Eiichi Kuramoto

Positron lifetime calculations on vacancy clusters and dislocations in Ni and Fe

Abstract Positron lifetime calculations have been performed on vacancy clusters (stacking fault tetrahedra (SFT), vacancy loops), such clusters on dislocation line, interstitial clusters, such clusters on a dislocation line and dislocation line itself in order to investigate the so-called intermediate lifetimes observed in the experiments, namely, positron lifetimes between that at a matrix and that at a single vacancy. Before lifetime calculations, various defects were constructed in the model lattices and were relaxed completely to obtain the stable atomic structure by using N -body potentials. Then positron lifetime calculation was carried out for each defect. It was shown that positron lifetime for a SFT in Ni dependes on its size and becomes smaller with increasing the size. The positron wave function is mainly localized at the corner of a SFT, which gives rather lifetime, e.g., 130 ps for V 28 , but when the cluster size is small, e.g., less than 10 vacancies, it gives a rather longer lifetime, e.g., 177 ps for V 6 because of the wave function localized at the inner space of a cluster. These behaviours are consistent with the experimental results. It was also found that the positron lifetime on a dislocation line and that at a jog are short (113 and 119 ps, respectively for Ni, 117 and 117 ps, respectively for Fe), close to the lifetime at matrix (110 ps for both Ni and Fe) and in these cases trapping potentials for a positron are shallow both for Ni and Fe.

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.

Computer simulation of fundamental behaviors of interstitial clusters in Fe and Ni

Atomistic features of small interstitial clusters in Fe and Ni have been investigated by computer simulation. The gradual change from an interstitial cluster to a dislocation loop was observed in detail from three points of view, (i) strain distribution along crowdion axis for each crowdion in a cluster, (ii) distribution of Burgers vector of peripheral dislocation line of a cluster, and (iii) recombination behavior of a vacancy of with a cluster. It is found that strain distribution was spread into two opposite directions on a crowdion axis with the increase of a cluster size. The irregularity on the curve of the distribution of Burgers vector of a smaller loop gradually disappears with the increase of a loop size. Recombination between a crowdion and a vacancy occurs on the peripheral position of smaller clusters, but this does not occur in larger clusters. These aspects show that the gradual change from an interstitial cluster to a dislocation loop occurs at a certain width of cluster size. Dynamic behavior was also investigated under cylindrical shear stress and the Peierls stress was obtained as a function of loop size. The results show that the Peierls stress decreases with increasing loop size down to the value of a straight edge dislocation. Activation energies of one atomic jump of these small dislocation loops were also calculated and small values of about 0.2 eV were found for loops of about 200 crowdions for both Fe and Ni.

INTERACTION OF SOLUTES WITH IRRADIATION-INDUCED DEFECTS OF ELECTRON-IRRADIATED DILUTE IRON ALLOYS

Electrical resistivity recovery spectra were measured between 77 and 200 K on Fe and Fe dilute alloys (Fe–Mo, Fe–Cr, Fe–Si, Fe–P, Fe–Be) irradiated with 2.5 MeV electrons at 77 K in order to study the interaction of substitutional solutes with self-interstitials. Oversized Mo and undersized Si solutes annihilated the recovery stage IE in pure Fe, whereas slightly oversized Cr, and undersized P and Be solutes yielded new recovery stages below IE, the positions of which stages shifted toward lower temperatures with increasing solute concentration. These stages are most likely ascribed to the migration and annihilation of mixed-dumbbells formed for the most part in stage ID. The characteristics of these recovery spectra have not been observed in FCC alloys, which leads to the indication that the mixed-dumbbell in these alloys has a higher mobility than the self-interstitial.

