Papers by Keyword: Tracer Diffusion

Abstract: High entropy alloys (HEAs) are equimolar multi-principal-element alloys (MPEAs) that are different from traditional solvent-based multicomponent alloys based on the concept of alloy design. Based on initial work by Yeh and co-workers, HEAs were postulated to exhibit four “core” effects: high entropy, sluggish diffusion, lattice distortion, and cocktail effect. Out of these four proposed core effects, “high entropy” and “sluggish diffusion” effects were most debated in the literature as these core effects directly affect the thermodynamic and kinetic understanding of HEAs. The initial work on HEAs by several researchers utilized these effects to indirectly support the experimentally observed “unique” properties, without independent investigation of these core effects. The presumed implications of these core effects resulted in justification or generalization of properties to all HEAs, e.g., all HEAs should exhibit high temperature stability based on high entropy effect, high temperature strength owing to limited grain growth, good diffusion barrier application due to sluggish diffusion kinetics, etc. However, many recent studies have challenged these core effects, and suggested that not all HEAs were observed to exhibit these core effects.

Abstract: Tracer diffusion is one of most reliable techniques for providing basic kinetic data in solids. In the present review, selected direct methods, in particular the radiotracer measurements as a superior technique due to its high sensitivity, Secondary-Ion-Mass-Spectroscopy (SIMS) profiling, X-Ray Diffraction measurements and Rutherford Backscattering Spectrometry are presented and discussed. Special attention is put on the radiotracer technique describing the currently used sectioning techniques in detail with a focus on the experimental applications and complications. The relevant experimental results are exemplary shown. Furthermore, the most recent developments and advances related to the combined tracer/inter-diffusion measurements are highlighted. It is shown that this approach offers possibilities to provide the concentration-dependent tracer diffusion coefficients of the constituting elements in multi-component alloys in high-throughput experiments. Possibilities of estimating the tracer diffusion coefficients following different types of diffusion couple methods in binary and multicomponent systems are briefly introduced. Finally, specificity of SIMS analysis of diffusion in fine-grained materials are carefully analyzed. If applicable, a direct comparison of the results obtained by different techniques is given.

Abstract: Heusler NiMnGa alloys are often categorized as ferromagnetic shape memory alloys or magnetocaloric materials, which are important for both practical applications and fundamental research. The NiMnGa alloys undergo a series of diffusion and diffusionless transformation from high temperature to low temperature. Among these transformation, martensitic transformation from austenitic phase to martensitic phase is critical in determining the properties of the alloys. Although martensitic transformation is considered diffusionless, diffusion also has important applications in the research of NiMnGa alloysDiffusion couples along with equilibrium alloys have been used to determine the ternary phase diagrams in NiMnGa alloys. Phase diagrams are important in selecting NiMnGa alloys, in particular two-phase NiMnGa alloys for practical applications. Furthermore, the diffusion couples effectively assist in the determination of compositions that exhibit martensitic transformation temperature near room temperature. Diffusion coefficients have been assessed for NiMnGa alloys. Tracer diffusivity of Ni, Mn and Ga was reported in a wide temperature range and followed Arrhenius behavior. Two different activation energies were obtained, corresponding to B2 and L2₁ crystal structure, respectively. Interdiffusion coefficients for NiMnGa alloys with B2 crystal structure are measured, which showed that Ni diffuses the fastest, followed by Mn then Ga. The diffusion coefficients provide useful information for fabricating NiMnGa alloys through diffusional process.A combinatorial approach involving diffusion couples and advance characterization has been developed to investigate the mechanical properties, microstructure and crystallography of NiMnGa alloys rapidly and systematically over a large compositional range. The composition-dependent modulus and hardness for NiMnGa alloys was extracted from the diffusion couples with the help of nanoindentation. Martensitic phases with non-modulated and various modulated crystal structures, and austenitic phase were identified in the interdiffusion zones by transmission electron microscopy. The results demonstrate the capability of using diffusion couples to speed up the discovery of new NiMnGa alloys or other similar alloys showing martensitic transformation.

Abstract: In this paper some recent progress in the area of Monte Carlo simulation of diffusion via the interstitialcy mechanism in a randomly ordered binary alloy is reviewed. Topics discussed include the calculation of tracer correlation factors fA and fB as a function of composition and jump frequency ratio wA/wB and interstitialcy correlation factors fI; which play a crucial role in the interpretation of ion-conductivity data. The percolation behavior of fI when wA ≪ wB is analysed in detail and limits of the tracer diffusivity ratios bD A/bD B for alloy compositions below the percolation threshold are presented. Allowance for non-collinear jumps (partly) replacing concurrent collinear site exchanges leads to a reduction of diffusion correlation effects. This goes along with a shift of the diffusion percolation threshold to lower concentrations of the (more) mobile component B. Even stronger changes of mass and charge transport compared to an exclusively collinear interstitialcy scheme are observed for additional contributions of direct interstitial jumps. It is remarkable that for both extensions of interstitialcy-mediated diffusion the Haven ratio appears to be greater than unity in certain composition ranges poor in B.

Abstract: High entropy alloys (HEAs) are considered as a novel class of materials with a large number of components (five and more) available in equiatomic or nearly equatomic proportions. One of the characteristic properties of HEAs was believed to be so-called 'sluggish' diffusion that should be crucial for intended high-temperature technological applications. The faith on this myth instead of rigorous experimental analysis played such a dominant role that the first set of data on interdi usion, in fact based on an improper analysis, were cited in hundreds of articles to state the presence of sluggishness of di usion rates in high entropy alloys.

