Volume 88, Issue 4

The study of the discontinuous precipitation reaction in a Ni-4 at.% Sn alloy aged at 775 K revealed that the process results in alternating α and β plates whose growth is accompanied by the renucleation event of the new β lamellae at the reaction front. Energy dispersive X-ray analysis of thin foils revealed a step-like change of the Sn content across the reaction front, indicating that the process is governed by boundary diffusion. The parabolic behaviour of the Sn concentration profiles across the α lamellae with an equilibrium Sn content at the α/β interface is consistent with the solution of Cahn’s diffusion equation. The Cahn parameter C changes by a factor of 2 within the same colony of α and β lamellae.

Non-isothermal precipitation in 3003 alloys was monitored by thermoelectric power measurements in various conditions of manganese supersaturation and strain. Thermoelectric power revealed itself as a technique able to generate valuable information which permitted us to study details of the precipitation. The as cast samples are characterized by a maximum rate of precipitation at 693 K and the homogenized samples, by a maximum rate of precipitation at 773 K. The strained samples exhibit a new maximum which displaces itself towards the low temperature region, according to the magnitude of the strain. Each peak obtained has been associated with a precipitated phase.

Abstrakt Im dritten Teil dieser Übersicht über die in den letzten drei Dekaden entwickelten numerischen Verfahren für die Simulation von Erstärrungsvorgängen werden die Modelle zur Vorhersage der Gefügegeometrie diskutiert. Die stochastischen Verfahren auf der Basis der Monte-Carlo-Methode bzw. der Zellularautomatentechnik, Ansätze, die auf der Phasenfeldmethode beruhen, und weitere analytische und numerische Verfahren werden kurz besprochen und kritisch bewertet. Obwohl einige Ansätze sehr vielversprechend erscheinen, sind Vorhersagen der Gefügeausbildung als Funktion der Abkühlbedingungen für technisch wichtige mehrkomponentige Legierungssysteme noch nicht möglich. Probleme, die den meisten dieser Methoden gemeinsam sind, sind die Verwendung mindestens eines freien Parameters, der an das Experiment angepaßt werden muß, und lange Rechenzeiten.

Abstrakt Die Eigenschaften metallischer Werkstoffe und Produkte sowie diejenigen von Halbleitern werden wesentlich von der Stoffzusammensetzung und der Mikro- und Makrostrukturbildung beim Phasenübergang von der Schmelze zum Festkörper bestimmt. Wesentliche Einflußgrößen sind Wärme- und Stofftransport durch Diffusion und Konvektion. Die Experimente zum Konvektionsverhalten erstarrender Schmelzen wurden unter Schwerelosigkeit im Parameterbereich 0.001 g bis 1 g dürchgeführt. Eine tiefere Einsicht in das Konvektionsverhalten während der Erstarrung konnte durch die Verwendung eines transparenten Modellsystems für die metallische Erstarrung gewonnen werden. Erwärtungsgemäß wird die durch parasitäre thermische Gradienten quer zur Phasengrenze hervorgerufene thermische Konvektion bei reduziertem Beschleunigungsniveau deutlich verringert. Im Experiment ist die Schmelze stabiler gegenüber dem Einsetzen thermosolutaler Konvektion als theoretisch vorhergesagt.

The excess enthalpies of liquid alloys in the ternary system Bi–Pb–Te were measured at 1173 and 1073 K in a heat flow calorimeter. Five sections Bi y Pb 1−y –Te with constant concentration ratios x Bi / x Pb were measured. The enthalpy surface in the ternary system is characterized by a valley stretching from the exothermic minimum in the system Pb–Te to the exothermic minimum in the system Bi–Te. The analytical description of the excess enthalpies in the limiting binaries was made with the Redlich-Kister formalism. For the description of the values in the ternary system the equation of Bonnier was taken using additional ternary coefficients. These were obtained by adjusting the curves to the experimental data using the least-squares method.

