Band 89, Heft 8

Impurities of Al or B exert a strong influence on the absolute thermoelectric power of Pt and Pt–Rh alloys. There is a change to less negative values in pure Pt and to more negative values in alloys of more than about 6 wt.% Rh. Consequently, the thermoelectric voltage of thermocouples with one leg of pure Pt is much more sensitive to Al or B impurities than thermocouples made of two different Pt-Rh alloys. The dependence of the absolute thermoelectric power on the valence electron concentration of the alloys studied is correlated to the derivative of the molar susceptibility vs valence electron concentration, as called for by various models connecting thermoelectric phenomena to the band structure of the materials.

The phase diagram and thermodynamic data of the Co–Dy system were critically assessed by means of the computer programs THERMO-CALC. The Co 5 Dy and Co 17 Dy 2 compounds are treated as one phase denoted δ, and a three-sublattice model Co 15 (Co 2 , Dy)(Co 2 , Dy) 2 based on the crystal structure is used to model the δ phase. The other phases are modeled with the Redlich-Kister formula [liquid, fcc, bcc, hcp] or as stoichiometric compounds [Co 7 Dy 2 , Co 3 Dy, Co 2 Dy, Co 7 Dy 12 , CoDy 3 ]. Good agreement is obtained between the calculation and experimental results.

A basic description of the influence of temperature and pressure on the thermophysical properties of condensed phases is attempted. A thermodynamically rigorous and a generalised thermophysical equation of state is formulated. This equation serves to relate in an explicit manner the basic state variables namely, pressure, volume and temperature with important thermophysical parameters such as specific heat, isothermal pressure derivatives of enthalpy, thermal expansion, bulk modulus etc. The versatility of this generalised thermophysical equation of state is demonstrated by deducing from it various existing and new approximate relations regarding the pressure and temperature dependence of elastic and thermodynamic properties. This general formulation will be useful in the interpolation, extrapolation and the assessment of the internal consistency of an existing thermodynamic and thermophysical data base. The thermophysical properties of aluminium are discussed as an illustrative application of the relations developed in this paper.

The central part of the Mg–Al phase diagram was experimentally re-investigated. Between the γ and β phases only one additional intermediate phase could be verified. The other phases reported in literature may be explained by a commensurate modulation of the γ phase (γ') found recently by electron diffraction. The solubility limit of γ' near the melting temperature was investigated by wavelength dispersive X-ray spectroscopy. Thermodynamic descriptions for these phases are proposed.

In the Cu–Zr system, especially between 0.25 < x (Zr) < 1, experimental liquidus points are missing and many doubts arise concerning the invariant points. In 1986, Kneller et al. [1] pointed out the existence of three phases and two eutectoid reactions which had not yet been reported. This work has been questioned by other authors [2,3]. Differential Thermal Analysis (DTA) experiments have been done in almost the whole range of compositions of the Cu–Zr system, especially where the work of Kneller et al. [1] is ambiguous; results are compared with the previous ones.

Orientation imaging microscopy (OIM) was used to study hot isostatically pressed (HIP) 99.99 % α-Al 2 O 3 . A sample preparation procedure for nonconducting ceramics and a preliminary texture analysis are given. The sample preparation procedure was optimized to produce a thin conductive coating, a smooth well polished and at the same time nearly strain free surface. In these alumina samples, the grain area ranged from 350 to 4000 μm 2 . Several grains exhibited a [0001] orientation with misorientation angles between two adjacent grains near 60°. From contour inverse pole figures, a slight texture, two times random, was determined along the sample “rolling direction” [100] which corresponded to the extrusion direction of the green body.

