Microstructure and room temperature fracture toughness of Nb–Si-based alloys with Sr addition

Nb–Si-based alloys show great potential to surpass the widely used Ni-based superalloys. The element Sr is widely applied in aluminum and magnesium alloys, but reports about the effects of Sr on Nb–Si-based alloys are quite rare. So, Nb–Si-based alloys with nominal compositions of Nb–15Si–24Ti–4Cr–2Al–2Hf–0/0.05/0.15Sr (at%) were prepared by directional solidification and heat treatment. The microstructural characterization and room temperature fracture toughness of Nb–Si-based alloy were systematically investigated. Results show that all these alloys consist of Nb₅Si₃ phase embedded within Nb solid solution (Nb_ss) matrix. The Nb₅Si₃ phase becomes refined and more discontinuous after adding minor Sr. As for the fracture toughness, 0.05 at% and 0.15 at% Sr additions do not cause significant change. The discontinuous and refining mechanism of Sr element was studied, and the analysis of toughness decreasing with Sr addition reveals that the size of Nb_ss phase plays a crucial role in determining the fracture toughness.