Effect of Si on microstructure and fracture toughness of directionally solidified Nb silicide alloys

Nb-xSi(x=3,9,16)-22Ti-3Cr-3Al-2Hf (at.%) have been successfully prepared by directional solidification in an optical floating zone furnace. Microstructure analysis and phases identification of the alloys were examined by X-ray diffraction (XRD), Electro Probe Micro Analyzer (EPMA) and Energy Disperse Spectroscopy (EDS). Fracture toughness specimens without pre-crack were prepared, room temperature fracture toughness of alloys was tested by three-point bending method, and fracture mechanism was studied. The results showed that with increasing Si content, Nb₅Si₃ phase gradually increased and the phase transformed from γ-Nb₅Si ₃ to the stable α-Nb₅Si₃ phase and β-Nb₅Si₃ phase. There appeared the Ti-rich Nb ₅Si₃ phase when the Si content is 16 at%. In addition, more micro-cracks generated in the Ti-rich Nb₅Si₃ phase, which seriously affected room temperature fracture toughness of the alloys.