Francisco Eiichi Fujita

Tweed structures associated with Fcc-Fct transformations in Fe-Pd alloys

The tweed microstructure associated with fcc-fct martensitic transformation in Fe-Pd alloys containing about 30 at. pct Pd was investigated by transmission electron microscopy and the selected area diffraction. It was observed that the tweed contrast consisting of {011}/<01ī> striations and accompanying the diffuse streaks became distinct from a temperature nearly 100 K above the Ms with lowering temperature. The tweed contrast was found to be induced by the static displacement of atoms, δ{011}/[01ī], associated with the formation of disc-shaped martensite nuclei on the {110} planes of the austenite. The tweed microstructure persistently remained in the fct martensite in the early stage of the transformation, where the degree of tetragonality is small and the c-axis directions of the small fct martensite variants are tilted against each other, giving rise to the tweed pattern. A similar tweed contrast observed in the initial stage of ordering in a stoichiometric Fe-Pd alloy was also explained by a strain contrast effect arising from an elastic shear strain induced by the nucleation of short range order particles with tetragonal distortion.

57Fe Mössbauer study on oxygen desorption of high-Tc YBaCu oxide superconductor

Abstract 57 Fe Mossbauer spectra have been measured at room temperature for high-T c YBa 2 Cu 2.95 Fe 0.05 O 7−y oxide superconductor as a function of annealing periods and temperatures in vacuum. Mossbauer spectra are analysed using mainly four different components. Most of Fe atoms are identified to replace Cu atoms at the Cu1 chain sites. Changes in intensities of the components during isochronal and isothermal annealings are considered to be due to the oxygen desorption and the changes in oxygen coordinations around Fe atoms. From the kinetics of the changes in spectral intensities, the activation free-energy has been obtained to be 1.7−1.9 eV.

Electron microscope study on martensitic transformations in Fe- Pt alloys: General features of internal structure

Martensitic transformations of Fe-Pt alloys near the composition of Fe3Pt with various degrees of order were examined systematically mainly by transmission electron microscopy. This alloy system exhibits two types of martensitic transformations, fcc-bct (bcc) and fcc-fct. The present experimental results indicate that each transformation is independent and compctitive. As the degree of order of austenite (L12 superlattice) increases, the dislocation density of bct (bcc) martensite is remarkably reduced and the shape of internal twins is changed, which is closely related to the thermoelastic behavior and the reversibility of the transformation.

57Fe Mössbauer study under high pressure; ε-Fe and Fe2O3

Using diamond anvil cell, the57Fe Mössbauer spectra of pure iron foil and α-Fe2O3 powder under high pressure have been measured at room temperature.57Fe Mössbauer spectra of α-Fe were measured from 15 GPa to 45 GPa. Isomer shift value decreased and the quadrupole splitting slightly increased as the pressure increased.57Fe Mössbauer spectra of Fe2O3 under high pressure up to 72 GPa were observed. Above 52 GPa, the new lines appeared at the center portion of the spectrum corresponding to the new high pressure phase. The spectrum of new high pressure phase consisted of 6-line splitting and doublet, suggesting the existence of the two different kinds of iron states in it.

Dislocation loops and their depth profiles in He+ and D+ ion irradiated nickel

Abstract Effects of implanted deuterium and helium on the formation of dislocation loops in Ni have been systematically investigated in terms of the irradiation temperature and dose by means of transmission electron microscopy. Significant enhancement of loop formation is observed only for He+ irradiation. The number density of loops at 200° C for He+ irradiation is about one order of magnitude higher than that of D+ irradiation and the difference increases at higher temperatures. Significant decrease in the density occurs at about 300° C for D+ irradiation, but it appears at about 600° C for He+ irradiation. Below 200° C, continuous nucleation of loops is observed only for He+ irradiation. The change in the loop depth distribution suggests that the nucleation of loops for He+ irradiation is enhanced by some defects with a low mobility such as small He-vacancy complexes. The significant decrease in the loop density above 600° C is explained by the decrease in the concentration of the small He-vacancy complexes due to their absorption by large cavities or their escape to the specimen surface.

Interference Function of Crystalline Embryo Model of Amorphous Metals. I

A simple and possible structural model of amorphous metals based on the concept of crystalline embryos is proposed. The quasi-crystalline clusters are supposed to exist in the liquid state, be enhanced during supercooling, and be frozen as the crystalline embryos in the amorphous state by rapid quenching. A model assembly of atoms containing the crystalline embryos and the boundary regions is constructed, and the pair correlation function and the interference function are calculated. The interference function of the b.c.c. embryo model is in good agreement with experimental ones. It is concluded that the structure of the boundary connecting the embryos plays an essential role as well as the ordered part in the embryos in the diffraction phenomena of the amorphous structures. The importance of chemical clusters and metalloid atoms is also suggested and discussed.

Mössbauer Spectroscopy of Amorphous Fe–B Alloys

Binary Fe-B amorphous alloys containing 12–25 at%B were studied by 57Fe Mossbauer spectroscopy. The spectra were examined by Fourier analysis and near-neighbor configuration analysis. The characteristic concentration dependence of the internal field distribution was explained by the interactions between iron and boron atoms in different ways depending on the boron content and the microstructure of the amorphous matrix. Alloys with boron content below 20 at% exhibited well-developed short range order with the bcc-like near-neighbor configuration, while Fe-25 at%B and slowly-quenched Fe-12 and 13 at%B alloys showed a considerable fraction of chemical order of the Fe3B type. The existence of chemical clusters in Fe-25 at%B suppressed the nucleation of Fe2B precipitates, and this alloy was therefore the most thermally-stable.

Mössbauer study on mixing and kneading of metallic powders

Utilizing57Fe and119Sn Mössbauer spectroscopy, the microscopic behavior of metallic powders has been investigated during and after the mechanical alloying (MA) process. A conventional ball-milling method is employed and the following combinations of metallic powders have been studied for their MA; Al−Fe, Fe−Sn and Ag−Fe. Mössbauer spectra show the occurrence of mutual atomic dispersion during the MA process even for the Ag−Fe system which is known as mutually immissible even in the liquid state. Formation of an amorphous phase in the Al−Fe system has been detected.

Defects in Si irradiated with D-T neutrons, D and He ions

Deuterium and helium ion bombardments of silicon have been performed in the range of 300–673 K as simulation experiments of the effect of protons and helium produced by nuclear reactions with D-T neutrons. The irradiation-induced defects were investigated by transmission electron microscopy and positron annihilation techniques. The spatial distribution of the defects was compared with simulation calculations made using the TRIM-3D code. It was found that cavities were formed by He+ ion bombardment with doses greater than 2 × 1021ions/m2, but not by D+ ion bombardment. {113} interstitial type dislocation loops have been observed by bombardments both with D+ and He+ ions. It was found that the implanted He+ and D+ ions behave differently for the nucleation of irradiation-induced secondary defects.

57Fe mössbauer study of high-T c Y−Ba−Cu oxide superconductors

Abstract57Fe Mössbauer measurements have been performed for Fe-doped YBa2 (Cu1-x Fex)3O7-v (x=0.005–0.10) superconducting oxides. Fe atoms mainly substitute at Cul chain sites and the fraction of Fe at Cu2 plane sites is discussed thermodynamically. The spin direction of Fe at Cu2 was determined to be normal to thec-axis using an oxygen deficient specimen having a strong texture along thec-axis. A magnetically broadened spectrum at 4.2 K for Fe at Cul is compared with a simulated one which takes the distribution of hyperfine fields into account.