Band 97, Heft 1

The Pd–B system in the composition range of the solid solution PdB y has been the subject of a number of investigations as it provides an attractive model system for interstitial compounds. No consensus about the crystal structures and phase boundaries, particularly in the low-temperature regime, has been reached in the earlier works. In the light of very recent results on the crystal structure and composition ranges of the low-temperature phases, on the basis of X-ray, electron, and neutron diffraction analyses, a tentative phase diagram is presented which is compatible with practically all previous results.

The Li –Sn system was critically assessed by means of the CALculation of the PHAse Diagram (CALPHAD) technique. The solution phases (liquid, body-centered cubic (αLi), body-centered tetragonal (βSn), hexagonal close-packed (βLi) and diamond (αSn)) were modeled with the Redlich-Kister equation. The intermetallic compounds Li 7 Sn 2 and LiSn, which have a homogeneity range, were treated as the formulae (Li, Sn) 7 Sn 2 and Li(Li, Sn) by a two-sublattice model with Li and Sn or Li on the first sublattice, Sn or Li and Sn on the second one, respectively. The others were treated as stoichiometric compounds. A set of self-consistent thermodynamic parameters describing the Gibbs energy of each individual phase in the Li –Sn system was obtained.

Experimental data for phase equilibria and thermodynamic properties have been used for thermodynamic analysis of the high-temperature heazlewoodite. A two-sublattice model in the framework of the Compound Energy Formalism is proposed for the Gibbs energy of a second high-temperature heazlewoodite phase around the composition Ni 3 S 4 . Optimized model parameters have been obtained which reproduce all data simultaneously within experimental error limits. The Gibbs energy modeling is carried out consistently with the recent assessment by the author of the nickel – sulfur system at 1 bar pressure over the entire composition range for temperatures from 25 °C to above the liquidus.

It is shown that Arrhenius plots of diffusion coefficients of Cr in β-Ti can be represented under each pressure by two straight lines which are fitted in the temperature regions above and below about 1390 K. The activation enthalpies for high temperature are 167 kJ/mol under 0.1 MPa and 191 kJ/mol under 2.1 GPa, and those for low temperature are 147 kJ/mol under 0.1 MPa and 153 kJ/mol under 2.1 GPa. The ratios of activation volumes for impurity diffusion to the molar matrix volume are in the range of 0.38 to 0.45. When the phonon-softening model is applied, the activation enthalpy for the impurity diffusion in the absence of any phonon-induced lattice instabilities is 226 kJ/mol under 0.1 MPa and increases with pressure. Its pressure dependence indicates that the activation volume for the diffusion with no assistance from phonons is 66% of the molar matrix volume, and this result agrees with the activation volume for the diffusion in “normal” bcc metals.

Density and surface tension of liquid Ni –Cu –Fe alloys have been measured over a wide temperature range, including the undercooled regime. A non-contact technique was used, consisting of an electromagnetic levitator, an optical densitometer, and an oscillating drop tensiometer. At temperatures above and below the liquidus point, density and surface tension are linear functions of temperature. The concentration dependence of the density is significantly influenced by a third-order (ternary) parameter in the volume, while the surface tensions can be derived from the thermodynamic potentials E G of the binary phases alone.

In the present paper, the effect of electric field strength in the range E = 0.1 – 5.0 kV/cm applied during the solution heat treatment (SHT) of the AA6111 alloy at 475° –550 °C has been determined employing resistivity. An increase in the as-quenched resistivity ρ w occurred for all field strengths, the increase being relatively rapid up to E ≈ 1 kV/cm and only slight, if at all, for higher fields. Analysis of the results gave that the field reduced the enthalpy, entropy and Gibbs free energy of solution, the amount of reduction increasing with field strength. The electric current, which occurred with application of the field during SHT, ranged from (7 × 10 –3 to 4 × 102) μA, increasing with field strength and SHT temperature, but decreasing with time during the SHT. The role of the current in the enhancement of ρ w by the field is not clear.

