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Sankt Stefan ob Leoben, Austria

Toth F.,Vienna University of Technology | Rammerstorfer F.G.,Vienna University of Technology | Cordill M.J.,Erich Schmid Institute of Materials Science | Cordill M.J.,University of Leoben | Fischer F.D.,University of Leoben
Acta Materialia | Year: 2013

Tensile specimens of metal films on compliant substrates are widely used for determining interfacial properties. These properties are identified by the comparison of experimentally observed delamination buckling and a mathematical model which contains the interface properties as parameters. The current two-dimensional models for delamination buckling are not able to capture the complex stress and deformation states arising in the considered uniaxial tension test in a satisfying way. Therefore, three-dimensional models are developed in a multi-scale approach. It is shown that, for the considered uniaxial tension test, the buckling and associated delamination process are initiated and driven by interfacial shear in addition to compressive stresses in the film. The proposed model is able to reproduce all important experimentally observed phenomena, like cracking stress of the film, film strip curvature and formation of triangular buckles. Combined with experimental data, the developed computational model is found to be effective in determining interface strength properties.

Borchers C.,University of Gottingen | Garve C.,University of Gottingen | Tiegel M.,University of Gottingen | Deutges M.,University of Gottingen | And 7 more authors.
Acta Materialia | Year: 2015

Steel powders obtained by mechanical alloying of iron and graphite were compacted by high-pressure torsion. During high-pressure torsion, mean grain sizes rise from about 10 nm after mechanical alloying to about 20 nm. Vickers hardness reaches values of more than 10 GPa. Differential scanning calorimetry revealed superabundant vacancies present in concentrations up to 10-3. The results are discussed in terms of strain, strain rate, energy input and deformation mechanisms. © 2015 Acta Materialia Inc.

Ghosh P.,Erich Schmid Institute of Materials Science | Chokshi A.H.,Indian Institute of Science
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2015

During the transition from single crystalline to polycrystalline behavior, the available data show the strength increasing or decreasing as the number of grains in a cross section is reduced. Tensile experiments were conducted on polycrystalline Ni with grain sizes (d) between 16 and 140 μm and varying specimen thickness (t), covering a range of λ (=t/d) between ~0.5 and 20. With a decrease in λ, the data revealed a consistent trend of strength being independent of λ at large λ, an increase in strength, and then a decrease in strength. Microstructural studies revealed that lower constraints enabled easier rotation of the surface grains and texture evolution, independent of the specimen thickness. In specimen interiors, there was a greater ease of rotation in thinner samples. Measurements of misorientation deviations within grains revealed important differences in the specimen interiors. A simple model is developed taking into account the additional geometrically necessary dislocations due to variations in the behavior of surface and interior grains, leading to additional strengthening. A suitable combination of this strengthening and surface weakening can give rise to wide range of possibilities with a decrease in λ, including weakening, strengthening, and strengthening and weakening. © 2015 The Minerals, Metals & Materials Society and ASM International

Krawczynska A.T.,Warsaw University of Technology | Lewandowska M.,Warsaw University of Technology | Pippan R.,Erich Schmid Institute of Materials Science | Kurzydlowski K.J.,Warsaw University of Technology
Journal of Nanoscience and Nanotechnology | Year: 2013

In the present study, the high pressure torsion (HPT) was used to refine the grain structure down to the nanometer scale in an austenitic stainless steel. The principles of HPT lay on torsional deformation under simultaneous high pressure of the specimen, which results in substantial reduction in the grain size. Disks of the 316LVM austenitic stainless steel of 10 mm in diameter were subjected to equivalent strains ε of 32 at RT and 450 °C under the pressure of 4 GPa. Furthermore, two-stage HPT processes, i.e., deformation at room temperature followed by deformation at 450 °C, were performed. The resulting microstructures were investigated in TEM observations. The mechanical properties were measured in terms of the microhardness and in tensile tests. HPT performed at two-stage conditions (firstly at RT next at 450 °C) gives similar values of microhardness to the ones obtained after deforming only at 450 °C but performed to higher values of the overall equivalent strain ε. The effect of high pressure torsion on structural refinement and mechanical properties of an austenitic stainless steel was evaluated. Copyright © 2013 American Scientific Publishers.

Renk O.,Erich Schmid Institute of Materials Science | Hohenwarter A.,University of Leoben | Schuh B.,University of Leoben | Li J.H.,University of Leoben | Pippan R.,Erich Schmid Institute of Materials Science
IOP Conference Series: Materials Science and Engineering | Year: 2015

In contrast to the general notion about the annealing behavior of coarse grained materials, hardening phenomena in nanocrystalline materials can occur. Although the phenomena have already been recognized several years ago, the mechanisms behind are still controversially discussed. For example, the influence of solutes segregated to grain boundaries on the strengthening mechanism is unclear. We present a combination of atom probe tomography and mechanical data to reveal the role of segregations to the strengthening. The results show that despite large modifications of the boundary chemistry the mechanical behavior remains widely unaffected. Additionally, it will be shown that hardening upon annealing can only occur below a material-specific grain size threshold value. © Published under licence by IOP Publishing Ltd.

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