fraction of hydrogen being diffusible. In this context, the quantities diffusible hydrogen, lattice hydrogen, thermodynamic activity of hydrogen and chemical potential of hydrogen are interchangeable in a general way. New discontinuities are generated during hydrogen embritllement (fracture surfaces, voids, dislocations) and dislocations move by kink pair formation. The production rate of these discontinuities depends on the chemical potential of hydrogen within the defactant concept or the generalized Gibbs adsorption isotherm. Thus, the chemical potential of hydrogen determines both the amount of trapping and the defect generation rate. For a crack propagating by dislocations generation, the chemical potential affects its velocity independent of the accompanying concentration enhancement in front of the crack tip or the related adsorption on the freshly generated crack surface. This article is part of the themed issue ‘The challenges of hydrogen and metals’.

Nanocrystalline Fe-1.77at.%C and Fe-3.27at.%C alloys prepared by ball milling iron powders and graphite powders are annealed below 573K where the precipitation of Fe3C does not occur. Upon annealing, a significant grain coarsening is observed in Fe-1.77at.%C alloy, whereas the grain coarsening is inhibited in Fe-3.27at.%C alloy. Within the framework of thermodynamic theories, the inhibition of grain coarsening in nanocrystalline Fe-C alloys is discussed. It is demonstrated that the inhibition of grain coarsening in the nanocrystalline Fe-C alloys can be ascribed to a vanished driving force for grain growth which is caused by the interaction between carbon and the grain boundaries of nanograins.

Self-diffusion in face-centered cubic (fcc) iron and hexagonal close-packed cobalt as well as Ni diffusion in bcc iron and Co-diffusion in fcc nickel are all accelerated in the presence of dissolved carbon. This is attributed to an enlarged vacancy concentration due to an attractive vacancy–carbon interaction. The enlarged Ni diffusion in body-centered cubic (bcc) iron contradicts conclusions for self-diffusion in bcc iron obtained from first-principles calculations. This discrepancy is discussed in the present study.

In patented and cold-drawn pearlitic steel wires dissociation of cementite occurs during mechanical deformation. In this study the influence of the carbon decomposition on the diffusion of nickel in ferrite is investigated by means of atom probe tomography. In the temperature range 423–523 K we observed a much smaller activation energy of Ni diffusion than for self-diffusion in body-centered cubic iron, indicating an increased vacancy density owing to enhanced formation of vacancy–carbon complexes.

The objective of this study was to quantify the effects of mechanical and environmental variables on oxygen-modified accelerated fatigue crack growth of steels in hydrogen gas. Experimental results show that in hydrogen gas containing up to 1000 v.p.p.m. oxygen fatigue crack growth rates for X52 line pipe steel are initially coincident with those measured in air or inert gas, but these rates abruptly accelerate above a critical Δ K level that depends on the oxygen concentration. In addition to the bulk gas oxygen concentration, the onset of hydrogen-accelerated crack growth is affected by the load cycle frequency and load ratio R . Hydrogen-accelerated fatigue crack growth is actuated when threshold levels of both the inert environment crack growth rate and K max are exceeded. The inert environment crack growth rate dictates the creation of new crack tip surface area, which in turn determines the extent of crack tip oxygen coverage and associated hydrogen uptake, while K max governs the activation of hydrogen-assisted fracture modes through its relationship to the crack tip stress field. The relationship between the inert environment crack growth rate and crack tip hydrogen uptake is established through the development of an analytical model, which is formulated based on the assumption that oxygen coverage can be quantified from the balance between the rates of new crack tip surface creation and diffusion-limited oxygen transport through the crack channel to this surface. Provided K max exceeds the threshold value for stress-driven hydrogen embrittlement activation, this model shows that stimulation of hydrogen-accelerated crack growth depends on the interplay between the inert environment crack growth increment per cycle, load cycle frequency, R ratio and bulk gas oxygen concentration.

A local electrode atom probe has been employed to analyze the redistribution of alloying elements including Si, Mn, and Cr in pearlitic steel wires upon cold-drawing and subsequent annealing. It has been found that the three elements undergo mechanical mixing upon cold-drawing at large strains, where Mn and Cr exhibit a nearly homogeneous distribution throughout both ferrite and cementite, whereas Si only dissolves slightly in cementite. Annealing at elevated temperatures leads to a reversion of the mechanical alloying. Si atoms mainly segregate at well-defined ferrite (sub)grain boundaries formed during annealing. Cr and Mn are strongly concentrated in cementite adjacent to the ferrite/cementite interface due to their lower diffusivities in cementite than in ferrite.