Abstract: Recently developed method of atomistic modelling (SKMF) is applied to order-disorder transitions in FCC alloys and to tracer diffusion in the ordered L1₂ structure. Results correlate with Kinetic Mote-Carlo modelling. Difference of diffusion activation energies of two species is found. Activation energy of ordering is close to one of minority component diffusion.

Abstract: Aluminium is a key element in geological and man-made materials which has only one stable isotope and no radionuclides with half-life times suitable for standard experimental diffusion studies. Here we report on our method using the radioisotope ²⁶Al (t_1/2 = 7.4×10⁵ a) as a quasi-stable tracer for aluminium in combination with SIMS depth profiling. First, our data for the aluminium bulk diffusivity in a-alumina are discussed jointly with published oxygen bulk diffusion coefficients. They clearly show that the relation D_Al>>D₀ is valid in the temperature range 1200 °C ≤ T ≤ 1800 °C. In an analogous manner, the two rare stable isotopes ¹⁸O and ³⁰Si are used together with ²⁶Al in diffusion studies of generic examples of materials which either consist of aluminium, silicon and oxygen only, or where these three elements are key constituents of the structure. For the crystalline aluminium silicate mullite our diffusivity data for aluminium, oxygen and silicon are used to explain the kinetics of the solid state formation reaction of mullite and the segregation kinetics of alumina from mullite. Finally, the diffusivities of oxygen and aluminium in model aluminosilicate glasses are presented as a function of temperature for different Al³⁺/Na⁺ ratios. For the aluminium silicate mullite and for the aluminosilicate glasses the relation D₀>D_Al>D_Si is valid regardless of the exact composition. For the glass system the activation enthalpies of aluminium and oxygen diffusion decrease with decreasing Al³⁺/Na⁺ ratio.

Abstract: The self-or tracer diffusivity of one component in a binary alloy is often required when there is knowledge of the other component’s self-or tracer diffusivity and the interdiffusivity (and the thermodynamic factor). In the present paper, this problem is addressed for the random alloy model by applying three possible approximations having different levels of accuracy: Darken (low level of accuracy), Manning (medium level of accuracy) and Moleko, Allnatt and Allnatt (MAA) (high level of accuracy). There are unexpectedly large differences between the results of these approximations that sometimes are reflected in the high sensitivity of the vacancy-wind factor to the level of approximation. Generally, for the application of Manning and the MAA approximations, it is found that there is a difference in the number of self-diffusivity roots depending on whether the tracer diffusivity is available for the faster diffuser or for the slower diffuser and depending on how close the composition is to the forbidden (according to Manning’s description) region. Provided that the interdiffusion coefficient (divided by the thermodynamic factor) is greater than the available self-diffusion coefficient multiplied by its complementary composition, the application of the Darken approximation always results in one self-diffusivity root.

Abstract: Molybdenum disilicide (MoSi₂) is an interesting material for high-temperature applications. It has a high melting temperature, good thermal and electrical conductivity and an excellent oxidation resistance. For many years the primary use of MoSi₂ has been in heating elements, which can be used for temperatures up to 1800°C. Since the 1990s the potential of MoSi₂ as a high-temperature structural material has been recognized as well. Its brittleness at lower temperatures and a poor creep resistance above 1200°C have hindered its use as in load-bearing parts. These disadvantages may be offset at least partly by using it together with a second material in a composite or an alloy. Projected applications of MoSi₂-based materials include, e.g. stationary hot section components in gas turbine engines and glow plugs in diesel engines. For future research and development directions of MoSi₂-based composites diffusion is a crucial property because creep is closely connected with diffusion. This paper is devoted to the basic diffusion and defect properties of MoSi₂. Data of Si and Mo as well as Ge diffusion from the Münster laboratory for both principal directions are briefly summarized. For all three kinds of atoms diffusion perpendicular to the tetragonal axis is faster than parallel to it. The diffusivities of Mo in both directions are many orders of magnitude slower than those of Si and Ge. The huge asymmetry between Mo and Si (or Ge) diffusion suggests that atomic motion of each constituent is restricted to its own sublattice. Positron annihilation studies on MoSi₂ from the Stuttgart laboratory are reviewed as well. They show that formation of thermal vacancies occurs primarily on the Si sublattice but cannot exclude vacancy formation on the Mo sublattice at higher temperatures. Correlation factors for Si and Mo diffusion via sublattice vacancies in the respective sublattices of MoSi₂ have been calculated recently mainly by Monte Carlo simulation techniques and are also briefly described. Diffusion, in particular self-diffusion, is discussed in connection with literature data on high-temperature creep, which is diffusion-controlled. Grain-size effects of creep have been reported and can be attributed to Nabarro-Herring and Coble creep. Power-law creep is attributed to diffusion-controlled dislocation creep. Some details are, however, not completely understood, presumably due to a lack of theoretical concepts for creep in uniaxial, stochiometric compounds and due to missing information on grain-boundary diffusion.

Abstract: To determine the effect of the annealing atmosphere on oxygen diffusion through Ba_0.95La_0.05FeO_3-d pellets, ¹⁸O₂ tracer diffusion and high-resolution secondary ion mapping were performed. When annealing in air, the ¹⁸O concentration around the surface up to a depth of 40 µm was almost constant. On the other hand, when annealing in vacuum, the ¹⁸O concentration obviously decreased. High-resolution secondary ion mapping indicated that the ¹⁸O concentration around the grain boundary was reduced. These results suggested that the grain boundary of BLF annealed in vacuum prevents oxygen diffusion.