Defect and Diffusion Forum Vol. 333

Abstract: Starting from fundamental aspects of thermal vacancies and solid-state self-and solute diffusion, this paper reviews procedures for tracer-and interdiffusion studies and of the major techniques for vacancy studies by dilatometry and positron annihilation in metals. Equilibrium vacancy and diffusion studies performed on pure iron and aluminium are mentioned at first. We also comment some peculiarities of solute diffusion in aluminium. Positron annihilation and differential dilatometry studies for Fe-Al alloys with various compositions are summarized and new experimental studies by the authors are reported for vacancy migration in Fe₆₁Al₃₉. All these studies indicate a relatively high fraction of thermal vacancies with relatively low mobility in this type of iron-aluminides as compared to pure metals. Tracer diffusion of iron and of several substitutional solutes such as Co, Ni, Cr, Mn, Zn, and In in Fe-Al from the Münster laboratory are summarized. The diffusion studies of Fe-Al cover various alloy composition between Fe₃Al and FeAl and several structures such as A2, B2 and D0₃. Interdiffusion coefficients obtained from diffusion couples between Fe-Al alloys are discussed together with Fe tracer diffusion data. The Darken-Manning equation is used to deduce Al diffusivities therefrom. The latter are hardly accessible to radiotracer experiments due to a lack of a suitable Al tracer. Diffusion of Al is slightly faster than diffusion of Fe indicating diffusion mechanisms with coupled jumps of Fe and Al atoms.

Abstract: The transformation of the solidification microstructure in AISI M2 grade high-speed steel inoculated with powder addition of WC have been studied focusing on the effect of austenitisation. In order to investigate the kinetics of both the microstructure and phase transformations in eutectic carbides, primarily M₂C carbide decomposition, upon heat treatments with respect to diffusion processes, electron scanning microscopy, XRD analysis and EDS have been used.

Abstract: Li diffusion is investigated in Li₂O-deficient, (110) oriented LiNbO₃ single crystals in the temperature range between 523 and 673 K by secondary ion mass spectrometry. A thin layer of ion-beam sputtered isotope enriched ⁶LiNbO₃ was used as a tracer source, which allows one to study pure isotope interdiffusion. The diffusivities coincide with those of (001) oriented single crystals and follow the Arrhenius law with an activation enthalpy of 1.33 eV. The results prove the existence of a three-dimensional diffusion mechanism.

Abstract: Hydrogen absorption and diffusivity in high quality ZnO crystals were investigated in this work by X-ray diffraction combined with slow positron implantation spectroscopy and electrical resistometry. ZnO crystals were covered by a thin Pd over-layer and electrochemically charged with hydrogen. It was found that absorbed hydrogen causes plastic deformation in a sub-surface region. The depth profile of hydrogen concentration introduced into the crystal was determined by nuclear reaction analysis. Enhanced hydrogen concentration was found in the sub-surface region due to excess hydrogen atoms trapped at defects introduced by plastic deformation. Hydrogen diffusion in ZnO crystals with various orientations was studied by in-situ electrical resistometry. It was found that hydrogen diffusion in the c-direction is faster than hydrogen diffusion in the a-direction most probably due to open channels existing in the wurtzite structure along the c-axis.

Abstract: Early stages of precipitation were investigated in solution treated binary Mg-Tb and Mg-Gd alloys. The supersaturated solid solution of Gd (or Tb) in Mg was formed by fast quenching of the alloys from solution treatment temperature. Decomposition of the supersaturated solid solution and precipitation effects were investigated by positron lifetime spectroscopy combined with microhardness testing. During solution treatment at elevated temperature some thermal vacancies form pairs with solute atoms. In quenched samples free vacancies are quickly annealed out, while more stable vacancies bound to solute atoms remain in the sample and enhance the diffusivity of solutes. The hardness of solution treated Mg-Tb and Mg-Gd alloys aged at ambient temperature rises due to formation of small clusters of Tb and Gd atoms. Isochronal annealing of Mg-Tb and Mg-Gd alloys leads to precipitation of coherent β phase, semicoherent β phase and incoherent β phase particles. It was found that natural aging of Mg-Tb alloy at ambient temperature has beneficial effect on subsequent hardening by β phase particles formed during annealing.

Abstract: The concept for alloy design of Nbbased hydrogen permeable membrane is applied to NbWMo ternary system. The alloying effects of tungsten and molybdenum on the solubility of hydrogen, the resistance to hydrogen embrittlement, the hydrogen permeability and diffusivity are investigated in a fundamental manner. It is found that the addition of tungsten and molybdenum into niobium decreases the hydrogen solubility. As a result, the resistance to hydrogen embrittlement improves and higher hydrogen pressures can be applied to the NbWMo alloy membrane. It is shown that the designed Nb5mol%W5mol%Mo alloy membrane with single solid solution phase exhibits excellent hydrogen permeability together with strong resistance to hydrogen embrittlement. In addition, it is found that the alloying of tungsten and molybdenum with niobium enhances the hydrogen diffusivity. In fact, the activation energy for hydrogen diffusion decreases in the order, pure Nb > Nb5mol%W > Nb5mol%W5mol%Mo.

Abstract: A method has been developed for calculating diffusion profiles in ternary systems by using effective interdiffusion coefficients of components and Boltzmanns solution for diffusion equation with variable diffusion coefficient. Using this method the concentration profiles for several diffusion couples in the systems Fe-Co-Ni and Cu-Fe-Ni are calculated as examples and some peculiarities of these calculations are discussed, particularly, how to solve some possible difficulties, which may sometimes arise at calculation procedures. It is shown that having the data on effective interdiffusion coefficients and their concentration dependence for at least two components in a ternary diffusion couple, the concentration profiles for all three components can be calculated with good accuracy.

Abstract: In this paper we couple the bi-velocity with the phase field method. It deals with: (1) the different mobility of the components in the two-phase zone; (2) nonzero steps of molar volumes for each component from phase to phase and (3) the composition dependent interdiffusion coefficients. The method allows to determine the average stress field during the diffusion process, the kinetics of the reactions and estimate the entropy production. The paper presents the numerical computations of diffusion in th eNiAlCr system. The results can serve as a basis in designing gradient coatings of extended life time.

Abstract: The influence of yttrium on the corrosion resistance of four valve steels (X33CrNiMn23-8, X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20-8) in combustion gases of fuel oil containing 10 wt. % of biocomponents has been studied at 1173 K under thermal shock conditions. These experiments have been carried out by rapid heating of the studied materials up to 1173 K and after two hours cooling down to room temperature. These thermal shocks have been repeated hundreds of times, determining after every shock the mass changes of the corroded samples. It has been found that small amount of yttrium deposited on the steel surface using electrochemical treatment considerably improves the scale adherence and consequently corrosion resistance in the case of X33CrNiMn23-8 steel, containing the highest chromium concentration.