Fuminobu Hori

Positron annihilation lifetime study of irradiated and deformed Fe and Ni

Abstract In order to investigate the fundamental behaviors of radiation-induced defects, especially vacancy type defects and also deformation-induced dislocations and vacancies, positron annihilation lifetime measurements have been performed for Fe, FeCu and FeSi irradiated with electrons at low temperature (77 K) and also for Ni deformed at room temperature. From the isochronal annealing experiments it was found that vacancies become mobile above 200 K and form microvoids in Fe, but in FeCu and FeSi alloys the interaction between vacancies and solute atoms significantly suppresses the microvoid formation process. In FeCu alloy, instead of microvoid formation it was considered that vacancy-Cu complexes are formed by judging the value of positron lifetime. In the deformed Ni positron lifetime decreased gradually as the isochronal annealing temperature increased. From this result and positron lifetime calculation it was suggested that in deformed Ni positrons are trapped and annihilated at complexes of a dislocation and deformation-induced vacancies.

A study of vacancy-type defects in the B2-phase region of the Fe–Al system by positron annihilation method

Abstract Vacancy-type defects of B2-type intermetallic compound FeAl have been investigated as functions of composition ranging from 41.2 to 50.7 at.% Al and quenching temperature ranging from 610 to 1373 K by means of positron annihilation technique. At quenching temperatures lower than 873 K, both the mean positron lifetimes and S-parameters increase with increase in Al content. With increasing quenching temperature from 873 K, the S-parameters for 41.2, 49.0 and 50.7 at.% Al certainly increase. The results are discussed by considering changes in the defect type and the atom configuration around the vacancies. The composition dependence at lower quenching temperatures is explained partly as a change in the fractions of Fe-vacancies, VFe, and the di-vacancies on the Fe-sites, 2VFe, and partly as a change in the fraction of antisite Fe atoms, FeAl. The temperature dependence of the S-parameter is explained by change in defect type from di-vacancies, 2VFe, to different type of di-vacancies, VFeVAl, sitting on the nearest neighboring Fe and Al sites.

Positron lifetime calculation in FeCu binary alloy with lattice relaxation

Abstract Using the embedded atom type many body potentials for iron and copper, the lattice relaxation around copper impurity atoms in iron has been performed. The stable configuration of atoms around copper and copper vacancy complexes in FeCu alloy was simulated. By introducing a positron into the relaxed lattice of FeCu binary alloy with and without a vacancy, positron lifetimes have been calculated. The positron lifetime in a relaxed FeCu matrix was 119 ps, while in a relaxed Cu-vacancy pair it was 168 ps. The latter, which is shorter than the positron lifetime in a single vacancy, is in good agreement with the experimental lifetime observed in FeCu alloy irradiated with electrons at low temperature.

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.

Study on analysis of crystal structure in CeO2 doped with Er2O3 or Gd2O3

Abstract To simulate the effects of burnable poison doping in nuclear fuel UO 2 , Er 2 O 3 (or Gd 2 O 3 )-doped CeO 2 pellets were prepared. Changes in lattice constant and atomic disordering for CeO 2 due to the Er 2 O 3 and Gd 2 O 3 doping were measured by means of XRD and XAFS. By the Er 2 O 3 doping, the lattice constant decreased, and a disordering of lattice structure was induced in the samples. The doping with Er 2 O 3 also induced the disordering of atomic arrangement around Er atoms, which was observed through the change in XAFS spectra. In contrast, the effect of Gd 2 O 3 doping was smaller than that of Er 2 O 3 doping. The result was discussed in terms of ionic size of dopants in CeO 2 crystal.

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.

Defect structures and phase transitions of FeRh alloys deformed at high speed deformation

Abstract Fe–Rh alloys of Rh concentrations ranging from 40 to 50at.% and of B2 phase were deformed by use of a compression machine capable of running tests at high speeds of impact. Induced complicated substructures and phases were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), and positron annihilation spectroscopy (PAS). A characteristic arrangement of L1 0 domains was observed, along with very small transformed A1 domains and dispersed in the residual B2 matrix. On the basis of the experimental results, we suggest a coupling of a pair of shears along {112}〈11−1〉 B2 for the transformation mechanisms from B2 to L1 0 and A1 phases.

Carbonization and/or Nitridation of Titanium and Zirconium for Increasing Melting Temperature

Samples of Ti and Zr metals have been carbonized or nitrided at high temperature by means of reactive plasma processing. Thickness and hardness of modified layers increased with the processing temperature and time in an experimental range below 1500°C for less 5h. The maximum hardness of TiC-layer modified at 1300°C for 3h was about 5000Hv(kg/mm 2 ) and the maximum thickness of ZrC-layer formed at 1400°C for 5h reached about 100μm. A TiC-layer was analyzed with EPMA, XRD, EXAFS and PAS(Positron Annihilation Spectroscopy). It was confirmed that the sub-surface layer of Ti metal was the mixture of TiC ceramics and Ti metal, and that the thickness of TiC with NaCI type structure was very large.

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.

Study on ion-irradiation-induced ferromagnetism in FeRh intermetallic compound by means of magnetic Compton scattering

The magnetic Compton profiles of Fe–50 at. % Rh intermetallic compound were measured to study the ferromagnetism induced by 200 MeV Xe ion irradiation. The magnetic effect at 50 K increases with increasing the ion-fluence. The analysis of the experimental result revealed that the values of spin moment induced by the irradiation were close to the values of magnetization obtained by a superconducting quantum interference device magnetometer, suggesting that the ion irradiation mainly induces the spin magnetic moment. The difference in magnetic Compton profiles between the irradiation-induced ferromagnetism and the intrinsic ferromagnetism in pure Fe is also discussed.