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Various paths of equal channel angular pressing (ECAP) and conventional post-deformation via forging and rolling were performed to improve the mechanical properties (particularly, the strength and hardness) of a copper-free silver alloy. The microstructure was investigated with optical and scanning electron microscopy. The primary grain size of approximately 130 μm was significantly refined to ultrafine-grained (UFG) conditions with mean grain sizes of 0.6 μm and 0.5 μm via ECAP and ECAP with forging and rolling, respectively. The remarkable improvement of the mechanical properties was confirmed via hardness measurements and tensile and compression tests. ECAP and subsequent forging and rolling increased the initial hardness and tensile yield strength by +234% and +355%, respectively. Furthermore, ECAP led to a much more homogeneous hardness distribution over the entire cross-sections of the bars. The thermal stability of the UFG material at room temperature, 100 °C, and 150 °C was maintained for at least 27 months, 1000 h and 4 h, respectively. Finally, the UFG state exhibited a better tarnishing resistance than the coarse-grained state. Thus, ECAP can overcome the most significant weaknesses of lean, silver-based alloys (i.e. poor mechanical and tarnishing properties), thereby creating opportunities for their wider use. more...

The commercial magnesium alloy EZ33A was processed by Equal Channel Angular Pressing (ECAP). Two different tools were used: a conventional one with two channels and one intersection (90°), and a so-called double ECAP (D-ECAP) tool with two intersections (90° and 120°). Four passes were conducted at temperatures subsequently lowered from 350 °C down to 240 °C using route Bc. The as-cast, pre-deformed material and both ECAP conditions were investigated. The microstructure was revealed by means of optical microscopy, scanning and transmission electron microscopy (SEM, TEM), and X-ray diffraction. Mechanical properties were studied by Vickers hardness measurements as well as by compressive and tensile tests. The microstructure investigations show a significant grain refinement as well as fragmentation and redistribution of the hard intermetallic (Mg,Zn)12RE phase by ECAP. This effect is even more pronounced in the material processed by D-ECAP where this phase is two times finer compared to the size after conventional ECAP. Additionally, small Zn-Zr particles of a few nanometers size within grains or subgrains were observed. Microstructure changes evoked by the ECAP processing significantly affect the mechanical properties. This is especially the case after 4 passes in the D-ECAP tool where hardness (90 HV2) is almost 70% higher than in the as-cast condition (54 HV2) and the tensile yield strength is enhanced by nearly 300% (82 MPa up to 327 MPa). Hence, the D-ECAP tool is more efficient than the conventional one in improving mechanical properties. more...

Mg alloys are suitable materials for biodegradable osteosynthesis implants which stabilize bone fractures as long as the fracture is healing but are absorbed by the human body after they have fulfilled this purpose. However, high degradation rates and/or low strength values can be seen as drawbacks of such alloys. The present study deals with a lean Mg–Zn–Ca alloy which exhibits a desired, low degradation rate and explores the possibilities to improve its mechanical properties by Severe Plastic Deformation (SPD) via Double Equal Channel Angular Pressing (D-ECAP). The effects of different D-ECAP process parameters on the microstructure, mechanical properties and degradation behavior are investigated in detail. The results show that grain refinement during D-ECAP occurs via dynamic recrystallization. Lowering the processing temperature leads to a reduction of the size of the smaller grains of the initial bimodal grain size distribution while a higher degree of deformation (higher number of D-ECAP passes) reduces almost only the area fraction of the larger grains. While the increase in hardness is mainly due to the small size of the recrystallized grains, the tensile strength is additionally determined by the remaining larger grains. Thus, the mechanical properties of the lean Mg–Zn–Ca alloy can be tailored by D-ECAP according to the requirements of the final application. For example, the tensile strength can be increased to more than 370 MPa while maintaining the elongation at fracture at 7%. The degradation rate is not affected by D-ECAP as the process does not change the number or size of intermetallic particles. The optimized mechanical properties, in combination with the low degradation rate, make the D-ECAP-processed alloy very suitable for biomedical applications as absorbable material for osteosynthesis implants. more...

