Impurities In Alloys Research Articles

ARXPS and DFT studies of thermally induced Pb surface segregation on Au/Cu alloys

• Surface segregation of Pb impurities in AuCu alloys was studied by ARXPS and DFT. • ARXPS show a 23% surface coverage by Pb atoms. • DFT calculations predict a ¼ ML coverage of Pb atoms in the first atomic layer. • The results show that bulk systems are affected while leaving nanoparticulate systems unaffected. Surface segregation of Pb has been studied using angle resolved X-ray Photoelectron Spectroscopy (ARXPS) and Density Functional Theory (DFT) modelling in a polycrystalline Au 0.85 Cu 0.15 alloy. Au surface enrichment was found in the alloy surface, whereas Pb surface enrichment was detected after annealing. ARXPS depth profiles show a 23% surface coverage by Pb atoms. DFT calculations predict an approximate coverage of ¼ monolayer (ML) of Pb atoms in the first atomic layer, consistent with the experimental results. The results presented here are an example of the importance of low concentration of impurities in determining the surface composition of bulk alloys.

First Principle Study of the Diffusion of Oxygen and Oxygen Complexes in Si, SiGe Solid Solutions and Si Nanocrystals

In the framework of a unified approach the diffusion coefficient (the prefactor and activation barrier) of an interstitial oxygen Oi, the hydrogen molecule H2, vacancy, oxygen dimer in silicon crystals and Oiin Si1-xGexsolid solutions, silicon nanotubes and nanowires has been calculated. For all the above cases, the calculated values of the diffusion coefficient are in good agreement with the experimental data. The calculated equilibrium structures, electrical activity, the vibrational spectrum, the mechanism of diffusion of oxygen dimer fully describe the experimental results. Our study has revealed that the diffusivity of impurities (defects) in alloys can decrease considerably and this variation results from the fact that the prefactor depends on the concentration of component elements of the alloy.

Microstructural evolution and characterization of a ferroniobium alloy

Niobium in the form of a ferroniobium alloy is added during the steel-making process to improve the mechanical properties of steel, but it may contain phase(s) with high melting temperatures that may be slow to melt or dissolve. It has been suggested that these phases lead to the presence of coarse Nb-rich particles in the resultant steel, which may adversely affect the mechanical properties. In the present study, electron microscopy and differential scanning calorimetry (DSC) were used to identify phases and microstructural evolution of a commercial grade ferroniobium alloy. The ferroniobium alloy was composed of two main phases, that is, Nb-rich solid solution and μ phase (Fe7Nb6). The μ phase (Fe7Nb6) was formed as a result of three reactions and exhibited three different morphologies based on their formation temperatures (proeutectic intermetallic, eutectic intermetallic and eutectoid intermetallic). The intermetallic that formed via the eutectic reaction was slightly Nb-rich and was heavily faulted. The Nb-rich eutectic portion of the Fe-Nb binary-phase diagram was also modified based on the DSC results to incorporate the effect of impurities in commercial alloys. On the basis of solidification study, the estimated cooling rate for the as received ferroniobium alloy was 10 K/min.

Point defect concentrations of impurity carbon in tungsten

Our recent investigations (Y.-L. Liu, H.-B. Zhou, Y. Zhang, G.-H. Lu, G.-N. Luo, Comput. Mater. Sci. 50 (2011) 3213) show that impurity carbon (C) easily bonds onto vacancy internal-surface and a monovacancy is capable of trapping as many as 4 C atoms to form CnV complexes in W, but the concentration distributions of point defects such as CnV complexes are lacking. Based on first-principles calculations combined with statistical model, we predict point defect concentrations of impurity C in W and find that the concentration of vacancy in the form of CnV complexes notablely increase with the presence of C, although the concentration of interstitial C is still higher than that of CnV complexes in W. We believe that the present method can be also generally applicable for predicting the concentration distribution of other impurities in other metals and metal alloys.

