Volume 93, Issue 4

For the first time, self-diffusion was systematically investigated in well-compacted nanocrystalline (grain size d ≈ 80 – 100 nm) γ-Fe–40 wt.% Ni material in a wide temperature range (600 –1010 K) in all Harrison-type kinetic regimes. Samples were prepared by sintering the nanocrystalline Fe –Ni powder mixture produced by ball milling of the component oxides after reduction in hydrogen atmosphere. The samples revealed a frequently observed bimodal microstructure consisting of nano-scaled grains and micrometer-scaled agglomerates of the nano-grains. Two different types of short-circuit paths were found to control the diffusionflux in such material. Owing to the applied sensitive radiotracer technique Fe diffusion in both types of interface boundaries could be successfully characterized by combining the evaluation of the experimentally determined 59 Fe diffusion profiles with a Monte-Carlo simulation of grain boundary (GB) diffusion. Part I presents the results obtained at elevated temperatures in the type-B and A regimes. Due to the sample preparation process the GB motion during the diffusion anneal was proven to be negligible. For the first time, it was shown that there exists an intermediate stage between the well-known kinetic regimes B and A if Dvt≃d $\sqrt{{{D}_{\rm v}}t}\simeq d$, where D v is the bulk diffusivity and t is the time. The corresponding concentration profiles could be linearized in the coordinates of ln c¯ $\bar{c}$vs. y3/2(c¯ ${{y}^{3/2}}(\bar{c}$is the layer tracer concentration and y is the penetration depth) and the equation to extract the GB diffusion coefficient from these data was derived. The limits of the new AB-type stage were established. It was demonstrated that the processing of the nonconventional experimental GB diffusion profiles in a nanocrystalline material can be done properly but is more sophisticated than in a coarse-grained material.

For the first time, the grain boundary diffusion was investigated in a nanocrystalline material in the whole spectrum of possible kinetic regimes: types-C, B, AB , and A. The microstructure of the nanocrystalline γ-Fe-40 wt.% Ni alloy revealed two different levels of organization: the individual nano-scaled grains build up micrometer-large clusters or agglomerates of the grains. The existence of the two types of internal interfaces in the material – the boundaries between the nano-grains and between the agglomerates of these nanocrystallites (the inter- and intraagglomerate boundaries) – affected notably the C-regime of 59 Fe radio-tracer diffusion and resulted in two-stage diffusion penetration profiles. The careful profile processing according to formal criteria of the given kinetic regimes allowed, for the first time, to elaborate a consistent picture of the diffusion behavior in this nanocrystalline material with a complex bimodal distribution of equilibrium and nonequilibrium internal interfaces in the same material. The absolute values of the measured diffusion coefficients and the Arrhenius parameters of Fe diffusion along the nanocrystalline boundaries in nano-γ-Fe-40 wt.% Ni ( D 0 = 4:2 ⨯ 10 –3 m 2 s –1 , Q = 187 kJ mol –1 ) are very similar to the Fe grain boundary diffusivity in coarse-grained γ-Fe. The activation enthalpy of diffusion along the inter-agglomerate boundaries ( D 0 = 3:4 ⨯ 10 –3 m 2 s –1 , Q = 148 kJ mol –1 ) is notably smaller and the absolute diffusivity is larger by two to three orders of magnitude than the corresponding values of Fe diffusion via the nanoboundaries. The observed features are explained by a quasi-equilibrium structure of the nanocrystalline GBs, related to the grain growth during sample preparation, whereas the residual interagglomerate boundaries remained in nonequilibrium state. The potential contribution of the numerous triple junctions in the nano-material to short-circuit diffusion was investigated in detail and was found to be negligible under the present experimental conditions.

The thermodynamic parameters of the new phase CuTi 3 found in the Cu–Ti system were optimised using experimental data. A non-stoichiometric description of that phase was taken into account by using an appropriate sublattice model.

The Pd–Sc system was critically assessed by means of the CALPHAD technique. The solution phases (liquid and face-centered cubic (Pd)) were modeled with the Redlich-Kister equation. The intermetallic compound PdSc, which has a homogeneity range, was treated as the formula (Pd,Va) Sc by a two-sublattice model with Pd and vacancies, denoted Va, on the first sublattice, whereas Sc on the second one; the others as stoichiometric compounds. A set of self-consistent thermodynamic parameters of the Pd–Sc system was obtained.

Some new melting processes for the ash have been developed to solve the problems on increasing volume of ash generated from municipal waste incinerators. The metal phase formed in this melting process generally consists of Fe –Cu – Si –P–C containing a small amount of other heavy metals, but their phase equilibria and physico-chemical properties are unknown. The present work aimed at determining the thermochemical properties of liquid Fe –Cu –Si alloys, which establish the basic system in this melting process. The heat contents of liquid Fe, Fe –Cu and Fe –Cu –Si alloys have been directly measured with a drop calorimeter at mainly 2073 K in the present work. The observed heat content and the enthalpy of mixing of the alloys were assessed by a thermodynamic model. The input energy which should be supplied to melt the metal phase in the new melting treatment process was also discussed.

We present a model to describe the viscosity of liquid metals with known physical properties. The model involves two adjustable parameters to the experimental data of the viscosity, but can be related to an effective atomic diameter and the coordination number of nearest-neighbor atoms in the liquid metals. The model is applied to eleven cases of mono-atomic liquid metals. The model is capable of describing the overall experimental data of the viscosity in a wide range of temperature, predicting the mean free paths of an atom in accord to the experimental interatomic distances in the liquid metals. An extension of this model to multi-component liquid metals is also suggested.

