F. Eiichi Fujita

Electron Radiation Damage of Iron in High Voltage Electron Microscope

Aggregation process of irradiation-induced point defects in iron is examined between room temperature and 400°C with a high voltage electron microscope. In the whole temperature range examined interstitial type dislocation loops are formed. The loops formed between slightly above room temperature and 350°C have flower-like shape. Especially the petalous loops formed around 300°C are divided into many small loops at the latter stage of irradiation by absorving mobile vacancies. The formation of the loops in the pre-irradiated specimen also tells that the vacancies become mobile at about 300°C. Analyzing the nucleation of the loops by using the chemical rate theory considering the effects of impurity atoms one obtains the activation energy for interstitial migration as about 0.26 eV.

Mössbauer Effect in Iron-Carbon Martensite Structure

The Mossbauer spectra in iron-carbon martensite could be resolved into four components arising from the 1st, 2nd, (3, 4)th neighboring iron atoms for the carbon atoms and the remnant iron atoms. The carbon atoms were found to cause localized changes of electronic states of the surrounding iron atoms. The internal magnetic field, isomer shift and quadrupole splitting for the first neighbors largely deviated from those for pure iron, i.e. , 265±2 KOe, -0.03±0.05 mm/sec and 0.13±0.05 mm/sec, respectively, suggesting the covalent admixture between the iron and carbon atoms. The internal fields and isomer shifts for the 2nd and (3, 4)th neighbors also deviated slightly from those of pure iron, but with the sign opposite to those for the 1st neighbors. The quadrupole splittings for them were nearly zero, which mean that the positive charge of a carbon ion is almost screened out within a distance of the order of 2A.

A Study of the Mössbauer Effect during the Tempering of Iron-Carbon Martensite

The Mossbauer effect of 57 Fe in the iron-carbon martensite steels was measured during their tempering process, and the appearance of the e -, χ- and cementite phases and the bonding nature in them were studied. It was clarified that the χ-carbide is formed at the tempering stage III a, the formation of which had not been fully proved in previous X-ray or magnetic measurements. It was also concluded that the electronic states of the iron atoms co-ordinating the carbon atoms in the martensite and the e -carbide formed in the stage I are substantially the same, suggesting that the motive force in the stage I is mainly the relaxation of the large strain energy in tne martensite structure. The observed localized and additive nature of the iron-carbon interaction, which was concluded from the linear relation between the amount of the internal field reduction and the number of carbon co-ordination for iron atoms in the various phases, seems to indicate that the Alexander-Anderson-Moriyas idea is applicable to ...

Mobility of lattice vacancies in iron

Abstract Three series of experiments to measure the mobility of vacancies in iron have been performed with a high-voltage electron microscope. The temperature dependence of the growth speed of interstitial-type dislocation loops during electron irradiation between 400°C and 860°C indicates an energy of 0·62 ±0·07 eV, and this is assigned to one half of the migration activation energy of a single vacancy, namely 1·24·0·14 eV. The annihilation of electron–radiation–induced vacancies U detected by the shrinkage and disappearance of interstitial clusters during post–irradiation annealing at temperatures between 270°C and 350°C. The temperature dependence of the process yields the energy of 1–47 ± 0·05 eV. The extent of the annihilation of deformation-induced vacancies is detected by the variation of interstitial cluster formation by electron irradiation. This annihilation process gives the same energy, 1·50 ±0·08 eV, and the same annealing half–time as those in the annihilation process of the radiation–induce...

