Papers by Author: Takeshi Fukami

Abstract: A crystallization process in an amorphous state under isothermal condition is examined for binary alloys ZrNi and ZrNi2 by differential thermal analysis (DTA). Time dependence of DTA curves is measured at several constant temperatures just below crystallization temperature. The fraction of crystallized volume in amorphous state and its time evolution during isothermal annealing are measured. These data are analyzed by the Johnson-Mehl–Avrami formula. The Avrami exponent is 2.4±0.1 for ZrNi and 3~4 depending on the set temperature for ZrNi2. The activation energy for crystallization of amorphous ZrNi and ZrNi2 was estimated by plots of lnt1/2 vs. 1/T.

Abstract: In order to examine the correlation between a mechanical property and an excess free volume for metallic glass Pd44Cu31Ni8P17, longitudinal and transverse ultrasonic velocities were measured to estimate longitudinal and transverse elastic constants, c11 and c44. An as-quenched sample, an annealed one at a temperature just below a glass transition temperature Tg and a crystalline one were prepared. The as-quenched sample contains the excess free volume depending on the preparing process. The Young’s modulus E, the Poisson’s ratio, a bulk modulus and c12 are estimated using c11 and c44. The values of c11, c 44 , c12 and E of the as-quenched one are smaller than those of the annealed sample losing excess free volume by about 0.2 % and much smaller than those of the crystalline one. The Poisson’s ratio for the as-quenched sample and the annealed one are 0.389 and 0.387, respectively, which are much larger than that for the crystalline one with 0.349.

Abstract: High-strength nanocrystalline Ni-W alloys containing 16.9 at. % W with average grain size of about 6 nm in diameter has been obtained by electrodeposition. At room temperature, the nominal tensile strength of the alloy was attained to about 1600 MPa, while the plastic strain before fracture was a very low value of 0.05 %. In this case, highly localized shear bands were observed near the fractured surface of the tensile test specimen. When the samples were annealed at 300 °C under a static tensile stress of 327 MPa, the plastic strain was largely increased at the initial period of annealing and then tended to saturate, i.e., 0.54 % for 2 h, respectively. Grain size of the Ni-W alloys was almost saturated to 10 ~ 15 nm after annealing at 300°C for 2 h. It may be expected that the high tensile stress during grain growth might be effective to obtain the large uniform plastic deformation of nanocrystalline Ni-W alloys.

Abstract: Viscous flow behavior in supercooled liquid region of as-cast and pre-annealed Zr55Cu30Al10Ni5 bulk metallic glasses has been examined by using a penetration viscometer under high-speed heating rate of 20, 200 and 400 °C/min. Applied load for the cylindrical-shaped penetration indenter with a diameter of 1 mm was varied from 0.049 N to 0.294 N. Viscosity was quite independent of these applied loads. By pre-annealing the bulk metallic glasses at 400 °C, the density of the glasses increased, while the viscosity and the activation energy for viscous flow in their soopercooled liquid decreased with the pre-annealing treatments. Corresponding measurements of the differential thermal calorimetry (DSC) have been also done.

Abstract: In order to examine mechanical properties of a metallic glass Zr50Cu40Al10 in low temperature below room temperature, the temperature T dependence of mechanical resonance of ultrasonic wave are measured. The mechanical resonance frequency in an as-quenched sample shows an abrupt increase at 200K for longitudinal wave and 160 K for transverse wave with decreasing T. After this abrupt increase, the sound propagation cannot be detected below these temperatures but the wave propagation is restored with increasing T and there is an abrupt decrease at 260K for the both wave modes. The similar hysteresis is observed in temperature dependence of the electrical resistivity. These suggest a kind of structure instability of Zr50Cu40Al10 in low temperature region.

Abstract: Viscosity of a Zr55Cu30Al10Ni5 supercooled liquid alloy having bulk metallic glass forming ability has been measured by using a penetration viscometer with a cylindrical probe under high speed heating conditions at rates between 20°C/min and 400°C/min in the temperature range from the glass transition temperatures (Tg) up to above the crystallization temperatures (Tx). The viscosity of the alloy decreased with increasing the heating rate and tended to saturate at the heating rate of 200°C/min and above. Corresponding measurements of the differential thermal calorimetry (DSC) under the high-speed heating conditions have been also done.