Superconductivity of barium with highest transition temperatures in metallic materials at ambient pressure.

Pressure-induced superconductivity often occurs following structural transition under hydrostatic pressure (PHP) but disappears after the pressure is released. In the alkali-earth metal barium, superconductivity appears after structural transformation from body-centered cubic structure to hexagonal-close-packed (hcp) structure at PHP = 5 GPa, and the superconducting transition temperature (Tc) reaches a maximum of 5 K at PHP = 18 GPa. Furthermore, by stabilizing the low-temperature phase at PHP ~ 30 GPa, Tc reached a higher level of 8 K. Herein, we demonstrate a significantly higher Tc superconductivity in Ba even at ambient pressure. This was made possible through severe plastic deformation of high-pressure torsion (HPT). In this HPT-processed Ba, we observed superconductivity at Tc = 3 K and Tc = 24 K in the quasi-stabilized hcp and orthorhombic structures, respectively. In particular, the latter Tc represents the highest value achieved at ambient pressure among single-element superconducting metals, including intermetallics. The phenomenon is attributed to a strained high-pressure phase, stabilized by residual strains generated from lattice defects such as dislocations and grain boundaries. Significantly, the observed Tc far exceeds predictions from DFT calculations under normal hydrostatic compressions. The study demonstrates the importance of utilizing high-pressure strained phases as quasi-stable superconducting states at ambient pressure.

Creep Resistance of S304H Austenitic Steel Processed by High-Pressure Sliding.

Sheets of coarse-grained S304H austenitic steel were processed by high-pressure sliding (HPS) at room temperature and a ultrafine-grained microstructure with a mean grain size of about 0.14 µm was prepared. The microstructure changes and creep behavior of coarse-grained and HPS-processed steel were investigated at 500-700 °C under the application of different loads. It was found that the processing of S304H steel led to a significant improvement in creep strength at 500 °C. However, a further increase in creep temperature to 600 °C and 700 °C led to the deterioration of creep behavior of HPS-processed steel. The microstructure results suggest that the creep behavior of HPS-processed steel is associated with the thermal stability of the SPD-processed microstructure. The recrystallization, grain growth, the coarsening of precipitates led to a reduction in creep strength of the HPS-processed state. It was also observed that in the HPS-processed microstructure the fast formation of σ-phase occurs. The σ-phase was already formed during slight grain coarsening at 600 °C and its formation was enhanced after recrystallization at 700 °C.

The Effect of Predeformation on Creep Strength of 9% Cr Steel.

Martensitic creep-resistant P92 steel was deformed by different methods of severe plastic deformation such as rotation swaging, high-pressure sliding, and high-pressure torsion at room temperature. These methods imposed significantly different equivalent plastic strains of about 1-30. It was found that rotation swaging led to formation of heterogeneous microstructures with elongated grains where low-angle grain boundaries predominated. Other methods led to formation of ultrafine-grained (UFG) microstructures with high frequency of high-angle grain boundaries. Constant load tensile creep tests at 873 K and initial stresses in the range of 50 to 300 MPa revealed that the specimens processed by rotation swaging exhibited one order of magnitude lower minimum creep rate compared to standard P92 steel. By contrast, UFG P92 steel is significantly softer than standard P92 steel, but differences in their strengths decrease with increasing stress. Microstructural results suggest that creep behavior of P92 steel processed by severe plastic deformation is influenced by the frequency of high-angle grain boundaries and dynamic grain coarsening during creep.

Hydrolytic Hydrogen Production on Al⁻Sn⁻Zn Alloys Processed by High-Pressure Torsion.

Aluminium-tin-based alloys with different compositions were synthesized by a high-pressure torsion (HPT) method. The effect of different alloying elements and processing routes on the hydrogen generation performance of the alloys was investigated. The results show that Zn can enhance the hydrogen generation rate and yield by promoting pitting corrosion. The highest reactivity in water was achieved for an Al-30wt %Sn-10wt %Zn alloy. Detailed analysis of the Al-30wt %Sn-10wt %Zn alloy shows that increasing the shear strain and the resultant formation of ultrafine grains and phase mixing enhance the hydrogen generation rate through the effects of both nanogalvanic cells and pitting corrosion.

The Effect of Ultrafine-Grained Microstructure on Creep Behaviour of 9% Cr Steel.

