Papers by Author: Hans Jorg Fecht

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Authors: Ke Jing Yang, Yulia Ivanisenko, J. Markmann, Hans Jorg Fecht
Abstract: In situ tensile tests were conducted on ultra fine grained (UFG) pure Pd and Pd-x% Ag (x=20, 60) alloys of different stacking fault energies (SFEs) with the aim to study the general features of the deformation process of UFG materials as well as the peculiarities brought by the alloying. Grey scale correlation analysis (GSCA) was used to determine the true strain as well as the surface flow within the gauge length. It was shown that the largest values of strength and uniform elongation were obtained in Pd-20% Ag alloy. The GSCA revealed different macroscopic flow processes in this sample as compared with pure Pd and Pd-60% Ag alloy. In particular, pure Pd and Pd-60% Ag alloy demonstrated rapid localization of plastic flow in the neck area, whereas Pd-20% Ag samples showed a large contribution of homogenous deformation even after neck formation. It has been proposed that larger strain hardening capacity of Pd-20% Ag alloy is related to its lower SFE as compared with that of pure Pd: the lower is the SFE, the more difficult is the cross slip and climb of split dislocations, which leads to enhanced dislocation storage and, ultimately, to increased strain hardening. At the same time, further decrease of SFE in Pd-60% Ag sample leads to development of deformation twinning and consequent reduction of strain hardening. The dimpled structure of fracture surfaces in the samples will also be discussed in relationship to these findings.
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Authors: Julia Ivanisenko, Ian MacLaren, Ruslan Valiev, Hans Jorg Fecht
Abstract: Recent studies of nanocrystalline materials have often found that the deformation mechanisms are radically different to those in coarse-grained materials, resulting in quite different mechanical properties for such materials. The use of pearlitic steels for the study of the deformation mechanisms in bcc materials with ultrafine grain sizes is quite convenient, because it is relatively straightforward to obtain a homogenous nanocrystalline structure with a mean grain size as small as 10 nm using various modes of severe plastic deformation (SPD). In this paper we show that highpressure torsion of an initially pearlitic steel results in a nanostructured steel in which austenite has been formed at or close to room temperature. The orientation relationship between neighboring ferrite and austenite grains is the well-known Kurdjumov-Sachs orientation relationship, i.e. the same observed in temperature-induced martensitic transformation of iron and steels. It is shown that this must have resulted from a reverse martensitic transformation promoted by the high shear strains experienced by the material during severe plastic deformation of the nanocrystalline structure. This transformation represents an alternative deformation mechanism that can be activated when conventional deformation mechanisms such as slip of lattice dislocations become exhausted.
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Authors: M.P. Macht, N. Wanderka, A. Wiedenmann, H. Wollenberger, Q. Wei, Hans Jorg Fecht, S.G. Klose
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Authors: G. Bürkle, F. Banhart, A. Sagel, C. Wanke, G. Croopnick, Hans Jorg Fecht
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Authors: C. Moelle, I.-R. Lu, A. Sagel, Rainer Wunderlich, John H. Perepezko, Hans Jorg Fecht
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Authors: Lilia Kurmanaeva, Yulia Ivanisenko, J. Markmann, Ruslan Valiev, Hans Jorg Fecht
Abstract: Investigations of mechanical properties of nanocrystalline (nc) materials are still in interest of materials science, because they offer wide application as structural materials thanks to their outstanding mechanical properties. NC materials demonstrate superior hardness and strength as compared with their coarse grained counterparts, but very often they possess a limited ductility or show low uniform elongation due to poor strain hardening ability. Here, we present the results of investigation of the microstructure and mechanical properties of nc Pd and Pd-x%Ag (x=20, 60) alloys. The initially coarse grained Pd-x% Ag samples were processed by high pressure torsion, which resulted in formation of homogenous ultrafine grain structure. The increase of Ag contents led to the decrease of the resulted grain size and change in deformation behavior, because of decreasing of stacking fault energy (SFE). The samples with larger Ag contents demonstrated the higher values of hardness, yield stress and ultimate stress. Remarkably the uniform elongation had also increased with increase of strength.
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Authors: Yulia Ivanisenko, Hans Jorg Fecht
Abstract: We suggest a simple method to study the mechanical behaviour of nanocrystalline (nc) samples in compression-torsion mode. High applied pressure prevents the fracture of sample, and quantitative parameters of sample response during torsion test can be compared with developed microstructure. Here we present and discuss the results of systematic investigation of mechanical behaviour of nc Pd with a mean grain size of 12 nm in a wide range of shear strains (0<γ<200) and at strain rates γ& = 3 10-1 s-1 and 3 10-2 s-1. We show that in the studied shear strain range the notable changes in the microstructure, namely a strain induced grain growth occurs, and that controls the relevant deformation mechanisms. For lower strains when the grain size is still small enough, the plastic flow governs by twinning and probably grain boundary sliding. The flow stresses are lower as compared with the later stages of deformation, when the grain size becomes larger and deformation is controlled exceptionally by dislocation glide. Finally, a steady stage is achieved, when the grain size, dislocation density and flow stress are saturated.
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Authors: Agnieszka Daniszewska, Witold Łojkowski, Hans Jorg Fecht, Krzysztof Jan Kurzydlowski, Urszula Narkiewicz, Gennady A. Salishchev, Michael Zehetbauer, Mariusz Kulczyk, Małgorzata Chmielecka, Dmytro Kuzmenko
Abstract: Current and prospective trends in application of metallic nanomaterials have been studied. The study has been conducted within the Nanoroad SME European project – as the first step for a roadmap for industrial application of nanomaterials. The web page of the project is http://www.nanoroad.net/. The present report presents an analysis of patents, papers, national and European projects in the field of nano-metals, and also an analysis of the present state of research and expected trends in this domain. Based on the performed analysis a data base of nanomaterials has been developed as well as roadmaps with expected time to applications. It can be found under http://bourgogne.arist.tm.fr/nanoroadsme/home/. The roadmap is mainly addressed to SMEs to help them to decide about applications or production of nanomaterials.
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