Formation, growth and decomposition of precipitates in Zircaloy-2 and -4

The intermetallic precipitates in Zircaloy-2 are mainly Zr(Cr,Fe)2 and Zr2(Ni,Fe). Zircaloy-4 only has the former unless the nickel impurity level is high. These precipitates are based on the hcp ZrCr2 Laves phase and bct Zr2Ni Zintl phase with Fe substituting for Cr and Ni in proportions that depend on the alloy composition. Their structure, composition and morphology are determined by the metallurgical history and can be changed by n eutron irradiation. Zr(Cr,Fe)2 precipitates irradiated at 580 K are depleted in Fe and to a lesser extent Cr. Depletion below the stoichiometric value is coincident with transformation from a crystalline to an amorphous structure. The transformation starts at the surface of the precipitate and advances inwards with increasing fluence. No structural change has been observed in Zr2(Ni,Fe) precipitates irradiated at 580 or 675 K. At irradiation temperatures ≥ 675 K there is solute depletion and dissolution of Zr(Cr,Fe)2 precipitates but no crystalline-amorphous transformation has been observed. There is a distinct observable change in precipitate morphology with the formation of Fe rich precipitates at grain boundaries and in the matrix. Both the Zr(Cr,Fe)2 and Zr2(Ni,Fe) precipitates irradiated at 350 K undergo complete amorphous transformation although solute depletion has not been observed