High-purity (99.99 wt.%) Al containing 50 wt. ppm Cu was cold rolled to 98% reduction in thickness and annealed by varying the heating rate of annealing and annealing cycles. Textures developed at various stages of annealing were investigated with the orientation distribution function analysis. The observed results were compared with those obtained on standard high purity (99.99 wt.%) Al without Cu addition. In slow heating rate annealing, it was found that in high purity Al the addition of 50 wt.ppm Cu significantly enhanced the development of {100}<001> recrystallization textures. Intensities of {100}<001> developed in high purity Al with 50 wt. ppm Cu were about twice as high as those observed in high purity Al without Cu addition. {123}<634> R orientations were completely eliminated by the addition of 50 wt. ppm Cu. In rapid heating annealing, on the other hand, annealing at temperatures above 325 °C resulted in the formation of very coarse recrystallized grains with weak {100}<001> recrystallization textures irrespective of Cu content. Such grain coarsening could be suppressed by pre-annealing at temperatures between 225 and 300 °C. In the specimens subjected to this pre-annealing, strong {100}<001> recrystallization textures could be obtained, even if they were annealed with a rapid heating rate in final annealing.

In high-purity 4 N (99.99 wt.%) Al containing 50 wt.ppm Cu, very strong {100}<001> recrystallization textures are developed after 98% cold rolling and annealing at 500 °C. They are about three times stronger than those observed in standard high-purity 4 N Al without Cu addition. In this paper, the mechanism of the formation of such strong {100}<001> recrystallization textures were investigated in detail by using the EBSP (electron backscatter pattern) analysis. It was found that, at the earliest stage of recrystallization, {100} <001>-recrystallized grains were nucleated by forming a row lying parallel to the rolling direction and growing preferentially within the elongated deformed grain in which they were nucleated. Most of these {100}<001>-recrystallized grains had orientations very near to the exact cube orientations. {100}<001>-recrystallized grains in this materials were characterized by their rapid growth, which was observed at the later stages of recrystallization. Since they were surrounded by deformed regions having very strong β -fiber rolling textures formed by heavy cold rolling, their grain boundaries were high-angle grain boundaries with high mobility. Assisted further by high stored energy introduced by heavy rolling reductions, {100}<001>-recrystallized grains could grow very rapidly, traversing readily several deformed grains. In this material, {100}<001>-recrystallized grains were not nucleated so abundantly. However, {100}<001>-recrystallized grains were nucleated much earlier and grew much faster than recrystallized grains with other orientations. They were, therefore, always larger than recrystallized grains with other orientations. Due to such size advantages, {100}<001>-recrystallized grains could rapidly consume fine-recrystallized grains with other orientations during the subsequent grain growth process. As a result, microstructures observed after annealing at 500 °C for 2 h consisted mostly of coarse {100}<001>-recrystallized grains slightly misoriented with each other. It is, thus, rapid growth of few {100}<001>-recrystallized grains during recrystallization, and their preferential grain growth, that enhance the development of very strong {100}<001> recrystallization textures in this high-purity Al. Cu seems to suppress nucleation of recrystallized grains with other orientations.

The microstructure and compressive behavior of the NiAl – 28Cr – 6Mo alloy doped with various amounts of Nb have been investigated. The results show that the microstructure of all Nb-doped alloys mainly consists of three phases, viz. the gray lamellar Cr(Mo) plate, the black NiAl matrix, and Cr 2 Nb with C14-structure-type 1 Laves phase semicontinuously distributed at the cell boundary. All Nb-doped alloys exhibit relatively good room-temperature compressive ductility and higher yield strength at all temperatures compared to the NiAl – 28Cr – 6Mo eutectic alloy due to the presence of Laves phase. It is found that the NiAl – 28Cr – 6Mo alloy doped with 1 at.% Nb attains the optimum concerning high temperature strength and room temperature ductility.

