Investigation on the zirconia phase transition under irradiation

Zirconia, ZrO2, produced by the oxidation of zirconium alloys in nuclear reactors, possesses a high stability under neutron irradiation. No amorphisation of yttrium-stabilised zirconia has been observed even at high dpa values (≈100 dpa). In pure monoclinic zirconia, a phase transition monoclinic → cubic (tetragonal) induced by irradiation has already been observed. The aim of this work is to study in detail the mechanism responsible for this transition. For that purpose, different kinds of irradiations with electrons (to study point defects) and low energetic ions (to study clusters due to collision cascades) have been performed on zirconia samples. A local probe (Raman spectroscopy) and a non-local probe (grazing X-ray diffraction) have been used to characterise the phase formed during irradiation, which is clearly the tetragonal phase. For the ionic implantation, the grazing X-ray diffraction permits to separate effects due to the ballistic collisions and the implantation peak. Using this method, it was possible to show that the profile of the tetragonal phase was only linked to the dpa profile. This result associated to the results obtained by the Raman spectroscopy (broadening of Raman peaks) shows that the phase transition may be induced by clusters formed near the collision cascades.