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Evolution of microstructure and hardness in AZ31 alloy processed by high pressure torsion
Strá,ská,, Jitka,Janeč,ek, Miloš,Gubicza, Jenő,Krajň,á,k, Tomá,š,Yoon, Eun Yoo,Kim, Hyoung Seop Elsevier 2015 Materials science & engineering. properties, micro Vol.625 No.-
<P><B>Abstract</B></P> <P>A commercial MgAlZn alloy (AZ31) was processed by high pressure torsion (HPT) at room temperature, resulting in an extreme microstructure refinement down to the grain size of 150–250nm. The microstructure evolution during HPT was investigated by transmission electron microscopy and X-ray diffraction line profile analysis. The microhardness was measured as a function of the distance from the center of the disk and the number of HPT revolutions. The detailed analysis of dislocation contrast factors in X-ray diffraction line profiles enables to determine the population of the different slip systems as a function of the imposed strain. The influence of microstructure and defect structure evolution on microhardness is discussed in detail.</P>
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Zá,bori, J.,Rastak, N.,Yoon, Y. J.,Riipinen, I.,Strö,m, J. Copernicus GmbH 2015 Atmospheric Chemistry and Physics Vol.15 No.23
<P>Abstract. The Arctic is one of the most vulnerable regions affected by climate change. Extensive measurement data are needed to understand the atmospheric processes governing this vulnerability. Among these, data describing cloud formation potential are of particular interest, since the indirect effect of aerosols on the climate system is still poorly understood. In this paper we present, for the first time, size-resolved cloud condensation nuclei (CCN) data obtained in the Arctic. The measurements were conducted during two periods in the summer of 2008: one in June and one in August, at the Zeppelin research station (78°54´ N, 11°53´ E) in Svalbard. Trajectory analysis indicates that during the measurement period in June 2008, air masses predominantly originated from the Arctic, whereas the measurements from August 2008 were influenced by mid-latitude air masses. CCN supersaturation (SS) spectra obtained on the 27 June, before size-resolved measurements were begun, and spectra from the 21 and 24 August, conducted before and after the measurement period, revealed similarities between the 2 months. From the ratio between CCN concentration and the total particle number concentration (CN) as a function of dry particle diameter (Dp) at a SS of 0.4 %, the activation diameter (D50), corresponding to CCN / CN = 0.50, was estimated. D50 was found to be 60 and 67 nm for the examined periods in June and August 2008, respectively. Corresponding D50 hygroscopicity parameter (κ) values were estimated to be 0.4 and 0.3 for June and August 2008, respectively. These values can be compared to hygroscopicity values estimated from bulk chemical composition, where κ was calculated to be 0.5 for both June and August 2008. While the agreement between the 2 months is reasonable, the difference in κ between the different methods indicates a size dependence in the particle composition, which is likely explained by a higher fraction of inorganics in the bulk aerosol samples. </P>
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Č,í,ž,ek, J.,Janeč,ek, M.,Krajň,á,k, T.,Strá,ská,, J.,Hruš,ka, P.,Gubicza, J.,Kim, H.S. Elsevier 2016 Acta materialia Vol.105 No.-
<P>Interstitial free steel with ultrafine-grained (UFG) structure was prepared by high-pressure torsion (HPT). The development of the microstructure as a function of the number of HPT turns was studied at the centre, half-radius and periphery of the HPT-processed disks by X-ray line profile analysis (XLPA), positron annihilation spectroscopy (PAS) and electron microscopy. The dislocation densities and the dislocation cell sizes determined by XLPA were found to be in good agreement with those obtained by PAS. The evolution of the dislocation density, the dislocation cell and grain sizes, the vacancy cluster size, as well as the high-angle grain boundary (HAGB) fraction was determined as a function of the equivalent strain. It was found that first the dislocation density saturated, then the dislocation cell size reached its minimum value and finally the grain size got saturated. For very high strains after the saturation of grain size the HAGB fraction further increased. The PAS investigations revealed that vacancies introduced by severe plastic deformation agglomerated into small clusters consisting of 9-14 vacancies. The evolution of the yield strength calculated from the microhardness as a function of strain was explained by the development of the defect structure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>
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