Sadakichi Kitajima

Kinetics of point defects in electron irradiated MgO

Abstract The kinetic behavior of point defects in MgO has been studied through the observation of the nucleation and growth processes of dislocation loops during electron irradiation in a high voltage electron microscope. Results have been obtained on dislocation loop formation and growth in MgO single crystals obtained from three sources during irradiation at temperatures from 25 to 1000°C. Circular and elongated dislocation loops with 1 2 〈110〉 Burgers vectors lying on {110} planes are formed in Norton MgO and ORNL and Tateho MgO, respectively. The loop nucleation stage is complete early in the irradiation and the density of loops reaches a saturation value CL. The diameter of loops D increases with irradiation time t. The dependence of CL and D on t and electron flux φ at various temperatures T is expressed as follows: C L ∝ t 0 φ −1 2 and D ∞ t 1 2 − 2 3 at T ≲ 600° C and D tφ −1 2 at T ≳ 600° C . Experimental results below ≈ 600°C are successfully analyzed using a theory based on a nucleation mechanism controlled by interstitial motion in which one or two pairs of Mg- and O-interstitials serve as stable nuclei for interstitial loops. The growth kinetics of loops above ~ 600°C are understood in terms of the steady state behavior of interstitials and vacancies with high mobilities, and are controlled by the vacancy species (Mg or O), the product of whose mobility and displacement rate has the lower value.

Production of highly perfect copper crystals with thermal cyclic annealing

Abstract Dislocation behavior in copper crystals in the course of annealing was studied, using an etch pit technique. Two annealing methods used were as follows: (1) isothermal annealing and (2) thermal cyclic annealing in which the temperature was changed between 1050° and 850°C with a period of 30 min. In the isothermal annealing, some of the isolated dislocations formed new sub-boundaries, without much reduction in the total number of dislocations. On the other hand, the thermal cyclic annealing was recognized to be conspicuously effective not only in the reduction of dislocations without forming new sub-boundaries but also in breaking-up of old sub-boundaries. With this thermal cyclic annealing highly perfect copper crystals were produced from crystals with initial dislocation densities as much as 1 × 106/cm2.

Mechanism of disordering in FeAl under electron irradiation

Abstract The disordering process in FeAl with the B2-type superlattice has bean examined during irradiation in a high voltage electron microscope. The long-range order parameter decreases exponentially with time under irradiation at temperatures from -150 to 295°C. The disordering cross-section (K) for irradiation along the ⟨100⟩ direction is almost the same as that along the ⟨110⟩ direction in the electron-energy range 0.25–1.25 MeV. Experimental results are analysed using theories based on two disordering mechanisms; (1) ⟨111⟩ replacement collisions, and (2) atomic displacement and subsequent random recombination of displaced atoms with vacancies. The unsuccessful application of the former mechanism and the successful application of the latter mechanism to the results suggest that recombination of displaced atoms with vacancies on the opposite sublattice is the dominant process for disordering in electron irradiated FeAl. The dependence of K on the irradiation temperature is also discussed in terms of t...

Nucleation and growth process of dislocation loops in electron-irradiated β-Nb-3·1 wt% Zr alloy

Abstract In order to understand the kinetic behaviour of point defects in β-Nb-3·1 wt% Zr containing small amounts of impurities, the nucleation and growth process of dislocation loops has been examined during electron irradiation in a high-voltage electron microscope. Small dislocation loops of interstitial character are formed during irradiation at temperatures from 300 to 675 K. The density of loops saturates at the beginning of irradiation, and loops grow in proportion to the cube root of irradiation time. The temperature dependence of the saturated loop density under a fixed electron flux consists of five stages. These results, together with the dependence of loop density on electron flux, are analysed in terms of the effect of impurities on the nucleation process of loops and the mobility of point defects. Two kinds of impurity, probably oxygen and nitrogen, act as nucleation sites of loops. The binding energies between a Nb interstitial and each of these impurities are estimated to be (1·1 ± 0·1) ×...

The inhomogeneity of stress and plastic deformation in neutron-irradiated copper single crystals

Abstract The plastic deformation in stage I of neutron-irradiated copper single crystals in a uniaxial tensile test is studied at 77 K and room temperature from the view point of stress-inhomogeneity due to the constraint of grips. Primary slip bands appearing in clusters are examined on the two surfaces where the slip step height of a slip band is the maximum by using optical microscopy. Main results are as follows: 1. (i) in the specimen deformed at 77 K, the morphology of primary slip bands and the width of individual clusters of them on one surface are not the same as those on the opposite surface except around the middle part of the specimen; 2. (ii) in the specimen deformed at room temeprature, the difference mentioned in (i) is not found; 3. (iii) in both specimens, the growth of the clusters in width in the upper half of the specimen is larger in the direction of the upper grip than in the direction of the lower grip. This situation is reversed in the lower half. It is concluded that the heterogeneous plastic deformation of neutron-irradiated copper single crystals in a uniaxial tensile test is related with the stress-inhomogeneity caused not only by the constraint of the grips but also by the clustering of primary slip bands.

