Band 88, Heft 9

The present paper compares the dislocation densities as determined in a Co-based superalloy (CoNi22Cr22W14) after creep and tensile deformation by Transmission Electron Microscopy (TEM) and X-ray profile analysis (XRD). After creep tests the dislocation densities obtained by both methods are in good agreement, which is the result of a nearly homogeneous dislocation distribution. The relationship between the dislocation density and the flow stress meets the Taylor equation. After tensile deformation the dislocation densities determined by TEM and XRD differ systematically from each other, but in both cases also a Taylor relationship can be obtained. The constant α of the dislocation interaction derived by TEM is much larger than in the creep tests and also than that of the XRD, which agrees well with the creep data. The difference between the TEM and the XRD results is the consequence of the dislocation cell structure much more developed in the tensile specimens, which leads to an underestimation of the dislocation density in TEM because of overweigthing the cell interior. By fitting the Fourier coefficients of the X-ray diffraction line shapes with a bimodal distribution of the defect content (composite model), dislocation densities of the cell interior can be estimated that correspond well to the TEM data.

The constitution of the ternary Al–AlCo–AINi system was investigated in the region from 55 to 70 at.% Al. Differential thermoanalysis, magnetothermal analysis, optical microscopy and transmission electron microscopy were applied. A liquidus projection surface of the stable diagram has been determined from heating experiments. The Scheil reaction scheme is reported for the investigated phase reactions. Isopleths with 57.5, 65 and 70 at.% Al and 19 at.% Ni are reported. These are based on results of more than 30 samples of different compositions and on data from other isopleths.

The phase relationships of the BaO–CaO–CuO ternary system at 850 °C in air was investigated by X-ray powder diffraction, scanning electron microscopy and electron probe microanalysis techniques. The samples were prepared by solid state reaction method. The existence of three binary oxide compounds, Ba 2 CuO 3 , BaCuO 2 and Ca 2 CuO 3 was confirmed and no other intermediate phases were found in this system at 850 °C. The isothermal section consists of six single-phase regions, nine two-phase regions and four three-phase regions. No carbonate was detected. However, barium dioxide, BaO 2 , was found in BaO-rich corner of this system. At 850 °C, Ba 2 CuO 3 keeps its high temperature structure, being tetragonal with a = 1.2978 nm and c = 0.3992 nm. BaCuO 2 has a cubic crystal structure with a = 1.82855 nm, and Ca 2 CuO 3 is orthorhombic with a = 0.32573 nm, b = 0.37769 nm and c = 1.2234 nm.

Enthalpies of mixing of ternary liquid Cu–Mg–Y alloys were determined by a calorimetric method along five different isopleths. For one of these isopleths ( x Cu / x Y = 0.5), magnesium vapour pressures were measured by means of an isopiestic method, and the corresponding values of the magnesium activity at 1173 K were derived as a function of composition. The integral Gibbs energy of mixing was computed by a Gibbs-Duhem integration. An association model was applied to calculate the thermodynamic functions of mixing for ternary liquid alloys using the model parameters of the three limiting binary systems.

In this paper, we compare the experimental enthalpy of mixing of liquid quaternary (In, Pb, Sn, Zn) alloys with the ones calculated by Redlich-Kister fit based only on binary and ternary terms.

Al–Al 3 Ti functionally graded materials (FGMs) were manufactured by the centrifugal method with a commercial ingot of Al-5 mass% Ti master alloy. The alloy was melted at a liquid/solid coexisting temperature, at which Al 3 Ti remains as a solid, and then it was cast into a thick-walled ring. It was found that the Al–Al 3 Ti functionally graded material can be successfully fabricated by the centrifugal method. It was also found that the volume fraction of the Al 3 Ti can be increased by repetition of the centrifugal method. Since the shape of Al 3 Ti particles in a commercial alloy ingot is that of a platelet, the Al 3 Ti particles are arranged with their platelet planes nearly perpendicular to the radial direction. The orientation effects become stronger when the G number becomes larger. Although the final centrifugal casting was conducted under a very large centrifugal force for the specimen cast three times, the orientation effects were weaker than those in the specimen cast one time. From these observations, it is concluded that the origin of orientation of Al 3 Ti platelets can be attributed to the angular velocity gradient of the melt along the radial direction produced by the difference in the viscosity.

