Band 89, Heft 11

Experimental results on properties of point defects in platinum which were obtained during the last forty-five years are discussed in the light of the modified two-interstitial model. After a low temperature irradiation correlated 〈100〉-split interstitial–vacancy pairs annihilate in recovery stages I and II with activation energies ranging from very small values up to the activation energy of migration of free 〈100〉-split interstitials of EIIM = 0.73 eV $ E_{{\rm{II}}}^{\rm{M}} \; = \;0.73\;{\rm{eV}} $ . Free 〈100〉-split interstitial recover in recovery stage III at about 300 K and vacancies in recovery stage IV at about 600 K. The recovery of vacancies after quenching is also observed in recovery stage IV and their activation energy of migration is EIVM = (1.37±0.02) eV $ E_{{\rm{IV}}}^{\rm{M}} \; = \;\left( {1.37 \pm 0.02} \right){\rm{ eV}} $ . Di-vacancies obtained by quenching have an activation energy of migration of E2VM = 1.1 eV $ E_{{\rm{2V}}}^{\rm{M}} \; = \;1.1\;{\rm{eV}} $ annihilating in a temperature range below that corresponding to the recovery stage IV range. Correlated 〈100〉-crowdion–vacancy pairs and 〈100〉 crowdions recover in the recovery sub-stages I D and I E , respectively. The activation energy of migration of 〈100〉-crowdions is ECM = 0.063 eV $ E_{\rm{C}}^{\rm{M}} \; = \;0.063{\rm{ eV}} $ . The increase of the electrical resistivity of platinum during irradiation with 1.85 MeV electrons from a Van de Graaff generator was measured in situ for various irradiation temperatures at and below the temperature region corresponding to the recovery stage III temperature region. The increase was very small and almost not detectable, because the electrical resistivity of a Frenkel defect decreases from ρ 0 = 10 · 10 –4 Ωcm at the temperature of liquid helium to very small values at ambient temperature. After large irradiation doses only vacancies were present which recovered during an anneal of 24 hours at 300 °C.

Die Anwendung von Mikromodellen der Erstarrung auf technische Prozesse und Legierungen setzt voraus, daß die Wechselwirkungen von Vorgängen auf verschiedenen Längenskalen während der Erstarrung berücksichtigt werden. In dieser Arbeit werden einige Entwicklungen der Mikromodelle bezüglich Kopplung mit Phasendiagrammrechnungen, dendritischen und eutektischen Wachstumsunterkühlungen und Makromodellen vorgestellt und folgende Punkte diskutiert: Der wichtigste Eingabeparameter ist das Gleichgewichtsphasendiagramm. Der Erstarrungsweg legt Art und Menge der erstarrenden Phasen fest. Verunreinigungen und geringfügige Beimengungen sollten als Legierungselemente betrachtet werden. Bei hohen Abkühlraten oder beim Rückschmelzen müssen Abweichungen vom lokalen Gleichgewicht berücksichtigt werden. Unter Einbeziehung der Wechselwirkung zwischen Makro- und Mikromodellen kann die lokale Gefügeausbildung in großen Gußstücken simuliert werden. Mit Kopplung der Modelle für die verschiedenen Phänomene ist es möglich, Erstarrungsgefüge für eine Vielzahl von Prozeßbedingungen vorauszusagen.

Laser surface remelting at various velocities has been employed to study the selection of microstructures of high-purity Fe–C–Si alloys containing nominally 3.2 to 4.2 wt.% C and 1 to 3 wt.% Si. The microstructure of the remelted region consisted of metastable Fe–Fe 3 C eutectic (ledeburite) or austenite dendrites, with interdendritic eutectic. Furthermore, ledeburite presents two solid-liquid interface morphologies: planar and cellular. The competition between the austenite dendrites and the ledeburite eutectic as a function of solidification rate has been experimentally determined. The critical velocity which destabilizes the planar Fe–Fe 3 C eutectic with respect to primary austenite dendrites, was of the order of several mm/s and depends on the initial composition of the alloy. The critical velocity for the destabilization of the eutectic interface leading to two-phase cells was 0.44 mm/s, for the alloy containing 4.2 wt.% C and 1 wt.% Si. Theoretical calculations of the coupled zone have been performed using current microstructure selection models. The results of the simulation were then used to construct a microstructure map which was compared with experimental results.

Outstanding features of biominerals and some key characteristics of biomineralization processes, which inspired so-called biomimetic approaches of materials synthesis, are summarized. Previous results on the deposition of oxide thin films on organic self-assembled monolayers (SAMs) from liquid solutions at temperatures below 100 °C are reviewed. Because of deleterious effects of basic solutions on the structural order and adhesion of siloxy-anchored SAMs, the present technique is best suited to oxides of cations that can be precipitated in acidic aqueous solutions. This conclusion is supported by the results on films containing Zn 2+ . Compact thin films of ZnO could be synthesized under basic conditions, but the films showed less complete coverage and adherence than previously reported films (e. g. TiO 2 , ZrO 2 , Y 2 O 3 , etc.) that were formed from acidic solutions. First attempts to use SAMs in the synthesis of Si– C–N ceramic films (silicon-dicarbodiimide) are described.