During rolling contact fatigue of the inner rings of ball bearings three stages of material response can be distinguished, in terms of the volume that is plastically deformed upon overrolling. After a first stage of material strengthening during which a decrease occurs for the volume that is deformed plastically, an effectively stationary, second stage is entered which is eventually succeeded by a third stage exhibiting a pronounced increase of the volume that is deformed plastically upon overrolling, which leads to failure. It is suggested that carbon diffusion induced by local temperature peaks occurring at the moment of overrolling is the key mechanism leading to fatigue damage. The amount of decomposed retained austenite is a useful, practical parameter to assess fatigue life. It is shown that published ideas about the role of certain components of residual stress in enhancing fatigue life are not correct and that the so-called Palmgren-Miner rule, as applied in practice, and the risk volume defined by Lundberg and Palmgren are inappropriate for assessing fatigue life.

X-ray diffraction profiles of Bragg reflections were measured on cyclically deformed nickel single crystals of different orientations with a high-resolution double crystal diffractometer. The mesoscopic dislocation structures beneath the crystal surfaces were studied by scanning electron microscopy using the channelling contrast of backscattered electrons. Changes of internal stresses on different length scales can be inferred from the characteristic dependence of the diffraction profile broadening on the plastic strain amplitude. Short-range stresses are predominant in the matrix structure of crystals cyclically deformed at small strain amplitudes. At higher amplitudes, in the plateau region of the cyclic stress-strain curve, remarkable longrange stresses arise in crystals of all orientations due to the existence of ladder-like, cellular and other dislocation structures.

Tensile deformation induced phase transformations and microstructural changes in an extruded eutectoid Zn–Al based alloy were investigated using X-ray diffraction and scanning electron microscopy. Decomposition of the metastable η E ′ $\eta _{\text{E}}^{\prime }$phase and a four phase transformation, α+ε→ T ′ +η, $\alpha +\varepsilon \to {{\text{T}}^{\prime }}+\eta ,$were found to occur during tensile deformation. Tensile deformation resulted in spheroidizing of the coarse lamellar structure, within which rupture holes were initiated to form in the rupture part of the specimen. The fine lamellar structure withstood the tensile rupture.

Attention has been focussed on the influence of Al content on the physical, mechanical and sliding wear properties of Zn-based alloys. Aspects studied include microstructure, density, electrical conductivity, hardness, tensile strength and elongation as well as sliding wear response of the alloys. Microstructural features of the alloys showed the presence of primary α, eutectic/eutectoid α+η (depending on whether the alloy was hypereutectic/hypereutectoid with regard to the concentration of Al) along with the meta stable ε phase. The study suggests that it is possible to design and develop Zn-based alloys with a wide range of concentration of Al. The alloys in turn attain different combinations of physical, mechanical and wear properties which could suit a variety of engineering applications. Increasing the Al content in the alloy system proves beneficial within limits. In other words, there exists an optimum quantity of Al which could reap its advantage to the maximum extent. This of course varies with reference to a specific property of the alloy(s). The changing response of the alloys has been explained in terms of their microstructural features and the effects produced as a result of the test conditions maintained while characterizing the specimens.

Defect dynamics during early stages of plastic deformation of copper single crystals are investigated via spectral analysis of acoustic emission (AE) with the aim to clarify the role of a surface in AE. The crystals plated with a thin film of amorphous NiP were examined together with unplated samples of the same [110] tensile orientations, geometry and dimensions. Features of AE are described in terms of statistical treatment of spectral parameters. It is shown that the surface conditions greatly affect AE. AE significantly changes in a time domain from the continuous type in unplated samples to the burst type in plated ones. Substantial changes in the shape of power spectra are also found. The results are interpreted by assuming that the resulting AE is largely due to dislocation escaping from the crystal. Statistical models of individual and collective AE sources are found to be capable of describing most of the AE features.

The reaction occurring between iron panels and Zn-5% Al baths during hot dipping was investigated. Low-carbon steel and Armco-Fe panels were dipped at three temperatures (410, 450 and 490 °C) in Zn–4.92% Al bath for dipping times from 16 to 2048 s. The reaction kinetics was studied by measuring the weight loss of the samples before and after dipping. The loss change of Fe does not follow a parabolic time law. The mechanism of the iron–melt reaction was studied in two baths, Zn–4.92% Al and Zn–5.17% Al, at a temperature of 450 °C. The formation of the intermetallic phases has been observed at the substrate–melt interface as a result of the chemical reaction between the Fe from the steel and the Al from the bath. Microstructural, SEM and X-ray diffraction data are presented to support the above-mentioned conclusions.