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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.


Maierhofer J.,Materials Center Leoben Forschung | Ganser H.-P.,Materials Center Leoben Forschung | Pippan R.,Erich Schmid Institute of Materials Science
Materialwissenschaft und Werkstofftechnik | Year: 2014

Defects in axles lead to a significant reduction of lifetime. Deep rolling provides a simple way to prevent crack growth emanating from defects or at least to slow down the crack growth rate, provided the flaw size does not exceed the size of the zone affected by the compressive residual stresses. In this work, a simple process model is presented for estimating the penetration depth of compressive residual stresses from deep rolling. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


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.


Renk O.,Erich Schmid Institute of Materials Science | Hohenwarter A.,University of Leoben | Wurster S.,University of Leoben | Pippan R.,Erich Schmid Institute of Materials Science
Acta Materialia | Year: 2014

Ultra-fine-grained high-purity copper (99.99%) deformed by means of high-pressure torsion into the steady-state regime was subjected to additional rolling deformation. The microstructural changes as a function of the applied strain were analysed by means of orientation imaging microscopy. It was found that after a distinctive rolling strain a steady state with respect to microstructural features such as grain size, misorientation distribution and texture evolves again. A special spilt specimen technique was used to perform quasi in situ observations of the microstructure between additional strain increments. Profound insights into the local deformation and restoration processes within the steady-state regime were gained. The observations lead to the conclusion that grain boundary migration perpendicular to the rolling direction leads to the disappearance of certain grains, enabling the occurrence of a steady state. © 2014 Acta Materialia Inc. Published by Elsevier Ltd.


Renk O.,Erich Schmid Institute of Materials Science | Hohenwarter A.,Erich Schmid Institute of Materials Science | Pippan R.,Erich Schmid Institute of Materials Science
Advanced Engineering Materials | Year: 2012

The influence of severe plastic deformation (SPD) on the fatigue behavior of a modified 316L austenitic stainless steel is investigated. Different ultrafine-grained and nanocrystalline microstructures are obtained by changing the processing parameters and applying a post heat treatment procedure. Samples are fatigued using both, load and strain controlled experiments. High pressure torsion processing makes it possible to reach a saturation microstructure, which is cyclically stable up to a stress level three times higher than the stress level of the coarse-grained structure. Fracture surface investigations and surface damage clearly show that the failure behavior of the SPD states under cyclic loading is different to their coarse-grained counterparts. For these microstructures, localized deformation in shear bands seems to play a major role for crack initiation and propagation. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Renk O.,Erich Schmid Institute of Materials Science | Hohenwarter A.,University of Leoben | Eder K.,University of Sydney | Kormout K.S.,Erich Schmid Institute of Materials Science | And 2 more authors.
Scripta Materialia | Year: 2015

Hardening phenomena in nanocrystalline metals after annealing have been widely reported, and the subject of much recent debate. Solute segregation to grain boundaries and dislocation source hardening have been proposed to cause the strengthening. To shed light on the dominant mechanisms, we present results from mechanical experiments and atom probe tomography on samples with similar grain size but different amounts of solute segregation and different boundary chemistries. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


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.


Maierhofer J.,Materials Center Leoben Forschung | Maierhofer J.,Erich Schmid Institute of Materials Science | Pippan R.,Erich Schmid Institute of Materials Science | Ganser H.-P.,Materials Center Leoben Forschung
International Journal of Fatigue | Year: 2014

A typical fatigue crack growth curve consists of the threshold region, the Paris region (linear in a logarithmically scaled diagram) and the transition region from the Paris region to unstable crack growth. For cracks exceeding a certain material-dependent length, this curve depends only on the load ratio R and is well described by commonly accepted crack growth models such as the Forman/Mettu (NASGRO) equation. However, cracks below this length typically grow significantly faster due to the absence of crack-closure effects, leading to an additional dependence of the crack growth curve on the crack extension Δa. In this paper, a simple analytical model for describing the crack growth behavior for any crack length and load ratio R is presented. For the QT steel 25CrMo4, the model is applied to describe the crack growth behavior for different crack length and load ratios between -3 and 0.5. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.


PubMed | Erich Schmid Institute of Materials Science and University of Leoben
Type: Journal Article | Journal: Acta materialia | Year: 2014

Ultra-fine-grained high-purity copper (99.99%) deformed by means of high-pressure torsion into the steady-state regime was subjected to additional rolling deformation. The microstructural changes as a function of the applied strain were analysed by means of orientation imaging microscopy. It was found that after a distinctive rolling strain a steady state with respect to microstructural features such as grain size, misorientation distribution and texture evolves again. A special spilt specimen technique was used to perform quasi in situ observations of the microstructure between additional strain increments. Profound insights into the local deformation and restoration processes within the steady-state regime were gained. The observations lead to the conclusion that grain boundary migration perpendicular to the rolling direction leads to the disappearance of certain grains, enabling the occurrence of a steady state.

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