Papers by Author: Reinhard Pippan

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Authors: Gernot Trattnig, Christof Sommitsch, Reinhard Pippan
Abstract: To understand the crack growth in massive forming and to consequentially avoid crack growth in workpieces, it is necessary to investigate its dependence on the crack depth and thus on the state of hydrostatic stress. Prior work shows that the crack opening displacement (COD) for shallow cracked tension specimens with low stress triaxiality is twice as high as for deep cracked specimens with high stress triaxiality. This work examines the crack growth in compression specimens with pre-cracked cylindrical upsetting samples. The compression samples were cut in the stress symmetry plane in order to observe crack initiation and crack growth by a single specimen technique. In this way it is possible to observe blunting, crack initiation and crack growth inside the upsetting specimens. The resulting COD does not differ significantly from the values achieved in tension samples with short surface cracks.
Authors: Reinhard Pippan, Stephan Scheriau, Anton Hohenwarter, Martin Hafok
Abstract: The improvements in the design of the HPT tools lead to a well defined torsion deformation and permits, therefore, a comparison with other SPD-techniques. The design of the tools, the advantages and disadvantages of HPT, as well as the limitation in the sample size are discussed.
Authors: Anton Hohenwarter, Reinhard Pippan
Abstract: Motivated by the large variety of enhanced properties of ultrafine and nanocrystalline materials such materials are under extensive investigation. Besides focusing on classical material parameters, like strength and ductility, the fracture toughness of these materials is also of great importance, especially when the damage tolerance is required. In this contribution an overview of the fracture behavior of different metals covering ultrafine-grained iron and nickel as well as a nanocrystalline steel processed via high pressure torsion (HPT) will be given. It will be shown that the specimen orientation can have a tremendous influence on the fracture behavior and toughness. Due to this toughness anisotropy an unexpectedly good combination of high strength and high fracture toughness can be achieved very often in these materials.
Authors: Martin Hafok, Reinhard Pippan
Abstract: Severe plastic deformation, SPD, enables the grain refinement of bulk materials. However, at strains larger than a critical value, no further microstructural refinement can be observed. This regime is denoted as saturation region of the microstructural size. It will be shown that this regime can be divided into a thermal and an athermal part. The transition between these two regimes was examined in an Al-3wt.%Mg alloy. The single phase alloy was deformed by high pressure torsion (HPT) at various temperatures and different rotational speeds. During the HPTdeformation the flow stress was measured by a torque cell in a temperature range between -196°C (evaporation temperature of the liquid nitrogen) and 450°C. The temperature and the strain rate dependent behavior reveal a shift of the onset of the thermal activated regime towards higher temperatures by an increase of the strain rate.
Authors: Martin Hafok, Reinhard Pippan
Abstract: By using techniques of severe plastic deformation a metallic material can be subjected to an enormous strain that is not achievable by conventional methods of deformation. In this study nickel single crystals with different crystallographic orientation and nickel polycrystals were deformed by high pressure torsion. All nickel samples were processed up to the evolution of a saturation microstructure where no further hardening of the material can be observed. In this region all samples develop a similar microstructure and micro-texture. The differences in the fragmentation of the microstructure and the micro-texture development between the single crystals and the polycrystalline aggregate were examined using EBSD. A major difference between single crystals and polycrystals was not only the microstructure evolution at low equivalent strains, but also the development of a stable micro-texture, which is achieved earlier by the use of a polycrystalline aggregate.
Authors: Clemens Vichytil, G. Mori, Reinhard Pippan, M. Panzenböck, Rainer Fluch
Abstract: Purpose: Applications for highly corrosive environments and cyclic loading are often made out of austenitic stainless steels. Corrosion fatigue and crack propagation behaviour has been studied to determine failure processes and damage mechanisms. Approach: CrNiMo stabilized austenitic stainless steel and CrMnN austenitic stainless steel in solution annealed and cold worked condition are compared. S/N curves and crack propagation rate curves are recorded in 43 wt% CaCl2 solution at 120 °C, which resembles most severe potential service conditions. For comparison these experiments are also performed in inert glycerine. Additionally, the electrochemical behaviour of these materials has been studied. Findings: The CrMnN steels have excellent mechanical properties but are very susceptible to stress corrosion cracking in the test solution. The fatigue limit as well as the threshold for long crack growth are significantly reduced in corrosive environment. Moreover these steels exhibit a remarkable increase in the propagation rate, which is extremely pronounced in the near threshold region. This effect is enhanced by cold working. CrNiMo steels also show a reduction in the fatigue limit, but it is less pronounced compared to CrMnN steels. The threshold is significantly reduced in corrosive environment, but propagation rate is lower in corrosive environment compared to inert glycerine. Possible explanations of this surprising behaviour are discussed.
