Search

Hydrogen storage characteristics is studied in the Mg-alloy ZK60 after processing by different SPD (Severe Plastic Deformation) methods such as High Pressure Torsion (HPT) and Friction Stir Processing (FSP), applying various deformation extents and rates. The capacity and kinetics of hydrogen storage was investigated and analysed, up to 100 storage cycles. While the degree of SPD deformation is less important for the storage capacity, the SPD processing method itself matters, yielding about ~ 30% more capacity in FSP than in HPT. As shown by DSC and XRD analyses, it is the density of SPD-induced vacancy agglomerates which is significantly higher in FSP than in HPT (~ 10–3 instead of ~ 10–4) because of the enhanced dislocation slip activity. Thanks to their stabilization through Mg(Zn,Zr) precipitates, the vacancy agglomerates survive numerous cycles of hydrogen storage in spite of the high storage temperature of 350 °C, and can act as thermally stable heterogeneous nuclei for the hydrogenation. This latter mechanism was found in all SPD methods applied irrespective of the deformation extent, on the basis of Johnson–Mehl–Avrami-Kolmogorov analysis providing the Avrami exponent n = 1, already from the second up to the highest hydrogen storage cycles. [ABSTRACT FROM AUTHOR]

The work is devoted to a study of the structure and mechanical properties of two nonequiatomic medium-entropy nanocrystalline alloys, in which in a coarse state additional mechanisms act during plastic deformation — twinning (TWIP) in the F e 40 M n 40 C o 10 C r 10 alloy and phase transformations (TRIP) in the F e 50 M n 30 C o 10 C r 10 alloy. The nanocrystalline state in these alloys is achieved by high-pressure torsion (HPT) at 300 K and 77 K after different numbers of revolutions n = 0.25 and 5. In the nanostructural state in the TWIP F e 40 M n 40 C o 10 C r 10 and the TRIP F e 50 M n 30 C o 10 C r 10 alloys, a basically complete phase transition from the fcc lattice to hcp is observed, the content of which does not depend very strongly on the HPT temperature and deformation. For both alloys in the nanostructured state, there is a significant decrease in differences in the phase composition and microhardness Hv by comparison with the coarse-grained state. A decrease in the HPT temperature and an increase in HPT deformation for all the cases studied lead to an increase in the value of Hv. The F e 40 M n 40 C o 10 C r 10 TWIP alloy remains ductile under active compression deformation at 300 and 77 K, while there is no macroscopic plasticity in the F e 50 M n 30 C o 10 C r 10 TRIP alloy under similar conditions. For the F e 40 M n 40 C o 10 C r 10 TWIP the thermally-activated character of plastic deformation is retained during the transition from the coarse-grained to the nanostructured state. [ABSTRACT FROM AUTHOR]

The mechanical properties comprising the stress-strain characteristics under uniaxial tensile deformation, the acoustic properties from mechanical resonance spectroscopy and—in parallel—the microstructural evolution during deformation of a nonequiatomic high-entropy alloy (HEA) Fe50Mn30Co10Cr10 have been studied in a wide temperature range, including ultralow temperatures down to 0.5 K. In the temperature range 300 to 4.2 K, a strong temperature dependence of the tensile strength occurs, hinting at the thermally activated nature of plastic deformation. Within the range of extremely low temperatures (4.2–0.5 K), however, the alloy exhibits anomalies of the yield strength, as well as discontinuous plasticity. Over the whole temperature range, the dynamic Young's modulus of tensile deformed samples shows a reduction of absolute values compared to those of the undeformed ones, and at temperatures < 30 K a change of the temperature dependence from almost linear to power-law type. At all temperatures down to 0.5 K, the alloy's plasticity stays as high as 50% as a consequence of a deformation driven martensitic phase transformation from fcc to hcp lattice (TRIP effect). Considering the ultralow deformation temperatures, the tensile strength reaches record values of 1513 MPa at 4.2 K, and still of 1274 MPa at 0.5 K, each being paired with significant strain hardening. These results suggest the HEA Fe50Mn30Co10Cr10 as a promising structural material for use in cryogenic environments down to extremely low temperatures. [ABSTRACT FROM AUTHOR]

In-situ X-ray diffraction was applied to isotactic polypropylene with a high volume fraction of α-phase (α-iPP) while it has been compressed at temperatures below and above its glass transition temperature Tg. The diffraction patterns were evaluated by the Multi-reflection X-ray Profile Analysis (MXPA) method, revealing microstructural parameters such as the density of dislocations and the size of coherently scattering domains (CSD-size). A significant difference in the development of the dislocation density was found compared to compression at temperatures above Tg, pointing at a different plastic deformation mechanism at these temperatures. Based on the individual evolutions of the dislocation density and CSD-size observed as a function of compressive strain, suggestions for the deformation mechanisms occurring below and above Tg are made. [ABSTRACT FROM AUTHOR]

