Defect formation and irradiation response in Ti-5331 alloy under helium ion exposure

The radiation resistance of titanium alloys and their mechanical responses to irradiation-induced defects are still not fully understood, particularly among the materials being considered for advanced nuclear reactors. In this study, we combine experimental methods with first-principles calculations to explore the formation and evolution of helium bubbles in the Ti-5331 alloy (Ti-5Al-3V-3Zr-0.7Cr) subjected to helium ion irradiation across varying fluences and temperatures. Our findings reveal that vacancy-helium atom complexes (He_mV_n) form in the Ti-5331 alloy, which then evolve into helium bubbles. The size and distribution density of these bubbles correlate with both the irradiation fluences and the temperatures employed. Significantly, we quantitatively estimate the average number of helium atoms trapped within vacancy clusters for sub-nanometer bubbles observed at low irradiation fluences, using a combination of first-principles calculations and experimental data. Notably, we identify approximately 1–2 helium atoms trapped within clusters of 13 vacancy sites, representing the primary type of vacancy defects introduced by irradiation. Additionally, we discuss the significant irradiation hardening observed, primarily due to the strong interactions between larger helium bubbles and defect clusters. These findings provide crucial insights into the complex interplay among irradiation-induced vacancy defects, bubble formation, and the evolution of mechanical properties in titanium alloys.