Papers by Author: Akihiro Iwase

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: It is well known that intermetallic compound alloys possibly include more than two types of vacancies basically, that is A-vacancy and B-vacancy in A-B compound alloy. Although MeV ordered electron irradiation produces vacancies and interstitials, the threshold energy of displacement for each elemental atom in intermetallic alloys is not necessarily same. Then the defect type introduced by the irradiation depends on electron energy. In this study, we performed positron annihilation lifetime and Doppler broadening measurements for different energy of electron irradiated B2 type Fe50%Al alloy. The values of positron annihilation lifetime for this alloy after 2 and 9 MeV electron irradiation were 200 and 172 psec, respectively. Also, the Doppler broadening parameters S and W show difference for this alloy with different energy of electron irradiation. These results suggest that different types of vacancy were induced into Fe-Al compound alloy by electron irradiation with different energies.

Abstract: The effects of applied stress and test temperature on the environment-induced cracking (EIC) behavior of carbon steel in 5 kmol/m3 NH4NO3 solution were investigated by using constant load method. The three parameters (time to failure (tf), steady state elongation rate (lss) and transition time to time to failure ratio (tss/tf)) were obtained from a corrosion elongation curve. The relationships between applied stress and the three parameters were divided into three regions; the stress-dominated, the EIC-dominated and the corrosion-dominated regions. Under an applied stress of 225 MPa (in the EIC-dominated region), the test temperature dependences of the three parameters were investigated. Although tf decreased with increasing test temperature above 323 K, it became constant independent of test temperature under 323 K. Whenever the EIC takes place, the relationship between log tf and log lss become a good straight line with a slope of -1.1 irrespective applied stress and test temperature, which means that lss becomes a parameter for predicting tf. In addition, the fracture appearance in the EIC-dominated region was intergranular. From the results obtained, the intergranular cracking of carbon steel was discussed in terms of film formation, dissolution and hydrogen permeation.

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.