Cu-6wt.%Ni-3wt.%Al was solutionised at 1173 K, cold rolled to 50 % and aged at 773 K. Transmission electron microscopy indicates that ageing of this material results in an initial recovery of the deformed matrix followed by precipitation of β-NiAl and γ′ -Ni 3 Al and recrystallization. The γ′ precipitates show a uniform distribution everywhere except zones near grain boundaries and shear bands which serve as preferential nucleation sites for β-NiAl precipitates. The recrystallization fronts were found to cause dissolution of γ′ precipitates but during further ageing a secondary precipitation of γ′ phase in recrystallized areas was observed. The highest strength, i. e. σ m = 590 MPa, σ 0.2 = 520 MPa, was achieved after 6 · 10 3 s of ageing.

The ferritic heat-treated SG-iron showed a decrease in strength by an average of 15 %, relative to the respective as-cast ingots. Meanwhile, an average increase in ductility of 30 % for sand mould ingots and of 240 to 490 % for the metal mould ingots was observed. Additionally, toughness improved for all investigated ingots due to the heat treatment (for metal mould ingots it showed an improvement range of 155 to 320 %). The, comparatively, drastic increase in ductility and toughness of the heat-treated ingots is discussed. The slip traces observed on the as-cast sand mould and ferritic heat-treated ingots were dominantly localised in the neighbourhood of graphite nodules and among them. A butterfly-like pattern of slip covered neighbouring ferrite grains revealing their grain boundaries. Differences in plastic deformation between ferrite and pearlite revealed the phase boundaries.

(γ + β)-based TiAl alloys containing V and Nb have been selected for the development of a useful multiphase intermetallic microstructure. The evolution of microstructure was studied by optical microscopy, electron-probe micro-analysis (EPMA), scanning electron microscopy (SEM), transmission electron microscopy (TEM). It is indicated that a finer microstructure can be obtained in the TiAl based alloys with a small amount of β (bcc) phase by an appropriate heat treatment. The mechanical properties such as bending strength, plastic strain, creep resistance as well as oxidation resistance were also investigated on the basis of bending tests, compressive creep tests and thermogravimetric analysis (TGA). The results show that the room temperature strength of alloys with this kind of microstructure, particularly those containing 13 to 15 at.% V, can be increased considerably. Also the ductility is obviously improved. However, the introduction of β phase resulted in a poorer creep and oxidation resistance.

This paper investigates the possibility of in-situ formation of TiB 2 ceramic particulates via the process of mechanical alloying of elemental Ti and B powders in a system diluted with Al. Elemental Al powder was mechanically milled together with Ti and B powders in a high energy planetary ball mill. 30 stainless steel balls with diameter of 15 mm were employed. The ball to powder weight ratio was 20:1. X-ray spectra revealed the evolution of structural changes of the alloyed powder particles. No evidence of formation of TiB 2 was manifested when the milling duration was shorter than 30 h. A TiB 2 -like structure could be detected only after 40 hour of milling. Because of broadening and weakening of the X-ray diffraction (XRD) peaks corresponding to Al and TiB 2 structures as a result of refinement of crystalline size and the presence of large structural distortion, only very weak and vague TiB 2 XRD peaks could be detected. Clear evidence of formation of TiB 2 was, however, observed when the mechanically alloyed powder was annealed. Therefore, the results of the present investigation clearly indicate that formation of in-situ TiB 2 particulates within Al is possible although a longer milling duration is required. Mechanical testing shows that both the 0.2 % yield and the ultimate tensile stresses are increased with MA duration and sintering duration.

The effect exercised on the mechanical properties of Al rods by the forming methods comprising the processes of drawing and rolling and the following torsion at ambient temperature was investigated. It has been found that, unlike in the case of Cu and CuZn30, in particular the drawing of Al rods produces the “island-like” hardness distribution, measured by Meyer’s method, which is only to same extent similar to the “layered” distribution. Torsion of such rods induces microcracks in outer layers of the rod. The subject of the present study is the penetration of the microcracks into the rods. A change of the methods of forming causes only to a small extent unstable metal flow, defined according to the Stüwe-Asbeck criterion, observed in the rods drawn and formed by combined methods at greater pre-deformations. The results should be taken into account when using Al rods as constructional elements.