The phase equilibria between ferrite and austenite as well as between austenite and the hexagonal ∊ nitride “Cr 2 N“ were calculated for different compositions of Fe–Cr–Mn–N alloys. Corresponding phase diagrams show, that single-phase austenite can be obtained by heat treatment only above about 1200 and below about 1600 K. The difference ∆ G m of the Gibbs energies of ferrite and austenite causes martensitic transformation, when the critical driving force ΔGMs $ \Delta G_{M_s } $ is surpassed. Applying recently published thermodynamic data the corresponding martensite start temperatures of alloys containing different mass contents of chromium and manganese were estimated. They were drawn into [%N]– T -phase diagrams. From the diagrams the conditions can be red where martensite can be formed by quenching single-phase austenite.

Precipitation processes in as-cast, homogenized and deformed samples of an 8011 commercial alloy have been studied by thermoelectric power under non-isothermal conditions, verifying the sensitivity of the thermoelectric power to the kinetics of the iron atoms in solid solution. The thermoelectric power versus temperature diagrams show that rolling favors the phase precipitation and that the kinetic behavior of the dominant alloying additions in the 8011 alloy is similar to the ones observed in other Al–Fe–Si alloys. The characteristic temperatures for each studied microstructural condition are reported.

The influence of solidification conditions on the microstructure and phase composition of a hypereutectic Fe-4 %C-12 %Cr-13 %V alloy has been studied. The analysed alloy consists of the following structural components after equilibrium solidification: primary MC-carbides, two types of eutectic mixtures and metallic matrix. A wide spectrum of metastable structures was found in rapidly solidified powder particles as result of cooling rate increase during solidification. Some important changes in phase composition were also identified in the material as result of different solidification conditions.

In the present paper, attention has been focused to understand the influence of the characteristics of counterface materials on the wear behaviour of a zinc-based alloy. The selected counterface materials for the present study were 304 stainless steel and EN8 and EN24 steels. Wear tests (pin-on-disc) were conducted over a range of applied pressure and speed. The investigation clearly indicates that the characteristics of the counterface materials greatly control the wear behaviour of the (zinc-based) alloy in addition to the sliding speed and pressure. The results obtained have been further substantiated with the characteristics of wear surfaces, subsurface regions and the debris particles generated during the tests.

The β' phase obtained by quenching Ag-Zn alloys having composition around 50 at.% Zn were transformed to the ζ phase by mechanical deformation. Aging of the partially ordered ζ structure caused the reversion to the ordered body-centered cubic β' structure. The rate of the reversion which has been measured for alloys having compositions from 48.0 to 50.9 at.% Zn was found to be dependent on both temperature and composition. The rate of the transformation increased exponentially with increasing temperature. It was observed that the reversion rate was the greatest for the AgZn compound and decreased for the other alloys for a definite temperature. The activation energy of the process was found to lie between 58.8 and 99.4 kJ mol –1 for the alloys studied.

Die Evolution von Feldern der Lokaldeformation ist in Ab- hängigkeit von der Orientierung der Dehnungsachse in einkristallinem Stahl mit hohem Stickstoffgehalt experimentell untersucht worden. Es wird gezeigt, daß die Verteilungen der Distorsionstensorkomponenten bei Deformation in [111]-Orientierung eine Gesamtheit von equidistanten Zonen der Deformationslokalisierung darstellen. Sie unterlie- gen einer Verrückung mit konstanter Geschwindigkeit. Bei der Deformation der Proben mit [001]-Orientierung bilden die Verteilungen ebenfalls equidistante, aber stationäre Zonen. Die Interpretation der Ergebnisse erfolgt im Rahmen der Vorstellungen von der Bildung dissipativer Strukturen in aktiven Medien.