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  • 1
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 503-504 (Jan. 2006), p. 407-412 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: 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
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 503-504 (Jan. 2006), p. 455-460 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: To study the influence of shear deformation on the evolution of the microstructure and the mechanical strength in rail steels, three steels with different microstructure (two pearlitic, one bainitic) were deformed by High Pressure Torsion (HPT). In order to evaluate in addition the effect of the strain path, a cyclic form of HPT was applied. The mechanical strength was determined by means of in-situ measurement of the flow stress and microhardness measurements. The differences of the mechanical strengths between the monotonic and cyclic deformed samples clearly indicate that a monotonic deformation promotes higher dislocation densities and leads to the assumption that dissolution of the cementite takes place more pronounced
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  • 3
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 503-504 (Jan. 2006), p. 621-626 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: 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
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  • 4
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 550 (July 2007), p. 277-282 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: By using techniques of severe plastic deformation a metallic material can be subjected toan enormous strain that is not achievable by conventional methods of deformation. In this studynickel single crystals with different crystallographic orientation and nickel polycrystals weredeformed by high pressure torsion. All nickel samples were processed up to the evolution of asaturation microstructure where no further hardening of the material can be observed. In this regionall samples develop a similar microstructure and micro-texture. The differences in the fragmentationof the microstructure and the micro-texture development between the single crystals and thepolycrystalline aggregate were examined using EBSD. A major difference between single crystalsand polycrystals was not only the microstructure evolution at low equivalent strains, but also thedevelopment of a stable micro-texture, which is achieved earlier by the use of a polycrystallineaggregate
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  • 5
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 558-559 (Oct. 2007), p. 1345-1351 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: A pure OFHC copper is subjected to severe plastic deformation (SPD) by a well defined high pressuretorsion process at ambient temperature. The change in microstructure of samples deformed todifferent strains, up to ε=64, is investigated in-situ, during annealing at 170°C, within a scanningelectron microscope. The spatial distribution of nucleation sites changes significantly withincreasing strain from nucleation at triple junctions and grain boundaries to a random distribution ofsites for von Mises equivalent strains beyond ε=4. The resulting mean size of recrystallized grains isabout 6.75 times larger than the mean microstructural size of the corresponding as-deformed state.For strains larger than ε=16 the recrystallized microstructure appears to be independent ofpreceding strain. A detailed investigation of the nucleation of recrystallized grains following verylarge strains shows that certain microstructural elements are favoured as nuclei and wereparticularly taken into account
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  • 6
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 584-586 (June 2008), p. 215-220 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: A commercially pure niobium has been subjected to SPD at room temperature ( ~0.11TM) via ECAP (90º, route BC) up to 16 passes and via HPT up to shear strains γ =1000. ECAP-edsamples show an equiaxed structure after 8 and 16 passes with a decreasing average grain size. Theresults show that both the microstructure and mechanical properties of ECAP-ed samples do notreach a steady state up to at least 16 passes. HPT samples show at outer region a finer structural sizebut similar hardness values at similar equivalent strains. The nanoindentation results show anevident indentation size-effect even for the most deformed samples. The hardness values at the nanolevel converge for the recrystallized, the ECAP-ed and the HPT samples. This implies that, at thenano level, when the geometrically necessary dislocation density overcomes significantly the(initial) statistically stored dislocation density, hardness depends mainly on the physical intrinsicproperties of the material (Burgers modulus, bulk modulus...) and the contribution of bulkmechanical properties (i.e., bulk yield strength) to hardness is smoothed down. Strain-ratesensitivity (SRS) of plastic strength has been also measured by means of rate-jump nanoindentationtests. The SRS is proportional to the inverse of hardness
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  • 7
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 584-586 (June 2008), p. 655-660 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Severe Plastic Deformation (SPD) is known to be an effective method of producingnanocrystalline materials, for instance by HPT and ECAP. These techniques are also capable ofreproducing microstructures which arise naturally when high pressure and friction is involved, forexample in wheel-rail contact problems. The resulting deformation layers build the origin point forfatigue cracks. For that reason the knowledge of the mechanical properties of these deformationlayers are of vital importance. In the framework of this study a baintic rail steel quality wasdeformed by High Pressure Torsion up to distinctive equivalent strains at a nominal pressure of 6GPa up to a final equivalent strain of 16. Afterwards the evolution of the resulting microstructurewas investigated by Scanning Electron Microscopy, by microhardness measurements and X-raydiffraction. The bainitic structure showed a strong alignment and fragmentation into the sheardirection with increasing strain, which was accompanied by an increase in hardness as well. X-raydiffraction measurements showed that the amount of retained austenite decreases dramatically aftersmall amounts of strain, which indicates that retained austenite cannot be stabilized by highpressures. Torque measurements during deformation showed after strong hardening at thebeginning, a saturation behaviour for higher strains, whereas for instance pearlitic rail steel qualitiesshow further hardening
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  • 8
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 584-586 (June 2008), p. 938-943 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Severe plastic deformation, SPD, enables the grain refinement of bulk materials.However, at strains larger than a critical value, no further microstructural refinement can beobserved. This regime is denoted as saturation region of the microstructural size. It will be shownthat this regime can be divided into a thermal and an athermal part. The transition between thesetwo regimes was examined in an Al-3wt.%Mg alloy. The single phase alloy was deformed by highpressure torsion (HPT) at various temperatures and different rotational speeds. During the HPTdeformationthe 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 ratedependent behavior reveal a shift of the onset of the thermal activated regime towards highertemperatures by an increase of the strain rate
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  • 9
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 584-586 (June 2008), p. 923-928 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Industrial available FeSi, FeCo and FeNi alloys with an initial grain size of 20-50 [removed info]m weresubjected to Severe Plastic Deformation (SPD) up to strain levels where a saturation of themicrostructural refinement is observed. For both SPD conditions, ambient temperature (293 K) andliquid nitrogen temperature (77 K), the microstructure of the severely deformed state is analysed byBack Scattered Electrons (BSE) micrographs captured in a SEM. Additionally, samples that weredeformed at 77 K are examined in a Transmission Electron Microscope (TEM). The magneticproperties were characterised by means of SQUID-magnetography providing information about themagnetization behaviour of the material in the as-processed state. Depending on the SPD conditionsthe mean microstructural sizes in the steady state are about 100-150 nm and 30-80 nm at 293 K and77 K, respectively. The small microstructural sizes influence significantly the magnetic propertiesof these ferritic alloys. The initial soft-magnetic behaviour of the coarse grained state shifts towardsa hard-magnetic with decreasing crystallite size. For crystallite sizes smaller than ~80 nm themagnetic properties become again more soft-magnetic. The experiments show that very lowcoercitivity can be obtained by SPD if the grain size is smaller than ~50 nm
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  • 10
    Electronic Resource
    Electronic Resource
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 584-586 (June 2008), p. 16-21 
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: The improvements in the design of the HPT tools lead to a well defined torsiondeformation and permits, therefore, a comparison with other SPD-techniques. The design of thetools, the advantages and disadvantages of HPT, as well as the limitation in the sample size arediscussed
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