Takaomi Itoi

FERROMAGNETIC CO-FE-ZR-B AMORPHOUS ALLOYS WITH GLASS TRANSITION AND GOOD HIGH-FREQUENCY PERMEABILITY

A Co-based amorphous phase with glass transition and supercooled liquid region before crystallization was formed in Co70−xFexZr10B20 and Co72−xFexZr8B20 alloys containing more than 14 atu2009% Fe. The crystallization temperature (Tx) is 899 K for the Co–Zr–B alloys and remains unchanged in the Fe concentration range up to 20%. The glass transition temperature (Tg) decreases with increasing Fe content, and the ΔTx(=Tx−Tg) increases from 25 K at 14% Fe to 34 K at 21% Fe. The amorphous alloys with glass transition crystallize with a single stage precipitation of bcc Fe(Co) and Co3ZrB2 phases. The Co-rich amorphous alloys exhibit good soft magnetic properties, i.e., saturation magnetization of 0.58–0.83 T, low coercivity of 4.7–8.3 A/m, and high permeability of 5500–18u2009300 in the frequency range of 1–103u2009kHz and low magnetostriction between −1.5×10−6 and +10×10−6 including zero. The success in synthesizing the soft magnetic amorphous alloys with high stability of supercooled liquid is promising for the future dev...

Fabrication of Co40Fe22Nb8B30 bulk metallic glasses by consolidation of gas-atomized powders and their soft-magnetic properties

Abstract Ferromagnetic Co 40 Fe 22 Nb 8 B 30 bulk glassy compacts having a high relative density about 99.6% and good soft magnetic properties were synthesized by consolidation of gas-atomized glassy powders. The high density was achieved by hot pressing under a pressure of 70 MPa using a decrease of viscosity in a supercooled liquid region.

Superplasticity in Fe-based metallic glass with wide supercooled liquid region

Abstract We have investigated the deformation behavior and the influence of deformation on the soft-magnetic properties for a ferromagnetic Fe 72 Hf 8 Nb 2 B 18 (at.%) metallic glass that has a wide supercooled liquid region of 76xa0K before crystallization and good soft-magnetic properties. The metallic glass was found to exhibit superplasticity at the supercooled liquid state. The maximum elongation to failure was 240% that was obtained at 873xa0K (T g +17 K ) and at 1.7×10 −3 s −1 under a flow stress of about 100xa0MPa. In this condition, crystallization occurred at elongation above 100%, resulting from the lack of thermal stability. The metallic glass was, however, deformed up to an elongation of 100% with retaining the good soft-magnetic properties and glassy phase. We succeeded in establishing the constitutive formulation of the flow stress in the supercooled liquid region. Its formulation was expressed well by a stretched exponential function σ flow =3C e exp (H/RT)[1− exp (A/{B e exp (H ∗ /RT)} 0.85 )] .

High-frequency permeability of (Fe, Co, Ni)62Nb8B30 amorphous alloys with a wide supercooled liquid region

The increase in B content to 30 at.u2009% for Fe–Co–Ni–Nb–B amorphous alloys caused an extension of a supercooled liquid region before crystallization, and a wide supercooled liquid region greater than 80 K was obtained in a wide composition range of 5–42 at.u200a% Co and 0–10 at.u200a% Ni for Fe62−x−yCoxNiyNb8B30 amorphous alloys. High B-containing amorphous alloys with low Ni content below 10 at.u200a% have high electrical resistivity above 230 μΩu2009cm and exhibit good soft magnetic properties, i.e., a low coercive force of 2 A/m, low saturated magnetostriction of 2–7×10−6, and high permeability of 21u2009000–29u2009300 at 1 kHz and 5000–7500 at 1 MHz. The high-frequency permeability characteristics at 1 MHz are much better than those for any kind of soft magnetic materials, and have been presumed to result from the decrease in eddy current loss caused by the increase in electrical resistivity. The synthesis of the Fe–Co–Nb–B amorphous alloys exhibiting a wide supercooled liquid region greater than 80 K and high-frequency permea...

