New permanent magnet materials

Under certain conditions, hydrogen can degrade the mechanical properties and fracture behavior of most structural alloys; however, it also has some positive effects in metals. Several current and potential applications of hydrogen for enhancing the production and processing of materials are reviewed. These include thermohydrogen processing (THP) and forming of refractory alloys, processing of rare earth-transition metal magnets by hydrogen decrepitation (HD) and hydrogenation–decomposition–desorption–recombination (HDDR), hydrogen-induced amorphization (HIA) and microstructural refinement, extraction of elements from ores and alloys, and the use of hydrogen as a reducing gas for welding and brazing. Hydrogen is found to enhance the formability, microstructure and properties of a large variety of materials, including steels, Ti-based alloys and metal matrix composites (MMCs), refractory metals and alloys, rare earth-transition metal alloys, metalloid-containing metallic glasses, etc.

Hydrogen-assisted processing of materials

Hydrogen-induced amorphization of intermetallics

Simultaneous TDS–DSC measurements in magnesium hydride

The radii of tetrahedral holes occupied by hydrogen atoms in the amorphous hydrides a-RFe2Hx (R = a rare earth metal) are larger than the empirical minimum hole size, 0.040 nm, of the general stable hydrides AxB1−xHy. However, the radii of the holes in the corresponding crystalline hydrides c-RFe2Hx are smaller than this value. Thus, the higher stabilities of the amorphous hydrides which are considered to be the origin for the driving force of hydrogen-induced amorphization (HIA) in the C15 Laves compounds are interpreted on the basis of the differences in the hole sizes. Furthermore, the occurrence of HIA in C15 AB2 Laves compounds has been analyzed in terms of the tetrahedral hole size, the stability of the compounds, the atomic size ratio, the valence electron concentration and the contraction or the expansion of the constituent elements and the holes. The atomic size ratio is the single most important factor controlling the occurrence of HIA in these compounds, and the compounds with the ratio above 1.37 are amorphized by hydrogenation.

Factors controlling hydrogen-induced amorphization of C15 Laves compounds

Amorphization of the CeFe2 Laves phase compound by hydrogen absorption

Magnetic properties and pressure effect on the curie temperature of Fe-Nd and Fe-Nd-B amorphous alloys

Fe-Nd and Fe-Nd-B amorphous alloys were prepared by dc sputtering in order to investigate the magnetic state and the thermal expansion anomaly. The pressure effect on the Curie temperature indicates a linear relationship between (l/Tc)dTc/dP and Tc, implying that the magnetic state is heterogeneous and it is not changed by addition of B. The pressure effect on the Curie temperature of Fe-Nd amorphous alloys decreases with increasing Nd content and the magnitude is comparable with that of Fe-Ni crystalline Invar alloys. The thermal expansion anomaly is remarkably large, accompanying a shrinkage in a wide range of temperature. It should be noted that a distinct anomaly is observed even well above the Curie temperature. This peculiar behavior is closely related to the magnetic clusters existing above the Curie temperature.

Pressure effect on the curie temperature and thermal expansion anomaly in Fe-Nd and Fe-Nd-B amorphous alloys

Multi-track recording utilizing multi-level partial response

Extremely high areal density utilized submicron trackwidth recording was proposed. To overcome the difficulty in tracking servo to follow the narrow track, multi-track parallel recoding is effective with a wider track read haed. A new encoding/decoding scheme introducing multi-level partial response is investigated. It is demonstrated by an experiment that recorded information on two tracks can be descriminated even with a single widetrack read head by means of this scheme.

Yasuharu Satoh

Papers

Amorphization of the CeFe2 Laves phase compound by hydrogen absorption

Magnetic properties and pressure effect on the curie temperature of Fe-Nd and Fe-Nd-B amorphous alloys

Pressure effect on the curie temperature and thermal expansion anomaly in Fe-Nd and Fe-Nd-B amorphous alloys

Multi-track recording utilizing multi-level partial response

Multi-track recording scheme utilizing multi-level partial response