Evolution of Zr(Fe,Cr)2 second phase particles in Zircaloy-2 under heavy ion irradiation

The Zr(Fe,Cr)2 second phase particles (SPPs) found in Zircaloy-2 and -4 are known to amorphize and dissolve under irradiation. In the present work, their evolution has been studied in situ in a transmission electron microscope (TEM) under 600 keV Ar2+ irradiation at 320°C, taking samples to two different doses of 13 and 24 dpa. Using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDX), the samples were thoroughly characterized before and after irradiation, allowing subtle changes in precipitate chemistry to be detected. At both doses the SPPs are fully amorphous, with an accompanying release of Fe to the surrounding matrix. The Fe/Cr ratio is seen to decrease in the SPP core and increase at the interface with the matrix, a result of ballistic mixing across the interface coupled with outward diffusion of Fe. No alignment of dislocation loops or segregation of solute to defects in the matrix was observed, although Fe was found to diffuse further along the c-axis direction compared to the a-axis direction.