Morphological Heredity of Intermetallic Nb₅Si₃ Dendrites in Hypereutectic Nb-Si Based Alloys via Non-Equilibrium Solidification

For hypereutectic Nb-Si based alloys, primary Nb₅Si₃ phases typically grow in a faceted mode during equilibrium or near-equilibrium solidification, which damages the ductility and toughness. To address this issue, here we artificially manipulate the growth morphology of Nb₅Si₃ using electron beam surface melting (EBSM) and subsequent annealing treatments. Results show that such a non-equilibrium solidification pathway enables the transition from faceted growth to non-faceted dendritic growth of Nb₅Si₃, along with evident microstructure refinement, generation of metastable β-Nb₅Si₃ phases and elimination of chemical segregation. The transformation from β-Nb₅Si₃ to α-Nb₅Si₃ and Nb solid solution (Nbss) particles is triggered by the annealing treatment at 1450 °C for 5 h. Also, we find the annealing-mediated formation of inherited Nb₅Si₃ dendrites that maintain the dendritic morphology of the original as-solidified β-Nb₅Si₃ dendrites. This work thus provides a feasible routine to obtain thermally stable and refined α-Nb₅Si₃ dendrites in hypereutectic Nb-Si based alloys.