Direct observations of cascade defects byin-situheavy ion/electron microscope interface

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Abstract
This is a review of a series of experiments on formation, annihilation, structure or behaviour of cascade defects by in-situ observation using a 400 kV heavy ion accelerator/200 kV electron microscope link facility at the University of Tokyo. Most of the results described here are obtained in gold. The experiments take advantage of the high mobility of interstitials in gold and of the strong effect of surface sinks on the thin foil specimens, which reveal vacancy properties rather distinctively. The temperature of the in-situ observations ranges from 120 to 800 K. The cascade defect images are formed instantaneously during irradiation, being composed of several groups of images at lower temperatures especially below 470 K, reflecting sub-cascade structure. A new concept of sub-cascade energy is introduced. For irradiation at higher temperatures, the defect images become sharper and the majority are identified either as loops or stacking fault tetrahedra (SFT). The size of the defects shows that short range diffusion of vacancies has taken place and the vacancies have been collected into large defects. During irradiation above 470 K, the defects are annihilated with certain lifetimes. Vacancy loops and stacking fault tetrahedra have different lifetime distributions. The process of annihilation of the defects is discussed. As we are using rare gas ions, cavities are often observed in the later stage of irradiation. This results in a change in the sink strength, thereby influencing the behaviour of the cascade defects. These results are discussed to construct a general view applicable to other materials.

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