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Yang B.,Materials Center Leoben Forschung | Vehoff H.,Saarland University | Pippan R.,Austrian Academy of Sciences | Pippan R.,CD Laboratory for Local Analysis of Deformation and Fracture
Materials Science Forum | Year: 2010

A summary of experimental results from nanoindentation, strain rate-controlled tension, in-situ bending and high pressure torsion on bulk electrodeposited nanocrystalline nickel, focusing on the effects of grain size on the mechanical behaviour and deformation mechanisms is presented. The interaction between dislocations and grain boundaries was locally examined by studying the dependence of nanohardness on grain size and indentation size; this is done by always performing nanoindents in the center of individual grains and by varying the grain size and indentation depth systematically. The grain size effects on the different deformation mechanisms of nanocrystalline nickel were revealed by strain rate-controlled tension and nanoindentation experiments, which show that with decreasing grain size the strain rate sensitivity increases and the activation volume decreases, indicating increased grain boundary mediated deformation processes in nanocrystalline nickel. Creep experiments at room temperature revealed that in nanocrystalline nickel grain boundary sliding or diffusion along the interface may dominate at lower stress levels, but with increasing stresses the deformation process is mainly controlled by dislocation creep. In-situ bending experiments in an atomic force microscope revealed directly that grain boundary mediated deformation processes play a significant role in nanocrystalline nickel, which is also supported by the observation of grain coarsening and softening of nanocrystalline nickel caused by high pressure torsion. © (2010) Trans Tech Publications.


Hohenwarter A.,Austrian Academy of Sciences | Hohenwarter A.,CD Laboratory for Local Analysis of Deformation and Fracture | Taylor A.,Austrian Academy of Sciences | Stock R.,Voestalpine AG | And 2 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2011

High-pressure torsion (HPT) has been used for investigating the influence of predeformation on the fracture toughness of a fully pearlitic rail steel. The use of HPT enables one to investigate changes in fracture toughness as a function of predeformation over a wide range of strain while simultaneously studying the influence of the crack plane orientation on the fracture toughness. With increasing prestrain, besides a strong increase in hardness, a pronounced anisotropy in the fracture toughness was found. Both the increase in hardness and the anisotropic fracture behavior can be attributed to the shear deformation process leading to an anisotropic composite structure on the nanometer scale. © 2010 The Minerals, Metals & Materials Society and ASM International.


Gludovatz B.,CD Laboratory for Local Analysis of Deformation and Fracture | Gludovatz B.,Austrian Academy of Sciences | Wurster S.,Austrian Academy of Sciences | Hoffmann A.,Plansee Group | And 2 more authors.
International Journal of Refractory Metals and Hard Materials | Year: 2010

Tungsten and tungsten alloys show the typical change in fracture behavior from brittle at low temperatures to ductile at high temperatures. In order to improve the understanding of the effect of microstructure the fracture toughness of pure tungsten, potassium doped tungsten, tungsten with 1 wt.% La 2O3 and tungsten rhenium alloys were investigated by means of 3-point bending, double cantilever beam and compact tension specimens. All these materials show the expected increase in fracture toughness with increasing temperature. The experiments demonstrate that grain size, texture, chemical composition, grain boundary segregation and dislocation density seem to have a large effect on fracture toughness below the DBTT. These influences can be seen in the fracture behavior and morphology, where two kinds of fracture occur: on the one hand transgranular and on the other hand intergranular fracture. Therefore, techniques like electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES) and X-ray line profile analysis were used to improve the understanding of the parameters influencing fracture toughness. © 2010 Elsevier Ltd. All rights reserved.


Hohenwarter A.,Austrian Academy of Sciences | Hohenwarter A.,CD Laboratory for Local Analysis of Deformation and Fracture | Kammerhofer C.,Austrian Academy of Sciences | Kammerhofer C.,CD Laboratory for Local Analysis of Deformation and Fracture | And 2 more authors.
Journal of Materials Science | Year: 2010

Fracture toughness measurements on bcc iron (Armco-iron), which is subjected to severe plastic deformation (SPD), were performed. Through high pressure torsion, an ultrafine grain structure was obtained and with subsequent heat treatments the grain size varied between 300 nm and 5 μm. The combination of SPD and individual heat treatments allows for a systematic study of the ductile to brittle transition (DBT) in the fracture behaviour as a function of grain size. Additionally, the influence of different crack plane orientations was taken into account. The results show that the DBT moves for smaller grain sizes (<1 μm) to higher transition temperatures. Furthermore, large differences in the absolute toughness values for a given temperature for the different crack plane orientations and grain sizes were determined. The findings can be related to a change in the crack path from transcrystalline fracture for grain sizes larger than 1 μm to intercrystalline-dominated fracture for grain sizes smaller than 1 μm. © 2010 Springer Science+Business Media, LLC.


Hohenwarter A.,Austrian Academy of Sciences | Hohenwarter A.,CD Laboratory for Local Analysis of Deformation and Fracture | Pippan R.,Austrian Academy of Sciences | Pippan R.,CD Laboratory for Local Analysis of Deformation and Fracture
Materials Science and Engineering A | Year: 2010

Fracture toughness measurements with ultrafine-grained bcc iron produced by high pressure torsion (HPT) are reported. The measurements were performed with respect to three different crack plane orientations, which showed pronounced differences in fracture toughness as well as in the appearance of the fracture surfaces. The mechanical anisotropy was found to be a result of the elongated and aligned submicrocrystalline microstructure. This causes intergranular fracture for the crack plane orientation of lowest toughness, simultaneously favoring a higher fracture toughness for the other specimen orientations. Since this mechanical anisotropy led to one crack plane orientation with a limited fracture toughness, a strategy for increasing the fracture toughness of this orientation is also presented. © 2010 Elsevier B.V. All rights reserved.

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