The Mossbauer Effect of Fe-V and Fe-Cr Sigma Phase

The magnetic properties and bonding natures of the Fe- V and Fe-Cr σ phase were studied by the Mossbauer effect and saturation magnetization measurements. The Mossbauer absorption spectra of the Fe- V σ phase alloys measured at 77 K showed the hyperfine broadening due to overlapping of small internal magnetic fields. Each spectrum was decomposed into four components: The internal magnetic field on one site (I site) is nearly zero, and those on the other three sites (III, IV and V site) vary from 61 kOe to 135 kOe, 16 kOe to 65 kOe and 38 kOe to 103 kOe, respectively, with increasing iron composition. Applying the modified Pauling valence model developed by Mori and Mitsui, the obtained values of the internal fields were reasoned and, therefore, the covalent bonding nature in this structure was confirmed. In the Fe-Cr σ phase containing 46.8 at% Cr, hyperfine broadening was observed at 4.2 K and a similar analysis and discussion are made.

Recovery of the Quenched-In Solute Hydrogen Atoms in Nickel

The recovery process of hydrogen dissolved in excess in nickel is studied by means of the liquid hydrogen quenching method which is a modified Schultzs method. A recovery stage of electrical resistance is found in the range 190K~380K in the isochronal annealing curves. It is found that during the annealing the dissolved hydrogen atoms diffuse to and escape out of the surface of the specimen producing the timewise and spacewise varying distribution of concentration in the interior. The resistance of the specimen is calculated by using an equivalent circuit of the parallel connection of concentric hollow cylinders having different values of electrical resistivity depending on the hydrogen concentration which can be obtained from the diffusion equation. Extremely good agreement is obtained between the experiment and calculation, and the diffusion coefficient is determined as D=2.14×10-3exp (-8880/RT) cm2/sec.

Recovery Process in Thin Films of Noble Metals Vacuum-Deposited on Low Temperature Substrate

Recovery of electrical resistivity of pure Au, Ag and Cu thin films deposited in vacuo on a mica sheet kept at liquid helium temperature was studied. Recovery process of Au films consists of several substages below 80°K, which are similar to those found in irradiated Au: The point defect recovery is strongly suggested. In the higher temperature region, both the point defect recovery and grain growth seem to be involved in the recovery process. The activation energies for recovery range from 0.013 eV (at 13°K) to 0.14 eV (at 55°K). The reaction is of the first order at around 25°K. Similar results have been obtained for Ag. In contrast to Au films, Ag and Cu films show remarkable reverse annealing between 20°K and 40°K, which presumably is closely connected with residual gas molecules such as oxygen. Some possible mechanisms of reverse annealing are also discussed.

Recovery of Pure Iron Quenched-in Liquid Hydrogen

A new method of hydrogenation and quenching of metals in liquid hydrogen, i.e. the hydrogen quenching method, is developed and applied to investigate the recovery of pure iron after the hydrogenation and quenching by measuring the electrical resistivity change during annealing at higher temperatures. A new stage corresponding to the migration and disappearance of solute hydrogen atoms in pure iron appears in the resistivity decay curve, and by an approximate analysis based upon the rate process calculation the migration energy of single interstitial hydrogen atoms is determined to be 2.2±0.2 kcal/mol. In the diffusion process, most of the solute hydrogen atoms are discharged at the free surface of the specimen, but, in order to fully explain the resistivity decay curves, the influence of trapping sites or sinks for hydrogen in the interior must be also considered.

Electrical Resistance Decay and Structural Change of Au–Si Thin Films Induced by Air

The electrical resistance of Au–Si films deposited at room temperature decreases very rapidly after having been exposed to air. The resistance decrease amounts to more than 60% of the initial value after the aging time of 1000 minutes at room temperature. Electrical resistance measurement and electron diffraction and microscope observation under increasing temperature in air or in vacuum reveal that oxgen plays a dominant role in this phenomenon. In an oxidizing atmosphere, low temperature migration of Si atoms toward the surface takes place preferentially. As a result of such migration, the electrical resistance of the film decreases and approaches that of pure Au. In vacuum, phase separation of Au–Si films into pure Si and pure Au takes place by heating, and the electrical resistance also decreases. The activation energies of Si migration and phase separation in Au–Si films are roughly estimated to be about 18 kcal/mol and 30 kcal/mol, respectively.