Nucleation and growth rates of coarse crystallites in the course of abnormal grain growth are studied by means of computer simulation. Two cases of fine-grained matrix stabilization are considered: the first one when normal grain growth is inhibited by a pinning force and the second one with “texture inhibition”. In both cases, the number of the crystallites first increases with time and then decreases. The linear growth rate of the crystallites also changes with abnormal grain growth evolution. The results obtained are explained by changes in both matrix grain size and pinning force magnitude (if any).

Es wurde ein pulvermetallurgischer Fertigungsweg zur endformnahen Herstellung von γ-TiAl-Automobilventilen entwickelt. Hierbei wird gasverdüstes γ-TiAl-Pulver mittels heißisostatischem Pressen endformnah verdichtet. Diese Arbeit beschreibt den Fertigungsweg, charakterisiert das erhaltene Gefüge und geht auf das erzielbare mechanische Eigenschaftsprofil am Beispiel der zweiphasigen γ-TiAl- Basislegierung Ti-46.5At.-% Al-4At.-% (Cr,Nb,Ta,B)-Legierung ein. Schwerpunkte bilden die für den Einsatz als Ventilwerkstoff relevanten Eigenschaften, wie z. B. das Festigkeits- und Ermüdungsverhalten.

An attempt has been made to develop dual-phase (d-p) steels from cold-rolled plain carbon sheet steels. Three plain carbon sheet steels with carbon content of 0.075, 0.095 and 0.2 wt.% have been selected. Intercritical heat treatments at 740 °C and holding times of 0.5, 1, 1.5, 2, 3, 4, 5, 8, 10, 15, and 30 minutes at this temperature followed by fast water quenching were used. A good combination of strength and ductility of d-p steels has been observed for the heat treatment cycle of 740 °C for 1.5 minutes and WQ. D-p steel with 0.075 wt.% carbon has shown an ultimate tensile strength of 560 MPa and a ductility (uniform elongation) of 16% whereas the starting material i.e. plain carbon cold rolled sheet steel, has a UTS of 382 MPa and a ductility of5.5 %. Similar improvements have been observed for other steels. These improvements in the mechanical properties are explained on the basis of the cold work energy and the kinetics of austenitisation during intercritical annealing.

Using differential thermal analysis, X-ray diffraction, microscopy and electron microprobe, phase equilibria in the Ti-rich corner of the Ti–Si–Ge–Al system were studied. Projections of the solidus and liquidus surfaces were constructed. At the solidus temperatures Ti 5 (Si,Ge,Al) 3 (Z) phase coexists with < β-Ti >-based solid solution (β), resulting in a wide two-phase β + Z region. The solidus surface has a temperature maximum at 1540 °C for 70Ti-5Si-5Ge-20Al. The isopleth at 5 at.% Si + 5 at.% Ge is given. The liquidus surface is characterised by the bivariant eutectic surface L ↔ β + Z. Microhardness of the primary phases, corresponding to the boundaries of the homogeneity ranges, and eutectic mixtures was measured. Partial substitution of silicon for germanium does neither change the character of the phase equilibria nor the temperatures of phase transformations.

The isothermal section at 500 °C of the phase diagram of the Dy–Fe–V ternary system was investigated by X-ray powder diffraction, electron probe microanalysis and metallo-graphic microanalysis. The ternary system consists of nine single-phase regions, sixteen two-phase regions and eight three-phase regions. The existence of five binary compounds were confirmed and only one ternary compound, DyFe 10 V 2 , was found in this isothermal section. The solubilities of Dy in the solid solutions of V in Fe, Fe in V and FeV are about 2.6 at.%., 3.5 at.% and 5.5 at.%., respectively.

This paper deals with studies on corrosion behaviour of two Al–Mg–Si alloys containing excess Si in 1(N) NaCl and 0.1 (N) inorganic acids. Intergranular corrosion and immersion tests were carried out as per standards (MIL-H-6088F, ASTM G-31). Intergranular corrosion was assessed using optical, scanning and transmission electron microscopy. The effects of isothermal and isochronal ageing on corrosion resistance in 0.1(N) HCl and on hardness changes were studied. The studies showed that the amount of intergranular attack in NaCl solution increases with ageing time and is maximum at peak aged condition. Segregation of Si particles and Mg 2 Si at the grain boundary contributes to intergranular corrosion. Corrosion rate was higher in HCl than in HNO 3 and H 2 SO 4 solutions, however it was maximum in 0.1 (N) HCl during its peak aged period.