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

Abstract The high-temperature alloys based on Nb-Si have been considered as potential alternative materials for Ni based super alloys. 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 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 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). The formation of high strength α-Nb5Si3 phase was attributed to such high strength value. Whereas, the increase in the Nbss phase fraction, coarsening of Nbss phase and 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 tortuousity 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.