Authors: Reinhard Pippan, A. Vorhauer, F. Wetscher, M. Faleschini, Martin Hafok, I. Sabirov
Abstract: Severe plastic deformation (SPD) has been applied to two classes of metallic materials, single phase and dual phase materials. The applied shear strain has been varied between 1 and 1000 and the homologous temperature between 0.08 and 0.4. The deformation experiments are performed by high pressure torsion (HPT). The resulting microstructures were investigated by backscattered electron imaging, orientation image microscopy, and in selected cases by transmission electron microscopy. It will be shown that the behavior of single phase material is relatively uniform. With increasing strain, the size of the structural elements decreases and reaches a saturation between a shear strain of 10 to 100. The temperature and the alloying are the main parameters, which controls the saturation size of the structural elements (grains). The behavior in the dual phase materials is more complex, it varies from simple homogenisation, fragmentation of one phase, to desintegration and supersaturation of the phases.
Authors: Georg B. Rathmayr, Reinhard Pippan
Abstract: Basic research considering the minimum achievable grain size of severe plastic deformation (SPD) materials is often performed with high purity metals. However, a literature study reveals large discrepancies in the microstructural size of high purity nickel processed by SPD. The focus of this work is the influence of small impurity concentrations on the resulting saturation microstructure. The microstructure of SPD nickel was systematically investigated using different purities ranging from 99.79 wt% up to 99.99 wt%. The materials were deformed by high pressure torsion (HPT) at different temperatures from liquid nitrogen temperature (-196°C) up to 400°C. It was found that carbon concentration is the governing element in achieving the finest microstructures and the highest strength. Therefore, further experiments with well defined carbon doped samples were performed. By changing the carbon content from 0.008 wt% up to 0.06 wt% tensile strength in the saturation regime increases by more than 700 MPa. It will be shown that even small variations (<0.01 wt%) lead to significant changes in ductility and tensile strength values.
Authors: I. Sabirov, Thomas Schöberl, Reinhard Pippan
Abstract: A coarse-grained W-25%Cu composite with a tungsten particle size between 2 and 10 μm is subjected to high pressure torsion (HPT) at room temperature to different strains. The evolution of microstructure with increasing strain is studied. At low strains (εeq ≤ 16), an inhomogeneous deformation of the material is found. This inhomogeneity is studied in detail by nanoindentation experiments. At larger strains (εeq ≥ 16), a fragmentation of tungsten particles is observed. The specimen deformed to a strain of 256 exhibits a homogeneous microstructure with a tungsten particle size between 10 and 20 nm. A further increase of strain does not cause a further significant tungsten particle refinement. A possibility of industrial application of HPT to produce nanocomposite materials is discussed.
Authors: Sergey V. Dobatkin, V.I. Kopylov, Reinhard Pippan, O.V. Vasil'eva
Abstract: At present, the possibility of the formation of high-angle grain boundaries upon severe cold deformation, in particular, equal-channel angular (ECA) pressing is reliably proved. The structure formation upon multi-cycle ECA pressing substantially depends on the route determining the shear plane in the sample upon repeated passes. The route is defined by the rotation of the sample around its axis upon the multi-cycle ECA pressing. There are four main routes: route A, in which the sample is deformed by many passes without any rotations; route Ba, in which the sample is rotated by ± 90°; route Bc, in which the sample is sequentially rotated in the same direction by 90°and route C, in which the sample is rotated by 180° about its axis before each subsequent pass. By the methods of SEM, TEM and EBSD analysis it was shown that the fraction of high-angle boundaries in a-Fe upon cold ECA pressing with an angle of 90° between the channels and N=4 depends on the deformation route and increases according to the route sequence: Ba-C-Bc.
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