Effect of heat treatments on the structure and composition of second phase particles in Zircaloy-4

The effects of heat treatments on the structure and composition of second phase particles (SPPs) in Zircaloy-4 have been investigated after SPPs separated from alpha zirconium matrix by electrochemical method. In these Specimens after air cooling from heating at 1050 degree C beta phase, the SPPs precipitated during cooling are Zr(Fe, Cr)2 with cubic structure, Fe/Cr ratio in Zr(Fe, Cr)2 is fluctuated between 2.1∼2.5. The cubic crystal structure does not change and only the Fe/Cr ratio decreases gradually to 1.9 in specimens re-heated at 600∼800 degree C for 3h. Small amount of precipitates with hexagonal Zr (Fe, Cr)2 can be detected, when specimens are re-heated at 700∼800 degree C for 3h. SPPs in Zircaloy-4 plates delivered by factory are hexagonal Zr(Fe, Cr)2 with Fe/Cr ratio decreasing from 1.9 to 1.5, when specimens are re-heated at 700∼800 degree C for 3h. It shows that the Fe and Cr atoms in SPPs will diffuse and substitute with those in surrounding matrix during specimens heated at a certain temperature. The solid solution content of Cr atoms in hexagonal Zr(Fe, Cr)2 is higher than that in cubic Zr(Fe, Cr)2, and the diffusion rate of Fe atoms is faster than that of Cr atom during the precipitation of SPPs from β phase cooling. this is why the Fe/Cr ratio of Zr(Fe, Cr)2 SPPs varies with the difference of heat treatments