Study on the Interaction between Dislocations and Helium Bubbles in Copper by In Situ Straining Experiments in Transmission Electron Microscopy

N. Nita (Sp), H. Matsui, Tohoku University, Sendai (Japan) 
 
Introduction
In fusion environment, helium induced by nuclear reactions will degrade mechanical properties of materials. Effects of the helium on the macroscopic mechanical properties have been studied rather extensively, while fundamental knowledge about the interactions between dislocations and helium bubbles are not sufficient. The objective of present paper is to investigate the dynamic interactions between dislocations and helium bubbles in ion-irradiated copper by in situ straining experiments in transmission electron microscopy (TEM).
Experimental procedures
The specimen used was pure copper (>99.999%). Prior to ion irradiation, TEM discs were electro-polished for the thin-foil irradiation. Helium ion irradiation was performed using Tandem type accelerator at room temperature. The energy of incident ions was 10keV and the fluence was 2x1017ions/cm2. The coarse microstructure was obtained by annealing at 650oC in high vacuum to observe the dislocation-defect interaction in detail. TEM observations were carried out with the in situ straining technique. Nano indentation tests were also performed to evaluate the change in mechanical property by irradiation.
Results and discussion
Microstructure of homogeneous distributed bubbles was observed after irradiation with helium ions and the successive annealing. Dislocations glided and interacted with helium bubbles with applying the stress at room temperature. The pinning-depinning behavior of moving dislocation was clearly observed by in situ straining experiment, which indicates that the helium bubbles contributes the irradiation hardening. The barrier strength of helium bubbles against dislocation motion was quantitatively determined by measuring bow-out angles of dislocation immediately prior to breakaway. The obstacle strength was discussed not only with the dependence of the bubble pressure, but also the size distribution of bubbles. Attractive interaction between dislocations and helium bubbles were also examined in terms of the modulus effect of the obstacles. Macroscopic mechanical properties by nano indentation tests were correlated with the values estimated by dislocation bow-out model.