Radiation-Induced α' Phase Formation on Dislocation Loops in Fe-Cr Alloys During Electron Irradiation

Abstract
Radiation-induced precipitates on dislocation loops in low and high purity Fe-9, -18 and -50 % Cr alloys were examined under electron irradiation in a high voltage electron microscope operated at 1 MV. Two types of dislocation loops on {100} planes with a <100> Burgers vectors and on {111} planes with a /2<111> are formed in high purity Fe-Cr alloys. However, only a <100> type loops are formed in low purity alloys, i.e. where carbon concentration is greater than about 60 wt.ppm. The growth rate of the loops formed in Fe-9 and -18Cr alloys is faster than that of Fe-50Cr alloy. All of the loops formed at temperatures ranging from 573 K to 973 K, even in the ( σ+α+α') phase regions in thermal equilibrium, have moiré fringe contrast due to precipitates identified as α' phase. The contrast is very weak in the Fe-50Cr alloy because of coarse distribution of precipitates, while the α' phase formed in low purity alloys has somewhat larger interplanar spacings that are modified by interstitial impurities. The α' phase formation on the loops can be understood by radiation-induced segregation of chromium and interstitial impurity atoms.