Characterisation of Laves Phases Zr (Cr, Fe)2 oxidized in an open furnace using Mössbauer Spectroscopy

F. SAPORITI; P. BOZZANO; R. VERSACI; C. P. RAMOS; I. RASPINI; C. SARAGOVI

Virginia

Simposio; Int. Symposium on the Industrial Applications of Mössbauer Spectroscopy (ISIAME 2000).; 2000

Zircaloy- 4 (% wt  Sn 1.45- 1.5, Fe 0.18- 0.24, Cr 0.07- 0.13, O 1400 ppm, Zr to balance) is widely used as fuel cladding material in nuclear reactors due to low neutron-capture cross-section, high mechanical strength, high thermal conductivity and good corrosion resistance in water and steam. Iron and chromium are essentially insoluble in Zr at low temperatures (< 600º C) therefore, they are present almost entirely in the form of precipitates . These precipitates are the Zr(Cr, Fe)2 intermetallic compounds, which occur in  hexagonal (C14) and in cubic (C15) structural forms. The structure, composition, average size and morphology of the mentioned second phase precipitates are closely related to the corrosion behaviour of Zry-4. Many studies were carried out in order to evaluate the growth and characterisation of oxide layers (ZrO2) and the evolution of precipitates when Zry-4 is oxidized or neutron irradiated. Previous studies showed that precipitates undergo chemical composition changes during open furnace oxidation. EDS (Energy Dispersive Spectroscopy) showed the presence of Zr, Cr and Fe within both the non-oxidized and partially oxidized precipitates. A progressive iron rejection towards the oxidized precipitate / oxidized matrix interface takes place. Three Laves phases, Zr(Cr0.4 Fe0.6)2,  Zr(Cr0.95 Fe0.05)2  and Zr(Cr0.15 Fe0.85)2, samples were prepared and oxydized with the aim of further contribution to the mentioned experimental evidence. The first composition is the most frequently observed and it presents the hexagonal C14 structure meanwhile the other ones present the cubic C15 structure. Mössbauer spectroscopy (MS) together with X- Ray diffraction (XRD) and Electron Microscopy (EM) technique were applied. Results allowed to following a 7 min. oxidation process and confirmed that in Fe-rich Laves phases the above EDS results are sustained.   In the case of the Cr rich Laves phase, 7 min oxidation is not enough to show the a-Fe presence.