Solid State Phenomena Vols. 203-204

Abstract: Carbonaceous – nanopalladium (C-nPd) films on insulating substrates, obtained by PVD (Physical Vapor Deposition) followed by the annealing method, are used for hydrogen sensors. In this paper we present the results of XRD, SEM and electrical measurements of C-nPd films grown by PVD and then either annealed in an inert atmosphere or modified by CVD (Chemical Vapor Deposition). The structure of palladium grains were measured in a H₂/N₂ gas mixture atmosphere containing H₂ in the concentration range 0-4%. Our measurements showed that C-nPd films prepared by PVD and annealing method were flat and they consisted of sphere-like palladium nano-particles, uniformly distributed in the low-ordered carbon matrix. Effect of hydrogen partial pressure on the crystal structure was studied in situ in GIXD measurements, using a specially designed measuring holder cell. It was observed that at low partial pressure of hydrogen, palladium fcc metal particles transformed into solid solution Pd(H) (α-phase), which occurs in the fcc structure as well. Lattice constant of that solid solution increases with increasing hydrogen partial pressure in the gas atmosphere. After exceeding the critical value of the partial pressure of hydrogen, palladium transforms into the structure of palladium hydride PdH_x (β-phase), which has significantly larger lattice constant. This critical value of hydrogen partial pressure depends on the initial structure of C-nPd film, and consequently on the parameters of PVD process and annealing. This value usually corresponds to a few percent hydrogen content in gas mixture.

Abstract: Modified unit cell of Fe₃AlC (K-phase) that can act as a substrate for a diamond growth was proposed. Spin-polarized calculations were performed to compare electronic structure of carbon atoms in K-phase substrate, diamond and graphite. Standard enthalpies of formation of different K-phase unit cells were calculated and compared with formation enthalpy of modified unit cell of K-phase.

Abstract: Plastic deformation of solid crystals is a complex process, mostly heterogeneous, due to the simultaneous effect of several deformation mechanisms. A dominating deformation mechanism depends on the properties of the material and external coefficients, viz. temperature, stress and strain rate. The applied Bridgman method permitted to obtain single crystal of the CuZn30 alloy adequate for plastic deformation investigations. Single crystal are characterized by selected crystallographic orientations from various areas of the basic triangle. In order to determine the influence of the crystallographic orientation on the Portevin-Le Chatelier effect selected single crystals were compressed at a temperature of 300°C at a strain rate of 10-3 s-1. Experiments confirmed the effect of the crystallographic orientation axis of CuZn30 single crystals on the observed differences in the intensity of stress oscillation on stress-strain curves.

Abstract: In the literature it is proven that thermal vacancies have a great influence on the mechanism of hardening of Fe-Al alloys. Moreover, in these alloys, we observed a long-range ordering, which can significantly affect the mechanical and physical properties and their stability. In this paper, influence of low-temperature annealing on elimination of excess vacancies was investigated. TEM observation of annealed specimens for the alloys with 28 and 38 at.% aluminum have helped elucidate the phenomena responsible for vacancies elimination due to the occurrence of particular interactions between point and linear structure defects. It was shown that the aluminum content influences significantly changes in defects structure. The alloy with 28 at.% aluminum has mainly superdislocations in the structure, while in alloy with 38 at.% aluminum, mainly unit dislocations and high-energy dislocation configurations, like dislocation loops, dislocation dipoles, and dislocation jogs, were observed. The results suggest that different defect types may control the diffusion process during low-temperature annealing and that it is affected by alloy composition.

Abstract: The paper presents the results of microscopic examinations of the growth of primary silicon crystals in hypereutectic A390.0 (AlSi17Cu5Mg) silumin. A diagram of the growth of primary silicon crystals in the melt without overheating and superheated to a temperature of 920°C has been presented. From the experimental results obtained on crystal morphologies of primary silicon, different sequences of crystal growth of the primary silicon can be expected. In the silumin without overheating, five-pointed star-shaped particles form, after superheating, the crystals of primary silicon take more compact shapes. This structure provides better mechanical properties and thus increases the area of application of alloys with hypereutectic silicon content.