Ziel der vorliegenden Arbeit war es, den Einfluss einer Last auf die Umwandlungswärme einer NiTi-Gedächtnislegierung zu untersuchen. Zur Durchführung dieser Messung anhand eines herkömmlichen Kalorimeters wurde ein spezieller Probenhalter entworfen, der eine Vordehnung während der kalorimetrischen Messung ermöglicht. Da unter festgehaltener Dehnung der Phasenübergang unter Last stattfindet, ließ sich auf diese Weise der Einfluss der Last auf die Umwandlungswärme untersuchen. Diese Untersuchung wurde auch mathematisch behandelt, und es wurde eine gute qualitative Übereinstimmung mit den Messergebnissen erhalten.

The magnetic properties of as-plated and heat-treated Ni – Fe –P and Ni –Fe –P–B films were investigated as a basis for developing new functional thin films. The amorphous Ni –Fe –P and Ni –Fe –P–B and crystalline Ni –Fe –P–B films were prepared by simply controlling the NaH 2 PO 2 concentration in the bath. The structure of all Ni –Fe –P films, prepared in an NaH 2 PO 2 concentration range of 18– 48 g/l, was amorphous, while the NaH 2 PO 2 concentration for the formation of amorphous Ni –Fe –P–B films ranged from 23 to 38 g/l. The increase of the Fe content is thought to be the reason for the increase in the atomic magnetic moment, and thus results in an increase in the saturation magnetization (Ms). The amorphous Ni –Fe –P films possess superior coercivity (Hc). Similar behavior can also be seen in the case of Ni –Fe –P–B films. The lower squarenesses of as-plated films, compared with those of annealed films, implies that the as-deposited films are inhomogenous. As the films were heated up to 400 °C, the homogeneity was obviously improved, and Ms decreased and Hc increased continuously for all the alloy films.

The paper deals with modelling of the shape and size of a crystal grain in a butt joint of ferritic high-alloy stainless steel made by TIG welding without the addition of filler material. The optimum, maximum, and minimum current intensities and welding speeds were experimentally determined. The welding speed is mathematically described as a function of the welding-current intensity. On the basis of macro specimens taken from three different directions with regard to the welding direction, the grain shape, size and location in the weld with regard to the welding direction were established. Individual characteristic crystal grains are shown in space. The average grain size was determined and described mathematically as a function of the welding parameters.

The paper treats welding of heat-resistant ferritic stainless steels alloyed with approximately 20% Cr and 5% Al. The major part of the paper is dedicated to welding of 20% Cr – 5% Al steel with 3 mm in thickness. Welding was carried out with five different welding processes, i. e., manual metal-arc, MIG, TIG, plasma arc, and laser beam welding processes, using a filler material and using no filler material, respectively. The welded joints obtained were subjected to mechanical tests and the analysis of microstructure in the weld metal and the transition zone. The investigations conducted showed that heat-resistant ferritic stainless 20% Cr – 5% Al steel can be welded with fusion welding processes using a Ni-based filler material.

A three-dimensional anisotropic finite element analysis was performed to calculate the hydrostatic pressure, elastic strain energy density and von Mises stress associated with periodically arranged cubic γ′ precipitates in the γ matrix of Ni-based superalloys. Inhomogeneous and anisotropic elastic media are considered, the effects of different lattice misfit and external loads are investigated. The results show that the asymmetry of the hydrostatic pressure, the strain energy density and von Mises stress could be used to predict the γ′ directional coarsening. The contribution of hydrostatic pressure to the total driving force is smaller than that of von Mises stress and the elastic strain energy. The reason was briefly discussed.

A quantitative method for analysis of aluminium alloys using secondary-ion mass spectroscopy has been tested. The sensitivity factor, S x /Al , for several elements ( x ) was determined using homogenised Al materials. Also other Al alloys were analysed using almost identical instrumental conditions. It was found that since the materials investigated contained much Fe and Mg, they contributed with molecular interferences when measuring Ti and Ni. The results showed that while keeping constant the analytical parameters, it was possible to use sensitivity factors S x /Al with a good accuracy for quantification of the elements which were mainly solved in the matrix. The results indicated that the metal was still slightly inhomogeneous in several elements even after annealing at 550 °C for one week. Another important factor is that the emission spectrometer has a high relative standard deviation for the analysis below 5 wt.ppm. This resulted in variations in the calculated sensitivity factor for elements like Na and Ca determined for different materials. The new method was used for calculating concentration profiles of elements in Al materials as a function of the sputtering depth. Samples of alloy AA5182 were homogenised for one week at 5 different temperatures between 450 and 580 °C. The solubility of K, Na, and B in Al was determined. They were of the order 0.2 wt.ppm Na, 0.3 wt.ppm B and 0.04 wt.ppm K at 550 °C.

The effect of the reinforcing particle size on the mechanical properties of 60 vol.% SiC reinforced aluminium matrix composites produced by the pressure infiltration technique was examined by compression, impact and wear tests. The compression strength and the impact resistance decreased with increasing reinforcing particle size. The composites exhibited different abrasion behaviour depending on the size of abrasive Al 2 O 3 grains. On fine abrasive Al 2 O 3 grains (85 μm), the abrasion resistance increased with increasing reinforcing SiC particle size. The contrary result was obtained on coarse abrasive Al 2 O 3 grains (250 μm). The wear tests conducted on M2 quality tool steel revealed that reinforcing of aluminium with coarse SiC particles has a very beneficial effect on the wear resistance with respect to fine reinforcing SiC particles.