Papers by Author: Yoshihiko Yokoyama

Abstract: The Zr-Cu-Al bulk amorphous (BA) alloys, which have no long-range ordered structure, possess various properties such as high strength and toughness with compositional dependence. In the present study, in order to estimate the natures of local structure change and relaxation behavior of hyper-eutectic BA alloys during annealing, positron annihilation measurements and density measurement have been performed for hyper-eutectic Zr-Cu-Al BA alloys with annealing. The enthalpy relaxation and structural relaxation was also measured by differential scanning calorimetry (DSC) measurement. These results show that the relaxation process of free volume containing in hyper-eutectic BA alloys is different from that in hypo-eutectic alloys.

Abstract: In order to investigate very high cycle fatigue properties of Zr₅₅Al₁₀Ni₅Cu₃₀ bulk amorphous alloy, rotating bending fatigue tests were conducted at room temperature by using 5 different series of Zr₅₅Al₁₀Ni₅Cu₃₀ alloy. The differences of material processing conditions are manufacturer, product year, material size and minimum diameter of specimen. Although all specimens fractured from surface, duplex S-N characteristics were observed for each series of the material. Time strength distributions at N=10⁴ for short life region and at N=10⁷ for long life region were well approximated by normal distribution. The entire S-N property accepting the normalized stress amplitude by the time strength at N=10⁴ or N=10⁷ has shown more clearly duplex S-N characteristics. In addition, P-S-N properties were also estimated from the standard deviation of time strength distributions at N=10⁴ or N=10⁷. Based on the observation of fracture surface by scanning electron microscope (SEM), it is confirmed that every fracture surface was consisting of typical three regions such as multi-facet region, stable crack growth region and instantaneous fracture region.

Abstract: Zr-Cu-Al bulk metallic glass matrix composite with the intermetallic compound ZrCu (B2) phase was fabricated. The effect of the ZrCu phase on mechanical properties and deformation behavior of the composite was investigated by compressive test. Also, phase transformation behavior of the ZrCu phase was analyzed by X-ray diffraction. Furthermore, macroscopic shape memory behavior was investigated using the composite by three point bending test. As the results, it was found that the mechanical properties of the composite depend on the volume fraction of the precipitated ZrCu phase. Compressive strength and yield stress of the composite decrease with increase of volume fraction of the ZrCu phase. On the other hand, plastic strain increases with increase of volume fraction of the ZrCu phase. In addition, it was confirmed that a stress-induced martensitic transformation of the ZrCu phase occurs by compressive stress loading. Furthermore, it was found that the composite with high volume fraction of the ZrCu phase exhibits shape memory effect.

Abstract: Structure of Zr₅₀Ni₅₀ Amorphous Alloy Was Analyzed by Anomalous X-Ray Scattering (AXS) Coupled with Reverse Monte-Carlo (RMC) Simulation. Topological Features in the Nearest Neighbor Atomic Configuration Clearly Suggest that the Strong Chemical Interaction between Ni and Zr Breaks the Icosahedral-Like Local Ordering Structure Common in the Dense Random Packing of Hard Sphere (DRPHS) Model. Nevertheless, the Structure of Zr₅₀Ni₅₀ Shows No Further Features Related to the Crystal-Like Chemical Short Range Ordering (CSRO).

Abstract: The crystal structure of the phase precipitated in a Zr-Cu-Al ternary alloy during quenching was investigated. The crystal structure is constructed by stacking several atomic layers periodically and is similar to that of the Laves phases. The reason why a Zr-Cu-Al alloy tends to crystallize into Laves-like periodic stacking order structure is discussed.

