Effect of In-PWR Irradiation on Size, Structure, and Composition of Intermetallic Precipitates of Zr Alloys

The corrosion behavior of Zr alloys depends on the kind, size, and distribution of the intermetallic second-phase particles. TEM examinations of Zr-Sn-Fe-Cr alloys irradiated in PWRs at temperatures between 300 and 370°C and fast fluences in the range of 5E21 to 1.3E22 cm-2 have been performed to study the irradiation-induced effects on the precipitates. The alloys contained different types of second-phase particles such as Zr(Fe,Cr)2, Zr2(Fe,Si), and Zr3Fe before irradiation. The influence of irradiation was found to depend on temperature and type of second-phase particles. At temperatures below 310°C, the Laves phase Zr(Fe,Cr)2, which normally is the most frequent precipitate in Zry-4, depletes in Fe, becomes amorphous, and dissolves completely at higher fluences. With increasing temperature, the rate of Fe depletion and dissolution decreases and new Zr-Fe phases are formed. At temperatures above 370°C, the Laves phase remains stable or even grows under irradiation. In Fe-containing Zr alloys with little or no Cr, rather large Zr3Fe precipitates are the most frequent particles. These particles are not dissolved by irradiation even at low temperatures. This was confirmed by annealing after irradiation. As a hypothesis, it was assumed that the different behavior of the various precipitates can be related to their melting or decomposition temperatures by using the homologous temperature (i.e., the temperature under consideration in K normalized to the melting or decomposition temperature in K). This interrelationship has been found to apply for irradiation-induced amorphization. The empirical approach to describe the thermal ripening behavior of second-phase particles before irradiation and to describe the transition from irradiation-induced dissolution to irradiation-induced growth by a normalized (homologous) temperature led to reasonable results.