Michio Kiritani

Growth of Interstitial Type Dislocation Loops and Vacancy Mobility in Electron Irradiated Metals

Interstitial type dislocation loops formed in Al, Au, Cu, Fe and Mo by electron irradiation in a high voltage electron microscope grow proportionally to a smaller power of the irradiation time than unity at lower temperatures under constant irradiation. They grow linearly with the irradiation time at higher temperatures at which vacancies are mobile. The balance of the product of the mobility and concentration between interstitials and vacancies explains the linear growth as well as the observed square root dependence of the growth speed on the irradiation intensity. The temperature dependence of the growth speed can be used to obtain the migration activation energy of vacancies. Temperature independent growth in a very thin specimen is understood as the surface dominant case as sinks. Sink efficiency of an edge dislocation to interstitials and vacancies is discussed.

Point Defect Clusters in Electron-Irradiated Gold

Defect formation in gold by the irradiation of 2.0∼2.5 MeV electrons was studied with a high voltage electron microscope. The formation of dislocation loops by the aggregation of interstitials generally precedes the formation of the defects by vacancies. Permanent sinks, such as specimen surfaces and dislocations, prevent the formation of interstitial type of defects and enhance the formation of vacancy clustered defects. A rapid formation of the vacancy type defects and the simultaneous shrinkage of interstitial type loops after the cessation of the irradiation are observed. Kinetics analysis of the point defect behaviour, standing on the random migration of interstitials and vacancies, explains all the observed various phenomena as the competition processes between two kinds of point defects under various circumstances. The irradiation intensity dependence of the density of interstitial type defects shows that the di-interstitials are their stable nucleus.

Anomalous production of vacancy clusters and the possibility of plastic deformation of crystalline metals without dislocations

High-speed heavy plastic deformation of thin foils of fcc metals, including aluminium, is found to produce a high density of small vacancy clusters, in the form of stacking-fault tetrahedra. The dependences of the density of the clusters on the deformation temperature and deformation rate indicate the production of vacancy clusters from deformation-induced dispersed vacancies. Neither dislocations nor any indication of the reaction of dislocations are present in the regions containing a high density of vacancy clusters. A possible model is proposed that describes, at extremely high strain rates where dislocation generation is difficult, how a high concentration of point defects is produced by a large number of parallel shifts of atomic planes without dislocations.

Electron Radiation Damage of Iron in High Voltage Electron Microscope

Aggregation process of irradiation-induced point defects in iron is examined between room temperature and 400°C with a high voltage electron microscope. In the whole temperature range examined interstitial type dislocation loops are formed. The loops formed between slightly above room temperature and 350°C have flower-like shape. Especially the petalous loops formed around 300°C are divided into many small loops at the latter stage of irradiation by absorving mobile vacancies. The formation of the loops in the pre-irradiated specimen also tells that the vacancies become mobile at about 300°C. Analyzing the nucleation of the loops by using the chemical rate theory considering the effects of impurity atoms one obtains the activation energy for interstitial migration as about 0.26 eV.

Analysis of the Clustering Process of Supersaturated Lattice Vacancies

A method to perform a computer simulation of the step-wise reactions among vacancies up to large vacancy clusters is contrived to analyze the clustering process in an exhausting system as quenched-in vacancies. The variations of the progress of vacancy clustering with aging temperature and with the size of stable nucleus are investigated. Initial transient decrease of mobile vacancies with a rapid increase of small vancancy complexes establishes mutual quasi-equilibrium. Stationary stage of small vacancy complexes following the initial transient corresponds to the nucleation period of vacancy clustered defects. Factors which determine the size distribution of final clusters are discussed. Existence of a definite stable nucleus for a dislocation loop in aluminum is denied. Agreement of the computed results with experiment is obtained when small binding energies of a vacancy to vacancy clusters are postulated, i.e. , less than 0.2 eV up to the cluster of forty vacancies.

Formation of Voids and Dislocation Loops in Quenched Aluminum

The existence of three dimensional defects, voids, is confirmed in quenched pure aluminum with an electron microscope. Influences of other defects on the formation of voids and dislocation loops are examined. Specific conditions of quenching temperature, quenching rate, aging temperature, and specimen size for the formation of voids and dislocation loops are determined experimentally. The differences in the type of secondary defects and in their size and density are explained from the size of migrating defects which changes with quenching rate and quenching temperature, and from the growth ability of the nuclei which changes with shape and aging temperature. Local irregularity of the distribution of the secondary defects is also explained. The effect of quenching stress is considered.

Defect interaction processes controlling the accumulation of defects produced by high energy recoils

Abstract Defect interaction processes involved in the microstructural evolution during irradiation with high energy recoils are discussed on the basis of experimental observations. First of all, an experimental procedure is outlined for identifying the nature of small defect clusters produced in cascades and subcascades. The procedure was the growth and shrinkage behavior of clusters produced in the cascades during subsequent irradiations with 1 MeV electrons in a high voltage electron microscope. It is shown that the analysis of the clusters produced during irradiation with high energy recoils can be used to identify the formation of sub-cascades. Various aspects of the one-dimensional glide of small self-interstitial atom (SIA) clusters and their role in defect accumulation are considered. The experimentally observed dose dependence of the cluster density is used to discuss the problem of fission-fusion correlation. Finally, some comments are made on the role of stochastic fluctuation of point defect reaction in the stability and lifetime of SIA clusters under cascade damage conditions.

Dislocation Loops with Stacking Fault in Quenched Aluminum

Defects in quenched and aged aluminum of various purities are studied. Almost all dislocation loops which are formed by the vacancy condensation in pure aluminum are Frank sessile type, containing a stacking fault. The shape of the Frank sessile dislocation loops is hexagonai on {111} plane bounded by directions. The annealing process of these dislocation loops and the change from Frank sessile dislocation loops to prismatic ones are studied experimentally. Discussions are given on the formation of Frank sessile dislocation loops and their change into prismatic dislocation loops.

Nucleation and Growth of Secondary Defects in Quenched Face-Centered Cubic Metals

A general discussion has been made on the factors which determine the type, number and size of secondary defects which are formed by vacancies in quenched face-centered cubic metals. A method of analysis, which uses the positive (growth) and negative (shrink) reaction ratios, has been introduced. This method is especially applicable for the analysis of the formation of such secondary defects that have weak bindings to vacancies at the initial stage of their formation. The existence of the large negative reaction during a wide range of nucleation and growth of faulted dislocation loops in quenched aluminum (the existence of large clusters which are apt to maintain equilibrium with migrating defects) is confirmed experimentally, and the formation of the loop is analyzed by the method introduced. Nucleation and growth of perfect dislocation loops, double layer faulted dislocation loops, voids and stacking fault tetrahedra are also discussed.

Shape of Voids in Quenched Aluminum

The shape of voids which are formed in quenched and aged aluminum is determined experimentally. Quenched specimens containing voids large enough to be resolved by the electron microscope, larger than 50A, can be produced by controlling quenching conditions. All images observed from various crystallographic directions are exactly those expected from a regular octahedral void surrounded by {111} planes. The image caused by the strain field of voids are also observed. It is found from the measured void size and density that, under certain experimental conditions, almost all quenched-in vacancies aggregate to form voids without forming dislocation loops or disappearing to preexisting sinks.