Microstructural evolution of Ti2ZrHfV0.5Mo0.2 refractory multi-principal alloy under low-energy, high-flux He ion irradiation

Abstract

The irradiation response of refractory multi-principal element alloys (RMPEAs) has gained increasing attention for the potential applications in future fusion devices. In controlled fusion reactors, plasma-facing materials have to suffer continuous low-energy high flux helium (He) ion irradiations. During high flux He irradiation, vacancy-interstitial pairs can be generated through trap mutation mechanism even without cascade collision. To simplify the model, Ti2ZrHfV0.5Mo0.2 RMPEA was irradiated with 50 eV He+ (less than the minimum He displacement energy of the alloy EdTi = 67 eV) under 1200 K to various fluences of 5 × 1024 - 1 × 1026 ions/m2, and the microstructural evolution was investigated. In the irradiated RMPEA, the intense sputtering of Ti, the growth and degradation of fuzz mainly composed of Zr, Hf elements on the surface, as well as the formation of V, Mo enriched band and high density large He cavities in the subsurface layer were observed. Although the sputtering threshold energy for He of V is similar to that of Ti (ETi = 24 eV, EV = 29 eV), V showed anomalous enrichment in the subsurface layer. Moreover, unlike the W fuzz growth process, the adatoms used to form fuzz in Ti2ZrHfV0.5Mo0.2 were derived from the Zr, Hf atoms diffusing through the vacancy-solute exchange mechanism, rather than from the atoms formed by the loop-punching mechanism. The anomalous enrichment of V atoms and the different fuzz growth mechanism were attributed to (i) different migration rates for different species of atoms in the alloy under vacancy diffusion mechanism and (ii) the formation of AB2 (A = Ti, Zr, Hf; B = V, Mo) phase blocking the continued formation of prismatic dislocation loops mainly composed of V and Mo.