I. Ishida

Confirmation of vacancy-type stacking fault tetrahedra in quenched, deformed and irradiated face-centred cubic metals

Abstract An efficient and reliable method for characterizing the nature of stacking fault tetrahedra in f.c.c. metals using electron microscope diffraction image contrasts is presented. By using the 220 reflection, and thereby eliminating the contrast from overlapping stacking faults, one can differentiate between intrinsic-type stacking fault tetrahedra due to lattice vacancies and extrinsic type due to interstitial atoms. The validity of the method is examined by the observation of vacancy-type stacking fault tetrahedra in quenched metals, and by the observation of interstitial-type faulted dislocation loops. Stacking fault tetrahedra introduced by plastic deformation, electron irradiation, neutron irradiation and ion irradiation are all confirmed to be vacancy type. It was found that interstitial-type stacking fault tetrahedra do not exist.

Defect structure development in 14 mev neutron irradiated copper and copper dilute alloys

Abstract Copper and twelve different copper base dilute alloys (Ni, Si, Ge and Sn) are irradiated with fusion neutrons from RTNS-II at 300–723 K, up to 6 × 10 22 n/m 2 . Roles of free defects are detected by the comparison of defect structures irradiated as thin foil and those as bulk, and the analysis on defect structure in pure copper gives 10 keV as sub-cascade energy. The direct formation of vacancy clustered defects from cascade damage is enhanced by oversized solutes. Oversized solutes as well as undersized solutes enhance the formation of the interstitial type dislocation loops by reducing the interstitial mobility and the stabilizing the loop nucleus. Existance or non-existance of dislocation loop formation near a dislocation gives a good criteria of the bordering condition for the loop formation. Well grown voids are observed in pure copper at 563 K, but they are not formed in all the alloys of 2 at%.

Mechanical property changes and microstructures of iron irradiated with fission and fusion neutrons

Abstract Mechanical property changes were observed for pure Fe irradiated with fission and fusion neutrons, and the irradiation-induced defect and deformation structures were correlated. Interstitial dislocation loops and voids were observed for specimens irradiated with both fusion and fission neutrons at temperatures higher than 563 K. The increase in the yield strength was well predicted from the number density, size and type of each defect for specimens irradiated at temperatures higher than 673 K with fission neutrons, while the observed increase in yield strength is larger than those observed for specimens irradiated at tempearatures lower than 573 K. These results show that the number of invisible defect clusters increases with decrease in irradiation temperature and is much larger in fusion neutron irradiation than fission neutron irradiation. From the increase in the yield strength, the number density of defect clusters was estimated by assuming their type and size.

Defect structure development in electron-irradiated Cu-based Si, Ge and Sn binary alloys

Abstract Systematic experiments of 1 MeV electron irradiation were made on Cu-based binary alloys above 300 K using a high-voltage electron microscope in order to study the effects of solute atoms on defect structure development. The solute elements examined were Si (+5.08%), Ge (+27.77%) and Sn (+83.40%), the volume size factors of which are given in parentheses; the amounts of these were 0.05, 0.3 and 2 at.% respectively. Interstitial-type dislocation loops and stacking-fault tetrahedra (SFTs) were formed in pure Cu and all the alloys. In pure Cu, the temperature dependence of the loop number density had a ‘down peak’ (i.e. loops hardly formed) around 373 K; below this temperature the majority of the loops shrank and disappeared during irradiation, while all the loops grew larger above it; and SFTs were unstable and repeated the formation and disappearance. In the alloys, the loop number density decreased monotonically with increasing temperature (no down peak was observed); loop formation was greatly enhanced except for complete suppression above certain temperatures in Cu-Ge and Cu-Sn alloys; and stable SFTs formed up to higher temperatures. The mechanisms for these effects were proposed, taking into account the trapping of point defects by solute atoms, the radiation-induced segregation of solute elements, and the bias effect on point-defect absorption at defect clusters owing to the segregated solute elements.

Defect structure development in a pure iron and dilute iron alloys irradiated with neutrons and electrons

Abstract The defect structure and mechanical property changes were observed for pure iron of 99.99% purity and a series of Fe–(0.1% and 0.4%) Cr and Fe–(0.1% and 0.4%) Mn dilute alloys irradiated with neutrons. From the comparison of the defect structures with yield strength change, a large contribution of the invisible defect clusters to the irradiation hardening was expected in the specimens irradiated in Japan Materials Test Reactor (JMTR), whereas these clusters are not found after irradiation in the Fast Flux Test Facility (FFTF). The electron irradiation experiments showed that addition of chromium and manganese to 0.1% in pure iron, development of large dislocation loops is suppressed, and frequent nucleation of small loops at the early stage of the electron irradiation is observed, similar to that in ultra-high purity iron of 99.9999% purity. The mechanisms of dislocation loop development in the early stage of irradiation for Fe–Cr and Fe–Mn are considered to be different.

