Norbert Jost

Friction-induced martensitic transformation in austenitic manganese steels☆

Abstract The sliding wear behaviour of various metastable Fe-Mn-C austenites as well as that of the classic Hadfield steel is investigated. The frictioninduced martensitic transformation and work hardening of the austenite in surfaces exposed to friction are analysed using X-ray diffraction, light microscopy and scanning and transmission electron microscopies. It becomes evident that the initial hardness of the austenitic Fe-Mn-C alloys is not a useful property to predict the wear resistance. Instead of this, the hardness of the work-hardened surface layer in connection with the particular wear mechanism has to be considered. Thin martensitic surface layers induced by frictional forces considerably increase the sliding wear resistance; thick layers which have been transformed to martensite, however, can initiate failure because of brittle fracture. This depends on the degree of metastability of the austenite as well as on the applied load.

Mesostructural Design and Manufacturing of Open-Pore Metal Foams by Investment Casting

The present paper describes the manufacturing process of open-pore metal foams by investment casting and the mesostructural/morphological evolution resulting from a new technique of modifying the precursor. By this technique, the precursor is coated with a polymer layer whereby a thickening of the struts occurs. Relative densities in the range of of open-pore metal foams can be achieved with high accuracy. The samples investigated have pore densities of ppi, 10 ppi, and 13 ppi. The relevant processing parameters needed for a homogenous formation of the polymer layer are determined for two different coating materials and the resulting open-pore foam’s mesostructure is characterized qualitatively and quantitatively. The alloy used for investment casting open-pore metal foamsis AlZn11. The microstructural evolution of these foams is evaluated as a function of the mesostructure. Differences in the microstructure are observed for foams with low and high relative densities and discussed in terms of cooling subsequent to investment casting.

Thermal fatigue of Fe–Ni–Co–Ti shape-memory-alloys

Abstract Thermal fatigue was investigated for the shape-memory-alloy system Fe–Ni–Co–Ti. Due to the lack of experimental data special interest was laid on the microstructures and transformation properties before and after thermal fatigue. Specimens were aged in the austenitic state in order to get maximum austenite-hardness and, thereby, a thermoelastic transformation behaviour. After this thermal cycling was carried out in the temperature range −196–+300°C. Dilatometric measurements as well as detailed microscopic investigations gave information about the transformation behaviour, change of transformation temperatures and the corresponding microstructures as well as the hardness of the austenite. It can be shown that the fatigue behavior corresponds in a characteristic way with the degree of the M s -temperature as well as the temperature hysteresis between complete forward and re-transformation Δ T H . Also, the microstructures which can be obtained after a different number of cycles show a distinct dependence on these parameters. The results give, for the first time, very useful information about the optimal conditions for the application of these new shape-memory-alloys and, thereby, for the maximum expected operating lifetime during fatigue.

Effective thermal conductivity of open-pore metal foams as a function of the base material

Abstract The effective thermal conductivity of open-pore metal foams in combination with the fluids air and water have been investigated in an extended range in relative density and selection of material. This study is conducted to estimate the influence of the thermal conductivities of the combination “metal foam — fluid” λs and λfl on the effective thermal conductivity λe of the open-pore metal foam. Therefore, open-pore metal foams (ρrel = 12.7 % in average) of different base materials are manufactured by respect of significant differences in the thermal conductivity of their bulk material in a range of 24.80 W × (m × K)−1≤λs≤ 402.13 W × (m × K)−1. These samples are saturated by air and water and the effective thermal conductivities of the corresponding combinations are determined. The thereto used method is a transient one and is based on the theory of inturbide temperature fields. The impact of the fluid type on λe is evaluated and its dependence on λs is identified, resulting in a simple expression for estimating the effective thermal conductivity as a function of λfl, λs and ρrel applicable for air and water.

Microstructural Characterization of Open-Pore Metal Foams of Pure Metals

Abstract This work presents open-pore metal foams made by investment casting from several pure metals. The resulting microstructure and its morphology will be characterized and compared to the microstructure of permanent mold cast round specimens. In addition to their manufacture, the preparation of the different samples for microstructural characterization is outlined. Finally, the individual microstructures are analyzed and compared by means of light microscopy and under consideration of relevant parameters. The comparison of metal foam and round specimen microstructures yields significant differences in the microstructural development.

