Papers by Author: Yulia Ivanisenko

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Authors: Ke Jing Yang, Yulia Ivanisenko, J. Markmann, Hans Jorg Fecht
Abstract: In situ tensile tests were conducted on ultra fine grained (UFG) pure Pd and Pd-x% Ag (x=20, 60) alloys of different stacking fault energies (SFEs) with the aim to study the general features of the deformation process of UFG materials as well as the peculiarities brought by the alloying. Grey scale correlation analysis (GSCA) was used to determine the true strain as well as the surface flow within the gauge length. It was shown that the largest values of strength and uniform elongation were obtained in Pd-20% Ag alloy. The GSCA revealed different macroscopic flow processes in this sample as compared with pure Pd and Pd-60% Ag alloy. In particular, pure Pd and Pd-60% Ag alloy demonstrated rapid localization of plastic flow in the neck area, whereas Pd-20% Ag samples showed a large contribution of homogenous deformation even after neck formation. It has been proposed that larger strain hardening capacity of Pd-20% Ag alloy is related to its lower SFE as compared with that of pure Pd: the lower is the SFE, the more difficult is the cross slip and climb of split dislocations, which leads to enhanced dislocation storage and, ultimately, to increased strain hardening. At the same time, further decrease of SFE in Pd-60% Ag sample leads to development of deformation twinning and consequent reduction of strain hardening. The dimpled structure of fracture surfaces in the samples will also be discussed in relationship to these findings.
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Authors: Yulia Ivanisenko, Hans Jörg Fecht
Abstract: Instrumented high pressure torsion, i.e. mechanical test in a torsion mode under high pressure, allows interesting possibility of materials testing, because materials mechanical response can be studied in a practically unlimited shear strain range. We have studied microstructures formed in initially coarse crystalline and nanocrystalline (nc) Pd and its alloys after instrumented HPT up to shear strain 300, and revealed signatures of similar processes occurring in all these materials. In particular, we found traces of cooperative grain boundary sliding in the form of aligned in parallel segments of boundaries of several grains with straightened triple points. Fracture surfaces contained shear bands. Texture measurements revealed lower dislocation activity in nanocrystalline state as compared with coarse crystalline one. Therefore we argue that cooperative grain boundary sliding is an important deformation mechanism at large strain which develops in both ultrafine grained (ufg) and nanocrystalline materials. In nc and ufg materials planes of cooperative grain boundary sliding act as precursors of shear bands and shear occurs along planes formed by numerous grain boundaries.
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Authors: Lilia Kurmanaeva, Yulia Ivanisenko, J. Markmann, Ruslan Valiev, Hans Jorg Fecht
Abstract: Investigations of mechanical properties of nanocrystalline (nc) materials are still in interest of materials science, because they offer wide application as structural materials thanks to their outstanding mechanical properties. NC materials demonstrate superior hardness and strength as compared with their coarse grained counterparts, but very often they possess a limited ductility or show low uniform elongation due to poor strain hardening ability. Here, we present the results of investigation of the microstructure and mechanical properties of nc Pd and Pd-x%Ag (x=20, 60) alloys. The initially coarse grained Pd-x% Ag samples were processed by high pressure torsion, which resulted in formation of homogenous ultrafine grain structure. The increase of Ag contents led to the decrease of the resulted grain size and change in deformation behavior, because of decreasing of stacking fault energy (SFE). The samples with larger Ag contents demonstrated the higher values of hardness, yield stress and ultimate stress. Remarkably the uniform elongation had also increased with increase of strength.
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Authors: Min Qi, Zai Qi Yao, Yulia Ivanisenko, Hans Jörg Fecht
Abstract: The equal channel angular pressing (ECAP) results in ultrafine-grained (~200–500 nm) Ti with superior mechanical properties without the harmful alloying elements, which is beneficial for medical implants. To improve the bioactivity of Ti surface, Ca/P-containing porous titania coating were prepared on the ultrafine-grained Ti and coarse-grained Ti by micro-arc oxidation (MAO) in Ca/P based solution. The phase identification, thickness, composition and morphology of the coatings were analyzed. The amounts of Ca, P and Ca/P ratio of the MAO coating formed on ultrafine-grained Ti were higher than those for coarse-grained Ti samples. Hydroxyapatite and α-Ca3(PO4)2 phases appeared in MAO coating formed on ECAP-treated Ti with increasing reaction time of 20 min (E20). Incubated in simulated body fluid within 2 days, bone-like apatite was completely formed on E20 surface, as evidenced of preferable bioactivity.
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Authors: Yulia Ivanisenko, Hans Jorg Fecht
Abstract: We suggest a simple method to study the mechanical behaviour of nanocrystalline (nc) samples in compression-torsion mode. High applied pressure prevents the fracture of sample, and quantitative parameters of sample response during torsion test can be compared with developed microstructure. Here we present and discuss the results of systematic investigation of mechanical behaviour of nc Pd with a mean grain size of 12 nm in a wide range of shear strains (0<γ<200) and at strain rates γ& = 3 10-1 s-1 and 3 10-2 s-1. We show that in the studied shear strain range the notable changes in the microstructure, namely a strain induced grain growth occurs, and that controls the relevant deformation mechanisms. For lower strains when the grain size is still small enough, the plastic flow governs by twinning and probably grain boundary sliding. The flow stresses are lower as compared with the later stages of deformation, when the grain size becomes larger and deformation is controlled exceptionally by dislocation glide. Finally, a steady stage is achieved, when the grain size, dislocation density and flow stress are saturated.
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Authors: Jiang Li Ning, Yulia Ivanisenko, D. Wang, Maxim Yu. Murashkin, Hans Jorg Fecht
Abstract: We processed a ferritic-pearlitic dual-phase steel under high pressure torsion for three and five rotations at room temperature, the results shew the concurrent processes of the grain refinement of ferrite phase and the decomposition of the cementite lamellae. After three rotations, a non-homogeneous structure was observed. The ferrite structure contained both cellular structure and banded nano-granular structure. The cementite was fragmented to fine particles, aligned along the longitudinal direction of the banded ferrite structure in the local region. After five rotations, a homogeneous mixture of nanoscaled equiaxed ferrite crystallites and cementite particles was obtained, without any visible trace of the former lamellar-type structure or particle alignment. The size distribution of the remaining cementite particles turned to a more narrow distribution skewed to finer sizes especially smaller than 10 nm, since the torsion increased from three to five rotations.
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