Experimental and theoretical approach for mobile defect clusters introduced under heavy ion irradiations

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Irradiation with ions and/or neutrons deteriorates properties of materials introducing lattice defects. One of the typical defects is displacement cascades in which "spontaneously melted" volumes are generated leaving point defects. The recent study on the displacement cascades revealed that bundles of interstitial atoms (crowdion bundles) are formed in the cascade process and may travel in the matrix with low activation energy. The defects are presumed to accumulate at defects such as grain boundary, precipitates, dislocations etc, inducing decorations and diffusion or retarding dislocation motion. In this work, the effects of the crowdion bundles on the microstructure evolution are investigated.This work consists of (1) in-situ observations in a transmission electron microscope (TEM) interfaced with an ion accelerator and (2) cross section TEM in ion-irradiated materials.(1) Electron transparency of fully-annealed copper and gold were prepared and set in the TEM for ion irradiations with heavy ions at the temperature from 670 to 870 K. Mobile defects associated with displacement cascades were observed. The typical traveling distance ranges 20 to 140 nm, however the upper limit was due to the limitation of observation area. The lifetime and the direction of mobility were systematically analyzed, and the formation of crowdion-type interstitial bundle was concluded.(2) Coherent-to-incoherent transition of strain contrast of coherent precipitates in dilute Cu-Co system was observed upon ion irradiation. The effect propagated over the range of ions, strongly indicating diffusion of crowdion bundles which is concluded by fluence, flux and temperature dependences.The above results were summarized as two research papers and an international conference in the fiscal year of 2007.