The effect of ultrafine-grained size on creep behaviour was investigated in P92 steel. Ultrafine-grained steel was prepared by one revolution of high-pressure torsion at room temperature. Creep tensile tests were performed at 873 K under the initially-applied stress range between 50 and 160 MPa. The microstructure was investigated using transmission electron microscopy and scanning electron microscopy equipped with an electron-back scatter detector. It was found that ultrafine-grained steel exhibits significantly faster minimum creep rates, and there was a decrease in the value of the stress exponent in comparison with coarse-grained P92 steel. Creep results also showed an abrupt decrease in the creep rate over time during the primary stage. The abrupt deceleration of the creep rate during the primary stage was shifted, with decreasing applied stress with longer creep times. The change in the decline of the creep rate during the primary stage was probably related to the enhanced precipitation of the Laves phase in the ultrafine-grained microstructure.

Transition from poor ductility to room-temperature superplasticity in a nanostructured aluminum alloy.

Recent developments of nanostructured materials with grain sizes in the nanometer to submicrometer range have provided ground for numerous functional properties and new applications. However, in terms of mechanical properties, bulk nanostructured materials typically show poor ductility despite their high strength, which limits their use for structural applications. The present article shows that the poor ductility of nanostructured alloys can be changed to room-temperature superplastisity by a transition in the deformation mechanism from dislocation activity to grain-boundary sliding. We report the first observation of room-temperature superplasticity (over 400% tensile elongations) in a nanostructured Al alloy by enhanced grain-boundary sliding. The room-temperature grain-boundary sliding and superplasticity was realized by engineering the Zn segregation along the Al/Al boundaries through severe plastic deformation. This work introduces a new boundary-based strategy to improve the mechanical properties of nanostructured materials for structural applications, where high deformability is a requirement.

High-pressure torsion for new hydrogen storage materials.

High-pressure torsion (HPT) is widely used as a severe plastic deformation technique to create ultrafine-grained structures with promising mechanical and functional properties. Since 2007, the method has been employed to enhance the hydrogenation kinetics in different Mg-based hydrogen storage materials. Recent studies showed that the method is effective not only for increasing the hydrogenation kinetics but also for improving the hydrogenation activity, for enhancing the air resistivity and more importantly for synthesizing new nanostructured hydrogen storage materials with high densities of lattice defects. This manuscript reviews some major findings on the impact of HPT process on the hydrogen storage performance of different titanium-based and magnesium-based materials.

Room-Temperature Superplasticity in an Ultrafine-Grained Magnesium Alloy.

Superplasticity, a phenomenon of high tensile elongation in polycrystalline materials, is highly effective in fabrication of complex parts by metal forming without any machining. Superplasticity typically occurs only at elevated homologous temperatures, where thermally-activated deformation mechanisms dominate. Here, we report the first observation of room-temperature superplasticity in a magnesium alloy, which challenges the commonly-held view of the poor room-temperature plasticity of magnesium alloys. An ultrafine-grained magnesium-lithium (Mg-8 wt.%Li) alloy produced by severe plastic deformation demonstrated 440% elongation at room temperature (0.35 T m) with a strain-rate sensitivity of 0.37. These unique properties were associated with enhanced grain-boundary sliding, which was approximately 60% of the total elongation. This enhancement originates from fast grain-boundary diffusion caused by the Li segregation along the grain boundaries and the formation of Li-rich interphases. This discovery introduces a new approach for controlling the room-temperature superplasticity by engineering grain-boundary composition and diffusion, which is of importance in metal forming technology without heating.

Optical Properties of Nanocrystalline Monoclinic Y2O3 Stabilized by Grain Size and Plastic Strain Effects via High-Pressure Torsion.

Yttrium oxide (yttria) with monoclinic structure exhibits unique optical properties; however, the monoclinic phase is thermodynamically stable only at pressures higher than ∼16 GPa. In this study, the effect of grain size and plastic strain on the stability of monoclinic phase is investigated by a high-pressure torsion (HPT) method. A cubic-to-monoclinic phase transition occurs at 6 GPa, which is ∼10 GPa below the theoretical transition pressure. Microstructure analysis shows that monoclinic phase forms in nanograins smaller than ∼22 nm and its fraction increases with plastic strain, while larger grains have a cubic structure. The band gap decreases and the photoluminescence features change from electric dipole to mainly magnetic dipole without significant decrease in the photoluminescence intensity after formation of the monoclinic phase. It is also suggested that monoclinic phase formation is due to the enhancement of effective internal pressure in nanograins.

Large enhancement of superconducting transition temperature in single-element superconducting rhenium by shear strain.

