Alla S. Sologubenko

Design strategy for microalloyed ultra-ductile magnesium alloys

This article describes a design strategy deployed in developing ultra-ductile Mg alloys based on a microalloying concept, which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the resulting microstructural and mechanical properties. After processing, the so-designed alloys ZQCa3 (Mg–3Zn–0.5Ag–0.25Ca–0.15Mn, in wt.%) and ZKQCa3 (Mg–3Zn–0.5Zr–0.5Ag–0.25Ca–0.15Mn, in wt.%) reveal very fine grains (<10 µm), high ductility (elongation to fracture of up to 30%) at moderate strength or high strength (ultimate tensile strength of up to 350 MPa) at reasonable ductility. These properties are explained based on thermodynamic modelling, microstructure analysis including transmission electron microscopy studies, and microstructural and mechanical testing after annealing, and are compared to a related commercial alloy (ZK31).

Design strategy for new biodegradable Mg-Y-Zn alloys for medical applications

Abstract The aim of this article is to describe the design strategy deployed in developing new biodegradable Mg–Y–Zn alloys. The development approach is based on a microalloying concept which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the characteristics of the new alloys using metal-physical experiments, thermodynamic calculations and transmission electron microscopy analysis. Our results show that after extrusion the alloys have very fine grains (< 10 m), exhibit high ductility (uniform elongation: 17 – 20 %) at considerable strength (ultimate tensile strength: 250 – 270 MPa) and reveal the presence of finely distributed intermetallic particles which are stable upon annealing. Due to an attractive combination of mechanical, electrochemical and biological properties, the new alloys are very promising not only for applications in medicine but also in other fields.

Single-phase high-entropy alloys – an overview

Abstract The term “high-entropy alloys (HEAs)” first appeared about 10 years ago defining alloys composed of n=5–13 principal elements with concentrations of approximately 100/n at.% each. Since then many equiatomic (or near equiatomic) single- and multi-phase multicomponent alloys were developed, which are reported for a combination of tunable properties: high hardness, strength and ductility, oxidation and wear resistance, magnetism, etc. In our paper, we focus on probably single-phase HEAs (solid solutions) out of all HEAs studied so far, discuss ways of their prediction, mechanical properties. In contrast to classical multielement/multiphase alloys, only single-phase multielement alloys (solid solutions) represent the basic concept underlying HEAs as mixing-entropy stabilized homogenous materials. The literature overview is complemented by own studies demonstrating that the alloys CrFeCoNi, CrFeCoNiAl0.3 and PdFeCoNi homogenized at 1300 and 1100°C, respectively, for 1 week are not single-phase HEAs, but a coherent mixture of two solid solutions.

Design considerations for achieving simultaneously high-strength and highly ductile magnesium alloys

A comprehensive scheme of phase configuration optimization in the Mg–Zn–Ca(–Zr) system by thermodynamic simulations and microstructural analyses is presented. A composition window of 0.2–0.4 wt% Ca and 5–6 wt% Zn is defined as optimal for establishing a complex heterogeneous microstructure allowing for enhanced ductility and simultaneously high strength of the material. Literature data analysis and our own results confirm the enhanced performance of alloys from this composition window.

Design Strategy for Microalloyed Ultra-Ductile Magnesium Alloys for Medical Applications

The aim of this article is to describe the design strategy deployed in developing new bioabsorbable Mg–Y–Zn alloys. The development approach is based on a microalloying concept, which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the characteristics of the new alloys using metal-physical experiments, thermodynamic calculations, and TEM analysis. Our results show that after extrusion the alloys have very fine grains (<10m), exhibit high ductility (uniform elongation: 17–20%) at considerable strength (ultimate tensile strength: 250–270 MPa), and reveal the presence of finely distributed intermetallic particles, which are stable upon annealing. Due to an attractive combination of mechanical, electrochemical and biological properties, the new alloys are very promising not only for applications in medicine but also in other fields.

Superior room-temperature ductility of typically brittle quasicrystals at small sizes.

The discovery of quasicrystals three decades ago unveiled a class of matter that exhibits long-range order but lacks translational periodicity. Owing to their unique structures, quasicrystals possess many unusual properties. However, a well-known bottleneck that impedes their widespread application is their intrinsic brittleness: plastic deformation has been found to only be possible at high temperatures or under hydrostatic pressures, and their deformation mechanism at low temperatures is still unclear. Here, we report that typically brittle quasicrystals can exhibit remarkable ductility of over 50% strains and high strengths of ∼4.5 GPa at room temperature and sub-micrometer scales. In contrast to the generally accepted dominant deformation mechanism in quasicrystals—dislocation climb, our observation suggests that dislocation glide may govern plasticity under high-stress and low-temperature conditions. The ability to plastically deform quasicrystals at room temperature should lead to an improved understanding of their deformation mechanism and application in small-scale devices.

