IRRADIATION-INDUCED PHASE-TRANSFORMATIONS IN ZIRCONIUM ALLOYS

Ion and electron irradiations were used to follow the irradiation-indu ced crystalline-to-amorphous transformation in Zr3Fe, ZrFe2, Zr(Cr,Fe) 2 and ZrCr2, as well as in Zr(CrFe)2 and Zr2(NiFe) precipitates in Zir caloy-4. Ar-40 and Bi-209 ion irradiations of Zr3Fe were performed at 35-725 K using ions of energy 15-1500 keV. The effect of the deposited -energy density theta(v)BAR in the collision cascade on the nature of the damaged regions in individual cascades was investigated. The amorp hization kinetics of Zr3Fe during in situ electron irradiation were al so determined. The electron fluence required for amorphization increas ed exponentially with temperature, and the critical temperature for am orphization was about 220 K, compared with 575-625 K for ion irradiati on. The difference between the heavy ion and electron irradiation resu lts is attributed to the fact that ion irradiation produces displaceme nt cascades, while electron irradiation produces isolated Frenkel pair s. The dependence of the damage production on the incident electron en ergy was determined for Zr3Fe and the results could be analysed in ter ms of a composite displacement cross-section dominated at high energie s by displacements of Zr and Fe atoms; by displacements of Fe atoms at intermediate energies; and by secondary displacements of lattice atom s by recoil impurities at low energies. An investigation was initiated on ZrFe2, Zr(Cr,Fe)2 and ZrCr2 to study the effect of variation of th e stoichiometry and the presence of lattice defects on irradiation-ind uced amorphization. The irradiation-induced amorphization of the inter metallic precipitates Zr(Cr,Fe)2 and Zr2(Ni,Fe) in Zircaloy-4 was also studied during in situ bombardment by Ar-40 ions of energy 350 keV. T he amorphization morphology was shown to be homogeneous. These results are discussed in the context of previous experimental results of neut ron and electron irradiations, and likely amorphization mechanisms are proposed.