The effect of he implantation on the tensile properties and microstructure of Cu/Fe nano-bicrystals

In situ uniaxial tensile experiments on as-fabricated and helium-implanted 100 nm-diameter Cu/Fe bicrystals unearth the effect of individual face-centred-cubic/body-centred-cubic (fcc-bcc) interfaces on improving radiation-damage tolerance and helium absorption. Arrays of nanotensile specimens, each containing a single Cu grain in the bottom half and a single Fe grain on top, were fabricated by templated electron-beam lithography and electrodeposition. Helium is implanted at 200 keV to a dose of 10¹⁴ ion/cm² nominally into the interface region. High-resolution, site-specific transmission electron microscopy (TEM) and through-focus analysis reveal that the interfaces are nonplanar and contain ≈5 nm-spaced He bubbles with diameters of 1-2 nm. Nanomechanical experimental results show that the irradiated samples exhibit yield and ultimate tensile strengths more than 60% higher than the as-fabricated ones, while they retain comparable ductility. Tensile failure always occurs gradually, along the interfaces, with no noticeable shape localization. The absence of brittle failure in He-irradiated metals might be explained, in part, by the inability of the small He bubbles to serve as sufficient stress concentrators for cracking. In addition, the non-orthogonal orientation of the interfaces with respect to the loading axes results in the development of both normal- and shear-stress components. Tensile loading along the pillar axes may cause those interfacial regions subjected to normal stresses to detach, while the inclined regions, subjected to shear, to carry plastic deformation until final fracture. Nanotensile experiments on as-fabricated and helium-implanted 100 nm-diameter electroplated Cu/Fe bicrystals shed light on the role of individual interfaces in improving radiation tolerance and absorbing helium. Nanotensile experiments on samples irradiated with He bubbles reveal a yield and ultimate tensile strength 60% higher than for as-fabricated ones while retaining a comparable ductility. Failure always occurs gradually, along the interfaces, with no noticeable shape localization.