A Three-Dimensional Microstructure Preparation Using Metallographic In-Depth Microsections

Abstract Efficient simulation techniques are conceived, amongst other things, for the modelling and visualization of microstructure configurations of materials and structural components in production processes, within a joint research project in which the Karlsruhe, Aalen, and Pforzheim Universities were involved. To this end, it is firstly necessary to determine the structure in all of the three spatial directions (three dimensions) which then will help to establish mathematical simulation algorithms. In the further course of the project, it is also understood that the simulation results are checked up and verified again and again by the respective real structures. These activities indispensably presuppose a precise localization of the structure on several planes and a definition as to what is realized via so-called in-depth microsections. This makes it necessary to develop and optimize techniques both for the exact maintenance of an x-y local constancy and absolutely accurate depth determination (z coordinate). For this purpose, the Pforzheim University formulated a simple metallographic process technology which is aimed at precisely localizing the structures at a well-defined material removal rate. It is on this basis that structural regions can now be detected on several planes at the same location and a sufficient collection of data can be created for the three-dimensional simulations of polycrystalline grain structures.

Production and Metallographic Examination of Precipitable Cu–Mg Alloys

Abstract Low alloy copper-based alloys are mainly used as conductive material in the field of electrical engineering. However, given a good electrical conductivity, they are characterized by low mechanical strength. Due to a low alloy content of currently used copper-magnesium, alloys rank among not precipitable homogeneous solid solutions. As a consequence, increases in strength can only be realized by means of solid solution strengthening and strain hardening. Up to now, the possibility of precipitation hardening remains largely untapped. In order to examine this potential for optimization regarding higher strength and enhanced conductivity, samples with differing Mg contents are prepared and systematically heat treated. This research work has the long-term objective of leaving the laboratory scale in order to make precipitable copper-magnesium alloys applicable for industry.

Heat propagation in computer designed and real metal foam structures

Purpose The purpose of this paper is to demonstrate a method for modeling of cellular structures by means of Voronoi tessellation and to conduct a validation by comparison with real metal foam structures. Design/methodology/approach Heat propagation behavior of open-pore metal foams is studied for both experimental as well as computer-modeled structures showing excellent agreement. The 3D open-pore structure of the real foam is reconstructed from 2D light microscope images in-depth. Findings An algorithm to create synthetic open-pore foam structures has been developed. Originality/value The algorithm for modeling synthetic open-pore cellular structures allows the random distribution of the individual pores close to reality.

Analysis and 3D-Modelling of Open-Pore Metal Foams Using Binarized Light-Optical Microscopy In-Depth Microsections

Abstract Open-porous metal foams are cellular structures that can be defined, in simplified terms, as interwoven networks of metal ligaments that are surrounded by a fluid (liquid or gaseous). They are characterized by a very low relative density of usually 4 to 12% of the density of a solid composed of an identical base material. As such they have a small volume along with a very large surface area, providing for numerous interesting physical properties with resulting practical applications. A detailed description of the physical properties of these materials essentially requires a characterization of their geometric structure. Within the scope of this work, microsections of these foam structures are captured in a defined depth distance of sz≤0,5 mm. In the course of the experiment, these microsections are software-converted to binary images. They are optimized in a way that artifact characteristics, which would have a negative effect on the analysis, converge to a minimum. Based hereon, it is now possible to generate realistic 3D models of individual microsections, allowing for a three-dimensional measurement of the material. Furthermore, binary format microsections allow for determining miscellaneous statistical characteristic values of the examined foam structures. Based on these results, it is consequently very easy to draw conclusions on the geometry of large-dimensional foam structures. When varying individual characteristic parameters, such a database also provides the possibility to generate differing geometric structures, which, in turn, provide for a sound basis in order to create scalable simulation models.

Modell eines Roboters aus Formgedächtnismetall

ZusammenfassungIm Fahmen diese Beitrags wird des Modell eines Roboters vorgestellt, der seine Bewegung ohne jegliche elektromotorische oder pneumatische Antriebe aufsührt. Entsprechende Bauelemente sind übedflüszig, da der Werkstoff, aus dem der Raboter gefertigt ist, die Greiffunktion selbst durchführt. Dies geschieht über das Signal einer Temperaturänderung, so daß die Bauteile lediglich erwärmt oder abgekühll werden müssen, um die Greifbewegung zu realisieren. Diese Eigenschaft von Werkstoffen nennt man «Formgedächtnis»: Der Werkstoff erinnert sich bei Temperaturänderungen an seine vorherige Form und nimmt diese selbständig an. Es handelt sich dabei, um eine völlig neue Werkstoffeigenschaft, nicht um eine lediglich verbesserte. Der Formgedächtniseffekt ist hinsichtlich seiner Ursachen sehr komplex und neu, so daß zum besseren Verständnis die Grundlagen des Effektes vertieft in diesem Bericht beschrieben werden.