The die cast Mg-5 wt.% Ca-6 wt.% Zn ternary alloy was exposed to 160 °C for different times of up to 40 days. Microstructural analysis and microhardness and hardness measurements of the samples after different exposure times enabled the thermal stability of the cast alloy to be monitored. The as-cast structure is composed mainly of α-Mg solid solution, elliptical grain boundary particles of CaMg 2 , and an intergranular eutectics of Mg and Ca 2 Mg 6 Zn 3 . The grain size of α-Mg and the structure of the alloy did not change during the treatment, but the amount of the intergranular phases increased slightly. Exposure to 160 °C resulted in a decrease in micro-hardness of the α-Mg grains, but no change in the overall hardness of the samples was observed. Microstructural analyses indicated diffusion of the solute elements from the α-Mg grains to the grain boundaries, resulting in a decrease in hardness of the α-Mg which is, however, compensated by an increase in hardness related to intergranular precipitates. Using a simple diffusion model, the diffusion coefficient of Ca atoms in Mg at 160 °C was estimated to be 2.1 · 10 –17 m 2 s – 1 , which is greater than that of Zn by two orders of magnitude.

In present work, reduction kinetics of Cu 2 O by hydrogen gas was studied by thermogravimetric analyses (TGA). The TGA experiments were carried out both isothermally and non-isothermally on shallow powder beds. It was established that additions of Ni or NiO did not have any serious effect on the kinetics of reduction of Cu 2 O. The composition and microstructures of the reaction products were analyzed after each experiment by X-ray diffraction (XRD) as well as by scanning electron microscopy (SEM). The activation energy for the reaction was evaluated from isothermal as well as non-isothermal reduction experiments and was found to be in good agreement. The impact of the stability of the oxide on the activation energy for hydrogen reduction is also discussed.

This paper addresses the decomposition kinetics of synthesized homogeneous expanded austenite formed by gaseous nitriding of stainless steel AISI 304L and AISI 316L with nitrogen contents up to 38 at.% nitrogen. Isochronal annealing experiments were carried out in both inert (N 2 ) and reducing (H 2 ) atmospheres. Differential thermal analysis (DTA) and thermogravimetry were applied for identification of the decomposition reactions and X-ray diffraction analysis was applied for phase analysis. CrN precipitated upon annealing; the activation energies are 187 kJ/mol and 128 kJ/mol for AISI 316L and AISI 304L, respectively. Isothermal stability plots for expanded austenite developed from AISI 304L and AISI 316 were obtained.

The results of the estimation of the viscosity for Ag – In and In–Sb liquid alloys are presented in this paper. Different theoretical models for viscosity determination, according to the Moelwyn–Hughes, Iida –Ueda –Morita, Kozlov– Romanov– Petrov, Hirai, Seetharaman – DuSichen and Kaptay, have been applied in the process of viscosity calculation at different temperatures – for the Ag– In alloys in the range 1073 – 1273 K and for the In–Sb alloys in the range 873 – 1073 K. The results for both systems, obtained using different models, were compared mutually and with available literature experimental data. The models, mostly capable to predict appropriate viscosity data for investigated binary systems, were defined.

Elevated temperature tensile properties of AlSi7Mg (Al-7 %Si– 0.7 %Mg) and its composites with 10 and 20 vol.% SiC p in the as-extruded form were investigated. The cast ingots of the matrix alloy and the composites were produced by the permanent die casting technique and extruded at 480 °C using an extrusion ratio of 10 : 1. The high-temperature tensile tests were carried out over the temperature range from 25 to 430 °C. Mechanical properties after extrusion show that the composite samples have strength values superior to that of the matrix alloy at ambient temperature. At elevated temperatures the results indicate that the composites exhibit good strength retention up to 300 °C, above which the strengthening effect of SiC p disappears as the temperature is increased. The strain rate sensitivity “m ” was observed to be 0.11 for the matrix alloy, 0.13 and 0.07 for the composites with 10 and 20 vol.% SiC p , respectively, at a test temperature of 430 °C in the strain rate range from 4 × 10 –5 to 4 × 10 – 2 s – 1 .