In cold-rolled Pd and Pd–H alloys the presence of hydrogen enhances the multiplication of dislocations and leads to an increase in dislocation density as compared to pure Pd. Subsequent removal of H from Pd–H alloys does not influence the dislocation density. H was identified as a temporary alloying addition (defactant) which increases the dislocation density of cold-deformed Pd.

A series of nanocrystalline Fe–C alloys with different carbon concentrations ( x tot ) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of α-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing x tot . Eventually the size reaches a lower limit of about 6 nm in alloys with x tot > 6.19 at.% (1.40 wt.%); a further increase in x tot leads to the precipitation of carbon as Fe 3 C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe 3 C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy.

By means of X-ray line profile analysis and positron lifetime spectroscopy, densities of deformation-induced defects in carbon-rich ferrite of a series of cold-drawn pearlitic steel wires with true strains (e) up to 5 are characterized. It is shown that both the dislocation densities and the vacancy cluster concentrations increase continuously with increasing e. On the basis of the measured defect densities, values of defect hardening are estimated. The result shows that contributions of the defect hardening to the total tensile strength of the wires reach nearly 40 pct, which is mainly ascribed to the dislocation hardening. Chemical surroundings of the defects in the carbon-rich ferrite are investigated by coincidence Doppler broadening spectroscopy. The association of carbon with the defects in ferrite is demonstrated.

The effect of dissolved hydrogen in a (1 0 0) vanadium single crystal was studied using compression tests of micropillars. It is observed that the shape of the deformed pillars changes with hydrogen concentration. At low concentrations the pillars deform on a few discrete slip planes and at high hydrogen concentrations the pillars deform to a barrel-like shape. Furthermore, the flow stress increases with hydrogen concentration. Both observations can be attributed to an elevated dislocation activity due to hydrogen.

The first direct observation, by atom probe tomography, of a deuterium dead layer is reported for Fe/V multilayered film loaded with D solute atoms. The thickness of the dead layers was measured to be 0.4–0.5 nm. The dead layers could be distinguished from chemically intermixed layers. The results suggest that the dead layer effect occurs even near the interface of the mixing layers, supporting an interpretation that the dead layer effect cannot be explained solely by electronic charge transfer but also involves a modulation of rigidity.

Mobile solute atoms will affect both kink formation and kink motion. Softening occurs if solute atoms segregate to kinks and if the rate determining step is the formation of kink pairs. Then double kink formation energy is reduced, which can be shown within thermodynamics. With increasing solute concentration the rate of dislocation motion will finally be determined by kink motion. Kink motion is retarded by solute drag, which increases by adding more solute. Under these circumstances hardening is observed.

Hypereutectoid steel wires with 6.35 GPa tensile strength after a cold-drawing true strain of 6.02 were annealed between 300 and 723 K. The ultrahigh strength remained upon annealing for 30 min up to a temperature of 423 K but dramatically decreased with further increasing temperature. The reduction of tensile strength mainly occurred within the first 2–3 min of annealing. Atom probe tomography and transmission electron microscopy reveal that the lamellar structure remains up to 523 K. After annealing at 673 K for 30 min, coarse hexagonal ferrite (sub)grains with spheroidized cementite, preferentially located at triple junctions, were observed in transverse cross-sections. C and Si segregated at the (sub)grain boundaries, while Mn and Cr enriched at the ferrite/cementite phase boundaries due to their low mobility in cementite. No evidence of recrystallization was found even after annealing at 723 K for 30 min. The stability of the tensile strength for low-temperature annealing ( 473 K) are discussed based on the nanostructural observations.

