S. Kojima

Microstructure evolution by neutron irradiation during cyclic temperature variation

Abstract Utilizing a technique to control the temperature which is not influenced by the operation mode of a reactor, an irradiation during which the temperature was alternatively changed several times between two temperatures ( T -cycle) has been performed. Some defect structures are understood as combinations of the defect processes at lower and higher temperatures, and some others are understood if the defect processes during the transient between the two temperatures are taken into consideration. However, the most remarkable characteristic of defect processes associated with the temperature variation is the reaction of point defect clusters induced by lower-temperature irradiation at the higher temperature. During lower-temperature irradiation, there is a greater accumulation of vacancy clusters as stacking fault tetrahedra in fcc metals than that of interstitial clusters as dislocation loops. Vacancies evaporated from the vacancy clusters at higher temperature can eliminate interstitial clusters completely, and the repetition of these processes leads to unexpectedly slow defect structure development by T -cycle irradiation.

Detection of the role of free point defects from the variation of defect structures near permanent sinks in neutron irradiated metals

Abstract The role of freely migrating point defects during neutron irradiation in the defect structure development with the presence of permanent sinks is studied using rate equations. Three specific cases are examined as a function of the distance from the surface: (1) only interstitial cluster existing system, (2) interstitial cluster and vacancy cluster coexisting system, and (3) only vacancy cluster existing system. The obtained results are as follows. In case (1): easy formation of interstitial clusters in thin specimen, and near the surface and the grain boundary of thick specimen; no clusters in the deeper region of thick specimen. In case (2): easy formation of interstitial clusters in thin specimen, and near the surface and the grain boundary of thick specimen just the same as those in case (1); the growth of both types of clusters in the deeper region of thick specimen. In case (3): easy formation of vacancy clusters near the surface of the specimen. These results are compared with the experimental results.

Role of solute atoms on microstructural evolution in neutron irradiated nickel

Abstract Fission reactor irradiation of nickel alloys with solutes of widely varying volume size factors 2 at% of Si, Cu, Ge and Sn, was performed for a wide range of irradiation temperature. At a lower temperature 473 K, a high density of small vacancy clusters are mixed with interstitial-type dislocation loops in all the alloys. At higher temperatures, in the alloys with solutes of negative volume size factor, Si, and extremely large positive factor, Sn, the unfaulting of interstitial-type dislocation loops during irradiation is suppressed, and the development of dislocation structures is moderate. In the alloys with solutes of positive but not large volume size factor, Cu and Ge, including pure Ni, unfaulting of loops strongly promotes the development of dislocation structures. As a consequence of the difference in these dislocation structure developments, void formation is almost completely suppressed in the first category and is enhanced strongly in the second.

Formation of vacancy clustered defects from cascade collisions during heavy-ion irradiation and their annihilation by freely-migrating interstitial atoms

Abstract Heavy-ion irradiation of several fcc metals and their ordered alloys was performed. The behavior of ion-irradiation induced point defect clusters during subsequent electron irradiation indicates that about 90% of the clusters are of vacancy type and 10% of interstitials. For any given ion and energy, a wide variation of the collision cascade zone size was found. In general, the larger zones were associated with a larger number of point-defect clusters. A collision zone of a given size was associated with the same average number of clusters even when they were produced by ions of different energy. The dependence of cluster formation on ion energy, target material, and angle of incidence of the ion beam can be correlated with a unique parameter, the depth in the foil of defect cluster formation. The defect yield increases linearly with irradiation dose for shallow depth distributions but then increases with a square-root relationship for deeper distributions. These results are analyzed in terms of a variation in the role of freely-migrating interstitials for the two cases.

Origin of unbalanced reaction of vacancies and interstitials during irradiation with cascades and influence on microstructural evolution

Abstract Based upon the underlying premise that all the microstructure evolution during irradiation results from the assymetrical reaction between vacancies and interstitials, the origin of the assymetry is sought and categorized, and the mechanism of defect structure evolution for each source of assymetry is investigated. The role of neutral sinks and the influence of dislocations are examined for the cases of irradiation with and without cascade damage. Vacancy cluster formation directly from cascades is found to favor the generation of freely migrating interstitials. Stochastic fluctuations of the point defect reactions under the balanced condition of vacancy and interstitial is experimentally detected, and the important role of the fluctuations is found in the determination of the fate of small interstitial cluster embryos produced by cascade damage. The influence of the unbalanced point defect reaction starting from difference in spacial distribution between vacancies and interstitials formed by cascade collisions is discussed as one of the important origins of vacancy dominant reactions.

Factors to influence the nucleation and growth of interstitial clusters during cascade damage

Abstract Small interstitial clusters produced by cascade collisions are known to serve as the nucleation sites of dislocation loops. However, their density is quite low compared with the number of large cascades. Factors which suppress the nucleation of interstitial type dislocation loops are explained and their relative importance is discussed. The first is the positional instability of small interstitial clusters. This effect is expected to enhance the dislocation structure development by agglomeration of the embryos, but it is also expected to suppress the formation of loops by easy escape to sinks. The second is the stochastic fluctuation of point defect reactions with embryos. This effect plays an important role when the point defect concentrations of a quasi-steady state are high. The third is the thermal instability of embryos of loops. The fourth is the cascade localization induced bias (CLIB) effect, which leads a vacancy dominant atmosphere in a cascade. At a low temperature at which vacancies are immobile and embryos are thermally stable, only the first factor is effective. At a very high temperatures on the other hand, the third and fourth factors are the main factors. In the intermediate temperature range, four factors are competitive.

Experiments to examine the contribution of gas atoms to void formation in irradiated metals

Abstract The use of vacuum melting has been employed to demonstrate that residual gases, especially hydrogen, strongly influence void nucleation of copper, copper binary alloys and various Fe-Cr-Ni base alloys during either neutron or electron irradiation. Void nucleation in nickel appears not to be strongly affected by residual gases, however. Solute-free and solute-bearing Fe-Cr-Ni alloys appear to respond differently to differences in gas content. When contamination of specimens with sodium occurs during neutron irradiation, void nucleation in both as-fabricated and vacuum-melted specimens is similar, suggesting that gas atoms re-enter the specimens during irradiation.

High Strain Rate Superplasticity of a Powder Metallurgy Pure Aluminum 1N90

Superplastic characteristics of 1 N90 pure PM aluminum and the microstructure evolution are investigated to make clear deformation mechanism of the High Strain Rate Superplastcity(HSRS). The 1N90 exhibits the m value of about 0.47 and the total elongation about 500% at the strain rate of 1.2x10 -2 s -1 and 893K. The 1N90 shows grain size of 10∼20 μm with the subgrains of 1∼2 μm after hot rolling at above 773K. ()n the other hand, 99.99% pure aluminum by squeese casting shows the total elongation less than 100% and m=0.2 and has a grain size of above 100 μm after hot rolling.