Recent investigations on palladium hydride (Pd-H) showed, for the first time, evidence of formation of vacancy-hydrogen (Vac-H) clusters during Severe Plastic Deformation (SPD) effected by High Pressure Torsion (HPT). Vacancy concentrations produced in Pd-H by this method are extraordinarily high. DSC-scans show that the thermal stability range of vacancies is extended by about 150K due to trapping of hydrogen leading to the formation of vacancy-hydrogen clusters. Recent experiments give evidence that the mobility of the H atoms and/or the vacancies is conditional for the formation of Vac-H clusters during HPT. Results furthermore indicate defect stabilization by hydrogen trapping not only for vacancy-type defects but also for dislocations and grain boundaries. more...

Since severe plastic deformation (SPD) has demonstrated its capability of producing bulk nanomaterials with highly advanced mechanical properties, research is increasingly focusing on the question as to whether functional nanomaterials can be achieved by SPD and in bulk shape, too. This paper presents promising results of reaching functional properties in SPD-processed bulk nanocrystalline magnetic alloys, bulk shape memory nanoalloys, as well as nanometals and alloys for hydrogen storage, and also reports on problems with other functional properties, like those of thermoelectricity, occurring in non-metallic nanomaterials. more...

Samples of partially hydrogenated palladium were heavily deformed at low homologous temperatures by means of high-pressure torsion and subsequently investigated by differential scanning calorimetry along with scanning and transmission electron microscopy. The results provide for the first time clear evidence for the formation of vacancy–hydrogen complexes from deformation-induced vacancies and for enhanced thermal stability of the nanostructure achieved under such conditions. more...

Pure palladium (99.95%) was hydrogenated, subsequently deformed by High Pressure Torsion (HPT) and analyzed by differential scanning calorimetry (DSC). For comparison some hydrogen-free HPT processed samples were also investigated. In case of the hydrogenated HPT Pd, the concentration of single / double vacancies is noticeably higher. The importance of hydrogen for the formation and stabilization of vacancy type defects and dislocations is discussed. more...

Nanocrystallization is known to yield several improvements for the storage and uptake rate of hydrogen in solids. Usually this process is achieved by ball milling to produce a nanopowders. However, handling of these powders is risky and difficult, and there is also a high risk of introducing impurities into the storage material. In this situation materials processed by Severe Plastic Deformation (SPD) yield several advantages: (i) the materials are produced in bulk shape with 100% density; (ii) the introduction of impurities is minimized, (iii) in addition to additional grain boundaries, many dislocations and particularly vacancies are generated which may further improve the hydrogen storage capacity and kinetics. more...

This paper reports on the development of a cryostat for neutron small-angle scattering experiments on alkali metals under uniaxial compression in a temperature range from 5 K to room temperature. The investigation was motivated by earlier observations of 'premartensitic' anomalies in metallic lithium and potassium which seem to suggest that nuclei of a low-temperature phase might be generated in the undercooled bcc structure either by cooling alone or by additional mechanical deformation. Cooling to 120 K in lithium and to 5 K in potassium, respectively, without applied load produced only very slight changes in the SANS patterns. However, after uniaxial compression of 3% at these temperatures anisotropic scattering patterns developed, suggesting the formation of small particles embedded in the bcc matrix whose orientation is related to the deformation direction. more...

In fcc crystals, dislocations are dissociated on the {111} glide plane into pairs of partial dislocations. Since each partial interacts individually with the Peierls potential and is coupled to its neighbour by a stacking fault, periodic variations in the separation distance d of the partials occur when dislocations running along closed packed lattice directions are displaced. This can drastically reduce the effective Peierls stress. By using the Peierls model the structure of 0°, 30°, 60° and 90° dislocations in a typical fcc metal with the elastic properties of Cu and a stacking-fault energy γ0 in the interval 0.04 ≤ γ0 ≤ 0.05 J/m2 was studied, and the magnitude of the Peierls energy ΔE P and the resulting kink energies E K were determined. Since the energies involved are of the order of 10−3 eV/b or less, their magnitude cannot be asserted with high confidence, considering the simplifying assumptions in the model. The difference in the changes of the core configuration during displacement of dislocatio... more...

