Volume 89, Issue 1

The excess enthalpies of liquid alloys in the ternary system Ge–Sn–Te were determined in a heatflow calorimeter along the sections Ge y Sn 1– y – Te with y = 0.2, 0.4, 0.5 and 0.6 at 1093 K, with y = 0.2, 0.4, 0.5, 0.6 and 0.8 at 1153 K and with y = 0.5 at 1213 K. The largest values of the exothermic enthalpies are found on a line stretching from GeTe to SnTe. The association model was applied for the analytical description of the ternary liquid. Additional ternary interaction parameters had to be used for the calculation of the excess enthalpies which were obtained by adjusting the curves to the experimental data with the least squares method. The thermodynamic functions of the quasibinary section Ge–SnTe were optimized and the phase equilibria calculated.

Crystallization of an amorphous ternary alloy with high aluminium content (Al 19 La 5 Ni 4 ) was investigated. Crystallization begins with precipitation of α-Al ( T x = 313 K, reaction enthalpy Δ H 1 = 1.4 kJ/mol), controlled by La diffusion. The diffusivity of La was estimated at 523 K as (8.8 ± 1.6)· 10 −21 m 2 /s. At higher temperatures, Al 11 La 3 and Al 3 Ni are being formed by eutectic reaction (Δ H 2 = 2.4 kJ/mol, reaction exponent 3.5 to 4). The activation energies for the eutectic reaction were obtained in isothermal (237 ± 8 kJ/mol) and in continuous heating conditions (229 ± 7 kJ/mol). The degree of crystallinity of the specimens is related to their hardness. The partial crystallization of α-Al nanoparticles (up to 30 vol.%) enhances the alloy’s hardness without significant loss of ductility.

The influence of hydrogen on the crystallization of meltspun Cu 50 Ti 40 Al 10 amorphous alloy was studied by DSC, TEM, X-ray and electron diffraction. The isothermal crystallization kinetics of the as-quenched glass revealed that the most probable crystallization mechanism is nucleation and subsequent three dimensional diffusion controlled growth. A strong influence of hydrogen on the crystallization of this amorphous alloy was found. Hydrogenation leads to drastic decreasing of the thermal stability, due to phase separation in the amorphous state and to formation of nanocrystalline structure during crystallization. The activation energy of the nanocrystallization reaction was determined by the Kissinger method.

The Lebowitz’s solution of the direct correlation function for binary mixtures with a square well attractive tail has been used to model the structure factors of Cu–Si and Ni–Si molten alloys. The computed total structure factors compare favourably with experiment. Results from structural studies indicate a need for a study of ordering phenomena in the system. This has been done by evaluating the long wavelength limit of the concentration-concentration fluctuations S cc (0). The computed S cc (0) was used in evaluating the Warren-Cowley short range order parameter α 1 for the two alloys. Our calculations indicate that the two systems manifest compound forming tendencies throughout the concentration range. Cu–Si, however, exhibits a greater tendency for compound formation than Ni–Si molten alloys.

REM-und EDS-Untersuchungen an sphäroidisierten Al-und Si-legiertem Gußeisen zeigen in Zwischenräumen der polykristallinen Kugelgraphitausscheidungen eine aluminium- und seltenerdmetallreiche Eisenlegierung. Kugelgraphit kann also in einer aluminiumreichen Umgebung wachsen. Der Wachstumsprozeß ist wahrscheinlich mit der Adsorption von Seltenerdmetallatomen an bestimmten Graphitflächen (d. h. an {101¯0}$\{ 10\bar 10\} $) verknüpft, da die Seltenerdmetalle nur in den Kugelgraphitausscheidungen, aber nicht in den Vermiculargraphitausscheidungen nachgewiesen werden. Eine signifikante Anreicherung von Al und Si durch Segregation aus der Matrix zu den Phasengrenzflächen von Kugel-und Vermiculargraphit wird nicht beobachtet.

The influence of carbon (0.1 and 0.3 wt.%) on the discontinuous precipitation reaction in a Co-14.8 wt.% Mo alloy has been investigated by means of X-ray analysis, analytical electron microscopy and quantitative metallography. It has been shown that the discontinuous precipitation is accompanied by the formation of 2 H type martensite with a 〈010〉 zone axis in the matrix. Energy dispersive X-ray analysis performed across the α lamellae of discontinuous precipitates has revealed that the carbon addition shifts the profiles toward higher Mo concentrations but the strongest carbon effect is observed for 0.1 wt.% C. This result may be attributed to two competing influences of the carbon addition: an extension of the solid solubility limit of Mo in Co and a blocking of the grain boundary steps at the reaction front. It has been also shown that the growth velocity of the of discontinuous precipitation reaction diminishes after doping the binary Co–Mo alloy with carbon. The carbon distribution within the α lamellae has been calculated. The results have shown that most of the carbon appears in MoC particles within the matrix and that the profile shape of the carbon distribution follows that obtained for Mo.

Controlled solidification experiments are carried out on Al-5.7 wt.% Ni eutectic alloys to investigate the solute distribution in both longitudinal and transverse directions of a directionally solidified ingot. The events at the solid–liquid interface are frozen by rapidly quenching in liquid nitrogen a partially directionally solidified ingot. Hunt and Jackson’s analysis for lamellar eutectics is modified for rod morphology to theoretically estimate the solute distribution ahead of the solid–liquid interface. The solute distribution profile as determined by microprobe analysis is found to be in good agreement with the theoretical solutions, provided a best fit choice is made for a variable, position dependent interface diffusion coefficient.

An experimental study on the grain growth of a medium carbon microalloyed steel with two levels of aluminum nitride (AlN) is carried out. Two types of precipitates (V-Ti carbonitrides and AlN) are expected in the alloys selected. The effect of both particles on the final grain size is analyzed, and special emphasis is given to the appearance of abnormal grain growth. The dependence of the volume fraction of particles and of their size on time and temperature is theoretically derived according to simplified models. The particle sizes are also experimentally determined and contrasted with the latter theoretical approaches. Classical models of grain growth control are compared and that proposed by Rios proportionates the best agreement with experimental results. The effect of the heating rate, i. e. overprecipitation, is also qualitatively included in the analysis.

Cu 81.8 Al 14.0 Ni 4.2 (Masse-%)-Einkristalle werden in einer selbstgebauten Anlage nach dem Bridgman-Verfahren gezüchtet. Die Phasenübergänge Austenit ↔ Martensit der einzelnen Martensitplatten in einem CuAlNi-Einkristall werden mit einem Lichtmikroskop beobachtet und zwei unterschiedliche metallographisch irreversible Phasenübergangsformen identifiziert. Der Phasenübergang Martensit → Austenit erfolgt bei Erwärmung durch Keimbildung von Austenit an der Mittellinie zwischen den einzelnen Martensitvarianten oder durch Verschwinden einer der Martensitvarianten. Die spannungsinduzierte Die spannungsinduzierte Umwandlung in dem CuAlNi-Einkristall wird mittels eines in-situ-Apparates mikroskopisch beobachtet. Die dabei entstehenden Grenzflächen, die man unter dem Lichtmikroskop gut feststellen kann, treten weit vor der Fließgrenze auf, während die martensitische Umwandlung erst bei einer Spannung, die viel höher als die Fließgrenze ist, vollständig beendet wird.