Analysis of dislocation characteristics and inside-outside contrasts in irradiated and annealed tungsten as a fusion reactor material

Tungsten is an important candidate of plasma-facing material for fusion reactors. Its irradiation response,especially the post-irradiation annealing (PIA) behavior needs further investigating. In addition, the practice ofthe "inside-outside" contrast method of determining the characteristics of irradiation induced dislocation loopshas not been utilized frequently, and the present research serves as an example to present some practicalconsiderations. In the present work, a tungsten thin-foil specimen is irradiated at 400 degrees C with 58 keV D+ to afinal fluence of 1x 1017 cm-2, corresponding to a dose of about 0.1 dpa. The specimen is prepared through theelectro-polishing method by using a NaOH based electrolyte. The ion irradiation is carried out directly on theelectro-polished specimen. The irradiated specimen is followed by isothermal annealing at 900 degrees C for 1 h. Theas-irradiated and post-irradiation annealing modified defects are investigated with a transmission electronmicroscope (TEM) operated at 200 kV. The irradiation defects are characterized by using TEM bright-fieldimaging for the same imaging field with different g vectors around the three major zone axes: the [001], [111],and [011] zone axis of the body-center cubic lattice of W. For each g vector, the +/- g are characterized, and thecorresponding contrast-extinctions and "inside-outside" contrasts of selected dislocation loops are identified. Theindices of the g vectors around different zone axes are assigned consistently with the Kikuchi map. As a result,the D+ irradiation increases a fine distribution of dislocation loops with an average size of (11.10 +/- 5.41) nm anda bulk density of 2.40x1022 /m3. Voids are not observed obviously. The post-irradiation annealing causes theloop size to increase and the loop density to decrease, with numbers of (18.25 +/- 16.92) nm and 1.19x1022 /m3,respectively. Through the contrast-extinction analysis, the dislocation loops in the irradiated and annealedspecimen are identified to be a/2-type dislocation loops. The post-irradiation annealing also causes thecoalescence of large loops and forms large irregular-shaped dislocation loops. Voids with typical sizes of 1-2 nmare also observed in the annealed specimen. The PIA modified microstructure is consistent with the stage IV orstage V characterization of classical PIA induced microstructures. Through the " one-step" inside-outsidecontrast method, the dislocation loops are identified as an interstitial type. The influcing factors for the "one-step" method are discussed and the importance of consistent indexing is also noted