Insights on grain boundary effects on crack formation and propagation in Nb3Sn coatings at low temperature and high strain rates: a molecular dynamics simulation study

Mechanical behavior of Nb3Sn coatings is inherently correlated with the anti-interference ability of the superconducting cavity system and the amplitude and phase stability of the cavity field. We report on atomic-scale simulation and analysis of grain boundary effects on the crack formation and propagation in Nb3Sn coatings at low temperature and high strain rates. For tensile-deformed single crystal Nb3Sn, accompanied by the occurrence of slips, micro voids propagate along the ( 210 ) and (210) planes (stretching the crystal along the [100] direction), and subsequently coalesce, leading to the cracks formation. For tensile-deformed single-crystal Nb3Sn coating on Nb substrate, it fails by crack propagation in the coating before showing significant yielding plateaus. In the Nb3Sn coating, the slip bands on the (201) and ( 210 ) planes meet and merge, inducing the breaking of atomic bonds and the formation of voids at the slip band intersections, and the subsequent micro-crack initiation. The misfit dislocations of Nb3Sn/Nb heterostructure interface and the dislocations nucleated on slip systems control the plastic deformation of the Nb substrate, causing the BCC-to-FCC and FCC-to-HCP phase transformation in the substrate, for which, ductile fracture occurs after necking. For polycrystalline Nb3Sn coating on Nb substrate, the simulations demonstrate that cracks propagate entirely along grain boundaries, exhibiting the intergranular fracture characteristics. The composite exhibits a significant strain rate effect. At high strain rates, crack propagation across grain boundaries in Nb3Sn is hindered, dislocations are emitted into the both adjacent grains from the grain boundary facets, they glide across the grain, and voids are nucleated at the intersections of dislocations inside the grains, which leads to the formation of voids and ultimately the occurrence of trans-crystalline fracture. The findings provide important information about the crack evolution in Nb3Sn coatings, which are critical to the fabrication and performance analysis of Nb3Sn superconducting radio frequency cavities.