Hideaki Iwaoka

Formation of FeNi with L10-ordered structure using high-pressure torsion

FeNi with the L10-ordered structure is formed over an astronomical timescale in meteorites. In this study, severe plastic deformation using high-pressure torsion (HPT) is employed for the production of the L10 structure in the laboratory. Its formation is confirmed by X-ray diffraction analysis and transmission electron microscopy. Processing of elemental nanopowders by HPT is an effective method for the formation of the L10 phase, which is enhanced by the addition of Co to FeNi or annealing after HPT. The formation of the phase must be associated with enhanced diffusion through HPT.

Solid-state reactions and hydrogen storage in magnesium mixed with various elements by high-pressure torsion: experiments and first-principles calculations

Magnesium hydride is widely known as an interesting candidate for solid-state hydrogen storage. However it is too stable and does not desorb hydrogen at ambient conditions. Although MgH2 suffers from slow kinetics, its hydrogenation kinetics can be significantly improved by addition of catalysts and/or decreasing the grain size. Reducing the thermodynamic stability of MgH2 is now the main challenging task. In this study, 21 different elements were added to magnesium in atomic scale by using the High-Pressure Torsion (HPT) technique and different kinds of nanostructured intermetallics and new metastable or amorphous phases were synthesized after HPT (Mg17Al12, MgZn, MgAg, Mg2In, Mg2Sn, etc.) or after post-HPT heat treatment (MgB2, Mg2Si, Mg2Ni, Mg2Cu, MgCo, Mg2Ge, Mg2Pd, etc.). In most of the compounds, the desorption temperature decreases by addition of elements, even though that the ternary hydrides are formed only in limited systems such as Mg–Ni and Mg–Co. Appreciable correlations were achieved between the theoretical binding energies obtained by first-principles calculations and the experimental dehydrogenation temperatures. These correlations can explain the effect of different elements on the hydrogenation properties of the Mg-based binary systems and the formation of ternary hydrides.

Effect of temperature on solid-state formation of bulk nanograined intermetallic Al3Ni during high-pressure torsion

A bulk form of nanograined intermetallic Al3Ni was produced by severe plastic deformation using high-pressure torsion (HPT). Powder mixtures of 75 mol% Al and 25 mol% Ni were processed by HPT at a selected temperature in the range of room temperature (RT) to 573 K under a pressure of 6 GPa. X-ray diffraction analysis revealed that the Al3Ni intermetallic formed after processing for 50 revolutions at RT but, as the processing temperature increased, less revolutions (i.e. lower imposed strain) were required for the formation of Al3Ni. Observations by transmission electron microscopy showed that the microstructure consists of ultrafine grains having a size of 300–2000 nm after 3 and 10 revolutions. Once the Al3Ni formed after a higher number of revolutions, equiaxed nanograins with a size of ~30 nm prevailed with a significant increase in hardness. The increase in hardness was more significant when processed at higher temperatures because of increasing the fraction of Al3Ni. It was shown that the solid-state formation of Al3Ni occurred due to enhanced diffusion (i.e. decreased activation energy for diffusion) through the presence of high density of lattice defects.

Hydrogen diffusion in ultrafine-grained iron with the body-centered cubic crystal structure

Abstract The role of grain boundaries in Fe on hydrogen diffusion has been investigated by electrochemical permeation tests using ultrafine-grained Fe produced by high-pressure torsion (HPT) processing. Permeation tests were also conducted on cold-rolled and water-quenched Fe to understand the trapping effect of dislocations and vacancies. Hydrogen diffusion was delayed in all these discs. However, the delay mechanism in the HPT-processed disc was different from that in rolled and water-quenched Fe. Grain boundaries do not act as trapping sites but slow the diffusion. The diffusion coefficients of hydrogen were significantly decreased by HPT processing on account of the high activation energy for hydrogen diffusion in grain boundaries.

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.

High-Pressure Torsion for Ring Samples in Different Thicknesses

High-pressure torsion (HPT) was conducted with ring samples of pure Al (99.99%) having different thicknesses. They were cut from an Al plate with 10 mm thickness to dimensions having inner and outer diameters of 23 and 30 mm with thicknesses of 2 mm or 4 mm. HPT was conducted at room temperature under a pressure of 1 GPa at a rotation speed of 1 rpm. It is shown that the strain was introduced more intensely in the mid part on the cross section. This strained region extended with increasing number of revolution and covered throughout the cross section in the 2 mm thickness ring after 30 revolutions. The intense-strained region extended to ~3 mm in the 4 mm thickness ring after 30 revolutions but remains almost the same even after 100 revolutions. The thickness of ~3 mm may be an upper limit to achieve homogeneous introduction of the strained region throughout the cross section.