Nobuyuki Nishiyama

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.

Extremely low critical cooling rates of new Pd-Cu-P base amorphous alloys

A new Pd40Ni10Cu30P20 amorphous alloy subjected to the B2O3 flux treatment was found to have a maximum thickness above 50 mm and a low critical cooling (Rc) of 0.100 K s−1, The Rc for the Pd-Ni-Cu-P alloy without the flux treatment is 1.58 K s−1 and the decrease in Rc is due to the suppression of heterogeneous nucleation, in addition to the high Tg/Tm of 0.72 and ΔTx( =Tx − Tg) of 98 K. These Rc values are smaller and the Tg/Tm and ΔTx values are larger as compared with those (Rc = 140 K s−1, Tg/Tm = 0.67 and ΔTx = 63 K) for an unfluxed Pd40Ni40P20 amorphous alloy. The new alloy appears to lie around the eutectic point with a low Tm of 804 K and the crystallization occurs through a single stage accompanying the precipitation of Pd2Ni2P, Pd15P2, (Ni, Cu)3P and unknown compound. The finding of the new amorphous alloy with the lowest Rc and the largest thickness is important for the future development of bulk amorphous alloys.

Stability and nucleation behavior of glass-forming Pd–Cu–Ni–P alloy with a critical cooling rate of 0.067 K/s

Abstract An undercooled Pd 42.5 Cu 30 Ni 7.5 P 20 melt having a eutectic composition exhibits the lowest critical cooling rate for glass formation of 0.067 K/s. Nucleation and crystal growth behaviors of the undercooled melt were investigated. For example, nucleation frequency and crystal growth rate at 683 K where the temperature is close to nose temperature were evaluated as 4.76×10 9 nuclei/m 3 s and 3.24×10 −7 m/s, respectively. These values are almost the same as those for the undercooled Pd 40 Cu 30 Ni 10 P 20 melt which has a slightly off-eutectic composition. However, the difference in incubation time for crystallization and nucleation mode was observed in both melts. The nucleation mechanism of undercooled Pd 42.5 Cu 30 Ni 7.5 P 20 and Pd 40 Cu 30 Ni 10 P 20 melt is compared. Based on these results, the prolongation in incubation time for crystallization is discussed in the present study.

Undercooled liquid-to-glass transition during continuous cooling in Pd–Cu–Ni–P alloys

The glass-transition behavior from the supercooled liquid of a Pd40Cu30Ni10P20 alloy was investigated by employing a power-compensated differential scanning calorimetry under continuous cooling. At cooling rates of 0.83, 1.17, and 1.67 K/s, the transition was clearly detected as an abrupt decrease in heat capacity. From the difference in heat capacity between the undercooled liquid and glass, the alloy obtained at the lower cooling rate was found to have a more relaxed structure. The thermodynamic parameters determined in the present study enable us to interpret the reason for the outstandingly high glass-forming ability of the alloy.

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.

Extremely wide supercooled liquid region and large glass-forming ability in Zr65 − xAl7.5Cu17.5Ni10Bex amorphous alloys

Abstract The temperature interval of the supercooled liquid region before crystallization in Zr 65 − x Al 7.5 Ni 10 Cu 17.5 Be x alloys is the largest for the x = 0 alloy. It is therefore said that the addition of Be with an extremely small atomic size as compared with those of the other constituent elements is not effective for further extension of the wide supercooled liquid region. The quenching of the melt in a quartz tube into water was found to enable the production of bulk amorphous alloy in cylindrical form with a diameter of 16 mm and a length of 150 mm for the Zr 65 Al 7.5 Ni 10 Cu 17.5 alloys with the largest ΔT x of 127 K. The finding of the extremely large glass-forming ability of the ZrAlNiCu alloy is very important for subsequent development of metallic amorphous alloys.

Thermal evidence of stress-induced structural disorder of a Zr55Al10Ni5Cu30 glassy alloy in the non-Newtonian region

The evolution of specific heat near the glass transition temperature, Tg, of a Zr55Al10Ni5Cu30 glassy alloy subjected to different degrees of non-Newtonian flow conditions was investigated. At conditions in which non-Newtonian flow begins, a gradual exothermic relaxation near Tg was observed. This is attributed to the change in configuration of the long-range liquid structure of the alloy. At conditions in which a high degree of non-Newtonian flow is favored, the alloy exhibits both high-temperature and low-temperature relaxation due to a change in local short-range order. The enthalpy of relaxation is found to scale as the logarithm of the normalized viscosity and is interpreted in terms of the free volume model of viscous flow. The decrease in viscosity in the non-Newtonian flow region is associated with a highly disordered structure and induced free volume.

Structural instability of metallic glasses under radio-frequency-ultrasonic perturbation and its correlation with glass-to-crystal transition of less-stable metallic glasses

It has been reported that the structural stability is significantly deteriorated under radio-frequency-ultrasonic perturbation at relatively low temperatures, e.g., near/below the glass transition temperature T(g), even for thermally stable metallic glasses. Here, we consider an underlying mechanism of the ultrasound-induced instability, i.e., crystallization, of a glass structure to grasp the nature of the glass-to-liquid transition of metallic glasses. Mechanical spectroscopy analysis indicates that the instability is caused by atomic motions resonant with the dynamic ultrasonic-strain field, i.e., atomic jumps associated with the beta relaxation that is usually observed for low frequencies of the order of 1 Hz at temperatures far below T(g). Such atomic motions at temperatures lower than the so-called kinetic freezing temperature T(g) originate from relatively weakly bonded (and/or low-density) regions in a nanoscale inhomogeneous microstructure of glass, which can be straightforwardly inferred from a partially crystallized microstructure obtained by annealing of a Pd-based metallic glass just below T(g) under ultrasonic perturbation. According to this nanoscale inhomogeneity concept, we can reasonably understand an intriguing characteristic feature of less-stable metallic glasses (fabricated only by rapid melt quenching) that the crystallization precedes the glass transition upon standard heating but the glass transition is observable at extremely high rates. Namely, in such less-stable metallic glasses, atomic motions are considerably active at some local regions even below the kinetic freezing temperature. Thus, the glass-to-crystal transition of less-stable metallic glasses is, in part, explained with the present nanoscale inhomogeneity concept.

Change in electron transport property after glass transition in several Pd-based metallic glasses

Abstract We investigated changes in the electron transport property of stable Pd 76 Cu 6 Si 18 and Pd 40 Ni 10 Cu 30 P 20 metallic glasses, prepared by a melt-spinning technique, following the glass transition. These metallic glasses had a supercooled liquid state after annealing with a heating rate of 0.67 K/s. The slope of resistance as a function of temperature changed after the glass transition for both metallic glasses. However, transmission electron microscope (TEM) observations and X-ray diffraction did not detect crystalline phases or amorphous phase separation in the supercooled liquid region. We discuss briefly the change in the electrical resistivity after the glass transition based on the estimation of the thermal expansion coefficient in the supercooled liquid.