A TEM study of the stability of intermetallic precipitates in Zircaloy nuclear reactor components

Transmission electron microscopy (TEM) has been used to characterise the radiation-induced changes in Zr(Cr,Fe)₂ intermetallic precipitates present in Zircaloy-2 and -4 nuclear reactor components. The results show that the precipitates become completely amorphous at low fluences (<1 × 10²⁴ n·m⁻²) during low temperature neutron irradiation (about 330 K) with no associated chemical composition change. At higher temperatures (about 573 K), a duplex amorphous-crystalline structure is produced. The precipitates retain a crystalline core surrounded by a peripheral amorphous layer that advances inwards with increasing fluence. The amorphous outer layer is coincident with a depletion of Fe that is dispersed into the surrounding hcp -phase matrix. Subsequent post-irradiation heat-treatment below the amorphous phase recrystallisation temperature results in the back-diffusion of Fe into the precipitates.