Transition from micro-rod to nano-lamella eutectics and its hardening effect in niobium/silicide in-situ composites

To improve the mechanical properties of niobium/silicide in-situ composites via rapid solidification, the evolution of eutectic geometry and the corresponding hardening effect in a prototype Nb–18Si (at.%) composite upon electron beam surface melting (EBSM), i.e., a rapid remelting and solidifying sequence, were studied. Results show that rod-like Nb solid solution (Nbss)/Nb₃Si eutectics prevail in the arc-melted state, yet evolve into lamellar arrangements after EBSM. Atomic scale scanning transmission electron microscopy (STEM) and near-atomic scale atom probe tomography (APT) were employed to characterize the three-dimensional stacking of nano-laminated Nbss/Nb₃Si eutectics and their compositions. Compared with the rod-like eutectics, the lamellar eutectics via EBSM demonstrate a prominent eutectic refinement (39.5 nm in spacing) and an increased volume fraction of Nbss (~41%). Nano-indentation testing reveals that with the microstructural transition from micro-rod to nano-lamella eutectics, a significant increment in hardness up to 13.9 GPa is achieved.