Plastic deformation of bcc metals at intermediate and low temperatures is dominated by [1 1 1] screw dislocations which macroscopically seem to glide on planes of high or irrational Miller indices. It is now well established that this non-crystallographic glide is composed of elementary steps of kink-pair formation on three symmetry-equivalent low-index planes. In the present investigation, the slip geometry of high-purity potassium single crystals was investigated by in situ X-ray measurements of the lattice rotation in uniaxial compression in the temperature range from 6 to 70 K. The different alternatives for the elementary glide planes, e.g., {1 1 0}, {2 1 1} or {3 2 1}, can be uniquely distinguished from changes of the primary slip direction within the unit triangle. The results showed that the elementary glide step in potassium occurs on {2 1 1} planes in the investigated temperature range. In addition, glide of [1 0 0] dislocations (obviously products from interactions between mobile and sessile [1 1 1] dislocations) becomes dominating in many samples at increasing plastic strain. Numerical simulations of the orientation change in 〈1 1 1〉{2 1 1} glide, based on the model of thermally activated kink-pair formation, were in good agreement with the experimental results. more...

The paper discusses methods for metallographic preparation of lithium, sodium and potassium single crystals. Because mechanical techniques and electropolishing cannot be applied, chemical polishing methods in organic solvents are considered in detail. A few methods suggested earlier in the literature were re-examined. Improved techniques were developed that yield an excellent metallographic surface quality for optical microscopy and scanning electron microscopy. First results of in situ optical microscopy investigations of the low-temperature phase transformation of lithium and of in situ slip line investigations in all three metals are shown. more...

The martensitic transformation of Li is characterized by a large temperature hysteresis and a complex low-temperature phase diagram. The phase transition is very sensitive to small mechanical strains. On the other hand, within the hysteresis region peculiar mechanical properties are observed. The critical resolved shear stress exhibits a pronounced anisotropy and asymmetry between tension and compression while the strong temperature dependence typical for other b.c.c. metals is absent. Investigations of the plasticity of Li single crystals above the Ms temperature indicated that stress-induced nuclei of the low temperature phase interact with the glide dislocations. This was partially confirmed by in situ deformation experiments during neutron scattering and seems to be in agreement with recent theoretical approaches which predict a stabilization of martensite clusters in the nucleation state above the Ms temperature. more...

It has been known for a long time that the low-temperature deformation behaviour of potassium is very similar to that of the refractory b.c.c. metals. Therefore it is generally assumed that the same mechanism, namely kink-pair formation in screw dislocations, determines the deformation rate below a certain knee temperature T k . A detailed investigation of the flow-stress of high-purity potassium single crystals showed that the knee-temperature is about 80 K, in contrast to values of 25–30 K reported in the earlier work. Between 80 and 4.2 K there exist three clearly distinct deformation regimes separated by bends or ‘humps’ in the curve of flow-stress versus temperature. A similar behaviour has been observed in the refractory metals. The close correspondence of these bends in b.c.c metals of radically different electronic structure strongly indicates that they are intrinsic properies of screw dislocation motion in the b.c.c. lattice and do not have their origin in details of the interatomic potential. more...

The elemental alkali metals as well as the earth-alkaline metal barium undergo phase transitions from an ambient bcc structure to close-packed structures (fcc or hcp, respectively) in a largely varying pressure range from 4.5 GPa in Cs up to more than 50 GPa in Na. Little is known about the mechanisms of these pressure-induced phase transitions. In particular, it is an open question whether the {\mathrm {bcc}}\to {\mathrm {fcc}} and {\mathrm {bcc\to hcp}} transformations are related to similar phase transitions at ambient pressure and low temperatures which occur in Li and Na but are absent in the heavy alkali metals and in Ba. In the present work, we have adapted the Paris–Edinburgh pressure cell for neutron scattering studies on single crystals of Li and Ba. It was found that under carefully chosen experimental conditions almost hydrostatic loading of Li and Ba up to about 7 GPa is possible. We describe the experimental methods which have allowed us to overcome the technical difficulties and report the first results of elastic neutron scattering on Li and Ba and inelastic neutron scattering on Li. more...