Papers by Author: Martin Hafok

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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: 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: 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: Martin Hafok, A. Vorhauer, Jozef Keckes, Reinhard Pippan
Abstract: Copper and nickel single crystals of high purity with a crystallographic orientation, (001) and (111) respectively, were deformed by applying high pressure torsion (HPT) at room temperature. Special interest was devoted to the structural evolution of the material, which was characterized by electron backscatter diffraction (EBSD) and X-ray texture analysis as well. In addition back scatter electron investigations were applied to characterize shape and size of the new formed structure. Furthermore the study is focused on the micro structural and micro textural evolution that lead to the increase of missorientation angle with increasing plastic deformation. We observed an increasing fragmentation of the structure with increasing plastic equivalent strain up to a level where the grain size is saturated. The saturation could be traced back to dynamical recovery and recrystallisation during the deformation process that is depending on the purity of the material.
Authors: Jon Alkorta, C.J. Luis-Pérez, E.N. Popova, Martin Hafok, Reinhard Pippan, J. Gil Sevillano
Abstract: A commercially pure niobium has been subjected to SPD at room temperature ( ~0.11 TM) via ECAP (90º, route BC) up to 16 passes and via HPT up to shear strains γ =1000. ECAP-ed samples show an equiaxed structure after 8 and 16 passes with a decreasing average grain size. The results show that both the microstructure and mechanical properties of ECAP-ed samples do not reach a steady state up to at least 16 passes. HPT samples show at outer region a finer structural size but similar hardness values at similar equivalent strains. The nanoindentation results show an evident indentation size-effect even for the most deformed samples. The hardness values at the nano level converge for the recrystallized, the ECAP-ed and the HPT samples. This implies that, at the nano level, when the geometrically necessary dislocation density overcomes significantly the (initial) statistically stored dislocation density, hardness depends mainly on the physical intrinsic properties of the material (Burgers modulus, bulk modulus...) and the contribution of bulk mechanical properties (i.e., bulk yield strength) to hardness is smoothed down. Strain-rate sensitivity (SRS) of plastic strength has been also measured by means of rate-jump nanoindentation tests. The SRS is proportional to the inverse of hardness.
Authors: Siegfried Kleber, Martin Hafok
Abstract: The investigated super duplex steel belongs to the group of stainless steels which exhibits an austenitic-ferritic microstructure with a phase fraction of about 50% austenite and 50% ferrite. The alloy shows excellent general corrosion resistance as well as a good resistance against stress corrosion cracking, corrosion fatigue and erosion corrosion. Due to these outstanding properties, the super duplex alloy is used in components for sea or waste water applications and in the offshore and chemical industry. In addition, the investigated super duplex steel exhibits a good weldability and a high strength in comparison to pure austenitic steel grades In order to optimize the production process and to provide a suitable microstructure to satisfy the customer’s requirements multiaxial forging test at various temperatures were performed in the Gleeble Maxstrain system. The force and the displacement after each anvil stroke were measured and used to distinguish the mechanical behaviour in the forging process at different thermal conditions. The recorded force and displacement is also compared with a multi step compression test to show the influence of change in the deformation direction. A certain number of samples were exposed an in-situ heat treatment after the deformation while other samples were immediately quenched after the forging to preserve the deformed microstructure, which was measured by optical microscopy and electron microscopy. Furthermore, electron back scatter diffractions scans were applied to characterize the degree of dynamic recrystallization during the forging process.
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