A review of microstructure evolution in zirconium alloys during irradiation

Abstract

The microstructure developed in Zr alloys during irradiation has proven to be increasingly complicated as specimens of varying composition and thermo-mechanical history have been irradiated to higher fluences. Cavities form in neutron and electron irradiated Zr at temperatures between 625 and 775 K. They are less common in the Zircaloys either because of vacancy trapping or because c- component dislocation loops compete for vacancies. There is a complex dependence on the purity of the material which is related to the presence of precipitates or interstitial solutes and on the presence of insoluble gases such as He. The latter is necessary to prevent the collapse of small vacancy clusters to form vacancy loops. Vacancy and interstitial dislocation loops having Burgers vectors of b = 1 3 〈11 2 ̄ 0〉 ( 〈a〉 type) coexist during neutron and electron irradiation. They are generally aligned in rows or layers parallel with (0001). There is a marked decrease in the stability of vacancy 〈a〉 type loops at temperatures ⩾ 723K. Dislocation loops having Burgers vectors of b = 1 2 [0001] (vacancy type), b = 1 3 〈11 2 ̄ 3〉 (interstitial type) and 1 6 〈20 2 ̄ 3〉 (undetermined character) have been identified in Zr following electron irradiation at temperatures between 573–773 K. Vacancy loops of b = 1 6 〈20 2 ̄ 3〉 have been observed in Zr and Zr alloys with a high interstitial solute content following neutron irradiation at temperatures between 560–773 K. Their formation and growth is determined by a combination of: (i) size effect interaction, dependent on their Burgers vector; (ii) stress, generated in collision cascades during neutron irradiation or in oxidised thin foils during electron irradiation; (iii) interstitial solute, which may lower the stacking-fault energy or segregate to the dislocations bounding the loops; and (iv) anisotropic interstitial diffusion (principally in the basal plane). The dislocation network in cold-worked materials is retained during neutron irradiation, at least for fluences up to 7 × 10 25 n m −2 at 573 K. Intermetallic precipitates of Zr with Fe, Cr and Ni undergo amorphous transformations (depending on the precipitate composition) during neutron irradiation at temperatures < 600 K. Radiation-induced dissolution and reprecipitation occurs at temperatures > 560 K. The secondary precipitates are often distributed in rows or layers parallel with (0001). Radiation-enhanced precipitation from solution has been observed in ZrSn and ZrNb alloys at temperatures between about 573–873 K.