Abstract: Zr-based bulk glassy (BG) alloys show high tensile strength and a high Charpy impact value. In this study, the free volume changes for the hypoeutectic Zr60Cu30Al10 BG alloy during isothermal annealing below glass transition temperature (Tg) have been investigated by positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements. The positron lifetime of hypoeutectic and eutectic BG alloys is almost the same, although the atomic volume ratio, estimated by the density for the eutectic BG alloy is different from that for the hypoeutectic BG alloy. The CDB spectra show a marked difference between eutectic and hypoeutectic BG alloys; that is, the spectrum of the hypoeutectic BG alloy is closes to that of Zr metal than that of the eutectic BG alloy. This result exhibits that the elemental fraction of Zr atoms around free volume in hypoeutectic BG alloy is greater than that in eutectic BG alloy. The CDB ratio profiles for the hypoeutectic BG alloy during annealing shows no appreciable change. The same trend was observed in the eutectic BG alloy.

Abstract: Viscous flow behaviour of supercooled liquids and mechanical properties in Zr55+XCu30-XNi5Al10 (X=0, 5 and 10 at. %) and Zr65Cu18Ni7Al10 bulk metallic glasses (BMGs) have been examined. Viscosity has been measured by using a penetration viscometer at a high-speed heating rate of 400 K/min. With increasing Zr-content, Tg tended to decrease and Tx tended to increase, resulting Tx (=Tx-Tg) increaseing up to about 170 K under a heating rate of 400 K/min by the DSC analysis. The viscosity lowered about one order of magnitude, e.g., minimum value of the viscosity decreased from 3~4x106 Pa-s for the Zr55Cu30Ni5Al10 and Zr60Cu25Ni5Al10 BMGs down to 5~6x105 Pa-s for the Zr65Cu20Ni5Al10 and Zr65Cu18Ni7Al10 BMGs under the same heating condition. Compression tests were also carried out at a slow strain rate of about 1 x 10-4 s-1 at room temperature. With increasing Zr-content, the apparent yield stress was decreased but the total strain at fracture was increased significantly, suggesting that there is a strong relationship between viscosity and the mechanical properties in these BMGs.

Abstract: The kinetics of structural relaxation in fragile glass former, Pd46Cu35.5P18.5 BMG, and strong glass former, Zr50Cu40Al10 BMG, was investigated by volume relaxation. The former exhibited a relaxation phenomenon that is well understood by the local topological instability model, while the latter showed monotonous relaxation behavior over a wide range down to Tg-60 K. The discrepancy may be closely related to the difference in the fragility of both glasses.

Abstract: Recent research works on bulk-metallic glasses (BMGs) have opened a window to create a new generation of structural materials for applications. Although the mechanical behavior of BMGs is being studied widely, the fatigue characteristics are poorly understood. The uniaxial tension-tension high-cycle fatigue (HCF) studies were performed on zirconium (Zr)-based bulk-metallic glasses (BMGs): Zr50Cu40Al10, Zr50Cu30Al10Ni10, Zr50Cu37Al10Pd3, and Zr41.2Cu12.5Ni10Ti13.8Be22.5, in atomic percent. The HCF experiments were conducted using an electrohydraulic machine at a frequency of 10 Hz with a R ratio of 0.1, where R = σmin./σmax., where σmin. and σmax. are the applied minimum and maximum stresses, respectively. The fatigue-endurance limit of Zr50Cu37Al10Pd3 was significantly greater than those of Zr50Cu40Al10, Zr50Cu30Al10Ni10, and Zr41.2Ti13.8Cu12.5Ni10Be22.5. In order to compare the fatigue property with the crystalline alloys, the same HCF experiments were also performed on Ti-6-4, drill tool steel, and Al 7075. The fatigue lifetime of Zr-based BMGs is generally comparable to those of Ti-6-4 and drill-tool-steel crystalline alloys and is greater than that of Al 7075 alloy. The fracture morphology of BMGs indicates that fatigue-crack-propagation region included the distinct rough striations and the fine striations. The possible mechanism for the striation formation was proposed.

Abstract: Resistance microwelding is an important joining process used in the fabrication of miniature instruments. The small-scale resistance spot weldability of Zr based glassy thin sheet alloys was investigated. Metallic glass sheets were successfully welded and amorphous structure was maintained in both weld nugget and heat affected zone. Joint strength that is evaluated by shear breaking force increased as the welding current increased, resulting from nugget growth. Fractured surface showed typical vein pattern.