Kenji Amiya

Ti-based amorphous alloys with a wide supercooled liquid region

Abstract TiCuNiCo quaternary amorphous alloys produced by melt spinning were found to have a wide supercooled liquid region before crystallization, though no glass transition was observed in TiCu binary amorphous alloys. The largest temperature interval of the supercooled liquid region ( ΔT x ) is as large as 90 K for Ti 50 Cu 25 Ni 20 Co 5 . There is a tendency for ΔT x to increase with an increase in storage modulus and with a decrease in loss modulus. It is therefore presumed that the increase in ΔT x for the multicomponent amorphous alloy is due to the suppression of crystallization for the supercooled liquid resulting from the increase in viscosity.

Mechanical strength and thermal stability of Ti-based amorphous alloys with large glass-forming ability

Abstract Ti-based alloy powders produced by the high pressure gas atomization technique were found to consist of an amorphous single phase in the particle size range below 32 μm for Ti 50 Zr 10 Cu 40 and below 25 μm for Ti 50 Zr 10 Ni 20 Cu 20 , though the cooling rate of their molten alloys for gas atomization is considerably lower than that for melt spinning. In addition, the TiZrCu amorphous powders exhibit a rather wide supercooled liquid region before crystallization. This is believed to be the first evidence for the appearance of the supercooled liquid region for atomized Ti-based amorphous powders. The significant increase in the glass-forming ability and the wide temperature range of the supercooled liquid region for the TiZrCu ternary alloys are presumably due to the simultaneous satisfaction of the two criteria of the significantly different atomic size ratios among the constituent elements and the difficulty of long-range redistribution of the constituent elements for the growth of crystalline phases.

Entropies in Alloy Design for High-Entropy and Bulk Glassy Alloys

High-entropy (H-E) alloys, bulk metallic glasses (BMGs) and high-entropy BMGs (HE-BMGs) were statistically analyzed with the help of a database of ternary amorphous alloys. Thermodynamic quantities corresponding to heat of mixing and atomic size differences were calculated as a function of composition of the multicomponent alloys. Actual calculations were performed for configurational entropy (Sconfig.) in defining the H-E alloys and mismatch entropy (Ss) normalized with Boltzmann constant (kB), together with mixing enthalpy (DHmix) based on Miedema’s empirical model and Delta parameter (d) as a corresponding parameter to Ss/kB. The comparison between DHmix–d and DHmix– diagrams for the ternary amorphous alloys revealed Ss/kB ~ (d /22)2. The zones S, S′ and B’s where H-E alloys with disordered solid solutions, ordered alloys and BMGs are plotted in the DHmix–d diagram are correlated with the areas in the DHmix – Ss /kB diagram. The results provide mutual understandings among H-E alloys, BMGs and HE-BMGs.

Magnetic properties of (Fe, Co)–B–Si–Nb bulk glassy alloys with high glass-forming ability

The glass-forming ability and magnetic properties of (Fe, Co)–B–Si–Nb glassy alloys have been investigated. The maximum diameter for the formation of a glassy alloy rod was 2.0mm for Fe72B20Si4Nb4 and 4.0mm for (Fe0.5Co0.5)72B20Si4Nb4. A number of local-ordered regions are recognized in Fe72B20Si4Nb4 bulk glassy alloy by high-resolution transmission electron microscopy. However, no local-ordered regions are observed in (Fe0.5Co0.5)72B20Si4Nb4 bulk glassy alloy. Saturation magnetization, coercive force, and maximum permeability were 1.14T, 1.5A∕m, and 32 000, respectively, for the (Fe0.6Co0.4)72B20Si4Nb4 bulk glassy alloy.