Effect of intensive melt shearing on the formation of Fe-containing intermetallics in LM24 Al-alloy

Fe is one of the inevitable and detrimental impurities in aluminium alloys that degrade the mechanical performance of castings. In the present work, intensive melt shearing has been demonstrated to modify the morphology of Fe-containing intermetallic compounds by promoting the formation of compact α-Al(Fe,Mn)Si at the expense of needle-shaped β-AlFeSi, leading to an improved mechanical properties of LM24 alloy processed by MC-HPDC process. The promotion of the formation of α -Al(Fe, Mn)Si phase is resulted from the enhanced nucleation on the well dispersed MgAl2O4 particles in the melt. The Fe tolerance of LM24 alloy can be effectively improved by combining Mn alloying and intensive melt shearing.

Removal of iron from recycled aluminium alloys

Iron is considered as an impurity in aluminium alloys due to its detrimental impact on strength and ductility. Much work has been carried out to reduce the iron levels in recycled aluminium alloys. In this investigation, the influence of Mn additions for reducing the Fe content in an Al–Si alloy has been studied. The experimental results and thermodynamic calculations revealed that a reduction of up to 62%Fe content can be achieved. The major mechanism of iron removal is the formation of the primary α-AlFeMnSi particles when held at lower temperatures. The characteristics and compositions of these particles, which settle on the bottom of the alloy melts, were confirmed using optical microscopy and X-ray diffraction techniques.Dans les alliages d’aluminium, le fer est considéré comme une impureté en raison de son effet négatif sur la résistance et la ductilité. Des travaux ont été menés pour réduire la quantité de fer dans les alliages d’aluminium recyclé. L’étude menée porte sur l’influence des ajouts de manganèse visant à réduire la teneur en fer dans un alliage Al–Si. Les résultats expérimentaux et les calculs thermodynamiques ont révélé qu’il est possible d’atteindre une réduction de 62% de la teneur en Fe. Le principal mécanisme d’extraction du fer est la formation de particules d’alliage α-AlFeMnSi lorsque des températures plus basses sont maintenues. Les caractéristiques et la composition de ces particules, qui se déposent au bas des fontes d’alliage, ont été confirmées à l’aide de microscopie optique et de techniques de diffraction des rayons X.

Coefficient of Diffusion in Crystals of Si<sub>1-x</sub>Ge<sub>x</sub>: Role of Preexponential Factor

In the present work the results of theoretical analysis of the process of diffusion of covalently bonded atoms (interstitial oxygen atoms) in Si1-xGex alloys are presented. The diffusion coefficient (activation energy and pre-exponential factor) was calculated by means of quantum-chemical simulations (Hartree–Fock, NDDO, PM5) and the dependences of the activation energy and pre-exponential factor on Ge atoms concentration (x) were analyzed with the use of the percolation theory. The study has revealed that the diffusivity of impurities (defects) in alloys can decrease considerably at low concentration (x<0.05) of a minor alloy component and this variation results from the fact that the pre-exponential factor depends on the concentration of component elements of the alloy. The alloy-induced decrease in the pre-exponential factor is associated with removal of the degeneracy of the number of equivalent diffusion paths. It is found that a sharp decrease in the pre-exponential factor causes experimentally observed decreases in the coefficient of diffusion of interstitial oxygen atoms and in the rate of formation of oxygen thermal donors in Si1‑xGex crystals at x~0.01.