14 MeV neutron irradiation effects on the yield stress of highly perfect copper single crystals

Abstract The dose-dependence of the critical resolved shear stress (τ crss ) of highly perfect copper single crystals irradiated by 14 MeV neutrons was measured for neutron doses of less than 9.5 ×10 14 n/cm 2 . The neutron irradiation was carried out at 298 K using RTNS-II at Lawrence Livermore National Laboratory. The tensile tests were carried out at 293 K. The increase in τ crss (Δτ crss , radiation hardening) due to 14 MeV neutron irradiation increased with increasing neutron dose and was proportional to the one-half power of neutron dose. Δτ crss in the crystals irradiated by 14 MeV neutrons was larger by a factor of about 2 than that in crystals irradiated by fission neutrons.

Effects of He+ irradiation on the mechanical properties of molybdenum

Abstract The effects of 100 keV He ion irradiation on the mechanical properties of molybdenum single crystals were studied by examining the temperature- and dose-dependence of tensile behavior at 77 and 293 K. The yield stress of single crystals increased with increasing dose and the fracture stress decreased with increasing dose at 77 K. A ductility loss occurred in irradiated crystals at 293 K. These results show that the surface damage layer about 200 nm thick causes embrittlement at low temperatures and ductility loss at higher temperatures.

Nucleation and growth mechanism of homogeneous radiation-induced precipitates in Cu-0.18 wt% Be alloy

Abstract A theory of formation of interstitial loops during irradiation is applied to the nucleation and growth of precipitates in electron-irradiated Cu-0.18 wt% Be alloy. The dependences of density and size of the homogeneously nucleated, radiation-induced precipitates on electron flux and irradiation time are analyzed. The conclusions are that a di-MDI, (mixed dumbbell interstitial) and/or a complex of an impurity and the di-MDI acts as the nucleus of the precipitate, and the precipitates grow as a result of the high mobility of the mixed dumbbell interstitials.

Influence of 100 keV helium irradiation on tensile properties of pure iron and JFMS steel at low temperatures

Abstract The effects of 100 keV He + ion irradiation on mechanical properties in polycrystals of pure iron and a ferritic-martensitic steel (JFMS steel) were examined at low temperatures. Pure iron polycrystals irradiated to doses higher than 1 × 10 21 ions / m 2 showed enhanced irradiation embrittlement and the DBTT increased by about 50 K at a dose of 1 × 10 21 ions / m 2 . The brittle fracture occurred preferentially along grain boundaries, and cracks were formed along grain boundaries emerging on the irradiated surfaces. The embrittlement disappeared by removing the irradiated surfaces thicker than the projected range of He ions. Microcracks initiated in the He-implanted surface layers trigger off brittle fracture of irradiated pure ion polycrystals. JFMS steel specimens irradiated to a dose of 4 × 10 21 ions / m 2 and elongated at 77 K also exhibited surface cracks along grain boundaries. The cracks, however, did not grow into the interior and the specimens failed in a transgranular manner. Grain boundaries of JFMS steel may be so strong that crack propagation along grain boundaries does not occur.

A method of determining the order parameter from thickness fringes in electron micrographs

A method of determining the long-range order parameter S in alloys with a high accuracy from thickness fringes in electron micrographs is proposed and applied to Cu3Au and Fe3Al. The extinction distance ξg and the intensity of a g (superlattice) reflexion at the exact 2g Bragg position in the systematic condition are calculated as a function of S at various accelerating voltages with the many-beam dynamical theory. In the case of Cu3Au the variation of ξg with S is predicted to be very large in the range 200-1000 kV, and the intensity of the g reflexion decreases with increase of accelerating voltage. In the case of Fe3A1 with a B2-type superlattice, on the other hand, S dependence on ξg and the intensity of the g reflexion are not sufficient to determine the value of S with high accuracy at around 200 kV, but they are sufficient at around 1000 kV. An experiment on Cu3Au reproduces the theoretical prediction and indicates that this method is useful for determining the long-range order parameter, especially in local regions.