High-frequency permeability characteristics of Fe- and Co-based amorphous alloys with high B concentrations

New Fe-based and Co-based amorphous alloys with a high B concentration of 30 at% were formed in the (Fe,Co,Ni)/sub 62/Nb/sub 8/B/sub 30/ system. The amorphous alloys exhibit a wide supercooled liquid region (/spl Delta/T/sub x/) above 80 K before crystallization in the range of 5 to 40% Co and 0 to 10% Ni and the largest /spl Delta/T/sub x/ is 87 K for Fe/sub 52/Co/sub 10/Nb/sub 8/B/sub 30/. The Fe/sub 62-x/Co/sub x/Nb/sub 8/B/sub 30/ alloys exhibit good soft magnetic properties i.e., low coercive force (H/sub c/) of 2-3 A/m, high effective permeability (/spl mu//sub e/) of 20000-30000 at 1 kHz and low saturated magnetostriction (/spl lambda//sub s/) of 2-8/spl times/10 d. The high /spl mu//sub e/ characteristics are maintained in a higher frequency range and the highest /spl mu//sub e/ at 1 kHz is 7500 for Co/sub 40/Fe/sub 22/Nb/sub 8/B/sub 30/ with a high resistivity of 237 /spl mu//spl Omega/ cm. The finding of the (Fe,Co)/sub 62/Nb/sub 8/B/sub 30/ amorphous alloys exhibiting large /spl Delta/T/sub x/, good high-frequency permeability and high resistivity is encouraging the future development as a new soft magnetic materials.

Evolution of Microstructures in Nanocrystalline VN Barrier Leading to Failure in Cu/VN/SiO2/Si Systems

We have investigated the evolution of microstructures in a nanocrystalline VN barrier of ?10 nm thickness to clarify the failure mechanism in a Cu/VN/SiO2/Si system owing to high-temperature annealing. Transmission electron microscopy observation reveals that the as-deposited VN barrier shows a uniform layer with a columnar structure composed of grains no larger than 10 nm in size. A negligible change in the morphology of the VN barrier is evident even after annealing at 600 ?C for 1 h. Annealing at 800 ?C brings about noticeable growth of VN grains in the lateral direction without any solid-phase reaction at each interface, resulting in a failure of the VN barrier owing to local discontinuity of the layer. This result is also consistent with the result obtained by Auger electron spectroscopy. It is revealed that the thin VN barrier fails after annealing at 800 ?C for 1 h owing to the loss of the continuity in the lateral direction without chemical reaction and intermixing at barrier interfaces.

Charge state of rare earth ions and glass transition in the glassy Al85Ni10R5 alloys

A glassy Al85Ni10La5 alloy has a supercooled-liquid region of 16 K between a glass transition temperature at 526 K and a crystallization temperature at 542 K, while Al85Ni10Eu5 has no supercooled-liquid region and crystallizes at 514 K. 151Eu Mossbauer spectroscopic study reveals that the Eu ions in Al85Ni10Eu5 are divalent, although the charge state of La ions is naturally considered to be trivalent in Ai85Ni10La5. It is supposed that the thermal stability of this glassy alloy system strongly depends on not only the ionic radius of rare earth element but also the electronic state.

Mössbauer Study on Crystallization Behavior of Glassy Fe58Co7Ni7ZrxB28-x (3 ≤x ≤18) Alloys with Variable Supercooled-Liquid Regions

Crystallization behavior of glassy Fe58Co7Ni7ZrxB28-x (3 ≤x ≤18) alloys was examined by means of X-ray diffraction and 57Fe Mossbauer spectroscopy in order to clarify the reason for the thermal stability against crystallization, where the thermal stability correlates strongly to the width of the supercooled-liquid region. Various crystalline phases appear in the specimens quenched from and annealed at temperatures 50 K higher than the crystallization temperatures. Crystalline Fe3B and Fe2B phases characteristic of the B-rich alloys appear along with Fe2Zr and unidentified Fe–Zr–B phases characteristic of the Zr-rich alloys in the quenched Fe58Co7Ni7Zr8B20 specimen that originally has the widest supercooled-liquid region of 66 K. The competitive frustration in the forming ability of these compositional short-range order is one of the reasons for thermal stability against crystallization. Both the quenched and annealed Fe58Co7Ni7Zr3B25 specimens do not contain any phase with the Zr constituent, while the glassy Fe58Co7Ni7Zr3B25 alloy has the supercooled-liquid region of 34 K. This fact implies that the Zr impurities in Fe3B- and/or Fe2B-type compositional short-range order also play an important role in the thermal stability of these glassy alloys.