A next nearest neighbor evaluation procedure of atom probe tomography data provides distributions of the distances between atoms. The width of these distributions for metallic glasses studied so far is a few Angstrom reflecting the spatial resolution of the analytical technique. However, fitting Gaussian distributions to the distribution of atomic distances yields average distances with statistical uncertainties of 2 to 3 hundredth of an Angstrom. Fe40Ni40B20 metallic glass ribbons are characterized this way in the as quenched state and for a state heat treated at 350 °C for 1 h revealing a change in the structure on the sub-nanometer scale. By applying the statistical tool of the χ2 test a slight deviation from a random distribution of B-atoms in the as quenched sample is perceived, whereas a pronounced elemental inhomogeneity of boron is detected for the annealed state. In addition, the distance distribution of the first fifteen atomic neighbors is determined by using this algorithm for both annealed and as quenched states. The next neighbor evaluation algorithm evinces a steric periodicity of the atoms when the next neighbor distances are normalized by the first next neighbor distance. A comparison of the nearest neighbor atomic distribution for as quenched and annealed state shows accumulation of Ni and B. Moreover, it also reveals the tendency of Fe and B to move slightly away from each other, an incipient step to Ni rich boride formation.

V–Fe5at.% 2 and 10-nm thick single layered films were prepared by ion beam sputtering on W substrate. They were loaded with D from gas phase at 0.2 Pa and at 1 Pa, respectively. Both lateral and depth D distribution of these films was investigated in detail by atom probe tomography. The results of analysis are in good agreement between the average deuterium concentration and the value, expected from electromotive force measurement on a similar flat film. An enrichment of deuterium at the V/W interface was observed for both films. The origin of this D-accumulation was discussed in respect to electron transfer, mechanical stress and misfit dislocations.

Pearlitic steel can exhibit tensile strengths higher than 5 GPa after severe plastic deformation, where the deformation promotes a refinement of the lamellar structure and cementite decomposition. However, a convincing correlation between deformation and cementite decomposition in pearlite is still absent. In the present work, a local electrode atom probe was used to characterize the microstructural evolution of pearlitic steel, cold-drawn with progressive strains up to 5.4. Transmission electron microscopy was also employed to perform complementary analyses of the microstructure. Both methods yielded consistent results. The overall carbon content in the detected volumes as well as the carbon concentrations in ferrite and cementite were measured by atom probe. In addition, the thickness of the cementite filaments was determined. In ferrite, we found a correlation of carbon concentration with the strain, and in cementite, we found a correlation of carbon concentration with the lamella thickness. Direct evidence for the formation of cell/subgrain boundaries in ferrite and segregation of carbon atoms at these defects was found. Based on these findings, the mechanisms of cementite decomposition are discussed in terms of carbon–dislocation interaction.

Addition of ternary elements to the D03 ordered Fe3Al intermetallic phase is a general approach to optimise its mechanical properties. To understand the physical influences of such additions the determination of the probability of site occupancies of these additions on the lattice site and ordering parameters is of high interest. Some common experimental techniques such as X-ray diffraction or Atom Location by Channelling Enhanced Microanalysis (ALCHEMI) are usually applied to explore this interplay. Unfortunately, certain published results are partly inconsistent, imprecise or even contradictory. In this study, these aspects are evaluated systematically by atom probe tomography (APT) and a special data analysis method. Additionally, to account for possible field evaporation effects that can falsify the estimation of site occupancy and induce misinterpretations, APT evaporation sequences were also simulated. As a result, chromium occupies most frequently the next nearest neighbour sites of Al atoms and local ordering parameters could be achieved.

Cold-drawn pearlitic steel wires are widely used in numerous engineering fields. Combining X-ray line profile analysis and positron annihilation spectroscopy methods, up to 10−5–10−4 vacancies were found in α-ferrite of a cold-drawn pearlitic steel wire with a true strain of e = 3. The formation of deformation-induced vacancies in α-ferrite of cold-drawn pearlitic steel wire was quantitatively testified.

Cold-drawn pearlitic steel wires were subjected to differential scanning calorimetry and resistivity measurements during heating. Both techniques yield two peaks, one at 500–600 K, the other at 400–450 K. Whereas the peak at 500–600 K can be ascribed to voids consisting of about 10 vacancies as confirmed by positron annihilation spectroscopy, the low-temperature peak is ascribed to V1C2 and/or V2C4 complexes.