Magnetic properties of Co–Fe–B–Si–Nb bulk glassy alloy with zero magnetostriction

The glass forming ability and magnetic properties of a Co–B–Si–Nb glassy alloy have been investigated. The maximum rod diameters for the formation of a glassy phase were 1.5mm for Co70B20Si8Nb2 and 3mm for Co70B22Si6Nb4. Zero-magnetostriction bulk metallic glass samples of (Co0.952Fe0.058)70B20Si8Nb2 were prepared in cylindrical form with a diameter of 1.5mm and in ring form with a thickness of 0.5mm. The (Co0.952Fe0.058)70B20Si8Nb2 glassy alloy samples exhibited a high glass forming ability and good soft magnetic properties, i.e., a saturation magnetization of 0.6T, a low coercivity within 0.1–0.2A∕m, and a high permeability of 104 000 at a frequency of 1kHz and zero magnetostriction. The success of the synthesis of the zero-magnetostriction Co–Fe–B–Si–Nb glassy alloy with good soft magnetic properties and a high glass forming ability is promising for the future development of sensitive magnetic sensors.

Damping properties of hydrogen-absorbed rod metallic glasses

Abstract Internal friction of hydrogen-absorbed Zr–Al–Ni–Cu–Pd metallic glasses has been investigated using rod samples. A broad peak is observed around 230 K in the temperature dependence of internal friction. Strain amplitude and frequency dependences of internal friction have been also investigated. The activation energy of hydrogen jumps and pre-factor estimated from the Arrhenius plots on the basis of the frequency dependence are about 0.4 eV and 3×10−13 s, respectively. Effects of hydrogen concentration on the temperature dependence of internal friction have been also clarified. It is found that the peak temperature decreases with increasing hydrogen concentration and that the peak internal friction increases linearly with hydrogen concentration up to 1 at.%.

Structural characterization of Cu–Ti-based bulk metallic glass by advanced electron microscopy

The atomistic structure of a high-strength Cu42.5Ti41.5Ni7.5Zr2.5Hf5Si1 bulk metallic glass prepared by copper-mould casting has been characterized by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). High-resolution TEM and high-angle annular dark-field observations revealed that crystalline nanoparticles with sizes less than 10 nm are embedded in an amorphous matrix. It was confirmed by energy-dispersive X-ray spectroscopy in combination with STEM that these nanoparticles possess higher copper content than the amorphous matrix. Nanobeam electron diffraction experiments indicated that their crystalline structure is basically face-centred cubic with ordered atomic arrangements.

Structure and mechanical properties of cast quasicrystal-reinforced Mg-Zn-Al-Y base alloys

The structure and mechanical properties of Mg–Zn–Al–Y base cast alloys containing an icosahedral quasicrystal phase (i-phase) as a main strengthening phase were investigated. Mg–8Zn–4Al– x Y base bulk alloys containing the i-phase were prepared by casting into a copper mold at moderate cooling rates. The Y addition was effective for decreasing the size of the i-phase and the increasing the homogeneity of its dispersed state. The mechanical properties examined by compression tests at room temperature were much superior to those of a conventional AZ91 Mg alloy. The creep tests at elevated temperatures indicated a promising high temperature creep resistance of the quasicrystal-reinforced Mg–Zn–Al–Y cast alloy. The strengthening mechanism was also discussed.

Change of Nanostructure in (Fe0.5Co0.5)72B20Si4Nb4 Metallic Glass on Annealing

Nanoscale structural change in (Fe0.5Co0.5)72B20Si4Nb4 bulk glassy alloy on annealing has been investigated using transmission electron microscopy. On annealing at temperatures above 773K, electron diffraction intensity analysis showed a clear structure change for a Cr23C6-type local atomic ordering. The local structure formation of Cr23C6-type nanophase was confirmed by nanobeam electron diffraction. A development process of dense precipitates of the Cr23C6-type nanophase was further studied by high-resolution electron microscopy. It was found that the formation of the highly-dense nanoprecipitates provides an increase in Vickers hardness.

Bulk metallic glasses for industrial products; New structural and functional applications

By use of excellent properties of bulk metallic glasses, some industrial products were practically prepared and their performances were investigated. Linear actuator constructed by a set of Fe-based BMG yokes generates large Lorentz force due to the large permeability and saturation magnetization of the BMG. Ni-based BMG microgear prepared by injection casting exhibits nano-imprintability against the surface roughness of mold. The newly developed alloy with a nominal atomic composition of Ti52Cu23Ni11Mo7Fe7 exhibits high yield strength of 1250 MPa, high fracture strength of 2740 MPa and large plastic elongation of over 20 %. These results for the industrial products made of BMGs are promising for future developments as industrial materials with high performance.