Recycling of aluminium scrap for secondary Al-Si alloys

An increasing amount of recycled aluminium is going into the production of aluminium alloy used for automotive applications. In these applications, it is necessary to control and remove alloy impurities and inclusions. Cleaning and fluxing processes are widely used during processing of the alloys for removal of inclusions, hydrogen and excess of magnesium. These processes use salt fluxes based in the system NaCl-KCl, injection of chlorine or mixture of chlorine with an inert gas. The new systems include a graphite wand and a circulation device to force convection in the melt and permit the bubbling and dispersion of reactive and cleaning agents. This paper discusses the recycling of aluminium alloys in rotary and reverberatory industrial furnaces. It focuses on the removal of magnesium during the melting process. In rotary furnaces, the magnesium lost is mainly due to the oxidation process at high temperatures. The magnesium removal is carried out by the reaction between chlorine and magnesium, with its efficiency associated to kinetic factors such as concentration of magnesium, mixing, and temperature. These factors are also related to emissions generated during the demagging process. Improvements in the metallic yield can be reached in rotary furnaces if the process starts with a proper salt, with limits of addition, and avoiding long holding times. To improve throughput in reverberatories, start the charging with high magnesium content material and inject chlorine gas if the molten metal is at the right temperature. Removal of magnesium through modern technologies can be efficiently performed to prevent environmental problems.

Quantitative analysis of tin alloy combined with artificial neural network prediction

Laser-induced breakdown spectroscopy was applied to quantitative analysis of three impurities in Sn alloy. The impurities analysis was based on the internal standard method using the Sn I 333.062-nm line as the reference line to achieve the best reproducible results. Minor-element concentrations (Ag, Cu, Pb) in the alloy were comparatively evaluated by artificial neural networks (ANNs) and calibration curves. ANN was found to effectively predict elemental concentrations with a trend of nonlinear growth due to self-absorption. The limits of detection for Ag, Cu, and Pb in Sn alloy were determined to be 29, 197, and 213 ppm, respectively.

New possibilities of modern methods for determining gas-forming impurities in metals

The main tendencies in the development of methods for determining gas-forming impurities in metals and alloys, and the physicochemical laws of the fractional separation of oxygen are considered. Examples of the analysis of model alloys and steels show that the method of fractional gas analysis can be used for the quantitative determination of oxygen bound in inclusions.

Why the Growth Rates of Alumina and Chromia Scales on Thin Specimens Differ from those on Thick Specimens

For a number of chromia and alumina forming high temperature alloys and coatings, recent studies revealed, that in some cases the specimen/component or coating thickness may substantially affect the growth rates of the surface oxides. For the alumina formers the thickness dependence is mainly governed by depletion of oxygen active elements such as Y, Zr, Hf, Mg which are either intentionally added alloying elements or manufacturing related alloy impurities. In the case of the chromia forming materials, which tend to exhibit a more substantial dependence of oxidation rate on specimen/component thickness, depletion of minor alloying additions is also an important factor to be considered. However, for these alloys relaxation of oxide growth stresses by plastic deformation of the metallic substrates seems to be the dominant parameter which governs the observed behaviour.

Fully automated analysis of grain chemistry, size and morphology by CCSEM: examples from cement produc tion and diamond exploration

Computer-controlled scanning electron microscopy (CCSEM) combines the advantages of energy dispersive X-ray spectrometry (EDX) with those of digital image analysis of back-scattered electron (BSE) micrographs. CCSEM analysis of a wide range of geological or non-geological materials has been introduced at the Geological Survey of Denmark and Greenland (GEUS) as a fast and reliable method to determine both the chemistry of individual grains and bulk samples. The chemical analysis is combined with measurements of the two-dimensional size and morphology of every single grain. The CCSEM technique was developed in the early 1980s for characterisation of coal minerals (Huggins et al. 1980; Lee & Kelly 1980) and studies of synthetic crystals for super-conductors and catalysts (Lin & Barnes 1984). Soon it found a broader application in the study of dust particles and fibres in lung tissue of mine workers (Friedrichs 1987), in the analyses of aerosols for air quality control and source emission characterisation (e.g. Heasman & Watt 1989) and the degree of sintering and consolidation of coal ash deposits (e.g. Huffman et al. 1994). CCSEM has been used in the earth sciences for the determination of the sediment budget of a lake (Yin & Johnson 1984), for the characterisation of soil and dust (Pirrie et al. 2004), for provenance analysis of ilmenite-bearing beach sands (Knudsen et al. 2005; Bernstein et al. 2008), and provenance studies on sandstones in oil-bearing basins (Frei et al. 2005). Other areas where CCSEM has been applied range widely and include characterisation of small inclusions, e.g. impurities in metal alloys or steel (Schwoeble et al. 1988), analyses of gun-shot residues (e.g. Steffen et al. 2007), and analyses of bladder stones obtained from a skeleton found in a Mesolithic cave-tomb (D’Alessio et al. 2005). In this paper, we demonstrate the benefits of the method with examples from the cement industry and from diamond prospecting.