Based on a novel defactants (defect acting agents) concept (R. Kirchheim, Acta Materialia 55 (2007) 5129 and 5139), a novel method of understanding and synthesizing NC material was proposed by introducing defactants (segregating solute atoms) into the materials to ease the formation of grain boundaries (GBs) and enhance the formation ability of nanocrystalline (NC) structures. The iron-carbon system was chosen as a model system where carbon acts as the so-called defactant. Iron powders mixed with different amount of graphite were ball milled to prepare NC iron-carbon alloys with different carbon concentrations (C0). After ball milling, the as-milled powder with relatively low carbon concentration was annealed at a certain temperature to achieve saturation of GBs by carbon atoms. The microstructures of the powders were investigated by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods. The mean grain sizes (D) of the powders were determined by analyzing TEM dark field images and X-ray line profiles. The results indicated that once the saturation of GBs is achieved, D of the NC iron-carbon powders will be strongly dependent on C0 and will follow a simple mass balance of carbon in a closed system, i.e. D=3ΓgbVm/(C0-Cg) with Cg the carbon concentration in grains, Γgb the grain boundary excess, and Vm the molar volume of iron. Based on the experimental results, the formation of NC iron-carbon alloys was treated in detail within the framework of the defactant concept. The increase of C0 significantly reduces the formation energy of GBs, leading to a substantial decrease of D.

The effect of hydrogen on dislocation nucleation in a vanadium (1 0 0) single crystal has been examined by means of nanoindentation. For electrochemically doped samples to different hydrogen concentrations within the α-phase, the pop-in load decreased with the increase in hydrogen concentration and multiple pop-ins were observed on the load–displacement curves. The interaction between the dissolved hydrogen atoms and the newly formed dislocation loops, resulting in the reduction of their line energy, is evaluated and explained based on the novel thermodynamic defactant concept.

Cold-drawn pearlitic steel wires are known to exhibit increasing strength with increasing elongation and are therefore highly interesting for a wide field of engineering applications. When isochronal heat treatment is performed at different temperatures, the tensile strength as well as the electrical resistivity decrease well before microstructural changes are observed. An Arrhenius analysis of both processes yield mean activation energies of about 0.3 eV. This is construed as interaction between carbon atoms and defects in ferrite, mainly vacancies and vacancy clusters.

Nanocrystalline titanium films of different thicknesses, sputtered on sapphire substrates, were charged electrochemically with hydrogen. Hydrogen absorption and the thermodynamics of the nanocrystalline Ti–H thin film system were studied using electromotive force (EMF) measurements. The phase boundaries obtained from the EMF–pressure–concentration curves were confirmed by X-ray diffraction, complemented by in situ stress measurements during hydrogen charging. The change in the stress increase with hydrogen concentration was found to be in good agreement with the obtained phase boundaries. In comparison to bulk Ti–H system, considerable changes, such as shifted phase boundaries, and narrowed and sloped miscibility gaps, were observed in Ti–H thin films. These changes vary among the films of different crystalline orientation and are attributed to both microstructural effects and stress contributions. The influence of the initial crystallographic growth orientation of Ti films on the measured thermodynamic isotherms, phase transitions and stress development is discussed in detail.

The influence of electric fields on the low temperature oxidation of individual nanoscale tungsten wires was investigated. In the experiments at room temperature, the nanowires were biased as anode opposite to a macroscopic cathode and H 2 O-vapor with a pressure of 10 −7 –10 1 mbar was provided as oxygen source. Under the influence of an electric field, a dramatic change of the oxidation behavior is observed with the formation of several 10 nm thick oxide layers for electric fields exceeding a threshold. The chemical composition of the layers formed is determined with laser-assisted atom probe tomography to be slightly understoichiometric WO 3 . After an initial period of fast growth, the oxidation rate later rapidly decreases to immeasurable low values. Evaluation of the electric field distribution in the vicinity of the sample by the finite element method reveals that oxide formation only proceeds if a critical field in the range of 0.7–5.0 V/nm, depending on the H 2 O-pressure, is present. This critical field is attributed to a field-activated reaction of H 2 O at the oxide–vapor interface. Besides for tungsten, field-induced oxidation is also observed for aluminum and p-doped silicon and thus apparently is a widely material independent phenomenon.

