Volume 96, Issue 4

Ferroics are materials which become ferroelastic, ferromagnetic, or ferroelectric at lower temperatures on cooling. They usually show a phase transformation which produces an ordered state (low-entropy phase). Their properties are partially due to a domain structure. The strain effects are found either by cycling around the transformation temperature (two-way effect, invar), by mechanical straining plus heating above the transformation temperature (one-way memory) or by cyclic mechanical loading (pseudo- or rubber-elasticity). Ferromagnetic or ferroelectric materials can be shaped by magnetic or electrical fields instead of mechanical stresses. In combination with a martensitic transformation, considerable strains are obtained in some ferromagnetic alloys, such as Ni 2 MnGa and other multiferroics. Polymers can show one-way behaviour like metals, and ferro- and piezo-electrical behaviour like ceramics. The largest amounts of reversible strains are obtained with polymers, the smallest with ceramics. Porous charged polymers show hysteresis curves like ceramic ferroelectrics, but larger amounts of strain: ferro-electrets. A systematic understanding of this group of materials permits to compare their abilities and limits. This, in turn, provides a basis for the selection of the best material for a certain application.

The characterization of the microstructure in superalloys is predominantly carried out with microscopic methods. Due to the low penetration of X-rays, electrons or light, the samples have to be very thin by using transmission methods and the investigation is limited to the surface. Neutrons enable to enlarge the investigated sample volume, both the crosssection, due to the large size of neutron beams, and the thickness up to a few millimeters, because of the favourable penetration depth. This article presents various examples in the field of superalloys, as in situ measurements at high temperatures, the detection of rare detrimental single plates randomly distributed in the sample, or the determination of lattice constants. This demonstrates how worthwhile it is using the non-destructive tools of small-angle neutron scattering and neutron diffraction to solve challenges arising in material science.

New experimental phase equilibria information about the Fe – C– Co – Cr – Mo– Si– V– W system and sub-systems are presented and used to modify the thermodynamic description of the system. The main interest was focused on the composition of the MC, M 7 C 3 and M 6 C carbides and the necessary adjustment between the carbide-forming elements. A new set of thermodynamic parameter values describing the Gibbs energy of different phases was obtained utilizing phase equilibria data from model alloys. A number of calculated sections of binary systems and ternary carbide systems are presented and compared to both new and old experimental information. Calculations of commercial alloys to verify the improvements in multicomponent systems are also presented. The present calculation using the Thermo-Calc software shows much better agreement with the new experimental results than previous assessments.

This work describes the development of texture in cast pure magnesium during unidirectional rolling, multi-step reverse rolling and multi-step cross rolling at different temperatures. The unidirectionally and the multi-step reverse hotrolled plates exhibit mainly an asymmetric R-type texture with basal-pole split. Hot-rolled plates were also coldrolled to a small reduction (10 %) by taking small strips of the RD-ND and TD-ND planes with two different initial textures. The shape and location of the main texture components in basal pole figures and ODF plots depend on the hot-rolling temperature and on the mode of rolling. The textures obtained in this study are compared with those of pure magnesium reported in literature.

Grain boundary (GB) solute diffusion of Ge and Ga in Al polycrystals was investigated by the radiotracer serial sectioning technique using 71 Ge and 72 Ga isotopes. The diffusion measurements were carried out under conditions of Harrison’s B-kinetics regime in the temperature range 450–680 K. The measured triple products P = sδD gb ( s is the segregation factor, δ the GB width and D gb the GB diffusivity) were found to follow the Arrhenius law with pre-exponential factors and activation enthalpies of 6. 36 × 10 –14 (5. 33 × 10 –13 ) m 3 /s and 53. 6 (54.8) kJ/mol for Ge (Ga), respectively. Alloying of Al by Ga results in a slight increase of the Ge GB diffusivity by a factor of two to three at 576 K within the total Ga content up to 1 at.%. This result does not confirm the appearance of a liquid-like GB layer in the Al–Ga alloys studied.

The precipitation behavior in an Al –Cu alloy isothermally annealed at 463 K was studied using Vickers microhardness tests, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). An additional endothermic peak found in the DSC measurements may be attributed to the θ″-phase, independent of the dissolution of the G. P. (II) and the θ″-phase at the aging temperatures. High-resolution transmission electron microscopy (HRTEM) revealed that the G. P. (I) is formed at a very early stage of isothermal aging, even at 463 K. Comparing the Vickers microhardness and TEM images, it was concluded that the stage at which the G. P. (II) and θ″-phases precipitated simultaneously is mainly responsible for the peak hardness.

