Radiation-induced solute segregation in metallic alloys

The subject of radiation-induced solute segregation (RIS) in metallic alloys is reviewed. RIS manifests itself in several different ways, including diffusion to point-defect sinks (dislocations, grain boundaries, voids, etc.), which can induce precipitation in undersaturated alloys, as well as self-organization of solute clusters and precipitation in defect-free material. Diffusion in dilute and concentrated alloys is highlighted, as are theories of RIS that include new ideas on diffusion of complexes involving coupling between fluxes of point defects and of solute atoms. Many important experimental observations are presented, including up-to-date findings using atom-probe tomography, with special emphasis on solute segregation in austenitic and ferritic steels. Results from computational modeling and theory are also presented and discussed in light of experimental findings. Examples illustrating the factors affecting RIS are shown and some important outstanding issues that impact the current understanding of RIS are described and discussed.     

Limits of detectability for clusters and solute segregation to grain boundaries

The abilities to detect and characterize the sizes and distribution of solute clusters, embryos, precipitates and solute atoms in the matrix by friends-of-friends methods in atom probe tomography are shown to improve with single atom position-sensitive detectors with high detection efficiency. In low solute matrices, clusters with as few as 5 atoms can be detected. A characterization method is presented that enables complete characterizations of grain boundaries and triple junctions by atom probe tomography in terms of the orientation relationships of the adjacent grains, as well as the local variations of the habit plane, surface curvature, and the solute excesses over the surface of a grain boundary with up to a 1 nm by 1 nm spatial resolution.     

Composition-dependent dynamic precipitation and grain refinement in Al-Si system under high-pressure torsion

Understanding composition effects is crucial for alloy design and development. To date, there is a lack of research comprehensively addressing the effect of alloy composition on dynamic precipitation, segregation and grain refinement under severe-plastic-deformation processing. This research investigates Al-x Si alloys with x = 0.1, 0.5 and 1.0 at.% Si processed by high pressure torsion(HPT) at room temperature by using transmission electron microscopy, transmission Kikuchi diffraction and atom probe tomography. The alloys exhibit interesting composition-dependent grain refinement and fast dynamic decomposition under HPT processing. Si atoms segregate at dislocations and Si precipitates form at grain boundaries(GBs) depending on the Si content of the alloys. The growth of Si precipitates consumes most Si atoms segregating at GBs, hence the size and distribution of the Si precipitates become predominant factors in controlling the grain size of the decomposed Al-Si alloys after HPT processing. The hardness of the Al-Si alloys is well correlated with a combination of grain-refinement strengthening and the decomposition-induced softening.     

Die Welt ist keine Scheibe

The world is not flat: from field ion microscopy to atom probe tomography Atom probe tomography has gained considerable importance in the past couple of years due to the steadily increasing demands on the capabilities of analytical tools for nanotechnology. Presently available preparation and instrumental methods allow the application of atom probe tomography also for technically relevant specimens. This affords a unique analytical approach for a quantitative and highly sensitive characterization of the chemical content of materials; by combining these features with the three dimensional registration of the elemental distributions, atomic spatial resolution can be achieved both laterally and in‐depth. For this reason, atom probe tomography is very well suited for the determination of the composition and morphology of crystalline and amorphous materials and nanostructures, and may have an enormous impact on research and development in various areas of material sciences. The first part of this contribution on modern atom probe tomography illustrates the evolution of the technique from its beginnings in the 1950s to the presently available commercial instruments. In the course of this overview, the basic physical and instrumental concepts will briefly be outlined. The second part will concentrate on the presentation of selected, current applications; these examples will serve also to emphasize some of the limitations of atom probe tomography.     

