Akito Ishii

Behavior of free volume in ZrCuAl bulk metallic glass after irradiation

Free volumes in Zr50Cu40Al10 bulk metallic glasses irradiated by 100 MeV and 200 MeV Xe ions and 2 MeV electrons at room temperature have been investigated by using positron annihilation techniques, the micro Vickers hardness and X-ray diffraction measurements. No crystallization took place due to both ion and electron irradiations. The coincidence Doppler broadening spectra of annihilation gamma rays also remained unchanged by ion and electron irradiation. Meanwhile, an increase and a decrease of positron lifetimes after electron and ion irradiation, respectively, were observed. This suggests the structural modification of free volumes. The response of positron lifetime to ion and electron irradiations may be connected with the tendency of hardness.

Free volume in Zr-based bulk glassy alloys studied by positron annihilation techniques

The eutectic Zr50Cu40Al10 bulk glassy (BG) alloys, which have a good glass forming ability, have been investigated for various properties, such as toughness, electronic resistivity and so on. Recently, the hypoeutectic Zr60Cu30Al10 BG alloys have been paid attention, because they exhibit no degradation of ductility and toughness after the isothermal annealing below the glass transition temperature (Tg). Although the toughness correlates with the free volume to be considered, the free volume of the hypoeutectic BG alloy has not been assessed in detail so far. In order to study the free volume in each BG alloy, therefore, positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements have been performed for Zr60Cu30Al10 (hypoeutectic) and Zr55Cu35Al10 (middle-composition) and Zr50Cu40Al10 (eutectic) BG alloys. The value of the positron lifetime for hypoeutectic BG alloy is virtually the same. Additionally, the CDB ratio curve for hypoeutectic alloy dose not match that for eutectic alloy in an electron momentum region around 0.015 m0c, indicating that the fraction of Zr atoms around the free volume in hypoeutectic BG alloy is greater than that in eutectic BG alloy. Moreover, the CDB results show that the local structure around free volume for hypoeutectic BG alloy is different from that for eutectic BG alloy.

Free volume change in Zr50Cu40Al10 glassy alloy by the annealing studied by positron annihilation spectroscopy

It is well known that structural relaxation in a bulk metallic glass affects some properties such as viscosity, electrical conductivity, and ductility. The free volume in glassy alloy, which is open volume retained by rapidly solidification from liquid state, has a significant roll for those properties. In order to discuss the nature of free volume in a Zr50Cu40Al10 bulk metallic glass, positron lifetime measurements have been performed for this metallic glass before and after annealing, and all positron lifetime spectra have been decomposed into multi components as a size distribution function by use of CONTIN-PALS II program. The positron lifetime distribution for this metallic glass has a broad spectrum comparing to crystal metal including a single vacancy, and its average lifetime corresponds to the free volume size. This width of positron lifetime (free volume size) distribution decreases by the annealing at 673 K. This change of the positron lifetime distribution can be attributed to a free volume relaxation.

A study of defects in electron- and ion-irradiated ZrCuAl bulk glassy alloy using positron annihilation techniques

Free volume changes in Zr50Cu40Al10 bulk glassy alloys irradiated by 200 and 2.5 MeV Xe ions, 180 keV He ions, and 2 MeV electrons were investigated at room temperature using positron annihilation lifetime and Doppler broadening techniques. In addition, a slow positron beam was used to probe the change in free volume in the 180 keV He ion-irradiated sample. X-ray diffraction revealed that no crystallization took place in any of the irradiated samples. The Doppler broadening spectra from the annihilated gamma rays remained essentially constant in all ion-irradiation cases; however, an extremely minor change of positron mean lifetime was detected in each case. For electron- and He ion-irradiated samples the positron lifetime increased, and the opposite was seen in heavy-ion irradiated samples. The Doppler broadening S parameter increased with He-ion radiation dose, and the depth profile correlated well to the damage profile.

Relaxation Behavior of Zr-Cu-Al Ternary Bulk Glassy Alloy Studied by using Positron Annihilation Techniques

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.

Positron annihilation study of free volume in electron irradiated Zr50Cu40Al10 bulk glassy alloy: Effects of thermal relaxation before irradiation

Change in free volume reflects various properties such as hardness and ductility of glassy alloys. The electron- and ion- irradiations affect the free volume of ZrCuAl bulk glassy alloys that is associated with the mechanical properties. In this study, as-quenched Zr50Cu40Al10 bulk glassy alloys and structural relaxed one by annealing for 5 hours at 673K below glass transition temperature (Tg) were irradiated by 8 MeV electrons with a maximum fluence of about 2×1018 e−/cm2 at room temperature. The behaviour of free volume in these samples was investigated by positron annihilation techniques. X-ray diffraction measurements showed that no crystallization occurred after the irradiation. We observed the positron lifetime increased by the irradiation for both as-quenched and structural relaxed samples. The increases in positron lifetime at 2×1018 e−/cm2 were 9 psec for as-quenched and 12 psec for relaxed sample, respectively. In addition, the increase in positron lifetime with irradiation fluence was clearly different for the two types of sample. These facts imply that the thermal relaxation before irradiation of Zr50Cu40Al10 bulk glassy alloy influences the radiation effects, especially the free volume change.

Free Volume Relaxation Process in Zr 50 Cu 40 Al 10 Bulk Metallic Glass Studied by Positron Annihilation Techniques

Structural relaxation around free volume in Zr 50 Cu 40 Al 10 bulk metallic glass (BMG) during isothermal annealing at 473, 573 and 673 K which are below glass transition temperature T g =675 K have been investigated by positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) measurements. The trends of change in positron lifetime, which correspond to the size of free volume at each annealing temperature, have a good correlation with their density change. These annealing processes obey a stretched exponential relaxation function (KWW: Kohlrausch-Williams-Watts law). Fitting parameters of KWW function, with relaxation time t 0 and β, in each temperature were determined. These relaxation parameters depend on the annealing temperature, suggesting the distribution of activation energy for structural relaxation. Moreover, the profile of electron momentum distribution around free volume derived by CDB spectrum during annealing showed no appreciable change at each temperature. These facts suggest that long range chemical ordering, particularly around the free volume, dose not take place essentially.