Mechanical property change in neutron irradiated Fe-Cr and Fe-Mn alloys, and their defect structures

Abstract The mechanical property change and defect structures were investigated on Fe-Cr and Fe-Mn alloys irradiated in the JMTR and the FFTF. Dislocation loop density increases much larger with manganese levels than chromium levels in irradiation in the JMTR, while in the FFTF, dislocation density of the Fe-Cr alloys is much larger than in the Fe-Mn alloys. This reverse trend is due to the saturation in the development of dislocation loops. The Fe-0.1 and 0.4% Cr with dislocation density much lower than that of 2% Cr, and that of the Fe-0.1 and 0.4% Mn much lower than that of 2% Mn showed the increment in yield strength almost the same as that of the Fe-2% Cr and rather larger than that of the Fe-2% Mn, respectively, in the JMTR. Yield strength increase without significant variation in strain hardening exponent is observed in the Fe-0.1% Cr and Fe-0.1% Mn irradiated in the FFTF and very small voids are observed in the Fe-0.1% Mn irradiated in the JMTR. These results suggest the existence of invisible defect clusters produced by irradiation.

Evolution of defect structures in copper by He+ ion irradiation under the control of free point defects

Abstract The role of free interstitial atoms and the effect of implanted helium were studied by a comparison of the defect structures developed in masked thin foils and in bulk samples of copper by 500 keV He + ion irradiation at room temperature. A minimum PKA energy of 23 keV for cascade damages to form a SFT has been deduced from an analysis of linear increase of the number of stacking fault tetrahedra (SFT) in thin foils at low fluence. The mechanism of defect saturation in the thin foils at higher fluence is analyzed from the number density of SFT. Point defect clusters in the bulk specimens are more readily annihilated at the depth where little helium is implanted, in contrast, the interstitial cluster formation is enhanced at the depth where helium atoms are deposited. The depth variation of the interstitial type dislocation loops observed in the bulk is understood in terms of the trapping of vacancies and interstitial atoms by the helium atoms.

Ferromagnetic Resonance in (NiFeCo/Cu/Co)-Multilayers.

Ferromagnetic resonance (FRM) measurements were made on [Ni 0.8 Fe 0.15 Co 0.05 /Cu/Co] multilayers in order to observe the coupling between ferromagnetic layers across Cu spacer. The resonant fields vary oscillatory with Cu spacer thickness. The FMR results were analyzed by the resonant model of a magnetically coupled two-layer system. By using a modified RKKY theory for interlayer exchange interaction and FMR results, the coupling constant, J , was determined as a function of Cu spacer thickness. The resonant field can be predicted for Cu spacer thickness and are well compared to the experimental values. In samples of antiferromagnetic coupling, the resonance mode of the magnetization flopping were observed at small applied fields.

Structure and Magnetic Properties of Co/Mn Multilayers Composed of Ultrathin Films.

In multilayers of [Co(10 A)/Mn( t Mn A)]20 deposited onto glass substrates under an ultra-high-vacuum (UHV), the saturation magnetic moment per Co, Ms Co, is observed to increase with increasing Mn thickness and eventually the magnetic moment to exceed that of bulk Co. The saturation magnetic moment also varies depending upon the deposition conditions of Mn-buffer layers, and magnetic moments are observed to be 1.5 to 2 times as large as that of bulk Co. This suggests that atomic layers of Mn adjacent to Co layers contribute ferromagnetically to the magnetic moment by forming ferromagnetic coupling between Mn and Co atoms at their interfaces.

Electron irradiation effects on Ti-Ni shape memory alloys

Abstract Effects of 1 MeV electron irradiation were investigated for Ti–Ni alloys of three different compositions; Ti–49.9, 50.2 and 50.7 at.% Ni, by using an ultra-high voltage electron microscope. Obvious amorphization by the irradiation at room temperature was observed for the Ti–49.9 at.% Ni alloy in the as-received state, while in the Ti–50.2 at.% Ni alloy, the amorphization proceeded only a small extent for much higher doses. No amorphization could be observed for both specimens irradiated at temperatures higher than 343 K. The Ti–50.7 at.% Ni alloy did not amorphize even after the irradiation of 8 dpa at room temperature. Higher nickel concentrations and higher irradiation temperatures suggested the retardation of the amorphization. Prior to the amorphization, the disordering takes place and brings about the shape memory characteristics loss, as well. The disordering process always takes place even if the amophization does not occur. Deformation of the alloys enhances the amorphization induced by the electron irradiation. No defect clusters could be seen in the present experiment.