Atom probe tomography characterization of solute segregation to dislocations and interfaces

The level and extent of solute segregation to individual dislocations and interfaces may be visualized and quantified by atom probe tomography. The large volume of analysis and high data acquisition rate of the local electrode atom probe (LEAP^®) enables the solute distribution in the region of and along the core of dislocations to be estimated. Solute segregation at precipitate-matrix interfaces of precipitates as small as 2-nm diameter may be quantified. Examples are presented of solute segregation to dislocations and clustering/precipitation in a neutron irradiated Fe–Ni–P model alloy and the neutron irradiated beltline weld from the Midland reactor.     

Influence of severe plastic deformation on the precipitation hardening of a FeSiTi steel

The combined strengthening effects of grain refinement and high precipitated volume fraction (~6 at.%) on the mechanical properties of FeSiTi alloy subjected to SPD processing prior to ageing treatment were investigated by atom probe tomography and scanning transmission electron microscopy. It was shown that the refinement of the microstructure affects the precipitation kinetics and the spatial distribution of the secondary hardening intermetallic phase, which was observed to nucleate heterogeneously on dislocations and sub-grain boundaries. It was revealed that alloys successively subjected to these two strengthening mechanisms exhibit a lower increase in mechanical strength than a simple estimation based on the summation of the two individual strengthening mechanisms.     

Interfacial segregation and grain boundary embrittlement: An overview and critical assessment of experimental data and calculated results

One of the most dangerous technical failures of materials is intergranular brittle fracture (temper embrittlement) as it proceeds very quickly and its appearance is often hardly predictable. It is known that this phenomenon is closely related to the chemistry of grain boundaries and to the difference of the segregation energies of the grain boundaries and the free surfaces (Rice–Wang model). To elucidate the effect of individual solutes on embrittlement of various materials such as steels and nickel-base superalloys, grain boundary and surface segregation was extensively studied in many laboratories. As a result, numerous data on surface and grain boundary segregation have been gathered in literature. They were obtained in two main ways, by computer simulations and from experiments. Consequently, these results are frequently applied to quantify the embrittling potency of individual solutes. Unfortunately, the values of the segregation energy of a solute at grain boundaries as well as at the surfaces obtained by various authors sometimes differ by more than one order of magnitude: such a difference is unacceptable as it cannot provide us with representative view on the problem of material temper embrittlement. In some cases it seems that these values do not properly reflect physical reality or are incorrectly interpreted. Due to the above mentioned large scatter of the segregation and embrittlement data a critical assessment of the literature results is highly needed which would enable the reader to avoid both the well known and less well known pitfalls in this field. Here we summarize the available data on interfacial segregation and embrittlement of various solutes in nickel and bcc iron and critically discuss their reliability, assessing also limitations of individual approaches employed to determine the values of segregation and strengthening/embrittling energies, such as density functional theory, Monte Carlo method, molecular statics and dynamics and tight binding on the theoretical side, and Auger electron spectroscopy, 3D tomographic atom probe, and electron microscopy techniques on the experimental side. We show that experimental methods have serious limitations which can be overcome by accepting reasonable assumptions and models. On the other hand, the theoretical approaches are limited by the size of the computational repeat cell used for the calculations of the segregation energy. In both cases, a careful critical analysis of the available segregation energy and/or enthalpy reflecting physical reality allows to assess the reliability of these values and their applicability in analysis of intergranular brittle fracture in steels and nickel-base alloys.     

Mechanism of Bainite Nucleation in Steel, Iron and Copper Alloys

During the incubation period of isothermal treatment(or aging) within the bainitic transformation temperature range in a salt bath (or quenching in water) immediately after solution treatment, not only are the defects formed at high temperatures maintained, but new defects can also be generated in alloys, iron alloys and steels. Due to the segregation of the solute atoms near defects through diffusion, this leads to non-uniform distributions of solute atoms in the parent phase with distinct regions of both solute enrichment and solute depletion. It is proposed that when the Ms temperature at the solute depleted regions is equal to or higher than the isothermal (or aged) temperature, nucleation of bainite occurs within these solute depleted regions in the manner of martensitic shear. Therefore it is considered that, at least in steel, iron and copper alloy systems, bainite is formed through a shear mechanism within solute depleted regions, which is controlled and formed by the solute atoms diffusion in the parent phase.     