Abstract For mass fabrication of thermoelectric generators a fast, cheap and simple method is required to produce high ZT leg material. It is well known that severe plastic deformation (SPD) enhances the density of defects and dislocations, and also refines the grains to nano-size. These are the essential key parameters for low thermal phonon conductivity. Consequently and as a first example, SPD via high-pressure torsion (HPT) at elevated temperatures in protective gas atmosphere was used to directly consolidate and plastically deform commercial p-type skutterudite powder, DD y Fe 3 CoSb 12 , (DD stands for didymium) into a dense thermoelectric solid. The HPT-sample exhibited a high figure of merit, ZT > 1.3 at 773 K, much higher than that of the hot-pressed reference sample (HP) from the same powder. The ZT achieved is comparable with ZTs of corresponding high energy ball milled and hot pressed samples (HBM-HP) as well as of ball milled, hot pressed plus HPT processed skutterudites (BM-HP-HPT). The thermoelectric efficiency is even 4% higher. Synchrotron measurements were performed at temperatures from 300 to 825 K in order to evaluate the changes in grain size and dislocation density of the CP-HPT samples before, during and after annealing. SEM and TEM images served to give a better insight into the temperature dependent diversification of the new material. Due to grain refinement and enhanced dislocation density, the CP-HPT material exhibits higher hardness values than the reference sample. The elastic moduli are, within the error bar, the same. With large high-pressure torsion facilities at hand, a new, fast and therefore cheap mass production of thermoelectric leg material directly from powders can be envisaged. This new production technique furthermore warrants high figures of merit but excludes time-consuming ball milling and hot-pressing steps. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract: Samples of γ-iPP were prepared by crystallization at elevated pressures using a specially designed pressure chamber. The γ-phase was subsequently investigated by dedicated in-situ X-ray diffraction experiments during deformation using synchrotron radiation. Parameters such as the crystallinity, the density of dislocations and the coherently scattering domain size (CSD-size) which in polymers corresponds to the lamella size, have been evaluated as a function of the strain by means of the multi-reflection X-ray line profile analysis (MXPA). Compared to the results for the α-phase, those for the γ-phase reveal an enhanced strength and a strongly reduced evolution of the dislocation density. The latter is explained in terms of a model comprising the formation of misfit dislocations. [Copyright &y& Elsevier]

Abstract: High pressure torsion has successfully been applied to produce low-porosity, bulk specimens from Al-based metallic glass ribbons (Al85Y8Ni5Co2, Al85Ce8Ni5Co2 and Al85Gd8Ni5Co2). The compacted disks possess higher hardness than the original glass and have substantial glass fraction with nanocrystalline precipitations. Mechanical and thermal impacts have only minor effects on the glassy structure as demonstrated by the stability of the X-ray diffraction halo positions. Unchanged halos reveal that medium range order is a key characteristic of the amorphous state. [Copyright &y& Elsevier]

In the temperature range 77–290 K, the microhardness of high entropy alloy CoCrFeNiMn (Сantor alloy) samples in three structural states was measured: (i) initial coarse-grained samples (ii) nanocrystalline samples obtained by high pressure torsion (HPT) at room temperature and (iii) nanocrystalline samples processed by HPT at liquid nitrogen temperature. With a decrease in temperature from 290 K to 77 K, the microhardness of all types of samples monotonically increased by about 40–60% which indicates the thermally activated nature of plastic deformation under the indenter. The highest microhardness was observed in nanocrystalline samples obtained by the torsion at room temperature. It is shown that the ratio of Vickers microhardness to the conventional yield strength remains constant in the range of 77–290 K, and that the ratio value depends on the microstructure of samples. In the coarse-grained Cantor alloy samples the normal indentation size effect was observed: the microhardness increased with a decrease of the indenter penetration depth; the obtained dependence is discussed in the framework of geometrically necessary dislocations model. In the HPT-nanocrystallized samples in the low load range, an essential microhardness drop with the load decreasing was observed, being a result of mechanical polishing of the sample surface, which destroys the microstructure created under the torsion and weakens the surface layer. • The temperature dependence of microhardness of a high entropy alloy is firstly reported for the temperature range 77–290 K. • Microhardness of Cantor alloy monotonically increases with a decrease in temperature. • The ratio of microhardness to yield strength is a constant in the range of 77–290 K. • For SPD processed nanocrystalline Cantor alloy, an anomalous size indentation effect was revealed. [ABSTRACT FROM AUTHOR]