Models explaining hydrogen embrittlement as caused by enhanced local plasticity are discussed in the light of a novel concept describing solute–defect interaction in a thermodynamic framework. Solute atoms segregating at defects with an excess Γ are called defactants because they lower the defect formation energy of defects γ , analagous to the action of surfactants with surfaces. The corresponding decrease is given by d γ = − Γ d μ , where μ is the chemical potential of the defactant. This equation is applied to recent nanoindentation results, and reveals a lower energy barrier for the generation of dislocation loops in the presence of hydrogen.

It will be shown that there is an analogy between surfactants in liquids which stabilize structures with large surface areas such as foams or microemulsions, and certain components in a crystalline solid, which stabilize defects such as grain boundaries and dislocations. These components are then called defactants. In the presence of defactants microstructural changes may occur because some of the defects compete more successfully for segregation of the defactants than others. Thus voids are formed at grain boundaries of Cu – Bi alloys because the formation energy of voids is provided by the interaction energy of Bi-atoms of the newly created surface. Or the beneficial role of rhenium on the high temperature creep of superalloys may be interpreted as a deficiency of defactants stabilizing dislocations and/or vacancies. These examples are discussed in the present work after a general treatment of the role of defactants.

Interaction of hydrogen with metallic multi-layered thin films remains as a hot topic in recent days. Detailed knowledge on such chemically modulated systems is required if they are desired for application in hydrogen energy system as storage media. In this study, the deuterium concentration profile of Fe/V multi-layer was investigated by atom probe tomography (APT) at 60 and 30 K. It is firstly shown that deuterium-loaded sample can easily react with oxygen at the Pd capping layer on Fe/V and therefore, it is highly desired to avoid any oxygen exposure after D2 loading before APT analysis. The analysis temperature also has an impact on D concentration profile. The result taken at 60 K shows clear traces of surface segregation of D atoms towards analysis surface. The observed diffusion profile of D allows us to estimate an apparent diffusion coefficient D. The calculated D at 60 K is in the order of 10−17 cm2/s, deviating 6 orders of magnitude from an extrapolated value. This was interpreted with alloying, D-trapping at defects and effects of the large extension to which the extrapolation was done. A D concentration profile taken at 30 K shows no segregation anymore and a homogeneous distribution at cD=0.05(2) D/Me, which is in good accordance with that measured in the corresponding pressure–composition isotherm.

Titanium films were prepared on sapphire substrates in an UHV chamber, by means of ion beam sputter deposition under Ar-atmosphere at the pressure of 1.5ּ10-4 mbar, with a deposition rate of 2,1 nm/min. The crystal structure was investigated by means of X-Ray diffraction using a Phillips X-Pert diffractometer with a Co-Kα radiation. For electrochemical hydrogen loading, the films were covered by a 30 nm thick layer of Pd in order to prevent oxidation and facilitate hydrogen absorption. The samples were step-by-step loaded with hydrogen by electrochemical charging, which was carried out in a mixed electrolyte of phosphoric acid and glycerine (1:2 in volume). An Ag/AgCl (sat.) and Pt wires were used as the reference and the counter electrode, respectively. XRD measurements were performed before and after hydrogenation in order to investigate the effect of hydrogen loading on the microstructure. The main characteristics of hydrogen's absorption behaviour, as well as the thermodynamics and phase boundaries of titanium-hydrogen thin films are discussed in detail with specific emphasis on the comparison to titanium-hydrogen bulk system.

Cold-drawn pearlitic steel wires are known to exhibit increasing strength with increasing elongation and are therefore highly interesting for a wide field of engineering applications. In a combined high-resolution transmission electron microscopy and atom probe tomography study, the strengthening mechanism is elucidated: first, there is a strong fragmentation of the original pearlitic microstructure, followed by partial decomposition of cementite accompanied by amorphization of the latter, leading to dispersion hardening. The concomittent mechanisms are discussed in detail.

Scherrer formula is often applied to X-ray-diffraction profiles for the size determination of small size clusters. However, for small clusters this often leads to conflicting results in comparison to other methods. A series of Pd-clusters of different size is studied by X-ray diffraction analysis and transmission electron microscopy. The influence of size and structure on the results is presented and discussed in comparison with theoretical calculations. It will be shown that the different structure of small size systems are one main origin of the conflicts. The structure problem can be overcome by using Fourier Transform of the X-ray diffraction pattern. The importance of the knowledge of the cluster structure was demonstrated by showing its strong influence on the hydrogen solubility.