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

离子和/或中子辐照会使材料的性能恶化，从而导致晶格缺陷。典型的缺陷之一是位移级联，其中“自发熔化”的体积产生留下点缺陷。近年来对位移级联的研究表明，在级联过程中会形成间隙原子束（群集束），并可能以低活化能在基体中移动。假定缺陷积聚在晶界、析出相、位错等缺陷处，诱导装饰和扩散或延缓位错运动。本文研究了聚砜束对微观结构演变的影响。这项工作包括：(1)在带有离子加速器的透射电子显微镜（TEM）中进行现场观察；(2)在离子辐照材料中进行截面透射电子显微镜（TEM）观察。(1)制备了完全退火的铜和金，并在670 ~ 870 K的重离子辐照下，在TEM中设置了电子透明度。观察到与位移级联相关的移动缺陷。典型的移动距离为20 ~ 140 nm，但由于观测区域的限制，移动距离的上限为20 ~ 140 nm。系统分析了其寿命和迁移方向，得出了群体型间隙束的形成规律。(2)离子辐照观察到稀Cu-Co体系中共格析出相的应变对比从共格向非共格转变。该效应在离子范围内传播，强烈表明了由通量、通量和温度依赖性得出的群集束的扩散。以上结果总结为2007财年的两篇研究论文和一次国际会议。

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.