The approach to equilibrium during tempering of a bulk nanocrystalline steel: an atom probe investigation

A local electrode atom probe has been used to analyze the solute partitioning during bainite transformation in a novel, nanocrystalline bainitic steel. Atom probe results show the absence of any partitioning of substitutional elements between the phases involved. The results are fully consistent with the diffusionless transformation of austenite to bainite. However, substitutional elements are expected to redistribute approaching an equilibrium phase boundary as the mixture of bainitic ferrite and retained austenite is tempered. The compositional analysis of the austenite/ferrite interface by atom probe tomography indicates that retained austenite decomposes during tempering before equilibrium is reached at the interface.     

Effects of the aluminum concentration on twin boundary motion in pre-strained magnesium alloys

We investigated the effects of Al concentration on the reciprocated motion of twin boundaries in pre-strained Mg-Al-Zn alloys through a combination of applied compression and tension,in-situ electron-backscattering diffraction observations,and high-angle annular dark-field scanning transmission electron microscopy observations.The twin growth was restricted by increased Al concentration,which resulted in the occurrence of smaller-sized twins.The reverse motion of twin boundaries was also restricted,resulting in the formation of higher fractions of secondary twins and 2-5° boundaries during reverse tension.The secondary twins and 2-5° boundaries mainly contributed to the increased ultimate tensile strength of the pre-strained Mg alloys.This effect is more significant in Mg alloys with larger pre-compression.Moreover,the increased amount of the Al solute atoms,rather than the precipitates,mainly contributed to the increased strengthening effect on the twin boundary motion.Our research contributes to development of high-strength Mg alloys by stabilizing twin boundaries.     

Combined atomic-scale modelling and experimental studies of nucleation in the solid state

The process of solid-state nucleation in highly supersaturated solid solutions has been investigated on the atomic scale by a combination of three-dimensional atom probe analysis and atomistic modelling using dynamical Ising models. In binary Cu-Co alloys, a simple atom-exchange model with a single thermodynamic parameter derived from phase-diagram data was able to reproduce the atomic-scale microstructures observed in the atom probe, and also match the measured peak precipitate density. Modelling solute effects in complex copper-bearing steels required a more sophisticated model based on a vacancy-hopping mechanism and a larger number of thermodynamic and kinetic parameters derived from independent experimental data and theoretical calculations. The model gave an excellent match to the experimentally observed microstructures, and it reproduced features such as the clustering of Ni and Mn before the precipitation of Cu. The model also allowed time-dependent behaviour to be investigated, and it showed that solute clustering of Ni and Mn occurs during the cooling of the alloy. These clusters then act as heterogeneous nucleation sites for the formation of copper precipitates. Understanding such complex solute interaction effects through combined experiment and modelling is an essential step to controlling nucleation and hence the fine-scale microstructures in advanced engineering alloys.     

Cu对Fe-NiAlMn合金析出强化过程的影响EI北大核心CSCD

将Fe-NiAlMn和Fe-CuNiAlMn合金在900℃固溶2 h后水淬,并在500℃时效不同时间。利用显微硬度测试和原子探针层析技术(APT)研究Cu的添加对Fe-NiAlMn合金析出强化过程的影响。结果表明:Cu的添加增强了时效初期的析出强化效果,加快了整个析出强化进程。时效过程中,Cu元素首先偏聚形核成为富Cu相,并促使Ni,Al,Mn元素在富Cu相与基体界面处偏聚形核形成Ni(Al,Mn)相,两相为核壳结构。随着时效时间的延长,富Cu相由BCC结构转变为FCC结构,与Ni(Al,Mn)相逐渐分离。     

Understanding formation of Mg-depletion zones in Al-Mg alloys under high pressure torsion

Redistribution of elements may take place in alloys during severe plastic deformation, which significantly alters the mechanical properties of the alloys. Therefore, comprehensive knowledge about deformationinduced redistribution of elements has to be established. In the present paper, the distribution of Mg in an Al-Mg alloy processed by high pressure torsion was examined using atom probe tomography(APT).With crystallographic information extracted by APT data analysis, this research reveals that the movement of dislocations plays an important role in the formation of Mg-depletion zones in the deformed microstructure.     

