He implanted Fe-Cr alloys

This paper discusses our recent experiments focused on the chromium influence on the microstructural changes of iron based alloys under radiation treatment. Our experimental method - the positron annihilation lifetime spectroscopy (PALS) enables an observation of size and density changes of the vacancy type defects in the material microstructure. We have created these defects by implantation of charged particles (He2+). The cascade collisions in the crystal lattice and following Frenkel pair creation have been in our work considered as possible approximation of the neutron flux damage. Our study of the chromium influence on the radiation treated of iron based alloys has been performed on the Fe-Cr binary alloys with four different Cr content (2.36; 4.62; 8.39 and 11.62 wt%). Thermal treatment of the materials fabrication process has been selected to ensure required martensitisation. To obtain cascade collisions in the microstructure of studied materials without neutron activation, accelerated helium ions have been used. This approach allowed us to observe vacancy behavior in the base material as well as possible helium voids formation. Helium implantations have been performed on the linear accelerator of the Slovak University of Technology in Bratislava. The ion energy (250keV) was chosen to ensure possibility of application of additional non-destructive methods, sensitive in near surface areas (MS, XRD, and SEM). To calculate parameter DPA (Displacement Per Atom), one has to consider the depth sensitivity of the experimental technique planned to be used. In our case, the used positron source 22Na provides continuous spectrum of positrons up to energy 545 keV. It corresponds, in case of used Fe-Cr alloys, to the maximal positron implantation depth of about 120 μm. However, more than 99 % of positrons annihilate in 100 μm and this value was used for calculation of average DPA. Vacancies per implanted ion ratio was taken from the SRIM simulations and almost doesn't dependent on the chromium content. There are almost no changes between pure iron, chromium and their variations of content in our alloys. The contribution of positron annihilation lifetime technique in the defects shows, that its highest density have been observed in the alloy with 8,39% of Cr. This could be interpreted via uniformly distributed defects in whole volume of impacted area and following increase of the probability of positrons to annihilate there. In the low chromium alloys the small vacancy clusters have been observed up to the size of 4 vacancies agglomeration. However, such clusters of the vacancies have not been observed in high chromium materials. It means, that Cr concentration on the level 9 wt% was confirmed as optimal having in mind the resistance to the radiation induced defects agglomerations. Mono- and/or di-vacancies are more acceptable than their agglomerations from the radiation embrittlement resistance point of view. (author)