Finding a physical approach for increasing the superconducting transition temperature (Tc) is a challenge in the field of material science. Shear strain effects on the superconductivity of rhenium were investigated using magnetic measurements, X-ray diffraction, transmission electron microscopy, and first-principles calculations. A large shear strain reduces the grain size and simultaneously expands the unit cells, resulting in an increase in Tc. Here we show that this shear strain approach is a new method for enhancing Tc and differs from that using hydrostatic strain. The enhancement of Tc is explained by an increase in net electron-electron coupling rather than a change in the density of states near the Fermi level. The shear strain effect in rhenium could be a successful example of manipulating Bardeen-Cooper-Schrieffer-type Cooper pairing, in which the unit cell volumes are indeed a key parameter.

Microstructural evolution and mechanical properties of biomedical Co-Cr-Mo alloy subjected to high-pressure torsion.

The effects of severe plastic deformation through high-pressure torsion (HPT) on the microstructure and tensile properties of a biomedical Co-Cr-Mo (CCM) alloy were investigated. The microstructure was examined as a function of torsional rotation number, N and equivalent strain, εeq in the HPT processing. Electron backscatter diffraction analysis (EBSD) shows that a strain-induced martensitic transformation occurs by the HPT processing. Grain diameter decreases with increasing εeq, and the HPT-processed alloy (CCMHPT) for εeq=45 exhibits an average grain diameter of 47nm, compared to 70µm for the CCM alloy before HPT processing. Blurred and wavy grain boundaries with low-angle of misorientation in the CCMHPT sample for εeq<45 become better-defined grain boundaries with high-angle of misorientation after HPT processing for εeq=45. Kernel average misorientation (KAM) maps from EBSD indicate that KAM inside grains increases with εeq for εeq<45, and then decreases for εeq=45. The volume fraction of the ε (hcp) phase in the CCMHPT samples slightly increases at εeq=9, and decreases at εeq=45. In addition, the strength of the CCMHPT samples increases at εeq=9, and then decrease at εeq=45. The decrease in the strength is attributed to the decrease in the volume fraction of ε phase, annihilation of dislocations, and decrease in strain in the CCMHPT sample processed at εeq=45 by HPT.

Thermal conductivity reduction of crystalline silicon by high-pressure torsion.

We report a dramatic and irreversible reduction in the lattice thermal conductivity of bulk crystalline silicon when subjected to intense plastic strain under a pressure of 24 GPa using high-pressure torsion (HPT). Thermal conductivity of the HPT-processed samples were measured using picosecond time domain thermoreflectance. Thermal conductivity measurements show that the HPT-processed samples have a lattice thermal conductivity reduction by a factor of approximately 20 (from intrinsic single crystalline value of 142 Wm(-1) K(-1) to approximately 7.6 Wm(-1) K(-1)). Thermal conductivity reduction in HPT-processed silicon is attributed to the formation of nanograin boundaries and metastable Si-III/XII phases which act as phonon scattering sites, and because of a large density of lattice defects introduced by HPT processing. Annealing the samples at 873 K increases the thermal conductivity due to the reduction in the density of secondary phases and lattice defects.

Evaluation of TEM samples of an Mg-Al alloy prepared using FIB milling at the operating voltages of 10 kV and 40 kV.

Transmission electron microscopy (TEM) samples of an Mg-Al alloy has been prepared using a Ga-focused ion beam (FIB) milling at two different operating voltages of 10 kV and 40 kV to investigate the influence of the FIB energy on the sample quality. The fine structures of the samples have been studied using a high resolution TEM, and the concentration of the implanted Ga was analysed using an energy dispersive X-ray (EDX) analysis. The result of the TEM observation revealed that point defects were introduced to the sample finally milled at 40 kV but not at 10 kV. However, crystal lattice images and electron diffraction patterns were clearly observed on both the samples. The typical influence of the FIB energy was indicated in the elemental analysis. The relative Ga concentration in the thin sample finally milled at 10 kV was 1.0-2.0 at% that is less than half of 4.0-6.0 at% of the Ga concentration in the sample finally milled at 40 kV. A comparison between the experimental results of the Ga concentration measurement with simulation was also discussed.

Determination of absolute thickness and mean free path of thin foil specimen by zeta-factor method.

The zeta-factor method is applied to the thickness determination of thin amorphous specimens where the convergent-beam electron diffraction method is not applicable. Characteristic X-ray intensities are first measured using standard specimens in order to determine zeta-factors. These zeta-factors are then used to determine local thicknesses of an amorphous Si and an amorphous Al alloy. Electron energy-loss spectroscopy (EELS) spectra are acquired at the same positions as for the X-ray measurements. Thus, using the thicknesses measured from the zeta-factor method, the electron mean-free path is determined through the EELS log-ratio method. The mean free path is measured as a function of the collection semi-angle, beta, and specimen thickness, and it is also compared with theoretical values. Furthermore, the mean free path of amorphous Si is compared with that of the crystalline Si.