Mechanical properties and deformation mechanisms of a novel fine-grained Mg-Gd-Y-Ag-Zr-Ce alloy with high strength-ductility synergy

Grain boundary precipitation and segregation play an important role in determining mechanical properties of Mg alloys. In the present work, we studied work focuses on the strengthening and deformation mechanism of coarse-grained(CG) and fine-grained(FG) Mg-Gd-Y-Ag-Zr-Ce alloy. The CG alloy is strengthened by means of age-strengthening with the formation of both basal plate γ" and prismatic plate β’ precipitates in the grain interior. While the strengthening of FC alloy is completed by intergranular alloying segregation and intragranular precipitates γ" and β’. The segregation of alloying elements at the grain boundary and formation of sub-micron particles can stabilize the grain boundary and suppress the intergranular deformation. Consequently, dislocations could be trapped near γ" and β’ precipitates in the grain interior. Unlike CG alloys, the FG alloys exhibit a heterogeneous transition from elastic to plastic deformation via the Lüders plateau. The rapid gliding dislocation multiplications and fine-grained size are necessary and sufficient conditions for the Lüders strains. Our work provides the insights on the evolution of fine-grained microstructure and helps for the design of Mg alloys with good mechanical properties.     

Evolution of the microstructure and solute distribution of Sn-10wt% Bi alloys during electromagnetic field-assisted directional solidification

The effects of forced flows at different velocities on microstructure and solute distribution during the directional solidification of Sn-10 wt% Bi alloys under a simultaneous imposition of a transverse static magnetic field(TSMF) and an external direct current(DC) have been investigated experimentally and numerically. The experimental results show that the solid-liquid interface will gradually become sloping with the increase of the forced flow velocity when the thermoelectric magnetic convection(TEMC)dominates the forced flow at solidification front. However, the interface will gradually become planar as the flow velocity further increases when the electromagnetic convection(EMC) dominates the forced flow. Moreover, when the flow velocity gradually increases, the primary dendrite spacing decreases from384 to 105 μm accordingly. The simulation results show that the solute distribution at the two sides of the sample can be significantly changed by the forced flow at solidification front. The rejected solute will be unidirectionally transported to one side of the sample along the TEMC(a low-velocity forced flow),thereby causing the formation of a sloping interface. However, the rejected solute will be returned back along the EMC(a higher-velocity force flow), which results in a planar interface. Furthermore, the solute content at the two sides of the sample under the forced flows at different velocities was measured. The results are in good agreement with the simulation results, which shows that the solute content difference between the two sides of the sample reaches the maximum when a 0.5 T TSMF is applied, while the solute content difference decreases to zero with a simultaneous application of a 0.5 T TSMF and a 1.6 × 10~5 A/m~2 external DC.     

Suppressed vacancy coalescence in dilute interconnect alloys exposed to electrotransport

Interconnect materials play a decisive role in present and future microelectronics technology. Alloying additions to improve the resistance to electromigration of interconnect metals are selected with the intention of maximizing the time to failure. We found that the intrinsic electronic charging state of vacancies and ‘vacancy–solute atom’ pairs in crystalline metals and dilute alloys is significant in the process of erosion void formation (or suppression). Supported by a number of own and published experimental electromigration data, a semi-empirical rule is established and theoretically justified, which allows a projection of the voiding probability of certain matrix element–solute atom combinations. Particular attention is paid to copper, aluminium, and silver as interconnect matrix metals, where chemical elements for solute atoms are singled out with a high potential of erosion void suppression.     