Quantitative analysis of impurities in aluminum alloys by laser-induced breakdown spectroscopy without internal calibration

To develop a fast and sensitive alloy elemental analysis method, a laser-induced breakdown spectroscopy(LIBS) system was established and used to carry out quantitative analysis of impurities in aluminum alloys in air at atmospheric pressure. A digital storage oscilloscope was used as signal recording instrument, instead of traditional gate integrator or Boxcar averager, to reduce the cost of the whole system. Linear calibration curves in the concentration range of 4×10 −5–10 −2 are built for Mg, Cr, Mn, Cu and Zn using absolute line intensity without internal calibrations. Limits of detection for these five elements in aluminum alloy are determined to be (2–90)×10 −6. It is demonstrated that LIBS can provide quantitative trace elemental analysis in alloys even without internal calibration. This approach is easy to use in metallurgy industries and relative research fields.

Rare-earth metals (REMs) in nickel aluminide-based alloys: I. Physicochemical laws of interaction in the Ni-Al-REM and Ni x Al y -REM-AE (alloying element) systems

The data on the Ni-Al-R (R = REM Sc, Y, La, lanthanides) binary and ternary systems and the interactions of three rare-earth metals (yttrium, lanthanum, cerium) with the main alloying elements (Ti (Zr, Hf), Cr (Mo, W) that are introduced into Ni3Al-based VKNA alloys are analyzed. The binary aluminides of REMs in the Ni-Al-R ternary systems are shown to be in equilibrium with neither NiAl nor Ni3Al. The solid solution of aluminum in RNi5, which penetrates deep into these ternary systems, is the most stable phase in equilibrium with Ni3Al. In the NiAl (Ni3Al)-AE-R systems, REM precipitation (segregation) on various defects and interfaces in nickel aluminides is likely to be the most probable, and REMs are thought to interact with the most active impurities in real alloys (C, O, N), since REMs have a large atomic radius and, thus, are virtually undissolved in nickel, aluminum, and nickel aluminides.

Morphology of Intermetallic Compounds in Al-Si-Fe Alloy and Its Control by Ultrasonic Vibration

Iron impurity in aluminum alloys forms coarse needle-shaped intermetallic compounds during solidification and hampers the recycling process. To control the morphology of the material microstructure, an experiment was carried out where ultrasonic vibration was applied to the melt during solidification. Aluminum-Silicon-Iron alloys containing 4 mass% iron were melted and solidified. The primary crystals that formed from the melts were intermetallic compounds that were identified as Al3Fe, � -AlSiFe (Al7:4SiFe2) and � -AlSiFe (Al9Si2Fe2). The refinement of these intermetallic compounds was thought to occur with the application of ultrasonic vibration. Accurate results were obtained when the vibration was applied at the liquidus temperature. The liquidus temperatures of the Al-615Si-4Fe alloys were in the range of 945 to 955 K regardless of the silicon content and that of the Al-18Si-4Fe alloy was 977.2 K. Coarse plate-like intermetallic compounds formed in Al-6 mass%Si-4 mass%Fe and Al-12 mass%Si-4 mass%Fe alloys, which can be refined by the application of ultrasonic vibration on crossing the liquidus temperature on cooling. The coarse columnar structure of an Al18 mass%Si-4 mass%Fe alloy was modified to a fine grained structure. [doi:10.2320/matertrans.F-MRA2007874]

Effect of thermomagnetic treatment on the damping properties of magnetically soft iron alloys

The effect of thermomagnetic treatment on the amplitude dependence of internal friction of magnetically soft Fe-Cr alloys and graphitized steels is investigated. The laws of variation of parameters of internal friction after thermomagnetic treatment of iron alloys with ferritic structure are determined. The effect of four structural characteristics (size of ferrite grains, graphite inclusions in ferrite matrix, content of impurities in alloys with ferritic structure, and content of vanadium and aluminum in alloys having no α-γ transformation) and of the composition on the variation of the damping properties of iron after thermomagnetic treatment is studied. The changes in the damping properties are analyzed depending on the changes in the crystal and magnetic structures.

