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Strength enhancement of high entropy alloy HfNbTaTiZr by severe plastic deformation
x10c,í,x17e,ek, J.,Haux161,ild, P.,Cieslar, M.,Melikhova, O.,Vlasá,k, T.,Janeč,ek, M.,Krá,l, R.,Harcuba, P.,Luká,x10d,, F.,Zý,ka, J.,Má,lek, J.,Moon, J Elsevier 2018 Journal of alloys and compounds Vol.768 No.-
<P><B>Abstract</B></P> <P>Refractory metal high entropy alloy HfNbTaTiZr with ultrafine grained structure and grain size of ≈80 nm was processed by high pressure torsion. The development of microstructure, lattice defects and mechanical properties with increasing strain was examined. Grain refinement of HfNbTaTiZr alloy deformed up to the equivalent strain <I>e</I> ≈ 50 resulted in a significant enhancement of strength while keeping sufficient ductility. However, further straining <I>e</I> > 100 led to a decrease of strength and the loss of ductility due to the decomposition of solid solution facilitated by vacancies introduced by severe plastic deformation.</P> <P><B>Highlights</B></P> <P> <UL> <LI> High entropy alloy HfNbTaTiZr was nanostructured by high pressure torsion. </LI> <LI> Grain size of 80 nm was achieved by deformation to the equivalent strain e = 5. </LI> <LI> Structure refinement resulted in remarkable enhancement of strength of the alloy. </LI> <LI> Ductility remains sufficient for good workability. </LI> <LI> Further deformation to very high strains resulted in loss of strength. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
x10c,í,x17e,ek, J.,Janeč,ek, M.,Krajx148,á,k, T.,Strá,ská,, J.,Hrux161,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>
Evolution of microstructure and hardness in AZ31 alloy processed by high pressure torsion
Strá,ská,, Jitka,Janeč,ek, Milox161,Gubicza, Jenx151,Krajx148,á,k, Tomá,x161,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>
Microstructure and corrosion properties of ultrafine-grained interstitial free steel
Hadzima, Branislav,Janeč,ek, Milox161,Estrin, Yuri,Kim, Hyoung Seop Elsevier 2007 Materials science & engineering. properties, micro Vol.462 No.1
<P><B>Abstract</B></P><P>The microstructure and the corrosion behaviour of ultrafine-grained interstitial free (IF) ferritic steel processed by equal channel angular pressing (ECAP) at room temperature following route C were investigated. Already after the first pass of ECAP, the microstructure was refined by a factor of approximately 200. On further passes, ECAP was found to lead to continuous grain refinement, while elongated grain structure produced in the first pass was retained throughout all processing cycles. After 8 passes, the microstructure comprised bands of elongated grains of average length of 500–1000nm and average width of 200–300nm. Corrosion characteristics of a non-deformed coarse grained specimen and an ultrafine-grained specimen that had undergone 8 passes of ECAP were investigated using electrochemical potentiodynamic tests. ECAP induced grain refinement does not appear to influence the electrochemical characteristics in neutral NaCl solutions. This encouraging finding demonstrates that ECAP, while enhancing mechanical characteristics, does not compromise corrosion resistance of IF steel. A protective oxidic layer is formed in alkaline NaCl solution, which reduces the corrosion rate significantly.</P>