The Nature of Stacking Faults and Partial Dislocations in Deformed Ni3Ga Single Crystal

The nature of stacking faults and partial dislocations in deformed Ni3Ga single crystal having the L12 structure has been analysed by transmission electron microscopy on the basis of the diffraction theory of the electron microscope images. The results showed that stacking faults are of the superlattice intrinsic type and that the majority of stacking fault ribbons are bounded by two superlattice Shockley partials with parallel and opposite a/3 Burgers vectors. This latter fact is incompatible with any formation mechanisms so far proposed. Most partials lay parallel to directions 30deg apart from the Burgers vector due probably to the sessile dissociation mechansim proposed by Giamei et al. The formation mechanism of stacking faults is discussed.

Plastic Instability of Tantalum Single Crystals Compressed at 4.2 K

Serrations due to slip were observed on the stress-strain curves of zone-refined tantalum single crystals compressed at 4.2 K. It was found on an oscillograph that each rapid stress drop was accompanied by a rapid temperature rise of about 40 K. This suggests that a discontinuous slip is caused by the plastic instability due to the heat generated during the deformation. Using a deformation model involving a temperature rise due to the work done, calculations of the stress-strain curve and the corresponding temperature change were made for the present crystal, which gave a rapid stress drop and a rapid temperature rise in agreement with the experiments within a factor of 1.5. The present calculation shows that shows that no triggering action is necessary for the initiation of a discontinuous stress drop. A criterion for the thermal instability to occur was also discussed.

Thermally Activated motion of a Screw Dislocation in a Model B.C.C. Crystal

Motion of a screw dislocation in a model b.c.c. crystal has been analyzed through the following procedure: (1) Peierls potentials for translations between stable and metastable positions of a screw dislocation are computed atomistically for a model crystal under various shear stresses on various planes, (2) activation energy of double kink nucleation for each translation is calculated using the theory based on the string model, and (3) the velocity and the integrated plane of thermally activated motion of a screw dislocation are analyzed for the steady motion. τ-χ and φ-χ relations obtained for a constant velocity at various temperatures show several characteristic features observed on transition b.c.c. metals; τ- T relations, however, give unrealistic discontinuous curves. τ-χ, φ-χ and also τ- T relations typical of less pure b.c.c. metals and alloys are reproduced by using a modified Peierls potential.

Computer simulation of defects interacting with a dislocation in Fe and Ni

In order to investigate the fundamental aspects of damage evolution under irradiation computer simulations of the interaction between a dislocation and point defects such as a self-interstitial atom (SIA), a vacancy and interstitial clusters have been performed for various configurations. It is found both in Fe and Ni that a crowdion with axis parallel to the Burgers vector of an edge dislocation interacts more strongly than those with other axis orientations. The same tendency was seen for the dumbbell. The capture range (capture area) within which SIAs are trapped by an edge dislocation is larger for Ni than for Fe, and that for vacancies is much smaller than that for SIAs, suggesting that the bias factor in Ni is larger than that in Fe.

On the characterization of graphite

We have tried to make a new characterization of graphite from an aspect of lattice ordering in directions both parallel and perpendicular to the basal plane by means of laser Raman spectroscopy, X-ray diffraction, positron annihilation and transmission electron microscopy (TEM). The Raman technique is found to give us information on the defects both at intra- and inter-basal planes, and disordering of the basal planes. X-ray diffraction easily sees the disordering, but not the defects in the basal plane. And positron annihilation is very sensitive to the point defect in the basal plane which is not so easy to be detected even in high resolution TEM. The results obtained with the present techniques are compared to material properties like electrical conductivity and outgassing to extract useful correlations between them.

Nucleation of voids in bcc metals

Abstract Measurements were made on formation of voids in high pure and He-doped(1~100 appm) bcc metals, iron, molybdenum and niobium, irradiated by electrons in the HVEM. It was found that nucleation of voids occurred in a very narrow temperature range (0.30–0.36 T m ) which is correlated closely with narrow dislocation density range. Helium at ~I appm enhanced the density and swelling of voids, though ~100 appm suppressed the swelling. These effects were interpreted by heterogeneous nucleation in the neighbourhood of dislocations where the local bias factor for defect flux depends on imperfection of sink strength of dislocations, and formation of vacancy clusters of dislocation loop type contribute to recombination.