Maciej Krystian

SPD processed alloys as efficient vacancy-hydrogen systems

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.

Metallography of alkali metal single crystals

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.

The plasticity of potassium

Abstract 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.

Martensitic transformation and mechanical deformation of high-purity lithium

Abstract 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 M s 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 M s temperature.

Lattice Defects in Hydrogenated and HPT Processed Pd

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.

Stabilization of Lattice Defects in HPT-Deformed Palladium Hydride

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.

Equal channel angular pressing (ECAP) and forging of commercially pure titanium (CP-Ti)

Pure titanium with ultra-fine grained (UFG) microstructure is an exceptionally interesting material for biomedical and dental applications due to its very good biocompatibility and high strength. Such bulk, high-strength UFG materials are commonly produced by different Severe Plastic Deformation (SPD) techniques, whereof Equal Channel Angular Pressing (ECAP) is the most commonly used one.In this investigation commercially pure (CP) titanium (grade 2) was processed by ECAP using a die with a channel diameter of 20mm and an intersection angle of 105°. Six passes using route B120 (in which the billet is rotated between subsequent passes by 120°) at a temperature of 400°C were performed leading to a substantial grain refinement and an increase of strength and hardness. Subsequently, a thermal treatment study on ECAP-processed samples at different temperatures and for different time periods was carried out revealing the stability limit for ECAP CP-Ti as well as the best conditions leading to an improvement in ...

Neutron scattering investigation of pressure-induced phase transitions in Li and Ba

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 and 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.