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Budapest, Hungary

Skrotzki W.,TU Dresden | Eschke A.,TU Dresden | Joni B.,Budapest Eotvos University | Ungar T.,Budapest Eotvos University | And 4 more authors.
Acta Materialia | Year: 2013

The evolution of microstructure and texture of a nanocrystalline Pd-10 at.% Au alloy (initial grain size 16 nm) subjected to severe plastic deformation by high-pressure torsion (HPT) at room temperature is investigated by X-ray line profile analysis and X-ray microdiffraction, respectively. In addition, changes in the microhardness are measured and the texture is modeled. During HPT the microstructure changes: the crystallite size goes over the maximum, the dislocation density goes through a minimum and the density of stacking faults decreases at/up to a shear strain of ∼1, corresponding to a grain size of 20 nm. Starting with a random texture, typical brass-type shear components develop at a shear strain above ∼1. The microhardness with decreasing crystallite size goes over a maximum at ∼20 nm. The correlated changes in microstructure, texture and strength strongly suggest the transition from a dislocation slip to a grain boundary sliding (GBS)-dominated deformation mechanism. The unexpected brass-type texture and its deviation from the ideal position can be simulated with the Taylor model assuming dominant partial dislocation slip and a certain contribution of GBS, respectively. Taken together, the results of many techniques applied to the same material, in particular those of the texture investigations, provide a more comprehensive and consistent picture of nanoplasticity than reported before for face-centered cubic metals. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Joni B.,Budapest Eotvos University | Schafler E.,University of Vienna | Zehetbauer M.,University of Vienna | Tichy G.,Budapest Eotvos University | Ungar T.,Budapest Eotvos University
Acta Materialia | Year: 2013

Niobium and tantalum are two body centered cubic metals with very different elastic anisotropy. The Az = 2 × c44/(c 11-c12) constant for Nb and Ta is 0.51 and 1.58, respectively. The submicron grain-size state of the two refractory metals was produced by the method of high-pressure torsion with different pressure values of 2 and 4 GPa for Nb, and 4 and 8 GPa for Ta, and two different deformations of 0.25 and 1.5 rotations, respectively, with equivalent strains of up to ∼40. The dislocation density and the grain size were determined by high-resolution diffraction peak-profile analysis. The beam size on the specimen surface was 0.2 × 1 mm, allowing the sub-structure along the radius of the specimen to be characterized. The strength of the two metals was correlated with the dislocation density and the grain size. It is found that, though the grain size is well below 100 nm, the role of dislocations in the flow stress of these two metals is significantly greater than that of the grain size. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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