Atom probe characterisation of high temperature materials

Abstract

An atom probe is capable of quantitatively analysing materials at the atomic level. Modern atom probes are derived from the field ion microscope, and are coupled with time-of-flight mass spectrometers, permitting identification of individual atoms. The introduction of position-sensitive detectors enables the reconstruction of a small volume of the sample owing to simultaneous determination of the x, y, and zcoordinates and the mass to charge ratios of individual atoms. This paper focuses on the application of atom probe techniques to the microstructural analysis of high temperature materials. Illustrations include carbide precipitation in creep resistant power plant steels and analyses of model and commercial multicomponent nickel based superalloys. It is demonstrated that atom probe field ion microscopy and atom probe tomography are valuable techniques in the development and understanding of technologically important alloys for high temperature service.     

Secondary dendrite arm spacing and solute redistribution effects on the corrosion resistance of Al–10 wt% Sn and Al–20 wt% Zn alloys

In general, aluminum alloys provide the most significant part of all shaped casting manufactured. An optimum range of properties can be obtained as a function of different cooling rate processes, such as sand, plaster, investment, permanent molds and die castings. It is well known that the dendritic network affects not only the mechanical properties but also the corrosion resistance. However, the literature is scarce on reports concerning the influences of dendrite arm spacing on corrosion resistance and mechanical behavior. The aim of this study is to investigate the influence of as-cast microstructure features, i.e., dendrite arm spacing and solute redistribution on the corrosion resistance of samples of aluminum alloys. In order to investigate the electrochemical behavior of solute and solvent of different aluminum systems, samples with the same order of magnitude of dendritic spacings were analyzed to permit comparison between Al–10 wt% Sn and Al–20 wt% Zn alloys. A casting water-cooled assembly promoting upward directional solidification was used in order to obtain controlled casting samples of these alloys. In order to characterize the dendritic structure, longitudinal sections from the directionally solidified specimens were analyzed by using optical and electronic microscopy techniques. The corrosion resistance was analyzed by both the electrochemical impedance spectroscopy technique and Tafel extrapolation method conducted in a 3% NaCl solution at room temperature. Although both systems present an Al-rich dendritic matrix, different responses to corrosive action as a function of dendritic spacing have been detected.     

Phase separation in PM 2000™ Fe-base ODS alloy: Experimental study at the atomic level

The coarsening of the three-dimensional microstructure resulting from phase separation during ageing at 748 K of a Fe-based PM 2000™ oxide dispersion strengthened (ODS) steel has been investigated by atom probe tomography and hardness measurements. Phase separation resulted in the formation of isolated particles of the chromium-enriched α′ phase. The aluminum and titanium were found to preferential partition to the iron-rich α phase. The partitioning of aluminum is consistent with theoretical calculations. The change in the scale of the chromium-enriched α′ phase was found to fit a power law with a time exponent of 0.32 in accordance with that predicted by the classical Lifshitz, Slyozov and Wagner (LSW) theory. The solute concentrations of the coexisting α and α′ phases were estimated from concentration frequency distributions with the Langer–Bar-on–Miller (LBM) method and proximity histograms. The hardness was linearly related to the chromium content of the α′ phase.     

Modeling of non-equilibrium solute diffusion upon rapid solidification: effective mobility versus kinetic energy

Abstract

The effective mobility approach is compared with the kinetic energy approach in terms of sharp interface modeling and phase-field modelling of non-equilibrium solute diffusion upon rapid solidification of binary alloys. The two approaches are equivalent for modelling of long range solute diffusion in bulk phases, but only the effective mobility approach can introduce the non-equilibrium solute diffusion effect to short range solute diffusion at a sharp interface or within a diffuse interface. Addition of the kinetic energy terms results in an unreasonable non-bilinear expression of the flux and thermodynamic driving force in the free energy production of interface migration or phase field propagation, whereas the effective mobility approach allows the thermodynamic extremal principle workable.