Abstract: Hydrogen-absorbing compounds of AB₅-type are widely studied in view of their application not only as anode material in fuel cells but also in high-energy Ni-MH cells. Within this work nanocrystalline MmNi_3.55Al_0.3Mn_0.4Co_0.75 compounds, where Mm is La–rich mixture of rare earth elements, synthesized by High-Energy Ball Milling (HEBM) of arc-melted alloy and AB₅-type alloy/carbon composites prepared by CVD method were studied. As the compounds subjected to high temperature during preparation their thermal stability has been established after annealing at various temperatures. X-ray phase analysis has unambiguously proven that over 400°C single-phase MmNi_3.55Al_0.3Mn_0.4Co_0.75 starts to decompose into a multi-phase mixture with Ni-based solid solution as a main phase. Similar behaviour has been observed for composite material. Moreover, carbon distribution on surface of alloy particles is inhomogeneous as seen in microanalysis. Nevertheless, a substantial increase of hydrogen capacity has been observed for composite material in comparison with pure intermetallic compound, when used as anode material in direct borohydride fuel cell (DBFC).

Abstract: Development of spatial microstructure imaging techniques (e.g. of automated serial sectioning) has made it possible to collect five macroscopic grain boundary parameters for sets of boundaries large enough for carrying out statistical studies. As a point of reference for future analysis of experimentally measured boundary data, various aspects of estimating the frequencies of occurrence of geometrically characteristic boundaries among random grain boundaries for the cases of cubic O_h, hexagonal D_6h and tetragonal D_4h point groups are discussed. Example frequencies, in particular for symmetric and improperly quasi-symmetric boundaries, are presented. Two approaches for verification whether a given boundary has a tilt or twist character are confronted, i.e. a method based on a distance function defined in the boundary parameter space and the widely known decomposition of a boundary into its tilt and twist components. The frequencies for tilt and twist boundaries calculated using both methods are compared.

Abstract: The specific heat capacity was calculated by DSC and the kinetics of oxidation was determined for alloys from the NiAl, FeAl and TiAl phase equilibrium system. The Ti48Al alloy contained an addition of 2 at.% of chromium and niobium. The highest value of heat capacity had the Ti-Al and Fe-Al alloys. At temperatures above 673K, the Ti48Al alloy showed an anomaly, that is, a lower value of the heat capacity. The change in heat capacity was related with an exothermic reaction. The oxidation of alloys was running according to either parabolic or cubic law. Alloys characterised by high thermal capacity showed a higher gain in weight. The slowest to oxidise was the Ni₃Al intermetallic phase.

Abstract: The paper presents the study results concerning microstructural characteristics of the Ni(Co)CrAlY type powders designed for the bondcoat spraying with the plasma method for TBC layers obtained by the atmospheric plasma spraying (APS). Three types of powders with the following chemical composition Ni-23Co-17Cr-12.5Al-0.45Y, Co-32Ni-21Cr-8Al-0.5Y and Ni-22Cr-10Al-Y were subjected to analysis. The assessment of surface morphology and inner structure of powders (SEM) was made. Chemical composition (EDX, OES-ICP) of the powders was determined. Phase composition by using XRD and EBSD methods was also analysed.

Abstract: The effects of processing conditions on the structure of polypropylene fibrillated fibres were studied using a combination of wide- and small-angle X-ray scattering methods. In particular the impact of selected stages of processing on the crystalline and lamellar structure of PP were analyzed. It was stated that crystalline phase is built from α crystals. The crystallinity index as well as the Herman orientation factor of the crystalline phase is found to have a correlation only with the draw ratio of the PP film. The lamellar structure also changes with the draw ratio.