Helium irradiation-induced ultrahigh hardening in niobium

In this work, using in-situ uniaxial tensile and compressive testing, microscopy, and theoretical analysis, we study the mechanism underlying the ultrahigh irradiation hardening in niobium (Nb). We show that irradiated Nb pillar exhibits a more than two-fold increase in the yield stress. With in-situ mechanical testing, we observe that He bubbles in Nb promote dislocation nucleation and multiple slip systems. The Nb pillars with 1.2 nm He bubbles fail by bubble coalescence and form a faceted fracture surface. In contrast, the Nb pillars with 8 nm He bubbles fail by bubble elongation and fragmentation. A theoretical analysis of the hardening contribution based solely on the density and size of He bubbles finds that it is less than one third of the experimentally observed hardening. To explain the large gap between the model and the experiment, we propose that the ultrahigh irradiation hardening originates from a large quantity of atomic-size, undetectable He-vacancy (He-V) complexes. The implanted He ions only account for less than 50% percent in the visible He bubbles, while most of them bind to vacancies to form stable He-V complexes that are distributed throughout the lattice. The strong interaction between dislocations and high density of He-V complexes is the chief source for the remarkable irradiation hardening observed in Nb.