Defect microstructure of a Ti64 alloy under Ti$^{2+}$ ion irradiation. Influence of dose and temperature

International audienceThe present paper focuses on the microstructural changes in Ti64 alloy after irradiation. Titanium alloys allow a significant decrease in activation and mass with similar mechanical strength and corrosion resistance than 304 stainless steel, widely used for core internal structures of Pressurized Water Reactors (PWR). The use of titanium alloys could be therefore a significant improvement for reactor exploitation and decommissioning. The nature, size and density of radiation-induced defects are likely to be important parameters governing the in-reactor behavior of the material. The aim of the study was to devise new achievements in thin foil preparation and Transmission Electron Microscopy (TEM) imaging in order to accurately identify and quantify the Ti64 irradiation defects, at temperatures representative of irradiation temperatures in a reactor. The method to image and count the precipitates was developed with samples irradiated by ions at the temperature of 600°C. Indeed, the microstructure of the sample irradiated at 600°C was characterized by a low density of tangled dislocations, and big precipitates, that were easily evidenced. Then the process was applied in samples irradiated with ions at 430°C and 300°C characterized by a microstructure of tangled dislocations, loops and small precipitates. In 430°C irradiation condition, the counting in dark field with a B~[11-23]orientation and with a high magnification micrograph provided similar results than APT analyses. In irradiation condition at 300°C, the precipitates were imaged by TEM as soon as the dose of 1 dpa. However they were not observed at the dose of 0.4 dpa. The precipitates were so small and their density was so high that the counting by TEM was very imprecise and Atom Probe Tomography (APT) analyses were needed. The results of this microstructural analysis allowed a discussion on the nucleation mechanism of the irradiation defects in Ti64 alloy