Band 95, Heft 1

The dihedral angle distribution is a useful parameter in the quantification of liquid-phase sintered alloys. Recently, Chhabra et al. have successfully developed a methodology based on digital image processing for dihedral-angle measurements on single-phase polycrystalline alloys. In this paper, the technique used by Chhabra et al. has been modified to evaluate dihedral angles of microstructures in a two-dimensional plane section of two-phase liquid-phase sintered alloys. It is shown that the procedure is extremely fast and accurate and, therefore, can be applied to obtain a significantly large statistical sample of dihedral angles. The validity of this method is demonstrated by evaluating the dihedral angles on samples of W–Ni–Cu alloys.

The paper deals with computer-aided repair welding of a damaged metal-working tool, i. e., a stamping tool. To repair an uncomplicated tool, TIG (Tungsten Inert Gas) welding with a filler material was used. In the process of welding, the welding parameters and the temperature at the workpiece were measured. The major part of the paper describes the use of a personal computer as an aid to calculate the temperature distributions in the workpiece in the process of welding and after welding, and the cooling rate after welding. To this end, the Microsoft Excel software was used. The cooling curves obtained for the individual parts of the workpiece after welding were plotted in a TTT (Time Temperature Transformation) diagram to determine the workpiece structure, hardness and, indirectly, other mechanical properties. The most important finding is that the method described permits to find the locations of maximum stresses in the workpiece that may be potential sources of crack formation.

The effect of T6 and retrogression and re-ageing (RRA) treatments on the corrosion and mechanical properties of a 7039 Al alloy was examined. T6- and RRA-tempered alloys exhibited a similar intergranular corrosion resistance for a retrogression time of 3 min at 220 °C. The results revealed that an RRA-tempered alloy (3 min retrogression at 220 °C followed by re-ageing) exhibited a higher hardness and strength and a lower ductility and toughness than a T6-tempered alloy with a significant improvement in the stress corrosion cracking resistance.

Electron microscopy studies on a creep-tested 10 % Cr steel have revealed the presence of a complex nitride precipitate which has been previously assigned as modified Z-phase. This paper describes how the Z-phase can be efficiently detected by energy-filtering transmission electron microscopy. The results of electron diffraction, electron energyloss spectroscopy and energy-dispersive X-ray analysis studies lead to a quantitative description of the modified Z-phase of the type Cr(V, Nb) N.

A Ni-25Al-25Cr (at.%) alloy was directionally solidified under Ar atmosphere in an Al2O3 – SiO2 ceramic mold by the standard Bridgman method. The microstructure of the as-fabricated alloy was studied using optical microscopy, X-ray diffraction and scanning electron microscopy. The alloy was composed of β-NiAl dendritic arms, α-Cr interdendritic regions, fine α-Cr particles in the dendritic arms, and β-NiAl + α-Cr eutectic in the interdendritic regions. The mechanical properties were studied by tensile tests in the temperature range 1123–1373 K. The results showed that the alloy exhibits superplastic behavior at appropriate testing conditions. The balance between work hardening (by dislocation glide) and work softening (by dynamic recovery and recrystallization) is responsible for the large tensile elongation of the alloy.

The thermodynamic parameters have been assessed for the systems of ZrO 2 – Y 2 O 3 , ZrO 2 –Al 2 O 3 and ZrO 2 –Y 2 O 3 –Al 2 O 3 using the Calphad technique. The solid solutions are described by the compound energy formalism. A two-sublattice ionic liquid model is used to describe the liquid phase. The phase diagrams of the ZrO 2 –Y 2 O 3 and ZrO 2 –Al 2 O 3 and Y 2 O 3 –Al 2 O 3 systems are in fairly good agreement with phase equilibrium data. Isothermal sections of the phase diagram of the ZrO 2 –Y 2 O 3 –Al 2 O 3 system at subsolidus conditions are calculated. Introducing ternary interactions in the liquid phase results in a better fit to experimental data of the liquidus surface and isoplethal sections.

The SrO–B 2 O 3 pseudo-binary system has been assessed. The liquid phase is described by means of a two-sublattice ionic solution model, (Sr +2 ) P (O -2 , BO 3 -3 , B 4 O 7 -2 , B 3 O 4.5 ) Q , where P and Q are constant. All the solid phases are treated as stoichiometric compounds. A set of self-consistent parameters is obtained by using the CALPHAD technique. A comparison between the calculated results and experimental data from literature is presented. It is found that the former reproduce the latter very well on both aspects of phase diagram and thermodynamic properties.

A thermodynamic assessment of the Au–Al system has been performed by using the CALPHAD method. The excess Gibbs energies of the solution phases were formulated with a Redlich-Kister expression, while the other intermetallic phases were described as stoichiometric compounds because of their limited composition ranges. Based on the experimental informations on the phase diagram and the thermodynamics, the thermodynamic parameters of all phases in this system have been optimized.

Chemical reactions and stability of Ti 3 SiC 2 in Cu during processing of Cu/Ti 3 SiC 2 composites in the temperature range of 900 – 1070 °C were investigated using X-ray diffraction and scanning electron microscopy. The results indicated that Cu reacted with Ti 3 SiC 2 above 900 °C, and the reaction products were closely related to the reaction temperature and the relative ratio of Cu and Ti 3 SiC 2 . At low Ti 3 SiC 2 content or temperatures below 1000 °C, Cu(Si) solid solution and TiC x were formed, whereas at high reaction temperature and high Ti 3 SiC 2 content, Cu–Si intermetallic compounds like Cu 5 Si, Cu 15 Si 4 and (Cu, Si) η' as well as TiC x were observed. The transporting process for the reaction was investigated and described. It was found that de-intercalation of Si from Ti 3 SiC 2 dominated the reaction, which dissolved in copper to form Cu(Si) solid solution or Cu – Si intermetallic compounds such as Cu 5 Si, Cu 15 Si 4 and (Cu, Si) η' . TiC x was always present as the decomposition product of Ti 3 SiC 2 .

The precipitation behavior of a high-purity Fe-1.03 wt. % Cu alloy during ageing at 650 °C has been investigated by transmission electron microscopy. The microstructure and the corresponding diffraction patterns of the {100} * , {110} * and {111} * reciprocal planes were analysed. A metastable phase with a B2-like structure was observed, which is quite different from the equilibrium copper phase shown in the Fe–Cu phase diagram. The formation mechanism of the B2-like structure is deduced based on the energetic point of view.