The response dynamics of AlSi eutectic microstructures on accelerated and decelerated directional solidification conditions is investigated with optical in-situ techniques during processing. In contrast to regular eutectics the microstructure in eutectic and off eutectic AlSi alloys is independent of the processing history. The spacing between the Si lamellae is completely determined by the locally achieved solidification velocity and can be calculated using well-known theories for stationary growth of irregular eutectics. The observation is important to industrial casting processes which are typically non-stationary, because the theory of stationary growth can be used to predict the eutectic morphology also under instationary conditions.

The surface of a medium carbon steel sheet was laser cladded with a Co –Cr alloy in two layers by a multi-pass technique. This study is concerned with the heat-affected zone (HAZ) created. Within this zone, austenitisation and martensite formation take place repeatedly twice or three times followed by later tempering. This change in microstructure is associated with a hardening and a softening. The microstructure consists of tempered martensite close to the heat source, an intermediate zone of tempered martensite/bainite and a partially transformed zone. However, the hardness is constant throughout the HAZ and decreases in the partially transformed zone to the substrate hardness. The analysis of the microstructure and the hardness are used to re-establish the temperature history encountered by the material in the HAZ. It could be shown that the HAZ has a translational symmetry that can be approximated by a 2-D sheet structure.

Titanium eutectic composite Ti–Si–Al–Zr alloy castings were subjected to high thermal loads and temperature gradients to simulate extreme conditions of a gas turbine component. The microstructure of the affected zones was studied and characterized. The results can be applied to the development of a new generation of heavy-duty components with properties exceeding known cast titanium alloys and intermetallics.

Mnce a room-temperature delay between the solution heat treatment and the paint-bake cycle is unavoidable in indus-trial practice, it is essential to counteract the adverse effect of natural ageing on the bake-hardening response of Al–Mg–Si alloys. This is often accomplished by an additional ageing cycle, referred to as pre-ageing, before the paint-bake treatment. While pre-ageing treatments shortly after the solution treatment are very attractive, long delays prior to pre-ageing produce undesirable effects on the bake-hardening response. The best combination of a sufficiently high T8X and a reasonably low T4P yield strength for a twin-roll cast (TRC) Al–Mg–Si alloy is obtained when the sheet is pre-aged for 5 minutes at 180 °C with only a 10-minute delay after the solution treatment. Several hours of natural ageing can be tolerated before the T8X yield strength drops below 200 MPa with a similar heat treatment at 200 °C. Pre-ageing at even higher temperatures must be employed only after substantial natural ageing, of the order of several days, to avoid a too high T4P yield strength that would impair stampability.

Turning experiments have been carried out with the cutting speed and feed as variables that were systematically varied between 10 m min –1 and 1200 m min –1 , and 0.1 mm and 0.5 mm, respectively, while all other cutting parameters were held constant. The arising residual stress distributions are presented and the influence of the varied machining parameters is investigated. Compressive residual stresses dominate the depth profiles but are often accompanied by a thin tensile residual stress layer at the surface. Microstructural investigations of near-surface cross-sections by means of transmission electron microscopy showed a zone where the grains had undergone plastic deformation, indicated by slip bands. On top of this layer, a surface layer exists where the grain size has radically decreased to only 50 nm to 130 nm. The grain size of the nanocrystalline layer is fairly constant for a certain cutting speed and feed, but depends on these cutting parameters. An increase in cutting speed and feed leads to larger grains in the nanocrystalline layer.

Winkelstrangpressen und Hochdrucktorsion lassen sich als einfache Scherung beschreiben. Die Geometrie der einfachen Scherung wird gezeigt. Die Ergebnisse werden angewandt auf Gefüge, die in Kupfer nach Verformung mit diesen beiden Verfahren entstehen. Man erkennt die Entstehung neuer Kristallbereiche während der Verformung.

Aluminium is a fairly recent metal (XIXth century). Investigations of its alloys developed in a military-industrial context. Duralumin was, thus, discovered at the beginning of the last century in Prussian Germany by the Metallurgist Engineer Alfred Wilm (1869 –1937) who had been explicitly mandated to look for an improved aluminium alloy in a military-owned research centre located in Neubabelsberg, just south-west of Berlin. The name Duralumin is as much due to the fact that it is hard as to the fact it has first been industrially produced in Düren. International connections, as well as harsh competition, accompanied the first research on this alloy in the beginning of the twentieth century.