Structure-property correlation and deformation mechanisms in ductile phase (Nbss) toughened cast Nb–Si alloys

High-temperature alloys based on the Nb–Si system have been considered as potential alternative materials for Ni–based superalloys. The focus of present research is to investigate the effect of Ti and Zr alloying elements and its combined addition on the microstructure, mechanical properties (compression, fracture toughness, hardness and modulus) and fracture behavior of the Nb–Si alloys. The microstructure of Nb–18.7Si and Nb–18.7Si–5Ti alloys composed of Nbss and Nb3Si phases. On the other hand, interestingly the Nb-18.7Si-5Zr and Nb–18.7Si-5Ti-5Zr systems exhibited distinctive microstructure and consisted of Nbss and α–Nb5Si3 phases. The present work reveals that the combined addition of Ti and Zr to Nb–Si (Nb–18.7Si–5Ti–5Zr) system resulted exceptionally high copressive strength (2236 ± 81 MPa), improved strain (10.16 ± 0.59%) and fracture toughness (10.59 ± 0.35 MPa.m1/2). The formation of high strength α-Nb5Si3 phase was attributed to such high strength value. Whereas, the increase in the phase fraction and coarsening of Nbss phase along with the presence of lamellar Nbss phase morphology are the contributing factors for its enhanced fracture toughness. The achievement of good combination of properties with stable microstructure without subjecting the Nb–18.7Si–5Ti–5Zr material to any additional heat treatment process is the promising outcome of this research. Fracture in the alloys started with cracking of the silicide phase (Nb3Si/α–Nb5Si3) and the crack path tortuosity increased by the crack bridging, branching and deflection mechanisms at the Nbss phase. Further, the constraint of the brittle silicide phase on the ductile Nbss phase impeded the ductility or deformation of the Nbss phase which resulted in interface debonding in the alloys.