Nano-Sized Structurally Disordered Metal Oxide Composite Aerogels as High-Power Anodes in Hybrid Supercapacitors

A general method for preparing nano-sized metal oxide nanoparticles with highly disordered crystal structure and their processing into stable aqueous dispersions is presented. With these nanoparticles as building blocks, a series of nanoparticles@reduced graphene oxide (rGO) composite aerogels are fabricated and directly used as high-power anodes for lithium-ion hybrid supercapacitors (Li-HSCs). To clarify the effect of the degree of disorder, control samples of crystalline nanoparticles with similar particle size are prepared. The results indicate that the structurally disordered samples show a significantly enhanced electrochemical performance compared to the crystalline counterparts. In particular, structurally disordered Ni xFe yO z@rGO delivers a capacity of 388 mAh g-1 at 5 A g-1, which is 6 times that of the crystalline sample. Disordered Ni xFe yO z@rGO is taken as an example to study the reasons for the enhanced performance. Compared with the crystalline sample, density functional theory calculations reveal a smaller volume expansion during Li+ insertion for the structurally disordered Ni xFe yO z nanoparticles, and they are found to exhibit larger pseudocapacitive effects. Combined with an activated carbon (AC) cathode, full-cell tests of the lithium-ion hybrid supercapacitors are performed, demonstrating that the structurally disordered metal oxide nanoparticles@rGO||AC hybrid systems deliver high energy and power densities within the voltage range of 1.0-4.0 V. These results indicate that structurally disordered nanomaterials might be interesting candidates for exploring high-power anodes for Li-HSCs.

Cyclic loading for the characterisation of strain hardening during in situ microcompression experiments

Abstract Various parameters from fabrication and testing are known to influence the behaviour for microcompression experiments, especially the work-hardening behaviour. In this regard, the most important factor is found to be the availability of unconstrained slip planes. The second-most important factor is the lateral constraint acting on the sample, which usually arises from friction between indenter and pillar in a combination with a laterally stiff indenter setup, which can generate significant grain rotation and deviation from ideal single slip behaviour. The effect of lateral constraints on the strain-hardening rate is demonstrated on single crystals of gold and a novel solution by cyclic loading conditions is suggested, which could provide comparable conditions for different types of indenter hardware. In this work, the cyclic loading method is shown to minimise the influence of lateral constraints and provide more accurate measurements of strain-hardening behaviour than commonly applied microcompression methods by preventing grain rotation due to frictional constraint.

Reflectance anisotropy spectroscopy as a tool for mechanical characterization of metallic thin films

In the present work reflectance anisotropy spectroscopy (RAS) is evaluated as a new tool for the mechanical characterization of metallic thin films on viscoelastic substrates. Cu and Cu–Zn thin films of thicknesses in the range from 50 to 1000 nm were sputter-deposited onto a viscoelastic polyimide substrate and subjected to uniaxial tensile loading. The changes in the mechanical, electrical and optical response of the films upon loading were monitored by simultaneous acquisition of total strain, electrical resistance and the RA-signal. The RA-spectrum of pure copper reveals a feature at a photon energy of ~4.0 eV that linearly increases with strain at the beginning of loading (elastic regime) and saturates at later stages (plastic regime). Post-mortem SEM studies of samples loaded to different strain values confirmed that this saturation corresponds to the onset of plastic deformation, defined by the appearance of slip lines. Concurrent measurements of the electrical resistance confirmed the absence of cracking at the onset of the 4.0 eV RA-signal saturation. Therefore we claim that the RAS technique can be employed for yield point determination. Besides the applicability of the RAS technique for pure metals, chemical sensitivity of RAS in terms of peak position was observed in the case of Cu–Zn thin films.

Synthesis, Spray Deposition, and Hot-Press Transfer of Copper Nanowires for Flexible Transparent Electrodes

We report a solution-phase approach to the synthesis of crystalline copper nanowires (Cu NWs) with an aspect ratio >1000 via a new catalytic mechanism comprising copper ions. The synthesis involves the reaction between copper(II) chloride and copper(II) acetylacetonate in a mixture of oleylamine and octadecene. Reaction parameters such as the molar ratio of precursors as well as the volume ratio of solvents offer the possibility to tune the morphology of the final product. A simple low-cost spray deposition method was used to fabricate Cu NW films on a glass substrate. Post-treatment under reducing gas (5% H2 + 95% N2) atmosphere resulted in Cu NW films with a low sheet resistance of 24.5 Ω/sq, a transmittance of T = 71% at 550 nm (including the glass substrate), and a high oxidation resistance. Moreover, the conducting Cu NW networks on a glass substrate can easily be transferred onto a polycarbonate substrate using a simple hot-press transfer method without compromising on the electrical performance. The resulting flexible transparent electrodes show excellent flexibility ( R/ Ro < 1.28) upon bending to curvatures of 1 mm radius.