Effects of minor additions and impurities on oxidation behaviour of FeCrAl alloys. Development of novel surface coatings compositions

In the present work the effects of single or combined minor additions of Zr, Hf, Ti and C on the oxidation behaviour of Y-containing, FeCrAl alloys have been studied. For this purpose high-purity, model alloys with single or multiple minor alloying additions were used. The results of long term discontinuous oxidation tests and detailed kinetics studies using thermogravimetry were complemented with extensive microstructural characterisation of the formed alumina scales using SEM and STEM. Hence, the oxidation kinetics and scale spallation rates and failure modes were correlated with the oxide composition and microstructure. The results demonstrate that the frequently reported positive effect of Zr, Hf and Ti on the lifetime oxidation behaviour of FeCrAl alloys can only be fully exploited if the concentrations of the above elements are carefully adjusted and the interaction with typical alloy impurities, such as carbon, is considered.

X-ray Fluorescence Spectroscopy of Novel Materials

This review focuses on the capabilities and potential of soft x-ray fluorescence spectroscopy for the study of the electronic structure and chemical bonding of novel materials. The basic principle of x-ray fluorescence measurements using synchrotron radiation and the corresponding instrumentation issues are outlined. Particular attention is given to x-ray spectroscopic studies of the electronic structure and characterization of nanostructures, thin films, interfaces, adsorbates, half-metallic ferromagnets, and impurities in alloys, superconductors, and semiconductors and the chemical bonding in biomaterials and liquids.

In-Situ Electron Microscopy Studies of the Effect of Solute Segregation on Grain Boundary Anisotropy and Mobility in an Al-Zr Alloy

ABSTRACTThe presence of impurities in aluminum alloys is of great interest with respect to microstructural properties, specifically, the effect of solute on texture and anisotropy. This paper presents new evidence of the pronounced effect of solute drag based on in-situ annealing and Electron Backscatter Diffraction experiments of Zr-rich Al alloys subject to prior strain. A compensation effect was found for grain boundary mobility maxima for specific boundary types. Trends in activation energy as a function of boundary type support the observations of a compensation effect with respect to temperature. Evidence for irregular motion of boundaries from in-situ observations is discussed in reference to new theoretical results that suggest that boundaries migrating in the presence of solutes should move sporadically provided that the length scale at which observations are made is small enough. A study of both boundary motion and solute segregation to specific boundary types using Scanning Transmission Electron Microscopy and in-situ TEM is presented.

Radiochemical reprocessing of vanadium–chromium–titanium alloys after fusion reactor demo decommissioning

Activated structural materials of fusion reactors after the reactor decommissioning may be sent either for burial or for radiochemical reprocessing and consequent material refabrication. This paper considers an opportunity of radiochemical extraction reprocessing of activated vanadium alloy, which is a potential structural material for Russian demonstration fusion reactors (DEMO). The nitrate solutions of the alloy components were used as the aqueous phase. The solution of di-2-ethyl-hexyl-phosphoric acid (D2EHPA) in a hydrocarbon solvent was chosen as an extractant in the experiments. The extraction of alloy impurities (till the dose rate level of 12.5 µSv/h) may be attained in an extractor cascade with 50 stages. End products of the extraction reprocessing are nitrate solutions of vanadium and chromium and titanium hydroxide. The expediency of the radiochemical reprocessing of the alloy or its burial may be determined after the technical and economic comparison of these processes.